How did you set up Brake Bias?

[Russ Noble]

however if your brakes are too large they may not get to operating temp and have less braking force than a setup which is slightly smaller and reaches operating temp.

Trevor, I have NEVER had cars that were overbraked in terms of heat dissipation,(Edit:except my F Fords which were light, had skinny tyres and not much power!) I would love a setup that was inefficient due to low temps! At least that could be easily fixed in terms of softer pads, presumably.

In terms of bias it should be set so you get MAXIMUM braking front and rear.

Now maximum braking does not mean lock up, it means when lockup is impending.

And at that point I have found that the difference between impending front lockup and impending rear lockup is as little as, or maybe less than, half a turn on the balance bar, hence my contention that the effective difference in the braking performance is very small....However I guess I could be wrong and half a turn could be significantly large.

Never the less, I like my setup and have had good results from it over the years, including a few class lap records. But when I get my current car optimised if I have time I may play around with more front and see what happens.....

 

[Clayton]

Trevor
I have changed the values in the charts that many times, looking at 'what if's' and was using the 0.8g for AO32R Yokohama's at a race track. (a little optimistic I suppose).

Steve
I used the same spread sheet on another 40 and was able to adjust the proportioning valve (tandem master) to either lock the fronts first or the rears. So I would say that the calc's have to be pretty close.
Ended up adjusting for the fronts to lock just prior to the rears.

Regarding which end to lock first, I would say the fronts. This is also based on the fact that when down changing and you don't quite match the revs with the ground speed, you end up with more rear braking (engine braking) and locked rears. Been there, done that with no happy ending !! IMHO.

 

[Craig Gillingham]

meeting the IVA machine criteria is a pain in the ars@.

having to weld up the bias bar adds to the ball ache... no independant machines exist i dont believe for pre IVA set up????

 

[Dave Bilyk]

Just one further point that only really comes into play with a non servo setup.
Pedal effort; With servos you dont need to worry about it, but without servos you need to go for good front to rear balance (ie well chosen mcyl and piston areas etc) to minimise pedal effort.
So when I did the calculations for my setup, I took this also into account.

In this case proportioning valves etc correct a poor front to rear balance at the expense of pedal effort.

Dave

 

[Trevor Booth]

Russ,
a pad that works at a lower temp will do it, There is some very neat material around these days that are not that soft that they wear away in a short time and will work at lower temps.

it will depend on how much % change one turn makes as to the effect.

I use a set of pressure gauges to see how much % change for one turn I have ISO dry break test ports in the lines to do this.

There are also pressure pads with a gauge that slips in where the pad is (after you take it out ) that are very useful tools

F&R -- well I have got you thinking -)

Clayton,
your 0.8 may not be that far out on a good track with plent of rubber on it.
032's when they get up to temp are very sticky.
however even if you used a lower CoF the balance is the same

also I did not check the rotor radius used. Ideally the radius at the centre of pressure of the pad not radius to OD of Rotor. Radius at centre of piston is very close to centre of pressure of pad.

 

[Clayton]

Trevor
I'll use a lower CoF in future.
Yes, I used the disc dia - the pad radial depth / 2 for the effective brake radius.

 

[Trevor Booth]

Clayton,
That would be very close, need a brake dynamometer to get exact torque and of course that is not practical. However the main aim is calculating the physical size of components and bias etc as a good starting point.

You have put a lot of work into your chart, well done

 

[EGLITOM]

Thanks Clayton

for that great chart.
Did put in the values of my RCR 40 setup. THe layout of this brake system is almost ideal according to your table (at a 0.7 CoF it needs to be theoretical bias bar balanced a little to the front 55%)
it uses two 1" willwood mains. total pedal force would be 75 lbs.
Sounds good to me.
The good reputation of the RCR setup is also a proof for the correctness of your chart. I would go ahead with it and do the layout accordingly

TOM

 

[Clayton]

Just to clarify, I cant take credit for the charts (I'm not that good on Excel). but as stated in my first post, I found them on the web while researching for my brake setup.
After getting the same results back from AP, I thought they must be pretty good.
Its makes for a much quicker answers than using the equations from the HP & SA Design books.

 

[EGLITOM]

Clayton

dont worry i runned through every formula in this charts and found nothing which wouldn´t make sense.

TOM

 

[SteveB]

Thanks for all the posts especialy Clayton for the spread sheet. I had two other versions of the same sheet but not in the same detail. The weight transfer information was especialy usefull to establish a target to aim for.

The set up I have when input in the spread sheet confirms that what I have is basicaly OK however the rear mastercylinder is probably oversized resulting in the pedal bias bar beeing well set to one side to regain the setting that was acceptable when the car was SVA tested.
The setting had to be fixed permanently at the test ( welding or drill and pinned ).
The result from the spread sheet suggests that a change of master cylinder size can give a split that is likely to give the right ratio with the pedal bias set central.
I will have to replace the bias bar if the drilled section is then inside the mastercylinder center lines.

The posts about brake test methods will also be a big help; unfortunately there are no local roads to do the tests safely, but now I have a better idea of what to aim for.

Thanks again for all the input

Steve
el

 

[Mick Ridley]

Hi,
Ive been reading this discussion, with great intrest, but im a little confused....Given that all the cars, are GT40s, with similar, weights, etc

Im supprised at the great differences, in the use of mastercylinders/caliper pistons

Russ said, he has used, Equal sized master cylinders, and the same sized discs, and calipers all round, and the brake split is near enough 50/50

Clayton, in his Excell sheet, uses .813 front, and .625 rear master cylinders, he uses 6 pot front calipers, and 4 pot rear calipers,with his front calipers, having double the area, of his rear calipers

Tom says he is useing, 1 inch master cylinders front and back, and as he has the delux RCR kit, he has 6 pot calipers front, and 4 pot calipers on the rear, assuming his caliper are similar to Claytons. his front calipers, will have double the area of his rear calipers

Russ, Clayton, and Tom,are all useing very different combinations, of master cylinders, and calipers........

I just want to understand about braking systems, so I can make the correct choise of master cylinders for my car

I hope someone can help

mick

 

[Dave Bilyk]

Hi Mick, notice that since Claytons front calipers have double the area of the rears, and so do his master cylinders (.813^2/.625^2=2.08). So everything is in proportion and all other things being equal he would have the same Front Rear balance as Russ.
There are plenty of choices to be made, both technical and economic, which can result in the same Front to Rear balance.
In my case I just started with the master cylinder size I had, and chose calipers and disc sizes to give good balance and pedal pressure. If it hadn't worked out I would have changed one or both master cylinders to make it right.
Using the spreadsheet and working out a range of options does help you to understand how it all works, so I suggest that you could do that.
The one pitfall I have noted is choosing too small a caliper for the rear; since we are talking mid (ish) engined cars here, the rear brakes need to be about twice as effective as they do on a front engined car, and with a small caliper and pad, even if you can get the front to rear balance, you will overheat and eat the rear pads as I did before I upgraded to a properly balanced 4 pot system.

Dave

 

[EGLITOM]

Mick

The piston area of the willwood 6 pot in front = 4,02 sqinch/side (pistons: 41,1mm and 2x 28,4mm
The rear calipers have 2 piston 44,5mm each side resulting in 4,821 sqinch surface.
So not to far apart from each other

as mentioned following the calculation one would have to bias a little (5%) towards the front.
This is the theory, praxis will show how it works and where i end up with the bias bar.

TOM

 

[Clayton]

Mick
I would of liked to have run the same size masters, or at least a larger rear one. But the calipers available to suit the disc that I needed (one with a 190mm internal handbrake drum) was very limited and had small pistons.
I could of went down the two piece setup path (removal bell) but stuck with a factory style one.
You live and learn.

Clayton

 

[Mick Ridley]

To Dave, Tom , Clayton

Thanks for replying to my question..

I now have a much better understanding, of what is required to achieve a balanced braking system on a GT 40

thanks

Mick