Q&A: Dealing with disc brake rubbing and bent rotors

A couple of questions about common disc brake issues came in recently. With discs on so many different bicycle types today, it's a good topic to kick off 2018. Happy New Year!

Q: "My gravel grinder road bike has a dragging disc brake. I don't really feel it when riding. But, if I pick up the bicycle and spin the wheel, it's obvious the pads are rubbing because the wheel stops. This has to be costing me energy and speed on rides, so it's frustrating. I've had a few mechanics try to fix it. But, no one can seem to stop it rubbing. Can you help?"

Yuri

A: The type of rubbing Yuri describes can be missed when riding because it’s slight. But, as he mentions, if you lift the wheel that’s rubbing off the ground and give it a spin, you’ll realize straight away that the rotor (the metal disc attached to the wheel) is slightly rubbing, because the wheel will stop spinning much more quickly than the other wheel (unless it’s rubbing, too).

This isn't the type of rubbing caused by a bent rotor. That creates a major rub that’s easy to see because the rotor wobbles when the wheel is spinning. For information on fixing bent rotors, see the second question (below this one).

The type of rubbing I’m explaining how to fix here happens on straight rotors. It’s highly annoying because, while it’s only slightly dragging, it’s constantly slowing you down and wasting your energy. You can try squeezing and releasing the brake lever repeatedly, removing the wheel and reinserting it, gently flexing the rotor to try to push the rubbing pad away, even loosening and repositioning the caliper – but the brake will usually still rub as soon as you brake again.

Fortunately, there’s a fix for this problem that almost anyone can do. All you need is the right wrench to remove the brake caliper from the frame and an ordinary business card. Don’t worry. You do NOT need to disconnect the brake hose/cable from the brake caliper or change any brake adjustments.

Tip: When working on disc brakes it's best to keep all oils (even from your hands), greases, lubes, etc. away from the brake pads and rotors. Lubes can contaminate and ruin the braking. If you make this mistake, to fix it, you may need to replace the brake pads and super clean the rotors.

To fix the rubbing, follow these 5 easy steps.

1. Remove the rubbing caliper from the frame by loosening and removing the two bolts holding the caliper in place (turn the bolts counterclockwise).

Before taking the bolts all the way out, be sure to note (take a photo) which one goes where and the order of any parts between the bolt head and the brake caliper. There may be washers or locking washers and one bolt may be different than the other. Be 100% sure you know exactly how the bolts and any parts go back on so you can get it right when you reinstall the caliper.

2. Now, hold the removed caliper in your hand. It’s still attached to the brake hose/cable, but there’s room to work on it.

Next, fold an ordinary business card in half and slip it inside the caliper as shown in the photo below. Depending on the size of the card, you might need to cut it down to fit.

3. Once the card is sized right and in place, wiggle and push the caliper back over the rotor so that the business card is in between the rotor on both sides and the brake pads, too (they’re tucked inside the brake caliper).

While keeping the caliper on the rotor, turn the wheel so that the caliper lines up to the bolt holes in the frame. Then, put the bolts back through the caliper, using care to put the bolts in the right place and any washers or tension devices in the right place, too. Refer to your photo. Screw the bolts clockwise until the caliper is loosely mounted back on the frame with the business card still in place inside the caliper.

4. Now, squeeze the brake lever and keep it squeezed while tightening both brake caliper bolts fully. If you're working on the rear brake, you might not be able to reach the bolts while holding the brake on.

So, ask a friend to help, or wrap something around the brake lever that keeps it firmly squeezed, like a toe strap or strong elastic band - the Bike Brake is handy for this.

5. Now that the caliper is back on the frame and tight, release the brake lever and squeeze it a couple of times as if braking.

Then, turn the wheel while holding onto the edge of the business card and the card will rotate with the rotor and come out. Once the card is out, you should find that your brake works nicely again with no more annoying rubbing!

Note that, instead of removing the caliper, you can try just loosening the caliper bolts. Then, removing the wheel and folding the business card over the rotor and reinstalling the wall and following steps 4 and 5 above.

The only thing is that it can be tricky on some bikes to get the rotor back into the brake caliper when the business card is on it - and to keep the business card in the right place on the rotor so that the card ends up exactly between all 3 things (rotor and both pads). You can do it if you're patient and careful but if you're not, you could get the card in the wrong place or even knock a pad loose.

In comparison, when you push the caliper over the rotor you can see what you're doing, the card will stay were you put it inside the caliper and it's less likely you will damage anything. But, both ways can work.

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Q: "Jim, not sure how this happened, but I bent my disc brake rotor. I know its bent because I can see the wobble and it wasn't like this when it was new. Is there a way to straighten bent rotors or do I need to have it replaced?"

Charles in the UK

Great question, Charles. Let's look at how rotors get bent first.

Causes
Rotors are the round, thin metal plates that attach to the wheel hubs. When you’re braking, the brake pads inside the brake calipers squeeze the rotors, providing excellent all-conditions slowing and stopping power.

If you’re lucky, your rotors will remain almost perfectly straight and you’ll never have to worry about them until they’re worn out, which can take a long time. But, if you’re unlucky, many things can bend the rotors.

For example, you could have a stick come up, get stuck in and bend the rotor. Or, on a long descent, you might hold your brakes on too long and heat a rotor enough that it warps. Another one is having someone at a rest stop mistakenly lean their bike against yours without realizing that their pedal is going to slam into your rotor when they let go.

Fortunately, these types of bends are usually fixable. What’s required is straightening the rotor. It’s a good skill to have because it’ll save money and let you keep riding instead of having to shop for new parts.

Tip: You can always replace bent rotors with new ones if you can’t or don’t want to try straightening them. They aren’t overly expensive. Just be sure to do your homework and get the same type you had.

Inspecting rotors
Before assuming a rotor is bent, check that there aren’t other problems causing it to only appear bent. Rotors are held on with lockrings or bolts. If these loosen, a rotor can wobble and look warped when all that’s needed is tightening the rotor bolts or lockring.

You might also have a wheel that’s not fully tightened in the frame. Check the quick release or through-axles to make sure. Or, a wheel might be off-center in the frame or fork, which can make the rotor look too close to the brake, causing you to think it’s bent. In that case, all that’s needed is loosening and centering the wheel.

Another glitch that can make a rotor wobble when it’s not bent is loose wheel bearings. To check for this, grip the wheel near the fork or frame stays (rear wheel) and push and pull gently sideways.

If the wheel bearings are correctly adjusted, the wheel will not move side to side when you do this test. But, with loose bearings, a wheel will move, and sometimes a lot. The cure is to remove the wheel and adjust the bearings to eliminate any play.

When inspecting rotors, if yours has a compound bend, or is bent so badly that it almost has a crease or fold in it, it is probably beyond repair. The bends that are fixable can be significant, but only smooth bends, not actual damage to the rotor. So, if you see twisted or folded or crunched rotors, you should replace them because they’re likely beyond straightening.

Straightening
Once you’ve determined that you for sure have a bent rotor and not a seriously damaged one, you can try to straighten it – or you can say “true it.” It’s a trial-and-error process that takes practice to master. It’s not difficult, but it can be frustrating and require patience and a good eye.

You can true rotors while the wheel is on the bike. For a tool, I use and highly recommend Park Tool’s DT-2 Rotor Truing Fork (about $20). You can see in the photo how this tool has two ends that slip over the rotor for excellent and precise leverage.

Park’s tool is heavy and fits perfectly so as not to damage the rotor even when you pry on it pretty hard. That’s usually what it takes to get a wobble out. If you’re new at it, you’ll get a feel for how you have to bend the rotor well beyond where you thought it would go straight to get it to improve, because the metal wants to rebound more than it seems it would.

You can slip the top of the tool over the top of the rotor to pull out or push in to remove a side-to-side wobble. You can also use the end with the short horizontal slot to remove a twist in the rotor by slipping that notch over the twist and pulling or pushing the tool as you hold it 90 degrees to the rotor. Another use of the horizontal slot is to slip it through the rotor and onto one of its “legs” to straighten lower bends.

Don’t rush it, and pay attention to the changes you’re making, and you can get good at straightening rotors. If it helps, you can mark the bent area of the rotor so you can keep track of your progress. Just be sure not to use anything that’ll compromise the braking surfaces.

Sighting the wobbles
One of the challenges of straightening rotors is seeing/finding the wobbles. With the wheels on the bike, you can sometimes sight through the brake and watch the rotor come through. If so, you’ll be able to see the rotor move left and right in relation to the brake pads, and be able to stop the wheel at that point. You then rotate the wheel to bring the wobble outside the brake so you can straighten just the right spot.

If you’re having trouble seeing the wobble through the brake, try putting a piece of white paper in your line of vision behind the brake caliper – or have a friend hold one up. Another trick that can work is holding a flashlight just so, to light the inside of the brake so that you can spot the bends.

Reading further, you’ll learn about the only tools I’m aware of specifically designed to make rotor truing much easier and super accurate. They’re great tools. But if they’re beyond your budget or needs, you could maybe copy how they work and rig up a pointer on your fork or frame.

Something as simple as a pencil for the pointer and a rubber band to hold it in place might do the trick in a pinch. If you come up with something that works well, please comment below and share it.

The gold standard in rotor truing tools
If you’re straightening rotors on a regular basis, you can save yourself a lot of time and do an even better job truing them with one or two more cool tools from Park, their DT-3 Rotor Truing Gauge (about $40) and DT-3I.2 Dial Indicator for DT-3 (about $40). See the photo.

Note that in order to use the DT-3 and DT-3I.2 Truing Gauges, the wheel with the bent rotor is removed from the bicycle and placed in either Park’s TS-4, TS-2.2, or TS-2 Professional Wheel Truing Stand ($230 - $372), which are all pre-drilled for the gauge to bolt onto.

If you have another brand of truing stand, it might be possible to drill it to accept Park’s gauges.

With the wheel held securely in a truing stand with one or both of Park’s gauges attached (the photo shows both), there’s no need to try to sight through the brake caliper. Instead, you locate the wobble with the gauge’s indicator and straighten each wobble with the Park Truing Fork.

It still takes time and patience to find and true wobbles, but because the indicator shows you exactly how true the rotor is becoming, the trial and error is greatly reduced. It’s super satisfying when you end up with nice true rotor and a disc brake that works perfectly again.

Here’s an excellent video on rotor truing with Park Tool tech whiz Calvin Jones


Here's hoping your disc brakes work like a champ from here on,
Jim