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Hello dtm,
I saw your post a while back and was going to reply but the more I thought about it the bigger the reply got! Just spotted it again and not really surprised no-one has tried replying.
My immediate thought was that .625" is a bit small especially with 8 pots (I'm assuming your 4 pots was 4 pots per side) and then realised a Skyline is a bit big and heavy. We had about 1" diameter on the Fiat Uno for just 2 pots but that did make the brakes damned heavy. So I did a few quick calcs and got a movement ratio from pedal to brake pad of 280:1. Assuming coefficient of friction for the pads = 0.4 and ratio of rolling tyre diameter to mean disc working surface diameter of 2.5:1 gave me 4500 lbf retardation for 100 lbf at the pedal.
Many years ago I did calcs for Ferodo but so long ago that there's no chance of having any data on my computer and it would be commercially sensitive anyway. You could do a very approximate calculation for the give in the pad material using the Young's Modulus of the material and stressed area under the pots. I used to know the Young's Modulus for one of the brake pad materials but that's long escaped my memory banks.
In any case I suspect it pales into insignificance compared to the other sources of movement. In no particular order these include - pull back of the pads due to the action of the seals, thermal shrinkage (and other distortions) as the brakes cool after application, deformation of the calliper bridge due to the forces trying to open up the calliper, pressure expansion of the brake hoses, lost motion due to clearances at the various joints such as pedal pivot, lost motion at the pushrod connecting the pedal to the master piston, lost motion at the master cylinder because of the feed hole that connects the working chamber to the reservoir, pad knock back including effect of wheel bearing stiffness etc. etc.
Some of these you can do nothing about. Others you might be able to do something to minimise - make sure all joints are free from excessive wear, use racing steel braid reinforced brake lines etc. I don't know if proper racing master cylinders have a lower stroke to cover the feed hole compared to bog standard ones but might be a factor.
Then use the largest diameter master cylinder you can get away with. If that doesn't sort it I guess you need an even bigger master cylinder and a servo! I've no idea what other people with big heavy race cars use but perhaps someone will chip in when they see this.
I think what I'm really trying to say is - there's a bit more to this than can be handled by calculations alone and the important thing is to handle the practicalities, including what do other people do on similar size cars?
Hope that's some sort of help,
Scott
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