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Braking and tires

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tom628

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I just installed and bedded in a set of Porterfield carbon/kevlar front pads and TSI kevlar rear shoes on our TR6.
Much better better pedal feel and noticeably more braking power, but now , under only moderately hard braking one or the other front wheel will suddenly break loose.( Maybe I'm braking harder than when the pedal felt like it was a hunk of wood). At first I thought I had goofed on the installation, or something was wrong with the rotors or calipers, etc. But it appears to be random as to which wheel breaks loose, depending I guess on road camber, surface condition, etc.
The tires are ContiForce, and are 8 yrs old although the tread is not worn much at all since the car was laid up for several years. I don't think I'm getting anywhere near the deceleration rate I should be getting.

Any suggestions? Do you folks think that the tires may just be too old and hardened to give optimum braking traction? Vigorous cornering seems to be OK.

TIA,
Tom
 
Can you get the rears to lock first by giving them just a bit of help with the hand brake ? If not, I'd say there's something wrong with them; like perhaps contaminated or glazed drums.

I'm not familiar with the Porterfield compound, but if it has a much higher coefficient of friction than the stock material, you may have simply upset the front/rear balance.

Of course you should also check front end alignment, and look for any worn joints allowing the wheels to deflect under hard braking. Had a TR3 once where one of the pivots had torn out of the frame, and that wheel would lock very easily (not to mention pulling horribly to that side).
 
Randall: Good comments. Thanks. The Porterfields were suggested recently by someone on, I thought, this forum but possibly another, as superior to some of the others like EBC Green Stuff. They are promoted as for street/autocross. They seem very strong with less pedal effort, and do not squeal. No pull to either side up to the point of a tire breaking loose.
I picked up the kevlar rears form Ted (TSI) at the Carlisle show. Drums looked like new; I deglazed and cleaned them and adjusted the shoes with no problems. No wheel cylinder leaks apparent.Wheels stop smartly when spun on jack stands, so at least things seem to be working. But I can't be sure, of course, that I'm getting full braking action out of the rears, as you point out. BTW, pulling on the hand brake while cruising at 20 or 30 mph produces only very modest braking effect. Don't know if that's typical. I'm not sure how to test the rears, however, other than finding a dirt road and seeing what kind of skid marks the 4 wheels leave.
Speaking of F/R balance, can one tell visually whether the rear wheel cyls. are the orig. 0.70" or the Sunbeam-type .0.75"?
Sorry for the long post.
Tom
Added: All new suspension bushings, etc. and aligned front and rear.
 
tom628 said:
I just installed and bedded in a set of Porterfield carbon/kevlar front pads and TSI kevlar rear shoes on our TR6.
Much better better pedal feel and noticeably more braking power, but now , under only moderately hard braking one or the other front wheel will suddenly break loose....Do you folks think that the tires may just be too old and hardened to give optimum braking traction? Vigorous cornering seems to be OK.
Click to expand...
There's an old expression that goes something like <span style="font-style: italic">Brakes stop the wheels; TIRES stop the car!</span> I wouldn't be at all surprised if your tires were a bit too old and hard after eight years!
 
I'm with Andy.

Eight year old tires and "spirited" driving are a VERY bad combo, BTW. Tires deteriorate. FIVE year old ones are dangerous, IMO.
Think seriously about some new bladders, Tom.

We do NOT want to learn of bent LBC's or broken pals, mmkay?
 
Probably the Porterfield R4S pads? They're excellent high performance street pads, and will have a lot more bite than most anything else you've been used to. Which means you're very likely exceeding the limits of those tires much more easily, as others have said.

As an illustration of this point, one of the guys I autocross with switched from high performance street tires to R compounds and found that his formerly excellent brakes would no longer lock the wheels. The difference was that the tires could transmit much more brake torque than before. He switched to more aggressive pads and picked up a lot more braking (or rather could make use of all the braking his tires could provide).

You've moved in an opposite way to the above, but the principle is the same. To make use of the brake torque available from the Porterfields, you need to be able to provide more grip from the tires.
 
Thanks guys. I'm looking at tire choices right now. I'd like to figure out a way, though, of testing the performance of the rear brakes (short of disconnecting/plugging the front circuit).
Randall, pulling on the hand brake didn't seem to change things.

Tom
 
To go along sideways with this topic and to follow up what doc said with a question. Di tires degrade in a garage if no ultraviolet light ca attack them? My tires are about 13 years old and only have about 5000 miles on them. They have spent almost 10 years in the shop only seeing daylight twice to wash the car.
 
One more consideration, re rears not locking up/modest braking with the hand lever: You just replaced the shoes. They may not match the drum diameter well enough yet to be giving their full effect. If you take the drums and shoes to a good brake shop, they can arc the shoes to match the drums. Or, you can just wait for them to wear-in -- but they won't give their full effect until the shoes make 100% contact with the drums.
 
DNK said:
To go along sideways with this topic and to follow up what doc said with a question. Di tires degrade in a garage if no ultraviolet light ca attack them? My tires are about 13 years old and only have about 5000 miles on them. They have spent almost 10 years in the shop only seeing daylight twice to wash the car.
Click to expand...

Age is age, Don. The things degrade with exposure to atmosphere, no way around it. Those tires are good for rolling it around in the garage now, but when you are ready to drive it, put new feet on it.
 
Don,
That's been in the news a lot recently. The fact that tires degrade over time whether they've been used a lot or not. There's even been a number of law suits due to low mileage tires failing. They basically look perfect on the outside but are rotting on the inside. Google "DOT tire degradation" and you'll get lots of hits like https://goliath.ecnext.com/coms2/gi_0199-4187033/The-invisible-danger-of-aging.html

They'll even tell you how to read the tire stamping to determine their age. I bet a lot of us are in the same boat with great looking low mileage tires that are over 8 - 10 years old.
 
ouch. I always heard it was the ultra violet that killed the rubber.
 
tom628 said:
I'd like to figure out a way, though, of testing the performance of the rear brakes (short of disconnecting/plugging the front circuit).
Randall, pulling on the hand brake didn't seem to change things.
Click to expand...
I'd say that's pretty conclusive evidence that there's a problem with the rear brakes.

Tires do get old and hard (even stored in a garage), but IMO you're looking at a more severe problem than can be explained by old tires. And besides, the rear tires are just as old.

Don't have a TR6, but both the TR3 & the Stag can lock the rear wheels using the handbrake. Even with some pretty sticky tires and 'stock' linings. It's harder to do it on the Stag (shorter handle plus bad angle), but still possible.

And I've never bothered having the shoes arced to the drums. Did that years ago on a car with drums all round, and it did make a difference, but IMO not as big as your problem.
 
Well, that makes sense Randall. I'm just not sure what to do about it right now. The car has essentially a conmpletely rebuilt brake system: lines, cyls, hoses, etc.I would have thought, and I discussed this with TSI Ted, that the new rear kevlars would improve the rear braking force, and so balance the improved front braking. Something must be wrong.
I appreciate everyone's help, and remain open to suggestions.

Tom
 
Just a thought : Do you have the PDWA hooked up ? It should tell you if the MC isn't building pressure in the rears for some reason (as might happen if it weren't correctly assembled). Wouldn't explain the lack of braking from the handbrake, though.

With respect to Ted (got some of his linings myself); I think my next step would be to pull the rear brakes apart, make doubly certain the drums are not contaminated or glazed (wipe them down with Arcyli-Clean or simiar after breaking the glaze with crocus cloth) and try a different set of shoes.

You could also try swapping tires front/rear; just to be sure it's not something about the tires.

FWIW, the rear brakes on my second Stag had apparently not worked for years when I picked up the car. Hydraulic problem with the MC and PDWA disconnected. But I didn't notice until a hard stop caused all sorts of havoc, and apparently the previous owner hadn't known either.

The first problem with the hard stop was that the left front locked up way easier than it should have ... the second problem was that after I came out of the store, the rear brakes were still locked on ! Pistons seriously stuck in both cylinders. Liberal application of a hammer on the drums caused them to retract enough that the car would move; so I disconnected them and drove the remaining 1300 or so miles home.
 
Did you bed in the new Kevlar rear shoes, per the instructions that TSI provided?
 
Boy did they hurt when I rolled over! :devilgrin: :devilgrin:
Couldn't resist that.
 
PeterK said:
Did you bed in the new Kevlar rear shoes, per the instructions that TSI provided?
Click to expand...
Hmm, Ted must've forgot to give me those ... anyone care to enlighten me, or should I call him ?
 
Thanks, Bob.
 
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