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Thread: Geometry Quiz

  1. #1
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    Geometry Quiz

    Hello all, This is my first attempt at posting here--please pardon any errors in placement or format.

    Last week, I irreparably bent and cracked the derailleur hanger on my ’96 Trek 8000. I decided to swap-out the XT components and Manitou SX fork onto a like-new Trek 4300 frame that I found on Ebay for $49.00. (At first I was hesitant to “downgrade” the frame, but then realized it was mostly an issue of foolish vanity concerning what digits are stickered onto my bike—by all accounts the new SL frames used on the 4300 are very nice—even built in Wisconsin.)

    The frame hasn’t arrived yet, and now I’m hung-up on what is perhaps an equally silly obsession—a slight difference in frame geometry. The head and seat angles will be .5 degrees different, and the top tube will be something like an inch shorter. The chainstays on the 4300 will be 16.9” vs. 16.7” on the 8000.

    Now my questions: Will I actually notice a difference in handling—or, do I just need to shut up and ride? What if any actual performance differences result from these differences in angles? Will my built up bike “look” like a “comfort-bike” (Heaven Forbid!!)? Is it possible to even see a difference in geometry like this with the naked eye? Is there a chance that any one single aspect of overall performance will actually be improved? A part of me hopes that these seemingly miniscule differences in geometry act more as a mental placebo that allow builders to charge more for “performance” geometry. Another part is saying, “…you idiot, those mathematical calculations are extremely important—each tenth of an inch is critical! You’re going to feel like your peddling an old Huffy….”

    Thanks for any input that will either confirm my fears or put my mind to rest. I hope to be a regular visitor here and will complete my profile as soon as I get a chance.
    Cheers,
    Epicurean

  2. #2
    TreadHead MtbVA's Avatar
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    The .5 deg head & seat angles will not matter much - at 21" from center of hub a 1 deg difference is only 3/8".

    Now, the top tube being an inch shorter could be a problem if your used to one that is an 1" longer..It could put hadlebars to close if using parts from other bike.

    As long as your legs have enough room, you could get a longer &/or higher rise stem to compensate for the difference if doesn't have the feel you're used to.

  3. #3
    Senior Member Callaway's Avatar
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    What year is the 4300, if it is relatively new at all I can assure you that it is not made in Wisconsin, I have contacted Trek about what bikes they actually make in the US. I don't know much about the swapping bit but let us know how it works, sounds interesting...
    '83/'85 Shoguns, Ridley Helium, Miyata 600 GT, '85 Peugeot PGN 10, Masi, Giant TCR Advanced (sold), Nishiki Prestige (stolen), Stumpjumper HT (sold) and Trek 830 Mtn XC (traded)

  4. #4
    DNPAIMFB pinkrobe's Avatar
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    You will barely notice a difference. The 4300 is based around a 3" travel fork, although your 8000 might have had a 2" travel fork. The top tube length is the biggest thing you will notice. Build it up, go for a ride. If it sucks, go find a used 8000 or 8500 frame [or Stumpjumper] and get that xc race feeling going again.
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  5. #5
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    well, I am not an expert... correct me if I am wrong.

    Having a slightly longer chainstay is good for comfort and going downhill.
    Having a shorter chainstay is good for handling and climbing.

    A slack headtube angle is better high speed braking steering, and a steep headtube angle is good for quick steering. Very responsive.

    The top tube length is mainly a comfort thing.

    Other things to consider are the wheelbase length? Short wheelbase is quicker steering and responsive handling.

    *remember depending on what fork you put on it will change your angles... you want a steep headtube- go short on the fork- you want a slack headtube- go looong on the fork.

  6. #6
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    Thanks everyone! I'll be sure to post how it handles once I get this thing built up. Perhaps I'll need a slightly longer stem because of the shorter top tube.

    Callaway, I haven't actually seen the frame yet, so the "made in Wisconsin" comment is made based on the info on the Trekbikes website. According to that, the 4300 is an "SL" Alpha Aluminum frame, which they state are made in Wisconsin. They list the 3900 and below as having an "Alpha Aluminum" frame, for which they do not claim US manufacture. I'll keep my fingers crossed--not that it really matters for $49.00 on Ebay!

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