Shimming brake discs

[Gfhug]

Fitting the brake calipers to the diff and I need 2 or 3 0.020" rear camber shims between the diff output flange and the disc to get the disc central in the caliper.

1. Is this acceptable?
2. How central must the disc be?

The diff is a 3.07 from an early S1 circa 1966 but is going into my S2. The calipers are XJ versions, so slightly later than the S2 caliper. When these calipers came off my S2 3.54 diff there were no shims between the discs and the output flanges, but 3 or 4 outboard between the brake discs and the half-shafts. Edited to add: the discs are standard later versions C26779 as fitted to all the later cars

Thanks for any comments or advice

Geoff

[JagWaugh]

Geoff,
IIRC 0.004 is the smallest shim in the original parts manual, so they didn't expect it to be any closer tolerance than that. What _is_ important is making sure that the pistons and discs are parallel - depending on who has blasted, plated, painted, filed and/or hammered what, the c/l of the pistons may no longer be normal to the disc c/l.

Within reason (say 0.040), not having the calipers centralised will not affect road braking, except that you will have to change the pads a bit earlier. The handbrake pads are probably the reason that they even mention it, being off center might affect the self adjuster mech, AND the Handbrake pads are pivoted, so you only have the geometry to wear about half the pad material away before the backing is scratching the disc. Parking, like idling, isn't what the car was built for anyways.

Andrew

[abowie]

Fitting the brake calipers to the diff and I need 2 or 3 0.020" rear camber shims between the diff output flange and the disc to get the disc central in the caliper.

Geoff

Ideally you want to shim the calipers, not use the camber shims to move the disc out.
While doing this is not a problem from a mechanical or safety point of view if you need to take those shims out to set camber you are stuffed. You can't get them out from behind the disc without stripping the IRS because you can't get the disc out.
Better to leave all your shims on the outside of the disc behind the half shaft flange. I have used about 2mm/80 thou of shims on the caliper bolts on one of my IRS to get the discs centred in the caliper; I never did work out why... It came that way.
The caliper centering isn't IMO critical; you just need them close enough so that you get even pad wear.

[PeterCrespin]

It's late and I may be talking out of my fundament, but...

I can't see how caliper/disc alignment has any effect on brake pad wear (not handbrake pad) in opposed-piston calipers. Pad wear is a function of friction, which is a function of pressure, area and surface finishes. None of these would alter with a parallel caliper displaced sideways by any amount.

The main reason for caliper centering AFAIK is to do with piston displacement/protrusion as pads wear. In theory, depending on seal arrangements and construction, it is possible for the piston(s) on the 'wide side' of an off-centre caliper to come too far out of the bore when the pad wears right down. In practice, the dimensions of parts don't allow this because of the catastrophic effect it could have if a pad wore down to the backing plate and a piston went out past its seal, but with a reasonably-centred caliper both pistons protrude the same amount and pads are easy to change by pushing them back in.

[JagWaugh]

Geoff,
Sorry, I misread your post, I thought you were talking about the shims on the caliper mounts. Normally, you would have no shims between diff and disc, they are all _supposed_ to be on the outside of the disc.

Is this just on one side? Double check that your diff mesh and backlash are correct. The pinion depth setting will shift the ring gear left and right, which is then corrected via the output shaft shims. If that isn't the case, check that you have the correct caliper brackets and that they are full up on the mating surfaces. In some combinations the later XJ calipers foul on the the cast in web on the diff casing nose, and need a bit ground off the area between the mounting lugs.
Also check that your new disks are correct. Do they have the same dimensions as the originals (hat depth in particular - don't forget to correct your measured values by half the wear loss on the old disc surface)
Finally, if everything else is dead right, it might just be a combination of tolerances and dimensional changes from swapping to the later calipers. Rule all the other possibilities out first though.

Pete,
I agree. What I meant about changing the pads earlier was that the pistons will be asymmetrically extended with the caliper uncentered. Given the design of the pistons seal, at some point of extension there isn't going to be enough piston left _behind_ the seal to stop it from canting. The pads aren't guided precisely at all (otherwise they would bind on the gude pins) so there is a turning moment on the piston. This concern is almost certainly purely academic - I've never seen a tilted piston in a lockheed caliper, except when someone removed a pad and allowed the piston to extend too far while doing some "Brake Wellness".

Like I said, I think the real purpose of the shims is to make sure that the C/L of the piston is 90 degrees to the disc surface. Either that or the whole thing is the vestigial remains of some fitting issue in the early days of disc brakes. At any rate, it it were a hyper critical tolerance, Jaguar would have stated it in numbers somewhere... IIRC the E, S and 420 Manuals all just say "replace the shims in the original locations" or words to that effect.

Andrew

[Gfhug]

Gentlemen, thank you for the replies.
I need 2 x 020" camber shims on the RH and 3 on the LH to centre the discs to the calipers. Yes, discs are same as old ones.

The diff has not been overhauled or anything else as it is a lowish mileage one in good condition. But as it is an early diff it doesn't accept the later calipers without the calipers being modified to fit. The mounting brackets on the calipers are thicker, one on each needing machining to fit. They have been a b****r to fit.

As I plan to check camber on the bench I may accept the shimming done so far and hope it won't be too far out to allow for shimming outboard of the discs. Prayer being said as I type.

Regards

[abowie]

It's late and I may be talking out of my fundament, but...

I can't see how caliper/disc alignment has any effect on brake pad wear

Yes, reading that you're probably quite right. Another mechanic's wive's tale.

[PeterCrespin]

I think Herr Waugh had it right about 'piston cant' (as the drunken Scotsman said when refusing his amorous wife).

The pad isn't located by the guide pins, other than dropping out. In all opposed-piston calipers I can recall seeing, the pad is restrained by a reasonably close (and binding-vulnerable) fit at the sides in the caliper casting. There's enough slack to make a 'click' sometimes on load reversal such as initial braking during forward/reversing manoeuvres but once that very slight clearance is taken up the braking load is 100% fed into the caliper/mounts/diff or upright. This applies even when pads are located (as on many moderns) by a spring clip into the inverted bucket-type piston or on some old-school designs like the S1 Dunlops where there's a retractor system. The old 'round pad' systems like the C & D type puck pads and pioneering CB750 Honda swing caliper were a bit different but still used the caliper structure to take all the braking load, rather than put side thrust or twisting loads into the piston beyond the first bit of fretting of the backing plate across the piston contact face.

Something sounds wrong with the OP setup though if caliper mounting lug thickness is an issue. Normally that would only affect fastener length not caliper fit if tge datum surface is the side of the lug nearest the diff casing. On the other hand, 1 mm misalignment is not an issue IMO, all other things being good. Even though the intention is to set things up on the bench I'd be strongly inclined to omit the inboard side shims because starting off with a bit too much negative camber is way better than having slight positive. The limitation on handling is usually not being able to dial in enough negative for performance work on modern tyres.

[PhilBell]

I'm reassembling the IRS after a diff rebuild and have found that that the brake discs are no longer centralised in the rear brake callipers. Given that I refitted the same discs, callipers and spacer shims, this is baffling, especially as this wasn't a problem during the two previous IRS rebuilds. And when I say the discs aren't centralised, they are both catching the calliper on their outer sides. Removing the discs and recleaning all disc and flange mating surfaces, then clamping the discs in position using the halfshafts makes no difference.

The only unknown is the diff rebuild. Maybe the rebuilder used different output flanges (unlikely, and neither looks new) or a different number of shims where the bearing housings mount on the diff casing. I'll give him a shout on Monday to find out.

If this is the case, fitting thicker/more calliper mounting shims is the answer, but I want to clear of the cause before I start trying to effect a cure.

Any thoughts chaps?

[christopher storey]

It's not the caliper /pads canting which is the potential problem, but the disc canting under cornering forces . This was , interestingly, a problem referred to in the Motor road test of the Mark IX in 1958, where the disc canting would push the pads on one side of the caliper back in further than was necessary , leading to a long pedal when the brakes were next applied . Presumably it was to prevent this sort of problem that, over the years , designed wheel bearing clearance has gradually been eliminated