Master Cylinder / Caliper Calc

[jimcroisdale]

Hi all,

Building a brake system on a gravity racer using a balance bar style setup. Original master cylinders didn't have enough volume to move the calipers enough, so we are buying new ones and dropping them in. The new master cylinders are Wilwood types with 0.75in bore. Here's my maths. Does it look right to you?

Caliper First:

Max gap between pads at rest - 4mm

Disc thickness - 2.5mm

Distance to move before pads clamp onto disc - 1.5mm

Piston diameter - 30mm

Volume required to move ONE piston 1.5mm is 1060mm cubed

--------------

Now the Master Cylinder:

Bore - 19mm

Stroke - 35.6mm

Displacement - 10,093 mm cubed.

-------------

So TWO calipers will need 2120mm cubed, and the master cylinder can provide 5x that. Should be fine? Will account for brake wear etc etc no problem?

Cheers,

Jim

[johnny comelately]

Hi all,

Building a brake system on a gravity racer using a balance bar style setup. Original master cylinders didn't have enough volume to move the calipers enough, so we are buying new ones and dropping them in. The new master cylinders are Wilwood types with 0.75in bore. Here's my maths. Does it look right to you?

Caliper First:

Max gap between pads at rest - 4mm

Disc thickness - 2.5mm

Distance to move before pads clamp onto disc - 1.5mm

Piston diameter - 30mm

Volume required to move ONE piston 1.5mm is 1060mm cubed

--------------

Now the Master Cylinder:

Bore - 19mm

Stroke - 35.6mm

Displacement - 10,093 mm cubed.

-------------

So TWO calipers will need 2120mm cubed, and the master cylinder can provide 5x that. Should be fine? Will account for brake wear etc etc no problem?

Cheers,

Jim

Gday Jim,
the gravity racer, is that what we call a billy cart here in Oz?

[jimcroisdale]

Yes, I believe it is!

[johnny comelately]

Yes, I believe it is!

Thongs, sandals, flip flops just dont cut it anymore so I will listen up Jim
There have been answers about this subject before, have you searched this forum?

[jimcroisdale]

Yes, have tried searching but no luck.

[johnny comelately]

Jim, it was the 13th March and it was you.
Jolle and Gambler and others answered some.
then there is "Rapid Brake Bias Adjuster", not sure what it relates to though
good luck cobber

[jimcroisdale]

Yes, that was a different issue that I managed to fix. But those master cylinders turned out to be too small, volume wise. So Ive got stuck in and learned/done the maths and really just wanted someone who knows this stuff to say "yep - youre on the right lines"

[Tim.Wright]

The calcs make sense to me. I'd be interested to know the previous setup given that the new dimensions are quite conservative. If you don't have any excessive caliper or hose flex you should be ok for volume.

If you want to take into pad wear you can add the combined thickness of the pad material of both pads to the 1.5mm in the caliper calculation. However, in an automotive braking systems, the brake disc remains in contact with the disc and the extra volume due to wear is replaced by brake fluid from the reservior between brake applications. So the master cylinder doesn't need to compensate for wear via it's own capacity.

If you are trying to lower friction by holding the pads off the disc via a spring then you need to account for the worst case which is the pads fully worn and fully retracted. This might turn out to be larger than the 4mm you have considered.

[jimcroisdale]

Yes, 4mm is the gap as it stands. It could get bigger. The previous cylinders had a bore of about 10mm and a stroke of maybe 15mm, so now ive done the maths they were marginal for one caliper with a 2.5mm disc, and so probably designed for a similar caliper but with a 3mm disc which would have made a lot of difference.

yes, we are using beefed up return springs to endure the pads stay clear of the disc - important in g racing..

Thankyou Tim for checking my sums

jim