Tire Pull: PlySteer & Residual Aligning Torque (RAT)

[Redline]

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."

 

[Chadman]

So.. My dealer was telling the truth when he told me my pull to the right was from my tires. Add the 255/85-16 Interco TRXUS M/T to the list. These tire are awesome off road and on gravel roads but SUCK badly on pavement. Hope they wear out soon.

 

[Redline]

You can't just add any tire to the list as it's the specific relationship of a certain tire with your vehicle and its alignment/chassis. Another vehicle with the same combination may or may not pull.

snip..............Add the 255/85-16 Interco TRXUS M/T to the list. These tire are awesome off road and on gravel roads but SUCK badly on pavement. Hope they wear out soon.

 

[R_Lefebvre]

I'd just like to comment... when an alignment shop tells you your vehicle is "In Spec" does not necessarily mean it's aligned properly. The alignment "window" from the factory is quite large, usually around 1 degree which is 10 times larger than the accuracy provided by a basic laser alignment machine. You have can have -0.5° camber on one side, and +0.5° camber on the other side, and be "in spec" but the car might pull because of the cross-camber. An alignment shop has the capability to align a car within a 10th of a degree side to side, but usually just get it "in spec". I don't accept this situation, but it's getting harder to get shops to do a proper job. I can do a alignment in my garage with string and a tape measure than some of these laser equiped professionals turn out the door.

Just keep that in mind before you just accept the alignment shop's suggestion that it's the tires, especially if you're running a major brand.

 

[Pskhaat]

Hmmm.....

 

[Redline]

You are very correct. Thankfully I have an alignment tech who is both very good and conscientious. Certainly I could have him redo the alignment to a different range within 'spec', both he and I are aware of this and discussed this option last Friday. It would likely involve re-clocking my adjustable upper ball joints on my Light Racing upper A-arms.

As currently aligned, the car doesn't pull with the tires I had on when he did the alignment (Maxxis Bighorn 255/85R16) or a set of 285/75 MT/Rs. It does with at least two other sets of 255s. Maybe I could push the issue and have them redo it, but in fairness if it didn't pull with the tires I had on it when I took it to them, them maybe it should be on me to pay for the custom alignment and re-clocking of the upper ball joints. I would not be surprised if I eventually get to this point, but since I just had to spend a fair amount of money doing some front end repair work/alignment after recent front suspension mods and Light Racing's A-arm bushings failing after only one year, I'm not excited about spending more money on the front end at the moment.

Your point is well taken. My intent with this thread of for people to be more informed about the relationship of their alignment and tires and how critical and interrelated it all is. It's very easy for the tire guy to blame the alignment and for the alignment to blame the tires. And when we are talking about modified vehicles there are lots of variables

I'd just like to comment... when an alignment shop tells you your vehicle is "In Spec" does not necessarily mean it's aligned properly. The alignment "window" from the factory is quite large, usually around 1 degree which is 10 times larger than the accuracy provided by a basic laser alignment machine. You have can have -0.5° camber on one side, and +0.5° camber on the other side, and be "in spec" but the car might pull because of the cross-camber. An alignment shop has the capability to align a car within a 10th of a degree side to side, but usually just get it "in spec". I don't accept this situation, but it's getting harder to get shops to do a proper job. I can do a alignment in my garage with string and a tape measure than some of these laser equiped professionals turn out the door.

Just keep that in mind before you just accept the alignment shop's suggestion that it's the tires, especially if you're running a major brand.

 

[Redline]

I knew you would weigh-in with something pskhaat :sombrero:

Hmmm.....

 

[Chadman]

You can't just add any tire to the list as it's the specific relationship of a certain tire with your vehicle and its alignment/chassis. Another vehicle with the same combination may or may not pull.

OK. :snorkel:

 

[Redline]

Here are my most recent (front) alignment setting:

Camber

Left 0.05* (-0.08 - 0.05 range)

Right 0.03

Caster

Left 3.32* (2.25 - 4.25 range)

Right 3.49*

Toe

Left 0.02" (0.00 - 0.03 range)

Right 0.02"


Do you see anything wrong with this? The 4Runner actually has extra caster on the right to try and allow for road crown, etc, but it still goes to the right. I have considered reducing caster, changing the camber... but don't know what the answer is. Change my wheels? I have a few sets (truly) of FJC TRD wheels that I love, 16x7.5", factory fit, good price, I really don't want to change wheels.

I do have another alignment appointment for this coming Monday to try and figure this out...

Baseline/Stock setting on new 4Runner w/ 2,000-miles were:

Camber

Left -0.26*

Right -0.49* (cross camber +0.23)

Caster

Left +3.00* (cross caster +0.06)

Right +3.06*

Toe

Left +0.00 (total toe)


S.A.I.

LT +12.0*

RT +12.5*


Right -0.05

I'd just like to comment... when an alignment shop tells you your vehicle is "In Spec" does not necessarily mean it's aligned properly. The alignment "window" from the factory is quite large, usually around 1 degree which is 10 times larger than the accuracy provided by a basic laser alignment machine. You have can have -0.5° camber on one side, and +0.5° camber on the other side, and be "in spec" but the car might pull because of the cross-camber. An alignment shop has the capability to align a car within a 10th of a degree side to side, but usually just get it "in spec". I don't accept this situation, but it's getting harder to get shops to do a proper job. I can do a alignment in my garage with string and a tape measure than some of these laser equiped professionals turn out the door.

Just keep that in mind before you just accept the alignment shop's suggestion that it's the tires, especially if you're running a major brand.

 

[Redline]

Two weeks ago I spent about three days doing trial and error testing to diagnose the tire pull and torque-steer my V8 AWD 4Runner seems to have. I drove lots of miles to find a new baseline. This is the story.

Since both BFG KM2 255/85 (had them only a few days) and the ******** Cepek Fun Country II 285/75R16 I replaced them with both pulled to right, I had to either adjust my alignment for the FCII or go back to Maxxis Bighorns. Not wanting to ditch the Cepeks I decided to search for an alignment solution.

As the alignment was set-up (specs in post above) I had NO tire pull with at least two sets of tires: Maxxis Bighorns in 255/85 and MT/Rs in 285/75R16. The car DID pull to the right to varying degrees of severity with: Toyo MT 255/85, Cooper S/T 255/85 (not much), KM2 255/85, and ******** Cepek 285/75R16.

Playing with full throttle on the AWD 4Runner with 4.88s made it clear that the car has some torque-steer/pull under heavy throttle regardless of the tire and alignment settings. I can get it to pull pretty hard to the right flooring the accelerator from a stop in the low gears, and letting off will bring it back to the center or left. Is this more to blame on the full-time, all-wheel-drive (no 2WD mode) or the 4.88s, or a combination of the two (my guess) along with all the other modifications? It's a guessing game, but there is some torque-steer. However, I don't drive the car hard most of the time and the tire pull I've experienced and wanted to cure was at light, steady throttle openings on relatively level roadways.

Settings for ******** Cepek FCII

Even when not hard on the throttle the Fun Country II treads would track to the right. Since road crown can have much to do with how a car tracks or needs to be steered, my goal was to get the car to track as true as possible in high gear (5th) on the freeway/highway, under light throttle, close to the speed limit 65-70.

To get the car to go straight instead of right, we needed to either add more caster on the right or take some away on the left, to change the cross-caster. Since the stock adjusters were maxed out on the right, getting more caster on the right would require 'clocking' the adjustable upper ball joints on the Light-Racing A-arms. These move in 1-degree increments, a big, unwarranted difference. This would also change how the tire rides in the wheel well, possibly causing more rubbing. Plus, the car already had plenty of caster on the right side.

My alignment tech reduced the caster on the left. This helped, but after several miles I wanted a bigger difference, even less on the left, to make the car go straight more often than not. He almost doubled his initial adjustment, which was too much. We backed it off a bit and settled on the cross-caster split I'm currently running, almost a full degree of cross-caster, which is about 1/4-degree more than the maximum allowable cross-caster for this car according to Toyota. My tech says there are lifted G.M. trucks that need 1.5-degrees of cross caster to travel streight down the highway.

After getting the cross-caster where it felt good to me with the current FCII tires, I took it back to my tech, he confirmed the cross-caster difference, and maintained that split when he finalized the alignment, also resetting toe and camber to spec as well.

Current Alignment Settings

Camber

L 0.03* R -0.12*

Caster

L 2.64* R 3.59*

Toe

L 0.02" R 0.03"

Try Another Mud Tire?

Having adjusted for tires that pulled to the right, I hoped that some of the other tires that had previously pulled right would not with the new settings. I would like to have a set of mud tires that would like these same alignment settings.

There was only one way to find out. I decided to try another set of Toyo MTs in 255/85, knowing that my local Les Schwab would take them back if they didn't work. To make the new Toyos less expensive, I traded-in three new Bighorns I had in my shop and took the plunge buying five new MTs. One of these Toyos needed "zero ounces" of weight to balance on my TRD wheel!

Amazingly, with almost a degree of cross-caster, the Toyo MT 255/85 still pulled to the right. Actually worse than the FCII did before the alignment adjustment. I returned them with next day with 50-miles on them. Obviously there is something strange about the design of the Toyo MTs and my 4Runner as the two clearly won't work together.

Different Cars, Different Settings

When it comes to tire pull and alignment, everything can be a factor. How the vehicle is modified and set-up, the tire tread, air pressure, tire width, wheel width, you name it. While we were working on a solution, my alignment tech said "now you know what we are up against sometimes", because he has some customers who are not very mechanically savvy and don't understand how a change in their tires can affect the handling/steering of their car so dramatically.

In the case of the Mall Crawler 4Runner, we have at least found a setting that works well for the current tires, and we know the old settings worked well for the Maxxis Bighorns, a tire I really like.

What is possibly most interesting is that the car has always been aligned within specification, but it appears that there is not one alignment setting that will work for all the tires we have run/tired in the past. The car needs to be adjusted for the tires currently mounted and it may have to be changed in the future when other tires are preferred.

 

[spressomon]

Has your torque steer become worse since lifting the front end a little more/since installing the new coil-overs?

 

[Redline]

I don't think so.

I first noticed the torque-steer/tire pull (mild) when the 4.88s were installed AND the front end was lifted an inch+ higher when the OME springs were correctly configured (they were not). However, that was also with Toyo MTs which apparently won't work on this car regardless of the settings. Never had 'issues' with Maxxis Bighorns, included with the added lift with the new coil-overs.

Has your torque steer become worse since lifting the front end a little more/since installing the new coil-overs?

 

[Dave Bennett]

Hmmmm. This may explain my slight pull to the right. Or I may just need another alignment...

 

[Redline]

I would be a little surprised if you had torque steer in 2WD on-highway.

Hmmmm. This may explain my slight pull to the right. Or I may just need another alignment...

 

[R_Lefebvre]

Sorry I missed your earlier post. Those alignment specs (original) don't look to be too bad. Given the problems you are having with different tires... I gotta bring up a term I've only seen used for cars: Scrub Radius. That is basically where your steering axis angle of inclination meets your contact patch. Modern car design has determined that it should be zero. The SAAI should intersect dead center in the contact patch to eliminate torque steer. This was a big revelation about 10 years ago. Assuming the rest of the suspension is fixed, the scrub radius can change due to different offset rims, or different height of tires.

http://www.miata.net/garage/offset.htm

Just an idea, I'm still relatively new to truck suspensions.