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Thread: Bike geometry

  1. #1
    Senior Member bidaci's Avatar
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    Bike geometry

    I had a 1997 GTLTS4 frame that I put together using parts from a 1997 GT LTS5. The geometry on the bike from what I can tell is the same except that one is aluminum and the other Cromo. I used the forks off the LTS5 (RST381m) which isn't a great fork but is still servicable. I noticed at slow speeds the front tire felt like it would want to tuck in on slow speed turns on the street. Looking at some specs I found online it appears the only real difference is the fork rake. The LTS4 is 1.6" and the LTS5 is 1.5". Headtube on both and is 70 degrees and the seattube is at 74. The original fork for the LTS4 was a RS IndyC.

    Am I correct in assuming that the headtube and the seattube angle are frame specific and do not take into consideration the forks? Would the difference in the Fork rake be attributed to the travel of the forks for which I have no specs on? Wouldn't using all the LTS5 components on the LTS4 in essence makes the bike ride and handle as a LTS5 would except for it being made out of aluminum?

    If needed I found the specs for each bike here.

    LTS 5

    LTS 4

    Thanks for any input

  2. #2
    Mostly riding...mostly NM-NewRoadie's Avatar
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    When I went shopping for a new fork (carbon fiber) for my 96 trek 2200 I found many people stressing the importants of fork rake when replacing or upgrading a fork. What I determined was best was NOT TO CHANGE IT.

    That said, there are (as I understand) two ways to change the angle of a front fork. 1) fork rake and 2) head tube angle. Head tubes are often at different angles on the same model of bike but in different sizes... for example a 52 cm frame of the same model might have a different head tube angle than the same bike in a 58 cm size. If the new fork doesn't feel right, i'ld either A) put the old one back on, or B) try to find a fork with the propper rake.

    Good luck! hope this helps.

  3. #3
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    I'd say with a 0.1" difference in rake, you aren't going to change the handling of the bike too much. You may be using a fork with more or less travel. This means that you may have increased/decreased the trail of the bike. Both frames have the same head tube angle.

    It sounds like you have put on a fork with more travel, which means that your trail has increased a bit. Bikes with larger trail #'s are more stable at high speed, but can sometimes be unstable at lower speeds (the wheel wants to "flop over"). Bikes with lower trail #'s are very stable at lower speeds, but can get a bit nervous at higher speeds.

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