Maximum Wheel and Tire Size?

[aj81spider]

The weight of the car is hanging on the mounting point of the wheel. As you move that mounting point out the weight of the car is hanging on a point further from the hub. This creates a lever action, so more force is exerted on the bearings. The bearing nearest the mounting point (the outer) will take most of this strain.

The center line of the wheel being in the same place may help the other suspension components (I haven't thought that through), but it doesn't affect that, as the hub doesn't care where the weight is transferred to the ground, it only cares where it is transferred to the wheel.

[Redline]

Simple statics dictates that the force couple on the bearings depends only on the tire/ground force, multiplied by the lever arm to the bearing, so whether this is achieved with an 15x8 ET10 wheel, or 15x8 ET20 + 10mm spacer, the net shear force and bending moment at the hub is identical. You can draw a free body diagram of the whole system to illustrate it. So I'm stilly rather confused why a FWD wheel with spacer should place any additional load on the bearings...

[aj81spider]

I'm not a mechanical engineer, so perhaps you can educate me. My reasoning is that the tire hits a bump and moves up. That upward movement is with force X. That force gets transferred to the bearing through the stack up of stuff between the mounting point of the tire and the bearing. The longer that stack up the longer the lever and the greater the multiplying force on the upward movement of force X. A large spacer means a longer lever arm.

I'm not sure I understand your configuration but am responding to the picture earlier in the thread of the very large spacer.

If I'm wrong I'm happy to be re-educated.

[DanD]

Stress on the bearings has nothing to do with spacers, and everything to do with actual offset of the wheel in relation to the hub. Create correct offset with a spacer, and the wheel will neither be too far out nor too far in, so there will be no inappropriate leverage applied. If I jack a car from the bottom of the hub, the weight of the car will be balanced between the inner and outer bearings. If I put a two foot extension on the hub, and jack from there, the leverage will be pushing the outer bearing up, and forcing the inner bearing down. It would also be straining the heck out of the spindle and trying to bend it before translating the weight up through the suspension.

A proper spacer is in essence just a piece of metal that acts as part of the wheel. Hubcentric spacers are the best, as they eliminate shear force on the lugs. Create proper offset, and you should have no worries.

[Redline]

Indeed, I was somewhat playing devil's advocate to question the assertion that a spacer would have any effect on bearing loads. As long as the effective ET (wheel + spacer) is the same, then the bearing loads should be the same. Without going into a very long lecture on statics, the internal loading at any portion within the wheel and spacer is a combination of vertical shear force and internal bending moment. The internal bending moment acting on at any point within the wheel / spacer / hub / bearing should depend only on the position of the force transmitted from the ground to the tire. For the static case, with equivalent ET, camber etc., then the bending moment at the bearings will be the same, no matter what combination of wheel and spacer.

Dynamically, I could see that a specific wheel and spacer combination could have a (very small) influence on the pressure distribution across the tire's contact patch, therefore theoretically could change bearing loads.

Probably the reason people might think that a spacer will increase bearing loads is because this is based on the effect of adding a spacer to an existing wheel, thereby decreasing the ET and increasing the lever arm of the tire force on the bearings.