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Backmarker: The Fastest Bikes on Dirt

Thursday, February 6, 2014
Kenny Tolbert shows one of the restrictor plates that was in use in Chris Carrs Harley-Davidson XR750  Prescott  AZ  2010 .
Kenny Tolbert shows one of the restrictor plates that was in use in Chris Carr’s Harley-Davidson XR750 (Prescott, AZ, 2010).
I get the feeling that almost everyone is happy that AMA Pro Racing Flat Track has eliminated all of the restrictor plate rules in the Expert Twins class. Until last season, there was a bewildering set of rules determining which bikes had to run the restrictor plates. (If you’ve seen a restrictor plate, it looks like a big washer and sits in the inlet tract between the carb or throttle body and the motor.)

Most of the riders at any National ‘Twins’ race are on Harley-Davidson XR750s, which are motorcycles that have hardly changed since the 1970s. Depending on the track, XRs ran restrictor plates with 32 or 33mm openings. Production-based motors of under 750cc were unrestricted, while production-based motors from 751-1200cc ran larger 38mm restrictor plates.

Confused? The fans weren’t; most had no idea that restrictor plates were even in use.

My reading of recent history is that the restrictor plates came in at Harley-Davidson’s urging, back in the ‘80s when the Honda RS750 dominated. Although the Honda was superficially similar to Harley’s XR750, the Honda had overhead cams and four-valve heads; restrictor plates hurt the Honda’s performance more than the Harley’s. (Years ago, Jerry Griffith—who developed the RS750 motor for Honda—told me, “They were fire-breathing sumbitches!.. [The AMA] let us win a couple of years, then they restricted us.”)

Fast forward 30 years or so to recent seasons. There haven’t been any RS750s in pro flat track lately, but there are (again) a number of competitive non-Harleys out there. Air-cooled Ducatis and over-bored Kawasaki 650 Twins have won races, and the Triumph Twins are threatening to do so.

The original goal had been to level the playing field, so to speak. But ironically, the rise of bikes like the Ducati (38mm restrictor on all tracks) and Kawasaki (unrestricted) put the Harley-Davidson XR750 at a disadvantage, limited as they were to 32 or 33mm restrictors. The problem: to match the apparent power of the ‘metric’ bikes, the XRs had to spin at up to 9000 rpm.

The XR’s an old motor, full of heavy, expensive, fragile parts. To run with the Ducati or Kawi on the mile tracks it had to be tuned to the very limit of reliability. Harley teams had to perform a costly rebuild after every such event.

That was frustrating, but the truth was that no tuner wanted to be limited by intake restrictions or flow. Flow is the essence of hot rodding. If you’re an old school engine building, getting the engine to breathe right is what it’s all about.

A great XR750 tuner like Kenny Tolbert may have tolerated restrictor plates if he thought they hurt his motors less than they hurt someone else’s motors, but any tuner worth his salt would rather build the best unrestricted motor. Even Bill Gately, who builds the Latus team’s Triumphs (which were ‘only’ restricted to 38mm last year) was really pleased by the new rules. “Now,” he told me, “we’re all limited by the same factor, which is tires.”

The good thing about being limited by tires is that unlike restrictor plates, the tire limitation is visible to fans, and becomes a big part of the show.



The newly de-restricted rules are really being welcomed by the Harley teams. Three seasons back, when Joe Kopp nearly won the title riding the Lloyd Brothers’ Ducati on the big Mile tracks, XR teams grumbled that there was no way they could match the Italian motor’s power. Bryan Smith made them piss and moan about power produced by Bill Werner’s Kawasaki. The truth is that those bikes looked faster than they were; Kopp and Smith blew past the Harleys at the end of the straight alright, but it was mostly because they were waiting longer to close the throttle.

But now, the Harley teams are busy building motors with longer cam duration, that will flow more, turn lower revs, pull taller gearing and hopefully go a lot longer between rebuilds. Will they erase the metric bikes’ apparent “advantage” on the biggest, fastest tracks? Maybe. We’ll know more at Springfield in late May.

The slowest bikes on dirt

Back in the mid-‘80s when the RS750 was giving the XR750 fits on flat tracks, Honda did something it hasn’t done since: it actively marketed a trials bike. I’m talking about the TLR200 ‘Reflex’, which was a street-legal trials motorcycle sold in ’86 and ’87.

Honda was trying to capitalize on a brief ripple of interest in observed trials. An American, Bernie Schreiber, won the World Championship in 1979 and competed at the top level for several more years.

Until the 1980s  the Spanish market was closed to the Japanese motorcycle manufacturers. In order to get access to it  every Japanese brand ended up buying a Spanish company. Honda bought Montesa. Since trials is still a major sport in Spain  Hondas chosen to leave the Montesa brand on its trials bikes.
Until the 1980s, the Spanish market was closed to the Japanese motorcycle manufacturers. In order to get access to it, every Japanese brand ended up buying a Spanish company. Honda bought Montesa. Since trials is still a major sport in Spain, Honda’s chosen to leave the Montesa brand on its trials bikes.
Now, trials is a niche-within-a-niche here in the US, so I was surprised to see American Honda promote a new Montesa trials bike here. The Cota 4RT is powered by a fuel-injected 260cc 4-stroke motor. It’s not a mass-market thing; you need to order it off the company’s web site, for shipment to a dealer near you. Still, it’s cool to see a major brand selling a trials bike at all, for the first time in decades.

The thing is, trials is way, way cooler than most motorcyclists think. (Partly because most motorcyclists don’t think about trials at all.) It’s tough to popularize, because it doesn’t seem that impressive on TV, where you can’t really appreciate the steepness or scale of the obstacles trials riders negotiate.

When I lived on the Isle of Man, they held a round of the World Championship there and let me tell you: if Jesus Christ appeared before you and made a blind man see, and then raised the dead, and then said, “Now, I’m going to ride my motorcycle off that cliff, turn at the bottom, and climb back up over that boulder without touching my feet to the ground,” you’d say, “Bringing that dead guy back to life was impressive, but there’s no way even you can do that on a motorcycle.”
Seriously. Top trials riders do things that are simply impossible.

The very first time I ever rode a trials bike, even I proved capable of getting it about six feet up a vertical tree trunk. Albeit by accident; that’s another story.

I don’t know what Honda’s plans are, in terms of Montesa sales volumes. But I really believe that with a solid, long-term promotional plan, recreational trials riding could make a little comeback.

It’s a great training and practice regime for any other kind of riding, from road racing to enduro. (Ask Kevin Schwantz, who started out as a trials rider, and still uses it as a form of practice.)

You can get away with practicing on private land where neighbors would never tolerate a motocross track. In an age of increasing restrictions on places where you can practice, it’s a sport you can do anywhere there’s a half-acre of boulders, or a hundred yards of creek bed. If you do have to travel to practice, you can easily carry a 165-pound trials bike across your back bumper on a hitch carrier; you don’t even need a truck or trailer.

Trials is the safest form of motorcycle competition, especially for novices who don’t have to deal with the steep obstacles that experts negotiate. It’s also a great option for older riders who still feel the need to challenge themselves, but who don’t heal so fast any more.

Honda claims that this new machine is perfect for both competitive trials riders and weekend hobbyists. At $9000, the Cota 4RT is not really priced for new riders; I’d love them to bring out a lower-spec machine priced for beginners, but if you’re on a budget, used two-stroke trials bikes show up on Craigslist in usable condition for $2000-3000.

Getting some trials chops is definitely on my personal motorcycle bucket list. There are some great places to learn, on both sides of the country. I can vouch for Gary LaPlante’s Motoventures program, which operates in San Diego County. He’s got trials workshops running on most Fridays and Sundays. Now that I live in Kansas City, the Trials Training Center in Sequatchie, Tennessee is closer. It’s a great place to get off on the right foot—which, of course, in trials riding is no foot at all.
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