Rotors are supposed to be flat. Right?

[blacknell]

So I've now encountered my third rotor in a row that has a slight warp - enough that my choice is between no rub OR effective braking. The first one came with my bike (we're talking about Avid BB7 mechanicals here), but the second two I bought brand new. I decided that I'd had enough of the brake induced shimmy with the last once, threw on a new one, readjusted the caliper mount, and . . . . rub rub rub. WTF?

I've been very careful in even application of torque, making sure the mating surfaces to the hub are perfectly clean, etc. Assuming that I couldn't have the bad luck of three warped rotors in a row, all I'm left with is there being something wrong with the hub itself (Shimano XT). Anyone else encountered a similar issue, or have ideas? Or do I just have unrealistic expectations?

[jabberwocky]

If you have a truing stand, its relatively easy to get rotors pretty straight. I put the wheel in, and attach a zip tie to the leg of the truing stand and cut it at the rotor. Using the zip tie as a guide, I see where the rotor is warping, and gently bend it straight with a large crescent wrench. I've never been able to get a rotor absolutely perfect, but that probably speaks more to my limited patience than anything wrong with the technique.

I bend rotors a fair bit in crashes, so I use this technique pretty often. It works well enough to eliminate rubbing, and reduces or eliminates the warbling noise warped rotors tend to make when you brake.

[werace424]

I just put the bike either upside down, or in the stand and push/pull the warped part the apropriate direction to eliminate the annoying rubing and speed robbing sound.

I doubt that you have bad enough luck to get three warped rotors. UNLESS.....you have made some bad MTB charma for yourself. ;)

Paul

[FIX_BKS]

I've installed a lot of disc brakes...especially now that the majority of Kona's we're getting in have them stock. RARELY is there a rotor which comes out of the accessory box which is 100% true even throughout the different manufacturers. Like werace424 suggested, I leave the bike hanging in the stand and watch the rotor travel through the pads while spinning the wheel. By applying a slight "flex" to the rotor in the opposite direction of the rub using just your thumbs (with a clean rag to avoid skin oil residue) you can eliminate the rub and get it 99% straight.

If it's waaaay out of wack, you would probably need the adjustable wrench as jabberwocky mentioned. This would be an "after a crash" repair as opposed to a first time setup.

I know it seems like they should definitely be 100% true 100% of the time out fo the box, but I can tell you it's rarely so.

[punga]

Because the rotors are stamped steel, you will rarely find them completely true, even out of the package. Just not a perfect process.

p!

[blacknell]

Thanks, all. I'll take a crack at the truing tonight. I'd held off on that in the belief that you need those truing forks and that I'd probably just make it worse, but between what's been said here and the thought that it can't get much worse, well, what's to lose?

Gracias.

[Brizn]

Haven't read the thread in its entirety yet.. but probably has some to do with the way you like to run your brakes. I often have the same exact issue with rotors.. because I like to have the pads really super close. Maybe try giving a little more slack on the cable by the pinch bolt... to give you more throw on the caliper's arm. I've found that this allows me to run the pads pretty close.. but still gives me enough clamping power when the going get's, well, steep.

[jabberwocky]

Thanks, all. I'll take a crack at the truing tonight. I'd held off on that in the belief that you need those truing forks and that I'd probably just make it worse, but between what's been said here and the thought that it can't get much worse, well, what's to lose?The key is to be gentle. It doesn't take much force to bend them. Make a small adjustment, check how it looks, make another small adjustment, check how it looks, etc.

As others have noted, I've never had a rotor come out of the box perfectly straight, although they are usually good enough to use. Hydros are less susceptible to rub because the pads retract on both sides, as opposed to mechs where the pad is fixed on one side.

[berrywise]

So I've now encountered my third rotor in a row that has a slight warp - enough that my choice is between no rub OR effective braking. The first one came with my bike (we're talking about Avid BB7 mechanicals here), but the second two I bought brand new. I decided that I'd had enough of the brake induced shimmy with the last once, threw on a new one, readjusted the caliper mount, and . . . . rub rub rub. WTF?

I've been very careful in even application of torque, making sure the mating surfaces to the hub are perfectly clean, etc. Assuming that I couldn't have the bad luck of three warped rotors in a row, all I'm left with is there being something wrong with the hub itself (Shimano XT). Anyone else encountered a similar issue, or have ideas? Or do I just have unrealistic expectations?

I wrote my dissertation on this exact problem so I feel fairly comfortable telling you that it sounds like it is user error. I spent about a year gathering data on disc brakes when applied to mountain biking. I tested rotors from over 20 different companies and my finding were completely different than what you are finding.

Manually bending the rotor yourself is a really bad idea and can cause catastrophic injuries. Yes, I have seen the results of people doing this in medical journals and it is not pretty.

What you should do is place the rotor under four or five heavy text books and let it sit for a three weeks. After this is done put the rotor in your oven and bake it at 400 degrees for one hour. Immediately after that place it in a bucket of ice water. This will fix the problem

[saxman]

The key is to be gentle. It doesn't take much force to bend them. Make a small adjustment, check how it looks, make another small adjustment, check how it looks, etc. .

I find that adjusting them on the spoke, not the actual braking surface works, and very, very small applications of force, otherwise you can bend the rotor too far in the opposite direction.

I use an old cast iron box wrench that was my grandfather's. It's small enough to fit between the spokes.

[someguy519]

What you should do is place the rotor under four or five heavy text books and let it sit for a three weeks. After this is done put the rotor in your oven and bake it at 400 degrees for one hour. Immediately after that place it in a bucket of ice water. This will fix the problem

I agree with flattening under books, but baking in the oven? I'd be nervous about losing any heat treatments applied to the metal - speaking from a "materials science" point of view, you can significantly change the physical properties of metals with heating, quenching, etc. Now it's been about 7 years since my last materials science course, but as I recall, rapid quenching will make a metal harder, but also more brittle. That said, 400F is probably not hot enough to dramatically alter the crystal structure, and they probably reach a couple hundred degrees anyhow under long heavy braking, but I'd still be careful about it - ductility is a good quality to have in a brake rotor.

[Brizn]

BW: you're obviously not to be discredited. It just seems like A LOT of work, and a long time waiting (3 weeks?!), for something that many of us have been able to do with an adjustable wrench in about 4 minutes. I've had to lightly tweak brand new rotors, too.

[saxman]

BW...any way you can upload the dissertation? I'm sure it would make for interesting reading for us types that like to tinker with our rigs.

[blacknell]

Seconding the request for the dissertation.

Not sure what the user error could be when we're talking about straight-from-the-box-to-the-mount rotors. Interested in hearing your thoughts, though.

~

Tried a bit of bending tonight, and while it initially seemed to be successful (i.e., eliminated the rubbing when the bike was flipped over), it reappeared over an hour of riding at Accotink. Try some more tomorrow. Thanks for the suggestions, all.

[themonkeyman]

Because the rotors are stamped steel, you will rarely find them completely true, even out of the package. Just not a perfect process.

p!

Just FYI, rotors are actually laser-cut, not stamped. They tried stamping them for a while, but the results were way too erratic in such a thin material.

But yes, almost every rotor on every bike I've ever built had to be tweaked. As for heating it, 400 degrees won't cause any problems, they will reach 400 and even 450 sometimes under very heavy braking.

[berrywise]

BW...any way you can upload the dissertation? I'm sure it would make for interesting reading for us types that like to tinker with our rigs.

I will see what I can do. The problem is I wrote the entire thing without the aid of a computer so it is all written out by hand. I might be able to scan some of the relevant pages and post but I cannot promise anything.

[eloach]

What you really need is a large bench vice from Home Depot. Put the rotor in the vice and crank it as tight as you can. While you are at home depot, pick up an acetylene torch. Now crank the torch so that it's got a wide flame and heat the vice and rotor until they are really hot, but not so hot you start to melt the vice. Cut the torch and allow to cool for 38.72 seconds.

Pour about 16 ounces of Vodka into a cooking sheet with a lip about 1 inch high. Now get a pair of tongs and grab the rotor from the vice and lay it in the Vodka. Don't get too close when you are doing this.

Note: If you use cheap Vodka the rotor ends up with a nasty residue on it, so use the good stuff.

[EJensen]

I will see what I can do. The problem is I wrote the entire thing without the aid of a computer so it is all written out by hand. I might be able to scan some of the relevant pages and post but I cannot promise anything.
Before following the stack/heat/quench advice, please review the author's posts in other threads. Highlights include:

I work full time for the army as an insurance adjuster so I probably shouldn't be commenting on this as I could be violating some work rules but eh what the heck.

What do you mean by epic? 10-15 miles?

I just got off the phone and it sounds like they are going to have course marshals at the top of SAB and it will be a mandatory dismount and run with bike area for all riders. I guess they were unable to insure the race otherwise.

What do you consider fast? I'm pretty new to mountain biking but I have been pushing an average 9.8 mph. Will that keep up? (from July 13, 2007)
berrywise: Irony at other people's expense is fun, but posting something that might get someone hurt is uncalled for.

Regards,
Eric

[jabberwocky]

What you should do is place the rotor under four or five heavy text books and let it sit for a three weeks. After this is done put the rotor in your oven and bake it at 400 degrees for one hour. Immediately after that place it in a bucket of ice water. This will fix the problemPlacing a rotor under books is unlikely do do anything. First off, steel has pretty good elastic properties, so the very slight correction pressing it flat will make are unlikely to stick. When you remove the steel from its press, it will spring right back where it was before. Second, hardback books aren't even remotely flat, so even if the rotor did conform to them you would probably just make the problem worse. And third, the temperature at which steel is stress relieved is typically 800-1300 degrees Fahrenheit, not 400.

berrywise: Irony at other people's expense is fun, but posting something that might get someone hurt is uncalled for.Thanks for pointing this out. I'll ask the other mods to keep an eye on this guy. I will say that this particular moderator is not amused.

[mark w]

For those of us who can't have enough tools, I present the following:

http://www.parktool.com/products/detail.asp?cat=14&item=DT-2

http://www.parktool.com/products/detail.asp?cat=16&item=DT-3

[g_barr]

Pour about 16 ounces of Vodka into a cooking sheet with a lip about 1 inch high. Now get a pair of tongs and grab the rotor from the vice and lay it in the Vodka. Don't get too close when you are doing this.

Note: If you use cheap Vodka the rotor ends up with a nasty residue on it, so use the good stuff.

I got to the vodka part and I'm thinking I need to check my rotors tonight :) . But since he's (1) superheating vodka and (2) advocating its waste, I'm a bit suspicious about the quality of his post.

Unless, I suppose, he is using the expensive French stuff squeezed from grapes which isn't really vodka anyway.

[rmac]

3 warped new rotors sounds like you could have a hub or caliper issue. Have all 3 been on the same bike, same hub? Do you have a spare disc wheel set up that you can run to determine if the issue is in the hub or rotor? You could put that rotor on another hub & see if you still have an issue.

I run bb7s & have had rub issues on the rear no matter how finely I tuned or backed off the pads. I tightened up the hub and this helped. I also straighten up my disc brake hanger (rear) much like you would straighten a der hanger. The spacing looked even, but it was leaning in a little.

Any chance the caliper is sticking or the cable is worn?

Every time I straighten a rotor it reverts backed to warped by the end of a long ride.

[blacknell]

Definitely use gin. Why in the world would you waste a good vodka like that?

~

Rmac, same bike, same hub. No alt wheel, unfortunately. Tightened everything up, new cables, etc. Sounds like we've much the same experience.

[saxman]

I will see what I can do. The problem is I wrote the entire thing without the aid of a computer so it is all written out by hand. I might be able to scan some of the relevant pages and post but I cannot promise anything.

People still turn in handwritten dissertations? Even when I was in school, almost 20 years ago, I had to turn in my papers typed. (Does anyone remember Applewriter?...I did a whole term paper - 21 pages on that).

Still, even handwritten information would be helpful.

[eloach]

Placing a rotor under books is unlikely do do anything. First off, steel has pretty good elastic properties, so the very slight correction pressing it flat will make are unlikely to stick. When you remove the steel from its press, it will spring right back where it was before. Second, hardback books aren't even remotely flat, so even if the rotor did conform to them you would probably just make the problem worse. And third, the temperature at which steel is stress relieved is typically 800-1300 degrees Fahrenheit, not 400.

Thanks for pointing this out. I'll ask the other mods to keep an eye on this guy. I will say that this particular moderator is not amused.

Well, I think he's a hoot. Just take it as what it is... nonsensical fun, served a bit dry. :)

[eloach]

I got to the vodka part and I'm thinking I need to check my rotors tonight :) . But since he's (1) superheating vodka and (2) advocating its waste, I'm a bit suspicious about the quality of his post.

Unless, I suppose, he is using the expensive French stuff squeezed from grapes which isn't really vodka anyway.

Yeah. Pointing out the original post(s) are/were clearly in jest with similar retort. I tried to make it pretty clear. The Vodka would likely catch fire immediately, with some slight chance that the rotor would not still be hot enough to ignite it. That would be wasteful.

I LOVE the hand written dissertation bit. :) :) :)

[jabberwocky]

Well, I think he's a hoot. Just take it as what it is... nonsensical fun, served a bit dry. :)If its supposed to be a joke, I would prefer it to be more obvious. Bike maintenance is confusing enough for newbies without intentionally misleading stuff posted up.

On the other hand Eloach's response was pretty obviously a joke.

[FIX_BKS]

If its supposed to be a joke, I would prefer it to be more obvious. Bike maintenance is confusing enough for newbies without intentionally misleading stuff posted up.

On the other hand Eloach's response was pretty obviously a joke.

Agreed! Sometimes it's hard to simply read the implied sarcasm. Though Eloach was clever in introducing Vodka to the equation at which point I knew it was real! I am just kidding. I think it was slightly more obvious that his was a joke!

I was thinking to myself regarding berrwise "this guy must be nuts" at which point my "if I don't have anything nice to say..." intuition kicked in. Since most of us haven't studied chemistry to the point of full comprehension, I hope nobody feels silly for having (almost) believed it...self included :confused:

[punga]

....The Vodka would likely catch fire immediately, with some slight chance that the rotor would not still be hot enough to ignite it.

Unless you were using 100 proof or better vodka, it won't catch. (not speaking from experience, :o but usually spirits aren't flammable unless they're at least 50% alcohol)

just sayin'

p!

[punga]

Before following the stack/heat/quench advice, please review the author's posts in other threads. Highlights include:

I work full time for the army as an insurance adjuster so I probably shouldn't be commenting on this as I could be violating some work rules but eh what the heck.

What do you mean by epic? 10-15 miles?

I just got off the phone and it sounds like they are going to have course marshals at the top of SAB and it will be a mandatory dismount and run with bike area for all riders. I guess they were unable to insure the race otherwise.

What do you consider fast? I'm pretty new to mountain biking but I have been pushing an average 9.8 mph. Will that keep up? (from July 13, 2007)
berrywise: Irony at other people's expense is fun, but posting something that might get someone hurt is uncalled for.

Regards,
Eric

I was wondering the same thing about this guy when I saw his other post about lifting gov't property. :confused:

Barrywise, do you expect people to take you seriously?

I don't even think there are 20+ brands of MTB disc makers out there currently, much less back when you had to hand-write your dissertation about disc brakes?

And seeing the "catastrophic results in medical journals?" You're kidding, right? There is very little tracking of bike related deaths and injuries as they pertain to road vs. MTB (as discussed here in these forums), yet you've seen medical journals discussing serious injuries related to people self truing their steel rotors?

When you get your voluminous, "hand-written" research scanned in so we, your peers, can review them, then you may be taken seriously. But until then, I call "Shenanigans"

p!

[eloach]

Okay, for starters:

"Berrywise"

Pun list -

Very wise
Wise from drinking wine
BS wise
wise guy
b erry wise
leery of wise
bleary eyes

and the list goes on... Seems like I even recall a character by that name in a written work that was pretty good.

So I gotta expect a bit of good dry humor. I happen to be fond of that stuff, other people hate it. It's a big world. I think he should send something in to Dirt Rag. They need a little better talent in their monthly fiction bit. :cool:

[paulson]

Berrywise sounds suspiciously similar to Leinenkugel's "Berry-weiss" beer.

[werace424]

I still vote for round rotors!!!!!!!!!!!!!

I have yet to see a flat rotor work. If it was flat it wouldn't be called a "rotor" now would it?

Scheeze la-weeze!