Tires: Wide or Narrow?

[devinsixtyseven]

http://www.a1.nl/phomepag/markerink/sandtire.htm

That thread was painful to read. The guys replying to the ranter must have an unending supply of patience...

What I got from the thread was this:

"blah blah blah argue RANT type curse talk talk lower your tire pressure."

Which is pretty much the bottom line . Go narrow, since you have to choose between narrow and tall or wide and small, and narrow/tall gives more contact patch at low pressure than wide/small.

Interesting to note: Scott's white paper recommends the 85% aspect ratio 33" tire. A longer discussion on restoring old military vehicles had a similar discussion and similar outcome from trying different NDT tires...100% ratio tended to fold at low pressure, 75%-85% was pretty good, below 75% didn't perform as well offroad but worked well on-road. Another article a I read a while back talked about the need to go wider on the road for stability at highway speeds, and on the road anyway low profile performs better. Most vehicles don't leave the road . BFG's Baja performer has an 80% aspect ratio...and Interco's 40" (39.5, actually) IROK has a lowest aspect ratio of 75% on a 20" rim...it rises from there. All tall and narrow...

-Sean

 

[Pskhaat]

All I can say is that the classical maths model for friction is roughly a linear function against the normal force.

Surface area does not come into play. Continuing to Brady's assertions is that wider tire will take less pressure to support the weight of the vehicle, the skinnier tire more pressure. As such, for additional flotation, the longitudinal flattening of a relatively skinnier tire will be easier to obtain than the lateral area. The higher aspect series (75/85/etc) tires it reasons are able to obtain more of an increase percentage of surface are for the same given air pressure drop than a lower aspect series tire.

Someone correct my logic?

 

[bigreen505]

The problem with the general "tall and skinny" is better advice is it is a little too nebulous. Might as well just say "Buy a tire in proper proportion for your vehicle" as it is almost as clear. If tall and skinny were the way to go we would all be running bicycle tires. I hear the chorus screaming "Not *that* tall and skinny"! Okay, fine. How do you figure out what is best for your truck?

I'm currently running a short 285/75 on my Trooper and in most conditions that I have been in, they provide too much float. So I was considering a 235/85 and was informed they were too narrow for the Trooper and *given the conditions that I wheel in* a 265/75 would be better. I ran 31x10.50 on the Pathfinder, providing roughly the same tread width as a 265, and they were too wide and too floaty in many conditions (Mike H. I hear you about a 31x9.5 for the Nissan).

So how can we get a good idea of where to start without buying lots of set of tires and trying them out? I understand we are looking for something in the 75-85% section width, but I think there is more too it than that. Running the same size tire Scott has on the Taco seems like it would be silly since his truck outweighs mine by easily 1,000 lb.

Anyone want to make a 255/80 r16?

 

[paulj]

My only other question is whether my stock wheels (the steel 16" "styled" wheels) are too wide to accomodate a 235.

The spec tables for the tire you have in mind should give the allowable rim width. This usually is a 2" range. I would guess for 235's the range is 6-8". 6 1/2 and 7 seem to be the recommended rim widths for 235's (at least in 70 profile size).

It has been argued here that it is ok to mount tires on narrower than recommended rims, as long as you keep any eye on how much the center of tread bulges out. Mounting tires on too wide of rims might be more problematic.

paulj

 

[teotwaki]

The problem with the general "tall and skinny" is better advice is it is a little too nebulous. Might as well just say "Buy a tire in proper proportion for your vehicle" as it is almost as clear. If tall and skinny were the way to go we would all be running bicycle tires. I hear the chorus screaming "Not *that* tall and skinny"! Okay, fine. How do you figure out what is best for your truck?

I'm currently running a short 285/75 on my Trooper and in most conditions that I have been in, they provide too much float. So I was considering a 235/85 and was informed they were too narrow for the Trooper and *given the conditions that I wheel in* a 265/75 would be better. I ran 31x10.50 on the Pathfinder, providing roughly the same tread width as a 265, and they were too wide and too floaty in many conditions (Mike H. I hear you about a 31x9.5 for the Nissan).

So how can we get a good idea of where to start without buying lots of set of tires and trying them out? I understand we are looking for something in the 75-85% section width, but I think there is more too it than that. Running the same size tire Scott has on the Taco seems like it would be silly since his truck outweighs mine by easily 1,000 lb.

Anyone want to make a 255/80 r16?

Most folks are betting on an intelligent discussion, weighing costs and benefits, making a decision, carefully evaluating results and also assuming all responsibility for the decisions.

My truck came with some sort of tiny Dunflop tires that I wanted to replace. I also wanted to stick with the stock rims, get better off-road traction, avoid fit issues and wheelwell clearance problems, gain a little more axle clearance, not increase noise too much and not wreck gas mileage.

The white paper referenced earlier was a huge help in the decision process and I never once considered bicycle tires. Makes me think of old Model T tires on wire spoke rims!

Anyone who wants to make a good tire choice has to put enough effort into the research and not swallow whole some nebulous advice about tall and skinny.

I ended up with the Bridgestones Revo tires referenced in my sig file below.

 

[bigreen505]

Most folks are betting on an intelligent discussion, weighing costs and benefits, making a decision, carefully evaluating results and also assuming all responsibility for the decisions.

<snip>

Anyone who wants to make a good tire choice has to put enough effort into the research and not swallow whole some nebulous advice about tall and skinny.

I think you took my post the wrong way. Tall and skinny tires in the 75-85% aspect ratio range can represent a huge range of tires for a given vehicle. On the Trooper for instance, I can run a 235/85 all the way to a 285/75 (completely discounting stock size or a tire that would require a re-gear). One is taller, one is skinnier, how do I tell which is best? Another way to say this is we are trying to run the tallest reasonable tires that fit our trucks, but still staying within a certain tread width relative to our vehicle weight to achieve or maintain a certain contact pressure. How do we do this?

Your suggestion, which makes perfect sense, is to read a lot and buy a lot of tires. Since I don't know of anyone with similar driving habits (daily travel, off-road travel in my area) to me also driving a Trooper, and I don't have the funds to buy a set of tires, hope I like them and buy something else if I don't, I'm left asking questions and hoping people can provide good answers. Since you have done the research, I'm looking for a tire that is about 32.25 - 32.5" diameter with roughly a 7.5" tread width. I don't honestly understand how section width factors into the equation. Not trying to be a jerk here (and honestly not trying to thread hijack either), just trying to grasp the factors involved and my only baseline are two sets of tires on two separate vehicles that I have deemed to have too little contact pressure for my average driving conditions. Then again, I also could be confusing "too little contact pressure" with "tires that fundamentally suck".

Is that any more or less clear?

EDIT: Wow, ask and ye shall receive. Found one, and it isn't even that expensive! 32.8" diameter, 7.5" tread width, AT tread pattern.

 

[Redline]

I love studying, shopping for, and buying tires... the only negative is the cost. I'm also a long devotee of a 'relatively' narrow tire. Have had 285/75 on a '05 Jeep TJ and '96 F350. I and preferred 255/85 on both of those rigs and tossed the 285s in favor of 255s. Except for some recent experimentation I have been using 255/85 on my old F350 since about 1998.

The tread face on my 4Runner's stock 265/70R16 Michelin Cross terrain SUV tires is 7-inches. The tread width on the Nos/Original spare (F350) Firestone Steeltex A/T 235/85R16 is 6 7/8-inches (both not on a vehicle at time of measurement). Mathematically the 235 should be narrower, but with these examples it is just barely so. There are so many variables. One could argue that going to a 235/85 would be the same width but taller with my above example.

I would like to run 235/85s on my 4Runner, at least on the street, but don't think I will like the typical/standard E-range sidewalls for mall-crawling. The 265/75s I have, Toyo A/Ts & Toyo M/Ts, do seem to be a very reasonable size for this vehicle. They may even be the perfect size, a little wider, a little taller, but I have too few miles both on dirt and pavement (only 5K) to know.

 

[hoser]

The tread face on my 4Runner's stock 265/70R16 Michelin Cross terrain SUV tires is 7-inches. The tread width on the Nos/Original spare (F350) Firestone Steeltex A/T 235/85R16 is 6 7/8-inches (both not on a vehicle at time of measurement). Mathematically the 235 should be narrower, but with these examples it is just barely so.

The 265 and 235 part of the size designation is actually the "Section Width" in millimeters and not the tread width you are measuring. 265mm / 25.4 = 10.43"

Also note changing rim width will also effect the tire's section width measurement.

 

[teotwaki]

I think you took my post the wrong way. Tall and skinny tires in the 75-85% aspect ratio range can represent a huge range of tires for a given vehicle. On the Trooper for instance, I can run a 235/85 all the way to a 285/75 (completely discounting stock size or a tire that would require a re-gear). One is taller, one is skinnier, how do I tell which is best? Another way to say this is we are trying to run the tallest reasonable tires that fit our trucks, but still staying within a certain tread width relative to our vehicle weight to achieve or maintain a certain contact pressure. How do we do this?

Your suggestion, which makes perfect sense, is to read a lot and buy a lot of tires. Since I don't know of anyone with similar driving habits (daily travel, off-road travel in my area) to me also driving a Trooper, and I don't have the funds to buy a set of tires, hope I like them and buy something else if I don't, I'm left asking questions and hoping people can provide good answers. Since you have done the research, I'm looking for a tire that is about 32.25 - 32.5" diameter with roughly a 7.5" tread width. I don't honestly understand how section width factors into the equation. Not trying to be a jerk here (and honestly not trying to thread hijack either), just trying to grasp the factors involved and my only baseline are two sets of tires on two separate vehicles that I have deemed to have too little contact pressure for my average driving conditions. Then again, I also could be confusing "too little contact pressure" with "tires that fundamentally suck".

Is that any more or less clear?

EDIT: Wow, ask and ye shall receive. Found one, and it isn't even that expensive! 32.8" diameter, 7.5" tread width, AT tread pattern.

Hey, don't worry! People here have no problem disagreeing and still being friends as we has so much in common.

My only nit to pick is I did not mean to imply "buy a lot of tires". I was saying "do a LOT of research" and then commit to one set. So keep asking questions until you are comfortable spending the bucks on the rubber.

 

[Redline]

Yep, I'm well aware of how tires are measured; i.e. section width and aspect-ratios, etc. But conventional wisdom, and reality in many cases, is that a tire with a wider section width is wider overall, and often has a wider tread face as well. Recently I had a set of Dean SXT (Cooper subsidiary) 255/85R16 with tread that was just barely wider than the above 235/85 tire. But I have also had sets of 255s that were quite wide.

I was attempting to make the point that a 235/85, or any tire for that matter, may not be as narrow as it sounds when you compare to other tires. My two sets of Toyo 265/75R16 tires both have a tread width about 8.25-inches wide. The Toyota's stock "265mm" wide tires have a tread width of only 7-inches. A 1.25-inch wider tread width for the "same" section width tire is a big difference. Yes, the overall tire is wider too, helping to increase the tread width, but the simple designation would make most think they are the same width. Because of variances in design, a 235/85R16 light-truck tire might be as wide as the stock 265-section width tire that came on a rig. In that case a 235 is not so much narrow, just taller than the stock tire. As an aside, the stock skinny 265 SUV all-season type tires were excellent on-road on the 4Runner. Smooth, quiet, rode well and handled light snow and ice very well.

The 265 and 235 part of the size designation is actually the "Section Width" in millimeters and not the tread width you are measuring. 265mm / 25.4 = 10.43"

Also note changing rim width will also effect the tire's section width measurement.

 

[hoser]

Sorry, it was late when I responded and I was thrown off when you said "Mathematically the 235 should be narrower." I've had the same experience as you. I went from a 285 to a 295 and the tread width increased by almost an inch. I can understand why they list section width rather than tread width. Fortunately, manufacturers specs are easy to find.

 

[bigreen505]

The tread face on my 4Runner's stock 265/70R16 Michelin Cross terrain SUV tires is 7-inches. The tread width on the Nos/Original spare (F350) Firestone Steeltex A/T 235/85R16 is 6 7/8-inches (both not on a vehicle at time of measurement). Mathematically the 235 should be narrower, but with these examples it is just barely so. There are so many variables. One could argue that going to a 235/85 form a stock 265/70 is just the same width but taller with my above example.

This is exactly what I'm talking about. We all know what size a tire is supposed to be, but in many cases reality can be quite different. My 285/75s have a tread width of 9.3 inches, about par for the course, but a diameter of 32.25" mounted on 7" rim, so probably 32" unmounted. I have found 235 tires claiming a 6.5" - 7.5" tread width and 255 tires running a tread width of 7" - 8.5".

 

[paulj]

Tread width varies with tire model, section (maximum) width, profile and rim diameter. Generally an AT or MT tire with a square shoulder will have a wider tread than a highway-allseason. There is also some variation across brands. For example I have found Coopers to be relatively wide in the tread, especially compared to comparable Goodyears.

Given a model, profile, and rim, treadwidth tends to increase with section width. Other things constant, treadwidth decreases with profile, since there is greater room for taper from the widest point to the tread.

In sizes that I am most familiar with, a 225/75/16 is likely to be narrower in the tread than 225/70/16, and 235/70/16, even though it is a bit taller than the 235.

The spec tables of some models include treadwidth, though this is not the norm. Some spec tables also have a footnote about how the section width varies with rim width, (e.g. 0.2" for 0.5" change in rim width). Tread width shouldn't vary with rim width. The section width listed in the table is for a tire mounted on the recommended rim. The nominal width (e.g. 265) is a rounded version of the measured section width.

paulj

 

[Scott Brady]

Tires and physics... One of my favorite topics!


Here are a few general concepts:

1. Coefficient of friction (Ff = Cf x Fv): This is where the whole logic starts. As a tire widens, the surface area increases, as it becomes more narrow, the vertical force increases. So on a piece of glass (perfectly smooth), a 33x12.5and a 33x9.5 have exactly the same traction.

2. Wide Tires for the street: Wider tires work great on the street because the road surface is highly tactile, which supports adhesion. Some racing tires have twice the adhesion rate of a passenger car (and also only last a few hundred miles). Cooling: Driving a car fast increases heat past the point that the rubber can maintain bonding to itself and the tire will quickly disintegrate. A wider tire presents more surface to airflow and a lower vertical force on the road. Wider tires can also take a higher lateral force for a tighter slip angle.

So in summary, wide tires are great for the street (performance cars) due to adhesion, cooling and lateral control.

3. Wide Tires for the trail: There are specific situations when a wide tire is good on the trail, and that is only in situations when you require high flotation (i.e., the maximum surface area to dissipate the vertical load). A wide tire with maximum deflation allows for this. Think Arctic Trucks. The AEV Arctic Trucks with 40" tires can float on 12+' of snow in the right conditions and running 2 psi with beadlocks. Big tires like that also require big HP.

So why do we want a narrow tire for expedition travel?

1. Low HP to weight ratio: Our trucks are heavy and typically running low HP stock motors for reliability and fuel economy. On the highway the width of tire you select directly translates to rolling resistance and wind resistance.

Low HP also limits the amount of frontal resistance we can push in mud and sand. A tall tire, aired down has excellent flotation, but minimal frontal resistance because to majority of the flotation gained comes from the carcass becoming longer, not wider (80/20)

2. Ground clearance with minimal lift: A vehicles wheel well is designed for a specific section width at full compression. Figure out what ground clearance you need for the trails you run or obstacles you expect to encounter and buy a tire with the same section width as the tires specified for you vehicle (typically 235-270mm for nearly all of our trucks) and then adjust the aspect ratio to gain the clearance you need. This allows full stuff in the wheel well for maximum articulation and full compression.

Some practical examples:

1. Camel Trophy. Land Rover might know a thing or two about 4wds
2. Military vehicles: Military engineers don't care about looks, only what works. Military tires are nearly all in the 85-100% aspect ratio.
3. I work very closely with Michelin engineers, and they asked me this question. Why does the American market want wide tires and huge rims when they don't work in any off-road testing model we have tried? I think we all know the answer to that...

But don't get too hung up on all this physics crap. Just look for tires in the 75-85% aspect ratio and you will be a happy traveler with a good compromise on performance. I have never found a situation in all of my travels that a 265/75 or 255/85 doesnt work perfectly.

For me however, I will find a way to fit 9.0 R16 XZL's on my truck with Staun internal beadlocks Just for testing of course :safari-rig:

 

[Scott Brady]

Sorry, it was late when I responded and I was thrown off when you said "Mathematically the 235 should be narrower." I've had the same experience as you. I went from a 285 to a 295 and the tread width increased by almost an inch. I can understand why they list section width rather than tread width. Fortunately, manufacturers specs are easy to find.

Great point!

For example, the 285/75 and 295/75 BFG AT's are only .1" different in width, but the 295 is almost an inch taller.