Why skinny tires

[LateNate]

I switched

I had 32x11.50s on my truck and they wore out. I decided to test the skinny tire theory and went with 33x10.50. One reason i did this was all the talk about tire size on this site. So I went out and purchased some skinny tires to see for myself. I put 15x8 black wheels on the tires and i think it makes them a little fatter looking. Not sure. I need to post some pics.

 

[DaveInDenver]

Well... what exactly do you mean by that?

Correcting oversteer in a RWD vehicle is slightly complicated. It depends on how bad the oversteer is, what your original throttle position was, etc.

If you have mild oversteer with a light throttle setting, the correct action is to hold the throttle, or maybe even increase it if you're in a low powered car. You need to get the weight to shift off the front axle, and onto the back axle. Countersteering goes without saying. Lifting off the throttle, or braking even, makes the situation worse.

Now, if you're talking massive oversteer, with huge amounts of wheelspin, that's another matter entirely. That will be cured by back off the throttle, but not completely.

FWD and AWD oversteer is always cured by planting the throttle and steering out of it. (again with the caveat of a very powerful AWD car at which case, some throttle modulation is needed)

It has more to do with shifting weight around than friction coefficients.

Oh, I know it's much more complicated, but in my truck with part time 4WD (i.e. 2WD normally), VERY underpowered, pickup (i.e. light rear axle weight), M/T tread, if I'm going down hill on a snow packed road in traffic, the rear end tends to float sideways and blipping (I dunno, wrong word, just giving it a quick, light throttle, whatever), is pretty effective at bringing the rear back in line without needing to touch the brakes, engage 4WD or really even steer much. If I am in 4WD the whole truck tends to drift sideways rather than wanting to swap ends. It's certainly putting more weight on the rear axle, but it's not much shift it seems. Not enough to feel anyway and I don't accelerate detectably. Probably more that the wheel speeds are not matched and I'm bringing the rear axle up to the same speed as the front tires are turning more than any change in traction or anything. Dunno, just thinking out loud. I am a very unsophisticated (some might use different pejoratives like terrible, horrible, inattentive or just plain bad) driver, so there's not much finesse in my style.

 

[JamesDowning]

Yes, not change in momentum of the dirt alone, but it is a factor. Otherwise, it's non-sensical that a car could accelerate faster on gravel than it can on pavement.

Most of those engines make most of their power at high RPMs. I assume the slip that the gravel provides allows the engines to rev into their power bands quicker. Even though they are only able to throw down a percentage of the power they are producing at the higher RPM, it may be more than the engine even produces at a lower RPM.

I've run soft terrain knobbies on hard terrain, and what happens is the knobs are inflexible, and they actually can get torn right off the tire.

+

Most of those engines make most of their power at high RPMs. I assume the slip that the gravel provides allows the engines to rev into their power bands quicker. Even though they are only able to throw down a percentage of the power they are producing at the higher RPM, it may be more than the engine even produces at a lower RPM.

- I beleive you just made a practical case where the shear stress limit internal to the tire became the limiting lateral force factor... not friction (this goes back to my prior post). The difference here is that you are shearing off a whole knob (due to stress concentrations around the knob base) and not just shearing off a thin sheet of rubber at the contact surface. +

 

[JamesDowning]

Probably more that the wheel speeds are not matched and I'm bringing the rear axle up to the same speed as the front tires are turning more than any change in traction or anything. Dunno, just thinking out loud. I am a very unsophisticated (some might use different pejoratives like terrible, horrible, inattentive or just plain bad) driver, so there's not much finesse in my style.

The rolling resistance inherent to a rubber wheel design and the mechanical nature of the vehicle will always apply a slight braking force to the tires. My guess is that the tires are slowing down enough to induce slippage, where you enter into dynamic friction (coefficient of dynamic friction is less than the coefficient of static friction). By bliping your throttle, you match the tire speed to the road speed, and enter back into static friction with the road.

At least, that's my understanding of what's happening.

 

[Cabrito]

I had 32x11.50s on my truck and they wore out. I decided to test the skinny tire theory and went with 33x10.50. One reason i did this was all the talk about tire size on this site. So I went out and purchased some skinny tires to see for myself. I put 15x8 black wheels on the tires and i think it makes them a little fatter looking. Not sure. I need to post some pics.

So what are the results after your test? I have 32x11.5x15's on my LC and plan to switch to the 33x10.5's in the future.

This thread is a great discussion but I can't seem to come to any conclusion other than my personal preference. I have worried about going too skinny because I have a 100 mile paved twisty road that we take every other weekend and feel like I need a little wider tire for traction and handling.

Where is the trade-off?

 

[Larry]

Great information here. But, remind me never to ask what time it is for fear we might get instructions as how to build a clock.

 

[Desolation]

Great information here. But, remind me never to ask what time it is for fear we might get instructions as how to build a clock.

ohhhhh... How do you build an atomic clock?:coffeedrink:

 

[Desolation]

I suppose that it is my turn. First incredible discussion!
What follows is my opinion and as such it is not worth the price of a cup of coffee, oh and there is No Math.
When I started exploring the PNW (US) in the 60’s as a kid we ran a then new International Travelall on them new radial tires, a 7.00Rx15 in a Michelin “Truck” Highway tires. Dad’s logic was that the manufacturer knew the best size tire… 200,000 miles when I sold that truck. I have come a long way since those skinny’s. After 30+years in various segments of the automotive field, predominantly the Off Road side, I have a grasp, and understanding of the Theoretical/Engineering reasoning. Fascinating stuff with friction in multi planes and both static and dynamic and slip angles, temperature, tread void ratios and stuff that I don’t fully understand…

In this conversation, a request was made for back to back experience, here is mine granted it has been a number of years ago. One of our past rigs, trail ready weighed 5800#US, give or take; V8 Powered, Manual Box, typical American “Truck based” SUV. We had the opportunity to run the same trail route of mixed terrain from deep mud to rocks on two sets of tires in about 8 hours. A set of 33-9.50 BFG MT and a set of the then new 35-12.50 BFG MT. Absolutely, the 9.50's I could not get off the truck fast enough! During the course of that day, there simply was not a single place where the skinny tire performed even adequately in comparison to the 12.50, including pavement! After that, I do not consider a 12.50 section on a 10” wide rim ‘portly’, it became simply “small” LOL!
Flip-side.
Today on our current rig (+4000#-empty) I chose the 33-10.50Rx15 BFG AT's on a lightweight OE alloy wheel for every reason listed in Scott's paper, however I did so because of the application, predominantly a Daily Driver(mileage), on pavement and biased to Snow and Ice on that pavement. When the weather gets real bad I’ll reach for the big truck on 37-12.50’s and fill the bed with sand AND chain all 4 wheels, but we are talking Feet of snow then.

It is not just the width that factors into the Ff, Cf, Fv; the diameter and tread void ratio plays a large part as well as the rubber compound and rim width. Further, it is as Scott notes, possible to exceed the Fv that a particular surface dynamic will tolerate. Due to surface irregularity/structural variability it is far easier then I feel is recognized once the tire leaves the tarmac. On the road down to a point that is load derived, I can agree with a thinner tire, never skinny thanks.

There is a dynamic that I have experienced with the tall skinny (yes I ran them back in the day) and that is rolling the sidewall. The Aspect Ratio coupled to the Rim Width are the primary determinants, and the taller the sidewall in relation to the width of the section can introduce a scenario where the tire will simply roll off the rim. That may likely never occur on the Rover or Jeep that had a top speed of 40-MPH (owned one of those) unless subject to a side hill that rates high on the pucker factor. However, because of the driver of our current rig will (has) sat at 90MPH for extended periods on pavement and covering ground in the desert the needle sees that same indicated speed exceeded though not for as extended of a time period; driving off the tire is a possibility, and avoidable by simply having some fat in the tire section.
It is interesting that the environmental realities of this conversation just came up. I can think of no terrain where the skinny tire causes less impact simply because of the greater unit loading that the smaller contact patch generates. This irrefutable dynamic being one of the “attributes” of the skinny tire. Anecdotal evidence on my part is simply the fact that I have both been stuck and pulled more vehicles out of “stucks” that were shod with Skinny tires then on wider tires. I dearly love the, “it will dig down to the solid stuff under the mud” argument. No not unless it is HUGE in diameter it wont! The only way you are going through that stuff with skinny tires is on the hook attached to a 15,000-SLP pound winch.
Simply based on my experience, which is valid only to myself, there is a functional limit to width on both sides. There is not a vehicle I can think of I would choose to run less then a 10.5” (250mm) section and I would not hesitate to run up to a 13” (325mm) section in a 36-38 inch OD.

Finally thanks for sharing the conversation here by the fire! However, I cannot see the fire anymore from here somebody moved it!:ylsmoke:
Agree or not on the aspect ratio, this has been fun!
Salute!

 

[78Bronco]

Mr Desolation has raised some great points. However, tires are a compromise of all sorts for varying terrain in terms of mud, sand, rock, snow or pavement. If you drive only one type of terrain, tire width, aspect and pattern are much easier to determine. I have always had good luck with an M/T (Max Traction) tread patterns. Even on compact snow/ice. So far I have been impressed with the 33x10.5's on my Mitsu. People tend to get stuck when they over estimate the capability of themselves & their vehicle and take a risk.




This is where skinnies aren't helping the journey. Not my picture, borrowing from anothe thread.

 

[DEEZLPWR]

Something I really do not understand is low-profile mud tires!

There is a very interesting discussion about why racecars went to wide tires in the 1960s in one of Carroll Smith's "To Win" books. I think it was Engineer to Win. Fascinating series, by the way.

some like the look of mud tires and may actually have a need for them but mainly they are easier to balance on a 20" rim than a 15 in rim for a tire of say 35" in diameter on each rim.

 

[michaelgroves]

This is a good read for those who are curious about the relationship between contact patches, vehicle weight and tire pressure. This obviously doesn't really provide insight into the skinny vs. fat debate currently broiling, but is interesting nonetheless....

http://www.performancesimulations.com/fact-or-fiction-tires-1.htm

Spence

Interesting find. It does have a bearing on the fat vs skinny debate, to the extent that if contact area is purely (or largely) a function of load vs pressure, then the idea that wide tyres necessarily provide significantly more contact area and flotation is bunkum. I hunted high and low for controlled tests to prove the idea that, for a given load and pressure, all tyre "footprints" will have the same area.

I did actually find some sites with vaguely relevant test data, but never directly measuring contact area, and/or not being rigorous about the load. I.e. not directly or primarily trying to test the relationship between load, pressure, and area.

Unfortunately, the Avon tests weren't precisely what was required either. They measured the change in radius of two different tyres, at various pressures and loads. The performancesimulations.com site converted this to a contact area by what I think is a dodgy assumption: "Ok, as stated already, Avon did not measure the contact patch size directly. However, it is probably accurate enough for our purposes to calculate the contact patch size by imagining the tire is a cylinder and penetrates the ground plane by the same depth as the vertical deflection that was measured on the tire tester. Agree? After all, any part of tire a that would be "underground" must be squashed against the ground and be part of the contact patch.".

So this wasn't actual measured data, at best it was theoretically derived from something else that was measured.

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.

Theere is no link to the original data, but it would be fascinating to see a test where they actually measure contact patch versus load/pressure.

 

[spencyg]

Theere is no link to the original data, but it would be fascinating to see a test where they actually measure contact patch versus load/pressure.

This woudln't be a hard test to perform Michael. I envision a small plate of thick plexiglass or lexan on some blocks, a small digital camera, and a tire pressure gauge. I would get the tire up on the plexi (mind you...I'm talking about 3/4" plexi here...tough stuff) fully supported around the perimeter with blocking except for a small void where a small camera can be inserted to take pictures of the bottom of the tread at various pressures. You could correlate the pictures to individual pressure data points, and then just figure out your ACTUAL contact area from there. I'm not sure I fully agree that Contact Area = Load / Tire Pressure due to the geometry of the tire, but this is where conjecture meets reality...a test! I might have some time and resources this weekend to try something like this, but I certainly encourage others to do the same. For a "quick and dirty", a simple tape measure could probably suffice for determining the rough contact patch.

I find Desolation's comments to be very interesting. We keep hearing about the limitations of a tall sidewall when a vehicle is "expedition" loaded. I wouldn't have thought this would have as much impact as it apparently does. This is all very interesting.
Spence

 

[michaelgroves]

This woudln't be a hard test to perform Michael. I envision a small plate of thick plexiglass or lexan on some blocks, a small digital camera, and a tire pressure gauge. I would get the tire up on the plexi (mind you...I'm talking about 3/4" plexi here...tough stuff) fully supported around the perimeter with blocking except for a small void where a small camera can be inserted to take pictures of the bottom of the tread at various pressures. You could correlate the pictures to individual pressure data points, and then just figure out your ACTUAL contact area from there. I'm not sure I fully agree that Contact Area = Load / Tire Pressure due to the geometry of the tire, but this is where conjecture meets reality...a test! I might have some time and resources this weekend to try something like this, but I certainly encourage others to do the same. For a "quick and dirty", a simple tape measure could probably suffice for determining the rough contact patch.

I find Desolation's comments to be very interesting. We keep hearing about the limitations of a tall sidewall when a vehicle is "expedition" loaded. I wouldn't have thought this would have as much impact as it apparently does. This is all very interesting.
Spence

That would seem an ideal test, Spence. FWIW, it's probably preferable to use as smoothly treaded a tyre as possible, to avoid the complications that big rubber lugs would introduce. Slicks would be perfect!

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.

To me, the only advantage of high sidewalls is that when they are run at lower pressures, deformation can take place over a larger area, and therefore harmful kinking can be avoided. Once they reach a height where this advantage stops increasing, it seems better to opt for a bigger wheel, if a bigger overall diameter is desired.

 

[michaelgroves]

This is where skinnies aren't helping the journey. Not my picture, borrowing from anothe thread.

A long narrow footprint is better in sand than a short wide one of the same size. I'd choose big diameter in preference to wide. And a smooth tread pattern. To be honest, though, I've found that none of those things matter much in sand - it's all about airing down, and the rest are minor considerations.

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!??

 

[dieselcruiserhead]

Excellent post all about it here that I just happened to find in another thread... In short, high centered and in 2WD due to mechanical problems... http://www.expeditionportal.com/forum/showpost.php?p=453174&postcount=17