Steering - a careful analysis

[JoeFromPA]

I guess whoever at FCA that wrote Star case S1819000003 that contains the instructions on how to check the steering gear box for excessive play, adjust toe to the max, check torque on the ball joints, track bar, drag link, date code on the steering stabilizer, etc. is perceiving the steering issue as well.

It's funny but that very same star case references two things:

1. If the steering gear is out of spec it's bad and needs to be replaced. A viable problem with instructions on detection and an unknown incidence rate.
2. That FCA has investigated many such cases and found the JL was operating just fine, as designed, and no problem was detected and they are looking for ways to better align the JL with customer expectations for driving experience (i.e. yeah they are going to change some things to tighten up the steering so these complaints go away in the future)

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[JoeFromPA]

@JoeFromPA, I don't know what those guys were doing in that study, but it's clear that doubling the load on a tire is not going to double the contact patch. This is because the tire pressure will change as the shape of the tire changes with increasing load which causes it's volume to change. So my question is, did they adjust the tire pressure to the starting value after each change in load? They don't say one way or the other, but from their data it seems they didn't. If they missed this, I wouldn't put much weight in any of their other conclusions. Here, we are talking about changing the tire pressure directly. Sure, in some tires the support provided by the structure of the tire itself (without any air) may have a larger effect, but in large off-road tires we're talking about here, it's not a big factor.

I suggest you go run some experiments on your Jeep yourself. Measure your contact patch at different tire pressures while sitting on a level hard surface.

I'm honestly unsure your question. When I read the study results by Avon it appeared they test the same tire, at fixed tire pressures, under different loads and measured contact patch size. So, by default, the tire pressure was pre-set at the start of each test (i.e. no load) and then the test was repeated at different pressures to see new results.

Why would they need to adjust tire pressure after each different load was applied within the same test cycle? Or are you asking if they re-measured resting tire pressure before the next subsequent load was applied? I'm unsure what that would gain.

 

[californiajeeping]

I wonder if the 2020 JL Wrangler steering gear box has a different part number than the 2018 and 2019 JLs. It is the same part number for the Gladiator and there is at least one car magazine review on the 2020 Gladiator that says the steering is terrible.

 

[vavaroutsos]

I'm honestly unsure your question. When I read the study results by Avon it appeared they test the same tire, at fixed tire pressures, under different loads and measured contact patch size. So, by default, the tire pressure was pre-set at the start of each test (i.e. no load) and then the test was repeated at different pressures to see new results.

Why would they need to adjust tire pressure after each different load was applied within the same test cycle? Or are you asking if they re-measured resting tire pressure before the next subsequent load was applied? I'm unsure what that would gain.

The author of the article seemed surprised that going from 28 psi with 600 lbs to 1200 lbs didn't result in a doubling of the contact patch area. My point is the contact patch area won't double because the pressure in the tire will be more than 28 psi because the volume of the tire has changed, yet it still contains the same amount of air molecules. If you look at their tables, you can see it appears they just calculated contact patch pressure from the same formula I used: contact pressure = force / contact area. But the author seems surprised that the calculated contact pressure is not the same as the initial tire pressure and does not remain constant with different force loading.

There are two things that provide support in the tire: the actual force it takes to deform the tire structure itself, and the air pressure acting on the contact patch. At one extreme you have a solid tire that requires no air pressure. At the other extreme you have a balloon that relies almost 100% on air pressure. Our tires fall somewhere between the two, but more towards the balloon end from what I have seen (my tires don't support that much weight without any air pressure). The force exerted by air pressure is a linear formula: lbs (tire load) = psi (tire air pressure) * sqin (contact patch area). The psi portion of this follows Boyle's law: PV = nRT.

 

[californiajeeping]

The AE version of the new JL steering gear box is now out - 68250506AE / 4 door. Previous versions where 68250506AC and 68250506AD. Confirmed with a Jeep dealer's parts department today. The 2 door steering box, which is different than the 4 door steering box, hasn't changed since last year.

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