Co-authored by Mr. Len Hensel, BSME, MS-PDD Wheel Engineer and Keith Heeres
The tire indexing, tire slippage, or tire spinning on rim problem we are looking at is as it relates to off the road equipment used in earthmoving, construction, mining and other heavy equipment applications. Tires spinning on rims is commonly talked about in automotive, off road recreational vehicles, bikes and motorcycles. Same type problem occurs on the big machines as well but for similar and some different reasons. Tire indexing is the topical term we use in the off the road industry to discuss the problem with tires and wheels.
Many wheel loader operators have experienced situations when they are pulling out of an aggregate pile, the tires are moving but not going very fast. At this point, the operator then realizes that the tire is slipping or tire is spinning on the rim/wheel. The end result can be a chewed up tire bead area which is expensive to replace. Other issues can result that will impact the availability and productivity of the machine. For over the road type of applications, tire indexing can lead to air loss and ride disturbance complaints.
Tire indexing is a problem of all pneumatic tire assemblies that are used with any type over-the-road vehicles or off road equipment. Off road equipment with high engine power/torque is more prone to tire indexing. Typically this would be all sizes of wheel loaders, rubber tired wheel dozers, and container handlers just to name a few. Construction equipment has been noted as experiencing tire indexing but other off road applications such as agricultural, industrial and mining will experience tire indexing as well. Tire indexing has been observed with non-drive axle application such as quarry and large mining haul trucks. Tire Indexing can occur because of high braking forces/torques. A non-drive axle would typically be the front steer positions of a haul truck.
So, what is tire indexing? What causes it? How can this be limited or stopped? What can be done about it? In this article we will address some of these basic questions.
The definition that is used in this article is “the permanent circumferential rotation of one or more of the tire beads relative to the bead seat(s) of the rim”. Along with this, the tire is not slipping on the ground. The tire traction with the ground is able to transfer all the static and dynamic forces from the equipment.
Other terms are used in the off road applications. Some of the terms are:
· Rim slippage or tire slippage or tire slippage on rim
· Rim spinning or tire spinning or tire spinning on rim or wheel
These terms generally mean the same thing.
It is important to note that the wheel is typically bolted to the drive axle so it cannot slip relative to the machine hub. The movement we see is the tire slipping on the rim bead seats. For a person observing tire indexing from the side of the road, it appears that the tire is stationary and the rim/wheel is turning inside the tire. In this article, the tire is the component that is indexing relative to the equipment driveline.
Tire slippage occurs with agricultural tires used in tractor applications. The drive tires have some intended slippage (especially in all-wheel drive configurations) with this type of application while they are under drawbar torque. A certain amount of slip at the ground is needed for efficient operation of the tractor. This type of slip is not addressed in the article.
Tire indexing can occur with passenger tire and wheel assemblies as well. In this application the driveline and braking forces are causing the movement of the tire. The consequences of the tire indexing is ride disturbance (change of tire assembly balance, change in the radial runout) that the driver and passengers that feel inside the vehicle. This type of indexing is not the primary focus of this article. However, cause 1 & 2 discussed later in this article do apply to passage car application.
There is a style of rim mounting called “demountable”. In this design the rim is clamped to the hub of the driveline. With this type of mounting the rim can slip on the hub because of drive or braking torques. We are also not addressing this situation in this article.
Tire indexing is an axle end system issue. Important note: The word system is being used because we have individual components that must work together so the drive or braking torques is transferred through the equipment to the ground.
The basic system components (tires, wheels, hubs, brakes) must work together so the equipment can move or stop when needed. When one component of the system does not function properly, tire indexing can be one of the consequences. When you have a system issue, factors and characteristics influencing it can be a combination of characteristics and not just a single characteristic.
Before we discuss the causes and factors that contribute to off the road tire indexing, we need to review the basics of tires and wheels.
The diagram on the right shows a typical cross section of a radial tire.
When we make mention of “the bead”, “bead area”, “Lower tire bead area” we are referring to item #1.
· When referring to the tire bead seat area we are referring to item #13.
Tires used in off road applications have several types of construction configurations. While many people know what these are, we will review them here anyway.
Tires, regardless of the application, can be separated into two very broad groups, tube-type (TT) and tubeless (TL).
Tube-type tires are constructed with rubber and other materials such as steel, nylon, and polyester. Rubber is a porous material from an air migration standpoint. An example of this would be inflating a balloon until it is big and taught. If we let it stand for a few days it begins to shrink because the air inside is migrating through the rubber wall of the balloon. In the early days of the pneumatic tires a special compound was developed to prevent air migration from occurring. This became the material used for tubes. Tubes have been used in tires for over 125 years and are still used today in over-the-road and off road applications.
The tube is located on the inside of the tire and acts like a big balloon. A flap, which is also made of rubber, is placed between the bottom of the tube and the rim. The flap protects the tube from chaffing against the rim and provides support around the valve slot add area. The tube is inflated through a valve stem which is generally located at the bottom of the tube.
When tire indexing occurs, the tube will move along with the tire because of the frictional forces between the tire and the tube. The valve stem however will not move and at some point the valve will separate from the main tube and cause immediate loss of air.
The tubeless tire was first introduced around 1947. This marked a significant technology advancement to reduce the tire system complexity and improve the reliability of the tire and wheel assembly. The tubeless tire was introduced into the off road applications around 1955. TL tires eliminated one field issue with tire indexing - the shearing off the valve stem. While indexing can still occur, the valve stem is part of the rim now that when the tire moves; it does not affect the valve stem. More than 90% of the tires produced today are tubeless construction. However, tubes can still be installed in these tires for special situations.
The next broad grouping of tires we will discuss is the type of internal tire construction. Each of the construction types we will discuss below can be made as a tube type with tubeless features. But as noted above, the vast majority of tires are made for tubeless operation.
Tires were originally made with rubberized cotton fabric (ply) to provide strength to the tire to resist the air pressure and support the load on the tire. Several ply layers were used with each ply having a different angle relative to each other. This allowed for the proper strength in the tire to be achieved. The plies are wrapped around to the tire bead. With this type of construction 1, 2 or 3 bead bundles will be used depending on the number of actual plies used in the construction. Modern materials used for the fabric plies have allowed the actual number of plies to be reduced, but the strength of the tire has increased. Bias tires tend have a stiff construction and provide good lateral load resistance.
Lateral loads (or forces) are those that push or pull against the sidewall or lower bead area of the tire. Typically these forces are caused by:
- Dynamic forces from maneuvering corners or other turning events
- Travelling on the side of an incline
- Dynamic forces from high center of gravity operations such as material lifts
When multiple bead bundles are used, a wide tire bead aids in resisting tire indexing because of a larger surface area to transmit the torque. Bias tires have a unique pressure pattern in transferring loads and pressures in the rim. More pressure is applied to the bead seat surface as compared to the radial tire construction. This also aids in resisting indexing.
The radial tire construction was introduced to the market place in in the 1950’s. This construction provided many performance improvements compared to bias tires and has allowed performance capabilities to be expanded beyond the limitations of bias tire construction.
Tire durability, TKPH (Tonne kilometer per Hour) ratings and other characteristics improved dramatically. Changes in the manner in which forces are transmitted between the tire and the rim and the lower lateral stiffness characteristics tended to cause an increase in tire indexing.
The main function of the rim is to provide support to the tire and to facilitate the passing of forces from the driveline of the equipment to the tire. There are several construction styles of rims used in off the road applications:
There are standard and heavy duty designs in the market place. Standard or heavy duty designs as it relates to the thickness of the steel material used to form the rim and the overall profile design of the rim. Standard duty designs are typically used in various agricultural applications. The heavy duty designs are typically used in construction applications (motor graders, small front end loaders) and high load applications such as harvester and logging applications. All designs are intended to be used with tubeless tires. However, inner tubes can be used. The rim profile has 50 taper for the bead seat surface to create a tight fit with the tire bead area. Knurling is included with some rim sizes to address tire indexing. Because this is a single piece construction, there are no loose components that exhibit circumferential movement.
These rims are commonly referred to as “flat base” style. They are typically used with tube-type tires in rim diameters from 15” to 24”. Normally a tube and flap is used with the tire to inflate the tire. The fixed flange side of the rim allows for some interference fit with the tire bead. Typically, there is no tire bead interference fit with the removable side of the rim. There is a limited ability to resist tire indexing. There some tube type rim designs that can be converted to a three piece configuration that are used for heavy load applications. Some of the rim designs can be converted to use tubeless tires if an “L” style polymer sealing ring is used.
There is lightweight and heavy duty versions used in the marketplace with the same basic components. These designs are intended to be used with tubeless tires. An O-Ring groove has been added to the gutter band so that an O-Ring can be added to create a tubeless seal. The rim profile has 50 tapers for the bead seat surfaces to create a tight fit with the tire bead area. Knurling is an included feature for the heavy duty design to help in reducing tire indexing.
There are standard and heavy duty rim and wheel designs in the marketplace that use the same basic components. These designs are intended to be used with tubeless tires. An O-Ring groove is used in the gutter band so that an O-Ring can be used to create a tubeless seal. The rim profile has 50 tapers for the bead seat surfaces to create a tight fit with the tire bead area. Knurling is included to help reduce tire indexing. The side flanges are separate components included for design flexibility.
In several of the above rim styles, a feature called knurling has been mentioned. Knurling is a lateral serration feature that added to the bead seat surface to assist is resisting the tire bead from indexing. A good way to visualize the function of knurling is that it provides a means for the tire bead area to grab onto an otherwise smooth surface of the rim bead seats. Specifics of the knurling and when they are recommended to be used are specified by various recognized industry standards bodies such as the Tire & Rim Association (TRA).
With multi-piece rim and wheel designs, the loose components resist circumferential movement through frictional force resistance only. In many cases this is not sufficient to keep the components from rotating. Mechanical locking features are used with all high torque applications. Two basic locking systems used on off road rims and wheels are shown below.
This is the simplest version and was the first one used in the industry. Notches are cut in the bead seat band and gutter band. A lug or driver is attached to the lock ring. The lug fits into the notches so the bead seat band cannot rotate relative to the rim. Notching of the rim components has an impact on the strength of these rim assemblies. Normally when this style of driver pockets is used the operating inflation pressure is limited to a maximum of 65 PSI (4.5 bar) or less.
Note – This tends to be the standard industry practice in North America. In other global regions end users are running at higher pressure levels
There are several versions of this style used depending on the strength that is needed for the application. One driver pocket is welded to the side flange (or bead seat band). A second driver pocket is welded to the outboard surface of the gutter band. A driver key is then placed in between to lock the side flange/bead seat band from circumferential rotation.
With five piece style rims, the flanges are free to rotate with the tire. A driver lug is welded to the vertical portion of the flange and two lugs are welded to the rim back band or bead seat band. These lugs fit and work together to keep the flange from rotating with the tire. Thus adding more resistance to tire indexing or slippage.
Tire indexing involves the Output End of the driveline system. We are defining the output end as the tire, wheel and hub/axle. All of the driveline power, all of the braking forces and all dynamic forces must be supported and transmitted by each element of the tire and wheel system. Each element of the system must then work in concert with each other so that all the forces are efficiently transmitted to the ground. The function of the tire and wheel system is:
The tire and wheel assembly is a pressure vessel. The amount of load that it will support is based on the air volume of the tire and the air pressure necessary to carry that load. The load index (LI) or ply rating (PR) of the tire defines the load capacity it can support. Important note: The rim must be chosen to match the tire capacity/capability.
In operation shock loads must be supported because of rough road surfaces, undulations of the road surfaces, a tire running over road debris and sudden loads being dropped into an equipment body or bucket. In many applications the tires are the only suspension system for the equipment.
Primarily turning forces when a piece of equipment is travelling around the work site. From a wheel perspective, turning (or twisting) generates the highest forces that the wheel assembly must support. This puts the greatest amount of stress on the wheel center as well as the bolts or studs holding the wheel to the hub. Other dynamic forces would be :
The dynamic and static forces from the equipment and driveline are passed through the flange and bead seat surfaces of the tire and rim interfaces noted as “A”.
There are no mechanical fastenings of the tire to the rim. Frictional forces generated by the inflation pressure on inside of the tire holds the tire in place and resists circumferential, transverse and lateral movement. More air pressure means more fictional force to resist the dynamic forces. Less air pressure means less resistive forces.
The bead seat area of the interface has additional features to aid in resisting indexing. The bead seat area of the tire is designed to have a certain amount of interference fit with the bead seat of the rim.
The interference is to create an air seal in the static and dynamic state and to aid in stopping circumferential and lateral movement of the tire bead areas. Knurling is an added feature to provide a mechanical means of holding the bead in place from circumferential movement.
Generally, the tire tread does not slip on the ground. While this can happen in emergency braking or other situation, the tire generally has good traction with the ground. Radial tires (seem to) have improved traction characteristics with construction and mining tires. The limitation of the tire and wheel system for transmitting torque to the ground is the tire and rim interface(s).
Anything that reduces the frictional forces of the inflated tire will allow or contribute to tire indexing or tire slippage on the rim. Even with ideal conditions between the fit of the tire to the rim bead seat surfaces indexing can still occur.
One of the simplest issues that can contribute tire indexing is tire air pressure that is too low. As mentioned above, frictional forces between the tire and rim bead area resist the dynamic forces that cause indexing. If the air pressure is too low, then the tire can move. Low air pressure can occur because of poor maintenance practices of a work site, or operating the tire with air pressures that are too low for the application. The following practices will help to avoid or minimize tire indexing.
Another universal cause regardless of the application is using too much tire lubrication when fitting the tire to the rim. A tire lubricant is used to reduce the frictional resistance of the tire beads on the rim bead seat surfaces so the tire beads will slide into the correct position. It is important to follow the tire manufactures tire mounting recommendations. It is also important to:
If too much lubricant is not good, insufficient amount of tire lubricant can be equally bad. If the tire does not slide onto the bead seat surface of the rim, then the tire beads may not be positioned as intended. If the tire beads are not in the correct position then the performance and durability of the tire will be impacted. Additionally:
As mentioned above, it is important to follow the tire manufacturer’s mounting recommendations and procedures.
Assuming we have used the correct amount of lubrication, the next issue in mounting the tire that can contribute to indexing is a tire bead that is not properly seated. It was noted above, that insufficient lubrication can keep the tire from seating. Insufficient air pressure (too low) to “seat” the beads can contribute to tire indexing because the tire bead will not be in the correct position on the rim impacting the interference fit and resulting in insufficient frictional forces resisting drive or braking torques. Most tire bead areas are designed to have an interference fit. Extra force is needed to push the tire into the correct position. The “seating pressure” is generally higher than the operating pressure for the tire. Seating pressure vary depending on the type of tire.
During the mounting process, before the assembly is inflated the driver keys must be installed. The outboard drivers are intended to stop the rim components from rotational movement. Many times, the keys are missing to begin with and they are missed as one of the steps for mounted a new tire on the rim.
With any problem solving activity, all aspects of the issue should be evaluated.
* Used with permission from Tire and Rim Association.
Below is a method used for measuring the rim bead seat diameters. Not typically available at the field level but is used at the factory level to verify dimensions.
* Used with permission from Tire and Rim Association.
* Used with permission from Tire and Rim Association.
Every brand of tire is designed and engineered to provide excellent durability and service life for the desired application. Each Engineering group has their own set of design standards and design principles that they believe are correct for a particular tire/application. The resulting differences can be observed in how the tires perform in the field and fit to the bead seat surfaces. With tire indexing, some brands or sizes within a brand can be better choices to minimize or eliminate tire indexing for the application. Factors that have an influence on tire indexing are:
The manner in which the equipment is operated can have an influence on tire indexing.
Equipment manufactures make a good attempt to have the various components matched so the operators have a good experience operating the equipment. However driveline efficiencies and tire technologies have improved, the drive and braking torques being passed to ground has increase. This is good for productivity of the equipment, but the forces being applied can be greater than the tire and rim interface can resist. In these situations it may be necessary to consider moving to an assembly with a larger bead diameter.
Tire indexing is an action that normally occurs in such small, minute amounts that it can be virtually undetectable unless the tire and rim components are marked on the tire, flange, bead seat band and rim/wheel. This way you can witness the physical movement of the tire and the wheel components.
The tire, side flange, bead seat band gutter band and hub have been marked prior to a tire indexing study. The paint marks are all aligned at the start of the evaluation.
A tire and wheel assembly component after the assembly has been used.
Tire indexing is virtually impossible to stop unless all the factors that cause it are addressed properly. The "tire is going to move no matter what you do". The question is, “how much”?
The term “fretting damage” had been mentioned above as a consequence of tire indexing with multi-piece rim and wheel assemblies. Fretting is an engineering term which describes the damage to contact surfaces when they are subjected to contact pressures and exposed to repeated movement (slippage) or vibrations. The movement does not need to be large, repeated minute motion is sufficient for fretting damage to occur.
Mr. Len Hensel, BSME, MS-PDD
Len has over 39 years of industry experience in providing wheel system solutions for the mining, construction and agricultural applications. He has worked extensively with the entire major off road equipment manufacturers and off road tire (OTR) companies to provide designs that meet the needs of new applications, pushing the envelope for greater load capacity and durability. He has worked in leadership positions in off the road wheel industry new product development, application engineering, new design validation, research and development projects, and quality assurance. He is a member of the Society of Automotive Engineers (SAE), participated in development of international standards (ISO) related to off the road wheels, and a past member of the Tire and Rim Association (TRA) representing the rim and wheel industry manufacturers.
Contact: [email protected]