Straightening brake rotor

[lragan]

After my unscheduled get-off at MOOTMag 7, my front brake would cause the lever to pulsate at low speeds.  Allen tilted the bike up on the back wheel and sidestand, and Greg spun the front wheel -- it stuck, and wouldn't turn completely around.  Rotor warped, or bent, or whatever.  So I added rotor to the parts needed list.  Received one from a fellow MOOTster a few days later. 

So I decided to check out the replacement rotor, and was surprised to discover it was warped, too.  Not as bad as mine, and in a more complex pattern than mine, but warped nonetheless.  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.  Notice the "soft jaws" addition to the vice, preventing marring the axle (thanks to MagnaDaddy of Hutto for showing me this cool addition to a vice -- magnets hold the jaws in place to the vice jaws.)

Brought it back to planar using a soft face (brass) hammer.  I practiced on one rotor before tackling the other.  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.  My rotor was mostly planar except for one spot that had to be whacked from the bottom (through the wheel), which I did by using the end of the hammer instead of the face.  The replacement I received required a few whacks in both directions.  So now I have two almost perfect rotors.

Caveat -- I have not tried either by riding the bike.  Stationary tests, just spinning up the wheel, show a huge improvement from where I started.  When I spin the front wheel now, it stops at more or less random spots as it spins down against the slight drag of the front brake pads.

It is possible that the rotor will go back to its warped state once it gets a bit hot from stops.  I will report more once I get the bike reassembled and ride it some.  Straightening with hammer blows (instead of steady pressure, as in a humongous hydraulic press) is less likely to leave internal stresses in the metal, so I am hopeful this will be a permanent fix...   

[mtrosclair]

You might consider having it resurfaced before use then it would be straight.

[Smoked U]

Good info.

I would have figured them for junk and pitch 'em in the trash.

[Curtis_Valk]

You might consider having it resurfaced before use then it would be straight.

The minimum thickness spec is not very generous on these rotors.  I wouldn't think that would give you enough to work with on a bent rotor.  Just a guess on my part though.

Curtis

[lragan]

I began to wonder "what is good enough?" in straightening the rotor.

Looked up the spec in the shop manual.  It specifies "runout" at .01 inches or 0.25mm.  Hmmmmm...  Not sure my eyeball is that calibrated...

So I rigged up the system in the picture.  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 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 on this repaired rotor.

Yes, it would be more accurate if I had

1) A thicker cross piece, or solder on a nut so I get more threads.
2) A larger diameter "Probe bolt"
3) Finer threads.

Still, I am satisfied with the results and intend to mount the wheel as shown and use the repaired rotor.

[Curtis_Valk]

4) a dial indicator

<grin> sorry, had to!

Curtis

P.S.  Any excuse to buy another gaget or tool, right?

[lragan]

I like the way you think, Curtis!  Let's see, a new rotor costs about $200...

Yep, I can justify it this way -- but for the less crazy, the method shown creates a "poor man's (non)-dial indicator".

[Chad in Michigan]

Lawrence, you've made it onto another site and a magazine (via your son): )

http://blog.makezine.com/archive/2009/07/motorcycle_brake_rotor_repair_kludg.html#more

found this on the Magna BBS site

[DG]

LOL.  I followed the link from the V4 board and after reading the article thought, 'weird I did not see a post on MOOT about that'.  Then I quickly put 2 and 2 together and guessed it HAD to be Lawrence and came straight over here to this site and confirmed it.  What are you some kind of engineer to think that up??   

[lragan]

Electrical Engineer -- which doesn't teach me much about mechanical stuff. 

My son, however, is a certified genius.  He gets paid to post interesting ideas, nearly all of which are his.  This is the only one of mine he has ever found interesting enough to post.

If we succeed in making a heat pipe for the R/R problem, he will post that, of course.

[Capt. Howard]

Electrical Engineer -- which doesn't teach me much about mechanical stuff. 

My son, however, is a certified genius.  He gets paid to post interesting ideas, nearly all of which are his.  This is the only one of mine he has ever found interesting enough to post.

If we succeed in making a heat pipe for the R/R problem, he will post that, of course.

It's just a matter of time before you get to make that post....I can feel it.....

[lragan]

So today I finally got to field test the straightened rotor.  It was hot -- like 38C or 100F.  Tried several fast stops from highway speeds using only front brake. 

Did not take it off and re-recalibrate the run-out, but it feels good, and there is no perceptible feedback to the front brake handle at any speed. 

I expect it to last, and will report if it does not.  Planning a long ride in the next couple of weeks in Colorado, so will report again after that is over.

[roboto65]

Now if I could just fix CRACKED rotors 

[Jerry G Turner]

Cracked rotors are easy to fix just replace them.

[lragan]

Yes, all it takes is $$$$