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  1. #1
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    Progressive Fork Springs

    Why canít we find some for MTB forks?

    The progressiveness and ramp up of air springs are understood by everybody but we keep on making straight rate springs for forks. Why?

    They even exist for shocks ... I do not understand.

    if you know any aftermarket or DIY options for trail bike forks please let me know.

  2. #2
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    Progressive spring isn't hard. Maths on the previous thread.

    I used 4/5 shorter coils bonding the joins with polymorph thermoplastic (which also acted as a glide ring).

    You need the springs to start to bottom out to get progression.

    I did this a decade ago so can't remember details, but work out where in the stroke you want progression and design one or two springs to bottom around that point. I believe springs generally stiffen anyway in last 10%

    Not hard... Otherwise how would I have done it =)

  3. #3
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    Quote Originally Posted by digev View Post
    Why canít we find some for MTB forks?

    The progressiveness and ramp up of air springs are understood by everybody but we keep on making straight rate springs for forks. Why?

    They even exist for shocks ... I do not understand.
    Perhaps it's a size restriction? You've got fairly limited room inside the fork legs, I guess the other difficulty is probably the range of different forks to fit them in or cover, different lengths, diameters etc.

  4. #4
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    Quote Originally Posted by Mudguard View Post
    ... the range of different forks to fit them in or cover, different lengths, diameters etc.
    The industry is making plenty for the motorbikes ... and they have different lengths, diameters, etc ...so it should not be a problem. A quick search on Google will give you hundreds of options.

    These guys have done it for the Boxxer. Itís just a matter of will!

    Code:
    The dual rate aspect of the spring is its main feature, with a 15% rise in the spring rate at 70% compression, its represents a long R&D and testing phase for the product over two years. The 15% rise in rate was agreed after testing on the World Cup circuit for the entire 2016 season
    https://www.raceonlysprings.com/home...e-fork-spring/

  5. #5
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    Quote Originally Posted by CaveGiant View Post
    Progressive spring isn't hard. Maths on the previous thread.

    I used 4/5 shorter coils bonding the joins with polymorph thermoplastic (which also acted as a glide ring).

    You need the springs to start to bottom out to get progression.

    I did this a decade ago so can't remember details, but work out where in the stroke you want progression and design one or two springs to bottom around that point. I believe springs generally stiffen anyway in last 10%

    Not hard... Otherwise how would I have done it =)
    Thx. OK so shorter springs so they can bottom out and leave remaining ones compress at their own rates, right?

    But that means the progression will start very/too early in the travel, no?

  6. #6
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    Quote Originally Posted by digev View Post
    Thx. OK so shorter springs so they can bottom out and leave remaining ones compress at their own rates, right?

    But that means the progression will start very/too early in the travel, no?
    It could theoretically be designed however you want. The amount each coil in series compresses is proportional to its spring rate. E.g. if you have two coils in series and one coil is 3x as firm as the other and you compress the fork 4 inches, then the softer coil will compress 3 inches while the firmer coil only compresses 1 inch. If the short coils are very firm and the long ones are very soft, it's possible that the long coils could bind before the short coils.

  7. #7
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    Progressive Fork Springs

    Based on that formula and example below, what should we need in order to get a 15% rise in the spring rate at 70% compression?! The free length should be about 300mm and the first 70% rate should be 45lbs. C'mon guys youíre clever-er than me

    Code:
    Keq = (k1)(k2) / (k1 + k2) 
    
    Keq = Equivalent Rate
    k1 = Rate of spring 1
    k2 = Rate of spring 2
    Keq = (20)(40) / (20 + 40) 
    Keq = 800 / 60 
    Keq = 13.333 lbf/in (pounds of force per inch)
    Source: https://www.acxesspring.com/calculat...in-series.html

  8. #8
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    That formula is just for calculating the effective spring rate of coils in series and does not account for the coils binding. In order to predict when the coils will bind we would need more information than just the spring rate, such as the wire diameter, length, and pitch (hot tightly it's wound).

  9. #9
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    Can't remember the exact method, but I have vague memories of plotting force/compression graphs using excel.
    Work out how much all springs compress every 10N. Once a spring compresses fully, it unsurprisingly won't compress further.

    There will be more sophisticated methods, but those use more than addition and graphing.

  10. #10
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    Quote Originally Posted by dlxah View Post
    That formula is just for calculating the effective spring rate of coils in series and does not account for the coils binding. In order to predict when the coils will bind we would need more information than just the spring rate, such as the wire diameter, length, and pitch (hot tightly it's wound).
    OK I'll try to find some more tech info about the coil that can match the one in my Push ACS3!

  11. #11
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    Quote Originally Posted by CaveGiant View Post
    Can't remember the exact method, but I have vague memories of plotting force/compression graphs using excel.
    Work out how much all springs compress every 10N. Once a spring compresses fully, it unsurprisingly won't compress further.

    There will be more sophisticated methods, but those use more than addition and graphing.
    I like your approach!

  12. #12
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    The OD should be around 29mm to fit inside the stanchion. I measured my 50lbs Push.

  13. #13
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    If you have a Push ACS3 kit and you want it to be firmer in the last 30% of travel, that's exactly what the air bump stop is for.

  14. #14
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    But, your damping may not match this ultra-progressive end of travel, so that may rebound badly and cause control/harshness issues without the damping being overwhelmed. Hydraulic anti-bottom-out systems are better IMO, but that's not really affecting the entirety of travel like progressive springs would.
    "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

    You're turning black metallic.

  15. #15
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    Quote Originally Posted by dlxah View Post
    If you have a Push ACS3 kit and you want it to be firmer in the last 30% of travel, that's exactly what the air bump stop is for.
    I know, itís just an experiment! For the sake of knowing what the differences are (if any). When I do some testing I always think about others who might benefit from it, and in that case .. a progressive spring only would surely be a lot less expensive that the 2 conversion kits on the market at the moment. Years ago it was about $100 to convert your pike to coil and now itís 3/4 times that price ... it seems a bit ridiculous. (Yes they have air or hydraulic bump-stops but do we need them with a progressive spring? I donít know!)

  16. #16
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    Quote Originally Posted by Jayem View Post
    But, your damping may not match this ultra-progressive end of travel, so that may rebound badly and cause control/harshness issues without the damping being overwhelmed. Hydraulic anti-bottom-out systems are better IMO, but that's not really affecting the entirety of travel like progressive springs would.
    Thatís exactly what happened with my NovyParts SPLUG, no matter the spring rate or damper settings I was unable to use full travel by a long shoot ... super difficult to use the last 1/3. I like to keep some for when things get hairy but that was way to conservative. Now with 0 psi in the Push ABS bottom-out system it feels way better and even on big compressions (think V shape not U shape) I still have about 10mm left, and itís not harsh. Progressive springs donít affect the entire travel, thatís the beauty of it ... now it may or may not be for me (with that damper) but surely itís great for other riders to know they have options. Thatís what I'm trying to discover ...

    Also people use them on MTB shocks, motorbikes, ATV, off-road rally cars, etc ... surely their dampers are not all custom tuned? Again I donít know, but I find it interesting and would like to find out a bit more about all that ... just because

    We know we canít rely on bike or suspension manufacturers to show us what all the options are because they have a different agenda and think about marketing, mass production and costs ... it doesnít hurt to think outside the box and more often than not we can all benefit from the outcome (feasible or not). Thatís the idea.

  17. #17
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    Quote Originally Posted by digev View Post
    Also people use them on MTB shocks, motorbikes, ATV, off-road rally cars, etc ... surely their dampers are not all custom tuned? Again I donít know, but I find it interesting and would like to find out a bit more about all that ... just because
    Maybe you misunderstood me, I wasn't saying you need "custom tuned" to run progressive springs, I was saying the damping needs to be tuned for it, meaning you can't just drop a progressive spring in a damper not tuned for it and expect great things automatically. If it is the intent from the manufacturer, then it may work just fine. Running a bunch of tokens and making a fork overly-progressive sometimes destroys the deep-travel and overpowers the damper.
    "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

    You're turning black metallic.

  18. #18
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    Quote Originally Posted by digev View Post
    I know, itís just an experiment! For the sake of knowing what the differences are (if any). When I do some testing I always think about others who might benefit from it, and in that case .. a progressive spring only would surely be a lot less expensive that the 2 conversion kits on the market at the moment. Years ago it was about $100 to convert your pike to coil and now itís 3/4 times that price ... it seems a bit ridiculous. (Yes they have air or hydraulic bump-stops but do we need them with a progressive spring? I donít know!)
    The ACS3 bump stop is not hydraulic. It is an air spring.


    Quote Originally Posted by digev View Post
    Progressive springs donít affect the entire travel, thatís the beauty of it ... now it may or may not be for me (with that damper) but surely itís great for other riders to know they have options. Thatís what I'm trying to discover ...
    The bump stop in your ACS3 kit does not affect the entire travel. It doesn't engage until about the last 1/3. A properly designed hydraulic bottom out control is typically also position sensitive and will not affect the entire range of travel (e.g. the Vorsprung Smashpot).


    Quote Originally Posted by digev View Post
    We know we canít rely on bike or suspension manufacturers to show us what all the options are because they have a different agenda and think about marketing, mass production and costs ... it doesnít hurt to think outside the box and more often than not we can all benefit from the outcome (feasible or not). Thatís the idea.
    The general consensus every time this topic comes up in these forms seems to be that progressive coils are not really feasible in mountain bike forks especially for mass production. There are quite a few companies offering coil forks or coil fork upgrade kits these days (MRP, Cane Creek, Ohlins, Push, Vorpsrung, and Avalanche to name a few). They all employ intelligent engineers who design these products for a living and surely have a lot more knowledge and experience with this subject than you or I, and I have to believe there's a reason literally none of them are offering progressive fork coils.

  19. #19
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    I don't think progressive fork springs are a good or needed thing.

    The weight on your fork doesn't change. It's not a case of a truck going from empty to full and needing to work at both extremes.
    A linear spring curve (i.e. straight curve) does very well at small bump, mid-stroke and bottom-out control. In comparison a progressive cuve will give you either a soggier mid-stroke or a firmer end-stroke. A soggier mid-stroke is an air-spring characteristic and storing and returning more force and energy at end stroke can require counter-productive rebound damping tunes to tame it.
    Owner of www.shockcraft.co.nz, Mech Engineer, Tuner, Manitou, Motorex, Vorsprung EPTC, SKF, Enduro
    www.dougal.co.nz

  20. #20
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    Quote Originally Posted by Dougal View Post
    I don't think progressive fork springs are a good or needed thing.
    Same logic applies to coil shocks?

  21. #21
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    Race Only Springs

    Look them up. I did and spoke to the owner who was super responsive.

    I was on the fence between a new damper or trying a progressive spring. I'm glad I went with a new damper instead, but maybe the prog coil may have worked too. It's just that going to a new coil wouldn't have addressed all the stuff I wanted dealt with.

    If you are properly damped and not bottoming out often a prog spring might not be a net gain. The feel of a prog air sprung fork is quite different than a coil, and I'm not talking about all that "off the top , small bump, difference" (Which I'm not sure I believe now with neg air springs getting so dialed). But as others alluded to, how a damper handles the rebound forces coming off a prog vs a linear spring, and end travel, I think makes a difference in how a fork rides.

    All that said, with a prog coil you might be able to back off on compression damping a bit. And coming back out of deep travel with a bit faster rebound near end travel might not be all bad.

    I'm no suspension expert by any means and all that was purely anecdotal.

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