Racing Pedal box

[simoncotton]

why do most race cars have a racing pedal box with no servo?

I have just bought a golf mk gti to race and it has no servo, just abrake bias pedal box with seperate cylinders for the front and rear.

Should I fit a servo or keep with the pedal box, want to race it on the track and sprint / hill climb events.

[Sam_68]

If you have separate cylinders for front and rear, you would need twin servos (or else fit the servo on the front circuit only and adjust the bias to suit).

Most race cars have unservo'ed brakes because:
1) Servos are extra weight
2) Pedal feel is better without a servo and it is easier to modulate the brakes for heel-and-toe gear changes.
3) Most racing drivers are men, not girls or limp-wristed wusses, therefore they have leg muscles that are capable of stopping the car without vaccuum assistance.

Unservo'ed bias pedal box with twin m/c should be fine, once you get used to the increased pedal pressures. I'd leave it as it is, personally.

[simoncotton]

Sam_68 said:

If you have separate cylinders for front and rear, you would need twin servos (or else fit the servo on the front circuit only and adjust the bias to suit).

Most race cars have unservo'ed brakes because:
1) Servos are extra weight
2) Pedal feel is better without a servo and it is easier to modulate the brakes for heel-and-toe gear changes.
3) Most racing drivers are men, not girls or limp-wristed wusses, therefore they have leg muscles that are capable of stopping the car without vaccuum assistance.

Unservo'ed bias pedal box with twin m/c should be fine, once you get used to the increased pedal pressures. I'd leave it as it is, personally.

Thanks for the advice. What set up (bias front / rear) would you recommend as a starting point, should it be more to the front then the rear?

[slinky]

simoncotton said:

Thanks for the advice. What set up (bias front / rear) would you recommend as a starting point, should it be more to the front then the rear?

[GreenV8S]

simoncotton said:

What set up (bias front / rear) would you recommend as a starting point, should it be more to the front then the rear?

It depends on the geometry of the wheels, calipers and master cylinders. The total braking effort at the road ignoring the bias valve should equal the static weight distribution of the car. If you're going to get it wrong, too much on the un-driven wheels is slightly preferable, but the more accurately you can get this the better. You can tell whether the balance is right by setting the car up in a steady corner close to the limit of grip (easiest in slippery conditions) and then feeling whether the car tends to understeer or oversteer when you apply the brakes lightly. This shows you the static brake balance.

Then you need to set the dynamic balance. You should have a bias valve which restricts the pressure to the rear circuit as the amount of braking increases, to compensate for the weight transfer under braking. The setting can be roughly calculated from the line pressure and the amount of weight transfer (which can itself be calculated from the wheelbase and CoG height). Once you have the static setting right, you test the dynamic balance by braking hard in grippy conditions and feeling whether the front or rear locks up first. Again the easiest way to tell which end is locking is to put the car on a very gentle curve before you hit the brakes and feel whether it understeers or oversteers.

Be aware that the weight transfer takes a while to settle down and if you stab the brakes hard before this has happened you will get a misleading result, for the heavy braking test you need to apply the brakes progressively so that the car has settled before you reach the limit of grip.

[JagwireXKR]

The biggest driver (excuse pun) for having no servo is hat the human foot is more sensitive to changes in pressure than changes in position and hence varying the applied brake force can be done with more finesse and accuracy.

[GreenV8S]

JagwireXKR said:

The biggest driver (excuse pun) for having no servo is hat the human foot is more sensitive to changes in pressure than changes in position and hence varying the applied brake force can be done with more finesse and accuracy.

I think there's a natural optimum force which is in the region of half your weight, which is a comfortable amount of force to apply and gives you accurate feeling and control over the amount of braking. Much less than this and you can control it but it is harder to feel how much force you are applying. Much more than this and you have to start straining and can't control it accurately. If you need a servo to get into this comfort zone, then you should fit one. If you don't then you shouldn't.

[bales]

green v8s said:

I think there's a natural optimum force which is in the region of half your weight, which is a comfortable amount of force to apply and gives you accurate feeling and control over the amount of braking. Much less than this and you can control it but it is harder to feel how much force you are applying. Much more than this and you have to start straining and can't control it accurately. If you need a servo to get into this comfort zone, then you should fit one. If you don't then you shouldn't.

Yeah I definately agree with this, I did the pedal box and braking system for the formula student car at uni for the previous 2 years. I actually did some testing on the amount of pressure we can exert just with a dummy pedal pedal box and a load cell. Most people can actually exert their own body weight onto the pedal (imagine doing a squat on one leg). However it is bloody hard to do this and I wouldnt imagine that you could modulate the pressure very well at this point or even concentrate on clipping an apex or anything like that. If you need a servo to be able to brake comfortably then I would use one because it makes everything so much easier.

I have driven both our race cars which had straight twin m/c's without any assistant and even in a 230kg car it is still very very difficult to lock the wheels, (in the formula student rules before you can compete you have to pass a brake test to show that you can lock up all 4 wheels and this can be suprisingly hard to pass).

The biggest thing that I have found makes the difference is just the pedal ratio, but i guess that if you have just bought a pedal box it will already have fixed length pedals, we always tended to go for between 4-5 : 1 for the ratio.

In terms of deciding the brake distribution, I have always started be calculating the total force required to lock the wheels, this has to be a bit of an approximation but it always turns out pretty accurate in the end. You just need to know the mass, C of G and wheelbase, you then assume the coefficient of friction will be 1.5 on slicks which is a bit optimistic and then just take moments about the car.

If you then have a set weight distribution that you know or are aiming for you can split the force between front and rear assuming that the tyres are equal size front and back which i suppose they probably wont be. If you know that force approximately then you can work back to the master cylinder sizes and size them appropriately so thet they give the correct forces with everything set central. Assuming that you have a balance or bias bar you should set that to the middle when calculating these things and then tweak it afterwards to get the correct bias.

It sounds comples but it isnt to hard to work out, only basic moment and maths and stuff really.

Alex

Edited by bales on Sunday 20th August 13:12

[Sam_68]

Regarding braking effort:

Remember that, apart from pedal ratios, you have the option of changing M/Cyl sizes to adjust pedal pressure, before you resort to installation of a servo. You really shouldn't need servo assistance to stop a car the weight of a stripped-out Golf though...we're all just getting soft, expecting feather-light controls because most modern road cars have power steering and servo brakes!

Regarding brake bias:
Yes, you should be adjusting bias so that the fronts lock just before the rears. You can adjust it fairly close for dry track use (ie. fronts lock only just before the rears), but it is normal to give yourself a little more leeway by setting slightly more front bias where grip is less certain (ie. wet conditions or variable track/road surface). How fine you can cut it depends to some extent on driver skill.

[falcemob]

simoncotton said:

What set up (bias front / rear) would you recommend as a starting point, should it be more to the front then the rear?

Pedal ratio should be 5 or 6:1 for good feel with no servo. A good starting point is to have a smaller M/cyl to the front callipers (gives more pressure to the front) and/or balance bar giving more bias to the fronts, this will all depend on the number of calliper pistons and sizes per corner.
You could use a remote adjuster so you can adjust the bias for different tracks or weather conditions.

Edited by falcemob on Monday 21st August 13:20

[thinfourth]

Or you could do what i would do.

Find a quiet industrial estate late at night and arse about until it feels about right.

but do bare in mind that ideal brake bias can change dependent on tyres, grip, weather, weight distribution and other factors so throw the calculator in the bin and see what works.

[simoncotton]

thanks for everyones help, some very sound advice.

I think one of the big problem's is that this is the first car I have owned without a servo/assistance so i'm sure its just me not being used to them. However, you have to stamp on it hard just to get the car just to stop normally - does not give much in the way of confidence.

I obviously need to look at the set up.

[Sam_68]

Just a thought, Simon, but do you know what pads the car is running?

Some race pads give bugger all braking effort until they are up to temperature; the more extreme race pads will never reach working temperature in road use. Could be that, in addition to the lack of servo, the high pedal pressures are due to use of too 'hard' a pad for the conditions you are driving the car under.

[simoncotton]

Sam_68 said:

Just a thought, Simon, but do you know what pads the car is running?

Some race pads give bugger all braking effort until they are up to temperature; the more extreme race pads will never reach working temperature in road use. Could be that, in addition to the lack of servo, the high pedal pressures are due to use of too 'hard' a pad for the conditions you are driving the car under.

I'm not sure what pads the car is running. I have been advised to use pagid fast road pads.

Any suggestions?

[thinfourth]

simoncotton said:

I think one of the big problem's is that this is the first car I have owned without a servo/assistance so i'm sure its just me not being used to them.

the problem is when you go to car with a servo once you are used to no servo.

Still convinced most other cars are broken now used to a car without a servo

[Sam_68]

simoncotton said:

I'm not sure what pads the car is running. I have been advised to use pagid fast road pads.

Any suggestions?

Try speaking to Pagid, Ferodo and Mintex directly, explaining your brake specification, the difficulties you have been having, and the type of use you put the car to. They will be able to recommend the most appropriate compound from their range.

Their Engineers are usually pretty helpful, and know more about brakes than almost anyone on here!

[simoncotton]

Sam_68 said:

simoncotton said:

I'm not sure what pads the car is running. I have been advised to use pagid fast road pads.

Any suggestions?

Try speaking to Pagid, Ferodo and Mintex directly, explaining your brake specification, the difficulties you have been having, and the type of use you put the car to. They will be able to recommend the most appropriate compound from their range.

Their Engineers are usually pretty helpful, and know more about brakes than almost anyone on here!

Thanks Sam, good idea probably best speak to them direct. I will hopefully use the car for sprint days and hill climb racing.

[Sam_68]

If your using it for hillclimbing and sprinting, you'll not be worried about the brakes overheating, then, and you'll obviously want something that bites from flat cold.

There shouldn't be any conflict between suitable pads and your requirement for minmum pedal effort, therefore.

[chappardababbar]

Is it possible to fit an electrical brake servo to a pedal box?

Thinking of fitting a pedal box to a road car. don't have the space for a hydraulic set up.