When to replace a rotor vs. when to re-true it?

[EB]

Here's one for the hivemind to argue over - when do you replace a rotor vs. try to true it back?

I'm an average mechanic and re-truing rotors has never been something I feel confident doing - always worried I'm going to make it worse, and I know the LBS can probably do it. But we're not being practical here, so let's throw out the LBS option for the moment.

Context: After two days of 7000' descents at Ashland Mountain, I discovered both rotors on my Forbidden are warped - the trails at Ashland are FAST and I'm a bit of a brake dragger sometimes, so obviously my fault. I can replace them, but the fancy XTR centerlock rotors I prefer are $88 each!

When do you bin the rotors and just get new ones, and when do you go in with a wrench and a prayer and try to get them straight yourself? What's your personal rule?

(If your answer is #savetherimbrake you lose the game)

[catchourbreath]

Assuming you haven't discolored them too bad from over heating and they're not below minimum thickness then using the rotor straightening tool is fine. I've found the higher end rotors tend to bend easier from knocks.

[tellyho]

6 bolt beefy rotors FTW.

[prototoast]

I retrue rotors all the time. It's not a big deal. Like catchourbreath said, I only replace if they're discolored or too thin.

If you have the tool, retruing rotors doesn't take a lot of skill. You just keep wiggling it around until they stop rubbing.

[Likes2ridefar]

I’m a very out of practice mechanic but truing rotors is one I even haven’t bungled. I use park tool rotor tool and carefully tweak them with positive results each time.

[KevinC]

We had the Morningstar Disc Truing set at the shop I worked at.
Two holder wrenches and a bending wrench.

I don't think they're sold anymore.
If anyone is interested, I can see about whipping up some laser cut parts.

[2LeftCleats]

This is sort of related. Why is the pad-rotor space on disc brakes so small that it takes very little deviation from perfect to cause rub? I assume there’s a hydraulics issue behind this and wonder if someone can enlighten me.

[Jaybee]

I do just fine with a crescent wrench and eyeing it up. But I'm risking SLX level rotors, not XTR

[FriarQuade]

There is a reason why the other end of our rotor truing tool has a bottle opener on it....

Seriously though; if you're looking at replacement anyway, what do you have to loose? Might as well try and fix them before you bin them and start over.

[JAGI410]

Originally Posted by KevinC

We had the Morningstar Disc Truing set at the shop I worked at.
Two holder wrenches and a bending wrench.

I don't think they're sold anymore.
If anyone is interested, I can see about whipping up some laser cut parts.

RIP Paul Morningstar. He made some cool stuff that no else makes now that he’s passed (not so peacefully either!). There’s a good opportunity to make up a new jig!

[yinzerniner]

Originally Posted by 2LeftCleats

This is sort of related. Why is the pad-rotor space on disc brakes so small that it takes very little deviation from perfect to cause rub? I assume there’s a hydraulics issue behind this and wonder if someone can enlighten me.

Has to do with leverage ratio. With the closed hydro system you can either design the brake lever pull to provide force or travel for a given amount of pull.
Since road hydraulic levers have such a limited amount of fluid the first gens were about providing maximum force but at the cost of travel / clearance of piston to rotor. Also heat from extended braking will expand the rotor and the pistons/fluid a bit, so the clearance space is further tightened or eliminated

What's cool is the latest Shimano road hydro levers (been this way for a while on MTB, but they have more room to work with) they're incorporated servo wave, where the ratio changes depending on the stroke.
https://bike.shimano.com/en-EU/techn...ve-action.html

Probably should have been designed that way at first but there's such limited space in road levers it's tough to fit everything in (as you can see from the bulk/size of most SRAM levers).

[EB]

For the record, these were servowave MTB brakes - the tolerances are definitely better than early road stuff, but warping MTB brake rotors beyond even those tolerances is a hazard for descending-heavy applications.

Ultimately decided to bin the rotor anyway because it was a good excuse to jump up to a 203mm rotor in back, which is something I've been wanting to do anyway. But I've got a Park tool rotor tool in the mail!