Going to have to disagree with you on this point. The wheel is held from shifting by the taper that is machined into the rim and on the surface of the lug nuts. Think of it like a taper pin. On an OEM application, the load is carried by the center of the rim against the hub. This was the problem with "Unilug" custom wheels a few years back. Designed to "fit many", these transferred the twisting (friction) and load carrying to the actual lug studs instead of the hubs. Dennis
Let's check that against field experience. Most, but not all wheels I have removed from cars over the last 48 years slip off the hub easily when lugnuts are removed. Some are firmly held by corrosion products at the wheel contact with the hub. Most recently, I pulled four alloy wheels from my Focus ST for replacement with other wheels and tire, and two have light witness marks in the pilot bore. The other two have corrosion products in the bore that interfered with the pilot on the hub. So maybe we have some support by pilot, but it does not look like much...
Let's review the loads on the wheels. There is the weight of the car downward trying to slip the wheel upward along the hub. There is braking or acceleration trying to slip the wheel fore and aft along the hub. There is torque applied either from power or braking that attempts to slip the wheel in rotation on the hub. And there is cornering which is compressing the joint between wheel and hub for part of the rotation and increasing tension on the joint during the other half of rotation. Let's see how the piloted hub works (or doesn't work) to restrain the wheel against movement on the hub.
I ran a simple calculation using specified wheel lug torque (100 ft-lbs), lug diameter (14 mm), and contact ring radius (2.625") for my Focus ST. I assumed coefficient of friction of 0.15 on the contact ring, which is low end, but gives us a minimum to work to. Using standard calcs (from Shigley), preload from tightening each lug nut is 10,700 pounds. Five lug nuts makes the total preload 53,500 pounds. Why do we suppose Ford put that much preload in?
- Even a low estimate of coefficient of friction of 0.15 means the wheel will slip across the hub when 8000 pounds is applied across the hub. The biggest loads I can get from vertical force plus max braking is on the order of 1200 pounds - a factor of safety of 6.7! This system is NOT slipping radially, so the hub pilot does not really provide any support against these loads;
- Checking wheel torque against slip of the wheel on the hub - tractive limits set torque at about 11300 in-lb while torque to slip the wheel on the hub is about 21000 in-lb, with a factor of safety of 1.86. Hmm, resisting torque has way less factor of safety than radial slip. Being as the pilot is about 1" radius, if there was 11300 lb of friction between pilot and bore, the pilot could resist these torques, but then you would need extraction devices to remove the wheel for service - not present on any car I know of. So, the pilot does not provide rotational support to prevent fretting and stud/wheel fatigue - it comes from friction that the stud system provides;
- I also checked the cornering moment from sideways traction at the tire patch and found that the joint stays loaded up with large margins, so fatigue of the studs is unlikely;
Pilot fit of hub to wheel centers the wheel for tightening the lug nuts, but once that is done, the design of the lug system and contact between hub and wheel is sized to have excess torque and radial friction capacity to prevent movement of wheel on the hub. Experience with these systems and analytical approaches bring us to the same conclusion - the wheel is held in place by friction from large preload, while the pilot serves to get things decently centered before we tighten the lug nuts.
Now if the wheels were to be held in place by the pilot, but friction was inadequate to hold the wheel in place, pilot clearance seen in the field would allow the wheel to orbit or nutate on the hub. The lug system would see substantial variation in load as the vehicle is driven, and I would expect fatigue and failures of the stud and/or the wheel. This is exactly the failure mode seen when lug nuts are inadequately tightened - there is fretting (wear from small but real slipping back and forth of surfaces) at the contact between wheel and hub, studs break off and/or lug nuts get pulled through the wheels, then wheels depart the vehicle.
One historical point that occurred in the 1980's IIRC. A car had been repeatedly brought in to the dealer complaining of large vibration on one corner, with the dealer unable to solve the problem. Eventually a wheel and tire departed from that corner of the car, and a tragic accident occurred. All five studs had failed in fatigue. During the subsequent teardown of the live rear axle, the axle shaft was found to have red paint on it and the hub face was found to be crowned so that wheel contact from tightening the lug nuts occurred near the pilot instead of out near the periphery of the hub, as specified in the part drawing. Red paint denoted scrap parts in the factory that built the axle. Other evidence included much fretting of the wheel, brake drum, and axle hub, all near the ID of the wheel, indicating much movement and slipping about under load. The wheel was firmly held on near the pilot, but this provided little capacity against small but real slipping of the wheel on the hub, and cyclic loading of the studs. It also did not support the wheel in a uniform manner, negating any dynamic balance of the wheel, and the resulting vibration. In this case we see friction resisting rotational movement was low, the pilot did not provide support, and the studs failed.
The attempts at universal fit aftermarket wheels with no piloting except the studs and lugnuts can result in substantial and non-repeating imbalance. Yes, the pilot fit between wheel and hub is useful and probably needed. To conclude that the pilot holds the wheels in place only covers radial loads (the easiest part) but provides absolutely zero support from slipping rotationally under accel and braking loads. Indeed supporting torque is the defining issue and one only reasonably achieved with preload and friction...