how to calculate front to rear brake ratio ?
 

How do I calculate my brake front to rear ratio?

I have 996 calipers in the front w/ rotors turned down to 305 mm (from 313). Calipers are as such moved closer to center.
Rears are stock 1987 Carrera.

No P/V valve anymore.

I'm a curious moron Horst. Not sure what ratio you're interested in? (Perhaps this is common brake-speak language that I'm unaware of.) Grüße an Sie in München von Miami.

This may help

http://www.stoptech.com/docs/media-center-documents/the-physics-of-braking-systems

I have been reading on this forum that a good front/rear ratio on a 911 is around 1.4. I have also been told that a front 996 upgrade leads to around 1.8, so a somewhat too much towards the front. As I have modified the front upgrade with a smaller rotor, I would like to calculate what I ended up with and if that is consistent with my practical experience.

Now that said ratio, I am not sure what it actually is. I am assuming torque front to torque rear, so both the piston sizes must go into that equation as well as the rotor sizes.

install a manual PV and adjust until balanced.

of course in a safe environment.....:eek:

Bob,

thanks for your reply, but first it does not answer the original question and second, a P/V valve should only be installed in the rear brakes, hence it will not help on any too high front bias.

Go to that site:
Jake Latham's 2000 Radical Prosport

Early 911s were about 1.6

I would agree with Max and the calculation I made a few years ago came up with around 61/39 front to rear.

The Stoptech White Paper has all of the details about how to calculate the bias ratio but there will always be some influence of brake pad types to consider so these numbers are only a rough guide.

Pressure Reducing Valves are useful in situations where rear wheels may lock up but they are not the greatest idea in terms of overall brake balance.

Most of these devices operate on a fixed ratio of input to output pressure and the adjustment does not change the brake balance but only the 'set point' at which they operate.

Up to the set point they have no effect on brake balance at all and the only real way to do the job is with an adjustable pedal box.

The ratio of pressures is normally stated on the valve which does differ from manufacturer to manufacturer.

It's easiest to do it at a chosen static line pressure line and to then calculate brake torque, the ratio doesn't change w/ line pressure but does change w/ a p/v in place so I do 2 calculations of brake torque, one below the knee and one above.

keep in mind that the ratio stays the same but the requirements change w/ conditions

you can do it for a wheel or for an axle doesn't make a difference except in the value of the torque derived being for a wheel or for an axle

brake torque at an axle is = wheel piston area for the axle x effective rotor radius x μ x line pressure
wheel piston area in M^2
effective rotor radius in M distance from the axle center line to the center line of the caliper pistons
μ coefficient of friction dimensionless
line pressure in N

Here are some that I put together, I did it 2 ways just to check results, the other way is much more complicated but starts w/ the applied pedal pressure in #s

http://forums.pelicanparts.com/uploa...1466375593.gif

Bill,

I must say that your pedal forces look huge, must be the spinach :)

At MIRA (http://www.horiba-mira.com/) in the UK the industry standard they look for in their Brake Test Division is a pedal force of around 60kg for a deceleration of 0.5g.

60kg is 135lbs.

I would be interested to know how you measured pedal forces.

When we designed our Bias Pedal Box we tried to use this standard and when we carried out basic testing we used a Taplow and tried to keep close to the MIRA suggested values.

Our 'leg' calibration was a bit subjective as we didn't have a suitable load cell but we did try to maintain a consistent 'push'.

The results we have had seem quite good and we have many pedal boxes being used in Race/Rally cars with good effect.

I used around 80# pedal pressure which is on the high end of what a healthy male can exert, but nowhere near the max

It drives me crazy when I see organizations, Mira in this case, composed of people that know better using loose terminology. kilogram(kg)is a unit of mass in metric speak Newton(N) is the correct unit of force. Yes 135kg of mass has a weight(force due to Earth's gravity) on average on the Earths surface of 132lb(~587N)

I thought you were using 355.8577 which is why I asked the question. Your chart shows this as the pedal pressure and I assumed this was pounds as your text does say it starts with pedal pressure in #s.

It's now clear that I was mistaken.

It is quite common for people to ignore the 9.8066 to convert kg to Newtons in the same way people ignore the difference between lbsf and lbsm in the Imperial system.

The problem is that most of us that are a few years old have grown up with Imperial Units and then the metric CGS system and then the MKS system which was finally replaced by the SI system.

In the MKS system kgf was the unit of force.

Depending on how old you are and when you graduated different people still favour different systems and when we work with US customary units it becomes a real muddle.

I still have some colleagues who are converting to the metric system inch by inch.

correct, I should have been more clear.

I am going to have to spend some time reading this.:confused:

what I did using a PV on the rear line was open it up until I was getting enough rear brakes, as I had upgraded my front calipers. with the stock PV, I could tell I was not getting much rear braking.

retired the 911 from DE, now thrashing a 951.

http://i197.photobucket.com/albums/a...pshcgt1jua.jpg

A typical reducing valve operates by changing the inlet to outlet pressure in a fixed ratio.

The slopes of the inlet and outlet pressures being identical at every set point.

By adjusting the valve - this diagram is for a valve with fixed 'click' adjustment all you do is change the point at which pressure is reduced ie the position of the 'knee'

You can buy vales that are continuously adjustable and valves with many different ratios between inlet and outlet, between 37% and 60% is typical but they all have the same basic characteristic.

They work most effectively when a car has rear biased brakes which need to be limited but if the car is badly front biased then they will not really add anything of value.

A bias pedal box changes that balance throughout the entire range of brake operation and are much more effective.

That seems quite high to me. I re-did my calculations for an Early 911 and come up with about 66 pounds per 1g deceleration with brake pad friction of 0.4 and a weight of 2200 lbs. Am I missing something big? I used Bill's rotor effective radii for that and the bias changes to 1.5 like he shows.

270 lbs per g seems quite high and it's certainly far away from what I calculate for a car that feels pretty good to me.

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.

+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.

I'm not sure what we're trying to do here.

At first I thought we were talking about brake bias. Now I'm not sure.

I do a lot with brake temps - if that has any relevance to this discussion.

Richard Newton