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While walking to work this morning my eye was caught by an odd movement on a bike passing me several metres away. The front wheel was far from true laterally, though it didn't look out of round. It was on a basic hardtail/hybrid with front suspension with disc brakes; it was fairly close but not rubbing on the rather wide forks.

If we assume that this wasn't due to impact causing weakening I couldn't see, how dangerous is it likely to be?

Chris H
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    Please don't introduce American spelling into posts written in British English. – Chris H Sep 25 '19 at 13:29
  • Making an assumption to start is bad practice. The wheel could have been improperly placed in the drop outs. The question is still a good question, but the first paragraph is irrelevant to it. – Deleted User Sep 25 '19 at 16:45
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    @DeletedUser did you mean 2nd paragraph? That's the one with the assumption. It's a physicist's (me) simplifying assumption as I lack access to data, and if the rim had been clobbered that would introduce further potential failure modes – Chris H Sep 25 '19 at 16:49
  • I meant the assumption it was out of true and not simply placed in the drop out incorrectly. But I suppose there is a second assumption there as well. – Deleted User Sep 25 '19 at 16:51
  • @DeletedUser it would have to be flapping around massively to show the level of side-to-side wobble it had, and the wobble looked rhythmic which I wouldn't expect if it was just loose. It certainly wasn't done up tight but crooked. I wouldn't have spotted that from 3-4m away – Chris H Sep 25 '19 at 18:28
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    @DeletedUser: Placing it in the drop out incorrectly wouldn’t cause a sideways wobble. – Michael Sep 26 '19 at 06:37

3 Answers3

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Another danger of riding an out-of-true wheel is cornering. When on the straight, the wheel may be "round" while wobbling from side to side.

But when leaning into a corner, the bike is no longer vertical, so that out-of-true in the rim is translating to an out-of-round as well. So the wheel of the bike will be rising and falling ( "oscillating" ?) sideways at roughly twice the wheel's RPM.

Effectively that could make the wheel loose traction much easier on the faster corners, or even at moderate speeds if the corner has any treacherous features like dampness, dust, sand, gravel, or even painted lines.

Criggie
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    Having clipped a stone with the back wheel while cornering on a wet road this morning, and felt the wheel slide out a bit, I'd hate to have that happen on the front on every corner. – Chris H Sep 25 '19 at 13:27
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    +1, this essentially corresponds to a point in my answer applied to cornering: "riding such a wheel should feel as rolling over periodic bumps from one side of the wheel". Traction loss is directly related to pulsing reaction force. – Grigory Rechistov Sep 25 '19 at 15:30
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    "( "oscillating" ?) sideways at roughly twice the wheel's RPM" That depends on the mode in which the wheel is out-of-true: If the rim is just not coaxial with the hub, you get once the wheel's RPM. If the wheel is taco'd, which is the typical mode of deformation, it's indeed twice. But you can also have a wheel that oscillates thrice per revolution, or even more. The limit is the stiffness of the rim. – cmaster - reinstate monica Sep 25 '19 at 19:30
  • @cmaster good point - I was imagining a round rim that rolls normally when straight, but has two points on the rim that are left, and therefore two that are right. A picture might have been better than lots of words. – Criggie Sep 25 '19 at 20:00
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Quite dangerous. The wheel can fail suddenly and without warning, causing an unavoidable and bad crash.

I assume the wheel had a broken spoke. Wheels can usually survive a single broken spoke (unless you have a really low spoke count). However, a second spoke is likely to follow and can cause the wheel to collapse entirely. On a rear wheel this can still be somewhat manageable, but on the front wheel it’s catastrophic.

Michael
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    It looked worse than when I've had a broken spoke on a 32 hole wheel and on my 36 hole touring wheels breaking a single spoke isn't enough for them to go visibly out of true – Chris H Sep 25 '19 at 09:39
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    I had a 16 spoke front wheel break one spoke, and it was instantly out of true enough to lockup on the inside of the front fork. Was almost an OTB ! – Criggie Sep 25 '19 at 10:40
  • I remember a bike that had 9 broken spokes in a 36 hole wheel, and it was still rideable. Of course, it was quite wobbly and needed immediate fixing... – cmaster - reinstate monica Sep 25 '19 at 19:36
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Riding an untrue wheel on a flat surface should be roughly equivalent to riding a perfectly true wheel on a correspondingly designed bumpy/uneven surface. At least from the point of what forces are acting on the bike and the rider. I.e., if there is a depression in a wheel, an equivalent terrain would be best represented by a BMX rhythm section with its periodical bumps:

rhythm section

It also applies for lateral untrueness: riding such a wheel should feel as rolling over periodic bumps from one side of the wheel.

It means the rider has to adjust to negotiate such "technical terrain" by redistributing own weight to maintain the balance and tire traction, even when riding a completely flat road. As anyone who rode a rhythm section can tell you, not going with its "flow" will quickly throw you out of the balance and possibly off the bike.

Essentially, it is similar to riding with a turned handlebar: it is possible, but you have to constantly adjust your riding style, and your range of steering control becomes asymmetrical and thus limited.

For an untrue wheel, for example braking will not be as effective and predictable because you essentially have periodically pulsing reaction force (both in direction and absolute value), and the friction force will pulse as well. Again, it is as if one is riding a mountain bike with straight wheels in a rock garden - the braking traction varies all the time as the wheels jump from stone to stone. When wheels are in air, the friction is zero.

In the MTB case, having a well tuned suspension (both front and back) helps keeping wheels planted on the ground. I assume the same applies for untrue wheels - having a suspension might hide some of the unevenness, but again it will spend a fraction of available suspension travel, making it less effective when you need it most, e.g. on big bumps.

Grigory Rechistov
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    This sounds valid, but probably only if all the spokes are present and tensioned. – Chris H Sep 25 '19 at 09:40
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    @ChrisH indeed, my answer only deals with the shape of the wheel as the whole, assuming it is untrue but has fixed shape under the ride (i.e. it does not disintegrate over the course). Going back to my analogy with the handlebar: riding with a turned handlebar is bad, but a loose or even self-detaching one is much worse. – Grigory Rechistov Sep 25 '19 at 10:26