Test­ing Mickey Thompson ET Street S/S Radials

Mopar Muscle - - Contents - TEXT AND PHO­TOS: CHRIS HOL­LEY

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.

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