Reducing the effectiveness of a brake servo

[Greystone]

Hi All,

After a break for an engine rebuild, I took my Rover Tomcat to Lydden for the BARC Tintops races last Sunday. Due to various problems I only managed practise but there is another issue that I need to address.

I am finding it all too easy to lock the brakes at almost any speed. This may well be due to incompetence but I am sure that part of the problem is the excessive servo assistance in the braking system, meaning that with the light pedal pressures required there is not a lot of feel as to what is happening at the other end. Having to run on road tyres (Toyo R888s) obviously does not help and the Tomcat is definitely not short of braking power.

One option, obviously, is to try it with the servo disconnected but the result of this may well be that it puts the fear of god up me.

Does anyone have any experience with ways of varying the amount of assistance provided by a brake servo? All suggestions welcome.

Many thanks

Andrew Cherry

[RED21]

Besides adapting your driving style, one of the factors that determine brake servo assistance is the diameter of the actual servo. A 330mm servo will have more assistance than a 200mm servo due to the greater surface area of the internal diaphragm.
So one possible solution is to fit a brake servo unit from ether a normally aspirated coupe or from another model like a Rover214, which are generally smaller and thus give less assistance so more brake pedal pressure is required.

You could also play around with brake master cylinder diameters etc, if rules allow.

[GORDON STREETER]

Greystone Has this just happened and was it ok before. Have you done anything to the brakes since you last used it. Is it front or rear problem ? Have a good think about it before you go shelling out on new parts that you may not need !
Tyres won't make a lot of difference if they are all the same.

[GordonG]

Just diisconnecting the vacuum to the servo won't work well, as the mechanics of the servo get in the way of the pedal action (and feel). You may be able to remove the servo entirely (as you have alluded to). Or, as already suggested, find another (smaller) servo from another model.

Finally, you can address the problem just as you would for an unassisted system:

reduce the mechanical advantage of the pedal
reduce the area of the caliper pistons
run harder (lower coefficient of friction) pads
Run a larger diameter master cylinder.

All the above will require a firmer push on the pedal, and are also areas that could be used to tune the effort if you do ditch the servo (though you will probably want to move in the opposite direction! I would suggest maybe a day at an airfield to p;lay with different options as this is not something you want to try for the first time in qualifying...

G

[zac510]

I am loathe to keep bamboozling you with reasons why it might be bad but you could have forgotten the one way valve after the engine rebuild or the diaphragm could be leaking when pedal is depressed. I guess GS is right - just be sure it is a bad setup before shelling out for parts you don't need!

[Notso Swift]

Have you changed pads?
In my experiance some pads that may have the same ultimate retardation (CoF) may have a very different initial bite, they come on quick, and when they lock they can be hard to unlock, producing a completely different feel.
(If the same manufacture has change formula it cam be quite baffling)

[Greystone]

Many thanks for taking the time to post your thoughts on this.

No changes have been made to the braking system. It has always felt over servoed. My brother used to share the car when we were sprinting it and he also feels the same about it.

What has changed is since the engine was rebuilt it is producing a lot more power and consequently the car is arriving at the corners a lot quicker. It tends to front end lock up. There is an adjustable pressure limiting valve fitted in the rear brake line which I have progressively backed off. This has shown some benefits. It has the standard set up for a car from the Tomcat race series of AP vented discs (330mm IIRC) and the AP four pot calipers on the front and the standard road car discs and pads on the rear and Mintex race pads all round.

Regarding your specific questions and points:

No changes to the braking system.
The one way valve is in place. (I have a spare I can try).
I have already considered looking at the mechanical advantage at the pedal.
I have a spare servo which I will loook at although I think that it may well be the same.
A harder pad with a lower coefficient of friction might be a cost effective option as it will soon be due for a new set of pads anyway.
I will investigate the possibility of using a less efficient servo from one of the lower spec models.

I am not sure about removing the servo assistance completely as it is a heavy car (almost 1100kgs) and the pedal effort without a servo might be excessive.

Whatever I do I agree with the suggestion that it the setup needs to be finalised away from the race circuit where changes can be made and tested progressively.

Andrew

[johnny yuma]

The more you back off the rear brakes the more work the fronts will have to do and if it's too touchy to modulate foot pressure they will lock.You can take out the vacuum diaphragm but leave rod etc in place to see what its like no servo.Then you may try more mechanical advantage at the pedal.In any event the rear brakes need to do 40%+ of the deceleration.

Try things on gravel at lower speeds.

[Notso Swift]

40%?
It is a production based FWD so the static weight would be close to 60/40, more like 20-25% of the dynamic weight is on the rear and the braking force should as close as possible match that percentage of dynamic weight.

[GORDON STREETER]

You could almost blank off the rear brakes and still do the same lap times !

[graham bahr]

Originally Posted by GORDON STREETER

You could almost blank off the rear brakes and still do the same lap times !

agrred infact after a rear brake failure a few years ago thats exactly what i did.

andrew (greystone) imho a non servo bias pedal box is the only real way forward.

[johnny yuma]

Originally Posted by Notso Swift

40%?
It is a production based FWD so the static weight would be close to 60/40, more like 20-25% of the dynamic weight is on the rear and the braking force should as close as possible match that percentage of dynamic weight.

Some wit once noted your brakes don't slow the car your tyres do.If you're only using 2 out of 4 tyres to stop you,and even if the you are right and weight transfer means 25% of the car's weight only is on the rear wheels thats still enough weight to work with but a big grab of deceleration you're giving up if you dont-gotta mean you're going to be outbraked.Maybe the dampers are too stiff or rollbars too fat? Or does it lockup in a dead straight line at high speed?

I ran a 1000kg (rwd) sedan for a season with servo boost to the front discs and no servo to the rear drums--too lazy to fit smaller slave cylinders to the rear drums after fitting the disc brakes on the front.Got sick of the inside fronts locking on turn in,changed the rear cylinder size,ran all the lines to the servo and problem solved.