Bias vs. Radial Performance Theory
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| 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.