First off, thanks to everyone who helped out with my caliper shims question, I'm doing much better with those now.

Just as I thought I was finished with my brake job, however, I've hit a bit of a stumbling block measuring runout.

First try, at the edge of the disc, I got 10 thou. I have since removed the disc, re-cleaned the mating surfaces with the hub (scraping gently with a sharp chisel also), and re-installed the disc rotated the other way. Still 10 thou, and the runout occurs at the same place relative to the hub, not the disc.

I also measure about 3 thou at the wheel mating surface of the hub, although this is not a great surface to get a read from.

So, I'm pretty sure the disc, and its attachment to the hub, is fine. I now suspect the cause is runout in the hub itself. So...

(1) Has anyone else encountered this? Do I just shim the disc, or try and get the hub corrected at a machine shop?

(2) Could the wheel bearings themselves be the cause of the runout? They appear to be in good shape, and I would have thought worn bearings would create wobble, but not the runout issue I'm seeing.

(3) Could it be the stub axle? Again, it does not appear in bad shape, I can see where the bearings were located, but cannot really discern an edge where they were riding (maybe, just, with a fingernail).

Surprisingly, I've not encountered much on the web. Would have thought this was a more common problem, given brakes are a pretty standard DIY job.

Thanks in advance,

cf

 

Bugga.

I have never encountered that in the many rotors I have replaced.

After you have had a drink or 3, remove the rotor again, I knowm and measure the actual mating face of the hub, where the rotor sits.

Also, when I do bother to meaure the runout on a problem car, I ALWAYS nip the wheel bearuings UP a tad, so as there is zero play, and then reset them after my doubt is satisfied.

These cars have a SERIOUS issue with wear of the spindle, mainly where the inner wheel bearing sits.

 

Thanks for the quick reply Grant.

I did tighten up the bearings a little in case, but got the same measurement. Not sure if I can get my dial indicator on the hub face itself while installed, but I'll give it a try.

I'm thinking about just shimming the disc, apparently the classic Corvette guys do this a bunch (with beer cans, no less -- maybe I can qualify for the "bodge thread!) My worry with that is that the wheel mounting surface will also be out, and I will never get a good alignment. I'd rather fix things properly.

How worn is too worn for the spindle? As I said, I can see a burnishing where the bearing sits, and with my fingernail I can just detect a lip at the top rear, and the bottom front, of the wear area. With my fingertip, it feels flat, so I figured the spindles were probably good until the next brake job.

I've been ringing around machine shops too. A new hub is not the cheapest thing, so I figure a new bearing and finding someone to true up the faces might be the way to go.

-cf

 

I really do not advise shimming, who wants a bodge on the front brakes in a hard stop from 130MPH?
I doubt the hub itself is out of true. Always possible of course, but it would have to have been out of spec from manufacture.
A new stub axle and bearing is not that expensive (say 50 USD should do it) and that would be the first thing I would do. Stub axles do bend.
A second hand hub should be pretty cheap also, every single HE onwards XJS has the same one, and they are not handed. Breakers must have loads.
Greg

 

Agree with Greg DO NOT shim.

10thou seems a lot and I would suspect you would have noticed this amount in the steering, not to mention the pad knockoff it would cause.

I had 1mm or 0.040" at the rim edge and this was noticeable as like bad wheel balance in the steering (bent rim, dam potholes)

How are you measuring the runout, where and how are you mounting the dial indicator.

 

Re: Stub axles

I would add that with my original worn stub axles the amount of play in my front hubs/bearings with wheels installed made an audible clicking noise when the suspension moved. When I removed the original stub axles I could no discern no visible ridge and could barely, BARELY feel any ridge with my fingernail but the noise went away with new stub axles and so that wear, however slight, appeared to be "enough."

 

The Corvette guys shim rotors on '65-82 Corvettes for unique and very particular reasons, none of which I believe apply here.

Those cars had rotors permanently attached to the hub/spindle with the friction surfaces being machined only after they were mated as an assembly. This resulted in a assembly absolutely free of run-out, but also meant that the rotor could not be replaced in the field without redoing the finish machining operation. No corner garage or dealership had the required specialized machinery, as a result GM never sold rotors as individual components, only as finished hub or spindle assembly.

The aftermarket introduced rotors sold as separate components and owners soon found themselves finding creative ways of dealing with varying amounts of run out

 

CF:


Nice job of presenting the issue. Many, no most, just put them back together, sans any use of a dial indicator. Works, most of the time. More than a bit of lateral run out will result in an annoying, yet relatively harmless pulsating pedal on application. later and smaller cars with less husky hubs and rotors suffer that malady. Why ? distortion from over torque by 'souped up" impact wrenches. No fix, other that new parts.


So, as I see it, .003 at the hub may not result in any difference in pedal and be harmless.


But, .00010 is a total, hub plus rotor run out. I might look at the mating face of the rotor for irregularities that would prevent it from seating on the hub.


The real test might not be within means. Securing each on a shaft that is perfectly level between "v" blocks and applying a known perfect dial indicator to the face of each component.


Oh, one more. Remove the inner bearings from the hubs. Apply to the stub shaft. Anything less than a firm seating will not work. No spin or wobble is OK!!


If you convict hub or rotor or both, all is not lost. Get good used parts from David Boger aka Everydayxj. Good guy and posts here.


Carl

 

Before you get to deep into hub and stub axle, are you sure you don't have a warped rotor?

It isn't unheard of that a rotor is defective from the factory. A couple of years ago on another vehicle, I had to return a pair of brand new rotors that were out of spec from the factory.

 

I've been thinking.

To check flatness of disk a sheet of glass (or even your kitchen benchtop) is flat enough to show a disk warped to this extent.

Put the old rotor back on and check runout.

0.003" at the hub will measure about 0.010" at the disk. When you measure with a DI I would mount the DI on a heavy metal plate on the ground, mounting the DI on the car I found it rocks too much. I have a thick steel plate 10x100x300mm, this is heavy enough not to move. Also pick the DI gauge head off the surface as you rotate the hub.

 

Ahhhhh. I always wondering why the rotors were riveted in place.



How prevalent/severe has the problem been? I've replaced a number of Corvette rotors but..so far...the cars have not been driven enough to present any problems.

What am I likely in for?

Cheers
DD

 

Sorry if its a silly question, but did you turn the rotor as you tightening it up? Maybe its jammed onto the wheel bearings in a wonky way.

You could always pull your other rotor (fun) and place it on this problem side to eliminate the rotor as the issue, as well as swap out bearings to see if they are the cause.

 

Apologies in advance for the off topic hijack.

I think you've got about a 1 in 8ish (?) chance that a replacement rotor will cause a problem due to runout. While the new rotor may be absolutely true and flat, the hub or spindle it attaches to may not be. GM was not all that careful with getting that particular surface perpendicular to the hub or spindle axis as they knew that the rotor friction surface would be finish machined to spec after mating and riveting. Again, there was no intent by GM to ever replace rotors in the field on their own.

Most field problems with rotor runout appearing first as a low brake pedal after a long run on the highway and with troubleshooting, air will be found most commonly in a rear caliper. A quick fix is too bleed the caliper but obviously the problem will reappear.

The reason the issue appears more frequently on the rear vs. front is the likely hood of greater rear bearing clearance (.008" vs. 0015") even if set to factory tolerance.

This greater tolerance, combined with the multi piston fixed brake caliper design, means that the rotor wandering laterally back and forth inside the caliper, will tend to knock the pads and pistons back into the calipers. This in itself will give a low pedal under initial application but in severe cases air will be drawn past the piston lip seal into the caliper housing making the low pedal a permanent and dangerous situation.

I was once of a 5K mile round trip road trip to a national convention where a fellow participant had to bleed his rear brakes at every lunch and dinner break. We took pity and organized a F1 style pit stop crew so that he was not late for every meal.

The fellow that restored his car was unaware of the issue whne replacing the rotors.

The fix is to measure total lateral runout after minimizing wheel bearing play and limit it to something under about .004" preferably by machining but shimming works in a pinch as discussed above.

We now return you to your regularly scheduled Jag Tech Talk

 

Just before we return our viewers, another correspondent has joined the discussion:
When disc brakes were developed for cars, in the early 1950s in the UK, they had been first used mainly in racing. When fitted to road cars, the phenomenon of "pad knock back" occurred, which as Mikey indicated, results in the pedal going to the floor. This happened because the pad had been pushed back into the calliper further than one master cylinder-full could push it back against the disc. Repeated pumping would restore braking. This phenomenon had not been apparent in racing as the brakes are used frequently and not too much knock back had occurred between corners.
It was overcome by tightening the bearing tolerances (those in live rear axles were particularly a problem) so that the disc could not move in and out against the pads to the extent it formerly did.
Interestingly, the Alvis TD21 was the first British road car to be fitted with disc brakes all round as standard, before this became standard on Jaguar models.
Greg

 

Good one Greg and Mikey.

And since I broke the mould a loooooong time ago, and the flame suit is NEW and fitting nice and snug.

I have always set "tapered roller" bearings to zero play when COLD. My early education, yes I did get one in the sticks, was that "taper roller" bearings are designed to operate in a "loaded" condition. If they were allowed to operate with"play" as it is called, then the rollers run the risk of being hammered, and get destroyed.

Road going wheel bearings are mostly "tapered roller" and they do get "hammered" violently.

I have never had a wheel bearing failure with that setting procedure, and that includes the Road Train, where the wheel bearings were at least 8" in diameter, and the payload on the thing was 120Ton, and Aussie dirt roads with corrugations, so a good hammering being delivered.

Bolting those rotors to the hubs on the XJ cars was always testy at best, and mating surface cleanliness was a priority, and then tightening the bolts in a "cross hatch" pattern, exactly as I do the wheel nuts. I have never had a pulsating pedal on any of them, and, YES, I did the runout test in the very early days, and then dropped it, as I never found an issue.

BUT

Rotors are now mass produced, and brands dont mean much anymore, and if you dig deep enough, they are possibly all Made in China, and thats enough on that subject.

I have an Iffy one on the Black car, that I fitted on Sunday, and I rang the supplier this mornig, and 2 new ones are being delivered today. Easy fitment compared to the XJ cars, but sometimes things go bang in the night.

 

Hi Grant

Nice info on tapered roller bearings.

One caveat about setting clearances to zero- this works well in situations where there is minimal temperature rise from ambient to working temp (like front wheels bearings in cars) as the clearance is not likely to change materially from it's initial setting. I have experience from my working career where the design guys got things very wrong with their initial calcs and bearings were letting go after just a few hours on the dyno. Turns out the housing the bearing assembly was mounted in expanded far more than anticipated which was reducing clearance while at operating temp to a substantial negative number.

The intermediate fix was to increase the cold clearance to compensate, but that lead to unacceptable vibration during warm up. The final fix was to modify both the housing and rotating assembly configuration so that their respective expansion rates were more similar both at ambient and running temps. This still required that the bearing be set at .005" cold clearance to avoid going negative once warmed up.

I believe this effect was taken into consideration with the original design of the Corvettes discussed above, hence the greater rear wheel bearing clearance vs. front wheel despite being very similar in appearance at first glance.

Hijacking again with Corvette rotors and possibly for Doug's benefit, I was just reading on another discussion board about an owner separating the original rotors from the hub/spindles to have them turned on a lathe to remove surface rust. Big mistake.

After re-installation the car lost it's brakes after just a few miles. Measurement of run out indicated that three of the rotors had around 0.025" max. which is well within spec, but the fourth had 0.014". The associated caliper had pumped itself full of air leading to loss of braking ability.

The owner had carefully reinstalled the correct rotor on the original hub/spindle and aligned the stud with the correct hole. Like many owners he was oblivious to the harm caused by having the rotor machined separately. His repair options are now being beaten to death between the practical group and the AR/OC never be too cautious zealots.

 

I've often wondered why tapered bearings are treated (that is, adjusted) differently in different applications and assumed that heat/cold expansion/contraction entered into the decisions but a lot of the rationale is above my pay grade.

The older Jag IRS, with inboard brakes, specifies a bit of end-float (.001-.003 as I recall) for the rear hub bearings. Later (such as my X300/XJR) outboard brake models....with a very similar hub design....call for .001-.003" pre-load. Yet (and I might be mis-remembering here) the XJ40 and X308 cars (again, same basic design) call for end-float.

One wonders about heat transfer from brake rotors to hubs and bearings....but is it enough to be of consequence? I guess not? Maybe only under the most severe circumstances?

Anyhow.....

As to front wheel bearings I long ago began taking them down just to zero and not an iota more and have had no ill results. My motivation was simple. My experience, consistently, is that they'll invariably loosen a tad when driven so setting them to the typical 'smidgen' loose (umpteen methods taught to us by older brothers and grand fathers) resulted in a 'smidgen' loose after driving....and the age-old castellated-keeper-and-cotter-pin system isn't all that secure.

As to Grant's "hammering" of bearings discussion, I often share the same worry. I am often amazed, though, at the Jags I've owned/seen/ worked on where the rear hub bearings seems extraordinarily loose yet soldier on and with no apparent ill effects when driving the car. Of course, this eventually must reach a tipping over point but, still, they seem to be very forgiving.

Enough rambling.....




I got a chuckle out of that

Mikey, I'm sending you a PM in a minute

Cheers
DD

 

Well, the saga continues...

Mikey: I am constantly amazed how knowledgable folks on this forum are. Mention some random factoid from Googling re Corvettes, and sure enough someone chimes in with full knowledge of the problems on that car also. Impressive!

warrjohn: thanks for the ideas about checking the disc. I agree 10 thou is a lot, and should be easy to spot. I do not have a way to measure runout other than on the car, but I did place the rotor on a flat stone countertop. It seemed to be dead flat.

Given there was some wear on the stub axle, I went ahead and ordered new axles and bearings, which arrived for this last weekend. Bearing races went in fine, but the stub axle is a problem.

To remove the stub, I had a piece of steel pipe cut to fit over the stub axle, capped with a couple of washers, which the bearing adjustment nut can tighten against. Loosened the stub axle retaining nut, tightened down the adjusting nut, banged on the back of the axle best I could with a hammer and punch. Apparently this technique has worked well for others.

It didn't do a thing for me! Moreover, the adjusting nut thread stripped during tightening, and it now just rotates in place. I can dremel it off, I suppose, but have no idea how to get the stub axle off now -- well, off the car, with a press, but I really don't want to go there.

Any ideas for removing the stub axle with the the hub carrier still on? I do not want to start dealing with spring compressors, etc, to remove the hub.

Thank you all so much for the continued help! Cheers,

cf

 

Oh Boy.

I have done a few spindles, and always off the car in a BF press.

Raise the car, place a sturdy jackstand, or whatever your world calls them, under the spring pan, with another set slightly lower, about 4", under the jacking point, lower the car. This will "preload" the spring and keep it away from your head.

Release the balljoints, and remove the vertical casting, press out the old spindle, and fit the new, lock the nut, refit to the car, and raise the car and move to the other side, simple.