GRAB AND GO
Testing Mickey Thompson ET Street S/S Radials
If you have hung around any drivers or crewmembers involved with any type of vehicle racing, the term tire testing has probably been talked about at some point. What do the drivers and crew mean when tire testing is mentioned? The short version is that tire testing results from a tire manufacturer developing a new tire compound, tire construction, or tire size that has passed all their internal research and development criteria, and now the tires are ready to be put through the paces of real-world testing by professional crews setting up the suspension, and professional drivers pushing the envelope. Most of the big-name teams have had an opportunity to test tires in hopes of finding that elusive couple hundredths of a second advantage over the competition, while the manufacturer anticipates developing the next top-quality performing tire that’ll be a profitable venture.
For the last 17 years, we’ve been successfully drag racing a 1969 Dodge Dart. The Dart has been run in various footbrake classes on a full- or pro Tree (.500). At Beaver Springs Dragway, a pair of track championships were earned while running their pro Tree bracket class (Pro-dial). During all the years of running the Dart, the rear tires have always been 26x9 bias-ply drag slicks on 15x7 Weld
wheels. With all the rage about the great success to be had with drag radial tires, it was a thrill to have an opportunity to run a pair of Mickey Thompson ET Street S/S drag radials on the Dart.
First, it had to be determined which ET Street S/S radials would fit in the tight, stock confines of the Dart’s wheelwells. The leaf springs were never relocated inboard, so the tire fitment would be a bit of a challenge. Working with the Mickey Thompson representatives, the tire selection was focused to two radial tire sizes. The 235/60R15 (26x9.50r15) was similar in size to the racing slick that had been used for years. The height and section width were nearly identical to the racing slicks; however, the 235 radials had a tread width that was almost 0.75-inch narrower than the slicks. The second radial was a 255/60R15 (27x10.5r15), which was an inch taller than the racing slick, but the tread width and section width were within 0.2 inch of the slicks. There was some concern that the 255 radials would grow with speed and come in contact with the front edge of the wheelwell opening, but the Mickey Thompson representatives told us the radial tires wouldn’t grow at speed like our bias-ply slicks. With the knowledge that our bias-ply slicks grew about 1.25 inches from the start of the run to the finish line and to maintain a similar finish line engine rpm with the radials, the larger 255/60R15 ET Street S/S radials were selected for the Dart.
With the tires on order, a pair of wheels that were similar to the Weld Draglite wheels that were on the Dart were needed. With a quick check on ebay, a pair of 15x7 Draglites with the correct 4-inch backspacing were found. An offer was made, and it was accepted. Now we had a pair of wheels that matched what was already on the Dart. A trip to Pennsylvania College of Technology was made to mount and balance the radials and wheels. A metal valve stem was screwed into each wheel, because the radial tires didn’t require tubes like our drag slicks. One at a time, the wheels were fitted on top of the Hunter rim clamp, and then with the machine, each bead of the tire was walked onto the wheel. Each radial was inflated to 25 psig and prepared to be balanced. A Hunter load-force balancer was used to balance each wheel. A static balance was selected to allow the use of stick-on weights (secured with duct tape) on the inner plane of each wheel rather than scuffing the inner and outer planes of the wheel with hammer-on weights. One of the two wheels required a large amount of weight to balance that was unacceptable, but the Hunter balancer indicated the high point of the tire and the low point of the wheel. The two points were marked, and the tire was broken loose from the wheel (using the rim clamp) and rotated on the wheel until the two marks were lined up. The tire was re-inflated, and the static balancing of the wheel fell into an acceptable range with a minimal amount of wheel weight.
The tires were rolled to the Dart, and the wheels were slipped on the long length studs of the Dart’s Moser axles. Once the lug nuts were torqued to spec, the Dart was lowered to the ground, and a check of the tire installation was performed. There was no interference between the radials and the leaf springs, the outer wheelwell lips, or the front edges of the wheelwell openings on either side of the
Dart. The tire clearance was between 5/8 inch and 1 inch at all three points of concern. The larger 86-inch circumference radials fit perfectly in place of the 82-inch circumference slicks. The Dart was backed out of the garage and taken for a short uneventful drive on the street. The tires felt firm without any harsh tire-related reverberations into the chassis. After the trip, the Dart was loaded into the trailer and hauled to Beaver Springs Dragway to execute our official tire test of the radials.
Once at the track, mother nature greeted us with an ambient temperature of 92 degrees F, a barometer reading in the 29.20-inch range, a dew point over 70 degrees F, and a density altitude (DA) of over 3,700’. It was an air you can wear day, and the track temperature was correspondingly hot with temperatures over 130 degrees F. The Beaver Springs track prep crew had a hard day of work getting the freshly scraped track hooking with such high track temperatures.
When the open time shots were announced over the PA system, the tire pressure was set to 15 psi (same as used with the drag slicks), and the Dart was rumbled up to the staging lanes. For the first run, the same procedures that have been used for years were followed. The routine consisted of a decent burnout, quickly staging, and making the run. Once staged, the tree flickered the ambers (pro Tree), and the accelerator was hammered to the floor as the brake pedal was released. The radials didn’t grip the track, rather the tires spun hard and for some distance before traction was gained. After the run, the time slip was studied, and the 60-foot e.t. was off by seven thousandths of a second. The Dart should’ve had a 60-foot around 1.58 or 1.59 seconds, but the radials had only managed a 1.666 second 60-foot. To make matters worse, the track temperature was 129.6 degrees F, which would work against any tire. For the second run, the same procedure of a burnout, staging, and making the run was followed. The air pressure of 15 psi in the radials resulted in an even worse 60-foot time of 1.722 seconds. Back in the pits, we felt a bit lost as to what the tires wanted. The third and fourth runs (all in the same lane) resulted in progressively worse 60-foot times. For run three, the air pressure had been dropped to 14 psi, and run four had an air pressure of 22 psi.
After run four, Beaver Springs Dragway first-year track owner/operator Mike Mccracken came over and mentioned that I was splashing water into the wheelwells with my blipping of the throttle in the burnout box. At Beaver Springs, a driver has to drive around a burnout board, back into the water box, and then pull forward to do the burnout. Years ago, backing into the water box didn’t always provide water on the full rotation of the rear slick, so a quick blip of the tire when pulling forward would coat the entire circumference of the tire for a proper burnout. This procedure works well with the slicks, but the treaded radials caught the water and slung it into the wheelwells. The water then dripped off the wheelwell onto the tires and the track as the Dart was staged. This contributed to the traction problems we were experiencing with the radials.
Armed with the new knowledge about the water on the tires, the burnout technique was modified. A last run in the open time shots was run with 12.5 psi of air pressure in the radials, and it too resulted in a poor 60-foot time. Between
6:12 p.m. and 7:26 p.m., a total of five runs had been made (all in the same lane), and the best 60-foot time of the test session was 1.666 seconds, which was done on run one. There was going to be an hour of downtime between the last open time shot and the first of two qualifying runs for eliminations, so that time was used to make some changes. It didn’t seem as though the radials were being hit hard enough on the initial launch, so the nine-step single-adjustable Rancho rear shocks were dialed down three clicks (softer setting to allow quicker extension of the shock absorber). The option of moving the Caltrac bars to the upper hole was considered, but instead the preload on each bar was turned two flats to increase the hit on the rear tires. Lastly, after a suggestion of another driver using Mickey Thompson radials, the air pressure was set to 17.5 psi. It seemed like a big swing, and with more than one parameter being changed at a time, we wouldn’t know what worked or didn’t work if the 60-foot times were still off.
On the first qualifying run, the adjustments to the suspension, the tire pressure, and the modified (and shorter) burnout generated a 60-foot time of 1.585 seconds and a quartermile elapsed time of 11.634 seconds, which was equal to the drag slick on a 110.6 degrees F track with a DA of 3,859’. For the last qualifying time shot, no changes were made from the first qualifying run. The results were similar to the first run, and it seemed as though we had stumbled upon a decent combination. When eliminations began, the Dart started clicking off rounds. The 60-foot times did not vary more than 4 thousandths (1.585 to 1.589 seconds) the rest of the night. The
tires propelled the Dart to the finals, and it would have been great to say we won, but a lazy reaction time put us on the trailer with the runner up money.
What was learned from the tire testing? First, the radials want to be struck harder than the slicks at the launch. The radials need much less burnout when compared to the slicks. In addition, with radials, a higher air pressure (16-22 psi) can be run compared to the slicks (1216 psi). The radials work very well on expertly prepared tracks, however, if the track is in poor condition or the suspension is not setup correctly for the car, radials don’t seem to recover when they spin from the starting line. While the radials are heavier than drag slicks, they have stiffer sidewalls that reduce the rolling resistance, so the finish line trap speed is about 1 to 1.5 mph faster than the drag slicks with the matching ET. Lastly, the radials don’t grow with speed, so make sure to order radials that are slightly taller than the current slicks; this will guarantee a finish line rpm is similar between the two types of tires.
Was the tire testing worth our time? Absolutely, the radials require some suspension modifications to find the settings that’ll work with any Mopar, but the radials work well, once the settings are found. Additionally, the radials are less expensive than our drag slicks yet provide the same performance. Furthermore, the radials can be driven on the road without having to swap tires, which is required with the drag slicks. The Mickey Thompson ET Street S/S radials will not disappoint on the road or at the track, so if you have a high-horsepower ride, contact the representatives at Mickey Thompson to see what tire will provide the best on-road compliance and on-track performance for your Mopar.
A pair of 255/60R15 Mickey Thompson ET Dart is dragstrip action. The DOT legal Street S/S radials arrived for our ’69 Dodge ET Street S/S radials will bridge the gap Dart. While the Dart may be occasionally between the limited street usage and used on the street, the main focus of the the dragstrip.