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|14th October 2010, 20:13||#1|
Join Date: Dec 2005
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Off-roading Tires, Wide Or Narrow,which Are Better?
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:
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.
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.
|The following BHPian Thanks mohan for this useful post:|
|14th October 2010, 21:04||#2|
Join Date: Jan 2005
Thanked: 14,313 Times
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.
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