Redline
Likes to Drive and Ride
Because of issues I have experienced recently with tire pull or more correctly, "ply steer" or "residual aligning torque" a thread is in order to discuss & expose the phenomena for those who may experience similar challenges.
With select tires (Toyo MT, BFG KM2, and less so w/Cooper ST) my '06 4Runner exhibits varying degrees of tire pull to the right even when the alignment is in spec, or 'perfect'. I learned of the term "residual aligning torque" (RAT) during a conversation with a tire engineer, and a quick web search located the information linked below. There is lots more out there depending on how hard you want to work your brain ☺
A quick glance at some of the data about RAT will reveal it's a bit technical to fully understand. The linked PDF with graphs and written explanations cleared some of the mud for me, though I would say that I'm now merely aware of the existence of RAT. Surely some of our more scientific members can completely understand what is occurring. For an enthusiast that modifies their vehicle and all seems well and within specification but they still can't figure why they're having alignment/tire/drivability troubles, just the exposure to the concept of RAT may be helpful. It may send us in the right direction to find a permanent solution to a problem, or help us realize that though everything is 'in spec', it's not in spec when different forces (road crown, chassis design, tires) are acting upon each other. We may choose to chase the corrections, live with it, or in my case try a different tire.
One thing appears clear to me. Manufactures likely spend plenty of time/brain power to get their vehicles to drive straight as they are equipped from the factory with the stock tires. Everything we do to modify them, particularly suspension, chassis, and tire changes can make huge differences in how our vehicles drive and handle. In general we are very lucky that our modified vehicles usually drive as well as they do.
http://210.101.116.115/fisita/pdf/G353.pdf (A PDF with graphs and data as well as helpful text)
"A crucial point for a good vehicle pull performance is to improve the matching characteristics of a vehicle and tires."
http://www.patentstorm.us/patents/6739185/description.html (from a Patent website)
"Unfortunately, achieving correct wheel alignment is a very complex problem to resolve due to the wide variety of factors affecting wheel alignment in a given vehicle with a given set of wheel/tire assemblies. These factors include the specific wheel assembly geometry that affect caster, camber, and toe in/toe out of the vehicle. Generally, these are the factors which are and have been measured by current wheel alignment apparatus. However, there are also a number of other factors that contribute significantly to vehicle performance and may vary from one vehicle to the next of identical design. Among these additional factors are the dynamic tire forces such as conicity, and ply-steer which may dramatically influence the handling of a given vehicle. These factors will vary from one tire to the next even when the tires selected for a vehicle may have been manufactured at the same location and time and even using the same tire mold."
http://www.freepatentsonline.com/5944082.html (another patent site with lots of big words but good explanations)
(a few paragraphs down)
"The design of the tread effects both the residual aligning torque and the residual cornering force. When the tread pattern changes, the stiffness of the tread blocks are modified. For example, changes in the circumferential grooves in the tread pattern will change lateral stiffness and effect the residual aligning torque and the residual cornering force. Changes in lateral grooves can also modify the stiffness of the tread of the tire and cause bending-twisting deformation changes within the tread. Changes in the lateral groove angles can result in less differential contact patch tangential forces between the various tread elements. Hence, the tread becomes more compliant as the tire rolls. U.S. Pat. No. 4,819,704 discloses how modifications in the size and shape of tread blocks produced by circumferential and lateral groove changes reduce plysteer."
With select tires (Toyo MT, BFG KM2, and less so w/Cooper ST) my '06 4Runner exhibits varying degrees of tire pull to the right even when the alignment is in spec, or 'perfect'. I learned of the term "residual aligning torque" (RAT) during a conversation with a tire engineer, and a quick web search located the information linked below. There is lots more out there depending on how hard you want to work your brain ☺
A quick glance at some of the data about RAT will reveal it's a bit technical to fully understand. The linked PDF with graphs and written explanations cleared some of the mud for me, though I would say that I'm now merely aware of the existence of RAT. Surely some of our more scientific members can completely understand what is occurring. For an enthusiast that modifies their vehicle and all seems well and within specification but they still can't figure why they're having alignment/tire/drivability troubles, just the exposure to the concept of RAT may be helpful. It may send us in the right direction to find a permanent solution to a problem, or help us realize that though everything is 'in spec', it's not in spec when different forces (road crown, chassis design, tires) are acting upon each other. We may choose to chase the corrections, live with it, or in my case try a different tire.
One thing appears clear to me. Manufactures likely spend plenty of time/brain power to get their vehicles to drive straight as they are equipped from the factory with the stock tires. Everything we do to modify them, particularly suspension, chassis, and tire changes can make huge differences in how our vehicles drive and handle. In general we are very lucky that our modified vehicles usually drive as well as they do.
http://210.101.116.115/fisita/pdf/G353.pdf (A PDF with graphs and data as well as helpful text)
"A crucial point for a good vehicle pull performance is to improve the matching characteristics of a vehicle and tires."
http://www.patentstorm.us/patents/6739185/description.html (from a Patent website)
"Unfortunately, achieving correct wheel alignment is a very complex problem to resolve due to the wide variety of factors affecting wheel alignment in a given vehicle with a given set of wheel/tire assemblies. These factors include the specific wheel assembly geometry that affect caster, camber, and toe in/toe out of the vehicle. Generally, these are the factors which are and have been measured by current wheel alignment apparatus. However, there are also a number of other factors that contribute significantly to vehicle performance and may vary from one vehicle to the next of identical design. Among these additional factors are the dynamic tire forces such as conicity, and ply-steer which may dramatically influence the handling of a given vehicle. These factors will vary from one tire to the next even when the tires selected for a vehicle may have been manufactured at the same location and time and even using the same tire mold."
http://www.freepatentsonline.com/5944082.html (another patent site with lots of big words but good explanations)
(a few paragraphs down)
"The design of the tread effects both the residual aligning torque and the residual cornering force. When the tread pattern changes, the stiffness of the tread blocks are modified. For example, changes in the circumferential grooves in the tread pattern will change lateral stiffness and effect the residual aligning torque and the residual cornering force. Changes in lateral grooves can also modify the stiffness of the tread of the tire and cause bending-twisting deformation changes within the tread. Changes in the lateral groove angles can result in less differential contact patch tangential forces between the various tread elements. Hence, the tread becomes more compliant as the tire rolls. U.S. Pat. No. 4,819,704 discloses how modifications in the size and shape of tread blocks produced by circumferential and lateral groove changes reduce plysteer."