Brake Calculations?

[ScottHughes]

I have been trying to calculate the piston sizes needed for my clubsport as I noticed on the Radical website that the older willwood calipers for the front had different size pistons, but the one's fitted to the front of mine have 4 x 1.75" pistons!

But during the calculations it looks like the rear brake pads recieve 2/3rds more force than the front due to the force at the brake peddle being distributed over a smaller area i.e. 2 pot calipers on the rear not 4 pots - also the rear pads are half the size! maybe thats where the re-balance come's from smaller pads = more force required to equal the front braking? My Calc's don't take into account the differant size master cylinders which looking at it reduces rear force/pressure by 10%, but this would still mean a lot more force at the rear rather than the front??? should I be aiming for a 50/50 split with bias being adjusted on the balance bar?

Does anyone have experiance of driving with cotpit bias adjustment and 2 pot rear calipers. Can you adjust the braking to the rear to the point that the rear locks before the front?

Thoughts........?

[GreenV8S]

To work the whole thing out you would need to know whether the front and rear circuits run at the same line pressure, or whether you have separate front/rear master cylinders. Then multiply together all the mechanical and hydraulic leverages from the pedal out to the contact patch i.e. m/c piston area, caliper piston area, caliper leverage relative to the tyre. Note that a bigger m/c bore reduces the line pressure, and bigger caliper bore increases the braking effort for a given line pressure - your comment seemed to imply your thought the opposite.

Edit to add: of course you also need to know the static weight distribution.

Edited by GreenV8S on Sunday 7th September 15:57

[BertBert]

I have got the wilwood 4 pot front and rears as per the part numbers on Rob's web site (5094/5093). They are 1.75" and 1.38". They work find and can be adjusted to front or rear bias. The bias is set quite far to the front. My pedal moves quite a lot and we think that the M/Cs need to be the next size up to give a firmer feel pedal.

Not sure that that answers the question though!

Bert

[ScottHughes]

Bert,
If you have the same calipers front and rear, and the same size M/C then you should have a 50/50 split? if you have a smaller bore rear M/C then you should have 10% more front brake pressure (If my thoughts are right 10% less if not)

How did you fit the 4 pots on the rear? did you get new uprights?

My Set-up is:
Front M/C bore is 0.7" and Rear is 0.625" (I thought that the larger bore would displace more fluid and increase line pressure?)

Rear pistons 1.75" x 2
Front Pistons 1.75" x 4
Front and Rear Disks are the same radius
I think the tyres are simular radius ish?
Front pads are twise the area of the rear
Both the M/C are operated from a balance bar positioned in the middle of the brake pedal so the force applied to each should be equal with balance bar set in the middle.

Edited by ScottHughes on Sunday 7th September 17:30

Edited by ScottHughes on Sunday 7th September 17:33

[GeoffW]

There's an excellent thread here on brake choices & specs, well worth a read:

http://socalpeeps.yuku.com/topic/608/t/On-the-subj...

cheers
Geoff

[lanan]

ScottHughes said:

Bert,
if you have a smaller bore rear M/C then you should have 10% more front brake pressure (If my thoughts are right 10% less if not)

My Set-up is:
Front M/C bore is 0.7" and Rear is 0.625" (I thought that the larger bore would displace more fluid and increase line pressure?)

Edited by ScottHughes on Sunday 7th September 17:30

Edited by ScottHughes on Sunday 7th September 17:33

Opposite way round.... smaller MCyl gives greater braking force, so is usually fitted to front system, depending on Calipers piston sizes.

[ScottHughes]

Radical list 0.7" for front and 0.625" for rear?

Oh weight distribution is 105kg front 135 kg rear

[ScottHughes]

Ah-ha
After reding how stuff works web site i have been doing calcs wrong? I was dividing the force by the area of the pistons but I should have been multipying it.

Front 1.75" 4 pots gives 6.35 x peddle pressure
Rear 1.38" 4 pots gives 5.6 x peddle pressure

Based on both using 0.7" m/c's if you change the rear to a 0.625 like radical suggest then you get 5.96 x on the rear which is only 7% less than the front a 53/47 ish balance.

But with my rear 2 pots I only get 3.34 x the peddle pressure to the rear so hardly any braking!

Note the bigger the m/c the less travel required by the pedal but as the area of the m/c piston is about a 1/10 of the caliper pistons I think you would struggle to tell the differance (1mm of pad movement = 10mm of pedal movement)

I think I will try and go for the same set-up as Bert but using the new forged Willwood calipers.

Edited by ScottHughes on Sunday 7th September 21:03