[mohan]

Wide or narrow,which tire ?
When I first got my Gurkha and looked at the tires,they looked odd,unusual as I was only street driving or at the most commute to my farm which still had good roads.
The stock tires looked tough but they weren't "cool".

So off I went to the tire shop and selected macho,"cool" alloy rims and beefy 270/70 Yokohama AT .The Gurkha looked fabulous and was an instant head turner.




People used to stop me and ask about the vehicle and I was congratulating myself.
Then came the proving grounds for my vehicle and gradually I woke up to realize my folly.It costed me several thousands,a broken ego,disillusionment and then the usual "blame-game".

There's one thing which I had done, a slim solace ,that I had still kept my stock tires.

I fitted them again and did more OTR's with them,shifted back to the wider Yoko's and then finally have shifted back again to the narrow stock tires.



The crawl on the narrow tires ,the handling off road ,the nimbleness was very evident doing the same trails.
I have realized that my machine does much better with narrow tires.
I decided to read about tires and personal anecdotes from professionals who actually did extreme off roading.


I have found very valuable advise in a book titled Sahara Overland, a Route and Planning Guide by Chris Scott (2004, ISBN: 1-873756-76-3):

After having failed to procure the right sized after-market tire, I found my Gurkha performing dismally. That's when I thought I should be reading about tires and trying to understand their functioning. Be it as it may,I am fully aware that it's experience that will teach me about tyre mechanics not mere reading.

Significant research has been done on racing tires and daily newer data is being added ,however no significant research has been undertaken to evaluate the behaviour of tires in off roading scenarios.
There can be no such thing as laws or a dictum regarding tire performnace off road,people would be found to be arguing about wider or narrower tyres.

Before we can discuss about the physics of off-road tyre performance we must know the following parametres which affect tire performance off road:

Definitions:

Friction: (Mech.) The resistance which a body meets with from the surface on which it moves. It may be resistance to sliding motion, or to rolling motion. [1913 Webster]

Adhesion: Intermolecular forces that hold matter together, especially touching surfaces of neighboring media such as a liquid in contact with a solid.

Momentary Bonding: The brief molecular connection of two surface elements under heat, pressure or mixing.

Deformation: The change in geometric size, shape, form or position due to force.

Mechanical Keying: The interlocking of surfaces.

A tarmac has a good tactile surface and hence offers good traction due to excellent adhesion and momentary bonding.In this scneario a wider tire would perform superbly.

On the other hand a OTR trail is made of ledges,rocks,ruts,sand and mud which dosent offer a good tactile surface ,hence adhesion and momentary bonding is poor.However on such a surface traction is achieved by Deformation and mechanical keying.

A wide tire distributes the vehicles weight over too large of a surface, preventing deformation from occurring at the same rate as a narrow tire with the same pressure (force).

A narrow tire will hold better than a wide one by keying to the surface aggregate due to the greater vertical force.

A narrow tire also presents less rotating resistance on a soft surface, like shallow mud, snow and sand.

Additional performance is gained by the assumption that most vehicles can fit a taller tire if it is narrower, which provides greater axle clearance.

Final arguments are made for the benefits related to reduced rotating mass and unsprung weight.

• Rolling Resistance: A narrow tire presents less rolling resistance on improved surfaces, increasing fuel economy and performance.

• Frontal Resistance: This is another key benefit of using a narrow tire. When driving through mud, snow and sand a narrow tire presents less surface area to the medium. A narrow tire will cut easier through mud, snow and sand than a wide one (due to resistance). The best example of this is when turning in sand. When the front tires turn, they present a wider surface to the sand. You can feel speed reduce immediately when a turn is initiated because of the resistance.

• Rotating Mass: A narrower tire weighs less than a wider one of the same height. The difference in weight between a 33x10.5 and 33x12.5 is about 8 pounds, coupled with the narrower, lighter wheel, the effect on rotating mass is significant. A lighter tire and wheel is easier to accelerate and stop.

• Size Fitment: All things being equal, a narrower tire is almost always easier to fit with less fender trimming and total suspension lift that a wider tire of the same height. Wider tires affect turning circle, compression travel (which needs to be limited by lowering the bumpstops, etc.).

• Weight: Without making this an article about suspension, one of the jobs of a properly engineered suspension is to control the cycling of unsprung weight, which is comprised of the axles (control arms, knuckles, etc. in an IFS), tires and wheels. The lighter those assembly's are, the easier it is for the suspension to control it, improving performance.

• Airing Down: This is another critical concept highlighting the advantage of a narrower tire. Airing down a taller tire will not reduce the GC significantly

As quoted from Sahara Overland, a Route and Planning Guide by Chris Scott (2004, ISBN: 1-873756-76-3):
"...Note that it's the diameter or height of the tyres that makes the difference in sand, and not, as many imagine the width... For the desert, you want tyres with a high aspect ratio of around 80 because this represents a taller sidewall so corresponds to added ground clearance when firm, and a longer contact area when deflated"

Traction in soft surfaces: It is a common misconception that airing down a tire for off-road traction only makes the tire contact patch wider. That is not the case. In fact, only 20% of the increased contact comes from the width. 80% of the increased contact patch comes from the tread patch becoming longer. A tall, narrow tire allows for a very long contact patch when aired down. That, coupled with the minimal frontal resistance (area), negates much of the downside to narrow tires in flotation situations. The taller tire allows for a long contact patch and still maintains good ground clearance.

Traction on rocky trails: Another common misconception is that when airing down it is the increased amount of tire on the rock (more contact patch), that allows better traction. It is not the contact patch that creates better traction, but the tires ability to conform to the surface irregularities (deformation and mechanical keying). When an aired down tire comes in contact with a rock on the trail, the tires tread collapses under the vertical and horizontal forces, causing the tire to wrap the rock, as opposed to sitting on top of it. The wrapping effect provides greater shear resistance, and in turn better traction. (Technically: the shear load is distributed over multiple planes, not just a horizontal one).

Tire spring rate: One of the great benefits of airing down a tire is improved smoothness. Less pressure allows the carcass to flex. A taller tire has greater sidewall compression, and in turn a better ride. (expressed as compressive strength=N/mm).

Real world example: When climbing a ledge with a jagged surface, the narrower tire will wrap the protrusions with more contact due to the increased deformation depth. The wider tire will rest on the surface of the protrusions and will have a greater chance of spinning (shearing).




Recently I have put 235/85 R 16 Cooper ST-T tires on my Gurkha.
These tires are perfect for my car,as they are taller and narrower.
They can be aired down without any decrease in the GC.











SOURCE/REFERENCES;

1-Scott Brady-Expeditions West 2005.
2-Sahara Overland, a Route and Planning Guide by Chris Scott (2004).
3-Mechanics of Pneumatic Tires, U.S. Department of Transportation.
4-The effect of inflation pressure on bias, bias-belted and radial tire performance (SAE) by B. L Collier.

[Samurai]

Excellent Article Mohan. Most of us stick to 235mm tyres on the Jeeps for it gives the most optimal performance on and off the road. Good to know the exact theory behind it.

[Ujjwal Bakshi]

Brilliant analysis. Thanks for sharing. Gives a great technical explanation to a lot of my questions regarding upgrading to wider tires on my crossover. I think I will stick to the stock tires.

Clearly, this is the reason why the Willys designed during the second world war for the European grasslands and slush was a timeless success with its slim tall tires.

And your analysis is a timeless piece as well. Thank you

[venkyhere]

Snow / slush :

There is a hard surface underneath that can provide reaction force to the weight of vehicle. Need is to displace the loose top layer and make the rubber meet the hard surface underneath, AND pack the loose top layer into lumps that can wedge between the curvature of the tyre and the loose layers either side of the main contact patch, thereby improving friction.
This means the need is to bring as much curvature of the tyre in contact with the surface. Which means long and thin contact patch, so that the center of the contact patch "sinks" as much as possible and the portions of the contact patch ahead and behind this sunken center, are forming superb "wedges" with the loose top layer - contact patch needs to be squarish rather than rectangular.


Sand :

Exactly opposite to above. There is no hard surface underneath. The grains of sand are having narrow contact miniature patches with other grains of sand below (like the ballast put on railway tracks). So a 50sq.cm contact of rubber might translate to some 30sq.cm of grain to grain contact area. So more rubber needs to be present , touching the sand, so that as much grains of sand are present to bear the weight. This means lower air pressure. But more importantly, there is no scope for curvature-packing like there was, in the snow case, because there isn't a hard surface anywhere. That means the loading has to be perfectly vertical as much as possible. This in turn means less curvature of wheel needs to be involved. Which in turn means wide tyres. That's why sand needs wide tyres with lower pressure - big contact patch, and the need to make it as rectangular as possible (instead of squarish)


Tarmac :

Contact patch shape doesn't matter in a straight line for constant speed travel. But for cornering, as much rectangulary patch as possible preferred - the wider (and softer) the contact patch, the more "twistable" it is, without losing contact with tarmac - wide tyres for better grip and lesser understeer.


Ice :

Completely different ballgame to anything above. The pressure of contact has to puncture the surface, "create" 10s of mini wedge formations between rubber and hard ice , because the surface is having much less corrugation (hills and valleys needed for friction) than even tarmac. So studs/chains, which melt the ice momentarily by increasing pressure massively (by reducing area by keeping the rubber "away" initially) and once a tiny swimming pool is established , the rubber can then go and wedge itself into the pool by displacing the water. 10s/100s of such small pools and subsequent rubber wedged inside ice "gutters" -- hills and valleys of friction.

[mygodbole]

Good grief, all that science proves I remain a Neanderthal in disguise as a 21st century Indian citizen.

99% of all off-roaders are weekend warriors. They need a home-office-home ride that can go to beach/mountain/farm/someplace-where-no-'road'-exists. The other 1% looks at the terrain and the challenges and configures the vehicle to suit the terrain and their requirement.

Different strokes for different folks.

While highly informative, the analysis remains just that: an analysis.

Narrow/wide, tall/short, under-inflated or over-inflated, deep groove or a bald tyre depends on the terrain. Every terrain needs an appropriate tyre. As venkyhere states, a different ball game in every variation. One size DOES NOT FIT ALL.

Weekend warriors need something that is suitable for all terrains. So, they will clobber the engine to ram their way through sand, float over snow and trample the flora on their way up the hill. But they will prevail, to Pluto with the wear and tear on human and machine.

Thank you for the information, but I afford only one set of tyres and they will have to carry me everywhere or perish in the process.

A neanderthal grunting all the way the destination.

[trippytragopan]

Originally Posted by venkyhere

Snow / slush :
This means the need is to bring as much curvature of the tyre in contact with the surface. Which means long and thin contact patch, so that the center of the contact patch "sinks" as much as possible and the portions of the contact patch ahead and behind this sunken center, are forming superb "wedges" with the loose top layer - contact patch needs to be squarish rather than rectangular.

A friend suggested me to look for the tires with silica added to tread. According to him they tend to perform better in snow.

[ruzbehxyz]

Originally Posted by mohan

Wide or narrow,which tire ?

A debatable topic on all fronts.
Tyre choosing on our 4WD is a very personal thing. Just go into any online forum, group, etc and ask “What is the best tyre for my 4WD ?” You will get 100 people offering 1,000 different opinions.

One thing that almost everyone agrees on is that you should get tyres with the highest profile (or tallest sidewall) that you can.

If we look at the various old Jeeps that were used in the wars, in the 50 and 60’s, the tyres on these were on tiny split rims so they had huge sidewalls and thin tread. We know that it worked in those days because they always got where they needed to go, on most occasions. Since then, 4WD tyres have gotten larger. Manufacturers have been using larger diameter rims to allow for better performance as well as allowing bigger and better brakes. Couple this with the desire for people to “stand out”, tyre widths have also gone wide. These days it’s nothing to see a 285 tyre as small, while people regularly run 305 or wider. While this can look really aggressive, in a lot of cases it just makes it harder for you car to roll and costing you more in fuel.

When you’re driving on a good sealed road, a wider tyre will normally offer a larger contact area, meaning greater grip. I say normally there because there’s a few times it does’t. The main times when it doesn’t is when it’s raining, or when your car isn’t heavy enough (yes, I know that’s not likely for some of our readers, but still…). In the rain, a wider tyre has a much harder time trying to push the water on the road out of the way, so there’s a greater risk of aquaplaning and losing control. If your car is too light, you won’t have enough down force to keep the tyres firmly planted on the ground and the slightest acceleration (or braking) will break traction.

When you’re offroad, it’s all about what terrain you’re in. If you’re driving a lot of mud bogs or salt pans a wider tyre can help with flotation as well as distributing the downward pressure over a larger area, which puts less force to the ground and can really help you “float” across brittle areas that other vehicles may crack through. If you’re driving on hard sand, the same goes. The wider footprint will give you less resistance. When you’re driving in soft sand or uneven busted up terrain, a more narrow tyre will be better. In soft sand, your tyres push the sand forwards before it gets pushed out, so the wider your tyres are the more work you have to do to push the sand out. This really does make it harder for your car.

This video from Mr. Robert Pepper explains some good stuff about offroad tyres.

https://youtu.be/spOUcaUzIHY

Source: https://l2sfbc.com/is-a-narrow-or-wi...est-for-a-4x4/

About Robert Pepper: Robert Pepper is a motoring journalist, author, photographer and driver trainer, specialising in offroad vehicles, towing, camping, tech and sportscars.

[venkyhere]

Originally Posted by ruzbehxyz

When you’re driving on a good sealed road, a wider tyre will normally offer a larger contact area, meaning greater grip.

Small correction, if I may :

Wide or narrow tyre, contact area remains same, if air pressure is same.
Let's say the weight pressing down on the tyre at one corner of the car is called w, then we have the air pressure that we have filled in the tyre, p :
If follows, that the contact patch area, a = w/p
(this equation has nothing to do with the tyre section, sidewall ratio or wheel size)


Thus, contact patch area doesn't change whether it's a 225/50/17 or a 195/65/16. What changes is the shape of the contact patch - in case of the wider tyre, it's more 'rectangulary' than in the case of the narrower tyre.
This aspect ratio change, doesn't make any difference when moving at constant speed in a straight line. However, for acceleration, braking and cornering, the wider tyre of the two offers better grip, purely because of the 'more rectangulary' nature. And if we want to dig deeper as to 'why', it's very complex.

[shankar.balan]

Speaking as a real-life, daily-user-owner of 2 Proper 4WD vehicles.

The OEM stock F78-15 Nylon Cross Ply tyres are perfect for my Gypsy. These are what the Army also uses and their Gypsys go just about anywhere and everywhere.
There is no real functional reason to change.

In my new Thar also the stock Ceat Czar AT 255’s are perfectly adequate for all that the Thar is asked to do under most circumstances.

I see no reason to change.

We should give the design engineers of these vehicles their due credit.