geometry question for new DH build- Mtbr.com
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  1. #1
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    geometry question for new DH build

    Question for people with more knowelege than I have.

    I'm in the process of building my new DH rig. Retired the old one and parted-out. New frame is a 2016 Kona Supreme Operator Carbon in 26".

    I have a 2016 Boxxer WC 27.5" fork (new in box) laying around. If I was to use this fork on my frame while running 26" wheels, would the rake be too much? If I use a Boxxer for 26" wheels (which I do not currently own), my HTA would be at 64 degrees and ST angle would be at 76 degrees.

    According to my calculations, and I could be way off, by utilizing the 27.5" Boxxer with 26" wheels, my HTA would go from 64 degrees to somewhere just north of 63 degrees and the ST angle would go from 76 degrees to a hair over 75 degrees. Wheelbase would grow a bit as well. Stack and reach would not change too noticably.

    Basically my question would be, is a 63 degree HTA too slack?

    Buying a 26" Boxxer is out of the question right now so it would be between the previous mentioned 2016 27.5" WC Boxxer or a 2013 Totem DH coil. I don't really want to use the Totem because of the 20mm less travel and also because full lock hits the downtube on both sides and that's no bueno.

    Any advice and or recommendations would be greatly helpful.

  2. #2
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    63 to 63.5 degrees is a pretty common range of HTA on modern DH bikes. My Jedi is 62.5, and I don't have any problem with it.

    That said, I'm not sure that HTA is the be all and end all of bike geometry. Slightly slacker seat tube angle wouldn't concern me much on a DH bike compared to a bike that I was pedaling uphill with seat fully extended. BB would also be slightly higher, but probably not that much higher that you'd be outside the range of what somebody that runs less sag could be at.

    The other difference could be what fork offset the 27.5 fork has vs a 26" fork. A few millimeters difference in fork offset can have an impact on handling, but that is more than I'm willing to guess at the effect of, as I've never tried forks with different offsets on the same bike with no other changes. There are calculators out there that you can use to look at HTA and offset to calculate the trail numbers but you have to be pretty knowledgeable on how all those numbers interact, and I don't get the impression that a lot of the so called geometry experts agree on ideal trail numbers.

    My guess is you'd be just fine running the 27.5" fork with a 26" wheel, and the most you have to lose is selling a slightly used fork instead of one that is new in the box.
    No dig no whine

  3. #3
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    I believe the Boxxer 27.5" has an offset of 48mm as compared to the 26" version at 42mm. So 6mm difference if my numbers are correct.

    I may just fire them on when I get to finishing the build and see how it looks/performs. If not good, I can try to sell/swap for a 26" set.

    I appreciate your help.

  4. #4
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    Also what I calculated is that the trail measurement between the two forks is 3mm greater for the 27.5". 114 vs 117.

    And actually I was a little off on the angles above. Caculated comes out to be around 63.5 HTA and 75.5 STA with the 27.5" fork and 64.0 HTA and 76.0 STA with the 26".

    I'm gonna give it a shot. At worst, I remove it.

  5. #5
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    So, final numbers calculated in a bike geometry calculator.

    with 26" Boxxer WC/26” wheels
    WB: 1148mm
    BB drop: -4mm
    fork length: 568mm
    fork offset: 42mm
    HTA: 64 degree
    STA: 76 degree
    trail: 117mm
    stack: 582
    reach: 398

    with 27.5" Boxxer WC/26" wheels
    WB: 1159mm +11mm
    BB drop: -3mm +1mm
    fork length: 581mm +13mm
    fork offset: 48mm +6mm
    HTA: 63.5 degree -0.5 degree
    STA: 75.5 degree -0.5 degree
    trail: 114mm -3mm
    stack: 589mm +7mm
    reach: 393mm -5mm

    If my calculations are correct, I think I'll be OK but if anyone has anything to add, by all means it will be appreciated.

  6. #6
    Big M, Little organ.
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    You can probably lower the crowns to the point where your HTA can be the same. Offset may be on the higher end but probably a minor thing in the grand scheme.

    If you are very particular, get Outsider bikes adjustable crowns. Not cheap at $235 but you’ll have all the offset and stem adjustment you’ll ever need.

  7. #7
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    So what was your old DH bike?

    I just recently swapped out a Fox 36 26 inch fork with a RS Pike (27.5). Also slackened it an additional 0.5 degrees via angleset. I don't remember if there was any fork offset difference between the two. From a feel standpoint, the difference was subtle and after a few minutes, indistinguishable.

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