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[tech] Rear Drag Tires
by George Klass of Accufab Performance Parts and Accessories
The American Autosports Heads-up classes have rear tire size limits. The PRO 5.0 class limits tire width to a measured 12.5". The Street Outlaw and Street Stock limits measured tread width to 10.6". And Trophy Stock limits total tread width of a "treaded" type tire to 9.5". As far as tire diameter is concerned, there are no restrictions, assuming that the tire fits into whatever the fenderwell modifications, if any, are allowed.
This is very important because a larger diameter tire will have a longer footprint, everything else being equal. A 28" by 10" drag slick will have more potential traction area than a 26" by 10" tire. On the other hand, a larger tire diameter will effect overall gearing so this has to be considered when choosing a rear tire size. My recommendation is to always go with the "smallest" in terms of width and/or diameter that works for your combination. Actually, the smaller the overall footprint on the pavement, the more pounds per square inch of footprint is available. And there will be less rolling resistance, also.
Some racers have the wrong rim width for the tire. All drag tire manufacturers list recommended rim widths for the different tire widths. If the rim is too narrow, the sidewalls will be pulled inward and bulge the middle of the tread out. The result is a car that feels "dirty" at the top end. Since air pressure plays an important part in total footprint, it is probably advisable to have the rim width and tread width close to the same width. This will allow the maximum amount of tire "tuning" with air pressure.
Let's talk about air pressure. A good tire gauge is the chassis tuner's best tool. A good tire gauge is defined as a gauge that reads in quarter pound increments up to about 20 pounds. A typical street tire gauge can't deliver this kind of accuracy. The chassis tuner's second best tool is a log book that records the results of each and every run, along with the tire pressure for that run.
A Mickey Thompson Tire engineer once told me that most racers are running tire pressures that are too low for optimum results. Somewhere between 10 and 14 lbs is about right. Anything less than 10 lbs may drastically effect handling at the top end. If your Mustang can't hook up unless you are running 7 lbs, you are using the wrong size slick or something else is wrong. Running 7 lbs in the rear is a gear way to meet Mr. Fence at half track.
To lower tire pressure, let some air out just prior to the next run, not just after the last run. The same holds true for adding air pressure. Wait until you are in the staging lanes before doing the final check. Even though the "big boys" may have an expensive air compressor in their trailer, you can have almost the same capability by buying an inexpensive compressed air tank at your local auto parts store. This way, you can add a little air late in the day when the sun goes down, if needed.
Talking about heat, let us discuss the great all American tradition known as "the burn-out". The need for consistency extends to the burn-out procedure. A good pass begins not at the starting line, but in the burn-out area. This is the place where the tires are prepared for battle. Finesse in driving through this area is related to the ability to heat up the tires with a minimum amount of tire smoke. Excessive burn-outs don't heat the tires much more but they do wear out the tread. Big burn-outs may look great but they are expensive. The trick is to create consistency with your burn-outs from run to run.
The hear generated by the burn-out isn't very deep and not that hot. Temperatures rarely get over 160 degrees on the tread. It doesn't take more than 3 or 4 seconds in front of the water box to achieve the maximum heat. Of course, I don't have to remind you that water burn-outs are for drag slicks only and not treaded type tires. If you attempt to do a water burn-out with your DOT tires, the water will accumulate in the treads and drip down under the tires when you are staging at the starting line. Dumb. In fact, many racers won't even drive the front tires through the water box. Instead, they drive around the water and then back up to the water. This assures a dry "path" for the rear slicks.
After the burn-out, proceed to the starting line. Many racers feel that it is necessary to "chirp" the tires, or to do a quick dry-hop before staging. I don't think that chirping the tires does anything other than put an unneeded shock load on the rear end and drive line components. Again, it may look good to the fans in the stands, but it doesn't help traction.
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