Hoosier Racing Tire
hoosiertire.com
Photography Credit: Perry Bennett
[Editor's Note: This article originally appeared in the May 2015 issue of Grassroots Motorsports, back when Bob Woodman Tires was still in business.]
Although the term “street tire” is stretched quite thin when it comes to the current crop of R-compound DOT track and autocross tires, there’s still a big difference between those nearly bald gumballs and true racing slicks.
Racing slicks tend to be lighter, stickier and more responsive than their street-bred counterparts. After all, slicks don’t have to meet DOT requirements for the things that make a street tire civil. Thanks to falling prices and increased availability—for both new tires as well as takeoffs—we’re also seeing more slicks in our world than ever before.
Despite the performance advantage offered by racing slicks, many drivers are still apprehensive about making the leap—even when their racing rules allow it.
Indeed, we were in a similar situation recently when we added a Formula 500 car to our project roster. In addition to people not thinking we were cool anymore, we faced the challenge of tuning and driving a car on true race slicks rather than the DOT rubber we’d been using for three decades. Racing slicks not only look different, but they’re a totally unique animal, right?
For some advice, we turned to Bob Woodman of Bob Woodman Tires. It’s no coincidence that he and the business share a name, and it’s no coincidence that he’s a wealth of knowledge on the subject of the differences between DOT stickies and slicks. He sells and services pretty much anything round, black and rubber that a person can race on. Here’s some of his advice.
Both DOT tires and slicks need a certain amount of tire temperature to work properly. However, slicks are much less forgiving of too little or too much heat. Each manufacturer has a desired operating range for particular tire compounds.
DOT tires tend to be a little more forgiving when cold—especially in autocross compounds, which are designed to work at near-ambient levels of heat. Slicks, on the other hand, don’t work well at all until they reach their minimum operating temp.
Both slicks and DOT tires suffer a lack of grip when overheated, but slicks can also suffer structural damage when they get too hot. This overheating can dramatically shorten the life of the tire or even lead to premature failure.
Proper tire pressures are always critical, but just as with tire temps, slicks are much less forgiving of improper pressures than DOT tires. (You may be seeing a theme here.)
This difference is due partially to how racing slicks are constructed, but also to the fact that a slick-shod car typically sports very high spring and damper rates, making for a very stiff chassis. Tire sidewall deflection then becomes a measurable factor in the total spring rate, making pressure that much more important.
Woodman also cautions that he often sees tire pressures that are too low. As with too much heat, this can lead to disaster. Better to start high and work your way down than the other way around.
Most of what you learned about tire tuning applies to slicks: You’ll still be looking for even tire temps across the tread face when tuning the suspension.
One caveat, though: Some radial-construction slicks produce their best grip when displaying a little more heat on the inside edges. High temps in the center of the tread usually mean tire pressures are too high, while higher temps on one edge or the other are usually the function of wheel camber. This logic applies to both DOT tires and slicks, by the way.
Slicks can be less forgiving on track, too, but not in the ways you might think. Slicks generally have a very high contact patch-to-weight ratio, so lateral grip will certainly be greatly improved.
Because you’re operating at higher cornering levels, however, breakaway can seem that much more abrupt. However, most good modern slicks are rather predictable when kept within their ideal operating temperatures.
Braking performance can actually be the biggest difference between DOT tires and slicks. Despite their additional grip, slicks usually have a much lower weight than similarly sized DOT tires, giving them less rotational inertia and making brake lockup less predictable. Non-ABS cars will have a tendency to flat-spot slicks, especially with an inexperienced driver just making the transition from DOT tires.
Just like DOT tires, tire wear removes rubber from the tread surface of slicks. As a result, the tread gets thinner with use.
Most slicks have molded-in dimples that make tire wear measurable, however. This feature is extremely valuable for monitoring their health.
Like uneven tire temps, uneven wear across the tread face can indicate the need for chassis adjustments. Wear rate can also give you an indication of overall tire health. For example, our light Formula 500 will probably never wear a set of tires down to the cord before they get heat-cycled to the point where they’re no longer effective.
Wear rates should be consistent throughout the competitive life of the tire. If you notice a tire not wearing as much over a given amount of track time, that means it’s getting harder and therefore less sticky.
If all you do is slap on some slicks, you’re not getting nearly the full benefit. Thanks to their highly grippy nature, slicks like a solid, predictable platform. Also, because of that same grip, you’ll be putting loads into your chassis that it never experienced on DOT tires. So chassis preparation becomes highly important, since you want to eliminate as many variables as possible.
In a production-based car, this usually means bushings are your weak link—and they’ll probably cause the most frustration when you’re making the swap. The grip from slicks can deflect OEM bushings far past the point where they can maintain stable alignment. In fact, slicks can push OEM bushings far enough that they risk failure or structural breakdown. A switch to slicks should also mean a switch away from rubber bushings as much as your rules package permits.
Since slicks allow for such high grip levels and quick reaction times, chances are you’ll need to retune the chassis to take full advantage of all this new tire capability. Luckily, the same techniques you used to tune your chassis on DOT tires—measuring tire temps, recording segment times and using data acquisition—transfer over nicely to slicks.
Just understand the importance of a solid platform and develop from there to find the right setup. It may be as simple as changing springs and shock settings, or it may take a lot of tuning. Long story short: This should not be a mid-season change. Make the jump to slicks when you can devote time to proper testing.
Don’t ruin those fancy slicks between races. Improper storage is one of the most significant limiting factors in their shelf life.
Two of the biggest killers of rubber are UV light and ozone. Of course, you can’t do much about these factors at an outdoor event, but away from the track it’s a different story. Slicks should be stored at moderate temperatures and away from UV light and ozone.
Place slicks in thick, black, plastic garbage bags to reduce UV exposure, and keep them away from ozone-generating devices like arc welders and heavy-duty electric motors. Never allow slicks to freeze—if they do, you should probably retire them.
Q: if my tires are running under/over temp as a whole (not an individual tire nor front/rear ends or left/right sides), what do I adjust? Spring rates? Other?
fearlesfil said:Q: if my tires are running under/over temp as a whole (not an individual tire nor front/rear ends or left/right sides), what do I adjust? Spring rates? Other?
A: Depends.
You don't just take A tire temperature, you take at least 3 at the inside, middle and outside edges of the tread. For example, if the inside edge is hotter then you could be running too much camber or too much toe out. Middle too hot? Maybe drop some air pressure. You see where this is going, there are thousands of conditions and fixes.
In reply to fearlesfil :
If the tires temps are even across the tire but over or under temp there are a couple of things specifically when it comes to slicks you can go do. You can go with a harder or softer compound. You can also change width. It is not uncommon for slicks to come in different widths for a given circumference. You can fit a wider or narrower tire without altering the gearing.
This was kind of addressed but the biggest thing I noticed was I lost the ability to steer with the throttle as easily. The window of throttle steer was very narrow which I really didn't care for.
In reply to JG Pasterjak :
The one thing nobody has spoke about yet is what slicks do to the car. Higher cornering speeds increase oil control difficulties. With street tires you might get by with baffles one way traps and an accusump.
Not with racing tires.
Not so much with cars but I've seen people fit slicks to motorcycles and it destroyed the balance. Mainly due to a lack of chassis rigidity.
In reply to frenchyd :
I didn't need baffles or an accusump with my 350Z on slicks. Just a deeper sump. Oil pressure never dropped at CMP or Roebling. However, I do plan to add both when I get the chance.
In reply to 350z247 :
If you have a camera, run a few hard laps with the oil hot and the camera pointed at the oil pressure guage.
Deep dumps and other band aides don't change the physics. Jaguar XKE carries 22 quarts in an 8inch deep sump with baffles. Turn 5 at Road America provides me with seconds of Zero oil pressure from 100 PSI as I stop from 155 to the 30? Mph cornering speed and accelerate up the hill to turn 4 Every lap the oil pressure would drop. Until I finished the race at 20 PSI luckily I had plenty of experience . changing bearings and it didn't take all that long Every race produced the same results. Road Atlanta I was forced to change the bearings between sessions after adding an accusump.
Following that I bit the bullet and put on a dry sump like every racing Jaguar has had since 1954.
Not only do slicks impact oiling control but they also put additional strain/shock loading on drivetrain parts like bearings, axles, differentials, etc.
If you all had proper two stroke cars like those of us running Formula 500s you wouldn't have oil starvation issues.
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