Why skinny tires

[Antichrist]

This woudln't be a hard test to perform Michael. I envision a small plate of thick plexiglass or lexan on some blocks....

Seems it would be a lot easier to jack up a wheel, place a piece of plywood or something that has a good amount of wet paint on it, lower the wheel, jack it back up, substitute a sheet of blank newsprint, lower the wheel, and you have a good "image" of the contact area.
Or jack it up once and use a roller and ink to prep the tire then lower it on to the paper.

This is an odd discussion though. When wide tires were starting to be all the rage in the 70's, several magazines did just that and printed the images in the article. For a given tire pressure on the same vehicle, wider tires had a larger contact patch than narrower ones. (I'll see if I can find some of the articles and scan them)
Or maybe I'm missing the discussion point.

BTW, it just occurred to me the main reason I've always run skinny tires. For the first 26 or so years of off-roading all I drove were Series Rovers with a bonnet mounted spare. It's easier to see over skinny tires.

 

[Scott Brady]

Michael,

You should do a few tests. We have done imprint tests with 265/75 R16 and 235/85 R16 tires. Essentially the same diameter. They did not have the same contact patch, though the variation was less than expected. About half of the % difference in the width.

It is often incorrectly stated that the PSI in the tire will match the PSI to the ground. That assumes an infinitely elastic toroidal membrane. The sidewall of a tire has structure, stretching and bending resistance in the carcass. That is why a light Jeep with 50" Michelin XZLs can run with zero psi in the tire, but certainly have PSI on the ground.

The same concept applies to the idea that a wide and narrow tire will have exactly the same footprint. The carcass has an influence.

 

[Pskhaat]

Sorry JamesDowning, I tried to reply but ended up mod'ing your post. I hope I restored it to some semblance of what it used to be :O

 

[R_Lefebvre]

My position is, if it's the air pressure that's holding the load up, then it must follow the rule contact_patch_area = weight / tyre_pressure. If it's not the air pressure that's holding the load up, then what is it? Some of it might be the stiff sidewalls of the tyre, but we know that it doesn't take much weight to push those down.

I think it's not purely the "wall strength" of the sidewall pushing down that is supporting the load (in addition to the pressure). There are other geometric forces happening. When a tire is loaded, such that the actual radius to the contact patch is reduced vs. the free radius, you have the vertical wall strength of the sidewall. But you also have to look at what happens to the actual tread surface. This is hard to put into words... The formula for circumference is 2*Pi*radius. Say we're talking a 35" tire. The free circumference is 220". Now, when you load it, and it's aired down, the bottom radius collapses, so the circumference needs to get shorter. But it can't, it's fixed at 220". So, that small piece of tread in the contact patch is actually being compressed into a smaller length. This would end up putting forces into the sidewall to resist the movement.

I'm really having trouble putting this into words. Suffice it to say, there are a myriad of forces in the carcass of the tire, resisting the downward force of the weight, beyond the simple vertical strength of the sidewall.

I can see why the instability of high sidewalls would be something of an issue for a heavy truck. But I'm not immediately sure why the aspect ratio would be important (i.e. why would a wider tyre make a high sidewall less likely to collapse?). I could be persuaded, though.

And I think you're correct. It's not the aspect ratio per-se, but it is the actual height of the sidewall that matters. However, there are some other effect going on related to how the tire sidewall is stretched in or out towards the rim. When the rim is much wider than the tire, the sidewalls are pulled out to meet the rim. This means that side loading is not simply acting in a cantelever fashion on the sidewall. The loads actually become tension loads, because you need to stretch the sidewall in order to get it to deflect away from the rim. That's why you see many performance cars set up this way.

I think the same things happens in truck applications when you have a very wide tire on a narrow rim. The worst case situation is when the sidewall is vertical, and the only way the loads are resisted is pure cantilever bending of the sidewall. So, skinnies are at a disadvantage here.

However, as I've mentioned, relative wall stiffness plays a huge role. You just can't compare stability of wide and narrow tires across different tire models. Tire manufacturers can make VERY stiff sidewalls if they want to. And I think they do on many of the "skinny" E-rated A/T tires many of us choose to run.

For example, my truck came with... 255/55/18's or something like that. These had good stability, because the sidewalls were short, and relatively stiff since they were a "sporty" tire. I traded a guy those wheels and tires for his 16's plus cash, and those 16's had some crappy 255/70/16 P-rated tires on them. It was absolutely TERRIFYING to drive like that. It had very low yaw-stability. When I put my 245/75/16 E-rated S/T-C's on, the stability jumped back up to the same, or maybe even BETTER than the short 18" tires had, even though the sidewall was roughly twice as tall.

I also recently bought a 235/75/15 C-rated Goodyear A/T for my trailer, and was very dissapointed to see how flimsy the sidewalls are.

This is an odd discussion though. When wide tires were starting to be all the rage in the 70's, several magazines did just that and printed the images in the article. For a given tire pressure on the same vehicle, wider tires had a larger contact patch than narrower ones. (I'll see if I can find some of the articles and scan them)
Or maybe I'm missing the discussion point.

No, I think you're completely on-point. You're just providing a counterpoint to the argument that skinny tires have the same contact patch size as a wide tire at the same pressure. It's just not the case.

 

[JamesDowning]

It is often incorrectly stated that the PSI in the tire will match the PSI to the ground. That assumes an infinitely elastic toroidal membrane. The sidewall of a tire has structure, stretching and bending resistance in the carcass. That is why a light Jeep with 50" Michelin XZLs can run with zero psi in the tire, but certainly have PSI on the ground.

The same concept applies to the idea that a wide and narrow tire will have exactly the same footprint. The carcass has an influence.

Indeed, the internal PSI of a tire is not the only force at play on the contact area of a tire. Contact patch size of a tire made of a balloon would alternatively be based almost entirely on psi (think rolligon as an extreme example of low pressure, high contact area tires).

Sorry JamesDowning, I tried to reply but ended up mod'ing your post. I hope I restored it to some semblance of what it used to be :O

No harm... I'd like to read your comments none-the-less.

I think it's not purely the "wall strength" of the sidewall pushing down that is supporting the load (in addition to the pressure). There are other geometric forces happening. When a tire is loaded, such that the actual radius to the contact patch is reduced vs. the free radius, you have the vertical wall strength of the sidewall. But you also have to look at what happens to the actual tread surface. This is hard to put into words... The formula for circumference is 2*Pi*radius. Say we're talking a 35" tire. The free circumference is 220". Now, when you load it, and it's aired down, the bottom radius collapses, so the circumference needs to get shorter. But it can't, it's fixed at 220". So, that small piece of tread in the contact patch is actually being compressed into a smaller length. This would end up putting forces into the sidewall to resist the movement.

I'm really having trouble putting this into words. Suffice it to say, there are a myriad of forces in the carcass of the tire, resisting the downward force of the weight, beyond the simple vertical strength of the sidewall.

I think you're trying to say that some vehicle weight is transferred circumferentially through the tread too, and not only vertically through the sidewalls.

You are very correct, and your point can be made by looking at the Michelin Tweel. The Tweel has zero sidewall and zero psi:

I actually sat through a lecture a few years ago about how the Tweel operated. The interesting take away was that the internal rubber spokes actually took very little compressive loading... as you would think it did. The stresses are actually transferred mainly through circumferential compressive loading of the tread, and the wheel actually HANGS off of the internal spokes, loading the spokes above the wheel in tension more than the spokes that are in compression above the contact patch. Think of a piece of paper. You can apply a tension load easily, but a compressive load will just cause the paper to buckle.

Sidewalls act similarly. Much of the tracking and side loading strength of a tire is in the construction of the tread. The sidewalls don't provide much vertical compressive support because they are actually in a buckling condition, not so much in a cantilever beam condition. The sidewall above the wheel actually carries most of the carcass load in tension (I know, kinda counter-intuitive) and transfers it to the bead, which acts in axial tension holding on to the wheel itself.

 

[R_Lefebvre]

I actually sat through a lecture a few years ago about how the Tweel operated. The interesting take away was that the internal rubber spokes actually took very little compressive loading... as you would think it did. The stresses are actually transferred mainly through circumferential compressive loading of the tread, and the wheel actually HANGS off of the internal spokes, loading the spokes above the wheel in tension more than the spokes that are in compression above the contact patch. Think of a piece of paper. You can apply a tension load easily, but a compressive load will just cause the paper to buckle.

Sidewalls act similarly. Much of the tracking and side loading strength of a tire is in the construction of the tread. The sidewalls don't provide much vertical compressive support because they are actually in a buckling condition, not so much in a cantilever beam condition. The sidewall above the wheel actually carries most of the carcass load in tension (I know, kinda counter-intuitive) and transfers it to the bead, which acts in axial tension holding on to the wheel itself.

Yes, that's exactly the kind of thing I had in mind. I was actually thinking of a spoked wheel. The wheel rim transfers most of the loading through the rim itself, and then the axle is effectively *hanging* on the spokes at the top, since we know the spokes on the bottom can't really take any compressive strain.

I have an old 1983 Suzuki ALT 125, that takes the low pressure thing to extremes. It has NO suspension, and just relies on balloon tires at 3psi.

Fun little machine, and safer than modern 4 wheelers, IMO, won't kill you if it rolls on you. But that's a whole other discussion.

 

[RHINO]

why dont we settle this and just run mattracks :sombrero:

 

[the dude]

Have any of you guys ran a bigger then 35x12.5 tire on an expo rig that was set up properly to handle the bigger tires?

I just ask because I was on a recent trip with two rigs that had 38" or over tires. One did brake the steering knuckle (on my truck) but that's not a "tire" issue, that's an equipment issue.

I find it odd that I get thrown into "show trucks and tundra buggies" class.

I guess I also find it odd that some think a 35x14.5 tire is a big tire. :coffeedrink:

My progression of tires on my HJ61

31x10.5R16
255/85R16
9.00R16
11.00R16
325/85R16
38x14.5R16

I'll take as big and as wide as I can get. The 11.00R16 and 325/85R16 being my favorite but after seeing the 39.5x13.5 Irok perform, I may have to investigate those further.

In all my wheeling/overland time I don't recall ever having a truck with a smaller tire drive by me that could pull me out if I got stuck.

I have seen lighter trucks get through things, but never because of a smaller, skinnier tire.

 

[boblynch]

Scott, maybe it's possible to set up a bunch of different obstacle courses at next years ExPo Rally and do some field testing. With your marketing skills you could probably get someone to sponsor a few train cars of sand, dirt, and rocks from various regions. Deep snow may be out, but I'm sure a big mud pit would be fun.

 

[DiploStrat]

How you gain experience!

Since I recognise Diplostrat's truck, and I know he has loads of experience and would have aired down if necessary, I am kinda wondering why he's digging instead of just driving!??

Michael is too generous!- we were all young and stupid once. I was digging as I was high centered in the sand and I did not have 4x4.

-- High centered because of all of the big Mercedes trucks (you know, the ones with the dual wheels that don't work off road) that had worn deep ruts. (Look closely - you will see the dual traces.)

-- No 4x4 because at the tender age of 24 I didn't know about lubing the third bearing in a Dana 44 axle - the one that carries the drive shaft through to the wheel. As a result of the dried out bearing, I could not stay in 4x4 more than a few metres - couldn't even keep the hubs locked. So I basically had to run in 4x2 until I got stuck and then dig out, put the truck in 4x4 for a few metres, etc. (Positraction would have been nice as well.)

We camped for the night a little later and admired the Toyota that motored by in 4x4 with 7.50x16's.

Pictures of the same truck in South America show it much better equipped. (But that is where I learned that the high flotation tire guys in California were only half right - the big tires are great, but you still need lots of plys to protect against punctures and cuts, even at the risk of a harsher ride and less flotation. Guess who bought 4 ply tires?)

No Internet back then, so you paid for all of your experience with sweat and money. But as life offers few easy lessons, I have never regretted the hard ones.

 

[Scott Brady]

I know there are many trucks (excluding vehicles like Unimog, etc.) on this site that run larger than 35" tires, but those tires are fitted because of either highly unique overland terrain/requirements or that those trucks are also used heavily/mainly for recreational fourwheeling.

Please remember the intent of this forum, which is not 4+ trails or bottomless bogs. The goal is vehicle-supported adventure travel. If the intent is to drive from AK to Tierra del Fuego, or from London to Cape Town, the advantages of a tire like a 255/85 R16 will greatly outweigh the advantages of a tire like a 38x16.5 on a Toyota Land Cruiser. International travel by 4wd absolutely favors a modest tire size and a 75 or 85% aspect ratio (16" wheel).

This is a quote from my previous post in this thread

Less rotating mass- Easier to start and stop
Less reciprocating mass- Easier to dampen
Less wind resistance- Better economy and range
Less rolling resistance- Better economy and range
Easier to fit a taller/narrower tire with less lift
Lighter spare
Lighter tire
Lighter wheel
Less unsprung weight
Less weight and leverage on steering components, bearings, etc.

In terrain:
1. Less frontal resistance in mud and sand. Where is most of the increased contact patch (for flotation) gained? In the length, not the width. Tall and narrow allows for more length and greater deformation with less resistance.

 

[RHINO]

scott thats exactly what i was trying to say a few pages back, i have always thought the modest tires you normally see is much more a tire of circumstance.

to add, replying with the dude, i would not hesitate to drive my 55 round the world, outside north america i would make sure i had access to spare tires in my size (37/12.5), but i'd do it.

hey scott, get me funded and i'll be the test pig to see if its really all that. :victory:

ohh ohh heres the caption for the trip,, "the oink heard round the world" woohoo!

 

[the dude]

Less rotating mass- Easier to start and stop
Less reciprocating mass- Easier to dampen
Less wind resistance- Better economy and range
Less rolling resistance- Better economy and range
Easier to fit a taller/narrower tire with less lift
Lighter spare
Lighter tire
Lighter wheel
Less unsprung weight
Less weight and leverage on steering components, bearings, etc.

In terrain:
1. Less frontal resistance in mud and sand. Where is most of the increased contact patch (for flotation) gained? In the length, not the width. Tall and narrow allows for more length and greater deformation with less resistance.

I will not argue with your list. Those are all valid points BUT I am not sure they apply to tire "performance" in the purest sense.

rotating mass is a braking issue
reciprocating mass is a shock/suspension issue
wind and rolling resistance ----> yup, that could be important as a tire issue but CAN be resolved with power and fuel capacity.
easier fit is a fit issue
lighter ------> also valid but can be over come with stronger parts. If we look at it as an overall weight issue on the truck I can also agree it is a valid point, but many of us load our trucks to the max so I am not sure that tires are a big concern.

Now if we are talking availability, ease of replacement, ect. I would totally agree a modest tire might be the way to go.

Even as a overland/expo vehicle I would think that things like side wall thickness, grip, added ground clearance, the ability to run low pressures, and overall strength would be high on the list of must haves. A bigger tire generally gives you all of those things.

maybe we are both on the same track, is a 39.5x13 considered a tall skinny?? I would much rather be referred to as "unique" then a "show truck":sombrero:

I also think that there is merit in first determining what you are doing with that tire.

I wouldn't hesitate to drive my rig around the world either on a set of big XLs or XMLs. Rhino, maybe you could get us both a sponsorship...

 

[Life_in_4Lo]

Desertdude and Desertgirl ran AZ to Alaska and back on 35" MTs. I think they have done so much travel in so many differing terrain situations with a loaded 'expedition' rig, they aren't running 315's for looks alone.

There are practical reasons to stay close to or same as OEM original size for travel and economy but "the dude" also has great points in his last couple posts.

Take the debate to the trails, it's easy to see what is going on if you want to run pizza cutters versus to oem or +1, +2 sizing. Webwheeling doesn't mean anything.

 

[Scott Brady]

As I have said from the beginning. Look for a tire in the 75 or 85% aspect ratio for the height. The height is a consideration for the terrain you are driving much more than the width. That is the critical point.

The 315s on Pasquale's truck were 35" tall, and certainly needed for the mud they ran in Alaska.

Sure, you can take a Cruiser around the world on 38s. It could even be fun.

The AEV trucks are a perfect example. They run 35 or 37s on their trucks and they look great, perform excellent on the trail and really work on the road too. I drove one with 35's and a HEMI a few days ago, and it drove excellent.

I just know that my experience has shown that big tires (over 35") are just not needed (deep mud in the jungles during the rainy season or Iceland conditions excluded). Recreational fourwheeling is limited outside of the US and Canada, and most roads, even bad ones, can be driven by a modestly built truck.

My new Jeep, due to land in just a few days will have 34s on it. Although 255 wide