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Tuning front discs while on the LBC

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Winston

Jedi Trainee
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I stopped in to say hi to my everyday drivers mechnic,
(he does let me use the lifts on friday afternoons),
and we were discussing the turning of the front discs(67TR4a). He said the discs can be turned while still on the car with proper machine. I asked who he would reccomend as I threw names of shops at him and he really could not give me the name of a local shop that does this.
He's probably the best mechanic in town, but the Triumph scares him to death!
Can the discs be turned while still on the car?
It sure seems that would mean less headache for me while I'm redoing the front brakes.
Thanks.
 
I've never heard of that, usually a brake lathe is bench mounted.
Its so easy to take them off I don't think I would take a lot of time looking for a place that can do them on the car.
Simon.
 
Yes, turning while on the car takes care of any wobble there might be in the hub. However, and as an ex factory tech trainer, I always have to ask why you are going to turn them? Measure the run out - wobble, as it turns. The spec is probably .002" or so. Measure the thickness across six or so spots. Variation again about .002". If that's ok and there are not real bad grooves, then there is no reason to turn them. Even with grooves, all you're going to do is increase performance (because surface area is greater) but decrease pad life. The only other considerations are if the overall thickness is enough, and if there is a bad glaze. If too thin, you'll replace anyway. If there is a glaze, then use some light sand paper or crocus cloth to clean it up. I know that everytime you get a brake job in a shop, the discs are turned, but that's a function of labor hour generation. I also know that many people will argue with me, but I've yet to hear a legitimate reason to turn an otherwise good disc.
 
I agree with Paul on this one. In 30 years of looking after disc brakes on all my cars, I've never turned a disc that was within acceptable runout specs, and never had a problem. Most shops will turn your discs every time they change pads. They'll tell you its necessary, but my feeling is that its a profit centre for them and they get to sell far more discs that way. My discs last longer than most people's because I don't keep machining them away unnecessarily.
 
I tend to agree with the above statements. Turning is only necessary if there is a pulsation or bad glaze. It will help bed in new pads and reduce the chance of noisy brakes. But it is not an absoloute must with every brake job.
Here's My take on "on-car" vs. "off-car" turning.
Both types, if performed properly, will give the same result. The On-car brake lathe was designed as a time saving device. You can turn a hubed rotor much quicker if you don't have to take it off the car, remove the bearings, clean out the hub, mount it on the lathe, turn it, repack the bearings, and re-adjust the bearings on the car. Oh and still charge the same as if you did.
But you have now removed the bearing service and this really should be done. When was the last time you did that?When are you gonna do it otherwise? You're halfway there already. It's well worth the extra time to service the bearings and just have the rotors turned "off-car"
P.S. the rules change a little when you have a modern car with non-servicable bearings.
 
I turn rotors often at work. I only use an on-car lathe. It actually isn't faster because lathe can often "fight" you adjusting itself to the hub. However, it gives a much better cut than off the car. I have never had one brake pulsation complaint come back after using the on-car. We always resurface the rotors with brake service to prevent "noise" complaints. If its warranty, Chrysler requires us to resurface with the on-car.

As far as my cars go, I just replace the rotors. For what I can find them for and without the labor charge, its easier. The European brands (BMW,SAAB, VW) do not allow cutting of their rotors, only replacement. If you have to cut them, use an on car for a much better cut. I have worked at three car dealerships, and two of them had on car lathes. I have never hooked one up to anything as old as a Triumph though. THey come with a full set of adapters so it should work, as long as the head fits in the wheel well.
 
Can also depend on the car -- with TR3A rotors selling for $230 per pair that's pretty expensive metal to be lathing unless it really needs it.
 
<font color="purple"> >>>> Have you considered shimming the disk to true it? </font>

Huh???
 
Vettedog,
That's probably the route I'll take, the original shims on the car are ok but..........
I wonder though if I do go this route should I also use anti-sqeal material on the surface.
 
[ QUOTE ]
Huh???

[/ QUOTE ]

Me too.

There are shims to center the caliper and anti-rattle shims behind the pads but how would you shim to true the disk?

Of course I refer to TR3-4, perhaps TR6 differs in this regard.

BTW - I routinely use the anti-squeal blue soup behind all pads.
 
I'd not heard of "on the hub" turning, either, but it seems that it would take care of those cases where some slight amount of corrosion or whatever prevented the disc from matching up true with the mount. I ALWAYS take a wire wheel to both mounting surfaces, and thusfar have prevented any such problems. Others may not. The downside would be that the bearings may not be greased and adjusted as they should - So I would think "on the hub" turning should only be performed after the bearings are taken care of and readjusted.
 
[ QUOTE ]
[ QUOTE ]
Huh???

[/ QUOTE ]

Me too.

There are shims to center the caliper and anti-rattle shims behind the pads but how would you shim to true the disk?

Of course I refer to TR3-4, perhaps TR6 differs in this regard.



[/ QUOTE ]
Me three.
I've been an auto technician for many years, and I've never heard of truing a rotor with shims (a TR6 is no differen't than a Tr3a-4). Even if you could it woulden't fix any thickness variations that would cause the heaviest pulsations.
 
Well, let me be undiplomatic then and jump into the deep end. There is no such thing as shimming the disc to take care of run out, warp or thickness variation. Perhaps we can get a clarification on the topic.
 
Amen brother! /ubbthreads/images/graemlins/iagree.gif
 
Whoa guy's,
Let's not start a fuss over my stupid question. I realize that shims won't true things up, I just figured since they came out with the pads they should go back in with the new pads.
Honestly my concern is as always safety. I realize I should replace bearings, maybe put on cross drilled rotors and really do things the way I would like, only problem is $'s.
This has been a very revealing topic or so it seems and as always I am very very grateful, but let's not fuss.
I think the variance once checked will be within limits, the original PO raced 30 years ago and the second PO only had it 3 yrs. (covered in his garage) and aside from a couple of odds and ends he had done himself there was not time for him to mess things up to much.I'm pretty sure I'm going to be good to go.
One question.........Blue Soup?
Thanks again all.
 
Paul,
I had never thought much about shimming the rotor to true run out until I worked on a few C3 Vetts and found it is not uncommon as the hubs were not machined true when originally made. At the factory, after the disk was attached to the hub, the rotor was turned on the hub to true the rotor run out. That’s the way GM did the C3 Vetts, 1963-70 something,; don't know when they found a better way that considered field repairs. I, after many years of being an auto enthusiast, did not know until 3 years ago, there were machines that could perform the rotor truing while the rotor was ON the car, but there they are! A real good way to get the C3 trued (all of this is exclusive for the C3 REAR hubs).

Concerning the problem expressed with this TR6, I understood the rotors were placed on a brake lathe on two different occasions and were, without question, true from that plane. Also, I understand the hubs were new replacements. If there are varying thicknesses in the rotor, but I understood were resolved by two cuttings on a fixed bench top rotor/brake lathe, shimming would do nothing to relieve teh problem. However with confirmation that the rotors were true the shimming should be between the rotor and the hub (that would be my specific recommendation); apparently the facing on the new hub is not machined true where the rotor bolts up.

Hope that sheds a little light on my suggestion and can enlighten the a-men choir as well : )
 
Ok,Ok I never ment to attack anyones idea. I was only agreeing with Paul Johnson,and he was only agreeing with us.
I understand where vettedog72 is talking about placing the shims (between the rotor and hub), but that would be a very unusual solution, and Ive never heard of it being done. If a rotor/hub combo has enough runout to require shims between them then somthing is wrong. Any runout should be able to be taken care of by turning them On or off car.
I believe Winston is referring to the shims between the pads and the caliper piston. These are common, and yes you should put them back in. They help stop squealing and reduce heat transfer(blue soup is a slang term for anti squeal compound that you would apply to the back of the pads with the shims). And the shims between the caliper and the mounting brackets are needed to align the caliper to the rotor.
I think our "choir" started over a misunderstanding of which shims we were talking about.
Thanks for the hand on the shoulder. in the future I will read more carefully and be slower to take sides.
Again sorry to anyone offended.
All this really proves is that we have intelligent, thinking people that contribute here. A lot of other forums would just end up in the blind following of any opinion posted.
I do herby withdraw to the sidelines of this discussion /ubbthreads/images/graemlins/grin.gif
 
Same story; my post was worded harshly, and I'm not trying to discredit anyone. Still though, if I follow this corrrectly, shims are placed under the disc, but on only one or two of the stud areas? Pretty peculiar. The hub itself carries a good bit of weight, and if it is not true, will give feedback through the steering wheel. Oh well, I see what you guys are talking about, which was what I was asking for.
 
PLease tell us if you do have the rotors turned while on the car. I would be interested to know if the lathe works on an LBC. My question though, are these rotors 30 years old?? I would replace any rotor of that age if new ones are avaible.
 
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