Mere days remain until the 18th annual NASA Great Moonbuggy Race! As we continue our exploration of moonbuggy mechanics, we turn to the drivetrain. And immediately Dennis Gallagher, our race expert and NASA astrophysicist, puts a weary hand over his eyes.
“Chains,” he says. It sounds like a curse word.
What it is, What it Does, What a Ride
What constitutes the drivetrain? Get on a bicycle or dirt bike and look down, Gallagher says. The drivetrain is basically everything between the rider’s feet and the road, including pedals, gearing, axles, tires and wheels.
This Huntsville Center for Technology buggy puts the pedals high and forward
for extra pushing power… (MSFC/David Higginbotham)
Typically, moonbuggy drivetrains come in two flavors, says Tom Hancock, the chair of the American Institute of Aeronautics and Astronautics Alabama/Mississippi section and a longtime member of the Great Moonbuggy Race planning committee. There’s the fancier, direct-drive system — which employs a driveshaft and transmission to transmit torque and rotation — or a simple series of gears.
The drivetrain system transmits human power from the riders’ madly pedaling legs to the drive wheels of the vehicle. Most also have a gearing system to convert speed and torque.
Hancock has seen some pretty fancy rigs over the years, but says most dependable buggies use reliable gearing originally developed for dirt bikes or heavier off-road riders. “You’ll see stripped gears, sure,” he says. “But-“
“Chains,” Gallagher repeats.
His lament is a common one on the track. Chains can spell doom out here, where drivers see the jut and angle of obstacles ahead and over-exert themselves. In recent years, one of the most common sounds on the course (after the fire-whistle wail of the starter’s horn, the whoops of the kids from Lima and Middle Tennessee State, and the boogie-inspiring percussion of the Puerto Rico teams’ drums) has been the grinding SNAP! of an over-tensioned chain breaking on the very first mound of thick, wet gravel. And if the chain hasn’t snapped outright, chances are it was too loose… and slipped off the gear teeth. Goodbye, momentum.
A Delicate Balance of Speed and Torque
“You’re putting a lot of stress on this system,” Gallagher says. “The buggy is human-powered, but it’s really not like a bicycle at all. A rider’s legs can put upward of 300 pounds of force on each pedal with each stroke.” Add a second rider, a lot of adrenaline and awesome prizes and bragging rights at the end of the track… and now there’s nearly 1/2-ton of force hitting those chains on every obstacle.
“Standard bike chains cannot put up with that kind of force,” Gallagher says.
Those guys in the bike-racing movie BREAKING AWAY were pretty amazing on a
flat, circular track. We’re pretty sure our course and our racers would leave them
spitting dust. (MSFC/David Higginbotham)
“If you’re using chains, make sure to use lots of tensioners and chain guards, particularly around the bottom of the vehicle where you contact the terrain,” Hancock says. “Last year, the Huntsville Center for Technology buggies had three or four different tensioners and they really paid off.”
Gallagher concurs. He also emphasizes caution to those teams who employ derailleur gears — a kind of variable-ratio transmission system that includes a chain and sprocket system to change gears and widen the range of torque and power that can be applied.
But being able to change gears to increase speed shouldn’t be the ultimate goal, he says. Because of all the turns and obstacles, the fastest buggies on the course usually only clock about 15 mph on straight sections.
“It’s a challenge,” he says. “On straightaways, you want gearing conducive to speed. On harder sections, you want gearing conducive to torque.” But he believes the best vehicles maintain a fairly steady top speed across the course, without a lot of gear changes.
For others, their delicate drivetrain elements were never designed to take the kind of punishment doled out by the NASA Great Moonbuggy Race.
“Think beyond a regular bicycle,” he says. “I like planetary gears, gearboxes designed specifically and fabricated by teams aware of the requirements of the course. Do it yourself!”
Hancock agrees, and offers this key pointer for teams building their own drivetrain — or working on nearly any other element of the buggy. “Modular is key,” he says. “Come up with a system where the hardware is nearly all modular, so you can easily and quickly pop out broken or damaged parts and pop in replacements.”
Put this, as they say, to the metal. But know the strength of said metal when it meets
Pedals, Wheels and Tires
Rounding out the drivetrain portion of our buggy tour are the points where the rider meets the ride and the rubber meets the road: pedals, wheels and tires.
Don’t shirk on good pedals, Hancock urges. Find quality pedals that have locks on them, the kind used by professional racers, to ensure your shoes don’t slip off. “Velcro won’t work. Toe hooks won’t work,” he says. “Your feet go through 40-60 degrees of motion and they’ll just come off. You’ve gotta anchor the feet securely to maintain contact with the pedals.”
And drivers need to be mindful of their choice of footware, he says. “Even if you spend money on those bike pedals with the little cleats, if you’re wearing smooth-soled shoes… what was the point?”
Buggy wheels and tires need to be strong and durable. “Look at track bikes, ATV wheels,” Gallagher suggests. “They don’t have to be tall — just wide.”
Tall wheels can present their own brand of problems, Hancock agrees. “There’s a tradeoff. On the undulating terrain found on the course, if a wheel is too big, it can end up spanning a small crater or rille and get stuck. If it’s too small, it can’t get up out of a gap. Wide wheels work best. Anyone who shows up with regular bicycle tires on their buggy is going to lose a wheel.”
This never happened to those BREAKING AWAY kids… (MSFC/David Higginbotham)
Next time, we’ll assess brakes, steering and seats, and talk about those clever, clever buggy accessories…