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My dad recently took a minor tumble on his motorcycle. He's fine, but the bike was banged up a bit, including a bent brake rotor. Consensus among his buddies in the Magna Owners of Texas was that the rotor would have to be replaced, but of course they're pricey, and since the rotor was "shot" anyway, Dad figured he might as well try to straighten it and see what happened.
Here's what he did, in his own words:
Since I had mounted the tire/wheel on the axle in my vice to polish the wheel, it was a simple matter to rig up the "feeler" shown in the first picture to check out the rotor flatness. Just a piece of copper wire about AWG 7 to 9 or thereabouts -- I had in my electrical junk box. With a light behind the setup, one can use the reflection of the end of the wire from the rotor surface to obtain a very sensitive indication of warp when one spins the tire/wheel. Brought it back to planar using a soft face (brass) hammer. Go slow, it takes some time. "Sneak up on it" by whacking gently, measure, whack a little harder, measure, etc. until it yields just a bit.
Then, concerned that the rotor needed to be flatter than he could detect with the naked eye, he rigged up a second jig to test it:
He explains:
Looked up the spec in the shop manual. It specifies "runout" at .01 inches or 0.25mm. Found a 1/4-20 screw in the junk box, and a piece of steel from some long discarded bracket off something I don't even remember. Drilled and tapped for the screw and drilled a clearance hole for the mount bolt. At 20 threads per inch, that would be .050" per turn. So .01" would be 1/5 of a turn. Put on a standard six-flat nut for reference. Turn less than one flat would be .050/6 = .0083 inches, a little margin to the spec. So, to use it, you spin [the wheel] and gradually drop the screw until it just touches at the highest point. Turn to the lowest point, and tighten down. Took less than one flat, so I believe I am in spec.
He cautions that he has not ridden the bike any serious distance since he made this repair, and cannot vouch for its safety, but he is plenty confident to use it himself. There's more detail in his original MOOT forum post here.
What do you think will happen to that rotor once it gets a good bit of heat in it? My guess it'll come untrue by itself, but that only happened on a shimano 6" rotor with me.
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It's a good question. Dad mentioned that possibility. I'll update with a comment after he's had a chance to ride it, and let everybody know how it's working.
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I love tinkering with cars, bikes, and all other things wheeled. HOWEVER, I have a very simple rule. There are two things I never cheap out on, use used parts, or try and modify myself (outside of reason). That's tires and brakes, which are the two most important safety features on EVERY wheeled vehicle I have had the pleasure of enjoying. So, I very rarely "pad slap" brake systems if the rotor looks marginal and I never allow tires to go completely bald. I think this is an awesome basic idea of what he did, it shows some great intelligence and intuition, but I would highly recommend buying a new rotor, for safety sake. ESPECIALLY after dumping the bike. Just my $0.02 on safety.
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Thanks for posting this.
If your dad is confident to ride on this rotor, I'm sure he has good reason. Anyone handy enough to accomplish this is also probably bright enough to figure out if/when the rotor starts to warp, again. So don't mind the naysayers. They can feel free to regale us with their stories... "how to use a credit card to buy stuff."
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Money can't buy you happiness, but it can buy you peace of mind on occasion. Buy a whole new front end before risking your life to a few bucks. If a few bucks buys a new rotor, buy it.... any motorcycle part is cheaper than a trip to the ICU, even cheaper than a funeral.
Rubber side down friends!
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While it would be worth the "can I do it" function, you should not ever do it and rely on it. If there is a hairline fracture from the initial accident or from you (or aggravated by you) pounding it back to true, the disk will explode on you under any for of braking.
I did some consultancy work for an engine rebuilding company. I was watching them test for fractures in crankshafts. They would "paint" it with a liquid which glow under UV. You could see the bright lines where there were cracks. Naked eye could not see them.
If it did not show any up, they would xray it. Then you could see them.
These would most likely exist in the rotor. You'll know they exist because you'll hear it shatter about 1.5 seconds before you hit the deck / tree / car lamp post (delete as applicable) under hard braking.
Like others have said - it's fun to modify stuff but certain things you do not. Never worth it!
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If you have visible cracks, then replace. I've had a crankshaft blow up before. So I know what can happen.
Rotors are made of iron or soft steel alloys. They are very different from the hardened steel of a crankshaft. They are also subject to different method of stress. While a rotor can catastrophically fail from cracking, the likelihood of that happening in absence of noticeable signs and symptoms (like visible cracks) is remote.
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It will likely not fail outright, but it will likely untrue again real fast after a thermal cycle or two.
Hammering on it also work hardened it, you should anneal the rotor if you know how to do so properly to prevent rewarping and especially the possibility of cracking and catastrophic failure.
I would have just gotten a new rotor personally, but there is no reason this repair isn't doable if you really want to. Just do it right and stress relieve the metal.
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