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  1. #1
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    Fork travel effect on bike geometry: HELP

    I have an old single speed I built up last year out of a 90's trek 830, it works fine, but was a $200 try at single speeding. I am now thinking about my next single speed. I've decided, as I really like and believe in single speeds, that I will get a frame built up for me with an EBB.

    My questions about fork length are as follows:

    1) Will 80, 100, 120 mm travel forks all change the geometry, in that I need to choose the forks before the frame is built?
    2) Are all 80, 100 and 120 mm forks the same length, in that if I get a Fox 100mm fork can I change it for a Marz 100 mm fork later?
    3) Is there a website that can answer all my questions about this? A table of all 2003/04/05 forks with specs would be perfect right about now

    My fork choice will most likely be a Marzocchi or Fox, as I've had such good experiences with Marzocchi forks, and the Fox's seem to be similar. The only other option I'm looking at would be a manitou black of some description.

    Second area of question is about the spacing of the axel in front of the fork centre line, 'rake'? or is 'rake' the angle. But, anyways, is that standard amoung the general type of forks I'm looking at?

    I want to end up with a durable bike similar to the Phil Wood limited edition Ti EBB bikes they had, but much cheaper. A bike for off road riding and endurance racing (sport class). It is important that this bike last many years, so having a geometry or EBB size that makes it obsolete would be a huge disapointment.

    I am planning on buying the Chris King SS hubs (disc), as they seem to review very well. I've not found the prices of the quick release conversion or the iso disc adapter for them yet though, can anyone help with the price of this? I use CK hubs on my XC and road bike too, so it makes sence to stick with CK if I can.

    This is my first custom build bike (apart from my first SS, but that was built out of spares and second hand bits), and I really don't want to build a dog.

    -Chris

  2. #2
    Doesntplaywellwithmorons!
    Reputation: DeeEight's Avatar
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    The usual rule of thumb, which must have been someone who failed math's thumb, was 1" = 1 degree. This meant a 1" change in fork length (measured from the bottom of the headset at the fork crown race where the bearing part slides onto the steerer to the axle) resulted in a change of 1 degree in the head/seat angles of the bike. Increases produce slacker angles, decreases produce steeper angles..

    Now the 1"/1degree rule was first spread in Mountain Bike A(F)iction and its become the most well repeated rule/myth other than the no-grease BB spindle myth, and I say myth because it was never actually described as to how they measured the change, or what wheelbase bike they measured it on. Shorter wheelbase bikes (like BMX/mod trials) have a greater change in the angles for a given change in fork length, than long wheelbase bikes (like full suspensuon DH bikes). A later issue of MBA in a big suspension fork theory/review article, printed the numbers being as 1 degree for every 0.71" of fork length change, on a 42 inch wheelbase bike, but it seems 0.71" on 42" = 1 degree is too much harder for most bike store employees to remember, or nobody read that issue, so few people ever know it, and just keep going 1" = 1 degree ad nauseum. Also the rule was created at a time when even rigid forks varied in length between brands by a good inch, and that suspension forks were setup without any sag/negative wheel travel (so you took the measurements usually when the bike was unloaded by a rider).

    Syncros rigid forks for example were 15" long while FatChance BigOneInch forks were 16" long, so say you have a FatChance frame (designed for their own forks) and you switch from the original fork to a syncros fork and presto, you've reduced the fork length and then increased the frame angles. A Manitou 1 fork for example, was 15.9" long, and had a max 1.8" of wheel travel. So switching to it would be virtually no change at rest, and gets steeper as the wheel compresses.

    So you can't just say XX mm of travel equals something change, because in the 1.8" example above, that's like 46mm of travel fork being the same as some rigid forks. And as you asked, no not all 80mm travel forks are the same length. Many brands are taller than others (marzocchi's are typically the worst in this regard) and the differences can be an inch or more for a given wheel travel/fork type (single or dual crown, telescopics or linkages). Linkage forks tend to be taller than telescopic forks for a given travel, and single crown forks are taller than dual crown forks for the most part, because the loads are spready over two crowns, they can be half as thick as otherwise needed with one crown, and thus the bottom one being shorter reduces the fork length measurement. A Noleen MegaAir fork for example (very popular among folks on here), is an inch shorter when set for 75mm than an RST Mozo SL fork is at 76mm of travel.

    Then there's the whole fork sag/negative travel setup issue that changes the length of the fork when the rider is on the bike, and you never really know if you're supposed to measure the bike with this sag or without, since frame/bike makers usually don't put the actual lengths they've measured in geometry numbers usually, just what wheel travel they supposedly used for the fork when designing the frame. They'll tell you the frame was designed for a 130mm travel fork maybe, and has such and such angles with that wheel travel, but that's likely only true for a very specific brand/model fork.
    I don't post to generate business for myself or make like I'm better than sliced bread

  3. #3
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    DeeEight has definitely given you some good info, but I'll add this.

    1 - Yes, best to decide what travel you want, then design the bike around that fork. That said, many forks with adjustable travel do not change axle crown length (without sag) so that could be your best bet if you want one bike for different purposes - something like the Reba...

    2 - No, unfortunately not, but they are pretty close

    3 - Not that I know of, perhaps mtbr should think about doing something???

    Another thing to keep in mind is trail. IMO this is the most important element as to how a bike handles. Search through here (the 29"er board has had some great threads on rake, trail and steering geometry) and you should find some info which either makes things much more clear - or muddies the water more than ever....

    Sam

    1) Will 80, 100, 120 mm travel forks all change the geometry, in that I need to choose the forks before the frame is built?
    2) Are all 80, 100 and 120 mm forks the same length, in that if I get a Fox 100mm fork can I change it for a Marz 100 mm fork later?
    3) Is there a website that can answer all my questions about this? A table of all 2003/04/05 forks with specs would be perfect right about now

  4. #4
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    Thank you for your detailed answers, they are very helpful.

    Well, my plan of attack now will be to make my own table of fork densions for the forks that I am interested in (The whole sag thing is another problem for later). If I can find some trends so that at least if I get a Acme XX fork it is a common (as opposed to standard) size so that in X years when it needs replacing there is the highest chance of being a good replacement.

    I'm disapointed that Marzhocci is bad for different fork lengths, as me and my friends have had such good luck with their forks for lasting, that they really were my number one pick so far.

    At least with this new ss bike I have time on my side, my cheapie SS will keep my SS needs met until a perfect replacement is born.

  5. #5
    Recovering couch patato
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    I looking into such matters myself lately, longing for an FS bike that will offer me the option to race agressive tight singletrack as well as ultra plush long DH's and marathons. 29", of course...
    right now, I'm leaning towards finding a bike that will be right in the middle with stem and seat settings, when used with a 100mm fork. An 80 fork (setting) would quicken the bike up, and ask for just minor fore/aft seat adjustments and shorter stem to keep seat angle and reach exactly the same (if I even end up wanting that at all). Same with 120mm (I want a converted Maverick DUC32, 120mm in stead of 150mm/26"), just minor seat adjustment and a flipped slightly longer stem. With 120mm the bike would ideally be suited for all-day fun and light FR riding as well as marathon racing.

    As you're probably looking for 26", take a look at Manitou's announced infinite travel forks. Those allow you to adjust the fork's travel on-the-fly to get the bike handling aor bar ehight you're looking for.

    To further add to the 1"/1ยบ myth : if you use a fixed % of sag, say 25%, the 0,71" gets bumped past 0,9", making it an even more useful rule of thumb.

    Taking into account that sag and fork angle together reduce the effect of travel to bar height, and more agressive (long travel) riding often asks for a taller handlebar, my whole multiple bar plan could easily go down the drain. Just make sure that with (what your or your frame would consider) medium travel, the bike will work as a medium for your various riding, and will be in medium stem/seat settings, so they could go each way if needed.
    Klok - XC - Skate - Ski

  6. #6
    beer *****es n' bikes
    Reputation: seely's Avatar
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    You could get something like the Rockshox Psylo series fork. Its a tough fork, and adjustable from 80-125mm of travel. They aren't made anymore but you can find them new still for under $200, and its a pretty tough fork. Personally, for me I want my head angle at around 69-70 degrees for XC and trailriding, anything slacker and I feel the effect on the climb. My old Kona was about 73 deg. I believe and it climbed like crap.


    Quote Originally Posted by keen to ride
    I have an old single speed I built up last year out of a 90's trek 830, it works fine, but was a $200 try at single speeding. I am now thinking about my next single speed. I've decided, as I really like and believe in single speeds, that I will get a frame built up for me with an EBB.

    My questions about fork length are as follows:

    1) Will 80, 100, 120 mm travel forks all change the geometry, in that I need to choose the forks before the frame is built?
    2) Are all 80, 100 and 120 mm forks the same length, in that if I get a Fox 100mm fork can I change it for a Marz 100 mm fork later?
    3) Is there a website that can answer all my questions about this? A table of all 2003/04/05 forks with specs would be perfect right about now

    My fork choice will most likely be a Marzocchi or Fox, as I've had such good experiences with Marzocchi forks, and the Fox's seem to be similar. The only other option I'm looking at would be a manitou black of some description.

    Second area of question is about the spacing of the axel in front of the fork centre line, 'rake'? or is 'rake' the angle. But, anyways, is that standard amoung the general type of forks I'm looking at?

    I want to end up with a durable bike similar to the Phil Wood limited edition Ti EBB bikes they had, but much cheaper. A bike for off road riding and endurance racing (sport class). It is important that this bike last many years, so having a geometry or EBB size that makes it obsolete would be a huge disapointment.

    I am planning on buying the Chris King SS hubs (disc), as they seem to review very well. I've not found the prices of the quick release conversion or the iso disc adapter for them yet though, can anyone help with the price of this? I use CK hubs on my XC and road bike too, so it makes sence to stick with CK if I can.

    This is my first custom build bike (apart from my first SS, but that was built out of spares and second hand bits), and I really don't want to build a dog.

    -Chris
    bike dude, velocity employee (this is my personal account)

  7. #7
    Retro Grouch
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    Leave it to the experts...

    I had a custom SS made by Paul Sadoff at Rocklobster. Paul has been making custom bikes for over 20 years. When he built my bike he took everything into account; my previous bike (Bonty), what I liked about it and what I didn't; my handlebar (flat with bar ends); my shock (I choose a Marzocchi Atom 100 with ECC); and of course, me. I don't know everything that went into making my bike but it rides great. Change anything and it will probably not ride as well.

    I would definately decide on a shock before you plan your bike build. I really like the Marzochi EEC. The shock locks down 3 inches on climbs which changes the geometry drastically. It is like having two bikes. The ETA leaves you 30mm of travel; I have never used one but it should work as well as the EEC.

    1G1G, Brad

  8. #8
    Nouveau Retrogrouch SuperModerator
    Reputation: shiggy's Avatar
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    I agree with Brad. Just pick the fork you want to use and then let the frame builder worry about the actual dimensions. That is one of the reasons to go custom. No need for you to go through all of the numbers unless you really want too. Even then just tell what fork you want and how you will use it.
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