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Tired questions

Treadless "racing slicks."
Treadless "racing slicks."
Adrian Hoffmann /

Here are several recent reader questions about race tires, who races in the rain, and why race cars only go in one direction. Taking them in no particular order, here are the answers.

Question: Why do race cars only go counter-clockwise? Why don't they go in the other direction?

Answer: In America, racing, and in particular oval racing, began on horse racing tracks at state and county fairs. In this country, horses have always run counter-clockwise, so when cars started racing on them, something over 100 years ago, they followed tradition and went in the same direction. Or maybe that's the direction in which the timing and scoring system worked, take your pick.

Since oval races are always run with cars turning to the left, race cars from go-karts to NASCAR and Indy cars have been built and optimized to run best while turning to the left. This usually means the suspension has been set up to be "neutral" -- that is, no steering effort required -- in a left turn; sometimes the driver  has to turn the wheel on the straight to keep from hitting the wall. Suspension modifications include camber, caster and steering, weight distribution and on occasion even using un-equal length suspension links. If you look at an Indy car from the mid 1960s, you'll see the suspension on the left side is several inches longer than on the right.

Mind you, this is not always the case. In Europe, road races are run clockwise, as are some oval races. Australia does the same, running their superspeedway races "backwards" by American standards. Again, this is as much tradition as anything else.

Question: Why do racing tires have solid treads without grooves?

Answer: Because the grip is better. That's the short-and-simple explanation, anyway. The full reason is somewhat longer. First, read my earlier article about Why We Have Wide Tires, which goes into detail about the interface between the flexible rubber on a tire's tread and the pavement. Put simply, the rubber on the tread fits into the gaps in the asphalt like pegs in holes, which is what makes the tire hold grip. When the force on the tread exceeds the strength of the rubber, it is scratched off the same way sand paper scratches wood from a board. But as always, things are more complicated than that.

Here's an experiment: Pick up a hand full of pencils that have erasers on them and rub all the erasers on a sheet of paper and watch closely at what happens. You will see each small eraser flex sideways as it grips the paper. You will also see a good deal of eraser dust and the edges worn off those small erasers.

Now repeat this with a big pink eraser and watch what happens. The big eraser may have the same area as the little ones, but it behaves much differently. As you rub it side to side, the rubber flexes a good deal less from side to side, and you may get less eraser dust. That is because each small piece of eraser is supported by the whole rest of it, rather than rubbing independantly, as the small ones did.

The same thing happens to tire treads. Street tires have to work in all conditions from snow to rain to hot, dry pavement, so they have grooves for water or snow to collect in; this lets it flow away so the blocks of rubber can cling to the pavement. The problem is, each of those small tread blocks acts like one of those small erasers -- it is unsupported by the rest of the tire, so it "squirms" from side to side, grinds its edges off and wears out quickly. It also builds up considerable heat, which melts the rubber and causes still more wear. In a "slick" tire, the whole tread, like the big eraser, is one block; there are no small bits wiggling back and forth, thus less tread wear and less heat build-up. So the tire runs cooler and lasts longer. Up to a point, anyway...

Question: Why don't racing teams rotate tires the way street drivers do?

Answer: Becuase racing tires have such soft tread compounds, they don't last long enough. Mind you, this is not the whole story. Racing teams that expect to make several tire changes during a race will sometimes "scuff" a set of tires -- that is, run them for just a few laps, until the stickers and the shine are worn off the treads. Then they're set aside for use during the race.

Why do this? Because "scuffed" tires last longer. When a tire gets up to operating temperature -- around 195 to 225 degrees -- some of the oils, polymers, binders and other volitile compounds in the tread boil off, leaving the harder componenets of the compound behind. This is no big problem while the tire is hot, but when it cools down, the harder elements remain and the tread is now "harder" -- and has less grip -- than before. After several cycles of getting hot, then cooling off, the tire gets considerably harder and loses a good deal of its grip. The one advantage of scuffed tires is that they last longer because the tread has gotten harder after a heat cycle. The driver sacrifices some grip for greater tire mileage.

Question: Why don't cars race in the rain?

Answer: Some of them do. Road racing traditionally runs in the rain; when the weather gets wet, they switch from "dry" slick treads to grooved "intermediate" or "wet" tires. The differences between the latter are that wet tires have deeper tread grooves and softer tread compounds to grab what little of the road they can while the tire is hydroplaning.

Oval races do not run in the rain because the speeds are so consistantly high. A road racing car may hit 150 - 200 mph on some straights, but braking and speeds in the corners are considerably lower. A car on an oval track, on the other hand, may run 180 - 200 mph all the way around the track. In anything more than a drizzle, that means tires will hydroplane so badly, they will not be in contact with the road, at all. Braking distances are much longer and cornering is like driving on ice. A sudden rain storm on an oval track can cause everybody to spin and hit the walls and each other.

The real crazies of the racing world are the performance rally drivers. Those guys drive on everything from glare ice and snow to mud, dirt and occasionally asphalt. If you're looking for someone with great car control and the ability to keep from spinning out in the most extreme conditions, that's where you'll find them.   That's because they know how to recognize when a car is about to go lose control and keep just this side of a spin. Dirt track drivers run a close second. Two-time Indy 500 winner Al Unser Jr. -- who got his start racing sprint cars on dirt ovals -- says the best pavement drivers are those who started on dirt because they know how to control a half-out-of-control car and keep it going. You know, sort of like a New Englander watching a southerner in snow...


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