Why are they used only in the back if there is no risk of winds at all?

  • What happens when the wheel turns sideways? – Daniel R Hicks Jul 1 '19 at 23:13

Because it's not permitted under UCI regulations.

Rule 1.3.018 applies, which includes this section:

In track competition, including motor-pacing, the use of a front disc wheel is only permitted in the specialities against the clock.

Disc wheels are commonly used in the sprint qualifying round (i.e. the flying 200 metre TT).

  • 2
    The follow-up question would then be: Why do UCI regulations allow rear disc wheels, but not front disc wheels? – cmaster - reinstate monica Jul 2 '19 at 6:52
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    I can't answer for the UCI technical commission but if I had to guess it's because the handling characteristics of a front disc wheel makes them less suitable for rapid response to steering input, a trait desirable in mass start and match racing (IOW it's a safety consideration). Using a disc on the rear doesn't affect steering response. The difference (slowness) in steering response of a front disc was my experience when using them on the track. Apart from individual timed events I used them during team pursuits. – alexsimmons Jul 2 '19 at 8:04
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    As to there being no wind, while that's true in an absolute sense, it doesn't mean a bike and rider experiences zero yaw at the track. You experience wind yaw when in the turns, and when entering a turn a rider experiences a change in yaw angle which also contributes to handling issues with a front disc. On average the yaw experienced on the track is a few degrees. Timed events are ridden under controlled and predictable conditions with inherently less risk than match and mass start races. – alexsimmons Jul 2 '19 at 8:11
  • I would add to the answer what you wrote in the comment to set it as resolved, as cmaster commented I wanted to know the reason behind. Thank you @alexsimmons – nck Jul 20 '19 at 10:48

Sprint often requires quick direction changes. With a quick turn of the wheel a disc becomes a rudder in the wind and can behave unexpectedly. Even in TT, many riders will elect something other than a disc in the front simply because it's a little more skittery. A disc in match sprint c/would be dangerous. UCI recognizes this.

  • Great first answer, but you're assuming that the reader is reading your answer after Alex's. You should edit your answer to stand alone better. – rclocher3 Feb 16 at 18:50

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