machined or ground

[Tack]

I was checking out the discs and pads on the GPX cause the pads are getting a bit low and close to replacment. I have noticed that the discs have quite a few circumferential grooves in them although they arn't deep.

Anyway, the question is when you guys replace pads do you have your discs machined (as in on a lathe) or ground (as in on a mille)?

I know from racing (cars not bikes) that milling gives a much better finish (new car discs are ground in manufacture dunno about bike discs though) and is much more accurate (much less run out than on disc brake lathe machines although having said that I dunno whether bike shops have much better quality brake lathes then car shops do).

So what you reckon?

[Smitty]

Tack
cupla things here
firstly how deep are the scores or grooves?
secondly how much material is left on the rotor?

Brakes places use a disk brake lathe (which is actually a type of milling machine) which gives a NON circumferential finish..this is very important!
have a look at a new bike in the showroom, the finish is across the disk
(car manufacturers do the same)

bike discs are different from car discs in that you cannot take off 1-2mm when you machine them and you are also dealing (generally) with a harder material (stainless steel) when working with bike discs (not cast iron like a car)

another issue is the ridge you get on the outside of the bike disc..that has to be removed when fitting new pads

re bike shops???
I am off to BKwaka shortly, will ask 'em what they do....

ciao

[Tack]

I haven't measured the disc yet...although to measure "into" the grooves I would need a micrometer (or vernier) with a needle point to get an idea of groove depth. Ofcourse I realise that undersize discs have to be replaced but was wondering whether bikers machine or grind their discs....like grinding is different in that it is done with a large circular stone on a mill bed...if the type of disc or pad requires it they put it on a brake lathe and put a light finish on it.

[Smitty]

Tack
spoke to the guys at BKwaka
their practice is to look at the grooves/score marks
and see whether getting rid of these will bring the disc 'undersize'
if they do end up wanting to fix the grooves, they use a brake grinder to get rid of the grooves. The rotors are ground if bad, otherrwise they do not bother..it costs about $70-$80 to do each rotor
some people will even buy new rotors (although more expensive)

imo
if the grooves are not deep, scratch up the rotors with some emery ..across the disc and fit new pads
and then bed them in....

ciao

[Neka79]

hmm just ride the bloody thing...lol..stick new pads in, and bed em in..most discs will score to a degree.....she'll be right mate...i hope..lol

[idh959]

Didnt someone, somewhere say that bike discs can't be machined??? Someones telling porkies!!!

[zxrarron]

well i was told u couldnt get them machined?