Top-1 Ack Attack Motorcycle Streamliner Aiming For 400 Mph In Bolivia

High-altitude Salar de Uyuni salt flat to allow 10 miles of acceleration

cockpit of Top-1 Ack Attack Motorcycle Streamliner
Top-1 Ack Attack Motorcycle StreamlinerCourtesy of Ack Attack

We've all heard about the problems of the Bonneville Salt Flats. Sometimes the course is under water and other times it's too rough for the highest speeds, but biggest problem of all is that its seven-mile length offers marginal room for spin-free acceleration and controlled deceleration. Don't hit those mountains! That's why the Ack Attack team chose "the best salt in the world," Bolivia's 10,000-square-mile Salar de Uyuni, for this year's record attempt.

Much is made of drag coefficient and engine power, but maybe the biggest problem of all is acceleration, because the salt surface limits tire grip to maybe 40 percent of what would be available on asphalt. Ack Attack’s wind-tunnel studies show its aero drag to be around 400 pounds at 400 mph (587 feet/second). That means aero will consume 400 x 587 = 235,000 foot-pounds/second of power, which translates to 427 horsepower. The rest of the machine’s nearly 1000 bhp is there for acceleration and rolling resistance (ready for action, the streamliner will weigh about 2200 pounds).

Mike Akatiff, the very experienced builder, has wisely provided enough engine to make the desired power without pushing too hard: a pair of 1299cc Suzuki Hayabusa engines turbocharged to 900-1000 hp total. This is 365 hp per liter—only 60 percent of the 600 hp/liter at which BMW's "Megatron" turbo Formula 1 engine of the late 1980s raced. It is also milder than the 450-500 hp/liter usual for 'busa-powered turbo bikes running for records at Maxton and other decommissioned US air bases.

Engine peak rpm is 10,500, or a (today) conservative piston speed of 4300 feet/minute. On the other hand, Ack Attack’s engines will be on continuous substantial throttle for around three minutes, something no circuit racing engine ever experiences. Testifying to the value of conservatism, this team states it has made 25 runs at more than 300 mph without a catastrophic engine failure. That is not only impressive, it is also essential. High speed on the salt throws surprise variables at every team that tries for a record. Handling them is every bit as important as horsepower. Conservative is good.

salt flats in bolivia
Salar de Uyuni salt flat in BoliviaCourtesy of Ack Attack

Why is the team confident it can reach 400 mph? They have already gone 376.363 mph on September 25, 2010, so they need another 25-plus mph to hit the big number. Aside from the effect of a longer acceleration distance, a 12 percent drag reduction is expected from Salar de Uyuni’s 12,000-foot altitude. Drag is directly proportional to air density. Also, in the past the streamliner’s tapering tail has been incomplete. A pair of “doors” located under the drag chute tube at the rear have “for various reasons,” according to Akatiff, not been able to be used during previous record runs.

People tend to think of streamlining as a pointed nose and not much else, but the real task is not just to push the air aside, but also to put it back where it was without giving energy to it. This explains why the tapered tail is so important—and the doors that complete it at the rear. After the nose of the vehicle pushes the air aside, the tapering tail is what puts the air back together, without trailing a storm of whirling vortices that carry away energy. Air pressure, acting evenly on this taper, is like a hand squeezing a rounded wet bar of soap; it produces thrust.

Think of the twin J-58 turbo-ramjet engines of the famed SR-71 spy plane: In every description of this aircraft, it is gleefully pointed out that most of its thrust comes not from the engines' exhaust momentum, but from the pressure of air as it slows inside their gradually widening intake diffusers. This is conceptually the tapered tail of Ack Attack, turned inside-out. That air pressure, acting at an angle against the inside surfaces of those diffusers, generates most of SR-71's thrust. What do the engines do, then? They are there mostly to pump the diffusers. It seems crazy, but that's how it works.

The wind tunnel showed that putting Ack Attack’s aft doors in place reduced drag by 40 pounds—a nine percent reduction. Tunnel figures showed that each percent that drag can be reduced has the potential to become four extra mph of speed, so there’s the potential for 9 x 4 = 36 more mph.

sunset over the bolivian salt flats
Sunset over the Salar de Uyuni salt flatCourtes of Ack Attack

The combination of a longer acceleration distance (which means the pilot, Rocky Robinson, doesn’t have to push rear tire traction as hard), the reduction in air density, and the hoped-for drag reduction if the doors can be used should handily put the machine through the timers at more than 400 mph. We’ll cross our fingers and hope the surprise variables behave nicely.

An interesting sidelight on this operation is that Bolivia’s extensive salars are estimated to contain as much as 60 percent of the world’s lithium salts, essential for the manufacture of lithium-ion batteries for phones, laptops, and electric cars. The salts are believed to have been leached out of volcanic flows by rain and then trapped in landlocked high lake basins. Evaporation leaves a salt surface so flat that orbiting satellites use it to calibrate their systems.