THEMíS THE BRAKES

DR TIM OFFERS SOME SIMPLE TIPS ON HOW TO GET THE MOST OUT OF YOUR BRAKE SYSTEM

STORY TIM BARTROP PHOTOS SM ARCHIVES, ANDREW BROADLEY & TB

THE foundation of engineering is a systematic and logical approach to problem-solving. Engineers start with the desired target before analysing and then optimising the system required to

When it comes to brakes, the target is to achieve the desired line pressure. We want enough fluidline pressure so that there is enough squeezing force at the wheels, with the brake pads gripping the disc with just the right force for optimal braking. A commonly accepted line-pressure target for a four-wheel disc brake set-up is 1200-1400psi. Personally, I like 1350psi as an ideal pressure.

Starting at the brake pedal, letís say that a comfortable average foot force on the pedal is 80lb. With a 7:1 pedal ratio, that gives us 560lb of force at the pushrod. So pedal ratio is important for good braking; clearly, a low ratio can limit the performance of the whole system.

The brake pedal is often the source of poor brake performance for other reasons, too. Whether itís an underfloor pivoting type or hung from under the dash, the brake pedal should be set up so that as it is squeezed it moves towards the point where it is at a right angle to the pushrod, and then marginally on from that position for maximum efficiency. If maximum brake force is at that right-angle point, youíre off to a good start. If, however, the brake pedal is set up to start at the right- angle point or past that point, the pedal becomes less efficient as the brakes are applied and, in the worst case, risks going over centre to a position of no return. Lengthening the pushrod or changing to a different pedal design are simple solutions to this problem.

The pushrod connects to the booster Ė the next stage in the system. A good dual-diaphragm in-line booster that is receiving at least 20 inches of engine vacuum will give you another 750lb of force into the master cylinder. So vacuum is important, too. Check the vacuum being generated by your engine; if thereís not enough you wonít get to that desired 1350psi of line pressure. If your engine canít develop the target vacuum due to a big cam, you can solve this by simply changing the vacuum source to a vacuum pump. But if the low vacuum is due to some other issue, youíll need to solve that before moving on.

Put a vacuum meter on the inlet manifold and see what reading you get. If itís low, check the inlet manifold for leaks by spraying brake cleaner around the gasket areas. If thereís a leak, the fluid will make the engine run rough as it gets into the nearest cylinder. Fix these problems and youíre on the way to braking excellence.

For the sake of this discussion, letís imagine that we have a 11 /8-inch-bore master cylinder. This master cylinder just happens to have about a onesquare-inch piston area, so our 1310lb of force (560lb from the pedal booster) at the pushrod becomes 1310psi of line pressure. So weíre a bit short of my ideal 1350psi, but not by enough to cause a problem.

To now get the brakes up to optimum pressure, you can increase the pedal ratio and/ or the booster size, or decrease the bore size of the master cylinder. For every 16-inch reduction in the master cylinder bore size, you can get a line pressure increase of up to 100psi, depending on the bore diameter. Remember though that decreasing the bore size increases the pedal travel required to displace the same amount of brake fluid, so thatís a compromise that must be taken into account. In reality, 7:1 pedal ratios are not that common, but that is often made up for by a big productioncar booster that pumps out 900lb of pushrod force, plus the pedal force. I see so many seveninch single- and double-diaphragm boosters that have very poor performance and are the source of brake problems, but this has been ignored due to the convenience of using a small-diameter booster in tight spaces. If the booster canít be changed, youíll need to get to the line pressure target by other means.

If you understand these simple principles and start at the brake pedal and finish at the wheels when optimising your system, good brakes are only a few steps away.

UNDER PRESSURE

THERE are other ways to measure brake line pressure if you are having trouble with the calculation method Iíve outlined. Pressure gauges and adapters are available from Wilwood, Speedway, Auto Meter, Kent-Moore and others. These are usually fitted in place of the bleed nipple at one wheel, and when foot pressure is applied the true line pressure is shown on the gauge.

Similarly, engine vacuum can be checked by attaching a 0-25inHg vacuum gauge to a hose thatís connected to manifold vacuum on the engine, such as at the brake booster take-off point. Ensure that you use a proper vacuum hose so that it doesnít suck in and give a false reading.

BOOSTER DELETE KITS

This Mustang race car has been fitted with a booster delete kit, which is not recommended for street use. A firmer pedal with less initial bite and more progressive feel is arguably preferable for motorsport applications

FACTORY PEDALS

Factory brake pedal set-ups have a great deal of engineering smarts invested in them at an OE level. Unless an engine conversion creates booster clearance issues or youí building a show car and aiming for a clinically clean firewall, sometimes ití best not to try and reinvent the wheel s