Bicycle Mechanics - Crooked rim : banging it as means of initial truing
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12-28-10, 02:41 PM
I have a quite-badly crooked rim. By that I mean, that I could not true it only by tensioning spokes correctly. This being said, I am very hesitant to put it in the trash.
I wanted to use it on a beater bike. I do not need it to be dead-straight.
A friend told me to bang it back to almost normal, then true it. :crash:
By that he meant, bang it on a wooden surface and not with a tool ; bang it where it is most out of true.
Another friend says he's been running on a banged-back-to-life front wheel for part of the summer and didn't need to even true it since.
What is there to be said about this method ? I've been searching the net, but w/o any luck.
12-28-10, 03:04 PM
I started looking more closely... some people claim success using the technique I mentionned.
I am a bit concerned about safety though.
12-28-10, 03:16 PM
Back when I was in college another bicyclist ran into me from the side and it put a bad bend into the rear rim. The rim was bent about 20 degrees out of true and was way past anything that could be fixed by regular truing via spoke tension. I disassembled it completely and then took the bare rim to the top of a stairway. Positioning the bent portion over the edge of the top step, I had a friend stand on top of the part that was fairly flat while I put some weight on the bent part. Once I thought it was close enough to all being in the same plane I laced it up using the other wheel as a guide and trued it in the bike frame with my spoke wrench. That was the first wheel that I ever built and it lasted for several more years and thousands of miles before I sold the bike.
I've repaired literally hundreds of wheels as follows:
After determining that a wheel can't be trued via tensioning, i.e. after it is certain that a straight rim can't coexist with uniform same-side spoke tensions, I'll put a mark on the rim sidewall corresponding to the location that the rim is bent worst, then lay a 2X4 on the ground with the rim's high-spot against the board, then push down to either side of the high spot.
This is followed by a round of evening out the spoke's tensions, and the entire process repeated untill the rim is true and spoke tensions even.
Note if the bend is near to the rim's non-welded seam, the seam may become mis-aligned, and that can be fixed lastly by light swatting with the 2X4 BEFORE any "sanding".
I always use audible plucks to most quickly establish relative tensions, but note that the adjacent, crossed spoke must be damped with a finger while the inspected spoke is plucked, or they'll ring together.
This can be very time consuming in some cases, but a very strong, evenly tensioned wheel can result. Practice improves speed and quality of the finished work.
Best of all, if the wheel is perfectly tensioned pre-trauma, the board-heave method restores spoke tensions as it straightens the wheel. I've had to fix a wheel (against a trashed coffee can in this instance) while racing off road, and the previously well-tuned wheel came back immediately to an almost perfect and reliable condition.
Best of luck,
12-28-10, 03:55 PM
I saw truing-by-banging done pretty expertly at Burning Man, when a friend pranged her rear wheel pretty badly. Took it to one of the fix-it camps, where one of the guys told me he used to race downhill and did this kind of repair pretty frequently.
He held it with the bend down and away, and whacked it really hard against a workbench a few times. That was enough to make the wheel usable again.
One thing about downhill rims, and I've fixed a lot of them, they are often extremely burly in terms of the rim's cross-section, and combined with their mere 559mm diameter, can be impossible for a lightweight like me to bend easily using body weight.
I use the more-controlled body-weight-against 2X4 for any rim that will yield to that, before resorting to impact methods. Also, the downhill bikes have disc brakes so there's no reason to be worried about any tweaking of those rim's flanges from "slapping" the wheel against a wooden bench.
A friend of mine was on a cross-state organized ride when her wheel turned into a taco. It would turn rotate on the bike. She took it to a support mechanic. The mechanic took the wheel to a big tree and holding opposite sides of the rim slammed the wheel against the tree. Put the wheel back on the bike a told her to ride on. Got her home just fine.
12-29-10, 04:16 PM
What is there to be said?
It can work to an extent.
Requires a lot of either time or talent.
Only works on symmetrical bends (rather than S bend, which requires insertion in some sort of slot and twisting it back).
12-29-10, 06:13 PM
Yes u can do that, it is not hard but u have to be carefull, why? well if the aluminum is very soft with one bang of the hammer you can screw the rim, at the oposite side if you have a good rim like a gp4, nisi or a velocitu you will need more than a good HIT to get it back to position.
U need a piece of very hard wood that wont cheap, a piece maybe 4x2 inches or so. Actually u need 3 of those. put two under the wheel and to the sides of the problematic area nd the 3rd one in the middle and whack it, take a look in the truing stand, then again and again untill u get the rim to the place u want. I remember a GP4 rim i had, the mechanic hit very very hard and the sucker did not move a mm, in a matter of fact that one tok at least 10 hits to get it right after an accidet, retrue the wheel, all perfect afterwards.
If you have a hop U need to cut a piece of wood with the shape of the rim and wack the rim from the interior part of the rim with another piece of wood, have seen this once all my life (there is a tool for this anyways). The other fix i have seen it and done it myself a few times with very good results.
12-29-10, 10:33 PM
On this page
scroll down to the January 3, 2010 blog and see his technique.
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