Steering - a careful analysis

[Markham]

I posted a question in the super long thread and didn’t get an answer so I want to post here.

I also have a 19 Ram 1500 in the household and it essentially has the same steering setup, different steering box, dampener, drag link etc.) I understand different vehicles, but it’s also not an apples to oranges comparison either. Both are larger vehicles, larger tires, same type of steering but the Ram 1500 has tight steering. What is so different or what am I missing?

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[richk225]

You can`t even compare a Jeep to Jeep let alone to a Dodge Ram. Rubicon has a wider track then other models along with other steering components
Everything from tire pressure to tire balance to tire size alone to rim offset makes a difference. you would think that they would all be the same but they are not. The steering alignment spec`s are different between models too

 

[DeVoTee]

IFS vs Straight Axle....

 

[Arrowhead]

@JoeFromPA, I don't think anyone is claiming that tire pressure has NO affect on steering performance. Sure there were a few Jeeps that had no steering issues other than coming from the dealer with way to high of air pressure in the tires. However, this is not the root cause for the vast complaints about sloppy steering. In fact, most of the door tags I have seen call for tire pressure in the 35 to 38 psi range. People are going quite a bit lower than this to improve things on Jeeps with bad steering. IMHO, it is just masking the issue. It creates a larger contact patch and makes the steering heavier so that the dead zone doesn't seem so twitchy.

I guess until you have owned or driven a Jeep with steering problems it's just hard to understand. My 2018 had so many things wrong with the steering, including high tire air pressure, finger tight ball joint nuts, flexy track bar, soft track bar bushings, poorly welded track bar bracket, bad steering pump programming, and bad steering box. Correcting the tire air pressure to the door tag specification had very little effect.

My thoughts exactly.

 

[JoeFromPA]

My thoughts exactly.

Lowering tire pressure from 40-50 PSI to 35 PSI or similar does not increase contact patch.

That’s a fundamental misunderstanding (common) of tire contact patch dynamics.

 

[vavaroutsos]

Lowering tire pressure from 40-50 PSI to 35 PSI or similar does not increase contact patch.

That’s a fundamental misunderstanding (common) of tire contact patch dynamics.

Really? Why do you think people air down for more traction/float? The lower the air pressure, the longer (not wider) the contact patch.

 

[JoeFromPA]

Really? Why do you think people air down for more traction/float? The lower the air pressure, the longer (not wider) the contact patch.

Because on soft surfaces more of the tire sidewall and tire tread will make contact and spread out the weight at very low tire pressures

At normal tire pressures and above on the road that does not happen. You have the same total surface area contact patch. Sure you may increase longitudinal contact patch at high tire pressures and decrease lateral. But the overall contact patch remains tb same.

 

[vavaroutsos]

Because on soft surfaces more of the tire sidewall and tire tread will make contact and spread out the weight at very low tire pressures

At normal tire pressures and above on the road that does not happen. You have the same total surface area contact patch. Sure you may increase longitudinal contact patch at high tire pressures and decrease lateral. But the overall contact patch remains tb same.

Joe, I'm sorry you're just plain wrong. Time for a physics lesson. Would you agree that each tire on the Jeep supports the same amount of weight regardless of how much pressure is in the tire (assuming the level of the Jeep doesn't change)? For simplicity, let's assume each tire supports 1000 lbs. If the tires are at 40 psi, then that means the contact patch is 1000 lbs / 40 psi = 25 sq in. If the tire pressure is lowered to 35 psi, then the contact patch is 1000 lbs / 35 psi = 28.57 sq in. This is just plain physics and there is NO denying it. The only thing that can be argued is in what direction the contact patch expands. Many people think it gets wider, but really it mostly gets longer due to tire geometry and construction.

 

[JoeFromPA]

Joe, I'm sorry you're just plain wrong. Time for a physics lesson. Would you agree that each tire on the Jeep supports the same amount of weight regardless of how much pressure is in the tire (assuming the level of the Jeep doesn't change)? For simplicity, let's assume each tire supports 1000 lbs. If the tires are at 40 psi, then that means the contact patch is 1000 lbs / 40 psi = 25 sq in. If the tire pressure is lowered to 35 psi, then the contact patch is 1000 lbs / 35 psi = 28.57 sq in. This is just plain physics and there is NO denying it. The only thing that can be argued is in what direction the contact patch expands. Many people think it gets wider, but really it mostly gets longer due to tire geometry and construction.

I’m sorry but this is incorrect. I’ll expand later with links to explanatory websites where you don’t have to take my word.

For now I’ll just say this: you just posited that the total contact patch of a tire measured in square inches is linearly proportional to its tire pressure. That’s just not true.

To be continued I guess.

(Ps Maybe ten years ago I had to have an old dragster guy explain to me over and over how tire contact patch size works - and then independently verify it multiple times. Was an amazing experience)

 

[JoeFromPA]

Joe, I'm sorry you're just plain wrong. Time for a physics lesson. Would you agree that each tire on the Jeep supports the same amount of weight regardless of how much pressure is in the tire (assuming the level of the Jeep doesn't change)? For simplicity, let's assume each tire supports 1000 lbs. If the tires are at 40 psi, then that means the contact patch is 1000 lbs / 40 psi = 25 sq in. If the tire pressure is lowered to 35 psi, then the contact patch is 1000 lbs / 35 psi = 28.57 sq in. This is just plain physics and there is NO denying it. The only thing that can be argued is in what direction the contact patch expands. Many people think it gets wider, but really it mostly gets longer due to tire geometry and construction.

Ok, I promised a data backed link and argument and not just my own internet-persuading. Here's one of the best examples I know of using a study by Avon tire on contact patch area, tire pressure, tire load, etc.

https://www.enginebasics.com/Chassis Tuning/Tire Contact Patch.html

Let me give some snippets:

"I had a difficult time believing what I was seeing when looking at this data. According to the Avon data and these calculations, the contact patch pressure not only does not remain close to constant, it never even approaches that of the air pressure in the tire. Granted, sidewall deflection is not included here so in reality the contact patch pressures are likely to be higher than what is shown, possibly significantly. But what is immediately obvious is that doubling the load does not double the contact patch area. I.e., the contact patch pressure does not remain constant.

From 390 lb to 773lb (almost double the tire load) the contact patch size only increases by 25%, even assuming laterally constrained sidewalls! Additionally, the calculated contact patch pressure rises from 6.50psi to 10.30psi, a whopping 58% increase in patch pressure. And again, since it is likely that the contact patch size grows even more slowly than what the tables show due to sidewall deflection, the contact patch pressure likely increases even more rapidly than what we see here.

The point here is, even if the calculated contact patch data was wrong, the trend is still obviously in the direction that contact patch pressure and air pressure are not very intimately related at all. I.e., the equation

Camp A uses: contact_patch_area = weight / tire_pressure

....is not even close. Those contact patch pressures should be pretty close to constant and somewhat in the tire pressure's ballpark throughout the load range if the equation is really a reasonable approximation of reality."

"This data is the most puzzling of all and can be compared with the first table on this page. What is interesting here is that even though the air pressure was lowered, the amount of vertical tire deflection actually decreased at many loads. At 28 psi and 553 lb load the tire radius was 308.3mm versus the 24 psi tire radius of 311.5mm. The calculated contact patch size actually decreased from 67.35 square inches to 58.24 square inches. I don't understand it, but there it is:. They took 4 psi out of the tire and it had less vertical deflection at 550 lb load! It looks like tires are little more complex than balloons after all. The physical tire structure and construction obviously can not be ignored or assumed to be "only 5-10% of the picture.""

"
Through the last two tables things are basically as expected. As air is removed from the tire the deflection at each load is greater and the contact patch grows. The 24 psi data is bizarre, but the general trend over all four tables is as most people would expect. Drain out the air and the tire center sits lower (past ~24 psi anyway). However, note that the deflection and calculated contact patch area/pressure don't change nearly as much as tire pressure would indicate if contact_patch_area = weight / contact_patch_pressure gave a reasonably accurate picture of reality. At 550 lb load, lowering the air pressure from 28 psi to 18 psi should increase contact patch size by roughly 55% if that formula is even close to reality.

Now is that bizarre or what? The 18 psi tire actually had LESS vertical deflection than the 28 psi tire did at 550 lb load. Remember, the vertical deflection and force (first two columns) are actual measured, Avon test data, and are not subject to any calculations on my end at all so they can't possibly be "wrong." That data can be verified at Avon's web site. The remaining columns are calculated, however, but make sense in light of the strange, unexpected changes in vertical deflection observed here. Strangely enough, at 18 psi the contact patch size on this tire appears to be smaller than it was at 28 psi, and the contact patch pressure increased only very slightly, by about 7% rather than 55% as would be expected.

The 18 psi tire sits higher at these two loads than the 28 psi one does! There's less vertical deflection. That's an Avon fact. I don't pretend to understand why this is so, but there it is. And again, when they doubled the load the calculated contact patch pressure does not double along with it, regardless of the tire pressure. It increased by 53% (high pressure) and 41% (low pressure) respectively.

When lowering the narrow tire's air pressure from 24 psi to 22 psi, the contact patch size grows 4%, not 9%. And don't forget about our constrained sidewalls! In reality the patch will grow even less than this. What else is interesting is that at this load, the wide tire's contact area increases by the same percentage as the narrower one does as pressure is reduced from 24 psi through 18 psi. I suspect that the constrained sidewall assumption skews the data in a way that means that the real wide tire grows its contact patch more rapidly than the narrow one. How much so is not known.

Contact patch pressure is constant and equal to air pressure? No, not even close. "


.....


Read at your own leisure. Bottom line is this:

ON THE ROAD (not in sand/mud) and at normal or high tire pressures, lowering/raising tire pressures (i.e. from 35 to 45 or from 45 to 45 PSI) does not increase contact patch. It changes the shape of the contact patch, which is relevant to steering feel.

OFF ROAD and especially in soft surfaces which contour to the shape of the tire, the dramatically lowering tire pressures will allow the tire to deflect dramatically and apply load (i.e. weight) over a more diffuse area. But you cannot take that reality and apply it equally to a hard surface (i.e. road) and at normal or high tire pressures.

Hope this helps.

 

[Arrowhead]

@JoeFromPA

Sorry bud, your really missing the boat here, you might just want to chill a bit. The dynamics change a bit with the tires we are dealing with on our Jeeps. Google "chalk test" and you'll plainly see that reducing the air pressure for sure increases the contact patch on larger off road tires.

 

[AnnDee4444]

@JoeFromPAGoogle "chalk test" and you'll plainly see that reducing the air pressure for sure increases the contact patch on larger off road tires.

I see how the chalk test can measure across the tread of the tire, but how are you supposed to measure the in direction of the tread?

 

[californiajeeping]

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.

 

[vavaroutsos]

@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.

 

[JoeFromPA]

@JoeFromPA

Sorry bud, your really missing the boat here, you might just want to chill a bit. The dynamics change a bit with the tires we are dealing with on our Jeeps. Google "chalk test" and you'll plainly see that reducing the air pressure for sure increases the contact patch on larger off road tires.

Your argument appears to be "I get your point but our tires are special."

I don't mind being challenged but this is a foolish challenge. Chalk tests are common in every tire. A chalk test does not measure contact patch. It measures tread width exposure to road surface. IT DOES NOT MEASURE TOTAL CONTACT PATCH AREA which is a function of the length, width, and tread design of the area touching road surface.

So, to use an extreme, you can chalk test a tire at 40 PSI and 5 PSI and find much more chalk touching the ground at 5PSI because the tire deflected out. Meanwhile, the longitudinal contact patch size shrunk dramatically. Doesn't mean the total contact patch increased or decreased on a hard flat surface - just means the tire deflected alot and more lateral tread is touching the ground.

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