TR4/4A TR4 Rear Brake Parts?

[Tabcon]

I'm getting ready to rebuild my rear brakes and I noticed that the guy who previously restored the car had 2 different types of wheel cylinders. One was the original Girling type without a slot at the end of the piston and having a piston diameter of .70". The other is an aftermarket type with a slot and a diameter of .75".

I see both offered at various retailers, both Girling and the generic brand types, but I was curious if the .75" diameter sized cylinder adds any additional stopping force over the .70". Also, is the OE Girling cylinder a better part? It certainly costs more.

 

[martx-5]

All other things equal, the larger bore piston will apply more force. It will require more travel on the master cylinder, but since the sizes are close, it may not be very noticeable. I believe that the .75" was a TR3, early TR4 size, and the .70" came later. Check on the Moss site, they should have the commission number breakdown.

 

[Tabcon]

Thanks.

I'm not so much worried about the braking pressure as I am about quality parts. I'm worried that if I order a cylinder from Moss, which makes no distinction in the catalog of the manufacturer, that it will be one of those Chinese or Indian jobs. I guess I answered my own question and I'll have to get the Girling cylinder just to be certain of it's quality.

 

[TR3driver]

Note that in this case, more force may not be a Good Thing. You are effectively changing the front/rear brake balance; and too much braking in the rear can make the rear wheels lock too soon (making the car want to swap ends during a panic stop). That's why there were so many different sizes of rear slaves (and drums), the factory kept trying to solve problems with premature rear wheel lockup.

A 64 should have the .70" cylinders, which came in at the same time as the front brake disks got smaller (reducing braking force up front).

 

[RJS]

Hey Tab,

Most definitely, check your commission number and the Moss catalog will tell you the original bore size. Very early TR4's had the 0.75" bore while later TR4's and TR4A's (like mine) had the 0.70" bore wheel cylinder.

My humble understanding is that, given an equal amount of pressure at the pedal, the smaller bore size wheel cylinder generates more pressure on the brake shoes.

I had to replace both wheel cylinders on my TR4A this summer. The wheel cylinders were original (Girlings) and while doing my bi-annual brake fluid bleed, I had an "Oh sh_t" moment when entire side of the wheel cylinder tore off after tightening the bleeder screw. After examining the crystilized metal in the crack, it appeared as if that crack had existed already for some time. Whew! Better to fail in the garage on jack stands than on the open road.

Anyway, not wanting to replace only the one side, I had to source two wheel cylinders quickly. My local NAPA parts store was able to get only one Raybestos "Professional Grade" wheel cylinder. I bought it and no doubt in my mind this was an OEM piece (0.70" bore and not made in China or India). It was absolutely identical in every way to the part which failed including being stamped "Girling" with the part numbers WC37355 and 64673305 just like the original. The labeling in the package also showed a Girling part no. of 63-37355 UK. The only downside is that he could only get me one and it was $68 each with tax. If you Google "Raybestos WC37355" you'll see it for about $45 and photos with Girling on the part.

So, ultimately, I returned this one and bought two pieces from TRF at $25 each. They appeared aftermarket, were 0.70" bore and carried a part no. of GI64674185. Don't know where exactly they were manufactured but, I do put a lot of faith in TRF parts.

Hope that helps.

Bob

 

[martx-5]

...
My humble understanding is that, given an equal amount of pressure at the pedal, the smaller bore size wheel cylinder generates more pressure on the brake shoes...

Actually, it's the other way around... see this explanation.

A smaller bore size on the <span style="font-weight: bold">master cylinder</span> will generate more force on the slave, if the slave remains the same size, but will require more stroke.

 

[TR3driver]

My humble understanding is that, given an equal amount of pressure at the pedal, the smaller bore size wheel cylinder generates more pressure on the brake shoes.

Nope, Art had it right.

Think of it in terms of pressure in the brake fluid : pressure is measured in pounds per square inch. When it acts on a piston, the more square inches the piston has, the more pounds of force will be exerted on it.

.75" piston is about .44"^2
.70" piston is about .38"^2

So, if the pressure is 100 psi, the .75" piston exerts 44 pounds force; the .70" piston only 38 lbf.

 

[RJS]

Art, Randall,

Thanks for the correction and setting the record straight. I confused wheel cylinder bore size with something I read about master cylinder bore size.

Cheers,

Bob

 

[trfourtune]

Tab,
consider the following:
If you increase the performance of your front brakes, you want to do the same to the rears. You want to keep the braking balanced. Many racers go to the morgan 7/8" rear brake cylinders (available from TRF)to balance the additional stopping power of the front brakes. Balance bars are only meant for fine tuning so sizes of masters and slaves will deal with the major brake balance issues. You might want to re-read Kas's books on this.
Rob

 

[Tabcon]

There is no way the rear drum brakes could ever even come close to the stopping power of the front disc brakes. The only way to compensate in this situation would be to place similar rear discs and calipers on the rear. I can see going to larger rear wheel cylinders on a mostly stock system, but bumping the piston size of the rear wheel cylinders a tad is really not going to make much, if any difference in comparison to brand new front Wilwood calipers and 12" rotors.
I think that by using the dual braking system with a proportioning valve will give me enough adjustment.

I have ordered a set of rear brake shoes from Carbotech which should help a little though. I've heard some great things about their pads.

 

[TR3driver]

I disagree; my rear drums have no trouble locking the rear wheels (and obiously the front brakes cannot do any more than lock the wheels). The advantage of disc brakes is increased heat rejection, not better stopping power.

In fact, when I owned a TR3 with front drums, my biggest complaint with them was that one would always lock first.

The point of 'balancing' is that you want all 4 wheels to reach maximum braking (without locking the wheels) at the same time. Anything less means you are not slowing the car as fast as possible, regardless of how good the braking is on a particular axle (or wheel).

 

[PeterK]

Balance bar or hydraulic proportioning valve is a good thing to have for road racing, even some autocrosses.

But it depends on the course, elevation change etc. Sometimes you just want the rears hard to help swing the rear end around a tight hairpin as you touch the brakes. Or not. Locking up the rears on a downhill turn is probably not OK.

But for the street, the "defaults" are probably best.

 

[Tabcon]

I would have to argue that with you Randall, respectfully of course. To say that a 12" modern cross drilled and vented rotor combined with a racing built caliper has no more stopping power than a 9" brake drum with 45 year old technology just doesn't seem to make much sense to me. Granted, one of the the main advantages of disc's over drums is primarily heat dissipation, but this also links directly to stopping power when you take fade into consideration. You also need to take into consideration that about 60 to 90% of a cars braking is handled by the front brakes regardless of what you do the the rears. With this in mind, you are completely right about balance. Without the ability to balance the hydraulics from front to rear, braking would be a mess after adding the new front brake system.

 

[TR3driver]

Granted, one of the the main advantages of disc's over drums is primarily heat dissipation, but this also links directly to stopping power when you take fade into consideration.

Exactly, and improved resistance to fade (aka better heat dissipation) is exactly what disc brakes, slotted & vented rotors, etc. is all about. But if you aren't using the brakes hard enough to get into fade, all that improved cooling is useless.

If you don't have an overheating problem, then a bigger radiator doesn't help

BTW, earlier TRs had 10" rear drums; the factory actually went smaller because the drums had<span style="font-weight: bold"> too much </span>stopping power compared to the front discs. Then with the smaller rear drums, they were able to go smaller on the front brakes as well (early TR4).
<div class="ubbcode-block"><div class="ubbcode-header">Quote:]You also need to take into consideration that about 60 to 90% of a cars braking is handled by the front brakes regardless of what you do the the rears.[/QUOTE]Definitely. Which means the rear brakes don't need to dissipate as much heat as the front brakes (because they don't work as hard).

I have driven hard enough on occasion to overheat my TR3A's brakes (enough to get into noticeable fade anyway) and it was the front brakes that faded first.

So, with rear brakes that both supply more stopping power than the tires can handle, and more fade resistance than the front brakes; there just doesn't seem to be much room for improvement!

 

[Tabcon]

Point taken...lol.

No argument with you there Randall, but what about the part where you said the disc brakes will have no more stopping power than the drum brakes?

Just kidding, I think I get your drift.

Since I have never actually raced, auto-crossed, or even really driven a TR4 hard before, I have no idea exactly what it will require insofar as braking, handling, power, etc. I think the car handled fine before I tore it apart, but I never really tested it due to the frame situation. All I'm attempting to do is to make improvements in those areas which I believe I can. It's been a great journey so far. I've learned a lot about these cars from you guys and I've taught myself a good deal in the process also. One of the best parts has been fabricating the tools and parts needed for the restoration that I was either too cheap to buy or due their unavailability. I've also gotten a few cool tools in the process.

I'm sure that once it's all back together and I actually get to haul it down to the local track, I'll be better able to tell if all my efforts made much difference. I'm really looking forward to that day.

 

[swift6]

If the drum brakes will lock the wheels just as easily as the discs, there is no real increase in initial stopping power. Locking the wheels is locking the wheels. Now if you go to a larger and grippier tire, afterall it is the tire that actually stops the car, the increased braking ability of the larger disc with multi-piston calipers may well be more effective if they can still lock the wheels with the larger/grippier tire if the drums can no longer lock them down.

 

[TR3driver]

Right on, Shawn.

FWIW, I am running "larger and grippier" tires (Bridgestone Potenza 205/55) and the tires are still by far the limiting factor; as I think it will always be. The laws of physics make it very difficult to have a coefficient of friction more than 1.0 ... when I was in high school, they taught that it was impossible.

Hasn't happened since I switched to Potenzas, but when I was running some 195/50 Yokos, a Volvo wagon startled me by turning left from oncoming traffic just in front of me while I was doing about 50 mph. My biggest problem was getting the message to my right foot, to back off the brakes so I could steer! There were very definitely 4 big black stripes leading to the intersection (and fortunately no one else in the lane she had just vacated)

I can still lock the Potenzas, but so far it has only been deliberately during testing. I'll save playing with front/rear balance until after I'm done with ride height (the TR3 could stand to be about 1/2" lower in front, which may affect brake balance as well).

 

[swift6]

Yeah, I think it would take a really wide tire that was really sticky as well, or really huge wheels (hoopty like wheels) to defeat the brakes on any of the body on frame TR's. The cars are too light to need really big brakes. Unless you plan on doing a LOT of repeated heavy braking and you need the better fade resistance.

 

[Tabcon]

I'm going to try these:

 

[trfourtune]

Tab,
what tires are those? (forgive my ignorance)
Thanks
Rob