Skinny Tire Shootout: 18×9.5 Versus 18×4.5 At The Dragstrip


There are numerous methods that can be employed to get faster times at the drag strip. The first that usually comes to mind is adding more horsepower. The second is traction and the third is weight. As speeds increase down the track aerodynamics will also play some role. Rolling resistance is also another factor not often thought of but can play a crucial role in reducing a car’s elapsed times. Depending on a project car’s goals as well as the owner’s level of comfort, and budget all of these areas may eventually need to be addressed.

Our Project Wild E Coyote isn’t lacking in the horsepower department with over 590 rwhp wild supercharged ponies at the beck and call of its accelerator pedal. We’ve set up the rear of the car to give us decent drag strip traction running stock 2011 Mustang wheels with Mickey Thompson drag radials, 265/40/18’s on the back. 

So with traction and horsepower covered we turned to weight, aerodynamics and rolling resistance as areas we needed to address.  Providing us with solutions to these issues as well as some of the background on achieving our goals are Weld Racing Wheels 18×4.5 RTS wheels and Mickey Thompson tires Sportsman SR tires. We are going to compare the Weld and M/T combo against our stock set of 18×9.5 GT500 wheels to see what we can pick up at the dragstrip.

Wheels

High Pad

Weld offers the RTS in three different pad clearance configurations. Due to our massive Baer Brake system we’re using the High Pad model which thickens the pad area where the wheel bolts to the flange and also curves the wheel spoke slightly. This gives us adequate room for our big calipers while still allowing us to run the lighter weight wheel and tire combination.

Weld Racing Wheels sent us a pair of their RTS series wheels in size 18×4.5, with the high pad clearance option. Just like what we needed for our huge Baer brakes, they developed these wheels due to market demand. Today’s muscle cars with their heavy curb weights and massive braking systems had been left without a drag racing wheel that could fit the car properly.

The forged alloy design of the wheel is not prone to delamination because the grain is aligned. Forged alloy wheels such as these are also 40% stronger than a cast wheel (such as the SVT wheels we’ve been using) this allows them to carry the heavier weight of the car in spite of their smaller width.

The wheel is available with three pad area options, allowing it to fit a variety of applications. Weld has an application guide available on their web site that shows what applications the RTS is available for. The standard low pad is also the lightest weight version of the RTS, trading off available clearance for calipers in favor of weight. The low pad offers, 0.89” of caliper clearance. Medium pad wheels add a little more weight but also offer a maximum caliper clearance of 1.19”.

With the high pad option that we specified on these wheels, there is 2.05” of caliper clearance. This is due to the thickness of the pad area where the wheel mounts to the hub or face of the rotor, and the fact that Weld gives the spokes of a high pad wheel a curved profile which helps make clearance and correct fitment possible. There is a bit of a trade off in weight, but the savings are still significant, also that weight is concentrated in an area of the wheel where it will have the least impact on the car’s performance, read on for more details regarding that. Kyle Fickler of Weld Racing told us  “We can concentrate the weight at the center, rather than out on the rim shell and still build a very strong rim for a street car. Most gains [on the track] are made at the rim shell and in the tire.”

Tires

For tires we went with Mickey Thompson Sportsman SR 26 x 6 x 18”.  These tires feature a radial construction which matches our MT drag radials that we’re running out back. Mickey Thompson does not recommend mixing a bias ply tire with a radial. We also chose these because they are street legal, meaning if we want to throw them on before we head to the strip we can do so with no worries. The smaller width of the tire will improve several areas including, weight, aerodynamic profile and rolling resistance. The tires also have a tread width of just 6.3” each.

In the past we’ve noticed driving cars with the big ‘n’ little tire combination on the street is a hassle. Your brakes don’t seem to work right, and modern cars may misbehave as well. Driving on the street with the Sportsman SR’s wasn’t bad at all. We hardly noticed any issues with braking or normal driving. While we won’t be carving corners with these on the car, it’s a welcome change from some past experiences of what can feel like ice skating in a combat zone when running skinnies on the busy streets of southern California.

Cutting Weight

Unsprung weight refers to the weight that is not supported by the vehicle’s springs. This can include brakes, wheels, tires, axle shafts, brake calipers, and various suspension components to name a few. The easiest way to reduce unsprung weight is to swap some of the affected components for lightweight replacements.

There is no set number that if you remove X number of pounds of unsprung weight it equals Y number of pounds of sprung weight – Scott Rider

Reducing the unsprung weight in the front end aides the reaction of the car’s suspension. By having less weight to  move, the car’s suspension will react/respond faster. It’s similar to carrying a bucket, if you’re carrying a five gallon bucket that’s full of water, it’s fairly difficult to lift that up at a high rate of speed. However if your five gallon bucket were say less than half full, it would prove much easier to move. 

Similar principles apply here as the car’s suspension reacts during starting line launches and hard acceleration down the track.  This reduction in unsprung weight can also help the suspension react more quickly to an irregularities in a tracks’ surface such as a bad seam. This too can improve safety and control as the front end is less prone to being upset since the front suspension can react faster and easier to such changes.

There is no magic ratio

We ran into an interesting fact during our reseach and testing for this article. For years we have read and beleived ourselves that there is a ratio that for every pound of unsprung weight you remove, it is equivalent to the removal of an even greater amount of sprung weight. There are various theories and numbers thrown about depending on who you ask, 1:5 and 1:7 are popular on the internet. These essentially state that every pound of weight you remove from the wheels and tires, or unsprung components would be equal to pulling out 5 pounds of weight inside the car.  However in our discussion with Weld Racing engineer Scott Rider we learned, “There is no set number that if you remove X number of pounds of unsprung weight it equals Y number of pounds of sprung weight.” There are too many other variables to consider, and the laws of physics play a huge role in this. This is because the further you get from the axle the greater the multiplier for the weight savings is in terms of moment of inertia. 

With respect to performance and weight savings in terms of wheels and tires it actually has more to do with the center of mass and moment of inertia. The closer the weight is to the centerline that it spins on, in our case the spindle or hub the easier it is for that weight to move, the more you reduce weight on the outside edge, farther away from the centerline of rotation the the more of a performance gain you will see.

This is where things can get a little confusing. “If you saved five pounds on a tire but added five pounds to the axle, the race car would see no difference in rotating weight, but the car would be faster because of the location of that rotating weight,” said Rider. This is because you’ve lost weight in the important area affecting the moment of inertia, the energy it takes to move the wheel forward and keep it moving down the track. 

Weld has literally spent thousands of dollars and countless hours researching this data. They can create computer models of a racer’s wheel and tire combination to show whether or not that combination would then gain anything over one previously used.  In the end what this really boils down to for the average enthusiast out there is that you could spend your money on a lightweight driveshaft, gun drilled and star flanged axles, but you may not see as significant a gain in terms of bang for the buck as you would in simply changing your wheel and tire combination for track use to one that is lighter, especially towards the rim shell area and a lighter tire.

Rolling Resistance

We have also reduced the amount of rolling resistance of the front of our car with this swap. This is the amount of friction the front tires have.  By reducing the total surface area that is contact with the pavement as we’ve done with this front wheel/tire combination, we have lessened the rolling resistance of the front of the car. The Sportsman SR tires should have less rolling resistance than our typical street radials which are designed to be sticky and give us maximum bite in high speed maneuvers. 

The design of the Sportsman SR tire and the reduction in contact patch should greatly reduce the rolling resistance of the front wheel/tire combination.

Remember a Mustang is a rear wheel drive car, this means it has to push the front wheels. Just like improving traction to the rear tires speeds up acceleration, decreasing the resistance of the front tires translates to less work for the rears which should equate to lower ET’s. Our street going tires have a treadwidth of 275 mm which converts to 9.25” while our MT Sportsman SR’s have only a 6.3” treadwidth. 

Aerodynamics

Mention aerodynamics and racing in the same sentence and for most people two things come to mind. The sleek bodies of Pro-Mod or faster cars, or the slippery designs of NASCAR and F1 racers. In a setup such as ours aerodynamics may not play such a huge role as it would on a car traveling down the track to a trap speed of two hundred miles per hour. However, it is important to note that we have decreased the surface are that air has to move around the tire as well as lowered the front end of the car, both of these play some role in the overall aerodynamics of the car and may affect our performance slightly. Remember the faster you try to go, the more mother nature will try to hold you back, so we’ll take any advantage we can get.

Left you can see Wild E Coyote with the track wheel and tire combination and right with the street tire combination. The front end sits slightly lower with the track combo which may be giving us a tiny bit of aerodynamic gain, especially at top end.

On The Scale

The scale in our case tells part of the story. Since we had neither the time nor the funds to have Weld deconstruct our street combination and perform computer modeling we can only provide the numbers from our racing scales. We can tell you that the numbers are significant with our front wheel and tire swap. Rider told us “The front is where you can really pickup because of the rolling efficiency of the tires and the weight of the tire and wheel.”

Both assembled wheels and tires were placed on the scales and checked for total weight differences.

We’ve broken down the numbers as follows:

Street Combo:

  • Wheel weighed in at – 28 lbs
  • Tire weighed – 26 lbs 14oz
  • Total weight each side – 54 lbs 14 oz
  • Total front weight – 109 lbs 12 oz

Weld and Mickey Thompson Track Combo:

  • Wheel weight – 17 lbs
  • Tire weight – 18 lbs 12 oz
  • Total weight each side – 35 lbs 12 oz
  • Total front weight – 71 lbs 8 oz
  • Total weight savings –  38 lbs 4 oz

It really is astonishing that there is a nineteen pound per side difference between our two wheel tire combinations.

This equates to a total weight reduction of over 19 pounds per side at the track. While this is significant what is more important than the overall gain is perhaps that drop in tire weight – a reduction of just over eight pounds in the weight on the tire. While we are not discounting the 11 pound drop from the wheel, it is important to go back to what we discussed earlier, that the real gain may be seen in the reduction of rotating mass on the outside of the wheel because it changes the moment of inertia. That change is going to be what makes up our gains.

At the track

Notice that even with the front sway bar disconnected our front end barely lifts at all to transfer weight to the rear tires when the heavier street combo is installed on the front end.

At the track we ran the car two ways. We first ran a set of passes with our street tire and wheel combo mounted on the front of the car to establish a baseline for testing. Our best ET of the day with our street tires was 7.452 at 98.16 mph. Not too bad for drag radials and street tires. We were only running the car completely through the eighth mile since we lacked a roll cage to go full bore on the top end.

We conducted both of these tests with our Lakeweood Qwik Links disconnected to help with weight transfer, our rear tire pressures set the same and the front suspension dialed in to transfer weight as best it could. Keep in mind Wild E Coyote is not a dedicated track star, this is a daily driver with lowered suspension, designed to handle the curves as well as it is to accelerate in a straight line. 

Top: We swapped the tires in the pits and made another set of passes. Bottom: You can see how much more weight transfer the car is getting and how much better the front and rear suspension can work with the Weld RTS and Mickey Thompson Sportsman SR combination installed.

Swapping to our Weld RTS and Mickey Thompson tire combination in the pits we went back to the starting line for some more passes where our best pass netted us a 7.357 at 98.34 mph. The lighter skinnies allowed for better weight transitioning under acceleration, which in turn gave us better 60 foot times. Matter of fact, we have never got the Mustang into the 1.6s prior to this test. This helped us pick up about a tenth in the eighth mile. From this data, we figure a 2 tenths drop at the quarter mile is very probable on the skinnies.

We ran the right lane for both passes. What you can see from the photos translates on paper. The lighter skinnies allowed for better weight transitioning under acceleration, which in turn gave us better 60 foot times. Matter of fact, we have never got the Mustang into the 1.6s prior to this test. This helped us pick up about a tenth in the eighth mile. From this data, we figure a 2 tenths drop at the quarter mile is very probable on the skinnies.

 

Anytime Wild E. Coyote hits the dragstrip we always run a 50/50 blend of VP Fuels 100. It helps fight detonation with the high air intake temperatures from the weekend. The fuel is oxygenated with ethanol and CARB-legal in all States. It contains no metal compounds and won’t harm catalytic converters or oxygen sensors.

Conclusions

The front is where you can really pickup because of the rolling efficiency of the tires and the weight of the tire and wheel – Scott Rider

At the end of the day the car reacted better allowing the suspension to do a better job transferring weight to our rear tires. At the same time the nose of the car was lower as is evidenced in the photos with the Weld and Mickey Thompson tires installed.

There is the old addage that for every one hundred pounds of weight you remove from a car you gain about 0.10 seconds in total ET. If that adage is true then our wheel swap is equivalent to a static weight loss in the car of over 200 hundred pounds. At the end of the day the timeslips don’t lie and as far as bang for the buck goes, to lose two tenths with a simple wheel and tire swap is something that every weekend racer should be looking into. Even better, we were able to drop times off our car without adding any additional stress to the engine and is reversible in about 30 minutes.

 

Article Sources

About the author

Don Creason

Don Creason is an automotive journalist with passions that lie from everything classic, all the way to modern muscle. Experienced tech writer, and all around car aficionado, Don's love for both cars and writing makes him the perfect addition to the Power Automedia team of experts.
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