THE RUBBER MEETS THE ROAD
Choosing the Right Enduro Tire Setup
By John Copeland
Walk through the pits at any enduro event and you'll see an astounding variety of tire sizes and compounds. And it's not just big tires on shifters and skinny ones on Yamahas, there's a lot of variation between otherwise identical karts. How come? I thought you'd never ask. Like most things, to reach the right answers, you have to start by asking the right questions.
First of all, what is it that you want an enduro tire to do? Ideally you want it to handle all the acceleration, braking, and cornering forces your kart and engine can deliver. But if that was all there was to it, you'd just choose the biggest, softest rubber you could buy and that would be that. Of course, we know that's not right. You also have to ask "What is it that I don't want an enduro tire to do?" Quite simply, you don't want it to do anything to hurt your performance, or to slow you down (other than under braking). That's where those big, soft biters can get you. Let's look at the things you want (or don't want) one at a time.
With the exception of 125cc and 250cc gearbox karts, no enduro karts or sprint sit-up karts on enduro tracks generate enough rear axle torque to spin the tires, even geared for the shortest enduro tracks. If acceleration traction were all we were concerned with, hard compound 4.50s would be plenty of tire 95% of the time. In fact, wider, softer tires can actually hurt acceleration. When a tire rolls on the track it actually consumes energy (in this case horsepower from the engine) in two ways. First of all, the weight of the kart compresses the tires slightly to create the flat "footprint" of the tire on the surface. In doing so it deforms the tire at the leading edge of that contract patch and that takes energy. The wider the tire, the wider the deformation, and the more energy consumed. Likewise, the softer the inflation pressure, the larger the contact patch and hence, a greater angle of deflection and more energy consumed. So, to begin with, we are losing horsepower in the deflection of the tread-face and sidewalls of the tire. Secondly, the softer the rubber compound, the more mechanical and chemical adhesion there is between the tire contact patch and the track surface. The soft rubber actually conforms to the irregularities in the track surface and provides mechanical traction. And, once it's up to operating temperature, the rubber chemically bonds momentarily to the track surface like glue. In both these cases, the wider and/or softer compounded the tire is, the greater the energy loss as it rolls down the track.
But what about braking? The issue here is a little more complicated. With today's advanced braking systems it is well within the karter's capability to lock up the rear wheels (and the fronts too with 4-wheel brakes) with moderate to high pedal pressure. Of course, braking just short of lock-up provides the maximum deceleration and still maintains driver control, so that should be the goal. Bigger, softer rubber must be better here, right? Not necessarily. As we go from a skinny tire to a wider one, the footprint gets bigger and, assuming the weight of the kart doesn't change, the pressure, in pounds per square inch, that the tire applies to the track surface decreases. You end up trading contact patch size for contact patch pressure. On karts with rear brakes only, the larger contact patch wins. When you apply the brakes the kart transfers weight to the front, decreasing contact patch pressure on the rear tires. Wider tires give the driver greater "feel" as the limit of traction is approached, largely because the wider area of tread face deflection gives the driver more feedback as it unloads and the contact patch shrinks. It gives a more gentle "breakaway" effect that translates to better control under braking. But when you add front brakes to the equation, the increased braking power available means that the driver is able to decelerate more quickly than with rear brakes only and, in most cases, the deceleration forces exceed the traction of the wider rears. You're in a different realm of braking performance now, and, as you make those front tires assume some (actually over 50%) of the braking responsibilities, the weight transfer from rear to front is greater. The lower contact patch pressure of the wider tires simply is overwhelmed by the loss of pressure as the weight moves to the front. Here a narrower tire, with it's correspondingly higher contact pressure, helps balance the kart better under braking. But does adding front brakes mean that the front tire configuration needs to change too? Of course! Sending all that weight to the front under braking dramatically increases the contact patch pressure on the front tires. Where with a rear brake only kart the fronts are just along for the ride until you turn, now you're asking them to assume the lion's share of the braking chores. The difference, however, is that with a 4.50 front tire, you may already have enough rubber to handle the braking responsibilities. Individual differences are more of a factor here. If you are running 4 wheel brakes in a heavy class, or one of the faster classes, you may find that a 5.00 or 5.50 front will give you better control under braking and more braking power. But don't forget that you're probably hurting acceleration. (See above) Only the stopwatch and the seat of your pants will know for sure. Remember, maximum braking power is rarely called for in enduro racing, where keeping speed and momentum up are the paramount concerns.
Then we have the question of cornering performance. In many ways tire cornering performance is ruled by the same forces that effect braking performance. Again, weight transfer is a critical factor, but now, instead of that transfer being front to back, it's lateral. But, just to make things more interesting, it's not just side to side, it moves around. When you enter a corner, even a flat-out turn with no braking required, you first load the outside front tire. Of course, this means a corresponding unloading of the inside rear tire. As the turn stabilizes the balance of front to back weight should return to normal. But as long as you are applying steering input in the direction of the turn, the outside tires will be carrying the bulk of the side load. Ideally, during the central portion of the turn, you should be able to bring the steering back to center for a perfect 4 wheel drift. This will tend to bring the side to side weight distribution back closer to normal, although still a little loaded on the outside. At the exit of the turn, as the kart begins to track down the next straight, the final phase sees the unloading of the outside front and more loading on the outside rear before the kart stabilizes on the straight again. Once the kart is totally straight after exiting the turn, the loading returns to the normal acceleration mode.
OK, OK, so the weight distribution of the kart moves around some during cornering. What does that mean in terms of tires? As the weight distribution of the kart changes, so do the deflection angles of the tires. Technically the difference between the direction a tire is pointed and the direction it is actually traveling is referred to as the slip angle. When a tire is subjected to a sideways load, the sidewall deflects in somewhat the same manner as the tread face deflects while the tire rolls. In fact, since you're still (hopefully) moving forward while your cornering, both happen at the same time. But because the cross section of the tires is not round, but rather square-ish, this deflection of the sidewall tends to reduce the size of the contact patch by trying to pick up the inside portion of it. Try that corner a bit too fast, or find yourself in the wrong place and try to "pinch" the kart tighter into the turn, and the combination of too much weight transfer and too much sidewall deflection will take the slip angle too far and around you go. Just like with braking, bigger and softer will help here, but the price you pay in performance is much greater. Today's sticky tires are more than a match for all but the stiffest chassis, and scrubbing them sideways will seriously eat up horsepower. Even the highly desirable 4 wheel drift, while the least power robbing of all cornering techniques, demands that the kart develop a moderate slip angle and "float" across the track.
So does that mean that we're back to skinny, hard tires? Only if you're prepared to drive within the limits they impose. Just as we discussed about braking, the wider, softer tires generate a more gentle "breakaway" during cornering. They eat up horsepower doing it, but the margin of safety is greater. Those skinny, hard tires will make the kart very "light on it's feet" but you'll get little warning when it's about to get away from you. On the other extreme, too much tire can cause the chassis to "bind up", twisting against the axle bearings and drive train. That horrible sensation that the longer you're in a turn the slower you're going is a good indication that you may be placing too high a value on "grip" and going slower for it.
One last thing before we summarize; you'll have to decide if you want your driving style to dictate your tire choices, or the other way around. If you're compelled to "stuff" the kart as deeply into the corners as it will stand, jump on the binders hard and ride them into the turn, or to throw the kart through the turns like Sammy Swindel, mount up your big "meats" and go at it. But successful enduro driving requires finesse. It can be aggressive, but, to be really quick, you have to maintain momentum, maximize smoothness, and use every bit of horsepower you have to get you on down the track. That means that sometimes you'll find yourself at tracks that require that you "take care" of your tires. A fast entrance to a long, slow turn, particularly if you have front brakes, can be murder on the outside front tire. Anyone who has every "overcooked" their tires knows what a horrible feeling it is to have the kart slide right out from under you, or to push hopelessly toward the outside. A slower entrance with a later apex will help that tire last longer and perform better. It's a common misconception that harder compounds run cooler. It's not necessarily so. Friction is what builds heat. That "slip angle" we talked about before is generating lots of friction, and that generates lots of heat. It may sound strange, but hard tires will heat up faster on a cold track that soft ones. They'll also overheat faster, particularly in front. Choose wisely.
So, to wrap this up, run the narrowest, hardest tires your skill and nerves will allow, right? Not necessarily. Choose the tires that are best suited to maximize your performance. If you determine that there is some disparity between what seems to be fastest and what is most comfortable to drive, only you can decide which is more important. I'm not suggesting that anyone race a setup that is so spooky to drive that they can't concentrate on what they're doing. All I'm saying is that, if you expect your kart to ride like an Oldsmobile, expect to get thumped by the competitors willing to trade some of that security for more performance. It's been my experience that most enduro drivers run alot more tire width, and in softer compounds, than they really need. They choose, consciously or not, to give away performance for the sake of , what? In some cases, they've tried running less tire, driven with exactly the same driving style, and been unhappy with the results. Running narrower, harder tires on an enduro or sit-up sprinter will pay off. You have to be willing to commit to the testing time and be willing to make the necessary adjustments in setup and driving to take advantage of it. So go ahead, dare to try something a little different. See if you don't agree that, at least in this case, less can sometimes be more.