Pedal Power

mac · Post by **mac** » Mon Mar 05, 2007 2:51 pm

Any one know or tell me how I could calulate the PSI of force exerted on the rear pads when braking hard?

Mac

MacK · Post by **MacK** » Mon Mar 05, 2007 2:54 pm

Would it not depend on how hard you press the pedal?

Assuming non-servo system.

mac · Post by **mac** » Mon Mar 05, 2007 3:15 pm

Non servo - and yeah dependant on pedal power hence the comment on "braking hard"

Mac

MacK · Post by **MacK** » Mon Mar 05, 2007 3:29 pm

How hard can you stamp

I was going to suggest fitting some sort of pressure gauge if that would be possible..

mac · Post by **mac** » Mon Mar 05, 2007 3:32 pm

Hmm - measuring device.......

I was just looking for a ballpark figure that assuming blah,blah then it would be x psi required to lock the wheels etc.

Mac

thinfourth · Post by **thinfourth** » Mon Mar 05, 2007 4:53 pm

I wander what mac might be thinking off?

mckeann · Post by **mckeann** » Mon Mar 05, 2007 4:57 pm

happy handbrake??

Rich H · Post by **Rich H** » Mon Mar 05, 2007 5:01 pm

I think that to work that out would have too many variables to calculate accurately.

You could work out relative piston sizes and then the bias front/rear and have a guess at pedal force exerted.

Rich

mac · Post by **mac** » Mon Mar 05, 2007 5:32 pm

Am I that transparent?

Mac

robin · Post by **robin** » Mon Mar 05, 2007 8:24 pm

mac wrote:Am I that transparent?

Mac

It's not really very hard ...

(1) Take a guess at force on brake pedal - say you can lift 200Kg weight with one leg (I haven't been to a gym since I was 16, so cannot say how many kgs a leg can lift - but I know I can just about lift me + one son on one leg, so 200Kg is high ballpark). That's 2000N, ish.

(2) Take a guess at the gearing from pedal to push rod into master cylinder - say 5:1 (i.e. the pedal is 5x further away from the pivot than the master cylinder rod is) - so that's 10,000N into the master cylinder.

(3) Take a guess at the cross sectional area of the master cylinder. Say 20mm diameter = 10mm radius -> pi*r^2 = 3.141 * 10 * 10 = 314 sq.mm.

(4) Take a guess at the cross sectional area of the pistons - now here you need some fudging depending on how many pistons you have (Elise has six) and how the front and rear pressures relate to one another (if you have a brake bias valve of some sort), but ignoring that and just assuming that the pressure will be the same for all four corners, you could for a moment pretend you had 6x50mm diameter pistons each with surface area of pi * 25 * 25 = 1963 sq.mm.

(5) Now realise that the force is geared up by the ratio of the surface areas - so 10,000N applied to a 314 sq.mm will generate a pressure of 32N/sqmm in the master cylinder - oil doesn't really compress, so pressure in each chamber must be the same, so force applied to rear of brake pad by piston is 32 * 1963 = 62,800N. Now your brake pad has surface area of approx 50mm x 100mm = 5000 sq.mm. and you're leaning on the back of it with 62,800N so the pressure is 62,800/5000 = 12.6N/sq.mm.

(6) Convert to rubbish units.

http://www.google.co.uk/search?q=12.6N% ... &oe=utf-8

Answer, 1,800PSI or so. Remember at the rear you have two pads, one piston, so that will be shared between the two of them = 900PSI each.

That should be right to within 2 orders of magnitude

Robin

mac · Post by **mac** » Mon Mar 05, 2007 8:32 pm

so a fuel pump wouldn't be strong enough then

Mac

thinfourth · Post by **thinfourth** » Mon Mar 05, 2007 8:59 pm

Not really

i looked at various things before happy handle Mk1

One of which was two electric line locks so you could push a button and switch off the front or back brakes but decided that might be too busy for auto testing. Would be better then happy handle as you could switch off the back brakes for reverse spins or for "tyre warming"

If it was a pure autotest car i would set the brakes up so that happy handle does only the back brakes and the foot pedal does only the front brakes. I might see if i can do this on the banana with a line lock before the happy handle

Either way I must sit down at some point and design mk2 happy handle and talk to stu and see what he can magic up

Also i might be fitting happy handles to the landy as i would like to try fiddle brakes where you lock up 1 rear wheel to either improve traction of skid steer.

Rich H · Post by **Rich H** » Mon Mar 05, 2007 10:21 pm

WTF is a happy handle?

I realise I may live to regret the responses...

r055 · Post by **r055** » Mon Mar 05, 2007 10:26 pm

is this one in the beemer?

robin · Post by **robin** » Mon Mar 05, 2007 10:41 pm

Happy handle is usually a term used to refer to an improved handbrake, e.g. hydraulic handbrake system. It has to come with a big lever so you can lock up the rear wheels quickly and efficiently.

Actually, the handbrake on the TSE honda that Lawrence kindly let me use at the start of the year was so good that no additional mechanism could have been better, except perhaps a 90-degree extension on the end of the handle to make it easier to reach whilst flailing the wheel and head, with slack jaw and tongue hanging out (my normal auto-test look

).

Cheers,
Robin