TR2/3/3A TR3 Rear Brakes -- A Bold Experiment

[Moseso]

It has long been accepted that the 10" Girling rear brakes, fitted to TR3s until sometime in 1959, lock up too soon -- well before the front discs are doing what they could be.

People have suggested swapping out the 10"ers for the later 9" parts, but that's an expensive pain in the rear.

Needing to have my shoes relined, it occurred to me that the new linings didn't HAVE to be the full 2-1/4" width of the shoes. So, I asked my reliner to put 1-3/4" linings on 'em. He said he could do that.

The car is still not back together from it's winter of tweakage, and it will be while before I've got those rear shoes bedded-in, but a report on brake balance/efficiency will be forthcoming...

 

[tdskip]

Let us know how it goes

 

[TR3driver]

My high school physics teacher would tell you that it won't make any difference, except in how easily the brakes overheat. The equation for friction depends only on normal force (ie how hard the piston is pushing) and coefficient of friction.

If you really want to cut braking power in the rear, then either use shoes with a lower coefficient of friction, or install a proportioning valve to reduce the hydraulic pressure to the rear.

However, at least as yet, I've not found the 10" rear brakes on my TR3 to be a problem. Of course, my car isn't exactly stock and that may make a difference; but I would expect the wide low profile tires and lowered front suspension to require more brakes in the front than in the rear.

The 3A had been converted to 9" rear brakes (before I got it), and it had a definite tendency to lock the front brakes first. I recall a certain incident where a Volvo station wagon turned left in front of me ... I slid both front wheels for what seemed like a lifetime until my fore-brain got control over the hind and pulled my foot up so I could steer around him!

 

[Moseso]

Well, I wouldn't want to argue with a high school physics teacher, but it seems to me that reducing the number of square inches of friction material in contact with the drum will, in fact, reduce the coefficient of friction at a given pressure.

When you say, "it won't make any difference, except in how easily the brakes overheat," what is your prediction? The narrower shoes will heat the drum more quickly, or less quickly?

I had thought about the proportioning valve, too, but thought the difficulties of plumbing it in, and dialing in the correct restriction, would be harder and more expensive than this simple (perhaps, too simple) idea.

 

[CJD]

You're right...the results will be interesting. On the one hand I agree with Randall's logic. On the other...the reduced friction surface may accomplish exactly what you are looking for.

Be sure to let us know the results!

John

 

[TR3driver]

Well, I wouldn't want to argue with a high school physics teacher, but it seems to me that reducing the number of square inches of friction material in contact with the drum will, in fact, reduce the coefficient of friction at a given pressure.

That was exactly the point of the lesson, changing the surface area doesn't change the friction (or the coF). The example used was a block of wood, approx 2" by 4" by 6", sliding down a ramp. Within the limits of our ability to time it, the block slid down in exactly the same time, no matter whether it was resting on the 4x6 side or the 2x6 side.

For overheating, less area for a given amount of heat means the temperature will be higher. At some temperature, the linings lose effectiveness. BTDT. Fortunately, there was no cross traffic !!
However, overheating the rear brakes is very uncommon (I had no front brakes at the time), so it's not actually likely to be an issue.

As you say, the results will be interesting. Unfortunately we have no real "before and after", so there's no real way to know if using the narrow shoes changed the friction.

 

[Darrell_Walker]

However, overheating the rear brakes is very uncommon (I had no front brakes at the time), so it's not actually likely to be an issue.

And to some extent drum brakes are self-limiting, as the hotter the drum gets, the larger it is, and more pedal is needed to maintain the same force between the shoe and the drum.

 

[tdskip]

As you say, the results will be interesting. Unfortunately we have no real "before and after", so there's no real way to know if using the narrow shoes changed the friction.

Picky picky picky Randall. Yeah, a baseline would have been good but anyway....

 

[TFB]

Are all the rear brake cylinders the same i.d.? A smaller bore might help.
Tom

 

[TR3driver]

And to some extent drum brakes are self-limiting, as the hotter the drum gets, the larger it is, and more pedal is needed to maintain the same force between the shoe and the drum.

Huh?? I'd not heard that one before. Yeah, the pedal might go a little lower as the drum expands; but the pressure at the rear brakes is the same as the pressure at the front brakes. The driver will just push the pedal down farther.

Unless maybe you're talking about the force to overcome the return springs? Not enough to notice, IMO.

 

[TR3driver]

Are all the rear brake cylinders the same i.d.? A smaller bore might help.

Nope, there are several different diameters available. In fact, the factory tried several different diameter cylinders with the 10" drums, before finally going to the 9". And some people put larger cylinders on the rear to go with the "dual pot" Toyota front calipers (although best as I can tell, the Toy calipers have exactly the same piston area as the TR ones do).

 

[Darrell_Walker]

And to some extent drum brakes are self-limiting, as the hotter the drum gets, the larger it is, and more pedal is needed to maintain the same force between the shoe and the drum.

Huh?? I'd not heard that one before. Yeah, the pedal might go a little lower as the drum expands; but the pressure at the rear brakes is the same as the pressure at the front brakes. The driver will just push the pedal down farther.

Unless maybe you're talking about the force to overcome the return springs? Not enough to notice, IMO.

Yep, you are right. Either you hold the pedal at the same position, in which case the pressure on both the front and rears drop, or you maintain the pressure, which means you need to move the pedal further down.

 

[Moseso]

Are all the rear brake cylinders the same i.d.? A smaller bore might help.
Tom

I believe that I am already running the smallest ones -- .625"

And, while I can't argue with the results of Randall's ancient physics experiment, I just can't wrap my head around the concept that a 10" x 1" drum brake could generate the same stopping power as a 10" x 2" setup could -- all other factors being equal.

 

[DNK]

I'm just amazed that Randall remembered a HS Physic lesson.
I forgot everything I learned the minute I walked out of the class

 

[smaceng]

The physics can sometimes be strange to the non-technical types, but as an engineer, the logic about the surface area discussion seems correct. I'll give you another interesting one:
If I put skinny tires on my TR6 and inflate to 30 psi and compare that to flat tires at 30 psi. Which tire has the larger contact patch? Neither, they are both about the same! The pressure in the tire is the factor to determine the contact patch, with is the wt of car divided by 4 and then divided by the pressure in the tire gives the contact area.
Interesting?
Scott in CA with nothing better to do! They make me remember this stuff at work.

 

[lbcs_r_fun]

You may never recognize a difference in braking performance. Many years ago, I drove a TR4 the entire summer, commuting 50 miles to and from work. Later that summer I was preparing to return to university and doing some maintenance. I soon realized that <span style="font-weight: bold">I had driven all summer with NO rear brakes. </span>

Regards,

T

 

[Don Elliott]

For 80,300 miles from new, I never had any problems with the 10" drums, shoes or linings which came with my 1958 TR3A when new. That's when I did the restoration in 1987 to 1990. At that time, I asked a clutch and brake shop to put new linings onto the original rear 10" shoes which they bonded into place. I'm now at 185,000 miles with these and have never noticed any grabbing or other early braking problem with the rear brakes grabbing first or with more stopping force.

I remember one spring after I adjusted the square adjusters for the rear linings a bit tighter, that the linings would squeal for a second or so - now and then. The next day, I backed off the adjustment till the noise was gone. But other than the annoying squeal, there were no mechanical problems.

 

[CJD]

I'll give you another interesting one:
If I put skinny tires on my TR6 and inflate to 30 psi and compare that to flat tires at 30 psi. Which tire has the larger contact patch? Neither,

With tires, the shape of the contact patch makes a huge difference with traction. Skinny tires at 30 PSI have the same sized contact patch as fat tires at 30 PSI, but they will not have the same handling characteristics, though. That's why I think the smaller brake linings will accomplish some change. Different temperature distribution...or whatever...might even grab more instead of less...


John

 

[tdskip]

Scott in CA with nothing better to do! They make me remember this stuff at work.

LOL

 

[Moseso]

Like Randall, I remember TONS about my jr. high and high school science classes. I like that stuff and it made an impression. Sadly, I never took a physics class -- all chem and biology. What physics I know came from my father, a structural engineer. We never dealt with friction much, however. I learned a lot about tension, torsion, ductility and hardness of materials, though. Ask me anything about how structures behave in the presence of lateral forces: wind and, especially, earthquakes.