The Finer Points of Flat Track

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American Flat Track has a grassroots feel, even if a winning program is often anything but homegrown. (American Flat Track/)

American Motorcyclist Association (AMA) dirt track racing, as it was during the 1970s and ‘80s, exerted a worldwide influence out of all proportion to its modest popularity at home. US riders with origins in dirt track moved to FIM Grand Prix roadracing and became 500cc world champions on the superpowerful and hard-to-ride two-strokes of that time. The early star of this success was Kenny Roberts, who brought to pavement the dirt track style of steering with the throttle rather than with the front end. After his three 500cc roadracing championships in 1978, ‘79, and ‘80, next up was Freddie Spencer with two such titles. Then came Eddie Lawson, Wayne Rainey, and Kevin Schwantz. American riders won 13 of the 16 championships, 1978–1993.

Related: A New Era Begins for AFT

Kenny Roberts was one of the first American dirt trackers who found success in FIM Grand Prix racing. (Cycle World Archives/)

This was a shock to European sensibilities, so motojournalists quickly attributed this wave of dominance to what American dirt trackers uniquely learned on dirt track mile and half-mile ovals, TT, and short track—to be comfortable and in control on a motorcycle that is sliding at both ends.

When in 2009 spectator tickets for the MotoGP roadraces at Indianapolis Motor Speedway also admitted the holder to the mile dirt track event at Indiana State Fairgrounds there came a collective gasp of excitement and admiration: The MotoGP paddock crowded into the Indy infield and were stunned by the close packs of machines sliding sideways through turns at 90 mph, sometimes inches from track retaining walls. The result was a sudden upsurge in dirt track popularity and training in Europe.

Yet AMA dirt track had suffered slowly declining spectator and rider attendance for years. Program after program to revitalize dirt track was announced—I lost count after six. Why was this spectacular and uniquely American sport not attracting more attention?

The elements that had made dirt track great remained—short, hard-fought races full of surprises plus lots of starts and finishes.

Back in 1972 Harley-Davidson had released its classic race-only (meaning not based on a production model) aluminum XR750, a bike that became so perfectly “shaped like dirt” that it took championships 36 times in the 43 seasons 1972–2015.

Harley-Davidson’s race-only XR75 took 36 championships between 1972 and 2015. (Harley-Davidson/)

Honda developed its own race-only engine—the RS750—and 1984–1987 took four titles in a row. Big Red was stopped by the AMA’s decision to apply intake restrictors. By reducing airflow to the engine, restrictors are the equivalent of running on part-throttle. Honda withdrew its support, yet a private team did take the ‘93 championship with a Honda RS. Always audible in those years were the natural questions—why should dirt track “belong” to Harley? What was “wrong” about Honda winning four titles, while Harley winning 84 percent of the titles was “right”? Doesn’t NASCAR preach brand identification—people come to see the very brand of car they drive win the big race? Yet AMA dirt track carried on as a largely one-brand show.

Complete XRs ceased to be offered but kits of parts could be bought. Building a competitive bike from those parts wasn’t a job for the home crafter—best to phone one of the recognized top builders and get it done right. None of this was cheap.

In the deeper past of the 1950s and ‘60s there had been a place in dirt track for the home builder, but professionally built engines and bikes have always had the edge. Seen any barn jobs run up front lately in the Indianapolis 500?

A fresh trend has been the recent appearance of extensively modified but production-based engines from Yamaha, Kawasaki, and one or two others. Veteran builder/tuner Bill Werner showed that the Harleys, by 2015 being tuned to the outer limits of their reliability, could be beaten. The tool he developed was extensively modified Kawasaki 650s, bored and stroked to 750 and made to run, breathe, and be reliable at 10,000 rpm or beyond. Bryan Smith was 2016 dirt track national champion on such a production-based engine. Achieving success this way required more than the ability to assemble flat-pack furniture. It called for deep dirt track experience plus the skills of machinists, welders, and fabricators.

Related: AFT Acts to Level the Playing Field in Twins Racing

Why was this kind of success so long in coming? The first answer is that the venerable XR had deep reserves of potential which had taken years to fully exploit. By 2015 seven-time national champion Chris Carr’s factory engines were needing a new con-rod big-end set every weekend. And that rear-cylinder exhaust cam gear, now working very hard, could sign off without warning. The second answer is that the US motorcycle market had long focused on big powerful bikes, neglecting midsized engines with potential as dirt track 750s. The Great Recession of 2008 reset the market, providing talented builders with fresh mid-displacement production designs able to breathe deep and live at high rpm.

Above I referred to the Harley XR as being “shaped like dirt.” A lot more than the engine has to be right to win dirt track races. The motorcycle must “hook up”—find traction on the dirt rather than be driven sideways by uncontrolled wheelspin. That calls for smooth, very controllable power, not for lots of power. I’ve been hearing for years about “killer” KTMs, built to 110–120 hp; on the track, they’ve gone sideways because power destroys hookup. And it’s not just the engine killing the hookup. Traction is also destroyed if the chassis is too stiff. And long-wheelbase bikes don’t work—look at the classic Harley XR: Its front and rear wheels are about as close to its engine as they can be (try to achieve that with a Ducati V-twin). A short-wheelbase bike strongly transfers weight to the rear tire during acceleration, keeping the drive wheel gripping rather than slipping. Make the wheelbase longer or the bike lower and the rear tire spins because it lacks the load added by that all-important rearward weight transfer.

Another element in staying hooked up is flywheel effect. If a bump momentarily softens the rear tire’s grip, engine power may so rapidly spin up the tire that its traction is broken and the bike goes sideways, off line. Extra flywheel mass can slow this traction-destroying spin-up. The best place to put this mass is in the engine’s fast-spinning crankshaft, but an alternative is in the much larger-diameter rear wheel. When I asked AFT rider Davis Fisher about the massive 7-7/8-inch steel flywheels of the old XR, he said that on a groove track with good grip they hold you back some (that is, power that could have accelerated the whole motorcycle is consumed instead in spooling up the heavy engine flywheels), but on a loose or slippery track they help you stay hooked up.

When riders have needed even more flywheel effect they’ve gone all the way to inflating the rear tire with water (now prohibited by rule), or by otherwise concentrating mass out at the wheel rim. So important has rear wheel weight become that it is now limited by rule.

Because each track has its peculiarities, and because the surface changes constantly, it’s valuable to be able to vary flywheel effect by having alternative external engine flywheels available in different weights; run the one that gives the best performance—heavy enough to control wheelspin, but not so heavy as to reduce acceleration. The use of external flywheels in this way has a long history, on both dirt and pavement.

Dirt tracks change constantly, and for many reasons. Race programs begin in afternoon sunlight and continue into darkness. The track surface is periodically reworked, and is watered as well. Personalities are important—some track prep people are respected and others not. The experience of established teams and riders gives them strong advantages in reading the track.

The bottom line is that more horsepower—beyond some moderate number in the mid-to-high 90s—just destroys the connection between the tire and the track. The horsepower you bring to the dirt must be dirt-shaped too—the torque curves that have worked well are flat or slightly declining as revs rise. A flat torque delivery gives the rider a controllable constant thrust to work with, rather than the nothing, nothing, blam of a sportbike or drag engine. That makes achieving a workable dirt torque curve an unfamiliar problem to many engine people. Dirt track torque is its own thing.

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