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Old 10-17-05, 10:50 AM
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TandemGeek
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Originally Posted by ElRey
Let me attempt to restate since I don't sem to have made myself clear: you can't determine trail and rake from teh frame's geometry. ... Rake and trail can be alterd by buying a new fork. You're stuck with frame geometry so that's where you look closely. Hence, I initially bought a frame set with 73 degree frame geometry (Cdale) and changed out the fork with something a little bit quicker. Changed the handling characteristics completely. Frame geometry didn't change.
Actually, you need the head tube angle based on a given fork length + the fork's rake + wheel diameter to calculate the trail. You can unintentially alter the frame's geometry when you change fork to obtain a different amount of rake if the fork length is NOT essentially the same as the original. With regard to the change you made, replacing a steel Cannondale fork (53mm of rake) with a Reynolds Ouzo Pro tandem fork (55mm rake) would not have been much of a change as they most likely have nearly the same fork length @ 395mm (+/- 5mm): a small change in trail. However, given that most tandems use forks that are longer than solo racing bikes, a lot of folks who switched over to AME's (now True Temper's) Alpha Q forks lost nearly an 1" of fork leg length which had the net effect of change the head tube & seat tube geometry by a full degree, as well as lowering the front bottom bracket.

Thus, the suggestion to discuss fork changes with your frame's designers or builders is always an essential step as some changes -- if not fully understood -- can really alter how a tandem will handle, possibly making it better or in some cases making it worse. Moreover, when head tube angles are altered by fork leg length, the net head tube angle needs to be understood before you can decide on what amount of fork rake to use to achieve the desired amount of trail. Burley worked with True Temper on it's V-brake tandem fork (essentially an Alpha Q CX fork that was beefed up for tandems) to give it the same fork length (and a larger wheel opening) as their steel forks so that they would retain their 73 degree head tube angles and used something even more aggressive than Co-Motion's steel fork spec. of 50mm of rake to achieve what they call their "race handling" package.

Again, as a consumer, the only thing you need to understand is that some builders use different steering geometry. Ride different models for yourself and then ride them again before deciding which you prefer and, remember, you're buying to suit your preference, i.e., what's best for you.

Last edited by TandemGeek; 11-03-05 at 12:55 PM.
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