What Is the Wheelie Limit?

[Hugh Janus]

Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

Here I’m not talking about “show wheelies,” in which the rider yanks upward on the bars while opening the throttle. I’m talking about pure power wheelies, in which the rider smoothly rolls on throttle until the front wheel rises off the pavement. When the front wheel rises, that sets the upper limit on acceleration. Further throttle opening just starts the process of flipping over backward. The rate of acceleration at this point is the wheelie-limited acceleration.

A wheelie occurs when two opposite torques become equal. One torque is that exerted by the mass of bike, fuel, and rider, multiplied times a lever arm whose length is the distance from the center of the rear tire’s footprint, forward to the vertical projection of the center of mass of bike, rider, and fuel onto the pavement. This torque presses the front tire onto the pavement.

The farther the center of gravity is from the pavement the less torque is required to lift the front tire. (Jeff Allen/)

Opposing that is another torque: that of the driving thrust of the rear tire, acting at ground level, acting on a lever arm that is the height of the just-mentioned center of mass. This tends to lift the front tire.

This thrust which barely lifts the front wheel is the maximum thrust that can be used to accelerate the bike.

Thus, to achieve and maintain maximum acceleration, the rider (or some electronic system) must adjust thrust to the amount that barely lifts the front—any more than that and the front wheel lifts faster and faster until the bike flips over backward, breaking an expensive and likely back-ordered taillight lens.

This is why drag bikes accelerate fastest with their front wheels just hovering, barely off the pavement.

It also explains why drag bikes are built as low as possible, for reducing center-of-mass height allows the wheelie limit thrust to be increased. Many a novice drag racer, after wasting entry fees in runs ruined by uncontrollable pop-up wheelies, has lowered the bike by removing spacers from the front fork tubes and fitting either shorter rear shocks or struts. This, by lowering the height of the center of mass (which is the “lever” on which rear tire thrust acts to produce wheelies), increases the maximum thrust that can be used without lifting the front. The lowered bike is easier to control because it’s harder to lift the front. Result: straight, quick runs, free of embarrassing pop-up wheelies.

Suspension travel in motocross has quadrupled over the past 50 years, with the result that today’s MXers are very tall. That raises their center of mass, giving tire thrust greater leverage to wheelie more easily, thereby reducing maximum acceleration. Because nailing the start is so important, someone found a way to temporarily lower the front of a bike just for the start. This was a latch that would hold the front suspension down (large helpers compress the front end until the latch engages). This, by lowering the bike’s center of mass a bit, allows the bike to accelerate harder—without lifting the front wheel—when the gate drops. It stays latched down until the rider unlatches it with a yank on the front brake lever. Ideally, this gives the rider so-equipped the holeshot, leading into the first turn. This latch system was given the name “holeshot device.”

MotoGP teams not only adopted this holeshot device, they did more. They also lowered the rear. If you follow MotoGP at all, you’ve seen photos of bikes with their bellypans nearly scraping the pavement. When lowered front and rear in this way, the bikes are too low to go around corners without grounding, jacking the bike off its tires, and low-siding. Riders need the bike high for corners, low for acceleration and braking.

Related: The Wheelie Is The Enemy

While the lower center of gravity helps riders get off the line, there needs to be traction as well. (MotoGP/)

If it gives an advantage at the start, it should also give an advantage every place on the track where significant hard acceleration takes place. But how can both ends of a bike be repeatedly lowered to make this possible?

There is also a MotoGP rule forbidding the use of electrical or hydraulic means to alter ride height on the track (I’m told the rule was made when the electrical rear ride height adjusters, once used to compensate for fuel burn-off during races, were found to be wearing out a lot more quickly than expected).

What could power MotoGP variable ride height (VRH)?

When Giulio Cesare Carcano (who designed Guzzi’s famed 500cc V-8 of 1955–57) needed to pump fuel from a low tank to a high carburetor, he used swingarm movement to power it. Something similar may drive VRH systems on current MotoGP bikes, possibly charging a pressure accumulator that “retracts the wheels” to lower center-of-mass height for harder acceleration or braking.

Rules to go into effect in 2027 will ban such systems, based upon the fact that because all teams have them, they are no longer an advantage to anyone—just an unnecessary expense. On the other hand, VRH has produced some impressive reductions in lap time.

The VRH systems now available on one or two ADV models serve a different purpose. All types of off-road bikes have become taller with the increase in suspension travel. Because some of these bikes prevent the feet of height-challenged riders from reaching the ground, a factory optional VRH system can lower the bike at low speed or when stationary.

Harley’s Pan America the first ADV with a VRH system. (Kevin Wing/)

Look much further back for the original VRH idea. In California many years ago the lowness of dragsters and other race cars drove a fashion for extreme lowering of street rods (check the height with a feeler gauge). When officers responsible for public safety began to issue tickets to operators of these cars, they responded in improvisational hot-rod fashion—by designing hydraulic systems to lower the car where being cool was important, and raising it where police observation could be expected. Lowering such a VRH-equipped car until it struck streams of sparks from the pavement was called “laying a scrape.”

Source

[Buckster]

This is really useful information for the sad cunts on here.

[Six30]

Harley riders .. move along , nothing to see here …

no chance of the front wheel coming off the ground.

[Buckster]

6 hours ago, Six30 said:

Harley riders .. move along , nothing to see here …

no chance of the front wheel coming off the ground.

There is literally a picture of a Harley in the article.