By: Jason Smith, general manager of TBM Brakes
One of the major questions we get is why do the wheels spin so freely in our brake kits?
First of all, a wheel should spin freely. There is some drag in the bearings, the seals, and hubs that will prevent the wheel from spinning slightly, but in general, the wheel should spin. So the question is, what is preventing the wheel from spinning on certain applications? When the wheel is not spinning freely, its typically due to brake drag. Brake drag is often the result of a few different issues.
One can be residual pressure in the brake system, possibly coming from a bad master cylinder. In this case, the fluid is either unable to flow back into the master cylinder reservoir quickly enough, so it’s building up pressure in the system, or the pushrod or piston in the maser cylinder might not be retracting all the way, in turn not allowing fluid to flow back properly.
Another possible issue is the pedal not pulling the pushrod all the way back, in which case a return spring would be needed to make sure the pedal pulls the pushrod back, preventing any residual pressure in the brake system.
Another cause of drag could be that the calipers are not mounted parallel or square to the rotor. This is when shimming and/or spacing the caliper is critical. You want to make sure the caliper is perfectly parallel to the rotor. If it is not, it will twist under load, and that twisting can cause the pistons to get wedged in the bores, in turn are preventing them from moving in and out of the bores properly, which results in them hanging up and dragging.
Another cause could be the rotor no longer being flat (or wasn’t flat to begin with). If the rotor is warped, the wheel won’t spin freely, and the rotor won’t spin freely in the caliper. You could also have built up pad transfer material on the rotor. If pad material hasn’t been transferred evenly on the rotor, that will result in the rotor appearing to be unflat. In this case you may want to scuff the rotors and start with a clean, new surface.
Another cause could be the caliper mounting brackets not being flat, or being weak, and flexing under load, causing the caliper(s) to bind up, resulting in one or more pistons being locked in the bores.
But one of the major reasons you see brake drag is due to the caliper flexing. If the caliper is flexing, it causes a few different issues. When a caliper us under pressure and flexing open, the pistons are no longer parallel with the pad and the rotor, and they tend to wedge themselves in the bores resulting in brake drag. This also wastes line pressure on fatiguing material, rather than allowing it to be used towards clamping force. When the calipers flexes open too far, the seal(s) can’t combat that flex, and can’t pull back the piston far enough.
At TBM we pride ourselves on delivering an extremely ridged caliper. With a rigid caliper, when the brake system is under pressure the caliper won’t move, resulting in a more consistent clamping force. This maximizes stopping power because you’re utilizing all possible line pressure and clamping force, which won’t fatigue material. Because the caliper is not flexing, the seal is able to do its job and pull back the piston 10-12 thousandths, giving you a free running, zero drag system. If you have a caliper that is flexing 60, 70, 80 thousandths, no O-ring can combat that and pull those pistons back, therefore the calipers will inevitably drag. So when we’re delivering a brake system, it is all built around having an extremely rigid calipers. This is what allows TBM Brakes to deliver a free running, zero drag system.