I need your help.

In my previous brake upgrades I just bought calipers from normal vehicles, installed them and went on enjoying it. Now I'm faced with caliper piston sizing. I'm going to order a set of Wilwood Dynapro Radial mount calipers, but what piston size?

I'm using a Wilwood copied reverse swing long ratio 6.25:1 pedal box, still need to purchase the masters for the front and rear brakes, would like to use the Wilwood Compact MC as I can purchase cheaper PBR units. Sizes avail is 0.75, 0.7 and 0.625

If you need more info please tell me!

Is the answer to this so easy that there is no info on the net or rocket science that no one have posted some info, please if anyone can assist, please do?

.625 front
.75 rear

Caliper piston size 1.62

Thx, Dattoman, any possibility you can tell me how you got to those figures as I would like to learn.

The ratio of the brake piston diameter to the master piston diameter gives you the mechanical advantage. So 1.62/.625 on the front. You use a smaller mechanical advantage on the back to get the brake force distribution right. Easy if it's disks or drums all round, harder if there's a disc front drum rear but I'm sure people here know the rules of thumb to figure it out. You want the front to lock first and can use this to tune the brake balance. (These rules hold for all hydraulics by the way.)

20 years of experience on brakes
And a licked my middle finger struck it in the air and pointed North... got the answers that way

Those sizes are fairly common for bias setups
And as for caliper pistons... yanks go overboard with their sizes... 1.75 is too big... 1.5 is too small for fronts (well.. not really... but 1.62 is "sweeter")

There is some science to this, but rule of thumb will probably get you there quicker.
I've been playing around with this at the moment for my Circuit racing car.

Science approach:
Usually you work forwards from calliper piston size and numbers, Disc Diam, wheel diam, Centre of Gravity location/height, weight distribution front to rear, max -G of the stop, Brake pad friction and tyre friction values. This will lead you to a ideal front and rear circuit pressure required.

Then you use your pedal pressure (i.e how much foot force you want on the pedal), pedal ratio and work out the required master cylinder size. Lastly, you need to ensure that the stroke of the master cylinder can move enough fluid for the brake size, so work out the total displacement of fluid required.

Whilst all this ideal stuff is great, when you only have the choice of three master cylinder sizes, what's the point of knowing you master cylinder diameter requirements to 10 decimal places if they aren't available or working backwards to piston sizes that don't exist, rotor diameters that you can't get.

I'm using S4 RX7 callipers on the front of my circuit car (Currently in build), which have a piston Diam of 1.28" and will be using a front Master cylinder Diam of 0.625".

The Key thing I am working on is keeping the same ratio of front to rear brake force. I.e I calculated the ideal pressure requirements front and rear and worked out the front braking %, when choosing the front cylinder based on what is available, I will choose the rear size based on what keeps the same overall front brake %. Therefore I may require more or less pedal pressure to achieve the required circuit pressure but at least I know the front and rear are still in the same %.
If I have miss calculated the max -g and hence the weight distribution front to rear changes, which will lead to an overall change to the front braking %, I can always bias this change out using the bias adjustments on the pedal.

Also, try http://www.stoptech.com/tech_info/tech_white_papers.shtml for some articles that might help.

Bryan

Quote:

Does the 1.62 caliper piston size apply to 4-pot calipers when used in-conjunction with the 0.625 front master?

.625 front
.75 rear
These are fairly standard
I've used them on 6 pot fromt and 4 pot rear setups too

Depends on pedal ratios , preferred heights , size of driver etc