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MG MGB GT V8 Factory Originals Technical - Rear brake cylinder choice

I am doing a CB BGT with a 2.8L Ford V6 and I have purchased kevlar lined shoes and pads. Since there is a choice of rear cylinder bores, which one might work best?

Bob Cline

The factory V8 GT used the roadster cylinder rather than the usual GT cylinder. The roadster cylinder applies less braking force than the GT cylinder by dint of its smaller bore.

Paul Hunt

This reduction in rear brake effort was in response to the front brakes being uprated and hence there was a greater risk of the rears locking. If you have increased the front braking system significantly more than stock then you may well have to reduce the rear cyl diameter further. Note though that the RV8 with 270mm dia vented fronts with 4 pot calipers still used the 20.3mm size rear cylinder.

Stock GT 22.2mm dia,
Stock Rdstr/V8 20.3mm dia,
next step down 19.0mm dia, Mini etc
next one 17.5mm dia, midget etc

Roger Parker

In other cars where I've upgraded the pads to hi performance pads I've noted much higher braking power for the same pedal pressure, as well as increased fade resistance. This increase in the coeffecient of friction can range from 25 to 50 %. The result is less pedal pressure for stopping. In an emergency stop max pressure is used no matter what. With big fat sticky tires there is more braking power and more weight transfer to the front, but the sticky tires on the back should still have more traction than stock tires. All in all it seems to me that to keep a balanced front rear balance with the lower pedal pressures required for the hi performance pads that the stock large size rear cylinders ought to be maintained. If the rears were changed to hi friction shoes that could change things again. I don't see people getting excited about upgrading the rear shoes. 1 the brakes are more than adequate for the rear as-is and 2 drum brakes are designed to be partially self energizing. As you apply the brakes the geometry causes the brakes to wedge against the drum and mechanically amplifies the cylinder power. (try applying brake drums rolling backward and you'll see much higher pedal pressure required). The hi friction linings would amplify this self energizing and make the rears much more likely to lock up. The disc brake is linear in action. double the pressure and double the friction and stopping power. Rear brakes are non linear because of the self energizing feature. Double the pressure and you may get 3 times the friction. With hi friction rear brake shoes this could result in rear brakes that unpredictably lock up under hard stopping conditions. Many of the hi friction materials grab better once they get hot. Having this sort of friction material in brake drums in a street application could be real scary.
Barry Parkinson

I have recently replaced both pads and shoes on my TR4 with the kevlar ones and noticed a dramatic increase in fade resistance and apparent stopping power. I have not tested the threshold of lockup but will attempt to do so in the near future. BTW all other aspects of the braking system are stock. I did replace the silicone fluid with Castrol LMB.

Bob Cline

I've been thinking about my comments in the post above and need to make a correction. The B rear brakes are single cylinder. The dual cylinder brakes are the ones that work great forward and hardly rearward. The single cylinder results in one shoe being self energized forward and the other backward. The rear shoe is self energized in the forward direction. You may notice that the rear brake shoe will show more wear than the front shoe. There is still a problem of the heavily loaded rear brake shoe getting hotter before the front shoe and the related problem of the coefficient of friction at the rear being unpredictable in relation to the front discs. One time you could have hot rear brakes and have lock up problems at the rear. Another time in a panic stop with cool brakes you could have no lock up problem at all.
Barry Parkinson

This thread was discussed between 24/08/2000 and 27/08/2000

MG MGB GT V8 Factory Originals Technical index

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