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spontaneous wheel warp

Old 10-07-16, 12:43 PM
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Chris Chicago
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spontaneous wheel warp

my brother was stopped at an intersection when he decided to turn. he says he may have overturned a little to the right and was just pushing the bike when his front wheel semi taco'd. he straightened it with his hands before pushing the bike the rest of the way to work. so it doesnt look taco'd now but it is seriously untrue.

what could have caused this to happen? he said he didnt jam it in a pothole or anything like that .

also, can i just true it back up and it's a-ok or should he avoid riding this wheel
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Old 10-07-16, 01:00 PM
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If it were me, I would replace the spokes, nipples, rim, rim-tape and possibly the hub. The skewer should still be ok.
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Old 10-07-16, 01:02 PM
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I can't draw on this computer, so you have to help with some imagination. Picture a big M shape graph where tension is in the vertical scale, and the horizontal scale represents rim deflections. A wheel built wheel is settled in the center V, and moving the rim to either side would increase tensions. But if it moves far enough, it reaches the top then spontaneously goes downhill to the potato chip shape.

Normally the V is deep enough that you never reach the limit, but if the tensions are uneven, or there's some local weakness in the rim, it's easier to go to the chip state. This kind of failure is much more common on rear wheels, but can also happen on a front.

I've saved many "chipped" wheels, but it takes work to ensure that the final product is properly and evenly tensioned. Whether it can or can't be saved depends on the rim more than the spokes. If the rim is evenly curved with no hard local bends, it's usually saveable. But if there's had bends anyplace, the differences in spoke tension needed will make a good outcome more difficult.
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Old 10-07-16, 01:04 PM
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The spoke tension is VERY uneven.
They need to be backed all the way off and retensioned EVENLY by someone with a modicum of competence and willing to do things SLOWLY if needed per skill level.
I build wheels rather slowly and get finished much sooner, with wheels that are "really good", tension wise.

I had a rear wheel do that on my then, new to me, hybrid.
I did a "nose stand" on a panic stop, with the rear wheel 2-3 feet in the air. I was a bit "occupied" at the time and didn't measure exactly.
Fortunately, I was able to safely "side step" to the left, but the rear wheel came banging down hard to the pavement with the bike leaned over about 1/2 way.
That led me to buying my Tension Meter and things got "worse" with a Park TS-2 truing stand.....
I got the wheel nice & true AND evenly tensioned, for what the low end rim would allow.

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Old 10-07-16, 01:08 PM
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Jobst Brandt said that it was more easy than you'd think to taco a wheel by side-loading it:



There's a decent explanation in his book. I hope @wgscott was just trying to be funny, because there's some chance that if you detensioned the spokes all the way around, the rim might not be that bent by itself, and the wheel could be brought back up to proper tension and true without needing to replace anything.

P.S. Also, I'm slow at composing posts...
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Old 10-07-16, 01:10 PM
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no bends in the rim!

so maybe i loosen the spokes until the threads show evenly then tension it up slowly?
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Old 10-07-16, 01:12 PM
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Originally Posted by wgscott View Post
If it were me, I would replace the spokes, nipples, rim, rim-tape and possibly the hub. The skewer should still be ok.
The hub is almost certainly not implicated as a cause or damaged as an effect. Spokes are very likely to be OK, with none having been stretched beyond the elastic limit, and even if it is, still OK, but will make a bit harder to build because it's starting length has changed. Nipples OK to reuse, but they're so cheap that I'd never bother.

So that leaves the rim, which as I said depends on it's relaxed flatness and how gentle the curves are. Assuming the worst case, I'd rebuild with a new rim and fresh nipples.
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Old 10-07-16, 01:17 PM
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You failed to address the rim tape issue!
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Old 10-07-16, 01:21 PM
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My rear wheel was warped worse than that exactly a month ago. (See "Warped rim salvageable?"). Sideways skid on loose gravel on asphalt, masked by recent blacktopping. The sideways skid and sudden stop when the rear wheel hit clean asphalt again caused the warp. No broken spokes and I didn't fall. Just lucky.

The LBS literally whanged the rim back into shape against a wooden work bench, then tweaked the spokes. I've been riding the bike for a month since then. No problems. And it's just a 24 y/o Araya P-45 single wall rim on a Shimano Exage hub. But I ride that bike mostly on pavement and try to avoid rough stuff.

However when the rear wheel on my other bike potato-chipped after a spoke broke during normal riding last spring, I opted for a new wheel. Because the spoke broke at the hub on the drive side I figured the old spokes would just keep popping and I'd end up spending more on spoke repairs than just buying a new wheel. I use that bike for rougher conditions, gravel and hauling groceries (up to 50 lbs), so I had the LBS install a modestly priced double wall rim, Weinmann Zac-19 with heavier gauge spokes and whatever hub the ready-made wheel came with. Good value compared with piecemeal repairs.
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Old 10-07-16, 01:22 PM
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Originally Posted by wgscott View Post
You failed to address the rim tape issue!

You failed to include the valve cap.
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Old 10-07-16, 01:23 PM
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You wont Know if the rim is physically bent until you loosen all the spokes .. Relaxed Flatness..

I suspect it will Not magically spring back Flat again . The stresses resulted in the rim being bent.

Bro check the spoke tension of that wheel regularly?






'/,

Last edited by fietsbob; 10-07-16 at 03:04 PM.
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Old 10-07-16, 01:54 PM
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Originally Posted by Chris Chicago View Post
no bends in the rim!

so maybe i loosen the spokes until the threads show evenly then tension it up slowly?
That's what I would do. Judging from your photo I'd be surprised if any of the wheel components weren't usable.

Loosen every spoke until only 1 thread is showing. With no tension on the rim, now is the time to check it for flatness or bends. Then I count turns as I tighten every nipple by the same amount. The more slowly you build tension into the wheel, the more likely your rim will stay round. When your wheel is close to full tension, do your final truing by tightening and loosening opposing pairs of spokes by an equal amount.

Good luck!
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Old 10-07-16, 02:47 PM
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Originally Posted by Dan Burkhart View Post
You failed to include the valve cap.
And the little nut on the threaded presta valve.
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Old 10-07-16, 08:30 PM
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Originally Posted by wgscott View Post
And the little nut on the threaded presta valve.
Who actually uses that?! If so, why?
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Old 10-07-16, 08:54 PM
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Originally Posted by SJX426 View Post
Who actually uses that?! If so, why?
I do. I screw it up against the cap when not in use, or I screw it all the way down to the rim before I put a pump head on the valve, to keep the valve stem from being pushed down into the rim.
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Old 10-07-16, 09:11 PM
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Originally Posted by wgscott View Post
I do. I screw it up against the cap when not in use, or I screw it all the way down to the rim before I put a pump head on the valve, to keep the valve stem from being pushed down into the rim.
Yeah, by centrifugal force due to my high speed.

Most here probably know this, but that actually happened with Schrader valves in dragsters. Turns out at 200mph, the Schrader spring isn't strong enough to keep the valve seated.
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Old 10-07-16, 09:16 PM
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Originally Posted by WizardOfBoz View Post
Yeah, by centrifugal force due to my high speed.

Most here probably know this, but that actually happened with Schrader valves in dragsters. Turns out at 200mph, the Schrader spring isn't strong enough to keep the valve seated.
I'll keep that in mind when I'm planning a hard sprint.
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Old 10-08-16, 08:12 AM
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Originally Posted by FBinNY View Post
I'll keep that in mind when I'm planning a hard sprint.
I actually think it has affected bike riders who have tried speed records. So, the next time you get on the Ocean Parkway bike path and attempt to hit 150mph, you should be mindful of this...
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Old 10-08-16, 01:51 PM
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Originally Posted by WizardOfBoz View Post
...Turns out at 200mph, the Schrader spring isn't strong enough to keep the valve seated.
I didn't know that, if I did the math right that's ~30g. (15 rim is ~100mph with a 30" tyre)
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Old 10-10-16, 11:05 AM
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I loosened the spokes to where threads just starting to show above nipples. rim still wavy, maybe moreso.

is it still fixable via truing?

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Old 10-10-16, 11:30 AM
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I didn't read all the posts here, so this may well have been said already: Spoked bicycle wheels are a stable assortment of rather heavily loaded members. In other words, there are large forces acting on the rim, the spokes and the hub flange. (Hub flanges are quite solid so they do not play into this picture except as an anchor.) Wheels work because all these forces are in balance. But if you push the rim to the side in one place and go past the balance point, the rim and spokes will pop into the other stable arrangement, taco'd. Ultimately the taco'd wheel is more stable than the true wheel and getting the taco'd wheel back to true without doing damage is very difficult. But, usually, the taco'd wheel has no damage done to it and with more brake clearance (say another 8" or so) ready to roll on!

The solution is simple. Loosen the spokes systematically. Go around and loosen each one turn. Keep doing this until the rim pops back to its true form, then systematically re-tighten and true. You may have tweaked the rim in tacoing, in which case, it will have to be straightened (by bending, NOT with the spokes IF it is safe to do with that rim - much more likely to be safe with the old '70s rims of very soft aluminum than today's). Also you may have a few spokes/nipples of compromised threads that need to be replaced.

Your brother's wheel not returning to true is not a good sign, but try the loosening anyway. That could be the result of uneven initial tension now showing its true colors.

Ben
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Old 10-10-16, 11:57 AM
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