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@nfarrar nfarrar/tires.rst
Last active Jan 29, 2016

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Tires

Because hey, it's a just a tire, right?

Types

There are no standards that dictate how a manufacturer labels their tire - and there are very wide bounds on these categories. An A/T from one company may technically be closer to an M/T, etc. Additionally, a major misconception is that because a tire handles better in offroad snow, that it will also handle better in on-road snow/ice/slurry conditions.

Type Characteristics Performance (Postive) Performance (Moderate) Performance (Negative)
Racing
  • extremely high surface contact
  • extremely low aspect ratio
  • extremely low flexibility
  • extremely high stability
  • pavement (dry)
 
  • all others
Passenger
  • high surface contact
  • moderate aspect ratio
  • moderate flexbility
  • moderate stability
 
  • pavement
 
All Terrain (A/T)
  • moderate to low surface contact
  • moderate to extremely high aspect ratio
  • moderate to extremely high flexibility
  • moderate to extremely low stability
  • trails
  • pavement
  • rain
  • ice
  • snow
  • mud
  • rocks
  • sand
 
Mud Terrain (M/T)
  • low to extremely low surface contact
  • moderate to extremely high aspect ratio
  • moderate to extremely high flexibility
  • moderate to extremely low stability
  • trails
  • mud
  • snow
  • sand
  • rocks
  • pavement
  • rain
  • ice
Extreme Terrain (E/T)
  • extremely low surface contact
  • trails
  • rocks
  • sand
  • mud
  • snow
  • pavement
  • rain
  • ice

Characteristics

Various tire characteristics and components that affect performance.

   
Aspect Ratio
  • the ratio of tire height vs. width
  • the greater the ratio, the more flexibility
  • the smaller the ratio, the greater the stability
  • tires with greater aspect ratios are generally desirable for off-road driving performance
  • tires with reduced aspect ratios are generally desirable for on-road driving performance
Sidewall
  • the vertical tire surface between the rim & horizontal tire surface
  • the more sidewall, the greater the flexibility, and less stability
  • some offroad tires extend tread onto the sidewall area
  • the sidewall is typically one of the weakest points in a tire (especially at low pressure)
  • beadlocker wheels are used to increase sidewall strength when running taller tires at lower PSI
Flexibility
  • the ability of a tire to conform to non-standard surfaces (i.e. rocks and obstacles)
  • greater flexibility generally equates to improved performance for offroad conditions
  • less flexibility generally equates to improved performance for on-road conditions
  • running tires at lower pressure improves flexibility
Stability
  • the ability of a tire to retain it's shape
  • generally desirable for on-road driving
  • improved performance while turning (especially at higher speeds)
Siping
  • machine slits, cut into the tires 'contact surface'
  • on offroad tires, these are present on the tread blocks
  • improves traction (specifically in water & snow)
  • improves heat disipitation, ride quality, breaking performance, and tire life
  • does not reduce tire strength
Tread
  • generally used to describe several more 'discrete' tire characteristics (i.e. aggressive tread)
  • for on-road conditions, tread reduces performance (reduced surface contact area)
Tread Blocks
  • the surface contact area on 'treaded' tires
  • these come in various shapes, sizes and patterns
  • the more surface contact the tread blocks have, the better they perform for on-road driving have
Void Spacing
  • spacing between the tread blocks
  • more spacing generally results in reduced on-road performance (reduced surface contact & siping)
  • reduces breaking performance & traction with pavement
  • generally results in increased noise when driving on-road
  • more spacing equates to better traction in 'slurry' conditions (water/mud/snow/sand)
  • the spacing design impacts how well the tires 'self-clean' (eject slurry from voids)
  • tires with poor 'self-cleaning' loose traction as voids become filled with non-adhesive residue
Tread Pattern
  • affects ride quality, tire wear, and on-road noise
  • affects how different types of residue are dispersed
  • an asymetric tire is designed to be used in a specific side-facing orientation
  • a directional tread pattern is designed to be used in a specific forward-facing orientation

Labeling

Tires are typically labeled with either a standard metric or imperial (high flotation) scheme. A standard (offroad) tire example labeled using both systems:

PB1556RE  (Metric)
│││   │└─────── Carrying Capacity
│││   └──────── Construction
││└──────────── Tire Width
│└───────────── Traction Grade
└────────────── Tire Type

35x1250R17LT/E (High Flotation)
│  │   ││ |  └─ Carrying Capacity
│  │   ││ └──── Tire Type
│  │   │└────── Wheel Size
│  │   └─────── Tire Construction
│  └─────────── Wheel Width
└────────────── Wheel Diameter
Value Notes
Tire Type
  • the type of tire (P = Passenger, LT = Light Truck)
Construction
  • the type of tire construction (R = radial)
Tire Size
  • diameter of tire
  • high flotation sizes are measured in inches from top to bottom of tire
  • on metric tires this is the sidewall height (in mm)
Tire Width
  • measured from sidewall to sidewall (either in milimeters or inches)
Wheel Size
  • the diameter of the internal rim spacing, measured in inches (not present on metric labels)
Traction Grade
  • performance rating for wet pavement conditions (rated from AA (best) - C (worst))
Carrying Capacity
  • carrying capacity in 'plies' (C = 6-ply, D=8-ply, E=10-ply)

Tire Size

  • Tires must be fit (mounted) to a wheel.
  • The tire size needs to be compatible with the wheel size.
  • The manufacturer typically defines the wheel-compatibility specifications.
  • Tires are built to fit on a wheel with a specific diameter (i.e. a 17" wheel)
  • Tires are built to fit on a wheel with a specific MWR (Measured Rim Width), but are compatible within a range
  • A more narrow wheel reduces surface contact with the tread towards the edges of tire
  • A wider wheel reduces surface contact with the tread in the center of the tire
Measurement Notes
Diameter
  • the distance from bead set to bead set across the diameter of the wheel
Width
  • the distance between inside of flanges
Offset
  • the distance between the wheel mounting surface (bolts to the drum) and centerline of the rim
  • this directly affects the wheel's backspacing
Backspacing
  • the distance between the wheel mounting surface vehicle-facing rim flange
  • more backspacing positions the wheel tighter into the wheel well
  • less backspacing positions the wheel further out of the wheel well
  • increasing the backspacing can cause the wheel or tire to rub on the sidewall
  • decreasing the backspacing can cause the tire to rub in the well or on fenders while flexing

There are a ton of factors that contribute to exactly how the wheel & tire will fit on the vehicle. The more extreme the setup, the more likely you'll run into sizing and rubbing issues. One of the safest (most commonly used offroad setups) use:

  • 17" x 8.5" wheel with 4.5" backspacing
  • 35" x 12.5" tire
  • 3.5" suspension lift

TPMS

Newer vehicles have integrated tire pressure montoring systems. To make this work, TPMS sensors are mounted in each tire that provide pressure monitoring data to the control system.

Tires typically don't come with TPMS sensors - however they can be swapped from existing tires into the new tires (or new manufacturer-specific TPMS sensors can be purchased and installed).

Tire Comparison

My Jeep is also my daily-driver (though I don't drive much). It's new and we're currently doing some light to medium trails (summer/mud/snow). Tires are going on a 3.5" MetaCloak Gamer Changer lift - so I'm looking specifically at 34-36" tires for this purpose. In colorado we end up doing some highway driving in the snow - making M/T's a poor choice - so I'm specifically looking at A/T's that'll do well offroad, without being horrible on the highway in the snow.

Tire Model Type Weight Max PSI Price Notes
BFG All-Terrain T/A KO 35x12.50R17 A/T     $280  
Cooper Discoverer A/T3 LT315/70R17 A/T     $280  
Goodyear Wrangler Duratrac LT315/70R17 A/T     $310  
Mickey Thompson Baja ATZ P3 LT315/70R17 A/T   50 $330  
Nitto Exo Grappler AWT 35X12.50R17 A/T   65 $300  
Nitto Terra Grappler G2 35X12.50R17 A/T   65 $300
  • max 34" on 17" wheels
Pitbull PBX A/T Hardcore 35x1250R17LT/E A/T     $345
  • on-road noise-reduction
Toyo Open Country A/T II 35x12.50R17LT A/T     $290  

References

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