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Tire 101 - Anatomy Of Traction

Posted in How To on June 1, 2013
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As the saying goes, a jack of all trades is a master of none. Generally speaking, tools designed to complete two very different jobs do neither job very well. Fortunately for us, Jeeps do not fall into that category. Jeep is one of the original American “do-it-all” tools. Originally marketed to the civilian market as a tractor that could plow a field, dig a hole, or be driven to town, Jeeps are one of the few tools that can do many jobs well. In modern times that means slow, heavy rock trail work and high-speed highway commuting. Okay, there are compromises. Most really comfortable Jeeps are not the best rockcrawlers, and most dedicated rockcrawlers can’t buzz down the highway with the flow of traffic. Much like the Jeep, the tire does many different jobs. And with modern compounds and designs, tires generally do a pretty good job no matter what gets thrown at them—but there are compromises. From street-legal drag tires to bias-ply mud tires with huge tread blocks and nearly bulletproof sidewalls, tires do it all. Somewhere in the middle is the real jack-of-all-trades: the all-terrain, all weather tire. What makes one tire different from the next? Differences in construction, tread design, and rubber compound determine where a tire will work or fail. Follow along as we show you a little bit about tire anatomy and delve into different tire construction, talk about different parts of the tire, and wrap up with a discussion on what all that writing on your tires means.

Tire Construction
Bias-ply: Bias-ply tires have textile based rubber-impregnated bands that run at an angle (30-45 degrees) up the sidewalls and under the tread from bead bundle to bead bundle. Bias-ply tires are known to flex better and ride smoother. They are also known to be stronger and more puncture-resistant especially in the sidewall because of the plies running at an angle or bias and because the rubber on the sidewalls is of the same compound as the tread. Bias-plies are also known for better self-clearing off-road. Bias-ply tires also tend to wander and follow ridges on-road, but are easier to repair when damaged than radials.

Radial: Radial tires are made with steel or textile-based cord plies that run at right angles to the direction of travel (with the most layers under the tire’s treads). These bands of steel or textile plies give radials a firmer feel and allow for more responsive turning. This also causes radials to ride a bit rougher. Sidewalls of radials are made with less material to try to compensate for their stiffer nature. Radials tend to last longer and have lower rolling resistance and offer better fuel economy.

Tire Tread Anatomy
Void ratio: The ratio of tread to voids (sipes, groves, dimples). Higher ratios indicate a tire with larger spaces between the tread for channeling water or clearing dirt and mud from the tread.

1. Sipes: Small cuts, slits, or grooves in a tire’s tread blocks, allowing the treads to move more easily. They also open up more biting edges within the tread to help with traction on ice, snow, and loose dirt.

Zig-zagged sipes: Zig-zagged slits that increase the effective length of a sipe, enhancing their effectiveness.

Interlocking sipes: These are S-shaped sipes that overlap. Common on ice and snow tires.

2. Void: This is the space between tread blocks.

3. Grooves: Voids that run circumferentially and are important in channeling water from the front of a tire to the back.

4. Dimples: Small round holes that allow heat dissipation for tire cooling and may serve as anchor points for studs in tires that are designed for ice studs.

5. Blocks: These are the parts of the tire that are in contact with the ground in the contact patch. Blocks are basically the tread between the voids, sipes, groves dimples—in other words, where the rubber meets the road.

6. Tread-cleaning bars: Bars at the base of the voids between tread usually near the shoulder of the tire. These break surface tension and help with ejection of mud, gravel, and dirt. They’re sometimes referred to as rock ejectors.

7. Tie bars: Bars that tie tread blocks together. These reduce flex of the tread blocks.

8. Ribs: Rings of tread that run circumferentially around the tire. These are rare in all-terrain tires, but help with steering and lateral (side-to-side) traction.

9. Tread wear bar: Small bars that indicate when a tire is worn out and ready for replacement.

10. Sidewall: The side of the tire between the shoulder and the bead bundle. These are generally made of multiple layers or plies in all-terrain and mud-terrain tires. The sidewall hopefully absorbs hits from curbs, rocks, and sticks without damage.

11. Sidewall tread: Tread blocks, sipes, dimples, and voids on the sidewall of a tire intended to provide traction. Good for climbing out of ruts. This also helps strengthen the sidewall to punctures.

12. Shoulder: Edge of the tire that transitions from the tread to the sidewall. The shoulder provides continuous contact with the road or trail (unless you are into jumping your Jeep). The shoulder and treads on the shoulder are important for steering, traction, and stopping.

13. Bead bundle: The part of the tire that seats on the wheel—made of rubber, textile, and a steel band that keeps the bead from stretching.

14. Bead protector: A lip or thick area of rubber just outside of the bead of the tire. This additional rubber helps protect the lip of the wheel from damage from curbs or trail debris.

15. Asymmetrical tread: Different tread profiles across the width of the tire (think Goodyear MT/R with Kevlar). These tires have an inside width of tread and an outside width of tread that differ in design, or differing left and right halves.

16. Directional tread: Designed to rotate in one direction. Think chevron treads of a tractor tire, Baja Claws, Boggers, Yokohama Geolanders and so on. These tires provide maximum traction when rotating forward and can dig excessively when reversing.

17. Symmetrical tread: Both sides, left and right halves of the tread are the same.

Reading a Sidewall
Have you ever looked at the side of a tire and seen something like LT295/70R17 123R and wondered what the hell all that means? If so, you are not alone. There is a ton of info about a tire on its sidewall, including diameter, wheel size, speed rating, weight rating, inflation information, DOT numbers, direction of rotation, how the tire should be mounted, and so on. Here is a little info on how to get the info out of all those numbers letters and maybe a symbol or two.

Metric tire sizing: LT295/70R17 123R The formula is (tire type (P for passenger, LT for light truck)) (section width)/(aspect ratio) (construction) (wheel size) (other numbers will be load rating followed by speed rating)

Standard sizing: 35x12.50R15LT 113Q The formula is (tire diameter) x (tire width)(construction)(wheel size) (tire type (P for passenger, LT for light truck)) (other numbers will be load rating followed by speed rating)

DOT Number: There is a lot of info in a DOT number, including a manufacturer’s code, batch number, and when the tire was made. We focus on the last two numbers, which indicates the year the tire was made. As tires age they get more brittle, and some tire shops won’t mount older tires on your wheels for legal reasons.

M+S: Indicates a mud and snow designation. This means it has a higher void ratio, or larger gaps between tread for better traction in looser materials, and can be used in all-seasons, such as an all-weather tire.

Snowflake: Indicates a minimum performance requirement in a snow tire test. That’s good if you plan on encountering snow with your Jeep.

Ply rating: This indicates the number of plies used in the construction of the tire usually this information is self explanatory and says the number of plies on the sidewall and tread. Some tires use a letter designation: B is 4 plies, C is 6 plies, D is 8 plies.

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