MotoGP: About Fuel Tanks

Would two gas tanks—one within the other—be better than one?

Cal Crutchlow race action photo

Rex and Cromie McCandless, motorcycle designers from Belfast, Northern Ireland, designed the "Featherbed" twin-loop frame for Norton in 1949-50. Its two horizontal top tubes made a perfect place to set the fuel tank. Through the classic 1960s, when riders sat far back "for traction," bikes were given lozenge-shaped "breadloaf" fuel tanks of the kind that gave Honda's famed 250cc Six its long, low look.

Once hired by MV Agusta, Giacomo Agostini learned the value of moving weight forward during acceleration to keep enough load on the front tire to make the bike steer. When the MV Triple appeared, it had a shorter tank and Ago's signature forward-offset clip-on handlebars.

In the 1970s, 750cc and then 500cc two-stroke engines made racebikes even more specialized for acceleration. Engines and riders were moved so far forward by 1980-81 that Suzuki had to notch its 500cc GP bike's radiator.

When four-strokes realized big torque gains from the use of tuned, resonant intake airboxes, a volume conflict arose between fuel tank and airbox. Everything had been so simple once! A flat-bottomed fuel tank above, horizontally mounted carburetors below—room for everything. But the new engines, with their high-flowing downdraft intake systems, needed intake airboxes directly above. For a time, as riders moved forward, fuel tanks grew shorter front to back and taller top to bottom. Carrying less fuel was one possibility, but when MotoGP began in 2002, room had to be found for 24 liters—6.3 gallons.

There wasn't room to make the tank any taller; the tank and airbox right ahead of it were already as tall as the presence of a rider could allow. It couldn't extend farther back, either. That would erase the gains already made by moving riders forward. And so the "Z-shaped" fuel tank was born. The upper, forward part of it occupied any space up high and right behind the intake airbox, and then, it extended downward, growing a "foot" that extended horizontally under the rider's seat. I just saw the latest version of this in the Repsol Honda garage at Indianapolis Motor Speedway; little has changed in shape, but very little fuel is now carried up high. Most of it is down and back under the seat.

Freddie Spencers Honda racebike

We know from what happened in 1984 that big mistakes can be made in fuel placement. That year, Honda slung the fuel of its two-stroke V-Four NSR500 under the engine and routed the fat exhaust pipes up and over, covering them with an insulated dummy "tank." It didn't work; rider Freddie Spencer found it slower in roll than the NS500 Triple on which he'd won the 1983 500cc world title. The NS500 had its fuel in the conventional place: on top. How could this be? Hadn't the wizards of English handling hailed the value of "low c_g_" as the secret of all that is good?

Then, the engineers realized their mistake. Intuitively, they had thought a motorcycle, rolling over to enter a corner, must roll (rotate around a fore-and-aft axis) around its tire footprints. If that were true, speed in roll would increase as major masses were moved nearer that axis (lowered). But if you can find a place on a racetrack where riders come straight at you, brake and then roll into a turn, you can clearly see that they do not roll around their footprints. What you see is the tires counter-steering for the outside of the turn, causing the upper part of the machine to fall to the inside of the turn. Only when the bike is at the desired lean angle do the tires steer into the turn, driving the machine around it.

The axis of roll rotation is therefore above the ground, and, in fact, passes through the machine’s center of mass, just as it does in the case of an airplane. Aha! Now, everything was clear. A conventional tank of fuel was closer to the roll axis of rotation than was a tank of fuel slung beneath the engine. That is why the conventional NS500 Triple was more maneuverable than the “advanced” NSR500 of 1984.

MotoGP has become extremely sophisticated, which is the natural result of having a number of gifted riders seeking some advantage, even a tiny one. Teams have found that very small changes affect rider feel and performance.

This season, Yamaha Tech3 rider Cal Crutchlow has had trouble with excessive forward weight transfer from a full fuel tank, making corner entry iffy. He has lobbied to be given the more recently developed fuel tank used by factory riders Jorge Lorenzo and Valentino Rossi. At Indianapolis, Crutchlow got his wish—and a fresh set of problems. Yes, the tank was definitely better in the first few laps of a race when heavy with fuel. But in the latter third of the race, there was too much weight to the rear, interfering with Crutchlow's style and slowing his laps.

Then, I wondered if the two tanks were just different shapes or if there could be more variables. Clearly, it’s in the rider’s interest to use the highest fuel first (it has the most leverage by which to overload the front tire during braking), and that would happen naturally. Fuel in the lower part of the tank could easily be kept from sloshing forward and up into the emptying upper part by baffling. And maybe, during the last third of a race, fuel could be kept in the forward half of the tank’s “foot” by baffling, as well.

Could you achieve something even better by using an active system? What if we created a tank-within-a-tank, located as close to the rider/bike center of mass as possible, and kept the remaining fuel mainly there by use of a pump and allowed the upper and rear part of the tank to empty? That would minimize the change in front/rear load as a result of fuel burn later in a race.

As long-serving and now-retired Suzuki teamster Stu Shenton said to me a few years ago, “If you can think of it, chances are someone is working on it.”