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MG TD TF 1500 - TD Rear Brakes - Benefit to adding 2nd Cylinder?

In refurbishing the brakes on my TD I'm looking at the backing plate for the rear brakes and notice it is the same as the front, only that in place of the 2nd wheel cylinder, there's the fixed mounting location for the shoes. It begs the question(s):

1) Could a second wheel cylinder be mounted there in place of the fixed shoe mount (I think that's easy - yes). It wouldn't be *that* expensive to accomplish. Another cylinder, a fitting to allow the fluid to carry from one cylinder to the other, just like the front, and you're there.

2) Would there be a braking effectiveness improvement? I think this one is tougher to answer. Obviously the rears carry much less of the braking chore given weight shift to the front as deceleration occurs, and there might even be a negative to it in that the rears might have too much effectiveness and lock up, but I'm wondering if anyone has ever tried it to see?

Plus you'd have a non-standard configuration, but then, most of our cars have at least some non-standard parts in them (c'mon admit it, you like the look of those "MG Car Company thresholds).

Ideas?
Stan Griffis

Stan - I doubt that a second cylinder added to the rear wheels would be very cost effective, since only 30-40% of the braking is done by the rears (perhaps less on the TD since the front brakes are twin leading shoes). It would also be at the cost of a somewhat demished reverse braking since both shoes would become leading shoes in the forward direction like the fronts are. Cheers - Dave
David DuBois

Stan:

To add to Dave's information, try an experiment. Back up at about 15 MPH and slam on the brakes. You'll think you'll never stop. That's because the car's only got two shoes really working backwards while it has 6 going forwards. Then imagine if you have no shoes really working when going backwards.

Leading brake shoes are self servo. That is they "dig in" by themselves once they contact the drum thus reducing the pedal effort. Trailing shoes don't "dig in" but are actually pushed away from the drum by it's rotation. This greatly increases the pedal effort.

Disk brakes do not have this self servo effect. That's why almost all disk systems have a brake servo. The only car I know of with disk brakes and no servo was the mga twin cam.

Blake
Bullwinkle

I've been dealing w/ disc brakes too long. My TR3 had discs doing most of the real work.

I'm having a hard time getting my brain around the "leading vs trailing" shoe idea on a system where the attachment point of the cylinders floats like on the fronts of the TD. It seems to me like the both shoe should engage all at once as the circumference of the shoes swept surface area increases (into an circle from thier resting oval shape actually) to engage the rotating drum.

On the rear I can see (in my head) how one shoe would be leading and the other trailing in each direction because the circumference of the shoe surface area does increase some, but because there is only a single point of pressure, the shape of the swept shoe area also is mishapen from a oval when at rest, to a distorted circle when the single cylinder (which also floats) drives the front of the shoes apart. I can see where in one direction one shoe would dig in more, while in the opposite direction the other shoe would dig in more.
Stan Griffis

The leading shoe has a little bit of a mechanical advantage, since the spinning drum pulls on it while braking. This acts as a force multiplier for the wheel cylinder. (Basically, I just repeated Blake's point. I really should read the whole thread before I start composing my replies.)

If you look closely at the rear brakes on a car with a single wheel piston, you should notice the leading shoe wears faster than the trailing one. Of course, on my MGB, the seals always fail and ruin the shoes well before they wear out anyway ...

Most classic motorcycles use dual leading shoes on the front wheel, if they are ancient enough to have drum brakes up front.

P.S. Both front wheel cylinders on each side of my TF front axle have fixed attachment points. The piston pushes the leading edge of the shoe into the drum, and the other end of the shoe pivots on the fixed back end of the other cylinder. Is the TD different?
Mark B.

Is the TD different?

No it is exactly the same.

Anothe disadvantage to increasing the forwar braking capacity of the rear brakes is that they will constantly lock up. 2/3 of the shoes on the fron cope with 2/3 of the braking effort, increasing this will put the balance too much at the back and require other changes to compensate.

My TD has an MGB rear axle with the larger MGB drums. I have had to fit hard competition linings to compensate for the increase in rear drum braking to correctly balance the car and to stop rear wheel lockup (which actually reduces braking efficiency)
Phil Stafford

Leading vs. trailing really irrelevant. Really bad idea. Highschool physics-Bernoulli's (sp? could be another guy also- but too early to look up)principle- pressure per area equals force. An extra same size cylinder would double the area under the same pressure, causing about twice the rear brake force, nice locking up, spin-outs, and probably a crash. So you would have to use a proportioning valve or half-diameter cylinders, which would also give you two more to leak. Most modern cars have a proportioning valve to balance.
George Butz

George (and others) - You pretty much confirmed my fears, mainly that the extra rear braking force wouldn't promote safety, and might even harm it.

Had to ask the question though once I saw the similarity in the backing plates.
Stan Griffis

This thread was discussed on 03/09/2003

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