Can I/should I Machine Only One Rotor?

[Abo Bob]

After the last track day I had bad brake shudder. It basically went away after a couple of weeks of light city braking.

I've had the wheels off and one rotor is quite grooved and obviously needs machining. The other side seems fine.

Can I and should I machine just that side or is this bad?

Thanks.

[GTRgeoff]

No problems machining just the one. Hope it sorts out.

[TokyoTaxi]

IMHO, "ALWAYS" do pairs ( fronts or rears ) when it comes to brakes !! Would you overhaul 1 caliper ? Always keep the braking even between the left and the right.

[GTRgeoff]

In my professional Mechanical Engineers opinion and with over 20 years motor racing and mechanical experience there is no point machining something that doesn't need it as the only imbalance that "may" occur is a slight differential on heat absorption capacity due to the small amount of material lost. As always you must maintain knowledge of the state of your configuration and repair/replace accordingly.

Overhauling a single caliper is an issue of "it depends". If it is several years old without ever having overhaul then yes. If it was done a few races ago and just one is causing problems then no.

[Abo Bob]

Cool.

Now I'm curious as to how it happened.

I always bed my pads in properly and I cool the brakes down properly on track days but on this occasion I was coming off the track and another person was also coming off in front of me. As we pulled in to the pit lane the Clerk of Course stopped the guy in front of me to tell him off for something which meant I was stopped behind him.

Thinking this would only take a few seconds I sat there with hot brakes and my foot on them for probably 1 minute.

It appeared to me that this was where the damage was done and the shuddering started but.. why only one rotor? And, why are there grooves in it and not the other? Do you think the pad had an uneven balance of material or something? I believe the grooves were there before the track day to a slight extent but were massive afterwards.

I run Bendix Ultimates street and track. Thinking of getting some Ferodo DS2500 (I think?)for the track and some other street pad that isn't so dusty.

[OFENSV]

just because its not as groved dosnt mean it hasn't got exessive run out causing shudder. Id be doing both if it was my car.

[GTRgeoff]

What occurs with brakes is the driving to aid cooling (air quenching) allows them to retain their homogeneous properties throughout the disc. When you are forced to use your barkes hard and then stop the air can quench the exposed regions and the area covered by the pads cools much slower. This can cause the formation of regions of cementite - a tough slow wearing form of steel which can result in a shudder in the pedal as the other areas wear faster. Often close inspection reveals an outline of the brake pad as a fine line of dark pits on the disc. It can wear out if it is not deep and likewise can be machined out if not too deep but often results in a wrecked disc, even after a really hard stop on the road and then waiting at a light for a few minutes.

The deep grooves would be caused by some hard particles in the pad that have not been evenly distributed during manufacture so causing concentrated wear. While the disc is off place some emery on a flat surface and take the pad back smooth(ish). Bedding is important but remember a few days of city driving can be enough for bedding in too. I always have a couple of laps on the track day to make sure the brakes are warmed and bedded anyway.

If you are at all concerned do both discs at once, but I tend to be able to feel which wheel is a bit funny under brakes so know where to start looking for the problem.

[Roy]

..stating the obvious. But a lot of people mistake pad deposits in the slot as a worn rotor. Technically you actually increase the level of friction if you have pad deposits on the rotor. Grab a small screw driver and run it down the slot. You may find the material in the slot just falls out.

[TokyoTaxi]

In my professional Mechanical Engineers opinion and with over 20 years motor racing and mechanical experience there is no point machining something that doesn't need it as the only imbalance that "may" occur is a slight differential on heat absorption capacity due to the small amount of material lost. As always you must maintain knowledge of the state of your configuration and repair/replace accordingly.

Overhauling a single caliper is an issue of "it depends". If it is several years old without ever having overhaul then yes. If it was done a few races ago and just one is causing problems then no.

Easy there Geoff.......... I did say "IMHO" ( in my humble opinion ). I didn't say you were wrong !!

And my opinion is not exactly an uneducated one either as I've been an active Motor Mechanic for 22 years ( cutting my teeth on Ferrari's, Lambo's, Porsche's etc ) and an Aircraft Engineer for 19. I have also been working with race cars for around 12 years.

As for the overhaul of one caliper thing, we're talking about an average street car ( no offence Abo Bob ) that would not normally receive the same amount of attention as most race cars ( in the brake department ) and therefore my comment.

Simply put, what I said is what I was taught and is what I practice, and again, is my "opinion" so chill !!

[GTRgeoff]

Yeah fair enough Scott. There is a lot of uneducated/unqualified opinion on here and while it is fine from a trade perspective to suggest doing a complete axle at once there is no supporting science to it, even if it is the way you were taught and continued to practice. If Adam or anyone alse is intelligent enough to ask can he do just one without asking why not then I'll respect that and give a yes or no answer. When someone wants to refute with a scientifically unjustifiable claim that then it's time to justify my answer. I also am a mechanic with over 20 years experience but I'm also an Engineer so I only give advice I can support professionally. Mind you if I'm wrong I'll wear that too.

BTW I've never been a fan of Bendix pads, even the ultimates but I do like RB74's from Racebrakes for a street/track pad. I hear good things about the Ferodo pads but haven't tried them. On road rubber on the track I haven't reached the limit of the RB74s during 5-7 lap sessions but with semi's and more power and decent suspension I feel I'm going to find the limit from advice I've received.

[Roy]

LOL...i would personally machine two. At the end of the day when they are worn you are goign to replace them together, so if you machine down a healthy rotor to the same size as your sick rotor it isnt that big a deal.

They are both going in the bin when one of them becomes undersized etc...

...runs and ducks for cover before 60mm artillary rounds hit my bunker

[TokyoTaxi]

...runs and ducks for cover before 60mm artillary rounds hit my bunker

LOL !!!

[GTRgeoff]

Waste of a Javelin missile to use it on a 32 but war makes monsters of us all

[TokyoTaxi]

Waste of a Javelin missile to use it on a 32 but war makes monsters of us all

Gee, I thought the purpose of this particular thread was to discuss opinions on the machining of 1 brake rotor vs. 2 at a given time and not to put down other car enthusiast's pride and joy ( even if in jest )

[GTRgeoff]

Oh so sorry to tread on your feelings there; personal joke between myself and Roy (who knows my background) but feel free to make a martyr of yourself. I considered the discussion finished and the sledging was free to commence judging by previous comments.

Anyway BOT, I'm certain the purpose of the discussion was about saving money and time by only working on what was necessary. So far we have personal thoughts without scientific basis on the matter vs Engineering opinion based on science. The choice now remains up to the reader.

It's probably easy to overlook the fact that one disc is considered fine but throwing a bone here and recalling Scotts earlier correct comment that street cars generally won't get the same attention as a race car there is some value in taking both in for inspection and measurement to determine wear remaining, especially if you are not confident doing this yourself.

Bris you should know better

When are you coming over to swill beers? Got anything on Saturday? I'm moving stuff into the new workshop so I can start pulling down an MX-5 head. Some extra muscle would be good and there's all the food and booze you can consume.