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Is fork trail or chainstay length is a bigger determinant in the overall steering of the bike?

Consider an arbitrary frame that I think is steers too slowly. I'm going to rebuild it because I want a fun project, have some scrap metal, all the parts, and I want to build another similar bike that steers a bit quicker and is more racy.

All other things being equal, mainly the head tube angle, would building another frame with shorter chainstays be a better way to go about it than using a fork with less trail?

I'm interested in the rate of change in the steering of the bike while changing those two factors, and which one affects the steering more, and by how much if it can be measured - i.e. 1mm change in trail could be offset by 2mm change in chainstay length (that's probably wildly inaccurate - I'm hoping to hear from some frame builders).

  • An experiment for you - get a steel bike (a junker) and compare a normal ride. Then bend the steel forks forward to increase rake (simulating a slacker headtube angle) and ride it. Then bend the forks back the other way (straighter) to simulate a more vertical headtube. Compare subjective feeling with measured numbers like "pressure required to turn front wheel statically when a XX kilo rider is in the saddle" - use a fishing scale for this. I have no suggestions around chainstay testing, though you could chop the junker's stays and stretch/squash it. – Criggie Jun 28 '16 at 1:00
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    Chainstay length has very little effect on how a bike steers -- it basically just affects turning radius, and that only slightly. The interplay between fork trail and tube angle determines how "twitchy" a bike is, in the trade-off between stability and quick response. – Daniel R Hicks Jun 28 '16 at 11:42
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    @Criggie that's an interesting idea about the fishing scale. Correct me if I'm wrong, but, bending the forks won't simulate any change in head tube angle, if they're bent forward the head tube angle stays the same and the trail decreases, backward, it's the opposite. I want to keep head tube angle the same as well across comparisons anyhow. – ebrohman Jun 28 '16 at 17:06
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    @DanielRHicks good to know. If I'm concerned with steering as well as overall handling/maneuverability the chainstay length is still important right? If lengthening it will make a bike more stable, that would seem to negate the effect of twitchier steering/less trail, or am I not understanding the interplay between these things correctly? – ebrohman Jun 28 '16 at 17:10
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    Study up on the interplay between head angle and fork rake. Very minor tweaks in the parameters can make enormous differences in stability vs steering responsiveness. Chainstay length, on the other hand, just affects turning radius, and with a factor that is about equal to the % change in wheelbase. – Daniel R Hicks Jun 29 '16 at 2:14
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Because you steer a bicycle with the front wheel (and weight offsets), the steering of a bicycle is almost entirely determined by the geometric parameters of the front wheel and where it is located. The most determining factors are:

  • Trail
  • Wheel flop factor
  • Front wheel radius

Trail and wheel flop factor are determined by steering angle, fork offset and wheel radius.

The chainstay length is irrelevant in this respect, but it does matter for the overall handling of a bicycle. For example it has an influence on how well a mountain bike climbs. For road bikes it has some influence on the weight distribution between both wheels.

There are tandems which handle like a good road bike (except for the turning radius). They essentially have "chainstays" that are almost 1.5 m long.

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