What are the failure modes of the brake rotor worn beyond limit

Question

Background

I have been maintaining two bikes for a few years. Both have disk brakes. One has around 10 000 km on it another one is well over 35 000 km. The disks on the bike with 35k+ are now reaching their end of life. They are cheap Tektro disks from early 2000s. I will be replacing them soon.

However I also maintain a relatively new bike that I have built up myself. This one has:

• Shimano XT flat mount callipers (front and rear)
• Shimano RT-86 rotors front and rear.
• Shimano resin pads (K03S initially and then switched over to L05A)

To my surprise the rear rotor has worn from new downto 1.3mm in under 6000km. The front one is now at 1.5mm (which is Shimano recommended scrap limit). There has been no brake rub, I used original Shimano resin pads only. Never used these rotors on any other bike or with metallic pads. Change of pads I made at around 3000 km was not due to pad wear but due to contamination. Overall I would estimate I have worn through about 1 full pair of pads in that time.

The calliper I used to take measurements is in a good state and I would rule out the measurement error. Also the lip on the edge of the 6000km rotor is noticeable and even more pronounced than the one on 35000km Tektro rotor...

There has been no substantial difference in riding conditions between old and new bike. If anything I would argue that new bike brakes had an easier life...

That situation made me think. Rotors on one bike have worn out much faster then the other and much faster then I expected them to. I tend to go on bikepacking trips in mountainous regions where I go multiple days (weeks?) unsupported with a loaded bike. It is very realistic that a rotor could fail under these circumstances.

Questions

• What are the failure modes of the brake rotors if you push them beyond limit (not that I intend to). I tried googling but did not find anything substantial.

• What are the critical signs to look out for on a trip (assuming you don't have a vernier calliper and don't even have a habit of checking your rotors regularly)? One thing that I noticed is that the sound changed when breaking. Instead of a continuous hissing noise that disc brakes normally produce, I get the hissing noise + pinging noise as if someone is touching the spokes. I am almost sure the spokes have nothing to do with this.

• Is it normal for brake rotors to wear out as quickly as pads??

Answer 1

What are the failure modes of the brake rotors if you push them beyond limit?

• Thinner rotors will heat faster and could warp, leading to inefficient braking.
• Excessive heat and a warped rotor could cause a pad to grab the rotor and cause the rotor to crack.
  • If it cracks, it could explode, ejecting bits of metal in random directions, possibly puncturing a tire, your leg, your arm, etc.
  • If a part of the rotor falls off, the pads will fall into the "hole" left behind and immediately lock the wheel causing skidding if it's the rear wheel or an endo if it's the front wheel.
• As noted in another answer, your rotors are a steel & aluminum sandwich. Once you wear through the steel layer, you will quickly wear through the aluminum layer and end up with pad-to-pad contact.
  • This could cause a wheel lock with consequences noted above.
  • It's possible that you might wear through evenly leaving the pads touching with the center part of the rotor spinning with the wheel and a ring of rotor outside the pads. This would give you no braking, but at least wouldn't lock the wheel.

Answer 2

I will headline this answer by saying that RT-86 is special in that it is an Ice-tech rotor. This means that it is made of a 3-layer sandwich: steel braking surfaces with an aluminum core. In addition to the thermal issues other answers address, the problem with wearing these rotors too thin is that you risk completely wearing through the steel layers. Aluminum is not suitable to be a braking surface material (it is too soft), and it is very possible that you will suffer catastrophic rotor failure should this occur. For extended touring purposes, the solid steel RT-76 may be more suitable.

As for the cause of the rapid rotor wear, it is possible that L05A and K03S are more abrasive than the pads in the cheaper Tektro brakes. This gives them more friction, and a higher metallic content could result in better heat tolerance for extended braking. However, pads and rotors really shouldn't be wearing at a 1:1 ratio like that. It's possible that you had dud rotors that were inadequately hardened or something.

On the road, you can observe how deep the braking surface "valley" is relative to the rest of the rotor to estimate wear. You can compare future rotors against these worn-out ones to calibrate your estimates. It would also be prudent to measure (and generally, inspect) your rotors before setting out on a multi-week trip. Keep in mind that your brakes will wear faster with a heavily loaded bike.

Answer 3

Rotor wear is a thing that most riders don't really consider. From what I have read, the bets I got was "2 to 4 years for a typical rider" - whatever that means. Obvious conditions you ride, speeds and style of riding means this will vary. XT components are built for weight and performance, with rotors being consider a consumable, so you 6000km might be expected. Cheap Tektro are aimed at bike less likely to be regularly maintained than XT equipped bikes, and weight is less important, so the discs probably started out heavier and likely harder material so they last longer.

The problem with worn rotors comes mostly from heat. Being thinner, they have less mass and will heat up quicker and 'hot spot' quicker. The hot spotting can lead to warping. Extra heat will increase risk of brake fade. A warped rotor is often still rideable, but the brakes won't be performing very well. There is increased risk the roto could crack.

If the pads are very worn, there is a theoretical chance the caliper cylinder will extend further than designed for and leak fluid (although I would be surprised), which could mean no braking at all.

The risk of all these kinds of failure exists anyway (Good reason why we have two brakes). It is hard to gauge how much the risk is increase by a worn rotor. Reading some web sites you would think the risk become almost certain the instant the disk dropped below 1.5mm. However, brakes are kind of important, not something to skimp on.

Signs things are going wrong at the same signs that you should be looking for anyway, as the failure modes can still happen. Increase lever travel, spongy brake feeling (especially after a long descent), disc rub when no brakes applied (warped rotor), pulsating feel in the lever. Look for cracks in the disks (around the vent holes).