Pelican Parts Forums - how to calculate front to rear brake ratio ?

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- - how to calculate front to rear brake ratio ? (http://forums.pelicanparts.com/porsche-911-technical-forum/918726-how-calculate-front-rear-brake-ratio.html)

Richard,

We started with brake balance but became sidetracked with pedal forces which was entirely my fault as I thought the numbers in bill's spreadsheet looked high and I asked a question which led to the move away from the main topic which is indeed Balance.

Quote:

Originally Posted by Bill Verburg (Post 9169071)

+1
trying to calculate g for a stop is a futile task, variables include but are not limited to; variable μ, variable aero, variable tire μ, variable weight transfer, variable trans/diff/engine effects, variable human effects, variable road surface

the best that can be done w/o instrumentation is to calculate brake torque to get the ratios, w/ instrumentation you can correlate to the ratios but it's not really a fruitful endeavor.

The other factor that can be readily calculated is the hydraulic pedal ratio, this has a large effect on the brake feel, you can include mechanical pedal ratio w/ that to get an even better idea of what the brakes will feel like. Fortunately in the Porsche 911 world flex isn't as big an issue as w/ some other brands.

For the purposes of spec'ing the brake system it is conventional to assume no tire lockup, and as a first approximation for non-hybrid and non-aero cars, no aero or engine braking effects. One still needs to make sure the bias is good at the same time, but the lbs/g is a useful parameter in my eyes. It is something that comes up in FSAE context.

It is a way of factoring in the fact that a lightweight car doesn't need as much mechanical advantage as a heavy one. The lbs/g is basically a final/overall mechanical advantage for you're leg effort. The tire friction will determine how many g's you can achieve but I'm interested in the ratio of leg force to deceleration, not the peak.

I think the original question was answered so I hope we don't mind this side bar? :confused:

I must admit that I'm looking at this for spec'ing a brake system from scratch on a small formula type car, so I'm just using 911 data for a reference of what feels good- which is why I am so interested in the lbs/g of deceleration. I can match that parameter and know the brakes should feel ok.

Quote:

Originally Posted by chris_seven (Post 9168965)

Max,

It does all start to become difficult to work out deceleration rates as there are so many variables involved and by neglecting rolling resistance and a number of other factors the errors may become significant.

66lbs for 1g seems low.

When we designed our pedal box and selected cylinder sizes we looked at as much data as we could sensibly find.

There was a study in 1970 by TRL that suggested a typical male driver exerted brake pedal forces of between 145 and 185lbs when braking 'hard'.

This work used a test 'buck' which was quite well instrumented and was the force that was applied without locking wheels. More than 200 drivers were 'tested'

The level of g that this force creates would vary significantly depending on road surface and tyres but seemed like a reasonable guide for cylinder size selection.

Would you mind if I sent you a spreadsheet so you could maybe tell me where we are differing? Did you take into account extra forces like aero and engine braking? Did I assume a pad mu too aggressive? Did I just do the math wrong?

I'm not too concerned about how accurate the number is in the end, so long as I can match the number that the spreadsheet gives for my 911.