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hello, could someone help me on this issue please? what would have better high speed stability, a large frame with short stem or a small frame with long stem? i notice the pro tour guys seem to prefer small frames with long stems. wouldn't a short wheelbase incur some high speed wobbles?
 

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Not so simple

tidi said:
hello, could someone help me on this issue please? what would have better high speed stability, a large frame with short stem or a small frame with long stem? i notice the pro tour guys seem to prefer small frames with long stems. wouldn't a short wheelbase incur some high speed wobbles?
There are a lot more factors than the ones you seem concerned with. As a general rule, a shorter stem makes a bike a little more twitchy, but not by much. Short wheelbase is NOT the cause of speed wobble. Stability is mostly about trail, but there are several factors, a very significant one is rider skill.
 

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Kerry Irons said:
As a general rule, a shorter stem makes a bike a little more twitchy, but not by much.
We need to be careful with this statement, as it leads to much confusion. On a given frame and for a given rider, a shorter stem MAY make a bike slightly more twitchy, because relatively more bodyweight is borne by the bars and so fine control is more difficult. That results in a more twitchy feel. However, that assumes that corresponding changes (such as moving the saddle aft or changing bar height) aren't also made, which could easily change the result.

If comparing two bikes with identical reach, drop, angles, and trail, but one having more reach in the stem, as compared to the frame, the larger frame will be less 'twitchy'. The longer wheelbase will slow the steering a bit. Also, contrary to the common cycling mythology, a shorter stem (weight and balance issues notwithstanding) will require more effort to effect the same movement, thereby effectively 'slowing' the steering. There's a reason that riders are taught to grab the bar tops when doing something like peeling a bar, grabbing bottles, changing clothes, etc.

If the question is about wobbles, a smaller frame has a probability of being slightly more rigid, AEBE, and so a bit less likely to get wobbly. However, wobbles are about the complete bike/rider/wheel/tire/pressure/fork/etc. picture and very complex, so there's no way to make a generalization that one bike will or will not be more likely to wobble than another.
 

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Here's my understanding of "Trail" as applied to bikes. If you took a line right down the center of the steerer tube and marked where it hit the floor/pavement and then marked where your tire actually hits the pavement...that is the "trail". The more 'trail', the less effect small steering movements have...(Think of a Hardly-Davidson Chopper with extended forks. You turn the bars and the front wheel flops over, but the bike keeps on going pretty straight) The Chopper, it has lot of fork "Rake" which is different from "trail" but connected to it.

Rake is the angle of the steering axis. That imaginary line we drew down throught the steerer tube. Bikes seem designed around appx. 73degree angles. If a fork was dead-straight from the steerer tube to the dropouts, you would have BIG trail (it would be a function of the front wheel diameter) but most forks have some "offset" or call it *bend* that moves the center of the wheel ahead, thereby moving the contact forward nearer where that imaginary center line of the steerer tube hits the ground...Making for a reasonable "trail"

There, I've confused you. You are welcome.
Don Hanson
 

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terminology...

The terminology used for bicycles is not the same as motorcycles. The term rake does not refer to the angle of the steering axis (the head tube angle). Rake and offset mean the same thing in bicycle terms. Offset or rake is measured parallel to the steering tube and it is the amount the front axle is in front of steering tube centerline.

The formula for trail shows that all amounts of offset reduce the trail and quicken the steering.

Trail = R/tanH - (rake/sinH), where H is the head tube angle and R is the tire radius. Without fork offset or rake, the second half of the equation would be zero and you would have a very large trail and slow steering.
 

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I'm currently considering the Excaliber frrom cc. I'm able to fit either L or M. However, I'm considering the L, with shorter stem, to get taller headtube. I'd say, that these days there aren't many dramatically steep angles, and super short wheelbases out there (ala Gios compact) and that things are relatively square,73/73 and won't be too twitchy.
 

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C-40 said:
The terminology used for bicycles is not the same as motorcycles. The term rake does not refer to the angle of the steering axis (the head tube angle). Rake and offset mean the same thing in bicycle terms. Offset or rake is measured parallel to the steering tube and it is the amount the front axle is in front of steering tube centerline.

The formula for trail shows that all amounts of offset reduce the trail and quicken the steering.

Trail = R/tanH - (rake/sinH), where H is the head tube angle and R is the tire radius. Without fork offset or rake, the second half of the equation would be zero and you would have a very large trail and slow steering.
Right on the terminology, not necessarily on the outcome. Inside a certain parameter, increased trail (via decreased offset/rake) will indeed 'slow' the steering. However, increase it too much, and we start to fight wheel flop. Small inputs suddenly yield large, nearly incontrollable steering results, and great efforts are required to keep the bike from diving into a leaned turn. Starts to be a problem about the time the steering axis gets ahead of the contact patch.

For a dramatic example, find and old bike and a bunch of body armor, flip the fork over, and go for a ride. There are a lot of terms to use for the result, but 'slow' isn't likely to be one of them. (yes that's extreme, but the directional results are valid.)
 
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