Why skinny tires

[T.Low]

Fabulous writeup Andre. This is exactly the kind of feedback I was looking for. Thanks!!

Spence

Oh gee, thanks Spence. I read Andre's "pardon the long" then scrolled down pardonning the entire thing with the touoch of a button.

Then I read your "Fabulous write up Andre" and now i'm scrolling back up to see what I was too lazy to read!

 

[T.Low]

Andre did indeed have some great points. Thanks.


In the midwest in the 80's I was all about the mud bog; 40x17's.

In the PNW & Canadian Southwest in the '00's, its all about picking lines thru the roots and rocks. A more narrow tire is easier to avoid rocks and roots with, and better for hard steering on those tight switch backs.

Q: At what metric size does a tire go from "narrow" to qualifying for the term "pizza cutter"?


FWIW: I run a trials tire on the rear of my Gas Gas instead of a knobby; better for rocks and roots that we have here.

 

[Desolation]

Q: At what metric size does a tire go from "narrow" to qualifying for the term "pizza cutter"?

I was just going to post a brake point on this so that we can all be clear.
I'll say 10.5" or 250mm is where the line is drawn on an Expo Truck. On a rally car it may be completely different.

 

[spencyg]

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

Cheers.

And for the record, I consider myself rather skinny so as to avoid ridicule, but also fat enough to hold my own in a fight and not be too concerned with my beer intake....isn't that what this all really boils down to anyway?



Spence

 

[RHINO]

lol @ spence.


desolation, i also think 10.5 is a good line.

my ealier reply about expo rigs running skinny tires out of necessity rather than performance is directed at tires less than 10.5, i think something like a 235-245 is pretty standard fair in most of the world, at least thats what i see in many world photos.
its that skinny-ness that i think is more necessity than performance. i also think 10.5 to 12.5 to be the perfect all around tire width based on height, i think up to 34-35" 10.5 and above that a 12.5, this is where you get that longer footpring for traction and incorporate some wide-ness for float.

of course this is just my thoughts and exp. i dont have mathematical equations.

 

[R_Lefebvre]

As written, Cf is the coefficient of rolling friction and seems only incidentally dependent on contact patch. The coefficient of static friction (say that is considered maximum coefficient) is independent of contact patch and is purely a relationship of the tire/surface interface. Coefficient of rolling friction is less than coefficient of static friction always (with the exception of hard ice) because it's the real dynamic interface and I think generally considered a function of slip (in that the coefficient is related on a force/slip curve).

I take issue with some of this, but maybe only because I'm misinterpreting what you are trying to say. (I love this mature debate, btw). So let me state this and see what you think:

Generally speaking, the coefficient of sliding friction is lower than the coefficient of static friction. However, the actual *maximum* load that a tire can achieve, happens not when it's purely static, but at very low levels of slip. (we're talking on a firm, dry surface here). I don't know that the actual physical phenomenon has ever been resolved down to the molecular level, but this is a well known fact. This is what makes for a really good driver. The slip can be felt through various other effects, and the car held at that point of slight grip, without going over. This is why "drifting" is stupid, because it's slow. But the best drivers will have the car actually sliding ever so slightly, it can only be seen in slow motion. This has also been measured on tire testing rigs, in the laboratory.

So IOW, the equation is better written as:

Your equation seems correct. The only point I'm trying to make is that it is a fact, I'll have to look for online sources to back it up, that the Mu of rubber increases, as the contact pressure decreases. Notice I'm saying contact pressure, not contact patch size.

I'm just reading refs like the Bosch Automotive Handbook and the SAE Handbook here as a non-professional, so I could certainly be misreading the description

Those references are not the state of the art. You should pick up Race Car Vehicle Dynamics by Milliken. Even this is probably not the state of the art anymore, since it was written about 15-20 years ago, but they is much more comprehensive than the other two, on this subject.

I have never seen dry friction related to contact patch size. The universal equation for dry friction is Friction = mu * N. Mu (coefficient of friction) is related only to the materials that are in direct contact, and N is the force (not pressure) acting normal to the contact surface. Contact patch size doesn't play into dry friction.

I didn't say it was related to contact patch size, I said it was related to contact patch pressure. Now, CPP will go down as the contact patch size increases if vehicle weight remains the same. However, the key point is we're talking about the pressure on the rubber. Your equation is overly simplified, that is what they teach in phsyics class, but it is not correct when talking about tires in the real world.

Rally vehicles have relatively wide tires for their vehicle weights. Granted, for dirt racing, they stick to somewhere around 215 width, but that's pushing it for their vehicles. When the terrain is being sheared due to high tire speeds, fluid friction (viscous drag) comes into play. In most applications (vehicle airflow design), you want to minimize fluid friction (drag). When it comes to rally tires, they want to maximize the fluid friction in order to provide more propulsion.

Ok yes, relative to the tire widths run on trucks weighing 2-3 times as much, rally cars do have "wide" gravel tires. I do like where you're going with the fluid friction thing. I'd take it a step further, and throw out one of my ideas I've had... I think in some cases, off-road vehicles with wildly spinning tires actually generate thrust from the mass acceleration of the dirt they are throwing backwards. Almost like a dirt jet-pump. It must be, when you consider some rally cars can accelerate FASTER on gravel than they can on dry tarmac.

Ice, is a different story altogether. I agree that super skinny tires are the way to go here, and as I understand it, its to maximize the force on each individual ice studs. You don't want float in that circumstance, you want to dig.

For snow, it's largely done for the same reasons we are talking about for off-road skinnies here. The idea is that the skinny tire can bite down through the soft surface layer of snow and mechanically grip the firm snow below.

IMO, the skinny tire thing on Expo trucks is simply a case of compromises. Since more of our time is spend on paved or gravel roads, as opposed to real trails, we compromise towards a more road-worthy tire, with better milage, drivability, etc. This is the narrow tire. And I think the debate is more about the skinny tire not being "as bad" as the mud boggers would think it would be, for all the reasons posted. But IMO, I also thing fat tires are better for pure off-road mug boggin', rock crawling, etc.

 

[DaveInDenver]

my ealier reply about expo rigs running skinny tires out of necessity rather than performance is directed at tires less than 10.5

It's about the right width tire. On my truck a 10.50 tire is fairly wide and 33x9.50 are nice for a lot of situations where float is no issue. Like on wet or snowy ('specially snowy) pavement with M/T pizza cutters (33x9.50) I have noticeably less white knuckles than 10.50 and particularly 12.50 wide tires. So the right width is the one that works with your truck. I feel the 33x10.50 BFG A/T to be a decent all around tire on Toyota pickups and the 33x10.50 M/T (slightly narrower than the A/T) to be perfect. The 33x9.50 M/T (no longer available AFAIK) was really skinny and the 33x9.50 A/T to be great all around, but less good in rocks on a wider rim. They are OK on stock 5.5" or even 6" rims, but on 7" after market rims they don't beat up my side walls and rim edges aired down. I am horribly squirrelly on any 12.50 wide tires commuting or driving on the interstate.

 

[xcmountain80]

I feel I have nothing to offer but I'm going to stick with my 295/75's.


Aaron

 

[DaveInDenver]

at very low levels of slip

Maximum coefficient is actually achieved with slight slip, IIRC about 10% slip but that's only from memory. So I believe we're on the same page here.

Your equation seems correct. The only point I'm trying to make is that it is a fact, I'll have to look for online sources to back it up, that the Mu of rubber increases, as the contact pressure decreases. Notice I'm saying contact pressure, not contact patch size.

I think we're saying the same thing. The mu is determined by many things, some of which certainly could be a function of various forces at the interface that I don't doubt are result of the patch physics. IOW, the rubber/rock/sand/pavement interface is molecular and mechanical. So the rubber composition alone doesn't fully encompass the coefficient like was presented in our ideal block-on-inclined-plane problems.

 

[JamesDowning]

(I love this mature debate, btw)

Same here!

Your equation seems correct. The only point I'm trying to make is that it is a fact, I'll have to look for online sources to back it up, that the Mu of rubber increases, as the contact pressure decreases. Notice I'm saying contact pressure, not contact patch size.

Ah, I beleive this is actually an interesting point. As I understand it, Mu doesn't change at all... however, the friction generated between the rubber tire and the surface of the road can actually be greater than the internal strength of the rubber. So the frictional limit is no longer F=Mu*N. The lateral load limit come about when Tau (shear strength of the rubber compound) < F (frictional force) / Contact Area. Tau is generally a fraction of the ultimate tensile strength of the material, so the units are force/area (but it's not a pressure measurement, its a shear stress measurement). If the frictional force generated at the contact patch becomes greater than the shear strength of the rubber, the tire will experience 'slip', but it's not because the tire is slipping, it's because the rubber is shearing away.

To increase the shear strength of the tire, the contact patch is increased in size. This is done with lower air pressures, less tread (think about drag slicks), or a wider tire.

I didn't say it was related to contact patch size, I said it was related to contact patch pressure. Now, CPP will go down as the contact patch size increases if vehicle weight remains the same. However, the key point is we're talking about the pressure on the rubber. Your equation is overly simplified, that is what they teach in phsyics class, but it is not correct when talking about tires in the real world.

I think this is basically what I explained above, but attempting to roll the phenomenon in with friction. Friction itself is essentially linear, but if you consider the shear strength limits of the tire as slipping and not shearing, then it does factor in contact patch size.

...I do like where you're going with the fluid friction thing. I'd take it a step further, and throw out one of my ideas I've had... I think in some cases, off-road vehicles with wildly spinning tires actually generate thrust from the mass acceleration of the dirt they are throwing backwards. Almost like a dirt jet-pump. It must be, when you consider some rally cars can accelerate FASTER on gravel than they can on dry tarmac.

That may also have something to do with it, but it can't be the change in momentum of the dirt alone that is propelling the vehicle.

IMO, the skinny tire thing on Expo trucks is simply a case of compromises. Since more of our time is spend on paved or gravel roads, as opposed to real trails, we compromise towards a more road-worthy tire, with better milage, drivability, etc. This is the narrow tire.

That sounds reasonable... I wonder if there is a similar relationship between an ice tire and a mud-terrain tire. Both have knobs (granted of different materials), but both knobs require a greater force (at the knob, so greater surface pressure) in order to bite into the terrain in which they travel... hmmm.

 

[R_Lefebvre]

I think we're saying the same thing. The mu is determined by many things, some of which certainly could be a function of various forces at the interface that I don't doubt are result of the patch physics. IOW, the rubber/rock/sand/pavement interface is molecular and mechanical. So the rubber composition alone doesn't fully encompass the coefficient like was presented in our ideal block-on-inclined-plane problems.

Yes, and so what I'm saying is, that the friction coefficient actually goes up as contact patch pressure goes down (there may be lower bounds to this, I'm not sure). Also, a wider tire of a given diameter, can be run with a lower pressure than a narrow tire of the same diameter. Racecars do this too. On pavement, or dry rock faces, a wider tire running at lower pressure should have more traction.

 

[dieselcruiserhead]

Great post. Thanks.

255 85 R 16 looks like a really interesting size. I looked it up on BFG's website and couldn't find it in a TA KO. Do you have a link to the tire you're thinking of?

Yes the Toyo M-55 in E-rated or the Open Country AT in D rated... I haven't run Toyo tires (other than a performance car tire that was on my old Volvo back in the day) but I've heard good things. The E rated 255 85 R 16 are what will probably be on my Super Duty as well. It will increase the mileage via putting the cruising speed at a slightly lower RPM (1800 - the sweet spot, best torque and lowest fuel consumption) and will also be narrow so it won't push all that wind. I googled it (there is another thread somewhere that is only a couple days old) and posted a link where quite a few people are reporting mileage increase with 255 85R 16. I look forward to them, but not their $225+ cost each so it will be a $1000 or $1250 purchase...

 

[R_Lefebvre]

Ah, I beleive this is actually an interesting point. As I understand it, Mu doesn't change at all... however, the friction generated between the rubber tire and the surface of the road can actually be greater than the internal strength of the rubber. So the frictional limit is no longer F=Mu*N. The lateral load limit come about when Tau (shear strength of the rubber compound) < F (frictional force) / Contact Area. Tau is generally a fraction of the ultimate tensile strength of the material, so the units are force/area (but it's not a pressure measurement, its a shear stress measurement). If the frictional force generated at the contact patch becomes greater than the shear strength of the rubber, the tire will experience 'slip', but it's not because the tire is slipping, it's because the rubber is shearing away.

To increase the shear strength of the tire, the contact patch is increased in size. This is done with lower air pressures, less tread (think about drag slicks), or a wider tire.

This is a very interesting idea. My knowledge of the science of tires is restricted to performance cars, which operate in a different realm than highly loaded truck tires. The idea that the traction could actually be higher than the shear strength of the rubber, on a highly loaded tire, definitely had merit.

Very good point.

I tried to find online sources to back up my statement that the coefficent of friction increases as contact pressure decreases (this now has be asterisked for the point you bring up above) but mostly turned up frigging patent pages that are not helpful.

That may also have something to do with it, but it can't be the change in momentum of the dirt alone that is propelling the vehicle.

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.

That sounds reasonable... I wonder if there is a similar relationship between an ice tire and a mud-terrain tire. Both have knobs (granted of different materials), but both knobs require a greater force (at the knob, so greater surface pressure) in order to bite into the terrain in which they travel... hmmm.

Since you bring it up, I should throw this out there. Dirtbike knobby tires: "soft terrain" tires are hard rubber, and "hard terrain" tires are soft rubber. Sounds backwards?

Soft terrain knobby tires are designed with very high void ratio, tapered knobs, and hard rubber. The idea is that the knobs bite into the terrain to achieve mechanical grip.

Hard terrain knobby tires are designed with more dense knob patterns, and softer rubber. They are designed to grip harder surfaces with more a classical friction/traction.

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. When running hard terrain tires on soft terrain, the knobs can't bite as deeply, and they also can wobble around a bit, so they just don't bite as well, but they don't wear out. So, on mixed terrain, it seems better to side towards hard terrain tires. You give up some traction on soft stuff, but least they don't fall apart.

As has been mentioned, enduro riders operating on hard terrain are experimenting with using "trials" tires. These are very high density knobs, with very soft rubber. When talking about running on pure rock, these are better than "hard terrain" knobbie tires, since the knobbies are actually designed for motocross, not enduro, and motocross guys don't run on rock. Their perception of "hard terrain" is different than enduro.

It used to be that enduro bikes shipped with tires that looked just like trials tires now do, but somewhere we lost the plot when the motocross craze hit. We're coming around full-circle.

 

[DaveInDenver]

Yes, and so what I'm saying is, that the friction coefficient actually goes up as contact patch pressure goes down (there may be lower bounds to this, I'm not sure). Also, a wider tire of a given diameter, can be run with a lower pressure than a narrow tire of the same diameter. Racecars do this too. On pavement, or dry rock faces, a wider tire running at lower pressure should have more traction.

Yup, which is why coefficient of rolling friction would be less than that of static friction or peak friction. I've also heard it referred to as adhesive coefficient of friction because the curve is dual sided for both acceleration and deceleration as the tire isn't necessarily going to symmetrical in both cases. I wonder if this stuff about slip also would explain why blipping the throttle helps you correct a fishtail.

 

[R_Lefebvre]

I wonder if this stuff about slip also would explain why blipping the throttle helps you correct a fishtail.

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.