(by Paul Clipper/Trail Rider mag,

from the monthly Art of Trailriding column)

In order to go fast on a bike, you have to have a fast bike. Stupid statement, huh? Well, bear with us. What we’re talking about is reaction time, not top speed. Think of an enduro bike with a real heavy flywheel, a bike that just pulls you out of the turns, not fast, just grinds you out into the next corner. You can sit down and ride a bike like this all day, just cruising around. What do you have to do if you want to go fast, much faster? Racing speed. Well, on this slow-revving bike you’d have to train yourself to ride at a higher engine rpm, you’d have to spin it faster. If you can’t get enough out of it stock, maybe you’d have to lower the flywheel weight and make the bike physically rev faster. In the old days, cutting down the flywheel— shaving weight from it on a lathe—was a popular thing to do to the old, heavy flywheels on our bikes. It was especially handy on four-strokes.

The key to going fast is having something there when you punch it. Mark Thompson at ISDE.

Doesn’t that make the bike harder to ride? The answer is yes and no. For a rank beginner, it would turn any mellow enduro bike into a handful, a real nasty little beast to ride. But for an experienced rider it would make the bike much easier to ride, and this is why: if you’re trying to go fast, the bike has to react immediately to input; and the faster it reacts, the faster you can go.

Now, in spite of what we just said, don’t blame your slowrevving bike on the flywheel weight. A lot of time and engineering goes into modern bikes, and chances are if you have a bike that’s just a few years old that never seemed to bust out of the hole, it is usually a tuning issue slowing it down. Improper carb jetting down low is a common problem. Having a faulty ignition is an uncommon problem (usually they either work or they don’t), but it does happen occasionally. Recently Randy Hawkins had a severe problem with reaction time on his race bike and it turned out to be a plugged spark arrestor.

When you open the throttle on a bike with a problem, the carb moans as the air velocity increases and you can feel the engine labor to overcome the initial weight of the ignition and other internal engine parts. When or if the engine develops enough inertia to overcome all this resistance to spinning, the revs climb rapidly and everything happens real quickly, just like you want it. The problem comes during that rev lag, while you’re waiting for it to “hit.”

What can happen? Well, think about this: when you come flying around a blind turn and there’s a tree splitting the trail, what do you do? You aim for the “best” side of the tree and hit the throttle. When the rear wheel reacts, the bike zaps forward, reacting to the turn input, and you shoot past the tree. Neat and clean; and then you go off to solve the next problem that comes up.

Now, what happens if you hit the throttle and nothing happens? You were expecting a surge forward, but all you got was a dull moan out of the carb. Chances are, you’ll either hit the tree or just manage to wallow around it, and then, at some point past the tree, the bike will come on the pipe and finally hook up. If you’re lucky.

And it’s all because of that momentary lag before the power develops. It could happen on any bike—a four-stroke with a “hitch” in the carburetion, a bike that’s jetted all wrong, or even a sticking throttle. The reason it’s so important is because of the speed you’re going and the limits of your reaction time. I read one time that the most alert, jazzed-up, wound-up, in-tune person in the world can only react as fast as one-thirtieth of a second to any sort of stimulation. That’s it; and that’s with training! You see the tree, for example, and the fastest you can crank on the bars and throttle is a thirtieth of a second later.

Okay. An enduro is a real intense situation, and if you’re trying to win your class, it’s feasible that your reaction time could be close to the limit. But, for the sake of argument, let’s say that we’re as slow as slugs, and we’re only going to react to seeing that tree in a fifteenth of a second; twice as long as a thirtieth.

How fast are you going in the woods? The enduro is a 24 mph, but maybe in that turn you’re going slower, maybe 20, or 18, but let’s say you’re going as slow as 15 mph. In the fifteenth of a second that it takes you to react, you’ve moved a foot and a half forward, because 15 mph is 22 feet per second, and a fifteenth of that is 1.5 feet. If cranking the throttle makes the bike react instantly, you’re all right, you got past the tree. But, if it takes, oh, say a half a second to get the back wheel spinning, you’ve just traveled 11 feet, and somewhere in there you either hit a tree or got wildly out of shape trying to avoid it!

Let’s try another speed. What about all those times when the trail opens up in a turn, and you’re 90 percent sure that there’s nothing around the corner, so you wick it on hard in third gear. How fast are you going? It’s hard to tell in a quick burst of horsepower, but how about we say that for a few seconds there you’re doing 30 mph. If you have to get on the brakes right away because there’s a deep stream right in front of you, or a fallen rider, if it takes 1/15 of a second you’re going to go three feet before you can even get off the gas. A half-second would equal the vast distance of 22 feet, and if it took you that long you either rammed a guy or are going swimming!

Man, it gets scary! Think about 40 mph, on a piece of two-track. No! Better yet, think about the time that your enduro computer recorded a top speed of 70 mph in a race. There’s only a few powerlines that you can do that on, or the sand roads down in South Jersey, but if you’re at the top of your form, your bike will go 3.4 feet in the 1/30 of a second reaction time. At a 1/15 of a second you’ll have gone 6.8 feet, and in half a second, which is not an awful long period of time, your bike travels 51.3 feet!

The moral of the story is that a mellow bike is great to learn on, but the faster you get the more you’re going to need a bike that reacts fast as well. Like the hermit crab, who won’t grow unless he gets a bigger shell, you won’t get faster unless you ride a faster bike.