THE NEED FOR SPEED
A SHORT-TRACKER TRIES ENDURO RACING
By John Copeland
All of us involved in karting share a common bond, we love to go fast. It's the combination of lots of things; the competition, the companionship, the personal challenge. But it's all wrapped up in speed, and the faster the better. So, what if you've spent your karting career thus far testing the limits at the asphalt sprint track nearby? Or probing the mysteries of ever-changing dirt? What if you wanted to check out this enduro thing? Do you have to buy all new stuff? Give up what you've been doing? What if you just want to check it out?
Well, the good news is, for a lot less effort and way less money than you probably expected, you can set your rig up, cruise down to the next big enduro event in your area, and take a close-up, first-hand look at karting in a somewhat faster lane. That's just what a customer of ours wanted to do, and I thought I'd take you along for the ride.
Meet Joe. Joe races pretty much every week at the FOP Park Sprint Track in Muncie, Indiana. He also mixes in a few street races, and finished 2nd in this year's Purdue Grand Prix (NKN June 1997). Joe's seen the pictures of enduro races in various karting magazines, and seen a laydown kart or two at the shop. But he's never been to an enduro event. He's heard us talk about going off to Daytona or Road America, and he's always been curious. But he's also been a starving college student and he could barely afford to keep his sprinter together. Finally, (with a little coaxing) he's admitted that he'd really like to give the enduro thing a try. There's a Midwest Series race coming up at Putnam Park Road Course in a few weeks. But, gee, what's it going to take to put his Yamaha-powered sprinter on a big track? Let's take a look.
The most obvious change we need to make is gearing. A quick check with someone from the club staging the upcoming event gives us a good starting point for the gear ratio. Instead of running ratios in the 8:1 range like most 2 cycler's sprint with, we'll need to be closer to 5:1. (!) That'll mean a different clutch drum for sure. The trick is to pick a clutch drum size that puts us in a position to use the axle gears that Joe already has. If he's already got most of the gears between 70 and 80 teeth, that means if we swap Joe's 9 tooth clutch drum for a 15 tooth drum, we'll be right in the middle of the range he already has. Of course, he could drop a big chunk of cash and buy belt-drive gears, a belt, and a belt-drive clutch drum. But, at this point, the idea is to get out there for the least expense and test the water, so to speak.
We'll need to make a effort to improve Joe's aerodynamics a bit too, if he's going to have a chance to run well when he gets out on a big track. This can be as elaborate and expensive as you want it to be, but it doesn't have to be. The single most important thing to do to make a sprinter more aerodynamic is to get the driver's profile as low as possible. Almost every enduro group has rules that limit how far you can go in this, which should be a clue as to how important it really is. We don't want to go out and buy a special sprint-enduro seat, and we don't want to do anything that we can't easily switch back so Joe can go back to racing at Muncie. So in this case we're going to lay the seat back to the limit of the rules. The Midwest Series says that the top of the back of the seat has to be at least 12" off the ground, and that the rearmost part of the seat cannot extend behind the rear axle. We'll unbolt Joe's seat, but not before marking the holes it was mounted in with a white tire paint stick. We'll mark where the seat mounts were too, using a permanent felt marker on the frame and seat mounts. That way when it's time to put the kart back the way is was, we can do it with minimum hassle. As we lay the seat down, we need to pay attention to what that does to Joe's driving position. Laying him down means moving him farther away from the steering wheel, and moving his weight toward the rear. To compensate, we'll move the seat itself as far forward as possible. If we can't get Joe close enough to the steering wheel to be comfortable, he won't enjoy the ride. Moving the seat forward and down may mean that the steering wheel is too high now. That's an easy adjustment to make. Just remember to check to toe-in when you're done. Now the basics are done.
Like most karters now days, Joe runs a nose cone and driver fairing. Those will pay big dividends on the enduro track, where top speed is generally aerodynamically limited. Just like with moving the seat, the goal here is to punch the smallest possible hole in the air. Cleaning up the air-flow over the driver is the biggest part of this. We'll want to make sure that the nose cone and driver fairing on Joe's kart work together to smooth the flow around him. Just looking at the kart from the front and the sides will tell us if we've got the bodywork right. Imagine the path the air will take as it hits the front of the kart. Will the natural flow take it around and over the driver? If not, we need to adjust the locations and mountings to improve it. This is also a good time to make sure that the mountings are strong enough to take the extended periods of high speed vibrations. We don't want anything falling off.
Finally, we need to pay a little extra attention to Joe's tires. The enduro track won't subject Joe's kart to the kind of intense, high-g side loads he's used to getting at the sprint track. But he'll probably be spinning those tires faster, and for a longer period of time, than ever before. If he hasn't bothered to balance his wheels for the sprint track, it's pretty important that they get a good balance job before he ventures onto the big track. An out of balance tire that would barely be noticeable on a short track will become an annoying (and potentially dangerous) vibration at the kinds of speeds he'll see at an enduro event. So we need to spend a few minutes with the wheel balancer to make sure this problem doesn't pop up and bite him.
The last thing Joe needs to do is check with his engine builder about recommended changes in his clutch and pipe set-up. For the most part, the engine set-up will be pretty much the same as Joe runs on the sprint track. Joe runs a Yamaha and, because he'll be pulling so much taller a gear, it would be wise to richen up the low end some. Remember, even with the clutch slipped up a little higher, the engine is going to be lugging off the slower turns much more than it ever would on a short track. That translates into more demand for fuel on the low end to keep from sticking. At the other end of the straights, Joe's Yamaha will be probably too rich, so he may want to lean down the high end needle some. I'd suggest, however, that he wait to do that until he's on the track and can evaluate what the engine seems to need on top end. If Joe was running a Briggs, he'd probably need to change jets to pour more fuel to it down those long straights. I mentioned the clutch before. Long tracks tend to favor higher engagement speeds on the clutch, at least with a Yamaha with a pipe. But Joe's engine clutch won't last long if he tries to slip it much above 10,400. In the interest of making sure Joe gets to spend his first time at the enduro track actually driving, instead of rebuilding a torched clutch, we'll leave it set where it usually is.
Finally, if the track is long on straights and short on tight turns, he may want to shorten up the pipe a bit to take advantage of more top end. But he'll have to keep an eye on the heat, since shorter pipe lengths tend to yield higher head temps. So now he's ready to head off and explore the high speed world of enduro racing. So far
all he's spent is the price of a clutch drum and a couple of hours. He'll need to plan on dropping another $100 a day or so for entry fees, fuel and oil, pit passes, track food, etc. Other expenses will depend on whether or not he's planning to stay overnight, or head home at the end of the day.
All in all, not really that expensive; not when you consider how much fun it can be. When he gets to the track he'll be amazed at how many other sprinters are there. There'll be 4 cyclers, and Yamahas with can mufflers, US 820s, Controlleds, and Yamahas with pipes like his. And once he's tried running the big track, he'll want to do it again.
Next month we'll follow Joe to Putnam Park and watch as he feeds the need for speed! See you then.