Tire Pull: PlySteer & Residual Aligning Torque (RAT)

[Redline]

I always welcome and enjoy your input.

My alignment tech recently told me about the Miata.net tire size calculator which I found pretty fun.
http://www.miata.net/garage/tirecalc.html

Your link is helpful for illustrating the possible issue. I think the wheels I have, Toyota FCJ TRD 16x7.5” do have a bit more positive offset than the stock 16x7 inches wheels. The thing to do based on what you just shared would have been to mount the second set of Toyo MTs on the original 16x7" wheels and see if that helped.

It would be great to know if this is the problem/solution, but even if the stock wheels did help with the right drift I don't want to go back to them. The 16x7.5" TRD wheels have too many other advantages on my 4Runner; more clearance, slightly wider track width, will accept a 285 wide tire, etc. (and I like how they look too).

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.

 

[Stumpalump]

Next time you have it on the rack throw a couple hundred pounds on the driverside floor to simulate your weight or just ask to sit in it. You should also have the typical load that you carry in the vehicle. Whats been modified on the front end or is it stock?

 

[Redline]

Just to be clear, the car drives fine now with the new alignment, with its current tires, and just a bit of extra cross-caster.

The car is very modified; full skids & bumpers (weight!), 4.88s, lockers, coil-overs, adjustable upper A-arms, different wheels, tires, etc.

Next time you have it on the rack throw a couple hundred pounds on the driverside floor to simulate your weight or just ask to sit in it. You should also have the typical load that you carry in the vehicle. Whats been modified on the front end or is it stock?

 

[R_Lefebvre]

It would be great to know if this is the problem/solution, but even if the stock wheels did help with the right drift I don't want to go back to them. The 16x7.5" TRD wheels have too many other advantages on my 4Runner; more clearance, slightly wider track width, will accept a 285 wide tire, etc. (and I like how they look too).

Is it possible you could fix it with spacers? I'm not sure which way your wheels are offset, as offset can be both negative and positive. And just a word of caution about that. The definition of which direction is negative is not an industry standard. I've seen professional OEM suspension engineers arguing about which direction is which.

I'm guessing your new wheels moved the mounting face inward, pushing the wheels outward, so spacers would make it worse.

 

[Redline]

You're correct. I said positive based on the Miata.net page description. The new wheels are indeed wider/further out and I don't care to run spacers.

Is it possible you could fix it with spacers? I'm not sure which way your wheels are offset, as offset can be both negative and positive. And just a word of caution about that. The definition of which direction is negative is not an industry standard. I've seen professional OEM suspension engineers arguing about which direction is which.

I'm guessing your new wheels moved the mounting face inward, pushing the wheels outward, so spacers would make it worse.

 

[ntsqd]

Seems to me that given the larger OD tires that the contact patch re-location was in the right direction relative to the SAAI. Just potentially was too much or not enough. Only way to know for sure would be to plot it out, and that might be telling but there may be nothing reasonable to be done about it either.

Scrub has been talked about and debated in desert racing forums since about 2000. There are off road advantages to having a lot of it (crawlers 'hunting' for traction by alternately loading the front tires) and to not having any (high speed impact with a rock). There is no perfect solution for every use.

 

[R_Lefebvre]

I can see where it might be desired. RWD racecars sometimes use it for weight jacking, etc. But for FWD, it was instrumental (along with equal length halfshafts) in eliminating torque steer in high power FWD cars. So it might have application in an AWD truck.

Now, there is a way to measure it empirically if you want to. I can't remember the exact details but it was something like chalking the contact patch, lowering it to the ground, turning the wheel back and forth and then observing the pattern. Something like that.

I may not have followed all of the details, but it seems to me the only constant that followed your pull issue was those wheels? If so, maybe this is the cause. If you don't want to change them then... there's not much you can do.

Now, another option might be, since you have solved your problem with alignment, then get some spacers to run with your *other* set of wheels, or buy new wheels for your other set of tires, to match the offset.

 

[Redline]

Though I have other posts in other threads talking about the tire pull/torque steer/scrub radius? issues my car has had, this seems like the most logical/technical thread to reactivate with an update. In other threads it was suggested I run my tires/wheels on another similar car. I never did, until today.

I have a friend with a very similar 4Runner, a 2004, AWD V8 that is lifted similarly. He is running my old uniball a-arms and the same TRD wheels and a set of 285/75R16 MT/Rs. We mounted my ******** Cepek Fun Country II 285/75R16 tires also on TRD wheels on his car. His car is aligned 'normally' (I believe with less cross-caster, like mine in the past) and he uses the same alignment tech. His car drifts to the right with the FCII and his current alignment settings as mine did before, and pulls harder to the right under hard acceleration just like mine has/does.

The good of this is that it helps me think that my car is not messed up (didn’t really think so) but that is has a quirk. There is something with this chassis, certain tires, and our set-ups that makes them drive similarly with certain tires, wheels, and settings.

A few days ago I installed my set of Cooper S/T 255/85s which had not been on the car in several months. They apparently also like the current cross-caster and drive truer, better than any tire to date. Some of this is the nice ‘skinny tire syndrome’ and their inherent superior straight line performance. But these tires used to track a little to the right as well, and they don't seem to now (at least not on the 200-miles of roads they have seen since returning to the car).

 

[jim65wagon]

Good news, in a sense, for you! At least now you do not have to tear through your ride to find a problem. Tire pull is not a myth....CONFIRMED
just like a good mythbuster. The sad part is, now every time you change tires (every 2-3 weeks?) you'll need to get an alignment for those specific tires.


as an aside, which do you like better; the FCII's or the ST's?

 

[Redline]

Yes it is good news! I'm very happy to find a similar car that behaves the same :victory:

I don't think I will need to change by alignment for ‘each’ tread as I think I might be able to find dial-in settings that will work with more than one tire. My current hope is that the Cooper S/T and Maxxis Bighorns will both drive well with the current alignment. I'll know after I get a set of Bighorns mounted.

For the record, I have been very conservative lately, running the Cepek FCII exclusively for 6-months! :sombrero:

Regarding the FCII vs Cooper S/T, I put those over in the FCII or S/T thread where they may be more helpful.

Good news, in a sense, for you! At least now you do not have to tear through your ride to find a problem. Tire pull is not a myth....CONFIRMED
just like a good mythbuster. The sad part is, now every time you change tires (every 2-3 weeks?) you'll need to get an alignment for those specific tires.


as an aside, which do you like better; the FCII's or the ST's?