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MG MGF Technical - Brake Pedal Ratio

Problem in designing something is finding the right numbers to put into formulaes.

Finding the theory and understanding it is one thing. Getting the correct numbers to put into the variables is yet another.

With the MGF/TF it is more or less a problem. When you really get going, there is jus too few design information at hand. Although some of us have an extensive online database wink:wink

This has to do with a 4-pot brake conversion that no boubt fits behind cup wheels.

Maybe guys at RLM know a little more?

thanks in advance,

David Peters

allrighty, measured it up and for sake of completeness for anyone who want to use MGF pedal boxes in their kit car. Or simply, as I am doing, changing calipers and want to know what theyre messing with:

L1 = Distance Fulcrum (pivotpoint) to master cylinder pushrod attachment: 6.5cm

L2 = Distance pushrod attach to pedal: 15cm

(L1+L2) : ( L1 )= 21.5 : 6.5

--->brake pedal ratio 3.3 : 1

which is I believe a normal value for a vacuum boosted brake application in passenger cars

vacuum boost ratio 4.6 : 1

I dont know at what vacuum value this ratio is applied.

Off to the Archives it goes


David Peters

David, I'm sure that'll be very useful info for someone...

Does it look like AP make a suitable caliper? I suppose a bias valve is another option, albeit more expense?
Mike Hankin

If you have ABS you don't need a bias valve.
David Clelland

David, this info is not really useful unless you also include the bore size of the MC and slaves, and the average distance of the pads from the centre point of the wheel
Will Munns

Ok Will, here we go. (I posted bore sizes in another threas)

Tandem Slave cylinder: 23.8mm bore (stroke unknown)
Front calipers (120 bhp, mpi version): Single piston: 48mm
Bias front/rear: unknown (60/40?)

anthing else I forgot or other could add?

BTW, AP do a kit which is boltable behind 15" VVC wheels. Off course this is radial mount, which makes the fixation point at the rear of the hub flange in stead of the front thanks rob) as the OEM items

As a side note: the booster has nothing to do with pedal travel, it merely gives us a reason for not doing physical traing as race car pilots do (to get muscle in the legs)

David Peters

keeping it alive for a while. IŽll be measuring the MC piston travel at 100lbs pedal pressure. Guess that would sum up all details then? At least for the front brake assy.

front OEM Pads are 6mm + 11mm in thick. 6mm being the metal part where the 11mm carbon stuff is adhered to.

When equiped with new pads the stock front piston has to travel for 2mm before to hit the brake disk.

If anyone can comment on rear piston travel that would be nice. If you have a rear assy laying around. Measuring distance piston to backside of pad would be enough;-)

David Peters

To complement the above:

Brake master cylinder piston travel @ 45kg (90lbs) of pedal load (tested at idle running): 20mm

Vacuum of booster assist at idle: not known
David Peters

This thread was discussed between 07/08/2006 and 18/08/2006

MG MGF Technical index

This thread is from the archive. The Live MG MGF Technical BBS is active now.