Thoughts on tires, tall or wide, advantages?

[Scott Brady]

Scott, genuine question: if the wider tire has more contact area would that not imply somewhat less impact on the carcass?

Not based on my carcass studies, where I measured like diameter tires of similar construction and load and did tread imprint testing. They lengthened to within 1/16" of each other at the same pressure.

I think that your assumption above would require that the carcass had a more significant influence on the elongation (deformation) of the carcass under load. In reality, for a standard truck tire (let's call it a load D), internal air pressure (PSI) has the greatest effect on load capacity. So the relatively minor variance in carcass width does not have a measurable impact on elongation. Physically, it must have some effect, but nothing that I have measured in my testing (i.e. negligible).

The most compelling reasons why are because of the small width variance, the small impact that the carcass width has on load capacity (most is in the carcass sidewall) and the significant portion of the load capacity that is borne by the air pressure inside the carcass/wheel.

 

[DaveInDenver]

Dave, I think this is at what you're getting, but almost all vehicles change camber throughout the steering cycle, not simply just straight-line camber setting. Narrow tires roll laterally better in steer. Wide and more squared tires should naturally realize lateral non-uniformity in pressure?

I was think in a straight line, too. The suspension cycles the tire through its camber angle, unless it's a solid axle truck. Seems as the tire cycles the way the tread is loaded will change quite a bit, as one sidewall is unloaded and the other loaded. The tire on a solid axle would remain loaded pretty much the same, but not on an independent suspension.

Anyway, my mechanically challenged brain just wants to tell me that he truck's mass is not linearly spread across the tread. This is as much due to the rim width against it's offset. Not to muddy it anymore, but I think that is as much a reason why wider vs. narrow, the way the truck's weight is translated through the tread width. Take a stock rim and go wider on the tires, less impact on handling since presumably the right tread (say the center) is still right in the design's load center. Go with different rims that have different offset, a wider tire might actually be better if it puts the right amount of key tread in the sweet spot of the load. But go to a stock width tire on a different offset, you move the bulk (or at least pavement important) of the tread out of the place where the normal forces are concentrated.

So if a contact patch is 8" wide, the majority of the force is concentrated in some portion of that tread, not uniformly across the full 8". If you make the contact patch 11" wide (keeping the same rim offset so that the tire widens in as much as out), then I wonder how the load changes. Does the force distribution change marginally or proportionally? IOW, if you have a non-uniform but symmetrical force distribution, does the pattern stay the same but just reduce in magnitude or does the distribution in the main load bearing surface remain about the same magnitude and the edges become less important? I would guess that according to Scott B. the magnitude reduces equally across the whole contact patch, but I'm not as sure. I say that because the friction of coefficient is not identical on all tires and that is dependent on the physical characteristics of the tire as it rolls through the road interface. It's got to do with the way the rubber/road interacts, since a slick has a lot of dry traction. Although the mechanism may not be the same on rocks, less about the rubber sticking to the rocks and more about the tread edges hooking.

At the same pressure and side wall construction the radial deformation has to be pretty similar, so the contact patches are not equal and directly related to the tread width. Since the traction comes from the way the rubber and road surface interact (there's abrasion, adhesion and maybe there's other terms in the coefficient) then a narrow tire might have less traction typically simply by the amount of interface. But since the mass per unit area increases with a smaller contact patch, the narrow tire tread should deform more and probably has a higher coefficient of friction (i.e., more traction). I just can't wrap my head around the two tires having the same friction anywhere but in theory. I agree with you Scott J. that a narrower section width should seem to be more reactive to pressure, too. Brady needs to recruit some automotive engineers, my reading of the Bosch handbook is leaving me wanting, here.

 

[BigAl]

None of the above really use especially wide tires.

We should define what wide is then, like width as a percentage of the height??? When I look at any race vehicle, I see a wider tire than whatever would have been stock on that vehicle style.


33x9.5 is 29%
33x10.5 is 32%
33x11.5 is 35%
33x12.5 is 38%
33x13.5 is 41%
33x14.5 is 44%

If you look at the BFG krawler series, they are in the 34-39% range. What do you think the ideal % is for Offroad? Dual purpose?

 

[Scott Brady]

Anyway, my mechanically challenged brain just wants to tell me that he truck's mass is not linearly spread across the tread.

And you would be right. There is nothing linear about the mass Fv interaction with the tractive surface. It is all highly variable, just like the terrain we drive on. For me the point of relaying the factors of friction is to first get people to stop thinking the just wider means more traction, which is a false assumption, because as a tread gets wider, it looses Fv.

So if a contact patch is 8" wide, the majority of the force is concentrated in some portion of that tread, not uniformly across the full 8". If you make the contact patch 11" wide (keeping the same rim offset so that the tire widens in as much as out), then I wonder how the load changes. Does the force distribution change marginally or proportionally? IOW, if you have a non-uniform but symmetrical force distribution, does the pattern stay the same but just reduce in magnitude or does the distribution in the main load bearing surface remain about the same magnitude and the edges become less important? I would guess that according to Scott B. the magnitude reduces equally across the whole contact patch, but I'm not as sure.

Magnitude of Cf would increase (nearly) equally if all variables are the same, with the exception of carcass width. For example, wheels are exactly the same (off-set), camber is exactly the same, pressure etc. As we both know, there would be some variability.

Since the traction comes from the way the rubber and road surface interact (there's abrasion, adhesion and maybe there's other terms in the coefficient) then a narrow tire might have less traction typically simply by the amount of interface. But since the mass per unit area increases with a smaller contact patch, the narrow tire tread should deform more and probably has a higher coefficient of friction (i.e., more traction).

Yes. On highly tactile surfaces, where adhesion is the significant variable to traction, then a wider tire would benefit (and so would a soft/smooth tire), like in a race car. Concrete and asphalt might have a rubber to surface Cf of 3.0 or higher, providing the opportunity to maximize adhesion with a soft compound and as much rubber/surface contact as possible. However, on a trail, the surface is not flat and highly tactile, but slick, broken, loose, edged, etc., which favor a higher deformation rate gained from reduced air pressure and increased Fv

I just can't wrap my head around the two tires having the same friction anywhere but in theory.

And you are correct again. On a perfectly smooth surface, like glass, the balance between Fv and Cf can be demonstrated. After that, the theory becomes influenced by a thousand other variables. I have attempted to break those out in my work with BFG and others, and we have been able to regularly isolate and demonstrate many of those variables influence on tractive force.

I think the most important point is to break out from just the friction model and review the influences that a heavy, wide tire has on an expedition vehicle. The performance and efficiency advantages of a narrower tire (think 75-85% aspect ratio) are pretty clear for the overlander. If you look at the real overland markets, where Bigfoot never really took hold, the vehicles are all specified with a tall, narrow tire.

Sidetrack here, not directed at Dave, just for general comment:

Can all those 70 series engineers at Toyota be wrong?

How about all of those military design engineers in Germany?

Did the Camel Trophy vehicle engineers miss the fat tire boat?

I agree with you Scott J. that a narrower section width should seem to be more reactive to pressure, too. Brady needs to recruit some automotive engineers, my reading of the Bosch handbook is leaving me wanting, here.

That is a good SAE resource, and so is Race Car Vehicle Dynamics, from the Millikens. So is the Racing and High-Performance Tire.

I have had the narrow tire ideas audited by a few tire engineers, which in all cases resulted in "we know, but the fat tires are what sells"...

 

[Scott Brady]

If you hold the road surface, tire pressure, rubber durometer and sidewall construction static, then you have non-identically sized contact patches but with otherwise identical material properties. So you have to have different coefficients of friction because a given material's property seem intuitively related to how the rubber reacts to different pressure (the proper pressure, force per unit area). With a smaller contact patch you have a higher pressure between the rubber molecules and road surface, so they'd have to react differently. That strikes me as how the coefficient of friction changes for a tire application, how the rubber changes its adhesion or abrasion to concrete or surface dirt.

That all reads correctly to me.

Cf is higher with the wider tire and Fv (higher load pressure as you indicate) is higher (along with resulting micro and macro deformation) with the narrower tire.

 

[BigAl]

Did the Camel Trophy vehicle engineers miss the fat tire boat?

Isn't the idea of the camel trophy truck to prove what that vehicle is capable of stock? More of a drivers test? If you were going to build a truck to compete on that same course in an "Umlimited class" would you build the same vehicle?

 

[Scott Brady]

Isn't the idea of the camel trophy truck to prove what that vehicle is capable of stock? More of a drivers test? If you were going to build a truck to compete on that same course in an "Umlimited class" would you build the same vehicle?

They still could have fitted a wider tire to the discovery for the event, as the 7.0R16 was not a factory fitment anyways (taller than stock).

 

[ntsqd]

Just stumbled onto this thread. Some informative reading going on here. Some of my thinking on the various topics brought up:

The simple physics example assumes linearity in "fsa" (force per square unit of area) vs. friction for a given Normal force. That being that the total friction between two surfaces remains the same regardless of surface area so long as the Normal force remains the same. As the area increases the fsa decreases, which seems reasonable to me.
What has always given me pause is if this is absolutely correct or only correct for very specific circumstances, or somewhere in the middle. My best guess is it's like the Theory of Relativity, it isn't absolutely correct, but for the world we normally live in where it isn't correct doesn't really matter.
As mentioned, the variables involved alter the relationship such that the basic example is really only useful for conveying the concept and not the actual detailed interaction.

As Scott points out, load capacity is air pressure and sidewall strength. Ignore the air pressure for a moment (consider a friend's Samurai which he trail runs on TSL's with no valve stem cores!) and think just about the sidewall. How far apart they are has minimal effect on their load carrying capacity. The total difference in capacity between a wide tire and a narrow tire of same OD would be the tread's deformation resistance. So a 7" wide tire and a 13" wide tire of the same OD & rim size should bulge the sidewalls very, very close to the same for a given load.

SA trucks sort of have a camber curve too. At least in single tire bump/dip incidences. It is the simple swing-arm of the axle housing and the anchor point of the far side leaf spring or linkage. Perhaps best termed a "Transitory Camber Curve"? Funny how such a simple design as a leaf sprung live axle can have so many complex interactions.....

Where wider tires get you in trouble is the oft necessary increase in rim offset. Rim offset being not the back spacing, as that usually has to remain fixed, but in how far from center the mounting surface is from the center of the rim. When the offset is increased a couple not so obvious things happen. There is a factor called "Scrub Radius". As simply put as I can think of, this is the distance from the contact patch center point to the point on the ground where the Steering Axis would intersect it. Mentally draw a line through the upper and lower ball joints (Steering Axis), and extend that line down to the ground. Though possible it is rather rare to see Scrub Radius in the negative, meaning that the Steering Axis ground intersection point is outboard of the contact patch's center point.

Scrub Radius gives obstacles leverage against the steering. It also gives rolling resistance leverage to aid in the return-to-center effect of Castor. Turn a still vehicle from centered to each lock while comparing the hood to some fixed point of reference. You'll note that the hood is the lowest when the steering wheel is centered. The more rim offset or more Scrub Radius a vehicle has, the more pronounced this is. If your vehicle moves up and down more than it did when stock the wheel bearings are under greater loading than when stock.

Kart racers don't have suspension to tune. They instead tune the front and rear track widths. It gets to be a mind bender, but the further out the tires are, the less traction they have. The solution is in thinking about the lever lengths. A tire really far away from the kart doesn't have to push up as hard as a tire right under the kart. Kinda messes with Summation of Forces? Not really, the tires are all pushing equal to the kart & driver's weight. However, consider a frozen point in time while the kart & driver are in a corner. Those outside tires don't have to work as hard to support the kart & driver if they are further out because they have a longer lever. That translates into less Normal force on the tire, which reduces it's traction.
If you extend this concept to tire sidewalls, then the outer sidewall has more leverage than the inner sidewall by a small amount. Since the tire is loaded uniformly by air pressure and non-uniformly by the sidewalls there will be a subtle gradient of fsa across the width of the contact patch.

As a side note on the usefullness of a frictionless surface, a friend on mine once worked on a crowd control project that sprayed a foam that denied the ability to stand up. The stuff was so slippery that it was impossible to remain standing. Kind of hard to be aggressive while laying on the ground with no motive ability.

 

[DaveInDenver]

That is a good SAE resource, and so is Race Car Vehicle Dynamics, from the Millikens. So is the Racing and High-Performance Tire.

Some additional books. I've got "Sliding Friction: Physical Principles and Applications" by B.N.J. Persson and "The Friction and Lubrication of Solids" by Bowden and Tabor to flip through. Those silly mechanical guys have all the cool texts. Problem is that I'm reviewing about half a dozen of my old EE textbooks for a professional exam I have in April, so I doubt I'll get any time to really read these books before this spring, though.

 

[Overland Hadley]

ntsqd,

Your posts make such great reading. Thank you.

 

[Scott Brady]

If you look at the BFG krawler series, they are in the 34-39% range. What do you think the ideal % is for Offroad? Dual purpose?

It really depends on the weight of the vehicle and the conditions you intend to travel in. As mentioned earlier, there is a tipping point, when the tire becomes too narrow to be effective in soft surfaces or handle high-speed cornering loads. For a traditional expedition/overland vehicle, you will want to be in the 30-35% range. If the vehicle is exceptionally heavy or driven mostly on soft surfaces, then a slightly wider tire could be considered.

 

[TxRider]

All the talk about friction and traction is interesting and all, but I don't see a lot of talk about tire diameter's function.

I'll use my motorcycles for a quick example. larger diameter definately makes for a lot easier negotiation of rough terrain of any kind. A 21" front 18" rear set my DS has does infinitely better than a 17" front 17" rear that most street bikes use.

Or to look at it another way, a 30" diameter tire will stop you if you hit a 16" tall wall, or are sunk 16" or any other time when what you are pushing against is higher then your wheel centerline as it will not try to roll up it (at least an undriven wheel), where a 36" tire would still tend to roll over a 16" obstacle.

Just thought I'd add it in as the original question seemd to be asking about tall and skinny vs not tall and wide etc. not to mention a taller tire has more surface area contact than the same width shorter tire.

 

[cshontz]

I wish BFGoodrich would bring back the 33x9.50.

 

[DaveInDenver]

I wish BFGoodrich would bring back the 33x9.50.

AFAIK the 33x9.50 AT is still available, despite rumors otherwise. The MT is 33x10.50 only. I don't know that the 9.50 is often stocked anymore, it's been a couple of years now since I got mine. But I agree that the 33x9.50 is such a nice option and so few other companies make them. You can get a 34x9.50 Swamper I know and that's about it in 15" rims.

 

[TxRider]

I wish I could find more tires for a 16" wheel that are 36"+ tall and 9-11" wide.