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  1. #1
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    Fork Length relative to Head Tube and Seat Tube - need advice

    I'm looking at getting a new cross frame and would swap components from my old one to the new frame - but I'm confused about how my current fork will work in the new geometries.

    My current bike has a HT length of 17.5cm , and Seat Tube of 55. I run a +17 degree stem with about 1 CM of spacers.

    One frame I'm looking at has the same HT (17.5) but the ST is 60.

    My question is - since the HT is exactly the same - would my fork and stem fit relatively the same same or since it has a much longer ST - would that effectively make the HT longer, thus I would use less spacers, or possibly need a stem with less rise?

    Thanks in advance!

  2. #2
    Senior Member Kopsis's Avatar
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    Head tube and seat tube length are completely unrelated. If you're currently riding a 55cm frame and it fits, then chances are a 60cm is too big for you. You need to find a frame that fits YOU first, then figure out if you can reuse your existing fork. The key dimension to look at is Effective Top Tube. Saddle height adjustment can let you get away with a wide range of seat tube lengths, but ETT controls reach an you only have a few cm of adjustment (by changing stem length) before you are "forcing" the fit and compromising weight distribution and handling.

    Now, to answer your original question ... if the HT is the same length AND uses a similar headset, then you can probably re-use your fork. However, 1cm of spacers doesn't give you a lot of margin to play with. Hopefully you have some additional spacers above the stem. What you're looking for is total stack height, which is the HT length plus headset stack plus stem height plus any spacers you want to run. That total has to be less than the length of your fork's steer tube from crown race to top.

  3. #3
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    Quote Originally Posted by Kopsis View Post
    Head tube and seat tube length are completely unrelated. If you're currently riding a 55cm frame and it fits, then chances are a 60cm is too big for you. You need to find a frame that fits YOU first, then figure out if you can reuse your existing fork. The key dimension to look at is Effective Top Tube. Saddle height adjustment can let you get away with a wide range of seat tube lengths, but ETT controls reach an you only have a few cm of adjustment (by changing stem length) before you are "forcing" the fit and compromising weight distribution and handling.

    Now, to answer your original question ... if the HT is the same length AND uses a similar headset, then you can probably re-use your fork. However, 1cm of spacers doesn't give you a lot of margin to play with. Hopefully you have some additional spacers above the stem. What you're looking for is total stack height, which is the HT length plus headset stack plus stem height plus any spacers you want to run. That total has to be less than the length of your fork's steer tube from crown race to top.
    If the 60cm frame has more BB drop than the 55 and thus the same standover height then the 60cm might not be too big. ETT might well be the same for both. But why buy a frame without a fork? Is it a carbon fork or other kind of high investment item? Just buy a frame with a compatible fork as OEM and you're good.

  4. #4
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    Quote Originally Posted by Leisesturm View Post
    If the 60cm frame has more BB drop than the 55 and thus the same standover height then the 60cm might not be too big. ETT might well be the same for both. But why buy a frame without a fork? Is it a carbon fork or other kind of high investment item? Just buy a frame with a compatible fork as OEM and you're good.
    My current fork is practically brand new - and is carbon. The one on the macho man is steel. No comparison there. The bikes have the exact same BB drop and ETT (though the TT on the Macho man is 1.5 cm longer - the 1.5 degree less Seat tube angle makes up for that). HT is exactly the same. I guess I'm just wondering how a 5CM longer seat tube could not somehow affect how much fork stack height I have. Everyone I spoke to also agrees that if the bikes have the same HT and BB drop - I should be good to go.

  5. #5
    Senior Member Road Fan's Avatar
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    I'm not sure how the BB drop fits in, if the TT is sloped. But I agree, you should be able to install that fork. Whether the front end ride height will be as intended is another question, based on blade length and offset.

  6. #6
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    Quote Originally Posted by Road Fan View Post
    I'm not sure how the BB drop fits in, if the TT is sloped. But I agree, you should be able to install that fork. Whether the front end ride height will be as intended is another question, based on blade length and offset.
    I did not know that the top tube was sloping. In any case, I agree. The only information that was relevant was the length of the HT. The different lengths of the ST have absolutely zero effect on anything happening up front. Zero. It is not a detail to spend even one minute considering. The ride height will be as intended because the fork is a known quantity from before.

    H

  7. #7
    coprolite fietsbob's Avatar
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    Hub axis to crown race seat + rake/offset of fork blades .. that is your steering geometry .. Trail.

    too long a hub to crown race and you lower the head angle as you push up the front of the frame .

  8. #8
    Senior Member Road Fan's Avatar
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    Quote Originally Posted by Leisesturm View Post
    I did not know that the top tube was sloping. In any case, I agree. The only information that was relevant was the length of the HT. The different lengths of the ST have absolutely zero effect on anything happening up front. Zero. It is not a detail to spend even one minute considering. The ride height will be as intended because the fork is a known quantity from before.

    H
    I guess you need to tell me if your TT is supposed to slope when the frame is level. I think I was confused in the first post, I thought you were talking about a frame with a TT that slopes up to the front. If someone talks about effective top tube length they are usually talking about a frame with a sloped top tube, but maybe you didn't know that.

    Can you show a picture of your frame, a large side elevation? Maybe there's a guesstimate than can be made. The distance from the rear brake bridge or canti pivots to the dropout (where the rear axle will be located should be very similar to the distance from the bottom of the fork crown or the canti pivots to the location that will hold the front axle. Then the front axle to ground and rear axle to ground distances will be equal.

  9. #9
    Senior Member Kopsis's Avatar
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    I raised the question of fit due to the significant difference in ST length, but if the ETT is roughly the same for both frames, then that's no longer a concern.

    So for the remaining question -- "will the steerer on my existing fork be long enough to fit the head tube on the new frame" -- none of this seat tube, BB drop, sloping top tube stuff is in any way relevant. HT length plus headset stack, plus stem stack, plus spacers must be less than or equal to fork crown to top of steerer. If it is, it fits. As to other effects on geometry -- Fork Lengths, by Damon Rinard. Carbon forks and steering geometry changes.

  10. #10
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    Thanks for all the replies. Also - check out this awesome frame geometry comparison tool.

    GearInches.com Bike Geometry Comparator

    I downloaded it and put in the numbers and it seems my front end will be exactly the same since the HT and ST are the same. I have to go over things again because I didn't have all the specs with me - but this seemed to be a good proof of concept.

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