XJ6 & XJ12 Series I, II & III 1968-1992

Rear brake drags

 
  #1  
Old 11-16-2014, 12:42 AM
chaparral2f's Avatar
Junior Member
Thread Starter
Join Date: Nov 2014
Location: Southern Oregon
Posts: 14
Likes: 0
Received 9 Likes on 1 Post
Default Rear brake drags

The rear brake on the left side is the at the moment locked up. I am pretty sure that the problem is shims behind the rotors are missing. I have a set of new Brembo rotors that I am going to put on, but I don't have any idea how to measure for the shim between the rotor and the flange on the differential. Any help would be greatly appreciated.
Thanks
 
  #2  
Old 11-16-2014, 01:13 AM
Veteran Member
Join Date: Mar 2008
Location: Pacific Northwest USA
Posts: 20,149
Received 6,548 Likes on 4,860 Posts
Default

Originally Posted by chaparral2f View Post
The rear brake on the left side is the at the moment locked up. I am pretty sure that the problem is shims behind the rotors are missing.

How can you tell?



I have a set of new Brembo rotors that I am going to put on, but I don't have any idea how to measure for the shim between the rotor and the flange on the differential. Any help would be greatly appreciated.
Thanks

First you'd install the new rotor and then the caliper. Then measure how far out-of-center (if any) the rotor is within the caliper. Then determine how many shims you'd need. As I recall they are about .015" each.

This would all be very tricky with everything still in the car. A bit less so if you drop the cage.

Cheers
DD
 
  #3  
Old 11-18-2014, 05:34 PM
Fraser Mitchell's Avatar
Veteran Member
Join Date: Dec 2009
Location: Crewe, England
Posts: 7,344
Received 1,573 Likes on 1,310 Posts
Default

It is not quite as simple as it sounds, because the shims to centre the rotor can alter the camber angle of the road wheel. That's because the drive shaft is also the upper wishbone of the suspension.

As I recall, if shims have to be added on one side of the rotor, you have to remove the equivalent thickness on the other side. However, I defer to Doug on this one as it is a long time ago, and when I replaced my rear calipers and rotors I got a Jaguar specialist shop to do it.
 
  #4  
Old 11-21-2014, 01:48 PM
chaparral2f's Avatar
Junior Member
Thread Starter
Join Date: Nov 2014
Location: Southern Oregon
Posts: 14
Likes: 0
Received 9 Likes on 1 Post
Default

Here in Southern Oregon, it would be easier to find a fuel tank for a flying saucer than to find a Jaguar specialist. I think I will set up the brakes, then do the rear camber. Thanks for the tip or else I would have probably forgotten.
 
  #5  
Old 11-21-2014, 06:45 PM
Tar
Tar is offline
Senior Member
Join Date: Sep 2012
Location: Portland, Oregon USA
Posts: 189
Received 45 Likes on 41 Posts
Default

Hey Chaparral,
For what it's worth, we have at least a couple of excellent independent Jaguar specialist shops up here in Portland (The Jag Shop and Consolidated). But I don't know how far down in Oregon you are- could be a long way!
Just a thought.

Andrew.
 
The following users liked this post:
o1xjr (11-22-2014)
  #6  
Old 11-22-2014, 06:39 PM
chaparral2f's Avatar
Junior Member
Thread Starter
Join Date: Nov 2014
Location: Southern Oregon
Posts: 14
Likes: 0
Received 9 Likes on 1 Post
Default

I've been to the dealership in Portland a couple of times. I live in Eagle Point, (Medford) and so I am almost 300 miles down the state. I wish there was a Jaguar shop down here.
 
  #7  
Old 11-23-2014, 09:33 AM
Veteran Member
Join Date: Jul 2012
Location: Walnut Creek, California
Posts: 5,732
Received 1,945 Likes on 1,569 Posts
Default

Yeah, I "chickened" out and hired the install of my rebuilt rotors out. Done in situ, by the tech. but, he had a lift. Huge advantage.


So, caveat.


But, is the idea to shim and thusly move the rotor to get it centered into the caliper. yes, that would change camber. But, how much?


Or, is it the concept to shim the caliper to cage location and thus get the relation within spec. In that case, no effect at all on camber.


Decades ago, son and I messed with adding disc brakes to the rear of his "tom's Low T" project. Solid rear axle. I forgot the source of the rotor and how we attached it to the axle flange. But, the caliper mount was cut and ground from heavy plate. Bolted to the axle housing in lieu of the drum brake backer. As I recall, we got lucky and it only took a small mount o shim stock to get a good center. Big disc brakes front and rear, it could really stop. No boost, just big foot!! Worked out a lot of leverage in the brake pedal arm. Again, cut from plate and ground.


Oxy acetylene, no plasma yet!1


Carl
 
  #8  
Old 11-23-2014, 10:41 AM
Veteran Member
Join Date: Mar 2008
Location: Pacific Northwest USA
Posts: 20,149
Received 6,548 Likes on 4,860 Posts
Default

Originally Posted by JagCad View Post

But, is the idea to shim and thusly move the rotor to get it centered into the caliper. yes, that would change camber. But, how much?

A .020 shim will alter camber about .25 (a quarter of a degree)


Or, is it the concept to shim the caliper to cage location and thus get the relation within spec. In that case, no effect at all on camber.

No.

If shims are added/removed to centralize the rotor then the camber is necessarily changed.

But.....

There are shims on both faces of the rotor. To centralize, you add/remove *inboard* shims.... and *then* add/remove the same number/thickness of *outboard* shims to keep the camber in spec.

(In actual practice darn few people do this. And it appears that little harm results. Either replacements rotor are amazingly consistent in thickness or the rear suspension geometry is very tolerant of small camber changes.)

Once the rotor is centralized it can be forgotten until the next rotor replacement comes due. During that period the rear camber is adjusted using outboard shims only....the ones between the rotors and the half-shafts

(In actual practice darn few people adjust rear camber. It's a pain in the *** to disconnect the half-shafts, add/remove shims, reattach, recheck the result. Hopefully it's Ok after the first time so you won't have to repeat-repeat the process!

Cheers
DD
 
  #9  
Old 11-23-2014, 11:37 AM
Veteran Member
Join Date: Jul 2012
Location: Walnut Creek, California
Posts: 5,732
Received 1,945 Likes on 1,569 Posts
Default

Clear!!!


I used cage when I meant differential housing, ie, the "pumpkin".


I had a feeling that the effect of the shims was minimal and as such in the real world,
"Que sera, sera".


Chores done. As soon as it warms up a bit outside, off to wire in a harness and relay for the "new" crank circuit. Son sent neat new stuff. so, it's cut, strip, solder and shrink. Making d... sure I've matched the pins right. New 40 amp intermittant duty relays. Close if not exact as to what I have. Why, it failed, I dunno. Failed relay or a connector issue. I suspect the latter.


Carl
 
  #10  
Old 11-23-2014, 11:37 AM
Veteran Member
Join Date: Jul 2012
Location: Walnut Creek, California
Posts: 5,732
Received 1,945 Likes on 1,569 Posts
Default

OOOPs. Sorry, wrong box. done!!
 
  #11  
Old 11-24-2014, 09:10 PM
chaparral2f's Avatar
Junior Member
Thread Starter
Join Date: Nov 2014
Location: Southern Oregon
Posts: 14
Likes: 0
Received 9 Likes on 1 Post
Default

I'm unable find any of the rotor shims so I'm going to have to order a new set. As long as I am ordering the rotor shims I may as well get enough to set the camber too. If anyone has a good source for these parts, and would make a recommendation of what to get, I would really like to know.
 
  #12  
Old 11-24-2014, 09:42 PM
Veteran Member
Join Date: Mar 2008
Location: Pacific Northwest USA
Posts: 20,149
Received 6,548 Likes on 4,860 Posts
Default

Contact David at....

Everyday XJ


Going back to your first post.......

Are you sure the shims are missing? I don't think there is any way to verify except to remove the brake rotor.

Anyhow, David can fix you up with shims.


Cheers
DD
 
  #13  
Old 11-24-2014, 11:15 PM
chaparral2f's Avatar
Junior Member
Thread Starter
Join Date: Nov 2014
Location: Southern Oregon
Posts: 14
Likes: 0
Received 9 Likes on 1 Post
Default

Doug,
There were shims behind the rotors when I got the rear end, but they managed to disappear somewhere along the way. The rotors were dragging really hard on the inside pad. There was about .015" between the rotor and the outside pads. Thanks so much for the tip on finding the shims. I'll be contacting him first thing in the morning.
 
  #14  
Old 11-25-2014, 12:40 AM
Veteran Member
Join Date: Mar 2008
Location: Pacific Northwest USA
Posts: 20,149
Received 6,548 Likes on 4,860 Posts
Default

The shims behind the rotor determine the position of the caliper over the rotor.

If the rotor is dragging hard on the pad (or, more correctly, the pad dragging hard on the rotor) it's probably because the caliper pistons are stuck.

The in-n-out travel (or lack thereof) of the pistons is what determines how heavily the pads drag on the rotors. If the pistons can retract normally and freely, there will not be excessive drag. If the pistons don't/can't retract then naturally there will be excessive drag



Cheers
DD
 
  #15  
Old 11-25-2014, 08:42 AM
Fraser Mitchell's Avatar
Veteran Member
Join Date: Dec 2009
Location: Crewe, England
Posts: 7,344
Received 1,573 Likes on 1,310 Posts
Default

These rear caliper pistons do seize up and cause brake drag, and I know this because it happened to me ! Initial symptom was the brake pedal going to the floor one night, but then the car recovered. Like an idiot, I ignored this. Then one day going on holiday, the pedal went to the floor when trying to slow down on a steep hill ! Fortunately pumping restored the brakes, but on pulling into a garage, white smoke was billowing out of the rear wheel arches. This garage had a mechanic there, and it was soon discovered that a piston had seized on, and heated up the disc so much that the heat destroyed the oil seal on the diff bearings. Caliper seals were all shot too.

I suspect it is well beyond the time the rear subframe needed dropping out and a full rebuild undertaken of the brake calipers.
 
  #16  
Old 11-25-2014, 08:49 AM
Veteran Member
Join Date: Mar 2008
Location: Pacific Northwest USA
Posts: 20,149
Received 6,548 Likes on 4,860 Posts
Default

Originally Posted by Doug View Post
The shims behind the rotor determine the position of the caliper over the rotor


Correction

The shims determine the position of the rotor within the caliper


Cheers
DD
 
  #17  
Old 11-25-2014, 08:56 AM
Veteran Member
Join Date: Mar 2008
Location: Pacific Northwest USA
Posts: 20,149
Received 6,548 Likes on 4,860 Posts
Default

Originally Posted by Fraser Mitchell View Post
These rear caliper pistons do seize up and cause brake drag, and I know this because it happened to me !


Indeed they do !

Caliper pistons seizing-up are nothing new at all, on Jags or any other. Lord knows how many calipers I've replaced for that reason.

One of the common clues I watch/feel for is simply how easily the car rolls when pushed by hand. Or, if rolling along at a walking pace will the car gently slow down? Or does it stop with a slight abruptness at the very end? If the later, a caliper is probably sticking.

Cheers
DD
 
  #18  
Old 11-25-2014, 09:09 AM
Join Date: Jan 2014
Location: Delaneys Creek, Australia
Posts: 23,868
Received 4,563 Likes on 3,472 Posts
Default

Originally Posted by Fraser Mitchell View Post
Then one day going on holiday, the pedal went to the floor when trying to slow down on a steep hill ! Fortunately pumping restored the brakes, but on pulling into a garage, white smoke was billowing out of the rear wheel arches.
I hear you, something along those lines happened to me. Don't ignore the message the brake peddle is sending you!

https://www.jaguarforums.com/forum/x...m-fire-115245/
 
  #19  
Old 11-25-2014, 10:28 AM
Veteran Member
Join Date: Mar 2014
Location: Tehama County, California, USA
Posts: 17,178
Received 3,472 Likes on 2,709 Posts
Default

Originally Posted by Doug View Post
A .020 shim will alter camber about .25 (a quarter of a degree)





No.

If shims are added/removed to centralize the rotor then the camber is necessarily changed.

But.....

There are shims on both faces of the rotor. To centralize, you add/remove *inboard* shims.... and *then* add/remove the same number/thickness of *outboard* shims to keep the camber in spec.

(In actual practice darn few people do this. And it appears that little harm results. Either replacements rotor are amazingly consistent in thickness or the rear suspension geometry is very tolerant of small camber changes.)

Once the rotor is centralized it can be forgotten until the next rotor replacement comes due. During that period the rear camber is adjusted using outboard shims only....the ones between the rotors and the half-shafts

(In actual practice darn few people adjust rear camber. It's a pain in the *** to disconnect the half-shafts, add/remove shims, reattach, recheck the result. Hopefully it's Ok after the first time so you won't have to repeat-repeat the process!

Cheers
DD
I am glad to know all this, Doug.
The old Michelins (the ones I shredded) were worn unevenly on the rear. It was Very noticeable but only when the tire was off the car.

I'm not the least bit surprised to know so many people ignore this. As I intend dropping the IRS this Winter anyway, it Will be dealt with.
(';')
 
  #20  
Old 11-25-2014, 12:30 PM
Veteran Member
Join Date: Jul 2012
Location: Walnut Creek, California
Posts: 5,732
Received 1,945 Likes on 1,569 Posts
Default

LnrB:


Not necessarily a true indicator. Suppose the front wore them that way and the PO swapped them to the rear to get a few more miles out of them. A crude rotation scheme if you will.


As the car is still together, put a bubble level on the wheels. Best if within the rim so as to eliminate tire shape. An indication of camber, not a measure. Merely +, 0 or -.


Off to mess with misconnected relay???


Carl
 
The following users liked this post:
LnrB (11-26-2014)

Thread Tools
Search this Thread
Quick Reply: Rear brake drags


Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service

© 2019 MH Sub I, LLC dba Internet Brands

We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to Amazon.com and affiliated sites.
 
  • Ask a Question
    Get answers from community experts
Question Title:
Description:
Your question will be posted in: