In this video, we'll show you how to true a disc brake rotor.
Hello, Calvin Jones here, Park Tool Company.
Rotors can become warped from use, abuse or simply by getting too hot.
One of the most common causes of a bent rotor is getting banged around when parking the bike
or stacking bikes tightly.
It's even possible for new rotors to have some problems.
If a rotor has been badly bent, especially from impact, it's best to simply replace it.
How much is too much? This example is far too bent,
but in most cases it's best to re-bend it and see what happens.
If you keep trying and trying, and there's no improvement, then it's like a warped cookie sheet:
you can push it around, but it will never be straight.
A note of caution. Anytime you are working around rotors, be careful.
The edges can be sharp and when they rotate, it can have quite the slicing effect.
Typical Tools include: A rotor truing tool.
A flashlight is useful as well, but even a white sheet of paper held behind the caliper
can help make lateral deviations, also called runout, visible as the rotor spins.
For advanced truing, we recommend a wheel truing stand,
a rotor truing gauge, and, for the highest level of accuracy, a dial indicator.
A rotor in need of straightening will likely show up as a rub or noise as it passes through the caliper.
But it's important to know there are many other things that can exacerbate this symptom.
Two quick things to check are wheel alignment if you have open drop-outs.
Seating this wheel properly solves the issue.
And bearing play at the hub is also easy to check for.
Just rock the wheel back and forth as shown.
If there's play, fix it.
After those quick checks, it's okay to start truing the rotor,
but it's always good to know other things that can lead to rotor rub.
These are: misaligned brake calipers, due to poor alignment by the installer,
in mechanical disc brakes, drag in the cable or housing can cause the pistons to not retract fully.
Hydraulic brakes can also have sticky pistons, but caused by dirt and muck.
Brake mounts on the frame may be misaligned.
This prevents proper pad alignment to the rotor face.
Professionals will use a facing tool such as the DT-5.2 to correct the issue.
Lastly, loose mounting bolts, or a loose lockring will keep the rotor from fully securing to the hub mounts.
A basic truing of the rotor can be sometimes be done with the wheel on the bike
by using the brake pads as a reference point.
Set the bike in a stand or upside down on the ground in order to get the wheel spinning freely.
Watch for wobble at the caliper between pads.
Look for a gap opening and closing.
Here, a flashlight can be useful.
A sheet of paper can also help make the lateral deviations, or runouts,
more visible as the rotor spins.
Find a section of the rotor that needs truing, and note which way it needs to be bent to correct the runout
Rotate this area out and away from the caliper.
Use a rotor truing tool such as the MTB-3.2 to flex the rotor in the appropriate direction
in order to improve the bend.
Always begin with a small amount of effort on the tool,
and then apply more effort if no improvement is seen.
Repeat as necessary.
When the rotor is not rubbing the pads as it spins, it is adequately straight.
It is often easier and faster to get a more accurate correction by pulling the wheel and
putting it in a truing stand equipped with a rotor truing gauge such as the Park Tool DT-3.
Something else I like to do is to mark the rotor arms in order to track the progress.
These can be easily wiped off later using acetone or alcohol.
Spin the wheel and slowly move the gauge end until you get a light contact.
Stop the wheel at a lateral deviation, and use the DT-2 to flex and then bend the rotor.
Using the DT-2 as shown will result in a narrower correction.
For wider corrections, use DT-2 at the arm, and press outward or inward.
However, if the rotor is in two pieces, do not attempt to bend at these arms -
bend only at the outer perimiter of the rotor.
Now we'll walk through the truing process in real time,
and we'll pick up when the DT-3 first contacts the rotor.
There we have our light rub.
This is the worst deviation outward.
and here we appear to be on about the ninth rotor -
arbitrarily picked, but we know that that's where the rub is.
Spin it one more time - we'll go this way
You can also see a large... ooh, a large deviation inward.
So here the rotor is moving inward, so here we're looking at number two or so -
number one and number two
we're seeing this motion this way.
So let's go back to that first rub right here again.
we need to bend this one inward.
So I'm going to bring that away from the indicator a bit,
engage the DT-2 fully on the rotor, and then flex it a little bit
and we're going to come back and see.
We did make an improvement.
That must be better. It was rubbing once, now it's not rubbing.
So we're going to come a little bit closer...
There's our rub, and yep, it's on the same place we we were before.
We're on that same number.
It's still a problem, but it's less of a problem so we are actually making progress right here.
I'm going to bring that up here and come in a little more.
Start with a little bit of effort - you could always add more but try not to overshoot.
We're still back between our 9th and 10th rotor.
We're going to make one more here.
One more correction.
Really light rub.
Now we should address this other one
and this is a judgment call and it does come with some experience
We're looking fairly good, but we can see we have that motion away from our indicator right through here.
We're going to bring this this one back outward.
So we're going to see it goes from about here - about 3 -
to about here - which is pretty much 12.
So here i'm going to engage on the arm and bring it out - again, just a little bit.
The noise here that we're not making is that large deviation we see.
We're back at that same spot but it does seem a little narrower.
I'm going to engage the tool on top this time.
We're going to come outward with this piece.
That's definitely better - we're moving closer and closer.
So how do we know when we're good enough?
It's nice to have things perfect, but we're going to use, as a test, a business card.
It doesn't go in here because it's zero, but we're going to find a light gap -
that very small gap there - that gap must be smaller than the business card - it doesn't go in.
We get to our bigger gaps - well, yes.
So here we want to keep keep working...
Notice we keep coming to back to the same places.
Number 10 is rubbing so we bend inward.
Now we've got good runout exept that one gap.
And sure enough, it's the number 2 so we bend outward, with a little more intensity.
And one more bend at number 9 inward,
and a final bend outward at number 2.
On a mechanical brake, this rotor would likely be good enough,
and if you want to be really picky, you can use a sheet of paper as a go/no-go gauge.
Professionals often use a dial gauge such as the DT-3i.2 to determine if a rotor is adequately straight.
When the total variance is defined, the dial can be rotated so the zero marks the center of the variance.
Then the target variance can be set as shown.
Values outside the target are isolated and adjusted.
When the variance falls within the target range, in this case .2mm, the rotor is true.
And that concludes the process for rotor truing.
Again, some repair is possible in our rotors,
but if the rotor does not show improvement in a few minutes, it has probably reached the end of its lifespan.
If ever in doubt, replace with a new one.
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