TECH TORQUE
> DR TIM OFFERS SOME SIMPLE TIPS ON HOW TO GET THE MOST OUT OF YOUR BRAKE SYSTEM
Get the most out of your brake system thanks to our in-house engineer Dr Tim
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 achieve it.
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, let’s 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 it’s 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 rightangle point, you’re off to a good start. If, however, the brake pedal is set up to start at the rightangle 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 – 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 there’s not enough you won’t get to that desired 1350psi of line pressure. If your engine can’t 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, you’ll need to solve that before moving on.
Put a vacuum meter on the inlet manifold and see what reading you get. If it’s low, check the inlet manifold for leaks by spraying brake cleaner around the gasket areas. If there’s a leak, the fluid will make the engine run rough as it gets into the nearest cylinder. Fix these problems and you’re on the way to braking excellence.
For the sake of this discussion, let’s 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 and another 750lb from the booster) at the pushrod becomes 1310psi of line pressure. So we’re 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 1/16-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 that’s 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 can’t be changed, you’ll 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.