Can You Ever Be Too Thin?

Way below spec! Should have been replaced a long time ago.

Well yes, when it comes to disc brake rotors. Everybody is disco-ing these days and lord help you if you try to find a new bike that has ye olde rim brakes. So whether you’re like Alvin Toffler and embrace these “new” technologic marvels being marketed like Cabbage Patch dolls or you’re firmly in the Luddite camp clutching your spoon brakes in a death grip, disc brakes for road bikes are here to stay like it or not.

I’ve carped about road disc brakes previously mainly focusing on the increased time, cost, and attention they need compared to rim brakes. I mentioned before that disc rotors wear out amazingly fast, way more quickly than an aluminum rim does (like an order of magnitude faster!). Doing your own bike repair and maintenance is a dying habit these days and it’s certainly not encouraged by the increasing complexity of equipment so much so that taking your bike to a shop for just about everything has become standard practice. But if you do your own bike maintenance, ignoring your disc brake rotors—which is unfortunately all too easy—is a bad mistake. Rotors wear down; for me they last about a year. When worn down they don’t always make odd noises or behave strangely—there just isn’t an obvious warning that you need to replace them. If you regularly take your bike to a shop, they’ll catch that because checking brake pads and rotors is standard practice and long before it becomes a problem they’ll tell you to replace the rotor.

Here’s what happens when you just don’t bother to check your rotors: Roger and I went for a ride and he commented to me that his rear brake felt like it wasn’t working—it wasn’t stopping the bike and he was relying almost entirely on the front brake. I didn’t think much of it—probably the brake pads had worn down yet again. We go through disc brake pads like candy. We have to change them about twice a year (note: on each bike!) In contrast changing rim brake pads is such an infrequent chore that I don’t even keep spare pads around. Maybe I have to change pads once every ten years or so. The other thought that occured to me was perhaps Roger’s bike had air in the rear brake hydraulic line, which can cause spongy and ineffective braking. I took a quick look at the rotor and it seemed fine and the lever feel seemed fine too yet the brake wasn’t doing its job. Hmm.

When we got home to the shop, I was able to pull the wheel out of the frame and inspect the brakes more carefully. The pads were actually okay. But a closer look at the rotor showed that it was quite worn. I couldn’t see it out on the road because I was looking at the edge of the rotor, which was still thick because his brake pads were wearing a track below the outside edge thus leaving it intact. I measured the thickness of the rotor and it was just 0.76 mm—half the recommended mininum thickness! In retrospect we were lucky the rotor hadn’t just cracked and split altogether being so thin. Good thing we weren’t going down Diablo!

A brand new Shimano rotor is 1.8 mm thick.

Every brand of disc brake rotors—Campagnolo, SRAM, Shimano, Tektro, etc. has a minimum thickness; when your rotors get this thin, they want you to replace them. Shimano for example recommends that its rotors be replaced when worn down to 1.50 mm. Brand new they are 1.80 mm thick, so you can see that there isn’t a lot that has to be worn down before they need to be tossed. This isn’t like eyeballing your rim brake pads or even your aluminum rims—you can barely see the difference between new and worn rotors. So to be prudent you need to invest about $20 in a vernier caliper which you can accurately measure the thickness of the rotor. You can buy analog or digital calipers but the digital ones don’t cost a lot more and they’re a lot easier to read. You should get in the habit measuring your rotors’ thickness every couple of months. When a Shimano rotor is down to 1.50 mm, replace it. (SRAM recommends no thinner than 1.55; Campagnolo says no thinner than 1.65 mm.)

A typical worn rotor; 1.36 mm is still below the recommended 1.50 mm for replacing.

Most rotors today use the Centerlock standard that Shimano invented for attaching rotors: the rotor is splined and fits directly onto the hub and held in place by a lockring. You’ll need a lockring tool to remove and install the rotor. The other, older standard is six-bolt rotors; for these you’ll need either a hex key or Torx T25 wrench depending on the kind of screws they use. Keep in mind that when you install the new rotor you’ll need to torque down the lockring or screws to the specified torque. Lockrings are 40-50 Nm and screws are usually 4 Nm. 40-50 Nm is a lot of force, so if you don’t have a torque wrench, tighten it as much as you can because you don’t want the lockring to come loose when you’re riding. If you have rotor bolts, you can buy a preset torque wrench with replaceable bits set at 5 Nm, which is close enough.

One thing to keep in mind when you’re measuring the thickness of your disc rotors: make sure you’re measuring the actual thickness rather than the outside or inside edge of a worn track on the rotor. Measuring the edges will give you an incorrect read of the rotor’s actual thickness. So place the caliper tips directly in the worn track as shown in the photos.


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