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-   -   Bent rim - Techniques? (https://www.bikeforums.net/showthread.php?t=1135702)

bcpriess 02-13-18 09:25 AM

Bent rim - Techniques?
 
Hi - I have a bent rim from my rear wheel sliding out from under me sideways in the mud and hitting a rail tie pretty hard on its side. When I tension the spokes to get it back in alignment, I end up with one super tense side, and one super slack side and the wheel is still bent. Looks to be about 5-7mm offset to the nds at its worst spot. The other 180 degrees of the wheel are still true. It's still round, but I can't keep my rear brakes from rubbing and still have a rear brake. What are some good techniques for bending the rim back closer to its original flat shape? I don't really want to get a new wheel or relace everything to a new rim if I don't have to.

fietsbob 02-13-18 09:48 AM

Slam the rim in the opposite direction as the bending action with equal force.. to get Newtonian on it..


Pringle bend or radial flat spot?






....

sch 02-13-18 10:02 AM

Depends in part on how much of the rim is out of sorts. A tacoed wheel, one with a rim
that looks like the edge of a potato chip is hard to fix with spoke tightening without
fixing the rim first. If the bend is over 4-5 spokes you may be able to adjust it out
by spoke tensioning, but likely the wheel will not be 'like new'.

this video and others obtaining by google might help: https://www.google.com/search?ei=Jge...P5bM#kpvalbx=1

Back in the day of really light rims a tacoed or pringled rim was easy to pop back flat
and something I did several times. Youtube has several videos on ways to fix these,
then use fine tuning with spoke tensions. Radial irregularities are a little harder to
fix than axial ones.

Youtube: https://www.youtube.com/results?sear...+tacoed+wheels

Iride01 02-13-18 10:07 AM

I'd loosen all the spokes to the last few threads. Then I'd try to bend it back into alignment as close as possible. Then I'd tension the spokes and see if I could pull it all back to true without too much tension on some or too little on other spokes.

It might be a futile effort if the bend is over a short section. The aluminum won't like flexing that much.

bcpriess 02-13-18 10:17 AM

As far as I can tell, the bend occurs over half the wheel. It is tacoed, not radial. Radially it appears true. I have 32 spokes, so the best way I can describe it is this way: The rim that corresponds to spokes 1-16 is true. The rim from spokes 17 through 24 goes from 0mm to 5-7mm out of true, and then over the course of spokes 25-32 returns back to 0mm/true. It doesn't look like a sudden bend, in fact I can barely see it from most views when I look across the rim braking surface held flat. The rim is an 18mm inner width Alex 26in mountain rim. Forget exactly which one.

The bigger question: Once I loosen the spokes, how should I go about straightening the rim? I have no prior experience and don't want to wreck the rim.

Retro Grouch 02-13-18 10:37 AM

Without having seen your wheel:

1. Can you loosen all of the spokes and bend the rim back straight - maybe in a doorway? If so, get it as straight as you can, tension and true the spokes.

2. (My bet) If the above doesn't work, cut your losses and buy a new wheel. If it's a front wheel you might have luck finding a used one. Trying to use the spokes to pull a bent rim back straight isn't likely to be a lasting repair.

FBOATSB 02-13-18 10:48 AM

Simple. Remove all spokes so bare rim can sit on a flat surface. If you don't have a bench big enough, use the floor. Use boards, body weight, leverage. No striking. Lots of videos on line for visual guidance. Get it as close as you can befor re lacing.

Bill Kapaun 02-13-18 11:26 AM


Originally Posted by bcpriess (Post 20167109)
Hi - I have a bent rim from my rear wheel sliding out from under me sideways in the mud and hitting a rail tie pretty hard on its side. When I tension the spokes to get it back in alignment, I end up with one super tense side, and one super slack side and the wheel is still bent. Looks to be about 5-7mm offset to the nds at its worst spot. The other 180 degrees of the wheel are still true. It's still round, but I can't keep my rear brakes from rubbing and still have a rear brake. What are some good techniques for bending the rim back closer to its original flat shape? I don't really want to get a new wheel or relace everything to a new rim if I don't have to.

The DS can have up to 2X the tension of the NDS. That's how the wheel is properly dished.
Trying to tension both sides the same will "pull" the rim way over to the NDS.
Notice the differing "angles" the spokes run between DS & NDS? The NDS has a MUCH greater effect on dish-

That said, I did something similar to my rear wheel when I did a nose stand with my new to me then Globe.
The wheel went about 3-4' in the air and banged down hard as I was "stepping off" to the side.
I had a rather nasty "Pringle".
I loosened all the spokes to the 1st thread and the rim WAS FINE. I just built the tension back evenly and had a "straight" wheel.
It was simply so poorly tensioned, it was looking for an "excuse" to go out of whack.

So- Don't start banging on the rim until you have loosened ALL the spokes first.

dabac 02-13-18 11:38 AM

I’ve never had much luck re-bending rims while laced.
My favourite approach is to unlace, place rim with low spots on a broom handle, hockey stick etc, then step on the high spots.
Once flat to <3 mm, you have a decent chance of getting it rideably true with acceptable spoke tension variation.

cny-bikeman 02-13-18 12:28 PM

First you need to determine what type of bend you have. The spoke tension will need to be more even than it is now in order to do so. Probably a good idea to release most of the tension to make sure the rim is actually bent, rather than temporarily distorted.

If the bend is U shaped then you need to just bend the middle while pulling or supporting the two sides. As you see above, there are a variety of approaches, from slam it on the floor, to push with your knee and pull with the hands, to using a doorway or levering it.

If it is an S shape (sharp right then left, and gradual return to the right) then you would need to place it in some sort of gap and carefully twist in the opposite directions - MUCH harder to correct.

andrewclaus 02-13-18 01:15 PM

I have had very good luck straightening bends (smaller than yours, but over 90 degrees of arc) while laced. Inflate your tire, swing it from about head height onto a hard floor. Watch for the rebound! Don't hit the rim on the floor. Gauge success by the tension in the spokes. Mark the center of the bend with an arrow showing which way to hit. Start with smaller swings. I have rescued quite a few wheels with this method, including on my daily rider. I would definitely try it on a larger bend like yours.

Deflate the tire before final truing with spokes.

bcpriess 02-13-18 08:30 PM

Thanks everyone. It appears to be true within about 1mm again. Tension even seems pretty uniform! EDIT: looks like I have it tensioned a couple mm to the nds. But rolling smoothly either way. Test ride felt good but I will probably check the tension again after the next couple rides.

sch 02-14-18 12:50 PM

Thanks for the followup. You may hear pinging in the wheel when you ride it, a manifestation of the tendency of tight
spokes to twist when the nipple is turned so the nipple just winds up the spoke instead of screwing down the spoke
thread. I used to take the wheel off the stand and put it on the floor vertically and bounce my weight on the rim
with my hands as the rim was rotated around a hemi-circumference. This results in a lot of pinging and then you
re-true and repeat til the rim is true and all pings are gone. Flat spokes can be gripped with a smooth jawed pliers
to minimize spoke windup. Bouncing on a wheel with tire on it doesn't work so well, need to have tire off.

Andrew R Stewart 02-14-18 11:54 PM


Originally Posted by Retro Grouch (Post 20167252)
Without having seen your wheel:

1. Can you loosen all of the spokes and bend the rim back straight - maybe in a doorway? If so, get it as straight as you can, tension and true the spokes.

2. (My bet) If the above doesn't work, cut your losses and buy a new wheel. If it's a front wheel you might have luck finding a used one. Trying to use the spokes to pull a bent rim back straight isn't likely to be a lasting repair.


+1 on both parts. Andy

bcpriess 02-15-18 12:12 AM

Here's what I did.

1) I loosened all the spokes until about 3 threads were showing. This was to ensure that all the nipples turned freely and I had enough slack for the rim to relax into whatever shape it was in.
2) I was then able to see the bend. It was king of a "j" shaped bend when the wheel was viewed in profile, covering about 120 degrees of the wheel. i.e. two bends in the same direction 120 degrees apart, just as one would assume with the type of side impact I experienced.
3) I blocked up under the inflection point of the bends, and stood on the rim on opposite sides, checking visually and on a flat surface until I felt it was pretty flat. After a dozen or so repeats, this got the rim straight enough to work with.
4) I reattached the wheel to the frame (no truing stand)
5) I numbered every other spoke with a sharpie on the braking surface since they all look the same and I needed reference points.
6) I began retensioning the spokes with full turns the first few turns until the obvious slack was taken up.
7) I began going by half turns at this point, taking care to keep the rim spinning straight.
8) As I was doing this, I flicked the spokes to see if they produced generally similar pitches. Knowing the rim was imperfect, I knew there would be some inconsistency as certain spokes needed more tension than others to compensate.
9) With the front wheel as a guide, I tensioned the spokes while visually checking alignment, until the spokes had a similar pitch to the spokes on the front wheel.
10) Eventually, by focusing more specifically on spokes in a couple locations, I was able to straighten the wheel.
11) After doing this, I checked all the other spokes' tension by feel and approximate pitch, and went about tightening up any spokes that had become relatively loose until all felt and sounded similar in tension.

Took me about two hours. For my first time, I think it went well.

SHBR 02-15-18 04:57 AM

Big bends can cause fatigue cracks.

Another big hit and that wheel is likely to be finished.

I keep a spare rim on standby, quicker and safer than making major corrections.

Andrew R Stewart 02-15-18 09:14 AM


Originally Posted by SHBR (Post 20170561)
Big bends can cause fatigue cracks.

Another big hit and that wheel is likely to be finished.

I keep a spare rim on standby, quicker and safer than making major corrections.


This type of repair really should be considered like a cat loosing a bunch of it's 9 lives. How many more are left? How important is it for your wheel to be reliable? How much is that reliability worth?


When I invest in new rims I buy 2 times the number I'll initially use (I am married to a rider so sometimes this means 8 rims) so I'll have back ups for a while after the rim model is no longer offered. Andy

bcpriess 02-15-18 05:03 PM

I don't think it was as big a bend as you're picturing. My best guess is it was bent out about 5mm. I think the spokes may have pulled it farther initially in kind of an "over center" effect, since it was in that 5-7mm range when the spokes were tight. It was hard to see the bend at first when I couldn't put it on a flat reference surface and with tense spokes interfering with my line of sight.

SHBR 02-15-18 08:12 PM

My rule of thumb, if it still turns in the frame, its usually salvageable, beyond that its too much of a risk IMO.

bcpriess 02-15-18 10:19 PM

Yeah, it was turning fine, just couldn't connect the brakes and also have it turn w 5-7mm of wobble. Since I'm pretty careful about obstacles, I haven't had to deal much with untrue rims. So that may have made it seem like a bigger deal than it was.

Arthur Peabody 02-18-18 11:33 PM

How long does it take to build a wheel? Price that at minimum wage; if you're as slow as I am, it swamps the price of a new rim. A wheel isn't iron; it has internal structure from its manufacture that is permanently damaged when it's been bent this badly: it never recovers (unless you heat it up to a high-enough temperature then anneal it properly).

I tried this once, the wheel wobbled (though it was true & round & tight) because it was no longer uniformly strong; it made unnerving high-pitched pings; the tightest spokes broke: it was a huge waste of time.

Le Mechanic 09-16-18 08:36 PM


cny-bikeman 09-16-18 09:36 PM


Originally Posted by Le Mechanic (Post 20569866)

The OP indicated in post 15 that he got the wheel back in shape - 7 months ago to the day.

AnkleWork 09-16-18 11:55 PM


Originally Posted by fietsbob (Post 20167156)
Slam the rim in the opposite direction as the bending action with equal force..
....

That always works for Barney Rubble.

Le Mechanic 09-17-18 07:27 AM


Originally Posted by cny-bikeman (Post 20569950)
The OP indicated in post 15 that he got the wheel back in shape - 7 months ago to the day.

Still a relevant post for someone else with the same issue, no?


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