MaxBoost!
Advice for fellow TR owners from Tom Shea Racing
Home phone until 9 pm 952-892-5669
Cell phone anytime (612)-209-9090.
Email ttshea@charter.net
"TSR" Switch Fuel System, save money, drive on pump gas!
Tires & Air Bags
What size tires can I run on my T.R.? The largest that will fit in the front on the stock rims are generally 235/60/15. Some 245/60/15 will fit. The largest for the rear are 26x10/15 for slicks or D.O.T.s, and 275/50/15 radials. Some 265/60/15 will fit. (28 inch tall tires can be used but they are less desirable for several reasons) Some 28 inch tall tires have a narrower tread width than their 26 inch tall counterpart. The fit of a 28 inch tall tire is a lot less forgiving, it has to be precise to avoid contact. Suspension travel may have to be limited.
The tire size should be kept near the original height of 26 inches. The width of the tire is limited by the width of the rim. 275 size is generally considered to be too wide for 7 inch stock size rims. The 275 number is roughly equal to a 10 inch wide slick or DOT tire. The 10 inch wide tire is commonly not used with a wheel width less than 8 inches wide. This is measured between the inner bead edges of the rim. The 10 inch wide tire can usually be used on a rim as wide as 10 inches.
The wider the rim the better. The reason for this is that the wider rim exerts more pressure further out towards the outer edge of the tire. This allows more air pressure to used in the tire for the same footprint. More air pressure inside the tire usually exerts this pressure in towards the center of the tread. The trick is to equalize these pressures for an even contact pressure. More overall pressure equals stronger resilience. This improves the stability and handling characteristics of the tire. In a wrinkle wall drag racing tire, this can mean better traction as well as better big end control at high speed. The tread wear will be more even and last longer. I like to use the widest rim in the range allowed by the tire manufacturer. The footprint of the tire should have an even contact patch across the width of the tread. This is evidenced by the tire wear eventually. This wear if uneven indicates the uneven traction exerted by the tire during its uneven wear.
What about the compound? Soft or harder? Our cars are considered relatively heavy by the standards of the racing tire manufacturers. Even the lightest of the stripped & lightened race-car versions of our cars are considered heavy by these standards. Except for a very few cars that come in under 3000 lbs. Our cars are HEAVY! We benefit the greatest by the hardest compound racing tires. This situation has the added benefit of longer tire wear. The harder tires also don't pick up as much debris. This property is more of a consideration on the street. I don't recommend driving D.O.T.s or slicks on the street. Slicks will get you an illegal equipment ticket. D.O.T.s will be an unstable tire for driving because of the soft sidewall and hydroplane at low speeds with very little rain. I recommend having a pair of 26 x 10 x15 D.O.T. racing tires mounted on 10 inch wide rims for racing. These tires will get traction that is close enough to slicks that the difference is minimal. These D.O.T.s will also allow you to compete in more races. Some classes do not allow slicks. Racing tires like D.O.T.s or slicks should shave off around 1/2 second in your ET.
The inner edge of the rear wheel well trim can catch & cut the outside edge, or upper corner of the tire in certain conditions. If the tire is a 26 x 10 / 15 slick or D.O.T. tire. If the car is driven up a driveway at an angle that causes the car body to be tilted. This is a common occurrence. The front corner tire that goes up an incline first will raise that corner of the car. The opposite corner in the rear of the car will tilt down. This sometimes causes contact between the tire and the fender lip. This situation can be avoided or minimized by avoiding or minimizing the angle at which you drive up into an incline.
The front sway-bar is much stiffer than the rear one. This causes the car body to be more closely aligned to the front suspension. Some cars that have the larger, stiffer rear sway-bar. This will see a more equal alignment between the two ends of the car over this type of condition. This may partially address this concern. Another thing that may help is the trimming of the inner fender lip. Not an option for those who prefer to maintain the original condition of their cars. So if you are running the large 10 inch wide racing tires on the rear. You may want to approach any inclines such as driveways at a head on square angle. This will avoid the tilting of the car body.
Air pressure? How can you tell what is the proper pressure to run? If you don't have a white liquid shoe polish applicator in your race kit, you should. This is what most racers use for numbers on car windows. I use it to make a mark on the tire near the valve stem. This allows a constant reference to the rim to tire. The tires can actually move by rotation on the rims. Some racers will make a more evident mark on racing tires to be observed when the car is launching.
This shoe polish can be used to mark the tread for a simple test. Make a nice thick even mark across the thread of your tires. Now drive on them for a short ways. Around the block should do it. Inspect the tires for the wear on the marks. If the marks are worn more in the center of the tread then lower the pressure. If the mark is worn more on the edge then raise the pressure. This test can be done with the rear racing tires but will have to be done under launching conditions. So make the mark then make a launch. The air pressure may not be optimally equal between the two rear tires. This is because of the torque of the car under launch.
Air bags. How do we use these? What is the proper pressure? When the engine is applying power against the drive-train & eventually to the pavement. That power is anchored against the car body ultimately. Remember that every force has an equal & opposite reaction. This is what causes the cars at launch to be twisted up on the drivers side. What is happening is the engine is twisting the driveshaft one way while being braced against the frame from the other.
The rear axle is actually pushing down on the drivers side and up on the passenger side. We try to counter this force by using an air bag in the right rear coil spring. The more air in this bag the more resistance to the compressive force from the axle twist. To determine how your car will react when you launch. Have a friend help. Use a yard stick to measure the height of the rear bumper at both ends, under launch torque. Use air pressure in the right air bag to level the car at launch.
Keep your eye on the transmission temp gauge. The brake standing required for this launch torque simulation builds heat quick. Keep this in mind when doing the measuring with the yard stick. If you need more than 20 psi in the bag, it might be a good idea to check what it actually looks like in the coil spring. They do tend to get distorted. The bag will bulge out between the coils. It may come too close or into contact with the exhaust pipe. Heat shields should have been included with the bags. You can address this problem, at least partially, with a larger rear sway-bar. If you have the engine foot braked tight against the converter, it will twist up the suspension. There needs to be some travel left over for launch. The suspension should be allowed to be reactive in order to be the most effective. Having the proper stall torque converter is the best way to address this issue.
© 2007 MGNTA.com
