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 08-20-07, 12:28 PM #1 daneil Senior Member Thread Starter   Join Date: Apr 2004 Location: Brooklyn, NY Bikes: Posts: 255 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) Interesting geometry question OK. So for all of you with vast knowledge of frame building, engineering, and the requisite mathematics I have a question to ask. Say that one had a funnybike, an older version with the 24" front wheel and 700c rear. What kind of geometry changes would occur if you replaced the front fork and wheel with a 650? This is assuming that the 24" is the more common small wheel which measures 520mm and that the 650 is the more common 650c which measures 571mm, for an overall difference of 5.1cm which would raise the front end by 2.55cm. Would the resulting change in geometry severely alter the handling of the bicycle?
08-20-07, 01:23 PM   #2
Nessism
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 Originally Posted by daneil Say that one had a funnybike, an older version with the 24" front wheel and 700c rear. What kind of geometry changes would occur if you replaced the front fork and wheel with a 650? This is assuming that the 24" is the more common small wheel which measures 520mm and that the 650 is the more common 650c which measures 571mm, for an overall difference of 5.1cm which would raise the front end by 2.55cm. Would the resulting change in geometry severely alter the handling of the bicycle?

Raising the front end will slacken the head tube angle quite a bit. You can get back some of the quickness if you go with a highly raked fork though. The bottom bracket height will also increase a goodly bit which may feel kind of weird. Whether or not these changes are "too much" kind of depends on how the bike was set up to begin with.

Good luck.
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08-21-07, 08:19 PM   #3
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 Originally Posted by Nessism Raising the front end will slacken the head tube angle quite a bit. You can get back some of the quickness if you go with a highly raked fork though. The bottom bracket height will also increase a goodly bit which may feel kind of weird. Whether or not these changes are "too much" kind of depends on how the bike was set up to begin with. Good luck.
I think it's going to result in less trail, which might really not be a problem. More trail makes it easier to keep the bike in a straight line, and makes the bike want more to "self-recover" if there's been a disturbance, such as sidewind, sharp bump, or rider fidgeting. However, trail is by no means all there is to say about bike handling.

08-23-07, 10:23 PM   #4
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 Originally Posted by Road Fan I think it's going to result in less trail, which might really not be a problem. More trail makes it easier to keep the bike in a straight line, and makes the bike want more to "self-recover" if there's been a disturbance, such as sidewind, sharp bump, or rider fidgeting. However, trail is by no means all there is to say about bike handling.
Why would there be less trail if the head tube angle is relaxed, that should increase trail.
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08-24-07, 02:57 PM   #5
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 Originally Posted by Nessism Why would there be less trail if the head tube angle is relaxed, that should increase trail.
Sorry, Nessism, you're right! He'll get a trail increase by decreasing the head angle, and an addtional one by making the wheel bigger (both terms are in the equation). He can reduce trail by increasing fork rake.

The added trail should slow the steering but improve stability.

The hard part is to decide what the geometry needs to be without extensive trial and error.