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  1. #1
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    Frame geometry: a top tube that slopes downward from seatpost to fork??

    Hello all, I've searched for something about this, and haven't had any luck..

    I have an old Trek 1000 that I had a LBS put a new fork on.

    Before installation they told me the handling might change so much that it could fell "un-rideable" and I "might not like it" etc. because it'd lower the whole front of the bike a few MM (i forgot now, it was something between a 4-10mm difference.)

    He explained that Trek forks are "special" and a bit higher than most other forks and it'd be best to use another Trek fork (I was going with a no-name cheap one < 1/5 the cost of a new trek fork).

    I accepted the risk, got the work done, and the bike handles/rides excellent with the new fork (also feels very "soft".. it's carbon from 2012 replacing aluminum)

    I rode it before getting the work done, but not very much or for very long..

    kicker:

    I have a commuter bike and a race bike, this bike serves as the backup for both (it's quick and light, but still has eyelets for rack mounting)..

    Every time i ride it, it feels great at the time and just after, but i wake up with incredible neck/upper back pain the next morning.. we're talking short commute rides 10mi round trips for work here.. around 20-25min on the bike each way..

    my question:

    could the slope of my top tube (a reverse slope or "forward" slope, down from the seat tube) really screw up my posture/body? i'm wondering if i just need to keep playing with the fit (i've already adjusted the stem length/angle and experimented), or if this bike just won't really work well because of the geometry. i specifically try to keep weight off my arms, and i'm accustomed to riding 60-100mi rides weekly and i don't own a car or ride the bus so all other transport is via bicycle.

    The frame size is the same as my other (also Trek) racing bike that fits perfectly.

    is this all in my head (a coincidence?)? a matter of playing more with fit? or do you think a "reverse" top tube slope is actually causing harm here?


    thanks, and sorry for the lengthy post.. maybe i should've put this in the frame builders section?
    Last edited by hajitosan; 07-04-13 at 02:42 PM. Reason: changed title to more accurately reflect situation

  2. #2
    coprolite fietsbob's Avatar
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    How about Data? What are the dimensions before and after this change..

    I can 'see' the difference by having numbers,

    where I cannot guess without you being present in the bike shop.

    I'd be inclined to get out the tape measure.. to be able to speak of 'how much'..
    Last edited by fietsbob; 07-04-13 at 10:20 AM.

  3. #3
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    This is the original geometry from Trek (in MM)
    Head tube angle =73
    Fork offset =43
    Trail =59
    Seat tube angle =73.5
    Effective top tube =573
    Seat tube =580
    Standover =808
    Wheelbase =1014
    Bottom bracket height =268
    Chainstay =417

    Head tube =140



    the new fork has lowered the whole front end by 5mm.

    many thanks for even considering this question!

  4. #4
    coprolite fietsbob's Avatar
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    Axle center to fork Crown race changing by just 5mm? should not make a Night and day difference.

    A machined washer 5mm thick Can , in theory, go under the fork crown race and the crown-race seat
    to make the difference zero ..

    but its not like substituting a Suspension fork for one thats 4"shorter.

    Head tube angle =73
    Fork offset =43
    Trail =59
    And a 5mm wider, and thus taller, tire can also make up the difference
    Last edited by fietsbob; 07-04-13 at 03:35 PM.

  5. #5
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    A sloping top tube, by itself, will not affect your riding position at all. Duplicating the 'effective geometry' of one of the other bikes should relieve your problems. Saddle height, setback above the crank, the drop from saddle to bar, reach, the works -- get it as close as possible to the more comfortable of the two bikes.

  6. #6
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    that's what i was thinking.. i want to take all three bikes to a professional fitting and have them match them all to my racing bike (perfect fit for centuries)

    thanks for the input people!

  7. #7
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    I'm not sure that matching fit between bikes makes any sense, unless they share the same geometry and key components (e.g. handlebar measurements).
    Chaad--'95 DeKerf Team SL, '02 Lemond Buenos Aires, '05 Novara Buzz, '73 Schwinn Collegiate, '06 Mountain Cycle Rumble, '09 Dahon Mariner D7, '12 Mercier Nano, '12 Breezer Venturi

  8. #8
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    I think The 1000, and 2.3 do have the same handlebar widths, but may be off by a tiny bit, and the crank arm legnths are the same.. i figure if the distance between my butt, feet, and hands are the same i should be good.. is that incorrect?
    Last edited by hajitosan; 07-06-13 at 11:25 AM.

  9. #9
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    It's a good starting point; ultimately, though, the END point is 'comfortable on each bike', even if they wind up measuring differently.

  10. #10
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    Quote Originally Posted by DX-MAN View Post
    It's a good starting point; ultimately, though, the END point is 'comfortable on each bike', even if they wind up measuring differently.
    I agree with this, because different geometries will place you on the bikes in different ways, engage different muscles, and have handling characteristics that change the way you ride. This is the reason why I said before if the geometries are not the same, it may not make sense to set them up the same.
    Chaad--'95 DeKerf Team SL, '02 Lemond Buenos Aires, '05 Novara Buzz, '73 Schwinn Collegiate, '06 Mountain Cycle Rumble, '09 Dahon Mariner D7, '12 Mercier Nano, '12 Breezer Venturi

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