I work at an independent auto repair shop and we seem to have 2 schools of thought, first thought, Mechanic A, one rotor under spec's, the other rotor on the same axle is machinable, replace only the rotor that is at or under min and resurface the other if within spec's after machining. Thought 2, same situation but Mechanic B says to replace both rotors to keep system performance equal.
Is there any documentation to backup either theory that we can refer to?
Replace both along with the brake pads.
You can probably get away with replacing one, but you replace them in pairs so that you can have brake balance between the two wheels. This is the same reason why you would replace brake pads in pairs. If one wheel brakes more than the other, it can cause your car to "steer" in one direction while hard braking. So just replace both to be safe.
I don't know of any official documentation, I originally started typing a comment but it became more of an answer.
Firstly,
You could create an imbalance in rotational mass across the axle.
For example, you could weigh a resurfaced brake disc that is just about within specification and compare that to a brand new disc.
This difference, no matter how minor, is an imbalance. Of course, there are bigger imbalances, such as fitting odd wheels/tyres to the same axle or in the case of cars without a centered differential (such as most FWD set ups), the drive shafts are different lengths. (See the Honda Civic Type R, I believe it has equal length driveshafts).
Secondly,
One side's caliper piston will be far more extended than the other side. Due to wear and tear, manufacturing defects, dust, and rust, this could mean you will have a braking force imbalance. In a perfect world, this wouldn't be the case, but other factors could influence the angle of incidence of the brake piston against the pad, thus the pad on the disc - creating a braking imbalance.
Thirdly,
You will eventually need to replace both discs at the same time. At some point, the resurfaced disc will come out of specification. In this case, the previous new disc might be close to the limit. Having one brand new disc and one disc close to the limit at all times will mean the caliper piston will never be used in its starting position. Over extending the piston can risk rust and dirt build up if the dust boot is stretched.
Conclusion
I won't get into the cost and time that goes into resurfacing a disc and the inherent risk that the resurface procedure could go wrong as I think its off topic. Bottom line is, wear and tear items should be done in pairs across an axle to avoid any imbalances or unpredictable situations. I once had someone tell me I should replace calipers in pairs too, I don't agree with this one however.
There is no reason to replace discs in pairs, if for some reason one disc has become worn soon after being replaced at an earlier stage, and the other hasn't. This situation happened to me this week - two new rear discs about two to three years ago, one worn beyond limits, worn only on the inner side of the disc, the other as new, why did this happen, because garages that simply fit parts, as opposed to being engineers, are not doing their jobs properly. The cause was most likely that one brake pad had not been replaced at all, or not replaced properly (cleaning the calliper and lubricating the contact points between calliper and brake pad), yet there was no brake binding, so I suspect that on brake pad had simply never been changed, hence the wear rate was so different; was it done deliberately to create new work, or was it just carelessness, I do not know, but the hugely inflated cost of brake repairs when compared to the actual cost of parts and the time it takes to do the work, makes it a real money spinner for garages.
All I actually needed was a new wheel bearing (cost £20), but they do not fit bearings that are pressed into the discs (not being engineers), they replace the disc itself, and only in pairs (cost £400 plus), I accepted that as it would have been the first time the rear discs would have been replaced, it made sense to go ahead and renew them. However, two to three years later I am being told that the same is required again, by a young fitter that was laughing to his colleague about the cost, as he sits behind the counter working out the proposed bill, nudging his colleague to get him to take a look at the bill as he giggles, and mocking the car as he rings around for quotes on parts - unprofessional or what! I didn't interrupt them, I just listened closely.
I challenged them when they told me what was needed, pointing out to them that the discs were not very old, and that wear on the one disc was unusual, and must have been the result of how the previous work by another 'fitters' was carried out; I was expecting the disc to perhaps fail, and had priced a new disc with bearing and ABS ring fitted, and asked if it is always necessary for discs to be replaced in pairs, when the other disc is in an as new condition, and was told by three people that there is no need to replace discs in pairs if one disc is still good, confirming what I already believed to be true.
Under no circumstances would the fitters accept that only one disc could be fitted (and two week old pads be used again also), claiming that it would cause brake imbalance (fitting new brake parts can always cause a degree of brake imbalance whilst brake discs and pads bed in, that is short term, and not a continuing issue)(the garage my cousin uses - engineers, not just fitters, and a small family run business, fitted a single new disc to her car, not seeking to make inflated profits), so I challenged them about the condition of a front disc and pad that amazed me when I replaced them myself a month ago - the disc was very badly scored (from before I owned the car), and the brake pad was broke in half length ways, so only 50% material in contact with the disc, and the same garage passed that for two years, which as I said amazed me - only 50% of the material contacting one side of the disc, and very badly scored on the other side of the disc, if anything was going to cause imbalance you would have thought that would have, yet it passed two MOTs. I asked them why, and they said the tester probably couldn't see it, to which I replied that I could see it - they said he can't be expected to notice everything, to which I replied that I hope he notices all the important things. So then, one new disc, and two week old pads causing brake imbalance, fade or failure, there is no reason why it should at all; drive your car accordingly whenever any brake parts that need to bed in have been replaced, and the brake efficiency figures for all will be just fine.
I try to do as much work as I can myself on my car, but time is always an issue, but this last week made me think that I would be much better off making time to do the work, and purchasing any tools I may need to do so, happy to know that I have cleaned and lubricated callipers properly so they can work as they should, and preserving the longevity of the parts fitted, and knowing it is all safe (that broken brake pad could surely have failed at any time, yet it didn't concern the tester - odd!).
I have fitted a new rear disc, and re-used the two week old brake pads, which were good (obviously), removing any raised edge, and booked another MOT with a different garage, one with a very good reputation that I used before I moved away from their area. The first MOT tester said I had a leaking power-steering reservoir, however I do not, it was just overfilled a little, yet they wanted to fit a new one - I have removed the excess oil and all is well.
Try and use a garage that are actually engineers, and not just fitters, they will help to keep your bills down, by replacing only what is actually needed (if they are a good one).
David.
