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  1. #1
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    650B Conversion Geometry Changes

    I only got a B minus in high school geometry, so you gotta help me out here…

    …other than a higher BB and stand-over height, are there any other change to the geometry when swapping front and rear 26” wheels for 650B’s?
    Last edited by u2metoo; 01-07-2009 at 03:50 PM.

  2. #2
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    as long as each wheel is the same size.... no, except for a change in trail due to the larger wheel diameter.

  3. #3
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    RE the trail - would you want more (a G2 fork) or less (Fusion) to 'correct' or re-establish the intended trail.

    I have on a few occasions experienced a bit of wheel flop on my blur XC w/ a 650B on the front. I know this has slackened things, but I found it steep to begin with and it is an improvement 98% of the time - trying to make it 100% w/ a fork offset adjustment.

  4. #4
    Mr.650b - Mr.27-5
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    Quote Originally Posted by reformed roadie
    RE the trail - would you want more (a G2 fork) or less (Fusion) to 'correct' or re-establish the intended trail.

    I have on a few occasions experienced a bit of wheel flop on my blur XC w/ a 650B on the front. I know this has slackened things, but I found it steep to begin with and it is an improvement 98% of the time - trying to make it 100% w/ a fork offset adjustment.
    More fork off set (rake) = Less trail.

    The added rake would reduce trail in a nearly (but not quite) linear fashion. IOW, if you add 6mm of rake you get nearly a 6mm reduction in trail.

    Cheers,

    KP
    “Those that say it can’t be done should get out of the way of those doing it.”

    PacentiCycleDesign

  5. #5
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    So, if I am understanding this correctly, I would want to increase the offset to make it steer a little quicker, esp.at low speeds.

    Thanks!

  6. #6
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    so i guess this discussion about rake and trail begs the question: are there opinions on what the "ideal" fork rake and and head tube angle should be for a 650b wheeled bike?

    is there a specific trail number that people shoot for regardless of wheel size or has it been found that as wheel size increases you do want additional trail? from the little bit i've followed regarding this topic in the 29er world the consensus seems to be that increased fork rake was needed (38mm to around 44mm) to bring the handling in line with what everyone was used to on a 26" bike.

    i guess i could do the math myself but this is the internet where it's always easier to just ask someone else for the answer. has this increased offset for 29ers brought the trail numbers back to what they would be on a 26" bike with a 38mm offset? or has it brought the trail back some but not all the way?

    my only point of reference is my own bike and the way it feels since putting on the 650b's. it's was designed for 26" wheels with a standard 38mm offset suspension fork. i swapped out the suspension fork for a salsa cro-moto 26" wheel fork. i chose this fork specifically for two reasons. it dropped my AtoC height by 2mm, thus steeping the HTA a little bit, and my fork rake increased from 38mm to 41mm, which brought the trail back a bit. this is all based on what peopled seem to be doing in the 29er world (slightly more HTA and increased fork rake).

    the end result is a bike that feels better in every situation. i expected it to roll and climb better for the standard reasons why larger wheels roll and provide more traction. my surprise was that it's actually better/quicker/more stable in the tight stuff as well. there are a few tight switchbacks i hit often and i seem to fly through them faster and with more stability now than when the bike had 26" wheels. it could be the wheels, it could be the changes to the geometry or in fact it could just be the fact that the fork is no longer compressing.

    thoughts?

  7. #7
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    Quote Originally Posted by reformed roadie
    So, if I am understanding this correctly, I would want to increase the offset to make it steer a little quicker, esp.at low speeds.

    Thanks!
    ha. that statement seems pretty damn consistent with what i just wrote above.

  8. #8
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    Quote Originally Posted by lanceh
    ha. that statement seems pretty damn consistent with what i just wrote above.
    Ha. Since I posted before you did, I think you got that the wrong way around.

    And while the numbers make sense, hearing it from a source that has experience w/ building or designing frames is more assuring than just doing the math.


    Anyway, my thought is that the best scenario for someone running a 650b only on the front is to get a fox steerer/crown w/ G2 offset for a 26" Fisher. Not sure if they are easily obtainable...might need a friend who works for Trek.

  9. #9
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    The differences in Trail are pretty minimal. If you want quicker steering, just lower your fork a few mm's....

  10. #10
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    i didn't realize the "G2" geometry affected 26" wheels as well, thought it only pertained to 29ers. so what is the offset of a G2 fox fork for 26" wheels?

    or are you saying take the lowers from a G2 29er and put them on a 26" fox fork? if so i would think this would be a bad option as the G2 offset would be way too much for a 650b wheeled bike

  11. #11
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    I did a quick check on the GF website and the 26" HiFi's have forks w/ 'special G2 offset'.
    Not sure what it is...I think the 29ers had like a 51mm offset and the 26" bikes had it in the high 40s.

    I would only shorten the fork if I could be sure it was from the 15-20mm of travel I wasn't getting from the Fox.

    It would seem that I could - as GF has tried to accomplish w/ the G2 geometry - keep the stability of the slacker angle w/o giving up the quick turning by adjusting the offset.
    Fork offset is an odd part of the geometry equation...it is dictated by the fork co., and the frame designers have had to deal with what was out there. Till someone w/ Fisher's (Trek's) clout comes along to change things.
    I am surprised C'dale, w/ their own forks, hasn't championed this first.

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