Tire Tech

Bias vs. Radial Performance Theory

Bias ply cornering dynamics radial ply corner dynamics
These drawings illustrate the difference between the bias-ply and the radial tire in a corner. The bias-ply tire, left, loses contact with the road as both tread and sidewalls distort under a cornering load. The radial tire, right, flexes mostly in the sidewall area keeping the tread flat on the road. The result is better adhesion for the radial.

Bias Ply vs. Radial performance

 Bias ply tires differ from radials on the following items.

  • Static negative camber requirements are less, usually about 1 to 1 1/2 degree negative is sufficient.
  • Rim width selection is more critical,  because the tread face is flexible, the rim helps support the tire.  The rim width should be as large or larger than the section width of the tire.
  • Air pressure can not be used to reduce sidewall flex (rollover).  Excessive air pressure will cause the tread face to bulge, reducing the contact patch.
  • Bias ply tires give more warning (than radials) about traction limits and have excellent feedback of what the contact patch is doing.
  • Bias ply tires operate at larger slip angles than radials.  This larger slip angle is what makes bias tires feel sloppy on initial corner turnin.  What this means to the driver, you have to 'lead' the corner more to account for the slip angles. 
  • Because of how bias tires react to cornering loads, the tread can be thicker than radial tires.
 Radials differ from bias ply on the following items.
  • Radials generally need more static negative camber than bias tires.
  • Radials generally provide more breakaway grip than a bias tire.
  • Radials give less warning before 'breaking away'.  This causes radials to be harder to drive at the limit.
  • Air pressure can be used to reduce sidewall rollover, without having the tread bulge like a bias tire.  The may allow you to use a larger sized tire.
  • Radials are usually heavier than bias tires due to the overwrap plys.
  • Radials operate at lower slip angles than bias ply tires.  The is the main reason Radials have better transient response than bias tires.  

Most racing radial tires are closer to a belted bias ply tire than a passenger car radial tire. This gives the racing radial tire traits from both bias & radial tires (good feedback & higher breakaway traction).  Which tire is best for you, it depends on your needs.  The bias tire is good for applications where negative camber is limited, but only if a wide enough rim is available.  Radials are good where transient handling is primary concern, and adequate negative camber is available.

To Decrease Understeer To Decrease Oversteer Adjustment
Higher Lower Front Tire Pressure
Lower Higher Rear Tire Pressure
Larger Smaller Front Tire Section
Smaller Larger Rear Tire Section
More Negative More Positive Front Wheel Camber
More Positive More Negative Rear Wheel Camber
Toe-out Toe-in Front Toe
Toe-out Toe-in Rear Toe
More Positive More Negative Front Caster
Soften Stiffen Front Springs
Stiffen Soften Rear Springs
Smaller Larger Front Anti-sway bar
Larger Smaller Rear Anti-sway bar
High Performance Handling Chart

Caster Angle Considerations

When setting up your suspension, do not overlook caster. Positive caster has the effect of passing road feel to the driver, and it aids in centering the steering wheel after a turn (it makes the front tires return to straight ahead quicker). Most cars respond favorably to caster angles between 4-6 degrees positive. Caster angles in this range have another benefit, when the tire is steered into a corner the outside tire gains negative camber. If the negative camber gain is large enough, you may reduce static negative camber (good for braking). Large caster angles can cause disadvantages in slick conditions, like rain, when weight transfer is limited. This is because if the weight transfer is not occurring, the body is not rolling, and the caster angle is still generating negative camber whenever the wheel are turned. In effect you're generating too much negative camber for the conditions.

Pyrometer Usage Tips

Pyrometers are excellent tools, everyone should have access to one.  They are most effective when steady state cornering is available (oval track or skid pad). This is because the surface temperatures of the tire changes very quickly. On a typical autocross or road course you should consider the last maneuver performed when reading tire temperatures. If the last corner was a hard right hand turn, check the left side tires first because they where doing the work and will be the most accurate.
Many people believe the proper tire temperatures a achieved when the temperatures are even across the tire. This is true if you are in a steady state corner all the time, but usually this is not the case. I normally want my inner temp to be 7-12 degrees hotter than the middle & outer temperatures. This is because of static negative camber & toe-out settings. If you use toe-in you'll see more heat on the outside edge. The thing you absolutely don't want to see is the center temp colder than the edges.
Another tip for stock vehicles with weak-knee posi-trac units, ignore the right rear temperature it will almost always indicate an over inflated condition. This is because the rear sway-bar, soft springs, and `limited' limited slip differential allow the right rear to spin excessively during right hand corners.

Using a Durometer

When taking durometer reading the tire should be mounted on the rim with the proper air pressure. This supports the tread to make your durometer reading more accurate. Remember a durometer is a pressure sensitive device. If the tire tread face is compliant when you attempt to use the durometer the readings are not accurate. Speaking of accuracy, two people could use the same durometer on a tire and get different readings based on how much pressure they applied to the durometer.
When using a durometer on Autocross compounds you would normally see a higher initial reading with the durometer reading dropping "settling". EX: the initial durometer reading is 60 but the needle drops until it stabilizes at 54. This characteristic is rebound, This type of reading indicates low rebound characteristics.
Using a durometer on a tire at room temperature is less than desirable. Ideally you want to test the durometer of your tire at the temperature the tire is going to see at the track. Manufacturers use compound heat curves to help them predict how a compound reacts during competition. Manufactures use really cool scientific instruments but you can check the heat curve of your tire at home. To chart a heat curve you need the following: pyrometer, durometer, heat lamp, paper, pencil. Take a durometer reading at room temp, record the temperature and durometer reading. Heat a spot on the tire with the heat lamp, while keeping tract of temperature readings using the pyrometer. Take durometer reading every 20 degrees. If it's an autocross tire chart the durometer reading from 100 to 180 degrees, road race tires 140 to 220 degrees. I would expect the durometer readings of autocross tires to soften at 100-110 degrees and stay consistent to 160 to 180 degrees. Road race compound tires the durometer reading should be consistent from 140 to 220 degrees.  
An autocrosser wouldn't want a tire which didn't soften until 140 degrees,  this tire would be almost useless the first couple of runs (unless the air temperature was hot). Charting heat curves are most useful when comparing different compounds or brands of tires.

Hoosier Radial Air Pressure Recommendation

Under inflation will kill the radials. A thin ring will form on the outside shoulder, it will look like you took your fingernail and scooped the shoulder off the tread. If you see this ring forming on your tires, add 6lbs of air immediately. Another indicator of under inflation is the Hoosier Radials will feel `mushy' in transient response. We have noticed a 10lb range where the pyrometer will report ideal air pressure settings. On the low side of the range, the tire was `mushy' in response and tire wear was not acceptable. The high side of the range produced the best times and tire wear. Recommendation, start high with the air pressures and work Down. This may result in a lack of grip if you grossly over inflate the tire, but you won't wear off the outer edge like under inflation will.
When setting the optimum air pressure for any race tire it's best to use a pyrometer! You CAN NOT use the method of `chalking the sidewall' to set the air pressure for Hoosier radials. This technique will cause premature wear from being under inflated. At no time should you ever have roll over onto the sidewall of a Hoosier radial. The bias ply tires however, chalking the sidewall will get you close to the proper air pressure. For maximum performance and tire life you should use a pyrometer.

Hoosier Radial Setup Considerations

When putting Hoosier Radials on the car for the first time, many people experience understeer (even cars which previously over steered). This is caused by increased rear traction and the need for more negative camber in the front. You need to tune this out as soon a possible to keep excessive front tire wear from occurring.
The Hoosier Radial does require more negative camber than brand X, but not as much as you think. Keep in mind excessive negative camber affects braking, and causes under steer during initial corner turn in.. If you require more than 3½ degrees static negative camber, you should look at stiffer springs/roll bar to limit body roll or change the camber curve to gain more negative camber.

Hoosier Drag Race Tube Use

It is left up to the racers judgment to run tubes in slicks. As a manufacturer, we suggest to leave it up to the customer unless the tires have an air retention problem. The added weight of the tube is not necessary.  By running a tube in a slick, it is actually like applying an extra piece of rubber to the sidewall of the tire, thus making the sidewall stiffer.
Use proper tube according to circumference of tire. Use following chart to determine proper tube for your application.

74/84-13  fits 74-84" CIRC.  with 8-12" tread width
80/88-15  fits 80-88" CIRC.  with 6-11" tread width
86/100-15 fits 86-100" CIRC. with 11-13" tread width
94/106-15 fits 94-106" CIRC. with 15-19" tread width

If the tube is not the correct size for tire, the tube will make a bulge or indentation in the sidewall of the tire. When seeing this problem, check tube for proper fit.  Natural rubber tubes, as compared to synthetic or man-made rubber tubes, are suggested for racing purposes. Natural rubber has a tendency to stretch more with heat and force applied, whereas synthetic rubber has a tendency over time become hard and brittle, thus causing the tube to split and deflate.
All drag D.O.T. Quick Time tires are tube type tires and as a manufacturer we recommend using tubes while running on the street. At the track when racing, Quick Times are safe to run without tubes. Once again, the racer is ultimately the one to make the decision about running tubes at the track.

Freeze Crack Advisory

This advisory addresses proper storage and use of Hoosier tires in cold conditions. Hoosier Tires, especially asphalt and hard compound tires, may experience Rubber Cracking if the tires are transported, crushed, flexed or stressed when frozen. The following guidelines are provided to avoid this problem.
1. Always store Hoosier Racing Tires indoors at temperatures above 32˚ F.
2. If tires have been subject to 32˚ F or less, allow them to warm to room temperature (about 70˚ F) for a minimum of 24 hrs before the tire is mounted, transported or flexed.
3. Always use spare tires not intended for future competition to store vehicles for prolonged periods of time or winter transport.
4. If below freezing temperatures are expected, please consider shipping tires once more favorable conditions exist.
Do not use tires that have evidence of Freeze Crack Damage.

Road Race Tire Mounting

Includes catalog numbers beginning with 43, 44, 45, 46
In most cases, Hoosier tires used in Road Race applications should be mounted with the serial code toward the center of the vehicle. Once a tire has been run in the proper orientation it is acceptable to remount the tire in the opposite direction to even out the wear.