My wife prefers narrow (Pizza Cutter) tires (9.50 or 10.50) on her Jeep and she usually gets better traction than I have on my Jeep using wide tires. (12.50). She has better luck in both deep sand and on loose dirt. The article below seems to agree that for most off-road situations we often encounter, narrow tires are better. That is even though they don’t look as “Macho”. – Roger
Tire Selection for Expedition Travel: The impact of tire width on traction
Article Type: Position Paper, White Paper
By: Scott Brady, Expedition’s West
Position: A tall, narrow tire is a superior choice for all off-highway surface conditions with the exception of deep, soft sand or mud.
This is an empirical summary of my position. The supporting details of this argument are contained in the body of the document. The start of my study came as a result of asking the critical question. What is better for most trail surfaces; a wide or narrow tire?
This initially led me to research the physics of tire traction, which is mostly related to dynamic friction as the vehicles are not just parked on a surface trying to maintain a static state. There has been some research on the subject related to performance vehicles and race cars, operating on highly tactile surfaces, and running at high speeds (and subsequent heat generation), but there was no significant research done for off-highway vehicle operation. So I spent several months researching the subject, testing the logic and formulating a position. Why a position, and not a conclusion or “law” of tire performance? For me to proclaim a conclusion would require much greater testing volume, samples, variables etc., which time does not allow. So, the following is my position.
When driving off-highway, the performance of a vehicles tires is influenced by the elements of friction as related to the following items: Adhesion, momentary molecular bonding, deformation and mechanical keying (Haney, 2003). Where a race car operates on a highly tactile surface, which allows wide, even smooth tires to perform exceptionally (due to adhesion and bonding), an off-highway vehicle operates in the exact opposite environment, with minimally tactile surfaces. Rocks, dirt ledges, dusty boulders, highly irregular surfaces, sandy washes, etc. occupy the fourwheeler’s environment. Surfaces that do not provide high adhesion rates or momentary bonding. However, these surfaces are highly irregular, which does provide the opportunity to take advantage of mechanical keying and deformation. These traction elements require high contact pressure, coupled with low air pressure to get the tire to flex with the terrain. 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.
It is important for me to note that this document is NOT about reducing tire air pressure “airing down”. Airing down provides its own performance benefits which are not covered in detail in this paper. For all examples assume that the wide and narrow tire are both at 15psi for the trail.
While the coefficient of friction (Ff = Cf x Fv) is linear and not affected by width (on a perfectly smooth surface, traction is consistent despite width), it is the variability of the road surface conditions off-highway that improves traction for a narrow tire. The greater the contact pressure, the greater of effectiveness of the friction elements of Deformation and Mechanical Keying. 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.
The argument is further bolstered by the assumption that a taller tire can be fitted to the vehicle if it is narrower. For example, a Toyota Tacoma can fit a 33×10.5 with 2″ of lift, but not a 33×12.5 with the same wheel off-set, suspension, etc. A taller tire allows the driver to air down further (maintaining sufficient ground clearance), increasing tire deformation on technical terrain and increased flotation on surfaces with low shear resistance (like sand).
The argument assumes that the surfaces encountered off-highway are not typically highly tactile, like concrete or asphalt, but are irregular, and loose, with poor surface condition (either dusty, muddy, wet, etc.)
This article is specific to heavy expedition vehicles with nominal lift and stock engines (as is desirable for extended travel reliability). The article is NOT about 1,500 lb. sand rails with high HP motors, or other competition platforms with one dimensional surface conditions.
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
Reading a flotation tire: Example LT 31×10.5 R15 C
“LT”= Light Truck. Intended to be used on a heavier vehicle, with a higher GVWR than a typical passenger car.
“31”= The tire height, represented in inches.
“10.5”= The tire width at the section (widest point of the carcass), represented in inches.
“R15″= Indicates the tire is a radial and has a rim size of 15 inches.
“C”= The load rating of the tire. It can also be D, E, etc. The higher the letter, the greater the load carrying capacity.
Reading a metric tire: Example LT255/85 R16/D 119Q
“LT”= Light Truck Tire
“255”= The section width in millimeters
“85”= The aspect ratio. Expressed as a percentage of the section width (so 255*.85=216mm). The result is the height of the tires side wall. To determine the overall height of the tire, the following equation is used [(side wall height)*2+(rim height)=total tire height]. For the example tire the equation reads [(216*2)+406=838mm] Note: A 16” wheel is 406mm tall
“R”= Radial Construction
“16”= Rim diameter in inches
“D”= Load Range . Indicates ply rating (example tire is 8 ply), and load pressure, or the recommended tire pressure at maximum load (example tire is 65psi) Available Chart= http://www.tiresafety.com/size_class/size_nav4b.htm#plyrating
“119”= Load Index, which is the load carrying capacity of the tire (example tire is 2998 lbs.) A good load index chart is available here http://www.tiresafety.com/size_class/size_nav2.htm
“Q”= Speed Rating (example tire is 95mph). Available Chart= http://www.tiresafety.com/size_class/size_nav2a.htm
Important note: For the sake of the following details, assume that the test vehicle is 5,000 lbs., and a narrow tire would be considered a 33×10.5 R15, and a wide tire would be considered a 33×12.5 R15, both run at 15psi for trail use.
The benefits of a narrow tire:
The Argument: A tall, narrow tire is a better choice for all off-highway surface conditions with the exception of soft sand, snow and soft mud that’s depth exceeds 110% of the vehicles minimum ground clearance. Here is the explanation.
• Contact Pressure: Contact pressure is expressed as the vehicles curb weight distributed over the contact surface of four tires. The contract pressure is not equal to all four tire road surface contact points as the vehicles weight is not perfectly distributed. To ease the description, let’s assume that the test vehicle weights 5,000 lbs and has a perfect weight distribution. Each of the vehicles four tires would be creating 1,250 lbs. of vertical pressure on the terrain. Let’s assume for the sake of this example that the vehicles tires are 10” wide, where the load and tire pressure results in a total surface area of 30 sq. inches. The total pressure per square inch (without equating the secant) would equal 40 lbs.
• 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 33×10.5 and 33×12.5 is about 5 pounds, coupled with the narrower, lighter wheel, the affect 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.).
Practical Application: I find a good rule is to use as tall a tire as possible with the same section width as the widest tire available from the factory for that particular model. That way the tire will compress into the wheelwell designed for that width, without rubbing due to width. Example: I was able to fit my Tacoma with a 33.4”x10.5” tire with only a 1 ¾” lift. A 33×12.5” tire would require additional trimming, less wheel offset and lowered bumpstops.
• 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.
As quoted from Sahara Overland, a Route and Planning Guide by Chris Scott (2004, ISBN: 1-873756-76-3):
Negative Effects: Nothing in the world is perfect, so there are some downsides to using a narrow tire…
The Benefits of a Wide Tire:
The flotation tire provides three benefits. Greater high speed handling safety and improved lateral traction on constructed roads (concrete, asphalts, etc.), greater section width for support of the heavy vehicle on soft terrain, and appearance. The engineering concept behind the flotation tire can be found in its name. These tires were designed to provide flotation on loose surfaces like sand. Flotation only comes at the cost of contact pressure. Flotation is achieved by minimizing the surface pressure per square inch exhibited by the vehicle. These features are important for heavy, full size trucks and SUV’s, but not most trail vehicles.
This chart represents the recommended tire width, based on the vehicles weight (GVWR).
|Several Real World Examples: (just to show I am not the only one who believes in a narrow tire)
The Turtle Expedition who has literally traveled around the world used a 255/85 R16 (33.3 x 10) for many thousands of miles on their full size Ford. Land Rover uses narrow 7.0 R16 XCL tires in most of their Camel Trophy events. The Rain Forest Challenge and The Trophy challenge have all been won by the aggressive Simex Trekker tire (35×11.00). Tom Sheppard often uses the 7-7.5 R16 Michelin XZL and XCL for many of his expeditions. All very narrow tires in relationship to their height…
|A Brief History of Tire Design and Engineering:
The History of Flotation Tires: Tires in the past were always tall and narrow. From the 4” wide solid train wheels to the earliest automobile, tires were always tall, and very narrow. It was not until the late 1970’s that this trend began to shift, with the popularity of radial tires increasing, along with the size of American full-size trucks. These factors are what created a change from the older bias designs and sizes (Like 7.5 16, etc.). The trucks were bigger, heavier and traveled at faster road speeds, so the application of a wide tire became more appropriate.
Metric Tire History: Metric Tire engineering became standardized in 1964 with the inception of the European Tire and Rim Technical Organization (ETRTO). This consortium pursued design and fitment standards that optimized tire performance and safety in the European market. One of the first results was the creation of a standardized tire range and labeling method. The range first included the sizes of 235, 265, 285, etc. with aspect ratios of 75% of the tire width. The engineers determined that the ratio of 75% created the best compromise between handling safety on the road and low surface traction performance. As a result, metric tires for tight trucks are typically found in that aspect ratio (example= 265/75 R16).
• Durometer, Viscoelasticity (Hysteresis)- I am going to touch on this lightly in this article, but a softer durometer rubber compound will have a much higher adhesion and deformation rate than a hard tire. There are several tires available with softer compounds, like the BF Goodrich Moab edition and the Goodyear MT/R. An easy test of the viscoelasticity of tire rubber is to press your thumbnail into the tread. The longer it takes to recover, the softer the durometer and higher the hysteresis. However, the softer tire will also wear out much faster.
• Siping- Siping is a great way to improve a tires traction. Siping provides greater tread deformation to the road surface, which is why it is so effective on icy and wet roads. The more the tire can conform to that very slick surface the greater the traction.
• Tread Design- On a high traction surface like dry concrete, a totally smooth tire has the greatest traction as the adhesion rate is highest between the two surfaces. On the trail however, surfaces are varied, including wet rocks, mud, dirt, etc. That is why off-road tires are so specialized and require a compromise of other capabilities. As a general rule, and aggressive all terrain tire with soft tread compound and siping will provide the best overall performance. A Goodyear MT/R or BF Goodrich MT with siping cut after purchase will also perform well in varied terrain.
Tread Design- Selecting the tread appropriate to the terrain you are driving.
Deserts: An all terrain tread without large shoulder block to displace sand under acceleration. Use a tall tire without heavy lugs and slightly wider than for hard surfaces (use a tire in the 75% aspect ratio range). Avoid rims taller than 16″ as it is necessary to have sufficient section height to allow for a long contact patch (for flotation) when aired down. (recommended tires: BF Goodrich (BFG) All Terrain KO, Yokohama Geolander A/T+II’s)
Mountains: The mountains will present the most rugged terrain, requiring the greatest clearance, traction and damage resistance. Choose a tall, narrow tire (in the 80% aspect ratio range). One that is a load rating or more above you GVWR requirement to ensure a sufficiently strong sidewall. Rocks, loose climbs and ledges are present on many mountain tracks. Use either an aggressive all terrain or a mud terrain tire in these conditions, and favor brands with stronger sidewalls. (Recommended Tires: BFG Mud Terrain and all terrain, Goodyear MTR).
Jungles: Jungle tracks require a tall, narrow, and aggressive tire. Large voids between tread blocks allow the tire to clear mud from the tire and maintain traction. The large voids also provide lateral control to help limit sliding and oversteer. Use the tallest tire possible (without excessive suspension lift), to allow axle clearance in deep ruts. (Recommended tires: BFG Mud Terrain KM, Interco Super Swamper Radial, Simex Jungle Trekker)
Section Width- A narrow tire will provide superior performance to a wide one. Wide tires are not appropriate for use on an expedition vehicle, unless they are crossing the Gobi. Contact pressure and its affect on adhesion, deformation and mechanical keying are the critical components of traction on rugged terrain.
Section Height- As a general rule, expedition vehicle tires do not need to be very tall, unless the vehicles break over, approach and departure angles require it. Typically a tire taller than 33-34″ is not required. Do not select a tire that is so tall that it requires lowering the bump stops, or fitting the vehicle with a suspension lift of greater than 2-3 inches. Most well designed SUV’s intended for extended, rugged terrain travel have sufficient clearance from the factory, or can be fitted with a minimal lift (1-2 inches). Of course, the larger the vehicle, the larger the tires that are mounted. Unimog’s for example use nearly a 35″ tire from the factory.