Precision Disk Brake Rotor Truing
A disk brake rotor truing gauge.
A disk brake rotor truing gauge.
For the mountain biker who needs perfect performance from equipment that's regularly subjected to high stresses, heat, and crashes: the ultimate rotor truing (and wheel truing) system.

With a wheel survey gauge tool, you can true a rotor or rim to within a paper's width of perfect. A true rotor gives true, predictable braking.

Rotor truing tools.
Rotor truing tools.
We got this system from Morningstar Tooling Products. The truing gauge (above) is the R2.O.C-TECH ("Rotors and Rims On Center") wheel survey tool. The truing levers are Drumstix rotor truing forks.

(Note: I haven't found a good use for the small "beer-bottle opener" hook on the side of the bending tool. I suspect it's just what appears to be: a beer bottle opener.) 

On this page, I'll show you the right way to true the rotor. For a quick-n-dirty method using a crescent wrench (sorta works, and lets you keep biking when you're away from the shop), click here .

First, decide if you should replace the rotor. If it shows a stress or "fold" line, chuck it! Feel the surface to see if it's concave (lost metal in the middle of the rotor), or has other signs of severe wear. Often, the reason your rotor wobbles is because it's become too thin to stay straight. If you're going to keep the rotor, file off any burrs and rough spots before you start work.
Installing the Wheel Survey Tool

Turn your bike upside down. (Or put it into a workstand.) To install the wheel survey tool, leave your wheel in the dropouts, but pull your skewer. Turn the skewer so the threaded end is on the brake rotor side. Now twist the skewer into the receiver of the wheel survey tool and lock it down with the quick-release.
The truing gauge "survey tool" locked into place on the wheel.
The truing gauge "survey tool" locked into place on the wheel.
After locking the survey tool into place using the quick-release skewer, slide the gauge until the indicator tip (the plastic part that "floats" along the surface of the rotor) is in the middle of the rotor.
Adjusting the pin so it sits on the rotor.
Adjusting the pin so it sits on the rotor.
Turn the gauge so the stem aims straight at the rotor, and the indicator tip hits the middle of the rotor, lying flat against the rotor surface.
Once the pin is touching, clamp it down.
Once the pin is touching, clamp it down.
Tighten the knob on the clamp slightly, just enough so the gauge stays in position.

Position the gauge so it's easy to read, yet out of the way of your tools.

Setting the "zero" position.
Setting the "zero" position.
Now nudge the gauge slightly towards the rotor, so the top knob (the end of the push-rod that goes through the gauge from the indicator tip) comes slightly away from the gauge as the indicator tip pushes against the rotor.

Give yourself about 2-3 mm of "float" so the indicator tip can move back and forth with the warp of the rotor.

Checking the Rotor
Ready to test the rotor.
Ready to test the rotor.
Make sure the indicator tip can "play" back and forth completely without hitting the end of its travel. Turn the indicator tip so it lies radially over the rotor's grip surface, flat on the metal.

This is the best position to detect all warps and bends. A circumferential orientation (the black tip lies along the direction of rotation) can detect the most common type of heat-warping.

Rotating the wheel to check the rotor.
Rotating the wheel to check the rotor.

Turn the wheel gently. The needle of the gauge will move with the warp of the rotor. When you see the needle deviate, identify three spots:
  1. where the motion starts
  2. the point of maximum excursion
  3. where the deviation ends

These are the spots where you'll apply your tools.

Finding where you need to apply the rotor tools.
Finding where you need to apply the rotor tools.
Note the needle motion on this rotor in just 1/8 turn of the wheel. This rotor is only 6 months old, and passes the "eyeball" test for being perfectly straight. (The eyeball test is watching the distance between pad and rotor as the wheel turns).

Any warp of the rotor decreases braking response and efficiency.

Straightening the Rotor
Correcting a rotor-strut bend.
Correcting a rotor-strut bend.
The best method for straightening the rotor is by using all three tools (Drumstix) as shown below. But often a slow, gentle warp (such as sometimes occurs during a flaming long downhill when the rotor begins to smoke) can be straightened by using the bending tool alone.

Here the long end of the bending tool is against a support strut. This corrects a "taco" type bend that involves a strut.

Correcting a warp in the circumference of the rotor.
Correcting a warp in the circumference of the rotor.
Here we're using the smaller side of the bending tool between the struts. This corrects a minor localized "tilt" type bend.
Using all three tools with only two hands.
Using all three tools with only two hands.
Outward bends and warps, three-tool method:
Find the spot where the rotor begins to bend outward, the spot of maximum deviation, and the spot where it returns. Put a stabilizing tool on the beginning and end-point of the bend. Put the bending tool in the middle of the curve, at the point of maximum deviation. Apply it so the tool leans outward.

Hold the outer (stabilizing) tools with your fingers while you push inwards on the bending tool with your thumbs as shown.

Test your correction and nudge it again.
Test your correction and nudge it again.
The bend will often rebound back a bit. Test the truing, and reapply the tools and correct as needed.

Many bends require a second application of bending with the tools closer together, as the original long curve becomes a short curve.

Using the three tools with an inward bend.
Using the three tools with an inward bend.
Inward bends and warps, three-tool method:
Again, find the spot where the bend begins, the spot of maximum deviation, and the spot where it ends. Apply the stabilizing tools on the beginning and end-point of the bend. Put the bending tool at the point of maximum deviation, turned so the tool leans inward.

For a widespread warp (the most common kind) put the stabilizing tools in the fold of your thumb while pulling back with your fingers on the bending tool.

Applying the correction force.
Applying the correction force.
Hook the outer (stabilizing) tools with your thumbs while you pull back on the bending tool with your fingers as shown.This view shows correction of a smaller bend, such as might occur from the rotor hitting a rock.
Adjust the spacing of the tools depending on the sharpness of the bend.
Adjust the spacing of the tools depending on the sharpness of the bend.
Tweak and reapply the bending force as necessary.

If a smaller curve remains after making the major correction, move the stabilizers inward and repeat.

Truing your rim precisely for rim brakes
If you don't have a wheel-truing stand, this can work.
If you don't have a wheel-truing stand, this can work.
Got rim brakes? Imagine how much better your brakes can perform if your rim is trued to within a paper's width of perfect.

By sliding the gauge out to your rim and moving the gauge-attachment rod so the indicator tip lies on your rim, you have a precise rim-truing gauge.

Truing works best with the tire off!
Truing works best with the tire off!
You can test the true of your rim with the tire on. But when it comes to actually truing the wheel, you should remove the tire from the rim to adjust the spokes.

After truing, remount the tire and test for true again. (A damaged tire can sometimes force a rim out-of-true when the tube is inflated.)