Skimming the disc or skimming the customer

[shadow79]

Generally I see people coming in all determined that while changing their brake pads they want their discs skimmed...need to Hear some change of air here....

Edited May 15, 2017 by shadow79
Typo

[Danny]

Not on every brake pad change you need to skim the disc, may be after 2-3 brake set changed and also depend on how driver braking habit was. If discs are closer to its finishing age of 4mm or less thickness, then better get it changed than skimming 

[Rahimdad]

Nowadays with most high end cars having EBD (Electronic Braking Distribution) the braking on these vehicles are every even and hardly needs the disks to be skimmed.

[treks]

29 minutes ago, Rahimdad said:

Nowadays with most high end cars having EBD (Electronic Braking Distribution) the braking on these vehicles are every even and hardly needs the disks to be skimmed.

EBD only works properly if the pads are in contact with the discs across the full surface area of the pad. Very often, the pads and discs wear at different rates, which means that the disc does not wear evenly. This means that the friction surfaces of the discs are no longer parallel with each other, or that the  sides of the disc may have high and low areas relative to the discs center line. So now if you put new pads on the unevenly-worn discs, the new pads are not in full contact with the discs, which means that the areas of the pad that are in contact with the disc overheat, which in turn overheats some parts of the disc, which finally, can destroy both pad and disc.

How unevenly a disc wears depends on whether the discs is OEM or aftermarket, if the discs were made in China or not, or if the wheel bearings are in good condition or not. It also depends on the quality and formulation of the pads, and the overall condition of the brake system. Driving style has very little to do with how brakes wear.

I do agree however that many garages skim discs when it is not really required, but then again, I would rather have  discs that match the pads than run the risk of having my brakes overheat. 

Edited May 16, 2017 by treks

[Barry]

It's a case of the customer knowing more about cars than a mechanic with many years of experience. The internet is to blame a lot for it. People thinking mechanics are dumb just because their hands are dirty is another part of it. Let's see them take an ECU apart, find a bad solder joint and fix it. A little knowledge can be very dangerous. People are always telling me this part is faulty or that part is faulty. I've learned not to listen and ask what the symptoms are and make my own mind up. However if someone doesn't want to listen and insists I change wrong parts even after being advised, I'll do it and take their money with a smile.

A particularly funny one a couple of weeks ago, I had two guys in. Driver and his friend. There was coolant in the engine oil, oil in the coolant. I knew straight away that the vehicle needed a new radiator, there was a leak in the oil cooler part. The friend proceeded to tell the driver that water in the oil was normal, all oil had some water in it from the manufacturing process. I stood back and listened, laughing in my mind as he justified how it was all possible.

I also had another guy last week who just bought a new car. He told me the brakes were finished and needed changed. I took a quick look through the wheel and told him the pads were brand new, probably less than 1000 kms, I'm honest, I'm not going to rip someone off. No, no he insisted and went to nabooda and bought a full set of brake pads and brought them back to me. I put the car on the lift and pulled the old pads out and showed them to him compared to the ones he just bought but he insisted they were finished. Ok no problem I said, give me 300 dhs and I'll change them.

Back to brake discs, they rarely need skimmed. I can count on one hand the number of times I've done it in U.K. but here, it seems to be every second car 'needs' to have it done. They should be visually inspected. The runout should be checked with a dial gauge. Thickness should be measured with a micrometer or a disc caliper, the minimum thickness is stamped on the disc. Only then can you make a decision as to wether they need skimmed. But UAE, it needs skimmed, send it to the machine shop.

[Gaurav]

Totally agree on this Barry.

I thanks Al Futtaim service advisor, for letting me do my phd on brake disc when I drove my Lexus GS430 for minor service and ended up with brake pad change and then brake disc change as it's less than 8 mm thick and cost 4000 dhs in agency workshop insteda of 600 dhs normal service.

Their service advisor must be trained very well to how to sell a boat to man who come for buying a fishing hook.

[Barry]

I recently went to a job interview for the position of service advisor. I was surprised at how much of the salary was made up from sales commission. If you're making 6k per month you can easily bump it up to 8-9k by selling unnecessary ac regas, engine flush, injector cleaner etc. not to mention the bigger stuff.

[treks]

@Barry, I am sure you will agree that some mechanics have dirty hands precisely because they are dumb.

[Barry]

Yes, there are some guys out there who enjoy rolling in their own dung.