The MGA With An Attitude
for Twin Cam - TC-250

On 6/25/2015, Edward Vandyk in the UK wrote:
"Here is how the Olthoff Twin Cam went for dual circuit brakes using, what I assume to be, a standard 7/8-inch bore MGA dual master cylinder (although I guess it could be a midget 3/4-inch one). I will not know until I am into the car over winter. It gives a very hard short pedal. In period it had a prototype cylinder apparently with a circular reservoir on top, but when it needed to be refurbished the then owner (still with us) replaced it with a standard unit (40+ years ago)".

The two pushrods have ball end joints, and the clevis pin is simply passed across the two ball ends and through the plain bore at top end of the pedal. Given that this is close coupled, there is no adjustment for front to rear pressure bias, which should give equal pressure for all pipes same as the original single line setup.

There must be some attention here to adjust pushrod length so the clevis pin will be positioned roughly straight across when the pedal is depressed with force. If the clevis pin was allowed to be canted it could cause significant wear on the pin and the pedal hole. Additionally, too much angle on the pin could cause the articulated joint to bind up resulting in unequal pressure, or possibly no pressure on one side. If pressure might fail in one fluid circuit only (like a broken brake line), then pressure on one side only would put a strong angular force on the clevis pin and pedal. By engineering standards this looks like bad design, but as a dual line brake system it may be better than single line if there is a problem.

Given that two pistons will displace twice as much fluid as one piston, the pedal would have short stroke and very high required pedal force. This would be better with smaller bore cylinders, at least down to 3/4-inch bore like the 1098cc Spridget cars. Perhaps better yet if it was sleeved down to 5/8-inch bore, as (0.625/0.875)^2 = 0.510. With 5/8-inch bore, half the piston area would give half as much fluid flow from each cylinder which would yield total flow and pressure equal to the original 7/8-inch bore single master cylinder.

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