Explaining why lowering tire pressure helps steering

[AnnDee4444]

I've seen this tip posted a few times, and recently figured out why it works. I am not suggesting this is a fix for every JL steering issue, but just attempting to understand why it has the effect it does. I will attempt to explain below, but feel free to ask for clarification or correct my mistakes.

A common suggestion for steering fix on the JL is to increase the caster. When you increase caster, the steering self-centering effect gets stronger because the trail distance has increased. Trail distance can be thought of as a lever between the point where the steering rotates and the contact point of the tire. Increasing the caster moves the pivot point further forward making the trail's lever arm longer. The longer trail is actually what aids in straight line stability, and adding caster is just the method to get the longer mechanical trail distance.

Now here's the tricky part: the total trail distance is actually made up of the mechanical trail (caster), and the pneumatic trail (tire). The tire's effective contact point is the pneumatic trail, and it is influenced by just about every aspect of the tire. Tire size, stiffness, compound, and tire pressure all factor into where the pneumatic trail is located. When you lower the tire pressure the contact patch lengthens and moves the pneumatic trail towards the rear of the Jeep. The changes in pneumatic trail increase the total overall trail, which is why decreasing tire pressure can aid in straight line stability.


TLDR; Mechanical trail & pneumatic trail are both components of trail distance, which together influence straight line stability. Lowering the tire pressure and increasing caster both have the effect of increasing the trail distance.

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[californiajeeping]

Good info. How do tire load range ratings - C, D, E, etc. affect steering, if at all?

 

[AnnDee4444]

Good info. How do tire load range ratings - C, D, E, etc. affect steering, if at all?

In regards to what I have posted above? I'm not entirely sure, but I would guess that the higher load ranges would make for a shorter pneumatic trail. With all else equal I don't think that this one factor could ever be noticed, and wouldn't recommend caster changes to address it.


There are other factors about higher load range tires that could influence steering though:

• Sidewall - Higher load range tires usually come with stiffer sidewalls, which should result in more responsive steering
• Weight - Higher load range tires are usually heavier, which would make the steering feel heavier from the increased weight and increased gyroscopic effect. The tire weight also is unsprung mass, which would cause the suspensions responsiveness to be worse than a lighter tire.

 

[DonBindas]

The previous information makes logical sense, but here is another perspective to add into the mix.

Wider and stiffer tires tend to follow the crown in the road more, so the vehicle feels dartier over those crowns in the pavement.

 

[roaniecowpony]

I recall buying a set of Kumho tires for my truck many years ago. The sidewalls were so flexible that the truck was all over the highway. Steering was a suggestion, not unlike the experience with my JLUR before I changed the trackbar.

Just my opinion, but regarding the JL steering issue I had, I think more caster than OEM is a waste of time. It doesn't address the steering issue at all.

 

[Kyanche]

My jeep came with the tires at 40psi and I'm kinda embarrassed to say I haven't touched the tires? My JLUR steers about as well as my JKUS did.

If I had to complain about two things, I'd say that when I make a turn, it doesn't pull itself back to the straight-ahead position as automatically as my old jeep did. It also has a bit of bump steer feedback on the freeway. Not that I haven't seen that on a car before.

That said, the hottest I've driven in is like 100f and coldest 55f? I haven't really done much freeway driving either.

 

[mysto]

As tires get wider, relative to their diameter, their contact patch gets rounder. A round contact patch has less of a tracking tendency than an oval one. This is why some of us buy snow tires that are a little narrower than stock.

Thank you @AnnDee4444 for singling a topic out and trying to understand it.

 

[AnnDee4444]

I'd say that when I make a turn, it doesn't pull itself back to the straight-ahead position as automatically as my old jeep did.

I noticed this on the test drive of my JLR. Additional caster will help with the self-centering.

 

[roaniecowpony]

I noticed this on the test drive of my JLR. Additional caster will help with the self-centering.

Agree. More caster will have a stronger centering. So, if your lower tire pressure = more caster theory is accurate, he should get stronger centering by just lowering tire pressure. Should be easy to verify.

 

[Jeep&dogs]

Same thing happens when installing a taller tire. As the hub moves further up the pivot point actually moves further forward, possibly the reason Jeep feels the rubicon with 33”s and a lift didn’t need longer arms from the factory. I have typically always been able to run less caster with a taller tire and get the same wheel return.

 

[roaniecowpony]

Same thing happens when installing a taller tire. As the hub moves further up the pivot point actually moves further forward, possibly the reason Jeep feels the rubicon with 33”s and a lift didn’t need longer arms from the factory. I have typically always been able to run less caster with a taller tire and get the same wheel return.

I think I follow...the caster angle stays exactly the same, but the functional caster moment arm (labeled Mech Trail in the depiction above) becomes longer as the tire diameter increases.

One thing about all this caster discussion is: it seems counter-intuitive to me that I would want more caster than the 4.8 (OEM) that I currently have. More caster tends to slow steering response in the center range. To me, the steering could use some more responsiveness in the center, not less (on my 4 door). As for the centering, meh. I've driven cars/trucks that were better and worse. But adding these super steering dampers fights the centering effect of caster too. I'm currently driving without a steering damper. It feels fine. Still not a lot of self centering. But it's not a noticeable thing either. I'm not willing to give up steering responsiveness in the center and change the pinion angle, potentially causing a wear issue, just for centering. Is it possible all the Jeep mania over 7-8 degrees of caster came from the old small, short, jeeps like CJs, TJs, etc? They are way quick on the steering and more caster will tone it down a bit. With a big long ass JLU, the responsiveness is light years slower than a little CJ and abnormally (compared to regular cars/trucks) high caster isn't needed to slow the steering in the center. I recall talking to my brother about his CJ build and he had the pads on his axle cut off and welded to give the right pinion angle while giving 7 degrees of caster.

BTW, After a Yeti Pro trackbar, tierod, draglink and steering box backlash adjustment, I'm completely satisfied with my steering. I'm running around 34 psi cold and 37-38 warmed up. Anyone in the Socal area with steering issues that would like to drive my JLUR to see what's attainable without buying a PSC system, I'd be happy to meet up with them.

 

[AnnDee4444]

Same thing happens when installing a taller tire. As the hub moves further up the pivot point actually moves further forward, possibly the reason Jeep feels the rubicon with 33”s and a lift didn’t need longer arms from the factory. I have typically always been able to run less caster with a taller tire and get the same wheel return.

Interesting point, I had not considered this. I'm not sure this works in all cases though. Consider the extremes:

• comparing a 33" tall tire to a 10' tall tire with the same mechanical trail. For some reason, I can't visualize that the 10' tire would respond the same as a reasonable sized tire. One difference I can see would be that the pneumatic trail would be much greater on the larger tire, due to a longer contact patch.
• Another extreme scenario would be a tire so short, that the caster approaches ridiculous numbers (think +45°).

 

[AnnDee4444]

More caster tends to slow steering response in the center range.

I'm not sure I've ever experienced this in any vehicle, including the JL. Do you have any documentation that supports this?

Is it possible all the Jeep mania over 7-8 degrees of caster came from the old small, short, jeeps like CJs, TJs, etc?

The XJ, MJ, ZJ & TJ factory specs are at 7°. Wheelbase shouldn't matter.

 

[Roky]

IMHO, the jl should’ve came from the factory with more caster. Especially since they incorporated the FAD. There’s no possibility of vibration from the front driveshaft, not that there would be much any way as long as you didn’t go past 6 degrees. I think this would’ve alleviated half the bad steering complaints. Again, just my .02, don’t have any science to support it.

 

[AnnDee4444]

IMHO, the jl should’ve came from the factory with more caster. Especially since they incorporated the FAD. There’s no possibility of vibration from the front driveshaft, not that there would be much any way as long as you didn’t go past 6 degrees. I think this would’ve alleviated half the bad steering complaints. Again, just my .02, don’t have any science to support it.

I totally agree. I bet FCA designed one control arm for all models specifically for the economy of producing a single part. Revising the alignment specifications per model was the cheapest way for everything to look OK on paper, and has nothing to do with what works best.

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