[bluemax_1]

Someone asked me about clarifying how spacers are any different from the rotor when it comes to introducing an angular component on the wheelstuds since both components are in essence free bodies sandwiched between wheel and hub. Since my long-winded explanation was too long for a PM reply, I figured I might as well post it here for future reference if anyone else might have trouble visualizing the forces in action.

Spacers < 5mm are usually OK on our cars, but I still wouldn't want to use them on a car being taken to road courses often (or often driven in the manner you would at a roadcourse).

Yes, the rotor is in a similar position as a spacer ... only if the spacer replaced the rotor and there was no rotor.

Examine the mechanics and placement of the components next to each other. The wheelhub provides the drive and connects to the car. The wheel provides the connection to the road. The rotor sits between and is what is acted on by the calipers to retard the rotation of the lot and all pieces are connected by being threaded through/pierced by the wheel studs.

Now when you're braking, the momentum/inertia of the car makes it continue to move forward and the grip of the tires forces the wheels to keep turning (no more momentum = car stops. Not enough grip = wheels lock up) and the clamping of the calipers/pads on the rotor is what slows down the rotation.

In the above case with no spacers, the forces acting directly on the wheelstuds are shear because the surface of the rotor's hub is directly on the surface of the hub of the wheel (not the same as the wheelhub) and the force on the wheelstud isn't angular, it's trying to shear the wheelstud.

If you still have trouble visualizing the forces at work, think of drilling a hole the exact size of your finger through a round kitchen table (which will be the wheel). Drill another hole through a cutting board (the brake rotor). Now stick your finger through the hole in the table from the bottom. The table is the wheel, your finger is the stud. Slide the hole in the cutting board over your finger so the board sits flat on the kitchen table and your finger is through the hole in the table and cutting board. Your finger is now through both and their surfaces are flush/flat against each other as in a wheel and rotor with no spacer.

To simulate the forces at work under braking, the whole assembly (table, finger, cutting board should all be rotating moving together as the wheel, rotor and wheelhub all move together before you hit the brakes). When you DO hit the brakes, your finger and the wheel keep moving, the calipers act on the rotor to try to slow that down relative to the rest. Since it would be a hassle to trying spinning yourself along with the table and everything else, we'll only look at relative movement and forces.

You can simulate that by pushing the cutting board to try to slide it along the table. (If everything is moving together, the force comes from the caliper trying to slow down the rotor which would then try to rotate at a slower speed than the rest but is prevented from moving relative to the wheel by the wheelstuds going through it). The force acting on your finger (the wheelstuds) is shear. It's trying to shear your finger off right where it sticks out of the table.

Now think about replicating this experiment, but this time you add a spacer between the cutting board and the table. This time when you try to retard the rotor (push the cutting board), the force on the stud (finger) is no longer simply shear. It has an angular component.

The easiest way to visualize this with the above experiment is to stick your finger through the table so your finger joint is right at the top surface of the table. In the first 'no spacer' scenario, the rotor/cutting board would be trying to shear your finger off right at the knuckle. In the 2nd scenario, because there's now a spacer between the table and the cutting board, the angular component instead is trying to bend your finger (the thicker the spacer, the greater the angular component). If you make the spacer about 1" thick so the pushing the cutting board now acts only on the end of your finger, your finger will bend at the joint sticking out of the table. that is the angular component introduced by spacers.

Here's some really crappy ASCII art to try to show the differences. these are the forces acting on the wheelstud right at the mating surface of rotor to wheel:

__l
__l
->l
__l<--
__l
__l

vs.

__l
__l
->l
SslsS
SslsS
SslsS
SslsS
__l<--
__l
__l


Hope this makes it a little easier to understand/visualize the angular forces introduced by spacers.


Max