Brake disc machining
If you find that your vehicle has a steering wheel shudder or a ‘pulsating’ brake pedal when slowing down from around 80-90km/hr, more than likely your brake discs or rotors will be warped or not running true. Before you take your vehicle to a workshop to have them machined there is a few things you should know.
Brake discs wear down over time, and for this reason each disc has a minimum thickness measurement. Once the disc gets below this measurement it is considered unsafe and must be replaced. Often when the brake pads have been worn down to their metal backing plate and worn into the disc, the disc will need to be machined to below it’s minimum thickness to remove the damage done by the pads.
But everyone checks their brakes every 10,000km so that will never happen, right?!! If the area where the brake pad contacts the disc is lower than the rest of the disc and there is a noticeable groove where the pad doesn’t run it may need machining beyond it’s minimum thickness as well.
The correct method of measuring the disc thickness is with a micrometer as pictured. The outside edge of where the brake pad runs will be the thinest and unless the discs have absolutely no groove in them straight edge calipers will give a false reading. If you don’t have access to the right tools it’s best left to your mechanic.
If you do have the right tools the next thing to do is find the minimum thickness for your discs. Most of the time it is stamped on the disc itself, although it may take some hard yakka with a wire brush to find it! Tune-up manuals or workshop manuals will have the information if you have one for your vehicle. Failing that a call to your local brake specialist should help.
Ok so you have found that your discs are undersize or your mechanic has told you so, the next step will be determined by the type of wheel hub that your vehicle runs. Most late model cars have a seperate hub/disc rotor. These are easy, remove the brake caliper bracket bolts and remove the brake caliper away from the disc and secure it to prevent stress on the rubber brake hose. The brake disc will now be able to be removed.
If you have a stubborn one a soft faced hammer will help here. If you are unlucky to have a wheel hub and disc all in one this is a bit more complicated. Remove the brake caliper as above and then remove the grease cap from the middle of the disc/hub. There you will be confronted with a split pin, a large nut, a washer and the outer bearing race.
Once you remove these the disc will be free to be taken off. Inside the rear of the hub will be another bearing race and a seal. This bearing race and seal will have to be removed by using a soft punch and hammer and tapping the bearing out from front to back. The seal will be pushed out by the bearing although it will most probably be damaged in the process.
The next thing to do is check whether your new disc/hub comes with the bearing cones already installed. If you are not replacing the bearings and seal at this stage (which is highly recommended) then you will need to remove the old bearing cones with a punch and hammer. Before any old bearings are refitted they will need to be cleaned thoroughly and repacked with grease. New bearings will also need to be greased so check out my Repacking wheel bearings post.
If you are having this work done by a mechanic the labour and parts prices for each hub setup will be vastly different. Heaps more work and parts needed in the old style hub/disc in one arrangement!
Nowdays there are plenty of workshops equipped with ‘on-car’ brake disc lathes. In my opinion this is the way to go as the disc is machined without removing them (saving time) and there is little chance for foreign material getting between the disc and hub which has the potential to cause disc run-out even though they have just been machined true. Here in Brisbane we have many guys running around doing mobile on car machining so check in your area as this may be more convenient for you.
In summary, be prepared for extra labour charges if you have the ‘old’ setup and take your mechanics advice if he thinks the wheel bearings should be replaced. Look for on car machining if available, saves time and money and be prepared for regular disc machining if you own a late model vehicle. The discs that they are putting on cars these days are nowhere near the quality or original thickness of say, pre 1990 vehicles.
Some may argue this point but too many times I have heard complaints about vibrations or shudders in late model vehicles all coming down to discs with excessive run-out. When we had our workshop we had a car in that had the discs machined three times in 75,000, twice by the dealership and once by us. The owner went back to the dealership to try and get a explanation and this is what they said; “Try not to drive through water as the discs are hot from braking and the water is cooling them down too quickly causing them to warp!” True story! Imagine that,”Honey I’m stuck at the train station and need a lift home” “Sorry darling but you know what the service advisor told us about driving in the rain, can you wait until the roads are dry!”. I’ve never heard a bigger load of bs in my life.
All I can tell you is the brand was bailed out by General Motors and started with a ‘D’ and ended with a couple of ‘O’s. Better not say any more on that subject!