Machining brake discs

[dannyp]

in general drilling disks is a bad idea ( it looks fast ) dut if you drill your bog std vented disk and then give it some serious stick it'll crack, you can radius the holes on the outside edge but not the inner edge, yay cast and a streess riser allways ends well.

ie found that small grooves sometines help but i've allways found the best brakeing performance comes from higher quality matched disks and pads. problem with uprateing brakes i found was as you move up the performance levels in pads there disk trashing propties rapidly increase. thr heat cycels get bigger and faster.

good plain disks, bloody good pads if that dosent work bigger disks

 

[Paul.]

I wouldn't trust a vented cast disc if drilled, I cut some up for a training centre recently and was surprised how brittle they are, cut into the edge with a thin disc as deep as it would go then a tap with a bolster chisel snapped them like a carrot.

 

[hotrodder]

HR, interesting reading, but really who are ATE tring to kid, that article reads like a load of BS. Lateral run out of 30microns, are they on this planet? The drive flange wont be that flat let alone the caliper mount being that square. Think they are bigging themselves up a lot there, we've all changed brake discs on old sheds of cars with rust everywhere and the lateral run out was probably in the order of tenths of millimetres with no adverse effects.

Agree that they're bigging themselves up, i'd be surprised of any marketing that doesn't. Don't agree with the rest though... obviously there'll always be runout at the hub and a running clearance for the bearings. Just looked up the specs for my daily driver, bearing end float 0.01 - 0.02mm, max axial run out measured at the hub flange is 0.03mm (30 microns), radial runout at the hubs centre register = 0.05mm. Mount a disc and measure the run out on that and it'll obviously increase due to the dia of the disc + it's tolerances hence the max run out measured at the disc according to the workshop manual is 0.08mm

If the new disc is p****d it's gonna be a lot harder to keep the total runout within specs unless you prat about clocking the disc one bolt hole at a time to try and get the tolerances to cancel each other out. Obviously most people don't check this, especially mechanics fitters chasing bonuses so if/when the disc runout is excessive it goes unnoticed as nothing happens initally. After a few thousand miles of one spot of the disc constantly rubbing a pad brake judder develops due to DTV which everyone blames on the discs "warping"

 

[Rocketman]

Agree that they're bigging themselves up, i'd be surprised of any marketing that doesn't. Don't agree with the rest though... obviously there'll always be runout at the hub and a running clearance for the bearings. Just looked up the specs for my daily driver, bearing end float 0.01 - 0.02mm, max axial run out measured at the hub flange is 0.03mm (30 microns), radial runout at the hubs centre register = 0.05mm. Mount a disc and measure the run out on that and it'll obviously increase due to the dia of the disc + it's tolerances hence the max run out measured at the disc according to the workshop manual is 0.08mm

If the new disc is p****d it's gonna be a lot harder to keep the total runout within specs unless you prat about clocking the disc one bolt hole at a time to try and get the tolerances to cancel each other out. Obviously most people don't check this, especially mechanics fitters chasing bonuses so if/when the disc runout is excessive it goes unnoticed as nothing happens initally. After a few thousand miles of one spot of the disc constantly rubbing a pad brake judder develops due to DTV which everyone blames on the discs "warping"

I hadn't mentioned previous but this is why the best method is to use a disc grinding machine that attaches to the car - these bits of kit grind the disc while on the hub and in theory give the best match you could do. Not a lot of places have them and it's a bit of a big purchase for the diy mechanic to say the least.

 

[AndyStobbs]

Agree that they're bigging themselves up, i'd be surprised of any marketing that doesn't. Don't agree with the rest though... obviously there'll always be runout at the hub and a running clearance for the bearings. Just looked up the specs for my daily driver, bearing end float 0.01 - 0.02mm, max axial run out measured at the hub flange is 0.03mm (30 microns), radial runout at the hubs centre register = 0.05mm. Mount a disc and measure the run out on that and it'll obviously increase due to the dia of the disc + it's tolerances hence the max run out measured at the disc according to the workshop manual is 0.08mm

If the new disc is p****d it's gonna be a lot harder to keep the total runout within specs unless you prat about clocking the disc one bolt hole at a time to try and get the tolerances to cancel each other out. Obviously most people don't check this, especially mechanics fitters chasing bonuses so if/when the disc runout is excessive it goes unnoticed as nothing happens initally. After a few thousand miles of one spot of the disc constantly rubbing a pad brake judder develops due to DTV which everyone blames on the discs "warping"

Thats all new tolerances, add a bit of rust and you can move the decimal point about a bit. Not trying to be argumentative, just realistic. Not always practical to rotate the disc relative to the drive flange, due to a retaining bolt, so that negates being able to pick the position, and if a new hole was to be drilled for the retaining bolt; well that brings us neatly back to the 'is it wise to drill a disc' question.

I know they are brittle, they're cast and cast is always brittle. Hard to escape the fact that thousands upon thousands of drilled/grooved/dimpled discs are sold in the UK alone, and there isn't a row of cars down every hard shoulder with discs in pieces. I do acknowledge they will crack, and I'm sure some do, but it seems fair to say its more exception than rule...

 

[PhillipM]

You didn't see the huge issues Renault had with cracking grooved discs then?

 

[AndyStobbs]

That's isolated though isn't it. Nissan have a steering wheel problem but it doesn't make every steering wheel under the sun defective.

 

[BrokenBiker]

i work in a factory that makes brakes, during testing we have to run discs with cracks and holes to account for every eventuality. Once you have seen a disc explode at 20k rpm you wont want to drill them. Very nasty if (when) it goes wrong

 

[AndyStobbs]

How many discs have you seen doing 20krpm on a car?

 

[PhillipM]

That's isolated though isn't it. Nissan have a steering wheel problem but it doesn't make every steering wheel under the sun defective.

No, but I've seen a lot of drilled discs with cracks in them.

 

[BrokenBiker]

How many discs have you seen doing 20krpm on a car?

the 20k rpm was an example based on our testing set up. Either way, the wheel will usually be spinning at a higher than the engine, and i wouldn't want to be anywhere near a disc that breaks apart whilst at motorway cruising speed

 

[AndyStobbs]

wheels go slower than engines

 

[BrokenBiker]

wheels go slower than engines

my mistake, but at 70 your wheel will still be spinning at roughly 2500 rpm, if you have to slam the anchors on it could still cause a lot of problems with a weakened or cracked disc due to holes being drilled when they aren't meant to be.

 

[AndyStobbs]

I get it to 954rpm at 70mph with a 215/45/17 wheel/tyre. Not being picky, but that is a much smaller number than 2500rpm. You can destroy anything if you spin it fast enough

 

[BrokenBiker]

. You can destroy anything if you spin it fast enough

you can also destroy a lot of things by drilling holes in them when they aren't designed to have holes drilled in them.

not trying to be funny but brakes are one area i wouldn't want something to go wrong no matter what speed i was travelling.

 

[AndyStobbs]

Equally I'm not being argumentative for the sake of it. I do like to thrash things out based on known facts rather than opinions though. Yes discs break, but how many of those that do break (in a road car application) can be directly attributable to grooving or drilling? Then is that percentage an acceptable loss rate?

Lawnmowerboy, says that they do a lot of disc work, after casting for race teams. Are they smashing loads of discs? Are they speccing/choosing a higher grade alloy? If so, why don't they get the maker to do the machining work - or for that matter cast the disc with the patterns already in it?

 

[MattF]

Only skimmed the thread, so apologies if I've overlooked anything applicable. I think the mechanical point is being pondered on too much. Anything can break under the correct conditions.

With regards brake discs however, pure and simple safety and liability should be the main concerns. If a manufacturer has made something in a certain way, it has usually been tested to be safe within its limits. Also, if it does crack or explode and some accident occurs, they're liable. Do it yourself though, and both the blame would lie with you alone in any accident and likely any insurance would be void. Up the creek without a paddle, so to speak.

 

[Tman]

As no energy can be lost or gained Brake discs work by convert kinetic energy into heat energy, by means of friction between the pads and discs. By grooving and drilling you are reducing the disc to pad contact, which will make for slightly worse braking under normal road use. Drilled and grooved discs are to dissipate the heat quicker to stop brake fade under heavy track use. So if it is for the road you would be better off with standard discs, if for the track would you want to use guestamated parts that may not crack.

 

[AndyStobbs]

I'm only concerned with road use, and as mentioned earlier its more for aesthetics than any performance increase. My discs are already bigger than standard, I can live with a small reduction in contact area.

 

[PhillipM]

Drilled discs were actually for outgassing reasons, grooves the same but with the benefit of deglazing, outgassing hasn't been an issue with pads for decades. All those holes are these days are massive stress concentrators right where the stress and heat is highest.