Front Brake Discs - Checking Runout

Checking the runout - checking to see whether the front brake discs are warped.

Measuring the Runout with a dial gauge

My front disc brakes looked quite a mess. Quite rusty. Not so much on the rubbing surface, but everywhere else - where they bolt on and around the rim edges. I guess this is partly down to road dirt when the car was my daily drive and partly down to many years sitting unloved in a barn.

Discs - June 2015

Back then I cleaned them up with a wire wheel on a drill and was fairly happy with the results. I didn’t give them a protective layer of oil for obvious reasons. Returning to them now, they had started to show a patina of rust all over again and generally looked a little tired. Replacement discs are widely available and, thankfully on a DS, are surprisingly easy to fit.  However I had heard that, because of poor steel quality, they tend to wear quite quickly. Especially when in combination with modern asbestos-free disc pads.

Another option is to have my current discs grit-blasted. However there was no point in doing this if the discs were already excessively worn or warped. As a first step therefore, I needed to carry out some checks on the discs. Brake specifications are listed in Operation DX. 451-00 in Manual 518 and Operation D.450-00 in volume 1 of Manual 814.

My disks were well within tolerance for disc thickness and with no obvious cracks, grooves or ‘hot spots’ on the surface. The other key test that you find in the manual is for disc ‘run-out’  - effectively the degree of any warp in the disc. Some degree of variance/ warp should probably be expected, but Citroen specify tolerance for this.

The way to test this is to fit a gauge to a stable surface, with it's needle on the edge of a fitted disc. Rotating the dic reveals the degree of warp or run-out.  

From the manual: fitting and using the run-out gauge

First job though, was to replace the studs in the drive shafts. When I’d removed the shafts, half of the studs had come out with their nuts. These studs needed to be reset in the shaft flanges. I double-nutted them on the opposite ends to the stubborn nuts to give me some purchase. The studs were cleaned up on my wire wheel and degreased. I ran a small wire brush over the female threads and followed up with a nylon brush dipped in isopropanol alcohol.

With the studs and threads de-greased, I applied threadlock to each and double-nutted them back on the flanges.

Studs cleaned and refitted

I made sure there were no contaminants (grit or burrs) on the surfaces where the discs would bolt. Now I was ready.

To test run-out I made up a simple rig to hold my dial gauge. I used the template for tool MR.630-52/21 already included in the Manual for this purpose. You can find it in the ‘Special Tools’ pages at the end of the first section of Volume 2 of Manual 814.

Bracket for measuring run-out with a gauge

I bought a length of 4mm steel bar from my local DIY store and bashed it around a bit in a vice. I hade to use an angle grinder to narrow the end where the gauge fitted so that it was reversible and could be used on both sides of the car.

Homemade bracket for run-out gauge

Gauge Fitted

This test was quite time consuming……..Following the instructions from step 23 onwards of Operation DX.330-4 of manual 518, (or Step 9 of Part II "Removal and Refitting a Brake Disc" at Operation 343-4 in section 1 of Volume 2 of manual 814) the discs were fitted to the differential shafts. The discs don’t bolt directly to the differential shafts. Instead they are sandwiched between the drive shaft and the flange of the differential shaft. Fitting them now, I had to use several washers on each stud/ nut to cover for the absence of the flange of the drive shaft. I tightened each disc down but not to the final torque figure.

Relative to the disc, the gauge needs to be fitted to something that doesn’t move. With a disc in place, the dial gauge bracket was bolted to the top hole where the parking brake calipers would eventually fit. I didn’t have a suitable bolt so used one of the caliper bolts – together with a socket as a spacer. The gauge needs to be set up so that it is contact with the outermost edge of the disc radius on the side of the disc that faces the wheel.

Dial gauge in place on the right hand disc.....

I rotated the other differential. Simply for ease of reading, I found the ‘low’ spot and zero-ed the dial. Rotating the disc again I measured the degree of change in the dial gauge. With the help of my son Tom, I made a video to show this process in action:

VIDEO: Testing Disc Brake Run Out (left hand disc)

I had a fair amount of needle judder (due to the gauges sensitivity and the discs roughness), but (as can be seen) the first test consistently gave a disc run out of about 0.22mm (yes – as much as that!). Joking aside, the Citroen specified tolerance is 0.15mm, so that test was a ‘fail’. The remedy is to remove the disc, rotate it 120 degrees clockwise relative to the studs (and so hub flange), refit and test again. That was the time consuming bit…..Allowing for machining tolerances on the mating surfaces of the disc and hub flange, presumably the aim is to see if there is a particular position (of the three possibles) that gives a reading within tolerance. If you find one - fit the disc in that position.

Before moving the disc, I used chalk to mark the ‘low’ and ‘high’ spots on the disk and hub for ‘test 1’. ‘Test 2’ gave the same results…..As did ‘test 3’…... The chalk marks were in broadly the same areas of the disc circumference – i.e. they moved around relative to a particular stud on the hub - indicating that the warp was on the disc itself and not a high spot on a particular part of the hub. That was good news as I didn’t want to have to pull the differential off again and get it machined.

I did the tests several times - so that’s several circuits - just to confirm my conclusions: run out on the disc and in excess of Citroen-specified tolerance. In this situation, the Citroen manual simply says ‘change the disc”, however the specifications also give tolerances for skimming (grinding) the discs – implying that for some problems, rectification is possible.

I went through the same process for the disc from the other side and found that one to be within tolerance – 0.10mm in all three positions. Just for good measure I then did it all over again trying the discs on the opposite sides! I arrived at a point where one disc was 0.10 and so within tolerance, but the other was 0.22mm and so out of tolerance.

Now 0.07mm (out of) out of tolerance didn’t sound a lot to me! But of course that did represent 0.07mm beyond a given figure – and I guess there has to be some kind of limit. I sought advice on various forums as to how critical these margins were in practice. I got some helpful answers but, as you can probably expect, these fell into both the ‘don’t worry’ and ‘do worry’ camps…..

One option for curing disc problems is to fit special abrasive brake pads. These are available from Der Franzose and so probably the other suppliers too.

"Brake Sharpening" Pads

Franzose say that these are/ were used in Citroen workshops though I’ve not found any reference to their use in in the workshop manuals as one of the remedies for brake disc run-out problems.  To use special pads the car needs to be assembled and running. I guess that they are a crude way of fixing problems and it would be difficult to measure the impact on disc run post-use with everything re-assembled.

In the end I sent my two discs of to my local engineer with the brake units for skimming and blasting. Decisions were needed:

Assuming that my out-of-tolerance disc is of uniform thickness and simply slightly warped, then any high/ low spots really need to be removed from both faces of the disc – to maintain uniformity. It would have been tempting to remove only enough to bring the disc back within tolerance – but that would have had the effect of lopping the top half off the warp ‘hump’ creating a flattened warp peak with a rise and fall both sides. Would that create new brake problems? To prevent this, the whole 0.22mm would have to come off. Same on the other side…..that meant reducing the overall disc thickness by 0.44mm. That was starting to sound like a lot….That was almost half the permissible wear on a disc – meaning that I was reducing the life of the disc and hastening the need for the new discs I was trying to avoid!

More decisions: to give unifom braking performance on both wheels, should the other in-tolerance disc be skimmed – just to give it a clean surface like the other disc?

The answers to these questions will have to wait as my engineer couldn't help me: when skimming discs they need to be cut on both sides at the asme time and he didn't have a big enough adaptor to be able to fit the discs to a lathe.