The Website Forum on www.schoolbusfleet.com/forum called Professional Garage has recently been hit with questions from various individuals about the differences between hub-piloted and stud-piloted wheels. There seems to be some confusion about these two types of wheels, their application and the number of visible hand holes or valve stem access holes in the disc wheel assembly.
At best, it has been very difficult to advise people as to these differences in the short amount of space allowed in the Forum pages. Subsequently, this article is intended to address these differences in a more detailed manner in hopes of answering these questions.
Clearing the confusion
There are many different types and sizes of wheels on school buses today. Many, such as multi-piece wheels and demountable rims, were manufactured years ago and are still in use today, mostly in aging fleets. For the purposes of this article, I will focus on tubeless disc type wheels that are in use today on Type C and D buses. Safety and training information about all types of wheels is available from Accuride Corp. on their Website at www.accuride.corp.com.
All wheels are stamped with identification numbers and letters. This is required by Federal Motor Vehicle Safety Standard 120. These stamps indicate several different things like wheel size, type, where the wheel was manufactured and that it was manufactured to meet U.S. Department of Transportation specifications. Wheel identification numbers also include the load and inflation rating allowable for the wheel.
There are not, however, any stampings that designate specifically whether a wheel is hub-piloted or stud-piloted. Identification between these two types of wheels is usually made by the visible differences in the wheels such as the stud bolt hole diameter and lack or existence of a chamfer around the stud bolt hole. Also, the center diameter of the large hole, which fits over the axle hub, will be slightly smaller on the hub-piloted wheels.
It is very important not to mix stud-piloted wheels (chamfered bolt holes) and hub-piloted wheels (smaller diameter bolt holes with no chamfer) for an application. Your tire people need to be trained to distinguish the two types of wheels and hub assemblies. The type that comes off must go back on, assuming the correct wheels were installed to begin with. It is especially important that one of each does not get installed in the dual position side by side because the inner wheel cannot be seen in the assembly. Mixing of wheel types will allow one wheel to be off center, and nuts will not properly secure the wheels to the hub. This is a safety issue that requires very close attention.
Setting up configurations
The proper mounting of dual hub-piloted wheels involves both wheels being in contact with the hub at their centers, and the proper two-piece flange nut being used (see Figure 1 on pg. 50). The improper method has one hub-piloted wheel on the inner position and a stud-piloted wheel on the outer position (see Figure 2 on pg. 50).
One way to determine if the inner wheel is being used correctly is by the stud size. Wheel studs for stud-piloted wheels in the dual position are three-fourths of an inch in diameter with 16 threads per inch in the left- and right-hand thread configuration. Hub-piloted studs are 22 millimeters in diameter in both single and dual positions, and therefore the inner nut of a stud-piloted system would be too small to fit on the 22-millimeter stud.
The second way to determine correct usage is by looking closely for a space between the inner and outer wheel mating surfaces (see Figure 2). Such a space is due to the inner nut not having a chamfer to seat against on the inner wheel. Also, the inner and outer nuts may appear to be bottomed against each other, which causes improper wheel clamping force of the outer wheel. This would be a very dangerous mounting combination.
A hub-piloted assembly with stud-piloted wheels is evident by looking at two locations: One, the chamfered larger stud holes on both wheels and two, clearance between the hub flange and the center pilot hole of the wheels. With a two-piece flange nut, one can identify a hub-piloted assembly with incorrect wheels attached (see Figure 3). Remember, stud-piloted wheels use an inner and outer tapered nut for fastening. These configurations, as shown in Figures 2 and 3, will cause the wheel/hub assemblies to fail. Nothing is holding the wheel centered on the hub or stud, and clamping force is minimized in both.
Two-hand vs. five-hand
I contacted Bill Noll, technical engineer for Accuride Corp., with some questions about bus wheels. I asked Noll if there was a reason behind the availability of two-hand hole or five-hand hole wheels. He responded by telling me that the original tubeless wheels in the industry were all welded under the drop center of the rim. When radial tires came out in the trucking industry in the early 1970s, they caused cracking of the rims at the attachment weld. At this time, all standard heavy truck wheels were five-hand holes.
Then a new design of tubeless wheel came out, which was approved for radial tires. To make sure everyone could tell the difference, they were made with two-hand holes. Now all wheels, two- and five-hand hole, are approved for radial tires, and the old wheels are out of service. Therefore, two-hand hole and five-hand hole wheels can be used interchangeably. Many fleets have stayed with the two-hand hole wheels because they don’t want the mixed appearance in their fleets. Theoretically, the two-hand hole would be stronger because it has fewer holes, but all wheels are capable of supporting their load rating regardless of the number of hand holes. Again theoretically, you would get more air circulation with a five-hand hole wheel, though testing does not support this claim because the difference is probably too small.
History of the market
I asked Noll what reasons were behind production of hub-piloted wheels. To my surprise, he said that this system has been around since the 1930s. The advantages for the system were better alignment on the hub (better run-out) and less nut chamfer distortion and wear. The hub-piloted system became the standard in Europe in the 1980s, but in the U.S., the truckers still liked the interlocking of the inner wheel separately from the outer wheel in the double cap nut stud piloted system.
In the early 1990s, the popularity of the hub-piloted system grew in the U.S. Two things helped this growth. The first was the non-continuous center pilot for the wheels, as seen in the photo above. Note the four ears protruding from the center of the wheel. Originally, these did not exist, but the hub had a continuous ring. A similar configuration exists on the rear axle hub. This helped eliminate the wheel sticking problem.
The advance in the design of the two-piece flange nut also helped the hub-piloted system gain popularity. The two-piece flange nut (see below) eliminated bolt hole chamfer wear and improved nut torque retention. But early two-piece flange nuts had some cracking problems in the nut flange. Confidence in this system grew, and it is the most popular today.
Noll says that the reasons for the market moving from stud-piloted wheels are simple: One, the system uses fewer components, and you do not have to worry about left-hand and right-hand nuts and studs, and two, because of the better alignment as stated above.
Torque, lubrication issues
I have heard the term clamping force used more with hub-piloted wheels than any other system. Clamping force is exactly what it implies, and a higher, more uniform clamping force is obtained with hub-piloted wheel/hub assemblies. Says Noll, "The original standard stud in the stud piloted system was grade five." One hub manufacturer, he says, stated that only grade five studs should be used because they stretch and give better nut torque retention.
One problem with stud-piloted wheels was that if the torque levels got too high, the stud would break. The literature always specifies nut torque (dry) for stud-piloted wheels to prevent excessive nut torque. Lubricating the nuts and studs eliminates friction, which is exactly what nut torque measures (resistance to rotation). However, lubricating the stud can cause high clamping force that can break a grade five stud. With the hub-piloted system, nuts and studs can be lubricated.
Excessive lubrication on the nut or stud will affect cosmetics of the wheel (oil streaks from the bolt holes). However, it is not recommended to lubricate the mounting surface of the wheel. This will affect the clamping force in the wheel. With the two-piece nut system, more of the clamping force is going into the wheel because it uses lubricated threads and lubricated rotating surfaces. Essentially, less torque is lost in friction.
Therefore, more clamping force is generated into the wheel. This is the reason that one nut can hold both of the wheels of a dual assembly on the rear axles. Prior to installing the two-piece nut, place a couple of drops of oil between the nut hex and the flange. This will help lubricate the two mating surfaces and help prevent corrosion from forming there.
I noticed after looking at the make-up of the two types of fastening systems that the improved clamping force of the new two-piece hub-piloted nut is obvious. Hold the two types in your hand and look at the contact area of the nuts. The stud-piloted nuts have a small chamfered area that contacts the wheel. This places force not only toward the hub for clamping force but also outward away from the hub. So in effect, a portion of the clamping force is used for centering the wheel to the hub, whereas the total contact surface of the hub-piloted nut is pressing the wheel to the hub.
In my observations of the two types of wheel systems, I noticed that the hub-centering pilot diameter appeared to be the same in late-model stud-piloted and hub-piloted configurations. When I mentioned this to Noll, he said that many hubs are made to be used with both. "However, there are many hubs that have different pilot diameters to fit specifically a hub-piloted or stud-piloted wheel," he said.
The hub-piloted bore diameter of the wheel is 8.66 inches. On the stud-piloted system, it’s 8.72 inches. The difference of 0.06 inch is not easily determined by the eye. This is why you must know the other means of identifying the differences between these two wheel systems.
Cleaning up corrosion
Many have commented that they have periodic problems removing the wheels from the hub due to corrosion. Suggestions have been made in the Website Forums of a few different methods of handling this situation. My suggestion is this: When the wheel is off, clean all corrosion from the hub centering pilots and center bore of the wheel by using emery paper or a Scotch Brite pad. A light coat of rust-inhibitive paint should be applied, allowing time to dry. Then apply a few drops of oil or similar lubricant to the centering pilots on the hub.
I prefer using disc brake caliper rail grease, which contains an anti-corrosive lubricant. Rail grease is heavier than anti-seize and does not fling off as easily. These lubricants may get washed off over time, and if removal is still difficult, you might have to invest in a wheel puller specifically made to do the job. Several are available on the market specifically for this purpose. One I know of is the Kiene Wheel Grabber, stock #84453, which is available through Myer's Tires Supply, or you can contact Kiene Diesel Accessories directly at (800) 264-5950. Attachments are available for just about any axle configuration on a school bus.
Studs, nuts and rims
A few of the buses in my fleet came with an odd set up of studs and nuts on the steering axle. This occurred at about the time of the transition period between stud-piloted and hub-piloted wheels. They had a hub-piloted hub and a stud-piloted wheel. The mounting studs were smaller than those normally found on either system, and the nuts were of the tapered stud-piloted design with a small-diameter threaded hole for the small stud. I assumed I could place hub-piloted wheels on these front axles if I replaced the mounting studs and used the hub-piloted mounting nuts.
When I mentioned this to Noll, he told me that there was an attempt to make a universal hub during this industry changeover period. However, I also may have been seeing some other variations, he said. "What you probably have is a 3/4-16 stud on the front, instead of the more common 11/8-16 stud. Some vehicle manufacturers such as Volvo preferred this arrangement," he told me. "You might also be seeing the metric stud that is 22 millimeters and not as large as the standard 1 1/8-16 stud."
Demountable rims, or Dayton-style rims, have been around for a long time. Demountable rims do not have a permanently attached center. They are mostly used nowadays in school bus configurations and semi-trailers. My recommendation is to stay away from them, if at all possible. If you have them in your fleet, I recommend phasing them out as soon as possible. When you buy new vehicles, specify disc- style hub-mounted tubeless rims.
In summary, training is the key to understanding wheel and rim differences, care and safety. Contact Accuride Corp. and they will help you follow the proper safety and training routines at no cost. Follow their simple training plan, and refer to the available wheel charts to match wheel components.
Brad Barker has been a school bus mechanic for 30 years and is currently shop supervisor for Park City (Utah) School District.