Wheels and Tires From Street to Strip

Picking the right wheels and tires for your car usually boils down to what fits and what’s on sale.  There’s a lot of monkey-see, monkey-do too — using what the local hero uses.  The reality is there’s a whole bunch more to drag race rolling stock than you expect.  And one area that seldom receives consideration is the fact that unsprung weight can actually affect performance.

And when it comes to unsprung weight, there’s very little data out there. In fact, it took a considerable amount of research to get to this point (and it’s presented here).  In the end, you might be surprised at how unsprung weight affects your car.  And if that’s not enough, we also have some hard and fast rules on how to select the right wheel dimensions for your ride.

First let’s define “Unsprung Weight”:  Unsprung weight is the part of the total vehicle weight that is not supported by the suspension springs.  Unsprung weight includes the wheels, tires, hubs, rear axle (if a conventional non-independent arrangement — independent systems also include the hub carriers, but these systems are few and far between in drag racing) and brakes.  Approximately 50% of the weight of the suspension links, drive shaft, springs and shock absorbers is also “unsprung.”   What it all means is that the total unsprung weight is what the shock absorbers are forced to control (in the bump direction) so that the tires remain in contact with the track surface.  The basics are easy too:  The less unsprung weight there is, then the easier it is to control.

Years (decades?) ago, we decided to tackle part of the unsprung weight equation by way of a computer simulation.  We used a couple of different examples (one real, one imaginary) and ran them through a Quarter computer race program.  The first was a 1969 Camaro complete with a healthy big block and a TH400 automatic with a transmission brake.  The Camaro weighed in at 3500 pounds and carried a 4.88:1 rear axle.  Wheels were heavy steel jobs while the rear tires were 9.00 X 30.00″ drag slicks (FYI, this was modeled after a car I owned at the time).  The second car was a theoretical Chevelle – your typical weekend warrior powered by a mild 350 V8 (345 HP @ 5600 RPM), backed by a wide ratio Muncie 4-speed gearbox.  The car weighed 3900 pounds ready to race and was fitted with a 4.11:1 rear axle.  Again, the wheels were heavy steel models, but in this case, the tires were 10.00 X 28.00″ slicks.  Both test cases were based upon the assumption that the respective chassis’ were sorted.  In short, the cars hooked, but the Camaro was right on the edge of blowing off the tires.

In both cases, the conditions were corrected to dynamometer “ideal” (zero elevation, 60° F ambient temperature, barometer reading of 29.92 In. Hg and zero percent relative humidity). After a series of baseline tests were run on each car, they were fitted with lightweight wheels.  With no other changes, the Quarter program predicted significant performance improvements.  Check out the results:

Camaro

Chevelle

Fair enough, but what about performance in the real world? It takes big power to turn a big rotating mass, and the heavier the rotating pieces, the more power it takes to turn them.  According to folks who do this for a living, for every ten pounds you take out of the rotational mass, it’s like removing 17-1/2 pounds from the racecar.  Ponder that for a second:  If you lighten each back wheel and tire by ten pounds, the car lost twenty pounds total.  Peel another five pounds from each brake rotor (front and rear), then remove five pounds from each front wheel and tire, you’ve removed a grand total of 50 pounds of static weight.  But when the rotational inertia is taken into consideration, it’s like removing 87.5 pounds from the car. And these figures are for a typical small diameter road race combination.  As the tire diameter increases, so does the inertia.  That means that a 33-inch tall tire under something like a Super Gasser stores plenty of it.

Speaking of Super Gas, when yours truly worked for the old Super Stock & Drag Illustrated, we became privy to testing by Sheldon Gecker.  Using a 2400-pound, tube frame, four link Dodge Daytona Super Gas car as the test bed, Sheldon decided to A-B-A test different wheels.  First, he had the old Monocoque Wheel Company build some “heavy” rear wheels (which wasn’t so easy).  The first set of 15 X 15-inchers weighed (coincidentally) 15 pounds.  Using his standard 16 X 33-inch Super Gas slicks (103-1/2 to 104-inch circumference), the car was base-lined.  It consistently ran 8.94 second ET’s.  Switching to a set of 11-pound wheels, the car ran consistent 8.90 laps.  Swapping the heavy combination back on the car, it slowed down 0.040-second.  When the light wheels were added, the MPH picked up, but it was proportional to the lower elapsed time.  In addition, the short times revealed that all the difference was seen down the track (more on this later).

Gecker still wasn’t finished.  Next, he tested a set of Monocoque bead lock wheels against the light combination.  Given the design configuration of a bead lock, the wheels proved to be quite a bit heavier than the more conventional versions.  Given the previous test, the results were predictable:  The car slowed by almost one tenth (actually 0.090-seconds) when the heavy wheels were added.  What had the most profound effect in the testing was the fact that the actual bead lock mechanism is in close proximity to the tire.  That means that most of the weight is moved to the outer edges of the wheel which in turn increases the rotational mass (and consequently, the rotational inertia) dramatically.

Those old tests came to a simple conclusion, and they happen to match our electronic test models:  There is ET in light wheels.  There is also some ET to be found in lighter slicks.

There’s one more consideration:  Running the wheels tubeless (there are still a lot of racers running tubes with slicks): When running drag wheels tubeless, you can lose as much as five pounds per wheel.  The only catch is you have to insure that the multi-part bolt-together wheels are properly sealed.

Over the next three parts, we’ll talk more about wheels, tires and a working knowledge of these aspects of your vehicle can improve performance – on the strip, and on the street.