I have never been one to shy away from asking for help when I am stumped, and I just ran into one of those occasions AGAIN!

The car is on the ground. It will roll backwards easily with little friction. However, it does not want to roll forward. I suspected the handbrake caliper pads - I hate those things, marginal design! It is difficult to get them to grab the rotor squarely so I assumed that the leading edge was grabbing in a forward motion. So I loosened the adjustment screw a bit. Got some marginal improvement but forward motion is still restricted.

I then started to inspect my rear primary calipers. I don't understand why the primary calipers/pads/rotors would be any different going forward or backward. Why would they offer more friction going forward than backward?

I jacked up the rear end. The pads are snug on the rotor but I can turn the tire by hand. When I assembled everything I knew they were snug but when one considers that when you are turning the rear wheels you are also turning the rear diff and the propshaft, I thought I was OK. Now I am thinking that I may need to look more closely at the shimming of the calipers, though again I thought I was OK. Before I start tearing everything apart I thought I would seek the guidance of the Forum. Anyone have any ideas? This is making me feel pretty silly.
Lin

 

Great question- sorry I don't have an answer!

 

Need more info... ?
Have you had the rear (and the front) sleeved ?

If it hasn't been done properly the piston can bind on the bore and cause problems.

The caliper shims don't affect braking, all they do is centre each piston on the caliper housing so you can slip the pads out when it's time to replace them.

Are the metal pad mounts binding on the caliper housing ?

They rust and bind.

You should be able to unbolt the single bolt, remove the pad keeper and slip the pads out with little effort.

 

Need more info... ?
Have you had the rear (and the front) sleeved ?
Brand new Coopercraft calipers front and rear.

If it hasn't been done properly the piston can bind on the bore and cause problems.

No binding.

The caliper shims don't affect braking, all they do is centre each piston on the caliper housing so you can slip the pads out when it's time to replace them.

I respectfully disagree. In the rear, they center the caliper housing on the rotor. If not centered then one side or the other can fit too tightly against the rotor causing excessive friction.

Are the metal pad mounts binding on the caliper housing ? No, all,brand new.

They rust and bind.

You should be able to unbolt the single bolt, remove the pad keeper and slip the pads out with little effort.

 

Lin, are you certain the roll resistance relates to the rear brakes? If there's any question you might pull all pads to confirm that's the problem. If not it could relate to the drive line or even excessive toe in the front. Just trying to think of possibilities.

 

I would try what Doug said then _ pull the pads.

At least that will tell you if it is brakes or drive line.

 

This is the odd thing, it resists rolling forward, brakes should not have that symptom, could be drive train, but need to rule out brakes first. Wouldn't the toe have to be WAY WAY off to cause excessive friction ?

Hey Lin, on the mad theme, you ain't parked on a hill are you !!!

 

Thank you one and all,
Actually John, going forward is ever so slightly downhill out of my garage. I have a list of things to check from those of you here who have volunteered ideas as well as some from the Saloon-Lovers Forum. I am now at the beach with grandkids for the week, so I will be back in the garage next weekend. I appreciate the help and will let you know how I come out. This is one I have never encountered before. Why in the world it moves backward easily but not forward is truly puzzling. Oh well, another day - another challenge! If anyone thinks of anything else that I should add to my list of items to check please don't hesitate to add to this thread.
Thanks again.
Lin

 

Lin
What an interesting question. I assume from your remarks that the problem is not with the front wheels.
Of course if you have a limited slip diff it is hard to figure out which rear brake side has the issue.
I guess I would see if I could get a feeler gauge between the pads and disc, if necessary mechanically persuading the pads to retract. There must have been clearance at some point for you to install the pads.
It does sound like a wedging action is at play here, maybe one piston is bottomed due to incorrect shimming.
Perhaps you should just pull the pads which would immediately identify the location of the problem

 

" In the rear, they center the caliper housing on the rotor. If not centered then one side or the other can fit too tightly against the rotor causing excessive friction".

The shims are there to simply centre the caliper housing on the rotor and nothing else.

The pistons move independently of each other and move up to the rotor surface for braking purposes regardless of where the caliper housing is mounted.

The only reason why the caliper is shimmed to the rotor is so you can remove the pads with out having to work the one piston (or in your case the two pistons) back in, to clear the caliper housing when replacing the pads.
Obviously though, that if it was off centre so much, that the caliper housing would come in hard contact with rotor; it would have to be out a lot and require some pretty thick shims to correct this.
On my car, the thickest shims totaled to around 30 thou.

At any rate, I don't think it's a brake problem.

I would check you front wheel bearings.

You could also jack up the rear end with both rear wheel off the ground and see if the engine will move the wheels forward and look for signs of dragging.

 

Well, I am back from a little beach vacation and while I did not have a lot of time to work on the Jag today I did make a little progress. Doug, Jeff and others got me thinking about whether my problem was really at the rear of the car.

I have not yet tried to do even any rudimentary alignment because I figured that I would get the car started first, get it turned around (now facing out of the garage), drive it onto my four post lift and then work on alignment when I can easily get under the car.

Just to recap, the problem was that the car would not roll forward though I could push it backwards without resistance.

So, after thinking about the issue all week while away, I was determined to first establish if my problem was the front of the car, the back of the car, or the driveline. I jacked up the rear and put the rear tires on the wheel dollies (see photo below). I don't have a lot of distance to push the car forward before I go out of the garage, but it appeared that with the rear dollies I still had resistance to rolling forward. So, then I put the rear back on the ground, jacked up the front and put the front tires on the dollies. Ah hah, the car seemed to roll forward without much resistance. Perhaps the problem is toe-in as Doug suggested?

I have new brake calipers in the front, new pads and new bearings. The tires/wheels spin freely when off the ground. Tomorrow I will adjust the toe-in and see if that solves the problem. At least it is an easy job. However, I remain puzzled by the fact that the car rolls rearward quite easily. If the problem was toe-in wouldn't it also be an issue going backwards?

Let me know if you have any other thoughts otherwise I will be adjusting the tie rods tomorrow and I will let everyone know how it comes out.

Thanks!

Lin

 

is the car in gear when you push it forward or backwards? I assume no it isn't.

is the emergency brake OFF when you push it forward or backwards? I assume it is.

I have my S type in a storage too, and the other day I was trying to push it forward, and it wouldn't move. I even grabbed it by the rear bumper, raising it somewhat and put all my strength into moving it, it wouldn't go.

then I checked the shifter and it was in PARK. Yes, haha! I forgot to move the shifter to NEUTRAL, by which time I was exhausted and gave up on pushing it again.

Not saying this could be your case, just saying that it is the little stupid things that make us think we have a problem.

 

No Jose,
The car is in neutral and the handbrake is not only off, it is now disconnected.
Lin

 

ok, then it is a transmission issue if the tranny is automatic.

not brakes, not alignment. The Transmission.

I found the same issue but reversed:

http://www.dsmtuners.com/threads/car...l-help.462337/

 

Correct the toe-in first. Just to rule it out if nothing else, before attacking the tranny.
If you imagine the front tyres are pointing inwards, when you go forward you will get great resistance because the wheels want to go together instead of straight, couple that with the tyres wanting to skip sideways (inwards) a little to release that pressure on the sideways tread force, and you have a tight feeling that won't be there when going backwards.

 

Yep,
Will adjust toe-in today. Jose, the car is a manual shift. 3.8 MOD
Lin

 

And the winner is:

Doug Dooren!

I believe that Doug was the first person to suggest that my problem might be excessive toe-in. He was right!

I am still a little surprised that this turned out to be the problem, but pleased that it was such an easy thing to adjust to fix the problem. At first I just "eye-balled" the alignment and thought I was fairly close, but the car while better would still not roll in a forward motion without some resistance.

So, I got out my floor jacks and string and did a "poor man's alignment." I put some grease on cardboard and put it under each front wheel to make it a little easier to adjust.

This method is just a preliminary to having the alignment done professionally, but lo and behold, it worked and yes, it solved the problem. The car now rolls as well in a forward direction as it does to the rear.

Special thanks to Doug, but thanks to all who offered a thought about what the problem might be. Everything helped. With reference to the subject line, today I am feeling less inept!

Lin

 

Nice one Lin, and good call Doug, still trying to understand why it affects forward movement and not backward, but very pleased you sorted it.

 

They must have been towed in quite a lot to affect how easy/difficult it was to push it forward.
I drove this guys 51 Rolls back to my house to work on his wood and the tires were severely scuffed.
I never did find out what was wrong with the alignment, but I know it was still pretty easy to roll the car around.

That's a neat trick with the cardboard and grease BTW. :-)

 

Sometimes the simplest things.................