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  1. #1
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    Explain "too linear" 2013 Fox forks to me.

    I'm trying to understand how Fox's 2013 fork offerings are "too linear", per multiple reviews.

    Recalling my intro physics class, coils are completely linear. The force to compress the coil is directly proportional the distance from the resting state. For the record, I've never ridden a coil fork regularly.

    Haven't we been trying to get air forks to "feel like coil" for years?

    What does "too linear" mean? I understand that this means that you may blow through the travel, but wouldn't this be a problem with coil forks as well?

    I have a 2013 Fox 160mm fork and do occasionally use all the travel. I feel that it is light-years beyond my older Fox fork that never got more that ~130mm of travel (even when going over the bars after big drops).

    Please explain.

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    An air fork has to support a wide range of weights, unlike a coil which uses different springs for different weights.

    This can cause an air fork to have the possibility of having too much initial spring force, or too little at the end of the travel. It is most likely to happen if the rider is very heavy or very light, but it also depends on the course. On a very smooth trail, an average rider-- right in the meat of what the fork was designed for-- may complain they don't use enough travel. On a trail with huge drops, that same rider may complain that they bottom harshly, unless the fork is so stiff as to be punishing on small bumps.

    This is why the air forks are often progressive. Fox heard people complaining that their forks were too progressive, and no doubt saw discussions on MTBR and elsewhere about people modifying their forks to be more linear.

    For me, I like the progressive rate of my bone-stock (so far ) 2012 Fox Float 32 RLC fork. It is plush on the trail, I have dialed in enough compression to strongly resist brake dive, and a combination of the air spring rate and compression damping ensures I don't bottom even when I catch 2' of air.

    However, I weigh 170 pounds geared up. I am neither overly light or heavy. The base fork works well for me and my riding conditions. And reading your post again, it sounds like your 2013 fork is working perfectly for you. So don't worry if another rider feels their 2013 is too linear.

  3. #3
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    Btw, you absolutely can have a progressive coil spring. It has different winding and/or thickness which results in a non-linear spring rate. This is very popular on passenger cars because it gives a nice ride on city streets, expansion joints, etc, but then the spring rate becomes higher to take a fast corner or absorb a big pothole.

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    Quote Originally Posted by ColinL View Post
    Btw, you absolutely can have a progressive coil spring. It has different winding and/or thickness which results in a non-linear spring rate. This is very popular on passenger cars because it gives a nice ride on city streets, expansion joints, etc, but then the spring rate becomes higher to take a fast corner or absorb a big pothole.
    Do the coils in contemporary coil forks have different windings or thicknesses in the same coil?

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    Quote Originally Posted by meph2 View Post
    Do the coils in contemporary coil forks have different windings or thicknesses in the same coil?
    No. Progressive coil springs for bicycle suspension do not exist. Although it would be a nice feature to have a slightly progressive coil for a fork. Instead, damping and bottom out bumpers are used.

  6. #6
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    A single progressive coil spring could probably be made for a bicycle at a high cost. The issue of mass producing them is that if you have too much sag or preload on a progressive spring, the softer section can be entirely collapsed, which then leaves you with a shorter main spring. You would gain no benefit from the softer section.

    To avoid that issue, you would need a greater number of progressive springs, each supporting a narrower range of rider weight, instead of 3-4 springs that cover 140 pounds to 260 pounds.

    It's also far easier to tune all aspects of your suspension if you have a linear spring, and many racers prefer linear because they want a consistent feel and are willing to give up plushness.

  7. #7
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    Quote Originally Posted by ColinL View Post
    A single progressive coil spring could probably be made for a bicycle at a high cost. The issue of mass producing them is that if you have too much sag or preload on a progressive spring, the softer section can be entirely collapsed, which then leaves you with a shorter main spring. You would gain no benefit from the softer section.

    To avoid that issue, you would need a greater number of progressive springs, each supporting a narrower range of rider weight, instead of 3-4 springs that cover 140 pounds to 260 pounds.

    It's also far easier to tune all aspects of your suspension if you have a linear spring, and many racers prefer linear because they want a consistent feel and are willing to give up plushness.
    Taking a stock spring and adding a little bit of tighter wind would not change over all spring, it would just at a little bit up ramp up at the end of the stroke. That said, the very few choices of coil springs for forks is a problem regardless of linear or progressive. its very easy to get caught between coils. I still ride coil forks because the mid stroke support and small bump compliance cant be matched by any air spring I have used.

  8. #8
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    Quote Originally Posted by mullen119 View Post
    No. Progressive coil springs for bicycle suspension do not exist. Although it would be a nice feature to have a slightly progressive coil for a fork. Instead, damping and bottom out bumpers are used.
    Years ago Stratos had progressive rear coil springs (still have one in my junk box). Marzocchi had progressive fork springs for their Monster T. For a given weight of 600lb the progressive Stratos spring felt a good 150lb lighter in tension than a straight rate 600lb coil. I believe the Monster T springs were also ranged for a lighter rider.

  9. #9
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    Quote Originally Posted by keen View Post
    Years ago Stratos had progressive rear coil springs (still have one in my junk box). Marzocchi had progressive fork springs for their Monster T. For a given weight of 600lb the progressive Stratos spring felt a good 150lb lighter in tension than a straight rate 600lb coil. I believe the Monster T springs were also ranged for a lighter rider.
    KTM motocross bikes had progressive coil springs for years trying to do away with linkage in the rear suspension. They eventually gave up and went with a linkage design because it works better. This makes me think that progressive springs for rear shocks are pointless because bike designed for coil rear shocks will incorporate a rising rate leverage ratio to combat bottoming. In forks on the other hand, I can see a market for a slightly progressive coil spring to help prevent harsh bottoming since only a handful of forks have a hydraulic bottoming circuit. I have always been somewhat surprised that they dont exist for modern forks.

    Anyway, we are hijacking the OP's thread with coil spring talk, Sorry about that OP.

  10. #10
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    I don't think that the "too linear" applies so much for forks greater than 120 mm of travel. But less than that, since you have so much less travel to start with, it becomes very difficult to get a fork that is not too firm without it blowing through travel.

  11. #11
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    Forks are usually not exactly linear, whether coil or air.

    Coil forks have a large "air chamber" within, and apart from that, usually either a charged space on top of a piston to prevent foaming, or air above oil on an "open bath" design. Both of these add progression as the fork progresses through travel and pressure is added to the oil column and onto the damping piston, usually resulting in more damping. This isn't a dramatic effect though, so often times a hydraulic cone is added to the bottom of the damping piston, as with some Fox designs and the Avalanche cartridges (not sure if marzocchi is doing this yet, but their newest cartridges are nearly a carbon-copy of the Avalanche stuff).

    The old "oil bath" system was easy to adjust the progressiveness via the oil height, allowing you to run a decent amount of sag and still ramp up to avoid bottom out.

    Most all-air forks have enough progression due to the air spring to get away without any of these additional systems or features, although sometimes they still inherently have some of them and in some cases can end up too progressive.

    Interestingly, it's usually the linear forks that feel over-sprung and too stiff when set up properly to avoid bottomout. They will run less sag when set up like this, and ride harsher. If it's a "linear fork" and it's truly "blowing through travel", you're going to be feeling it bottoming and you'll figure this out really fast and up the spring pressure or spring weight.

    The more progressive stuff can feel like it's "blowing through the travel" more, especially as long as it's not bottoming out. That real supple initial action can be nice, but must be supported with the right amount of damping. The faster an impact happens, the faster the oil is forced through the pistons and the more the fork is slowed down. This means the low-speed damping is usually lacking and where the fork falters a bit, but this is very subjective to the exact fork and damping system.
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  12. #12
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    Quote Originally Posted by mullen119 View Post
    No. Progressive coil springs for bicycle suspension do not exist. Although it would be a nice feature to have a slightly progressive coil for a fork. Instead, damping and bottom out bumpers are used.
    Marzocchi pro-wind springs existed, and I used them on several forks. Not really any improvement. Other forks used multi-stage coil springs of different rates as well, not sure if it was Risse, but it was one of those earlier small-production manufacturers. Heck, even the simpler manitou coil+elastomer stacks would qualify here (which were copied the next season by RS if I recall correctly). I had a few of these manitou forks, they were pretty terrible due to how sticky all of the parts were. I took my 97 FS and converted it to "mostly coil" like the FS Ti model, it was an improvement, but the chassi (lubrication) seriously held it back. It was hard to make any assessments of those based on the performance. The marzocchi forks obviously worked, but was there any benefit to using them in an open bath fork that could already be adjusted? No.
    "It's only when you stand over it, you know, when you physically stand over the bike, that then you say 'hey, I don't have much stand over height', you know"-T. Ellsworth

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  13. #13
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    Quote Originally Posted by mullen119 View Post
    KTM motocross bikes had progressive coil springs for years trying to do away with linkage in the rear suspension. They eventually gave up and went with a linkage design because it works better.
    Naw
    It's because their suspension components and design were crap. Their forks had the worst build quality bar none until a few years ago. It took them almost a decade to make their pds shock work near as good as the Ohlins version that they didn't want to pay to license. It's definitely not because of the spring that they went back to a linkage design.

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    Quote Originally Posted by kan3 View Post
    Naw
    It's because their suspension components and design were crap. Their forks had the worst build quality bar none until a few years ago. It took them almost a decade to make their pds shock work near as good as the Ohlins version that they didn't want to pay to license. It's definitely not because of the spring that they went back to a linkage design.
    Its not the spring itself, its the bad design of a linkageless rear suspension. There is no reason to use a progressive rear spring when you can use linkage to create the progressiveness. It was just a bad idea from the start to try to make a linkageless design.

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    So, why can't the new 2013 Fox air forks use the compression damping mentioned above to correct the "too linear" feeling.

    Isn't the Vanilla RC2 damper from last year the same as this year's Float damper?

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    Modern air forks really need adjustable air spring volume. Maybe even a staged air volume like the "DRCV".

  17. #17
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    Quote Originally Posted by meph2 View Post
    So, why can't the new 2013 Fox air forks use the compression damping mentioned above to correct the "too linear" feeling.

    Isn't the Vanilla RC2 damper from last year the same as this year's Float damper?
    Is the RC2 really progressive? I pulled the RC2 cartridge out of my 06' Vanilla RC2 and if you cycled the damper , by hand, it would have no resistance until the last 3/4" of travel then hit a hydraulic wall.

  18. #18
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    Quote Originally Posted by keen View Post
    Is the RC2 really progressive? I pulled the RC2 cartridge out of my 06' Vanilla RC2 and if you cycled the damper , by hand, it would have no resistance until the last 3/4" of travel then hit a hydraulic wall.
    I would think that it would stop hard bottoming. The reviews I've seen report hard bottoming even with the RC2 damper.

  19. #19
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    Quote Originally Posted by mullen119 View Post
    Its not the spring itself, its the bad design of a linkageless rear suspension. There is no reason to use a progressive rear spring when you can use linkage to create the progressiveness. It was just a bad idea from the start to try to make a linkageless design.
    Sure there is

    You shave around 3-4lbs off the bike and you lower maintenance time/costs for enduro bikes. I think the idea is great it was just a poor execution.

  20. #20
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    It is very easy to make the new forks more progressive. All you have to do is add oil to the air chamber. Should take all of 5 minutes.

    One of the reasons they made the fork more linear was to allow custom tuning of the spring rate.

  21. #21
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    Quote Originally Posted by ColinL View Post
    This is why the air forks are often progressive. Fox heard people complaining that their forks were too progressive, and no doubt saw discussions on MTBR and elsewhere about people modifying their forks to be more linear.

    For me, I like the progressive rate of my bone-stock (so far ) 2012 Fox Float 32 RLC fork. It is plush on the trail, I have dialed in enough compression to strongly resist brake dive, and a combination of the air spring rate and compression damping ensures I don't bottom even when I catch 2' of air.


    Correct me if I'm wrong, but all air forks are progressive based on the volume and well, they use air. It's a gas priciples basic property. Boyle's Gas Laws. Coils are not in this category. As already stated, they're linear.

    For me, a good coil, with a upper gas cushion to catch the big hits. Coils are often referred to as "buttery." Because they're linear.

    Adding oil makes the fork have a steeper "ramp" in the spring rate curve. If it's too steep the rebound damping control might not be able to control the higher rate as it was not tuned to this rate, or lower volume of air.

    Adding air to a coil makes it more progressive at teh top of the stroke. A desirable trait , imho.

    Been doing full sized suspension for years on off road race vehicles. Bypasses , gas springs and all disc valving. Progressive leveraging. The tenets are very similar.

    DR
    Last edited by dirtracer; 11-04-2012 at 11:24 AM.

  22. #22
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    Quote Originally Posted by dirtracer View Post
    Correct me if I'm wrong, but all air forks are progressive based on the volume and well, they use air. It's a gas priciples basic property. Boyle's Gas Laws. Coils are not in this category. As already stated, they're linear.

    For me, a good coil, with a upper gas cushion to catch the big hits. Coils are often referred to as "buttery." Because they're linear.

    Adding oil makes the fork have a steeper "ramp" in the spring rate curve. If it's too steep the rebound damping control might not be able to control the higher rate as it was not tuned to this rate, or lower volume of air.

    Adding air to a coil makes it more progressive at teh top of the stroke. A desirable trait , imho.

    Been doing full sized suspension for years on off road race vehicles. Bypasses , gas springs and all disc valving. Progressive leveraging. The tenets are very similar.

    DR
    Air springs can be engineered to be linear throughout the entire stroke. The reason coils are considered "buttery" is because they lack the initial stiffness that MOST air springs possess, and don't have the extra friction caused by more sliding surfaces.

    Coil springs alone are dead linear within their usable stroke, however without some form of ramp up (be that spring, damping or hydraulic lock anti-bottoming systems, or a leverage ratio that decreases to increase the effective spring/damping rates at the wheel), completely linear sprung suspension needs to be run at a level of stiffness most people would consider unacceptable in order to prevent harsh bottoming out on bikes that are ridden aggressively. The "plushest" feeling suspension (not necessarily the most stable or controlled) invariably comes from a system that has a soft spring rate (measured at the wheel) at the start of the travel and a stiffer rate at the end of the travel - in other words, a progressive system.

    Obviously, suspension that is TOO progressive lacks control in the early and mid stroke, and/or ramps up too savagely.
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    Quote Originally Posted by dirtracer View Post
    Correct me if I'm wrong, but all air forks are progressive based on the volume and well, they use air. It's a gas priciples basic property. Boyle's Gas Laws. Coils are not in this category. As already stated, they're linear.

    For me, a good coil, with a upper gas cushion to catch the big hits. Coils are often referred to as "buttery." Because they're linear.

    Adding oil makes the fork have a steeper "ramp" in the spring rate curve. If it's too steep the rebound damping control might not be able to control the higher rate as it was not tuned to this rate, or lower volume of air.

    Adding air to a coil makes it more progressive at teh top of the stroke. A desirable trait , imho.

    Been doing full sized suspension for years on off road race vehicles. Bypasses , gas springs and all disc valving. Progressive leveraging. The tenets are very similar.

    DR
    Still, can't you alter the progressive rate by changing rates of how much oil is allowed to pass through the shims by changing the diameter of the shims? It seems that not all coils are as linear as they seem, either. The property of a gas is progressive and the property of the coil is linear, but with augmentation to the overall system, these rules seem to have exceptions.

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    [QUOTE=ehigh;9842232]Still, can't you alter the progressive rate by changing rates of how much oil is allowed to pass through the shims by changing the diameter of the shims?

    Changing damper shims would alter lo/hi speed compression characteristics. Progression should be handled by the spring air chamber size. My Rockshox Pike coil bottomed out quite easily as the air volume filled the entire stanchion and no oil other than semi-bath was used (pop seals if you tried adding oil). I asked a Rockshox tech what would control bottomout for a properly sprung Pike ? He said the fork had bottomout cushions in the lowers. Ok so my fork is slamming the bumpers on 1' drops ? I was told to go up a spring rate. Ok now my fork has terrible small bump compliance - SOL.

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    Quote Originally Posted by Steve VS View Post
    Air springs can be engineered to be linear throughout the entire stroke. The reason coils are considered "buttery" is because they lack the initial stiffness that MOST air springs possess, and don't have the extra friction caused by more sliding surfaces.

    Coil springs alone are dead linear within their usable stroke, however without some form of ramp up (be that spring, damping or hydraulic lock anti-bottoming systems, or a leverage ratio that decreases to increase the effective spring/damping rates at the wheel), completely linear sprung suspension needs to be run at a level of stiffness most people would consider unacceptable in order to prevent harsh bottoming out on bikes that are ridden aggressively. The "plushest" feeling suspension (not necessarily the most stable or controlled) invariably comes from a system that has a soft spring rate (measured at the wheel) at the start of the travel and a stiffer rate at the end of the travel - in other words, a progressive system.

    Obviously, suspension that is TOO progressive lacks control in the early and mid stroke, and/or ramps up too savagely.
    Air springs are not linear. Period. The only way I can see that a air spring will become linear is if A) you bleed off air at certain points in the travel to create a linear spring rate or B) you have a rather large air chamber, and this will only make it less pregressive. High volume air cans for example.

    "Initial stiffness"? Please elaborate. I have seen air forks compress over the tiniest of rocks. Also, "more slidng surfaces"? Again, please elaborate.

    "completely linear sprung suspension needs to be run at a level of stiffness most people would consider unacceptable in order to prevent harsh bottoming out on bikes that are ridden aggressively" I have trouble believing this because motorcycle suspension doesn't use any air at all and has been working in off-road flawlessly for years.

    "The "plushest" feeling suspension (not necessarily the most stable or controlled) invariably comes from a system that has a soft spring rate (measured at the wheel) at the start of the travel and a stiffer rate at the end of the travel - in other words, a progressive system." Again, dirt bikes dont have progressive suspension and they seem to have the whole " two wheeled off-road thing" pretty dialed. Coil is the way to go. Plain and simple. The only drawback of coil in a pure performance situation would be weight. And titanium springs will fix that.

  26. #26
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    Quote Originally Posted by ehigh View Post
    Still, can't you alter the progressive rate by changing rates of how much oil is allowed to pass through the shims by changing the diameter of the shims? It seems that not all coils are as linear as they seem, either. The property of a gas is progressive and the property of the coil is linear, but with augmentation to the overall system, these rules seem to have exceptions.
    No, not unless you vary the orofice size or pressure acting on it at different points in the travel, then it becomes "progressive damping", except that "progressive damping" is very poor for bump control, because it means that you'll get good slow-bump performance, but horrible high speed, as the fork has to flow a lot more oil faster in that case, and if it's solely based on the position, it won't make a distinction between a high speed and low speed event. The 5th element and curnut CV/T system did just this, but they were pretty horrible damping systems in most respects. I owned both and I'd recommend steering clear They "pedaled well" and handled the "big hits" well (like a drop, or landing, etc), but were horrible otherwise. The fox DHX shocks had SPV elements, as there was an SPV type valve in the reservoir with a normal shim-stack, but it basically had characteristics of both, position sensitive damping that could make up for falling-rate suspension designs or be adjusted for a pedaling platform, better high speed damping control than the Progressive/Curnut shocks, but high speed performance still suffered a bit due to the shock just not being able to flow as much oil as is necessary.

    There are some good reads on high speed damping, so I suggest doing some more research. Realize that it has to be regressive, due to the fact that oil forced through a port creates a huge amount of "damping", so unless it's allowed to open further the faster the fork moves, it will "hydrolock". This doesn't have anything to do with the "progression" of the fork though, that is a separate effect, and again only slightly affected by piston pressure and volume of air above the piston.

    You don't want damping to control progression, there are too many variables and it's far to difficult to get the "correct amount" of damping for low and high speed events. You can have hydraulic cones at the bottom to control bottomout, but that's only the last bit of travel. I think there is an optimum rate at which a fork will "ramp up", and the fork has to be designed with that total picture in mind. As the fork is intended to work with riders from 125-225lbs as a fairly targeted range, that's going to be hard to achieve without extreme engineering/design/features or aftermarket tuning. Otherwise think about it, you set the air pressure low, but it ramps up at the end as if it had a 225lb rider on it, or you set the pressure high because you are heavier, and it doesn't ramp up enough at the end because you are heavier than the target. This was easier to solve with the oil-height forks like some old RS forks (boxxers, etc), many current DH/AM marzocchis, and a few others, but those forks were heavier due to that oil bath and foaming was possible, although not usually very detrimental with these forks. Manitou and others used specific bottom-out bumpers too, although again, this was just the end of travel.
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    Quote Originally Posted by Gawdodirt View Post

    "Initial stiffness"? Please elaborate. I have seen air forks compress over the tiniest of rocks. Also, "more slidng surfaces"? Again, please elaborate.



    "The "plushest" feeling suspension (not necessarily the most stable or controlled) invariably comes from a system that has a soft spring rate (measured at the wheel) at the start of the travel and a stiffer rate at the end of the travel - in other words, a progressive system." Again, dirt bikes dont have progressive suspension and they seem to have the whole " two wheeled off-road thing" pretty dialed. Coil is the way to go. Plain and simple. The only drawback of coil in a pure performance situation would be weight. And titanium springs will fix that.
    More sealing surface's in an air suspension increase stichion, plus you must overcome air preload. I spoke w/ a Tech. @ Manitou Suspensions and he described the operation of their MARS Air Spring. Basically a coil spring and air spring are run in series. The initial suspension movement is thru the coil spring (no air stichion or preload) then the secondary air spring come into play adding a progressive spring curve. For those that still remember coil sprung bikes you could rest you elbow on the seat and the rear would sag like butter, fronts required a slight weight and you might have to "stretch the front wheel" to extend the fork off the negative spring.

    Motorcycles use open bath dual coil setup's. Weight and the lack of LSC (platform) are huge turn offs for the MTB community.

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    Quote Originally Posted by keen View Post
    More sealing surface's in an air suspension increase stichion, plus you must overcome air preload. I spoke w/ a Tech. @ Manitou Suspensions and he described the operation of their MARS Air Spring. Basically a coil spring and air spring are run in series. The initial suspension movement is thru the coil spring (no air stichion or preload) then the secondary air spring come into play adding a progressive spring curve. For those that still remember coil sprung bikes you could rest you elbow on the seat and the rear would sag like butter, fronts required a slight weight and you might have to "stretch the front wheel" to extend the fork off the negative spring.

    Motorcycles use open bath dual coil setup's. Weight and the lack of LSC (platform) are huge turn offs for the MTB community.
    Marzocchi does the same thing. They use the coil up into the last inch or so of compression, then the air kicks in and prevents the bottom out. Weight is only a turn off for people who don't enjoy the effects of gravity.

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    Quote Originally Posted by Gawdodirt View Post
    Air springs are not linear. Period. The only way I can see that a air spring will become linear is if A) you bleed off air at certain points in the travel to create a linear spring rate or B) you have a rather large air chamber, and this will only make it less pregressive. High volume air cans for example.

    Air spring force at any given point is the sum of the negative spring force and the positive spring force. Without going into detail, I assure you that it is possible to create a very linear spring curve by manipulating those parameters - this is not to say, however, that items currently on the market actually do this.

    "Initial stiffness"? Please elaborate. I have seen air forks compress over the tiniest of rocks. Also, "more slidng surfaces"? Again, please elaborate.

    Air springs in their usual form (positive spring opposed by negative spring) typically suffer from a "preloaded" feel (caused by a high initial spring RATE - not spring force, spring RATE) that then drops away into low support in the mid stroke, usually followed by a progressive ending stroke (end stroke progression depends almost entirely on positive chamber volume vs piston displacement). Air springs obviously have to have a sealed piston running in a tube at the very least (eg Float forks with the coil negative springs), but normally also have an air negative springs (all air shocks, most air sprung forks) which has a seal head as well, and in some cases even a tertiary sliding seal (eg TALAS, Infinite Travel etc). A coil spring has none of these seals.

    "completely linear sprung suspension needs to be run at a level of stiffness most people would consider unacceptable in order to prevent harsh bottoming out on bikes that are ridden aggressively" I have trouble believing this because motorcycle suspension doesn't use any air at all and has been working in off-road flawlessly for years.

    Motorcycle suspension does however typically rely on progressive leverage rates in the rear end, and spring ramp up in the forks from air chambers that are initially at atmospheric pressure, and whose sizes are dictated by oil levels. Most open bath Marzocchi forks, among others, use exactly the same principles.

    "The "plushest" feeling suspension (not necessarily the most stable or controlled) invariably comes from a system that has a soft spring rate (measured at the wheel) at the start of the travel and a stiffer rate at the end of the travel - in other words, a progressive system." Again, dirt bikes dont have progressive suspension and they seem to have the whole " two wheeled off-road thing" pretty dialed. Coil is the way to go. Plain and simple. The only drawback of coil in a pure performance situation would be weight. And titanium springs will fix that.
    As above, dirt bikes do actually have progressive suspension characteristics (KTM used their progressively wound springs and position sensitive shocks to try to avoid using a linkage for years). Remember that all suspension characteristics need to account for ALL forces when measured at the wheel, with respect to both position and velocity, which means that a linear spring and purely speed sensitive damper (as in the coil sprung rear shocks) can produce a progressive spring rate and a position-sensitive (progressive) damping rate when measured at the wheel, as they are being manipulated by the progressive linkage between the shock and the wheel.
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  30. #30
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    Quote Originally Posted by Gawdodirt View Post
    Marzocchi does the same thing. They use the coil up into the last inch or so of compression, then the air kicks in and prevents the bottom out. Weight is only a turn off for people who don't enjoy the effects of gravity.
    Which Marzocchi ? I thought Marzocchi used coil w/ an air assist. MARS is not a coil w/ air assist, see the attached link (scroll 3/4 of the way down) : Coil compresses then the coil spring plunger compresses the piston in the air chamber - the initial fork movement is all coil. Even on full coil w/ air assist you add air which can create initial stichion and preload. I have a Manitou Tower Expert that is a coil /air assist and the more air preload you add the worse the fork performs.

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    Quote Originally Posted by Jayem View Post
    No, not unless you vary the orofice size or pressure acting on it at different points in the travel, then it becomes "progressive damping", except that "progressive damping" is very poor for bump control, because it means that you'll get good slow-bump performance, but horrible high speed, as the fork has to flow a lot more oil faster in that case, and if it's solely based on the position, it won't make a distinction between a high speed and low speed event. The 5th element and curnut CV/T system did just this, but they were pretty horrible damping systems in most respects. I owned both and I'd recommend steering clear They "pedaled well" and handled the "big hits" well (like a drop, or landing, etc), but were horrible otherwise. The fox DHX shocks had SPV elements, as there was an SPV type valve in the reservoir with a normal shim-stack, but it basically had characteristics of both, position sensitive damping that could make up for falling-rate suspension designs or be adjusted for a pedaling platform, better high speed damping control than the Progressive/Curnut shocks, but high speed performance still suffered a bit due to the shock just not being able to flow as much oil as is necessary.

    There are some good reads on high speed damping, so I suggest doing some more research. Realize that it has to be regressive, due to the fact that oil forced through a port creates a huge amount of "damping", so unless it's allowed to open further the faster the fork moves, it will "hydrolock". This doesn't have anything to do with the "progression" of the fork though, that is a separate effect, and again only slightly affected by piston pressure and volume of air above the piston.
    The "hydrolock" that occurs for high speed impacts is generally only a symptom in orifice dampers. Flow of oil through an orifice is similar to the aerodynamic principal that resistance increases at the square of the velocity. If the velocity doubles, resistance quadruples. This is why getting good low speed damping in an orifice damper causes harsh high speed performance. However, decent suspension forks have a separate shim damped high speed circuit. Shim stacks created a more linear damping curve because the shims bend as the oil flow increases. This is how forks create regressive damping. Multi-stage shim stacks also allow for some progression control on high speeds that is independent of the low speed circuit.

    I do, however, agree that spring progression has properties to be desired since it is essentially position sensitive rather than velocity sensitive.

  32. #32
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    Expanding/reiterating the OP's question

    This is the part I get:

    If you want to tune an air spring to two optimal behaviors, simultaneously -- sag at your trail weight, and just barely bottoming on your biggest hit -- then you have to adjust two variables: air pressure and volume. For every P and V you get a different curve. You can find a curve that meets your two goals, above.

    This is the part I don't get:

    Dampers control shock speed by bleeding off energy from the compression or the rebound of the shaft. The names are low-speed and high-speed.

    But you hear/read stuff like "tune out harsh bottoming w/ HSC" or whatever. Say we're talking about a DH shock, 3" stroke. 9" inches of rear wheel travel. "Controlling bottom-out" must be something you do during the last inch of wheel travel, during the last 0.33" of stroke... but whatever damper settings you have have been working through the first 8" of travel, 2.66" of stroke. So how can they "tune bottom-out"?

    Isn't a bunch of HSC that you might want at the end (if your shock is too linear and fails to ramp up) going to apply to the beginning- and mid-stroke too, and be harsh?

    Are LSC and HSC misnamed, or is the marketing/instructions not correct, or is there hidden position-sensitivity in the damping system? Halp!

  33. #33
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    Quote Originally Posted by Snfoilhat View Post
    This is the part I don't get:

    Dampers control shock speed by bleeding off energy from the compression or the rebound of the shaft. The names are low-speed and high-speed.

    But you hear/read stuff like "tune out harsh bottoming w/ HSC" or whatever. Say we're talking about a DH shock, 3" stroke. 9" inches of rear wheel travel. "Controlling bottom-out" must be something you do during the last inch of wheel travel, during the last 0.33" of stroke... but whatever damper settings you have have been working through the first 8" of travel, 2.66" of stroke. So how can they "tune bottom-out"?

    Isn't a bunch of HSC that you might want at the end (if your shock is too linear and fails to ramp up) going to apply to the beginning- and mid-stroke too, and be harsh?

    Are LSC and HSC misnamed, or is the marketing/instructions not correct, or is there hidden position-sensitivity in the damping system? Halp!
    Some shock designs, like Fox Boost Valve, change the damping when the shock is compressed more.

    Otherwise, you were right the first time. The damping sucks up energy and slows down the shock in the first place, so that it either doesn't reach the final bottom-out position, or at least does it more gently. It might be a little more harsh over all, but less harsh than hitting the bottom out bumper.

  34. #34
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    Quote Originally Posted by Steve VS View Post
    Air springs in their usual form (positive spring opposed by negative spring) typically suffer from a "preloaded" feel (caused by a high initial spring RATE - not spring force, spring RATE) that then drops away into low support in the mid stroke, usually followed by a progressive ending stroke (end stroke progression depends almost entirely on positive chamber volume vs piston displacement). Air springs obviously have to have a sealed piston running in a tube at the very least (eg Float forks with the coil negative springs), but normally also have an air negative springs (all air shocks, most air sprung forks) which has a seal head as well, and in some cases even a tertiary sliding seal (eg TALAS, Infinite Travel etc). A coil spring has none of these seals..
    Yup. Here's a spring curve for a Rock Shox Reba fork that has the Dual Air system. It doesn't look that bad until you take the derivative to find out the spring rate.




    The height of the blue bars represents the stiction in the fork.

  35. #35
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    Quote Originally Posted by Snfoilhat View Post
    This is the part I get:

    If you want to tune an air spring to two optimal behaviors, simultaneously -- sag at your trail weight, and just barely bottoming on your biggest hit -- then you have to adjust two variables: air pressure and volume. For every P and V you get a different curve. You can find a curve that meets your two goals, above.

    This is the part I don't get:

    Dampers control shock speed by bleeding off energy from the compression or the rebound of the shaft. The names are low-speed and high-speed.

    But you hear/read stuff like "tune out harsh bottoming w/ HSC" or whatever. Say we're talking about a DH shock, 3" stroke. 9" inches of rear wheel travel. "Controlling bottom-out" must be something you do during the last inch of wheel travel, during the last 0.33" of stroke... but whatever damper settings you have have been working through the first 8" of travel, 2.66" of stroke. So how can they "tune bottom-out"?

    Isn't a bunch of HSC that you might want at the end (if your shock is too linear and fails to ramp up) going to apply to the beginning- and mid-stroke too, and be harsh?

    Are LSC and HSC misnamed, or is the marketing/instructions not correct, or is there hidden position-sensitivity in the damping system? Halp!
    It helps to try to understand basic inputs to the suspension as being either based on displacement at the wheel with no fixed energy quantity, or energy from the rider with no fixed displacement quantity. There is almost always crossover between the two, but one is usually the dominant factor in any suspension motion event.

    When the wheel hits a bump, it has no option but to go up and over (or stop and throw you over the bars - we try to avoid that one!), and the amount of energy delivered to the suspension is determined by how much resistance (from both spring and damper) there is to the motion of the wheel.

    When the rider's weight is compressed into the suspension (g-outs, landings, corners etc), it is a transfer of kinetic energy (creating a transient force whilst dissipating energy in the damper) to the spring (until equilibrium is reached, which is usually only instantaneous) in order to change the momentum of the rider in a given direction.

    Generally speaking, if the spring rates are within normal ranges, the majority of hard bottom outs come from the energy-dominant inputs from the rider, such as hard landings, severe g-outs and so forth.

    Typical HSC damping is not position sensitive, as you correctly point out, however while increases in HSC affect the entirety of the stroke, the additional energy dissipated during high-energy compressions (particularly from top-out, in other words, heavy landings) means that small changes in HSC can have noticeable effects on travel use and bottom out.

    However, this needs to be considered very much fine tuning, because if your spring rates or progression levels are way off the map, trying to compensate with HSC settings can severely compromise other aspects of the ride.
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  36. #36
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    Quote Originally Posted by Matty F View Post
    The "hydrolock" that occurs for high speed impacts is generally only a symptom in orifice dampers. Flow of oil through an orifice is similar to the aerodynamic principal that resistance increases at the square of the velocity. If the velocity doubles, resistance quadruples. This is why getting good low speed damping in an orifice damper causes harsh high speed performance. However, decent suspension forks have a separate shim damped high speed circuit. Shim stacks created a more linear damping curve because the shims bend as the oil flow increases. This is how forks create regressive damping. Multi-stage shim stacks also allow for some progression control on high speeds that is independent of the low speed circuit.

    I do, however, agree that spring progression has properties to be desired since it is essentially position sensitive rather than velocity sensitive.
    Not sure what you are disagreeing with, hydrolock can still occur with a shim-stack setup. Ports aren't big enough, etc...
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    Quote Originally Posted by Jayem View Post
    Not sure what you are disagreeing with, hydrolock can still occur with a shim-stack setup. Ports aren't big enough, etc...
    Yes, even with shim-stack set-ups that are not properly valved or have proper port sizes hydraulic lock can occur. Keep in mind that long before hydraulic lock might occur the damping forces are ramping up, due to high shaft speeds and creating a very harsh feel giving the rider a "did not compress feel" on high speed square edged hits or in extremely over damped systems a premature low to midspeed bottoming feel 3/4 the way through the stroke.

  38. #38
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    Since someone started this question by dragging out intro physics, I'm going to drag out a formula from intro chemistry.

    PV = nRT
    Ideal gas law - Wikipedia, the free encyclopedia

    n is the amount of gas in a closed chamber in moles, R is a constant, and T is temperature. For purposes of a mountain bike fork, the entire right-hand side can be imagined as a constant - we're not changing any of this stuff mid-ride.

    The left side is pressure times volume, which people have already brought up. The fork will sag to a point where the force generated by the gas pressure is in equilibrium with the weight the bike and rider have over the fork. (So now we're back to physics.) Bang into something and the front wheel and fork try to accelerate the rider upwards. This requires force, and since the fork is not rigid, it compresses until either equilibrium is reached, the fork runs out of travel, or force stops being applied. In other words, little hits transmit some force through the fork but once it's compressed some, they don't apply force anymore, while big hits will cause bottom-out or alter the trajectory of the bike. Big words for stuff that all of us know anyway, but it bears repeating.

    So, pressure coming back from the fork is proportional to 1/V. That's not a linear response. For myself, I think a non-linear but tunable spring curve is great - when it's working well, it means a fork can be set up to feel plush over chatter but also not bottom out, and it doesn't even take that much travel. (Although my current one doesn't have a compression damper, so I've had to let go of that some. I find movement during climbs to be very annoying.)

    As far as stiction in air forks goes - well, if they were perfect, maybe there wouldn't be any high-end coil forks. I wonder if Fox confused people wanting air forks to be more like coil forks in terms of immediate small-bump response with people wanting air forks to have a more similar spring curve throughout.

    Regardless, if the size of the air chamber can be tuned with oil volume, it's not like someone who wants their spring rate to ramp up at the end can't have it - just needs more oil. I have to say that an external volume adjustment like Marzocchi is advertising sounds pretty cool.
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    Am I missing something here? I was under the impression that the positive/negative fork air chambers could be tuned to give an approximation of linearity that a simple one sided air spring cannot. The air at the bottom of the fork leg goes under the piston and it's pressure drops in a non linear fashion as the air on top compresses in a non linear fashion; the dislinearities cancel out, more or less, depending upon pressures and volumes used. At least that's what I saw the last time I took one apart.

    Linkages have the advantage of also varying the damping rate as a byproduct; a progressively wound spring won't. Falling rate linkages can be useful as a sort of 'propedal' effect at the expense of a harder ride which can be useful on an XC bike. The inverse of plush. Horses for courses, ponies for paddocks.

    The Reba graph shows a near linear rate with an air spring. Negative and positive pressures are recommended equal on the fork's label but you could drop the negative air and get a more rising rate and probably a stiffer initial preload.

    As to coil being the 'only way to go' on a bicycle, the spring rate for a 120 lb rider is about 60% of that for a 220 lb rider, for a 30 lb bike. OK, the wheels and tires aren't sprung, and only half the chain! But stocking and dealing with springs for every test rider was a nightmare. Air was free, fixed it, and weighed nothing. Coils are cheap, simple and maintenance free and a simple preload adjustment served most motorcycle riders well enough; their weight discrepancies as a percentage of the whole weren't a problem whereas in bicycles rider weight is , hopefully, almost the whole story.

    A coil has no stiction, but the newer air systems are so close to stiction free that nobody seems to care anymore. Certainly not enough to haul springs along, unless a motor's doing the hauling.

  40. #40
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    Fox 40 feels too linear

    When we set-up sealed damper cartridges for the Fox 40, we refer to these forks as too linear because neither side has an air over oil height set-up that would cause the pressure to ramp-up significantly. So we have to take this into account when valving the cartridge, i.e. it has more damping than a typical open bath cartridge that we use in the Marzocchi 888 and Rockshox Boxxer. For the coil spring (VAN) versions of the Fox 36 this also holds true, except for the one open bath model where you can raise the oil height to achieve this pressure ramp-up.
    Last edited by crseekins; 11-06-2012 at 11:08 AM.

  41. #41
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    Quote Originally Posted by ehigh View Post
    Still, can't you alter the progressive rate by changing rates of how much oil is allowed to pass through the shims by changing the diameter of the shims? It seems that not all coils are as linear as they seem, either. The property of a gas is progressive and the property of the coil is linear, but with augmentation to the overall system, these rules seem to have exceptions.
    This does not change the spring rate. This will slow the rate of change, but not the actual final load/distance value.

    There ae really no exceptions in each seperate system . But the proper combination/application can get the best of both.

    DR

  42. #42
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    Quote Originally Posted by AndrwSwitch View Post
    Since someone started this question by dragging out intro physics, I'm going to drag out a formula from intro chemistry.

    PV = nRT
    Ideal gas law - Wikipedia, the free encyclopedia

    n is the amount of gas in a closed chamber in moles, R is a constant, and T is temperature. For purposes of a mountain bike fork, the entire right-hand side can be imagined as a constant - we're not changing any of this stuff mid-ride.

    The left side is pressure times volume, which people have already brought up. The fork will sag to a point where the force generated by the gas pressure is in equilibrium with the weight the bike and rider have over the fork. (So now we're back to physics.) Bang into something and the front wheel and fork try to accelerate the rider upwards. This requires force, and since the fork is not rigid, it compresses until either equilibrium is reached, the fork runs out of travel, or force stops being applied. In other words, little hits transmit some force through the fork but once it's compressed some, they don't apply force anymore, while big hits will cause bottom-out or alter the trajectory of the bike. Big words for stuff that all of us know anyway, but it bears repeating.

    So, pressure coming back from the fork is proportional to 1/V. That's not a linear response. For myself, I think a non-linear but tunable spring curve is great - when it's working well, it means a fork can be set up to feel plush over chatter but also not bottom out, and it doesn't even take that much travel. (Although my current one doesn't have a compression damper, so I've had to let go of that some. I find movement during climbs to be very annoying.)
    You are only considering half the spring right now. As I mentioned before, the net spring force is the sum of both positive and negative spring forces, and depending on the configuration of the negative spring, it can have an enormous effect on the net force throughout the stroke, allowing designers to create pretty well any curve they want.
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  43. #43
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    I have a new 2013 Float R. 160mm ,open bath. it is noticably smoother, livelier, and I think better-performing than previous fox 36 forks I've used. These previsou forks had RC2 dampers. Those ar nice for fiddling with compression. But man...this new Fox fork feels really great.

    I must be experiencing the dreaded "too linear" feel of these forks. However, I am very happy with the 2013 fork's performance.

  44. #44
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    Quote Originally Posted by AndrwSwitch View Post
    Since someone started this question by dragging out intro physics, I'm going to drag out a formula from intro chemistry.

    PV = nRT
    Ideal gas law - Wikipedia, the free encyclopedia

    n is the amount of gas in a closed chamber in moles, R is a constant, and T is temperature. For purposes of a mountain bike fork, the entire right-hand side can be imagined as a constant - we're not changing any of this stuff mid-ride.
    As long as somebody is going to drag out chemistry, I'm going to drag out physics again. Suspension forks undergo adiabatic compression, which means that as the fork compresses, the gas also heats up, and it happens so fast that the heat cannot dissipate. If the volume gets squished by half, the pressure more than doubles.
    The correct formula is

    P(V^1.4)=constant

  45. #45
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    Quote Originally Posted by beanbag View Post
    As long as somebody is going to drag out chemistry, I'm going to drag out physics again. Suspension forks undergo adiabatic compression, which means that as the fork compresses, the gas also heats up, and it happens so fast that the heat cannot dissipate. If the volume gets squished by half, the pressure more than doubles.
    The correct formula is

    P(V^1.4)=constant
    Thanks. I ws going to mention the temp variable and its contribution.

    DR

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    Quote Originally Posted by beanbag View Post
    As long as somebody is going to drag out chemistry, I'm going to drag out physics again. Suspension forks undergo adiabatic compression, which means that as the fork compresses, the gas also heats up, and it happens so fast that the heat cannot dissipate. If the volume gets squished by half, the pressure more than doubles.
    The correct formula is

    P(V^1.4)=constant
    Thanks. I ws going to mention the temp variable and its contribution.

    DR

  47. #47
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    Quote Originally Posted by Steve VS View Post
    You are only considering half the spring right now. As I mentioned before, the net spring force is the sum of both positive and negative spring forces, and depending on the configuration of the negative spring, it can have an enormous effect on the net force throughout the stroke, allowing designers to create pretty well any curve they want.
    Not really. Within the physical limits of fork design (size of stanchions, weight of fork, heigh of fork, ...) designers cannot create any curve that they want. Forks have to be lightweight, have a certain stiffness and certain ride height which are all dictated by its physical limits. On top of this, an air negative spring initially has a much lower air volume than the positive air spring for two reasons: 1- the positive air spring needs to have the correct ramp-up and stroke 2 - the negative air spring also has the air piston push rod in it which takes up volume. So having equal positive and negative air spring pressures initially means that the positive air spring pressure ramps up much slower than the negative air spring pressure drops off.

    Yes, this is tuneable but only mildly so within confines of the fork structure.
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  48. #48
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    Quote Originally Posted by meph2 View Post
    I'm trying to understand how Fox's 2013 fork offerings are "too linear", per multiple reviews.

    Recalling my intro physics class, coils are completely linear. The force to compress the coil is directly proportional the distance from the resting state. For the record, I've never ridden a coil fork regularly.

    Haven't we been trying to get air forks to "feel like coil" for years?

    What does "too linear" mean? I understand that this means that you may blow through the travel, but wouldn't this be a problem with coil forks as well?

    I have a 2013 Fox 160mm fork and do occasionally use all the travel. I feel that it is light-years beyond my older Fox fork that never got more that ~130mm of travel (even when going over the bars after big drops).
    Please explain.
    Just curious, has anyone here actually looked at the air spring assembly of any of the longer travel Fox forks? The negative coil spring is drastically different than older Fox forks. I have pictures of a 2013 Fox 36 Float RLC Factory if anyone wants to see the guts. It is quite interesting that Fox went to so much expense to create a more linear spring rate,
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  49. #49
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    Quote Originally Posted by bog View Post
    Not really. Within the physical limits of fork design (size of stanchions, weight of fork, heigh of fork, ...) designers cannot create any curve that they want. Forks have to be lightweight, have a certain stiffness and certain ride height which are all dictated by its physical limits. On top of this, an air negative spring initially has a much lower air volume than the positive air spring for two reasons: 1- the positive air spring needs to have the correct ramp-up and stroke 2 - the negative air spring also has the air piston push rod in it which takes up volume. So having equal positive and negative air spring pressures initially means that the positive air spring pressure ramps up much slower than the negative air spring pressure drops off.

    Yes, this is tuneable but only mildly so within confines of the fork structure.
    Actually, yes they can. The current products on the market may not do it, but that does not mean it is impossible or even particularly difficult.
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  50. #50
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    Quote Originally Posted by Steve VS View Post
    Actually, yes they can. The current products on the market may not do it, but that does not mean it is impossible or even particularly difficult.
    Then maybe you can explain how. Just denying it with no explanation does nothing add credibility to your argument. The guys at Fox and SRAM are no dummies you know.
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