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Re: [T3] Converting Single MC to Dual MC

On 12 Dec 2002 at 0:41, Dave Hall wrote:

> When the Variant brakes are being checked on the rolling road for the annual
> test, I get about 200 units on the front before locking and 300 on the rears,
> and the guy said last time he'd never seen such high figures for an e-brake, at
> around 200 (but we all know they are good if working well).

The F/R numbers are probably just proportional to weight on those wheels and 
would depend heavily on the friction between your tires and the rolling road. 
This looks like a 40/60 static weight distribution, which is about right. From 
this we know the F/R position of the center of mass, so we could estimate the 
weight transfer as a function of deceleration if we knew how high the C of M 
was. I suppose we could make a reasonable guess....

I think the really interesting number from your test above would be to know 
what number you get for the rears with the same pedal force (fluid pressure) 
that locked up the fronts. This would give us the braking proportioning for our 
cars. I agree with you that it is probably less than 2:1.

I wonder if this is a test they could do?

One little appreciated thing about a rear engine car is that we CAN have really 
good rear brakes, and you're right. Type 3 brakes are GREAT, both front and 
rear. Front engine cars are forced to run with weak rear brakes because there's 
so little weight there, and even less when braking.

> The calipers produce more than 3 times as much force on the disc as the
> wheel cylinders produce on the drums,

I've never spend much time thinking about this, but this thread got me to 
thinking (ouch!) The difference between the 2 forces on disk pads vs. drum 
shoes is that the caliper piston force is applied perpendicular to the rotor, 
while the drum piston force is tangential. In essence, we are building up hoop 
stresses in the drum.

Have you ever worked over the physics problem of how much force it takes to 
slow down an ocean liner if you take it's rope and wrap it a few times around a 
fixed post? If you make very modest assumptions about the coefficient of 
friction you discover that a small force pulling on the free end of the rope 
will overcome HUGE forces trying to pull away on the ocean liner end.

I suspect that this is what's happening in drum brakes. In other words, there's 
a big mechanical advantage to applying that force tangentially. Then the 
leading shoe thing adds to this for one shoe, but takes away for the other. 
[Except for early type 3 fronts with 2 leading shoes.] 

I'm sure all this has been worked out somewhere, but I've never seen it.

Jim Adney
Madison, WI 53711-3054

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