Thoughts on tires, tall or wide, advantages?

[Scott Brady]

I've often heard that skinny tires give better traction, but I can't figure out why the pros use wider tires. Desert race trucks, rock crawlers, sprint cars, mud boggers, late models all use wider tires. You might say that race vehicles are big horsepower, but some rock buggys use 75 hp saturn motors???? Except for the steering wheels on a swamp buggy, I can't think of an offroad race vehicle with skinny tires. Generally racers use whatever will give them an advantage????

None of the above really use especially wide tires. Desert Race trucks require a slightly wider tire because they are typically 2wd, so flotation in sand is important. They also have the HP and drivetrain strength to turn them and are not at all concerned about MPG.

Rock crawlers also do not use especially wide tire (in the competitive circuit). Competitive rock crawlers use a combination of adhesion (soft tire compound) and deformation (extremely low air pressure to allow the carcass to wrap the rock).

The most important aspect of traction to understand is the coefficient of friction, which is (Ff = Cf x Fv). What that means is skinny OR wide, they have EXACTLY the same traction on a perfectly smooth surface (like a plate of glass). As the tire becomes wider, the Fv (vertical force load) decreases, BUT the Cf (coefficient of friction) increases. The same thing happens in reverse with a narrow tires <Fv and >Cf. So, all else being equal, a narrow and wider tire have exactly the same friction torque load capacity on a smooth surface.

Then why use a narrow tire on an expedition vehicle? The same reason they are used EVERYWHERE in the world except on big, bad mall crawling 4wds (you think someone might be trying to compensate for something?)

Look at military vehicles, look at Camel Trophy trucks. They all use tall, narrow tires, and for the following reasons:

1. Reduced rotating mass (easier to accelerate and stop)
2. Reduced reciprocating mass- think suspension up and down (easier to dampen and control with the shock and spring)
3. Lower rolling resistance (think better MPG and more power! Cool!)
4. Lower air resistance (both rotating and frontal)
5. Easier to store and change (hey, you can put it on your bonnet)
6. Much easier on the drivetrain
7. Much easier on the steering components
8. Easier on the bearing, spindles and knuckles
9. Less frontal resistance in shallow snow and mud
10. Hey, the are also CHEAPER to buy and better on the environment too!

And the above are without going into any of the real trail advantages from a physics perspective... :costumed-smiley-007

 

[MaddBaggins]

All that is fine and good Scott, but the 315/75 r16's look damn cool on my rig!

 

[Scott Brady]

It is also important to note, that there is a "sweet" spot for a tire aspect ratio and vehicle weight. I go into that a little more in the white paper, but for the typical truck and SUV on this forum, it will be around 85% aspect ratio. Tires get wider as they get taller to ensure sufficient tire contact (scrub and deflection) with turning.

It is all a balance. Just enough flotation for sand and mud and snow, but as narrow as possible to benefit from the items listed above. I have found that sweet spot to be at about 85% aspect ratio. If you want something "meatier", use the 75%. What I try to do is keep the width about what stock is, and just go taller. That usually means jumping from a 75% aspect ratio tire to the 85% aspect ratio tire, which gives you a 5% increase in ground clearance.

Lets compare a 265/70 and a 255/85, something that relates to a Tacoma, 4Runner, most Tundras and Sequoias too.

265/70 R16 = 30.6x10.6/16
255/85 R16 = 33.3x10/16

So, you have nearly the same width as factory, yet gain 1.35" in ground clearance. Significant...

 

[Scott Brady]

All that is fine and good Scott, but the 315/75 r16's look damn cool on my rig!

Noth'in wrong with going the "sexy" route

An M5 is not really that practical either, but I would sure love to have one in my driveway.

 

[Northern Explorer]

Look at military vehicles, look at Camel Trophy trucks. They all use tall, narrow tires, and for the following reasons:

1. Reduced rotating mass (easier to accelerate and stop)
2. Reduced reciprocating mass- think suspension up and down (easier to dampen and control with the shock and spring)
3. Lower rolling resistance (think better MPG and more power! Cool!)
4. Lower air resistance (both rotating and frontal)
5. Easier to store and change (hey, you can put it on your bonnet)
6. Much easier on the drivetrain
7. Much easier on the steering components
8. Easier on the bearing, spindles and knuckles
9. Less frontal resistance in shallow snow and mud
10. Hey, the are also CHEAPER to buy and better on the environment too!

And the above are without going into any of the real trail advantages from a physics perspective... :costumed-smiley-007

How much of that is strickly a result of the wider tire presumably being heavier? (numbers1,2,3,5,6,7,8) What if someone had a wide tire with a short aspect ratio on a smaller rim that weighed the same as a 255/85 16 tire? Obviously, a taller tire, to get the axel up over obsticals, is what's ideal for expedition travel. So this is more of just a point to ponder.

 

[Rexsname]

It seems to me that a higher aspect ratio would allow a longer footprint when the tire is aired down than a short tire/big wheel would.

REX

 

[Scott Brady]

Obviously, a taller tire, to get the axel up over obsticals, is what's ideal for expedition travel.

Exactly

 

[Pskhaat]

That is why wider tires are harder on bearings, the way the force is translated through the tire camber changes non-uniformly as you widen and narrow tread width, right?

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?

Scott B, one item you discuss but I don't see in this thread is also that a narrow tire will see much more contact area increase per pressure difference (good thing).

 

[Redline]

Is this the case even when the tires are essentially the same diameter? Does the narrow tire still become longer at the same (low) PSI?

Example: 255/85R16 vs. 285/75R16
Both are very close to 33-inches tall, but the section width is at least an inch different if not more depending on the specific tires in question.

...

Scott B, one item you discuss but I don't see in this thread is also that a narrow tire will see much more contact area increase per pressure difference (good thing).

 

[Scott Brady]

I mean it's not really a global friction problem, but a combination of many, many local rubber/road interfaces. So in that respect a wider tire would have more potential for localized minature spots of high coefficient of friction and the net sum of all the higher friction interfaces would yield a higher traction (and probably higher rolling drag) tire.

Certainly, it is not only a global friction problem, but each variable is still influenced by either the positive tractive force gained from Cf or Vf, and the variables are what make one tire better in one particular environment over another. For the sake of example, the more global approach to Cf is appropriate and meaningful to the casual reader. Your example above is also a variable, indicating that a wider tire has increase adhesion in specific environments, which it true, but the wider tire also lacks the same Fv of the narrower tire, reducing the depth (micro deformation) present with a greater Fv. In each situation, with the exception of maximum flotation, there are positive influences of both Fv and Cf on the rotational tractive capacity of the tire, for the given surface.

That is what makes all of this so dynamic and situational, further bolstering the concept of favoring the known advantages of a taller, narrow tire.

 

[Scott Brady]

Is this the case even when the tires are essentially the same diameter? Does the narrow tire still become longer at the same (low) PSI?

Example: 255/85R16 vs. 285/75R16

All things being equal (carcass structure, section height, etc) the carcasses will both lengthen the same amount. However, the 285 will have 11% greater total contact area and will distribute the load of the vehicle over a larger surface. From a performance perspective, there is always a parasitic loss to the vehicle from the wider tire, which is the increased frontal resistance to the sand/mud etc. The increased frontal resistance may not be a factor for a high HP application (like a sand buggy with a V8), but will have an impact on a small displacement (or low HP to weight ratio) expedition truck.

 

[Ursidae69]

Time for some testing in OJ. :26_7_2:

I can see it now, 4 white Tacomas outfitted with fridges and RTTs, each with a different tire size, all in front of deep sand. The whistle blows, they lurch forward and....

 

[Pskhaat]

Is this the case even when the tires are essentially the same diameter?

I'm trying to do some ridiculous experiments here with the kids' toys now that you ask that For any given diameter with let's assume the same carcass and compounds, I guess the first clarification would be that a higher aspect ratio tire (instead of simply saying narrow) will indeed exhibit a higher aspect patch.

I'll go way simplistic dynamics here (please correct?) but roughly if the track width is less than the carcass height I would at first think that there would be more gross tire material to deform in longitude than latitude and thus the contact patch may actually increase respectively (differential, not necessary absolute values) more per pressure unit drop; thinking said narrow tire would then buckle more of the carcass?

 

[Pskhaat]

All things being equal (carcass structure, section height, etc) the carcasses will both lengthen the same amount.

Scott, genuine question: if the wider tire has more contact area would that not imply somewhat less impact on the carcass? The inverse relation think about an infinitely skinny tire where all deformation would be in the carcass.

 

[Scott Brady]

The University of Nebraska also did an interesting study on the tractive capacities of narrow and wide tires, finding that narrow tires outperformed wide one in all surfaces but sandy and muddy ones (expected).

I will look for the original technical piece (pdf), but for now, here is a quick article summary