As I started looking at my wheels to have 3 mm spacers made, I found that the bolts have conical faces that seat into the wheel.
Almost every American wheel is centered by the conical faces of lug nuts.
Now I am wondering why the conical bolts when the wheel is supposed to be hubcentric.
Does anyone know this?
There must be a small amount of play so the wheel can slide over the hub. Is the hubcentricity just to make inserting the bolts easier? Is the wheel really centered by the bolts after all? If the bolts don't really center the wheel, then if they are off, wouldn't the bolts, being conical, pull the wheel in one direction and move the wheel in that direction to the extent of the play between the wheel and the hub?
This would make the tightening sequence of the bolts critical.
Can anyone speak on this with authority?
__________________
-Bart
2002 Bluewater/caramel
Any other car is a compromise
I don't know about speaking with authority, but I can provide a response.
First, conical-seat wheel bolts are used for several reasons. These include (1) to locate the center of the bolt hole of the wheel at the centerline of the threaded hole in the hub flange (within machining tolerances) after assembly on the hub; (2) to allow "loose" tolerances for the location and size of the bolt holes in the wheel; (3) to provide a no-clearance circumferential fit once tightened, in order to transmit acceleration and braking forces to the tire contact patches without concern for motion (or slop) between the wheel and the hub; (4) a small amount of local yielding at the conical bolt seats in the softer wheel when the bolts are torqued covers small variations in manufacturing tolerances and helps keep the bolts from loosening in service; and (5) conical bolt seats are the cheapest way to accomplish objectives (1) through (4) above. Those of us who have used the old-style aftermarket aluminum and magnesium wheels with shoulder-style lug nuts know all about the problems of manufacturing to tight tolerances, movement of the wheels relative to the hubs, and lug nuts becoming loose.
I think this post is really about the need for a hub pilot, and the relationship (if any) to shimmy problems with spacers.
There have been numerous cars that depended only on the conical lug nut/bolt seats to locate the wheels, with success. The early VW Beetle and Porsche arrangement is a clear example. Most cars have utilized some degree of hub piloting, at least since the mid-'60s, although often with relatively loose tolerance. BMW products, with more attention to engineering precision than most, use a fairly close-fitting hub pilot arrangement. Clearance appears to be in the range of 0.003 - 0.005 inch. This provides a better centering of the wheel than would occur with simply relying on the bolt seats, due to the weight of the wheel/tire assembly and the clearance between lug bolt threads and the threaded holes in the hub flange. Without the hub pilot, the wheel could be installed as much as 0.015 inch (my estimate) off-center - assuming the bolt holes are accurately located in both the wheel and the hub.
The relevant question at this point is how close to centered is good enough? In the bad old days wheels were seldom made this accurately, and we are not even talking about lateral runout. But today they are (or should be). So we try to get the wheel installed on the hub as accurately as we can, in the interest of minimizing a stack-up of tolerances that collectively can cause a problem.
The real issue with wheel accuracy, however, is the tire part of the equation. Tire manufacturers dream of getting tire circumferential (and lateral) runout down to these small values - when mounted on a perfect wheel. Further, and much harder to detect, there is often variation in the "spring constant" around the circumference of the tire. Belts don't always get put into the carcass properly aligned with the outside dimensions of the tire. Many of us have had experience with a tire that measured (relatively) true, was correctly and carefully balanced, and shimmied badly. There are test machines to measure rolling spring constant, but they are not easy to find. The only fix is to discard a perfectly good looking tire and get a non-defective replacement.
All the fuss over the wheel and centering of the wheel is to ensure that this is not the source of a shimmy problem.
I got a batch of bolts from H&R awhile back that had a small diameter seat (smaller than stock). These were a source of shimmy problems. Make sure your supplier gives you the correct bolts. SD
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