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The trade offs for 26" through to 29" with respect to rolling resistance, contact patch, obstacle rollover, etc... have been well discussed. However, it occurs to me that as the rear wheel gets larger the clearance eventually requires the chain stay length to increase to accommodate the wheel.

How does a chainstay length affect a mountain bike performance?

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All else being equal, longer chainstays equate to a longer wheelbase, and the rear wheel trailing further behind you. You are correct in your assertion that as wheel size increases so too does chainstay length, though many manufacturers have put considerable effort into minimizing this and as such the increased chainstay length is not directly proportional to increased wheel radius. For example, a 26" and 29" version of the same bike model are unlikely to have an inch and a half discrepancy in chainstay length. That said, there are tradeoffs to be had in chainstay length differences.

As an analogy, think of a drag strip or hill climbing motorcycle vs a standard sport or dirt bike. The first two examples have very elongated swingarms that put the rear tire well behind the rider and dramatically increase the wheelbase. Those long swingarms are great for preventing the bike from looping out and maintaining straight line stability, but neither can turn or corner like the latter standard motorcycle examples. That is a very exaggerated example, but the same principles apply to chainstay length differences on bicycles. Again, all else being equal, longer chainstays are going to be more stable at speed and typically provide better climbing traction, but they will cause the bike to not handle as sharply and to no turn as quickly- that much is just basic physics.

Keep in mind that chainstay length and wheelbase are just two factors in a multitude that determine how a bike will handle. Also keep in mind that just because you have a bike with slightly longer chainstays doesn't mean you can't rail corners or navigate switchbacks, and just because you have shorter chainstays doesn't mean you can't bomb descents- much of this comes down to technique, and you'll alter your technique to accommodate for any differences in your bike's handling characteristics.

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    +1 for altering your riding style! Also, you can end up spread between multiple obstacles, which can be good or bad. I have trouble riding my XXL 29er on a pump track, but my L DJ bike is perfect. It's all about purpose and geometry. – Aaron Aug 12 '13 at 15:25
  • I guess one takeaway would be that the larger wheel size might force a design trade-off by limiting the minimum chainstay length. – JeffV Aug 13 '13 at 11:39
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benefits of short chainstay:

  • very easy to manual/wheelie
  • very easy to corner (just by "thinking") with minimal body movement and without turning the bars

benefits of long chainstay:

  • stability on high speeds (due to longer wheelbase)
  • better/easier climbing
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I think this discussion fails to mention two important physical consequences of longer chainstays:

  • all else being equal, a longer chainstay will flex more, and your body weight has more leverage: this gives either improved comfort or squishy bike depending on your view.
  • with a longer chainstay, there is more weight on the front wheel, so it will have more grip. Basically it changes the weight balance and how you ride the bike in a turn or climbing.

Both are of practical importance on a mountain bike.

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