What is the best way to straighten a slightly bent derailleur hanger? Thanks

I'm a new fan of the Park der hanger alignment tool. It's simple and effective.

It's not necessarily the best $$ investment for an average, single road bike DIY'er, but ... if you don't want to buy one, find an LBS that has one, and knows how to use it, and have them check your aligment, and use it to straighten it, if necessary.

My $0.02. YMMV

Gently. If you don't want to spring for a hanger alignment gauge (mucho dineros), you can remove the RD and use a large adjustable wrench to tweak it home, being careful to straddle the bolt hole and not distort it.

Eyeball alignment won't be as precise as with a proper gauge, but can do a reasonably serviceable job, saving you a trip to the LBS, or the cost of the tool.

Park Tool DAG-2 = $50 - $70. If you shop around.

Bike-Shop shouldn't charge more than $10 to do the job. Often quite a bit less.

I have the DAG-2 - works very well.

I've used a spare rear wheel to straighten mine. It seems to have held up OK so far.

But -- what's the risk?

Poor shifting, and the possibility of shifting into the spokes or stays. But these last two are generally following accidents that the cyclist would be aware of - and did nothing to address. If it's just a bit out of whack, you'll be amazed in the improvement in shifts once it's back in alignment.

I guess what I meant to ask is, what's the risk I've run by aligning my derailleur hanger using a spare wheel rather than an alignment tool? For example, do the threads of the wheel axle and the hanger actually match, or were they merely close enough?

I know that it's got a large amount of leverage, and simply trying to eyeball it with the other wheel mounted in the bike is difficult since they're on different planes. I've tweaked quite a few metal objects by hand before, too, and know that patience plus small movements are better than trying to work fast.

Once aligned, yes, it shifts great; all the issues I couldn't fix via cable and screw adjustments disappeared.

Whatever works...Works. Not quite as exacting as a DAG-2, but your method seems to come close. Or, at least, you haven't bent the hanger much. I had my hanger (on frame type) bent, and the guy just eyeballed it back. Has worked fine.

Could you elaborate on what you mean by "used a spare wheel"? That doesn't make any sense whatsoever.

I'm thinkin he meant he screwed the axle into the derailleur mounting hole and twisted it into shape. An M10 x 1 axle will screw right inta that puppy. Actually, a 3/8 axle will too, but it will be a sloppy fit.

Ew. Well if it was a 10x1 that's fine but anything else = bad.

I know it doesn't, but I wouldn't have thought to try it unless I read about it (probably somewhere here on BF) --

Put the bike in your favorite workstand (or upside down on the ground, whatever). Unscrew the derailleur from its hanger. Leave the hanger attached to the frame. Leave the rear wheel mounted on the bike.

Take an extra rear wheel, or at least a wheel that has exposed threads on the end of its axle, and screw it into the derailleur hanger -- yup, in the same hole that the derailleur used.

Carefully and thoroughly check that the spare wheel is just as vertical as the wheel mounted on the bike. Account for any wobbling movement of the spare wheel if you wiggle it by hand -- if it wobbles down, make sure that it wobbles upward the same amount past vertical (I'm terrible at describing this, but you'll get it), and figure that halfway between the wobbles is the "center". The wobble comes from the "sloppy fit" that Dan Burkhart just mentioned as I'm typing this post (I got the email notification just now ).

Carefully push and pull on parts of the spare wheel to bring it in vertical alignment with the wheel on the bike. Get a piece of tape measure or ruler to make sure that the centerline of one wheel is equidistant from the other at various points. Since their axles aren't on the same axis, you won't be able to simply measure tire-to-tire, or rim-to-rim, except at two points; otherwise, you'll be measuring from the center of one tire to underneath the rim of the other wheel, etc. Whatever parts you choose to measure, do it the same each time, like always rim-to-rim, or tire centerline-to-centerline, etc.

Stand back, look at it some more, then see if you need to tweak it again. Get the top and bottom aligned, then get the front and back aligned. Check other parts to be sure, too.

Unscrew the wheel from the hanger and re-mount the derailleur. You might have to readjust the screws or cable a bit to account for the newly realigned hanger.

That's what I figured. I guess the only caveat is here is making sure you're slamming in the right axle size into that derailleur thread. Be careful with what you read here on BF, with the exception of a dozen or so people there's some seriouslly bad advice floating around here sometimes.

Using the spare-wheel technique, you can also use a ruler to measure the distance between the wheels. If you can get three points around to be equi-distance apart, then you're assured that the derailleur-hanger is in a parallel plane with the rear dropouts.

Right, and that's why I keep saying to be careful with it. It feels like the threads are at least very close to the same pitch, but I'll bet the diameter is off. It's not good for mounting wheels onto a car (look for video of an RX-7 whose wheels popped off because of lug nuts that didn't quite match the lug studs), but good enough for aligning a hanger a couple/few times.

Derailleur hangers are threaded M10 x 1. Quick release rear axles are typically this same thread, but usually have only about 4mm protruding past the lock nut. Most (but not all) solid axles are 3/8" x 26tpi. This is close enough to thread into an M10 x 1, but as I said before, the fit will be sloppy. I would be concerned about damage to the hole and threads twisting around on there with all that play.

First, do no harm.

I'm a believer in keeping the derailleur attached or screwing in a correctly sized bolt before attempting to realign a derailleur hanger. It's all-too-easy to ovalize the derailleur mounting hole. If you do that, you've got real trouble. I just clamp an adjustable wrench around the whole derailleur. During my mountain biking days I even did a couple of trailside realignments by hand.

I'm confident of my ability to eyeball derailleur hanger alignment on 7-speed bikes. As the number of rear cogs increases the cogs are spaced more closely together and hanger alignment becomes more critical. On a 9-speed or 10-speed bike I'd strongly recommend finding somebody who has the proper alignment gauge.

I'm going to recommend against using anything but 10x1. If you have to do it.

Stop writing what I was going to write :/