The importance of correctly setting up brake balance.

Incorrect brake balance can result in a car becoming unstable and spinning off the road. This is why brake balance is crucial not only for manufacturers of road vehicles but also for race cars, where race engineers can manually adjust the brake balance.

To visually explain this, we use the weight transfer curve to provide a clear graphic representation of the desired braking. This helps in determining how to alter the brake balance of a vehicle.

In any car, the green line shown on the graph represents the ideal line. This indicates a perfect brake system, optimising the use of brakes on both the front and rear axles. In a racing scenario, the goal is to follow this line perfectly. The actual brake equipment used will be represented by the straight blue line. The point where the blue and green lines intersect is known as the Zcrit point. Beyond this point, the rear brakes will lock up first and make the car more unstable. For any car setup, whether for racing or road use, the brakes must be as safe as possible, and this graph helps determine that.

In a motorsport context, the ideal Zcrit will depend on the preference of the race series manufacturer, our customer. You can control the balance initially through the corner brakes themselves by increasing the size of the caliper (more piston area, higher friction, higher disc effective radius) – all these factors will provide more brake torque on a particular axle. By balancing the brakes on the front and rear axles, you can alter the vehicle’s balance and the point where the blue line crosses the green line (Zcrit). In motorsport, you have added flexibility not available in road cars, as they usually have a front/rear split with two separate master cylinders and a balance bar. This allows for different sizing of the master cylinders. The smaller the bore in the master cylinder, the more pressure you get. If you have already fixed your brake sizes on the corners and want more pressure to the front, reducing the bore diameter to the front will result in more brake torque at the front. Increasing the bore will give you less. By changing the bore diameters and having them differential in a race series, you can fine-tune further than just the caliper size on the vehicle.

Balance bars allow us to adjust the amount of brake torque going to one axle or another by winding the bias one way or another. This provides the flexibility to fine-tune components such as the master cylinders, calipers, discs, and pad friction on a race car.

Further adjustments can involve proportioning valves. Each valve has a ratio, and the proportioning valves used on race cars can alter the Zcrit point. Some are lever-operated with 5 or 6 settings, meaning there could be 5 or 6 blue lines at a lower gradient. You can maintain the braking as it is on the blue line until you want the gradient to change to be either lower or flatter. The bent blue line on the graph below shows results with a valve fitted.

Road-legal vehicles may not have any methods for the driver to adjust the vehicle’s balance. If a balance bar were installed on a production vehicle, it would have to be locked in position and not adjustable. Where valves are used on production vehicles (pre-ABS, this was how the balance was controlled), the valves would have a fixed cut-in and ratio, so you couldn’t effectively alter the ratio.

Each race team will have its own idea of where they want the cut-in point on the graph. This can depend on many factors, a significant one being downforce. The Zcrit point essentially represents the adhesion between the tyre and the road. As more weight is placed on the rear axle, the Zcrit figure will increase, as will changes to the vehicle’s centre of gravity. For example, at a low Zcrit with a high centre of gravity, when you brake, the centre of gravity tries to move forward and lift the body off the rear axle, resulting in less downforce at the rear and less adhesion between the tyre and the road.

It is crucial not to set vehicle balance without all the necessary information and without consulting a professional. Being done incorrectly or having someone other than a brake engineer or race engineer to adjust or calculate the balance could lead to dangerous, unbalanced vehicles and be fatal. Talking to an expert who can understand the influence of a component change on the system as a whole, along with the performance requirements you are looking for, is extremely important.

If you’d like to know more, please get in touch with our team or visit our website: www.alcon.co.uk