kroozer 

I have a Nervar 631 crankset I picked up some years back in a trade, and that I'm just now using for the first time. It's the 5-pin 50.4mm bcd model, and I've set it up with TA Cyclotouriste rings. Alas, I've discovered that the drive-side crank arm is bent inward a few milimeters: not a lot, but enough that it's visible if you look straight down, and the pedaling feels just a bit wonky. I guess I could just get used to it, because I use the bike mostly for urban commuting over fairly short distances. But I don't like the bend and want to straighten it back.
1) Is this even a good idea? How much would I weaken the arm?
2) What would be the best way to do this? I'm thinking leaving it mounted on the bike, and just putting a piece of pipe around it and pulling it outward. But of course I don't want to #%& up the frame or BB.

3alarmer 

.
...I have never straightened an alloy crank arm, because I'm afraid of the thoughts that will go through my mind, when it breaks and I fall off the bike. Not so much the falling, because I've done that before, but the thoughts about how this was self inflicted.

scarlson 

These are forged, right? I'd go for it if you're sure they are cold forged, which it seems like they are. Doesn't sound like a big bend to do. If it's a dealbreaker for you, I'll take 'em off your hands and do it myself. I've bent back all manner of forged alu things, and I'm still here. Mafac brake arms for example. Put 'em on double secret probation for a while afterwards, inspect periodically, and let them regain your trust.

Cast, I would not do.

Also leaving the crank bent and riding it long distances could mess with your joints.

wschellen

I had a similar experience with an old Stronglight crank on a Motobecane. I took the pedal off, placed an old fence post pole over the arm and pulled it back. The arm is straight now and seems fine, but as they say, your mileage may vary.

repechage

Steel- no problem
aluminum?
my body is worth more
whether it be break or wear on my joints

daka

I'd be inclined to discard it before wasting any effort on straightening it. Aluminum crankarms are documented victims of fatigue failure and a bend beyond the elastic range may have generated microcracks that could precipitate a failure inducing crack. Since the complete breaks often cause a fall/crash/injury, it is more than a cosmetic issue and deserves some prudence. Even crankarms that haven't been bent should be inspected for cracking as they get old and have accumulated a lot of miles.

dabac

While I agree that there’s more risk to bending back a crank arm compared to replacing it, I have straightened crank arms - and ridden them for considerable mileage - w/o issues.
I’ve used a heating tip collected from a guy who built waist-steered recumbents using repurposed BBs and cranks for the pivot.

SJX426 

kroozer I bought a folding bike with one. At first I thought the pedal axle was bent, so I paid the price. After a couple of test runs during the clean up phase, I discovered the crank arm on the left side was bent.
I decided I had nothing to loose. Either it will straighten out or break. Either way I would have a crank set that was straight with a replacement if it broke.

I have a hi-lift jack or farm jack to support my other interest of off roading. I set up blocks of wood on either end of the crank with the bow up. I used the jack to press down on the bowed part. Initially I though a car would be enough weight to force displacement, not.

The second attempt, and successful one, was using the opening between my garages that has exposed foundation to set the crank with blocks on one and the jack on the other. Then just jacked until the crank was visually straight. I think it is off just a bit but really hard to tell. I tried to alight the flat around the pedal boss with the flat area of the BB spindle boss. They are close to parallel. Close enough for me. I could take measurements but it was not a collector type folder and not worth more effort, IMHO.

Some times you have to think out of the box if you don't have the right tools, in this case a hydraulic press. Didn't think to take pictures of this activity.

ollo_ollo

I wouldn't hesitate. My Winter commuter, a Centurion Pro Tour, suffered a bent drive side crank arm when I fell on an icy curve. Bent it enough to notice immediately, wasn't rideable at all. Mentioned it to framebuilder Bill Stevenson at the Bike Stand in Olympia, WA. Turned out he had a tool to make it good again for a minimal charge. Left a slight mark on the arm, but never had any problems thereafter and it was still gave good service years later when sold. (it was a forged SR Aerox triple crank) Don

CargoDane

I'd buy a new one. Not only is it safer, but you'll be lucky if can correct it perfectly. It is probably not merely bend inwards, but also twisted a bit - both will make them feel "wonky" when in use.

thook

make sure it's not a problem with your bottom bracket spindle. i'd mounted a deore lx drive side crank arm one time and thought it was bent. turned out the taper on the spindle wasn't matched well to the crank

steve sumner

when I worked in a shop in the 70s we had tools made specifically for straightening crank arms
Nervar arms are on the soft side so straightening it should be no problem

kroozer 

Thanks for the ideas. The arm is not exactly parallel to the chainrings so I don't think it's the axle.

juvela

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should you elect to have a go at straightening definitely do it off the cycle

no point in risking damage to spindle or frame

once you get it clamped up between blocks as described earlier you have excellent control

if you do not have an industrial strength jack as delineated above you can make a good job of the process with a beefy bench vise

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obrentharris 

I would guess that there is still a muffler shop or auto mechanic in Nayarit who bends exhaust tubing with a hydraulic press instead of one of those fancy programmable benders. The same tool, without the rollers, could be used to straighten the crank arm.
Brent

thook

heck....a bottle jack under the frame of a truck would probably suffice as a press