Discs for heavy duty applications, especially larger discs, should be mounted to allow axial float and radial heat expansion of the disc. This may be achieved by either of these four methods :-

Radial float allows differential expansion of disc and bell thus reducing stresses in the disc and minimising disc cracking and distortion.

The idea of axial float is to compensate for a certain amount of stub axle / upright flex by allowing the disc to take up its ideal position within the range of float thus avoiding "knockback" of the caliper pistons. However the float should not be excessive as disc gyroscopic loads can cause the same effect that the float is trying to alleviate. The amount of axial float will depend somewhat on the application.

In a "perfect" system with minimal disc movement relative to the caliper the amount of float need only be around 0.15 - 0.2mm.

In most actual installations where the mountings are less than ideally rigid a float of 0.40 - 0.45mm for "float in the disc" systems and 0.20 - 0.30mm for "float in the bell" systems is more appropriate.

It is often found that Balanced Brake systems require more float (0.5 - 0.6mm) for optimum performance.

A range of standard bobbins are available from AP Racing to achieve this.

Wide Bobbins
AP Racing has two options of wide bobbins for heavy duty disc arrangements offering improved stability in high torque applications.
- CP4135 a forged one piece bobbin and stud providing improved and unrestricted airflow.
- CP7016 a two piece alternative for thicker mounting bell flanges using seperate bolt.

- Bobbin tool to assist bobbin orientation when assembling for CP4015/CP4135 types
- Part No: CP4015-137

Note: Disc mounting tightening torque - 14Nm (10.5lb/ft)