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  1. #1
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    Question on BB height for Truth

    Hi fellow Ellsworthians! I intend to swap my fork for a 80mm on my 16" truth. as stated in the 05 website, the BB height is 12.8" for a 100mm fork. Just can't figure out the height for a 80mm without some CAD program. Did post an enquiry on their website ages ago, but i guess TE and guys are too busy to reply?

    on a sidenote, what do you guys think is an ideal BB height for a XC full sus using 80mm fork? My hardtails were 11.6"-12". Considering the rear travel for sussers, i guess they need more clearance, but how much without losing the racy feel? Many thanks in advance!

  2. #2
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    Reputation: chad1433's Avatar
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    You know, the BB height will depend on the Axle to Crown height of the fork you choose and not so much on the travel length.

    For example, you get a 80mm RS and say an 85mm Marzocchi and they'd give you very different A to C heights, not just a difference of 5mm (generally speaking).

    What I'd do is get a fork that's adjustable between the two heights and see where you like the travel/BB best. You may find that 90mm is best, yet no one actually makes a 90mm fork. With U-turn or TAS you could achive those numbers.

    So what forks are you considering?

  3. #3
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    good point...Mrs JM has the Sun Tour XC-Pro...adjusts from 60mm-120mm amd it didn't change the geometry of her little Dirt Jumper...she has the one with the remote travel adjustment

    some details here...lot of bang for the buck

    http://www.srsuntour-cycling.com/standard.xml?vpID=233
    Attached Images Attached Images

  4. #4
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    Quote Originally Posted by sparst
    Hi fellow Ellsworthians! I intend to swap my fork for a 80mm on my 16" truth. as stated in the 05 website, the BB height is 12.8" for a 100mm fork. Just can't figure out the height for a 80mm without some CAD program. Did post an enquiry on their website ages ago, but i guess TE and guys are too busy to reply?
    Here is a pretty quick and accurate way to claculate your BB height change due to fork change:

    Change in BB height = (chainstay length/wheelbase)*(change in crown-to-axle height)*sin(head tube angle)

    Quote Originally Posted by sparst
    on a sidenote, what do you guys think is an ideal BB height for a XC full sus using 80mm fork? My hardtails were 11.6"-12". Considering the rear travel for sussers, i guess they need more clearance, but how much without losing the racy feel? Many thanks in advance!
    A XC FS bike with 100 mm rear travel and 80 mm front travel should be around 12.25" and 12.5" to keep that racey feel.
    There is no added value to my participation - in fact, just more confusion.

  5. #5
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    pardon me but how do i measure the axle to crown height? from top of crown just below the headset or bottom where the stanchions start?

    Quote Originally Posted by GearHead
    Here is a pretty quick and accurate way to claculate your BB height change due to fork change:

    Change in BB height = (chainstay length/wheelbase)*(change in crown-to-axle height)*sin(head tube angle)
    Cool! i didn't know there's such a formula.. but my question is:

    1. which chainstay length? actual or effective?
    2. what dimensions? inches, mm ?

    I'm currently on a bomber 105mm wanting to swap for a SID 80mm

    Also, i ride both a hardtail and a Truth. Most measurements are the same or close, except the 11.75" BB hgt (HT) versus 12.8" (truth). I can't possibly have a Truth running 11.75" BB hgt due to rear trave and factory specs.... so was wondering what could be a good balance? say HT 11.75" while a full sus 12.5" or 12.25" or 12.75"???!! such that both bikes will ride as similar to each other.

    an example is an enlightenment 11.75" vs truth 12.8"

  6. #6
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    Quote Originally Posted by sparst
    pardon me but how do i measure the axle to crown height? from top of crown just below the headset or bottom where the stanchions start?
    It is measured from the top of the crown (underside of headset lower race) to the axle.

    Quote Originally Posted by sparst
    Cool! i didn't know there's such a formula.. but my question is:

    1. which chainstay length? actual or effective?
    2. what dimensions? inches, mm ?
    You would need to use the horizontal effective chainstay length in the formula. In the case of FS bikes, the effective chainstay and actual chainstay are pretty much the same thing.

    The units of measurement are what ever you are comfortable using, there are no constants in the formula so it will work with both inch and mm units. The angle you use in the formula needs to be degrees and not radians.

    The current formula would be most accurate on a bike which has both the actual and effective chainstay length being near equal (or BB height being close to wheel radius). If you were to use this formula for a bike with say a 10.75" BB height and 29er wheels, it would need a correction factor put against it to account for the chainstay angle not being very close to 90 degrees. If you want to use it for other bikes I can modify it for you.

    Quote Originally Posted by sparst
    I'm currently on a bomber 105mm wanting to swap for a SID 80mm

    Also, i ride both a hardtail and a Truth. Most measurements are the same or close, except the 11.75" BB hgt (HT) versus 12.8" (truth). I can't possibly have a Truth running 11.75" BB hgt due to rear trave and factory specs.... so was wondering what could be a good balance? say HT 11.75" while a full sus 12.5" or 12.25" or 12.75"???!! such that both bikes will ride as similar to each other.

    an example is an enlightenment 11.75" vs truth 12.8"
    It seems that racey FS bikes with 80 mm travel are using a BB height of 12.1"-12.25" and racey FS bikes with 100 mm travel are using a BB height of 12.6"-12.8". Since you are changing the front travel and not the rear travel, you only want the BB height to change height by 1/3 of the difference between a 80 mm travel bike and a 100 mm travel bike.
    There is no added value to my participation - in fact, just more confusion.

  7. #7
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    Quote Originally Posted by GearHead
    it seems that racey FS bikes with 80 mm travel are using a BB height of 12.1"-12.25" and racey FS bikes with 100 mm travel are using a BB height of 12.6"-12.8". Since you are changing the front travel and not the rear travel, you only want the BB height to change height by 1/3 of the difference between a 80 mm travel bike and a 100 mm travel bike.
    hmm since the truth is already 12.8" for 100mm, i guess 12.6" should be ideal for 80mm. but anyway the fork's up already here's a pic. BB height is around 13.3" with tires.. is that considered tall for xc?



    Gearhead, many thanks for your input, the formula's pretty useful. just another question: the C to A height is that the actual length(say measure the fork alone) or measured vertically with bike setup(effective vertical length)?

  8. #8
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    First you'll get used to the tall bottom bracket. Then you'll grow to love it. Then you'll have a hard time riding without it.

    My GF used my bike for about 6 months while I was recovering from a surgery. She got so used to the clearance and body positioning on the tall bottom bracket that when I re-claimed the bike a few months ago(prior to a second surgery- "don't get hit by a car" is my new motto ), she bought a Truth for herself because her old bike/other new bike options did not feel "right" and she couldn't pedal through techincal terrain (seriosuly, it helps a lot) with the same ease. This included some pretty extensive trials on Jamis, Marin,and Specialized offerings.
    Sweet looking bike.
    Mike

  9. #9
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    Quote Originally Posted by sparst
    hmm since the truth is already 12.8" for 100mm, i guess 12.6" should be ideal for 80mm. but anyway the fork's up already here's a pic. BB height is around 13.3" with tires.. is that considered tall for xc?
    I am confused? You put a 80 mm travel fork on and ended up with a BB height of 13.3"?

    Quote Originally Posted by sparst
    Gearhead, many thanks for your input, the formula's pretty useful. just another question: the C to A height is that the actual length(say measure the fork alone) or measured vertically with bike setup(effective vertical length)?
    The C-to-A height is the length of the fork measured parallel to the fork sliders/stanchions alone.
    There is no added value to my participation - in fact, just more confusion.

  10. #10
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    Quote Originally Posted by GearHead
    I am confused? You put a 80 mm travel fork on and ended up with a BB height of 13.3"?
    yeah i was bewildered too, thought maybe my tires were too inflated.. or the tire knobs were especially tall... then i recalled i measured from ground to center of BB!! haha what a blooper! all along i'm used to using ct of BB for measurement( tube lengths, chainstays etc), guess i got confused.. anyway new measurement is 12.4" with used hutch scorpions. i suppose could be taller for new tires..so say 12.5" thereabout.

    thanks again for the formula GearHead!

    Quote Originally Posted by Mike Brown
    First you'll get used to the tall bottom bracket. Then you'll grow to love it. Then you'll have a hard time riding without it.
    well at 12.5" i guess now it aint so tall afterall, since my last hardtail was at 12". for my truth i guess it's not so much of the clearance, but the ict that's making it a sweet ride. glad you're loving yours.


    Thanks to all who replied!

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