GP Classic Steel - 1989 ATK 604E | Pulp MX

This week?s selection from Greg Primm?s Classic Steel is the Ferrari of eighties thumpers, the ground pounding 1989 ATK 604E.

By: Tony Blazier

In 1989 this was one of coolest motocross machines you could buy. The ATK combined earth-shaking power with class leading handling and suspension. It was an unbeatable package for the lucky few that could lay their hands on these ultra-exclusive machines.

ATK was founded by Austrian-born engineer Horst Leitner in the early 1980s. Leitner had been a successful motorcycle racer in Austria before immigrating to the United States to pursue his dream of designing his own motorcycles. The name of Leitner?s new company came from his 1981-patented device to eliminate chain torque for improved handling. Known later as the A-Trax, Leitner originally called the device the Anti-Tension Kettenantrieb or ?ATK? for short. ATK started out building custom chassis? for Honda XR motorcycles. They were designed to turn the mild mannered play bikes into full-fledged race machines. After the success of the kit bikes, ATK decided it was time to try their hand at producing a complete motorcycle. ATK?s prototype machine used an Austrian built Rotax 562cc four-stroke motor bolted to a chromoly steel frame with White Power suspension.? The ATK was a revolutionary machine for its time, featuring Leitner?s innovative A-Trax , USD front forks and a single link-less rear shock. In an era where four-strokes were considered playthings, this was a 100% serious race machine. The reaction to the initial prototype was so overwhelming that Leitner set about making his dream of an ATK production machine a reality.

This is an ad for the original ATK 560. In 1984, this bike was revolution for four-stroke aficionados. If you were going to go valve and cam, this was the machine to get.

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Austrian-born engineer Horst Leitner started with an idea for an innovative rear suspension system and grew that into the sixth largest producer of off-road motorcycles within ten years. His passion for innovation has made him a major player in the automotive, mountain bike and motorcycle industries.

?In 1989 the ATK 604 (it still used a 562cc motor but ATK changed the name to 604 for marketing reasons) was the master blaster of the Open class. Even compared to contemporary 500cc two-strokes, the ATK was an absolute rocket. It was a 100% custom built, no compromise motocross machine. At $6950 the ATK cost twice what a new KX500 went for in?89. For that money you got a four-stroke that could rip the holeshot at a 500 National right off the showroom floor. The suspension was all premium Dutch White Power (now WP) components tuned just for the US market. The front brake was a KTM Brembo unit while the back was a custom disc mounted in their signature fashion to the countershaft. At an annual production of only 200 units, you were guaranteed each bike was hand assembled with care. This was a premium machine for a discerning racer.

Instead of designing their own engine from scratch, ATK turned to Rotax to provide the power for their motorcycles. The Austrian built motors were heavy and decidedly low tech, but claw hammer reliable and rocket ship fast. The 562cc thumper pulled hard from the first crack of the throttle and kept on pulling till the rev limiter kicked in. Compared to the two-stokes it competed with, it was both faster and easier to ride. When you add in the electric start option you had the ultimate open class power plant of ?89.

?The ATK?s 562cc Rotax air-cooled mill was hardly state of the art even in 1989. With manufacturers like KTM and Husky already producing liquid-cooled DOHC racing singles, the Rotax mill looked downright old fashioned by comparison. What the motor lacked in curb appeal, it made up for in rugged reliability and blistering performance. The 604 produced a thundering staccato of four-stroke torque from the second the throttle was cracked until the rev limiter kicked in. The motor had ground-shaking power and was fast enough to show even CR500 riders its tail lights. There was a bit of a learning curve for riders raised on two-strokes however. ?The motor revved slower than a two-stroke and the power was more Massey Ferguson than twin-turbo Porsche. There was also A LOT of engine braking under deceleration. Today?s four-strokes have virtually none of this old school trait, but rolling off the throttle on an ATK was like throwing out an anchor. On the plus side, the reward for learning to manage the ATK?s quirks was holeshots by the dozen.

ATK used a White Power link-less shock mounted to left side of the swingarm much like a 98-10 KTM. Unlike most Euro bikes that used the WP components, the ATK featured excellent suspension action. The combination of ATK?s patented A-Trax chain torque elimination system and carful set up gave the bike the best rear suspension of any Open class bike in ?89.

?One disadvantage the ATK had over its two-stroke competition was weight. In ?89 the 604 came in two configurations, one with electric start and one without.? If you got the kickstart model, the bike weighed a slightly porky 247 pounds. If you ponied up the extra $1000 for the e-start, the weight swelled to 262 pounds. That would put the 604E in XR600 territory and a good 30 pounds heavier than a two-stroke 500. Of course the tradeoff for the extra weight was that magic button and on a big four-stroke, that was no minor thing. Today?s 450?s make starting a piece of cake, but in the eighties starting a big thumper was no laughing matter. If you stalled a hot four-stroke you might end up kicking for an hour before the big beast would light. Overnight ATK negated the single biggest disadvantage thumpers had to their ring-ding competition.?

Gentleman Jim Holley puts the ATK604 through its paces for the Dirt Bike cameras.

?Weight usually puts a great deal of stress on suspension, but the ATK?s WP?s were up to the challenge. At the time, White Power did not have a sterling reputation in America. Their forks and shocks had graced many Euro bikes over the years, but their performance always left a lot to be desired. The problem was improper set up, as Euro brands tuned their bikes up for the faster, less rough European circuits. ATK rectified this problem by setting up the 604 for the American market. The suspension was stiff enough for our jump filled tracks while still remaining plush enough to absorb smaller hits. The combination of the WP shock and ATK?s patented A-Trax torque elimination device gave the 604 the best stock suspension of any open class machine in ?89.? The forks were likewise firm without being harsh and set up nicely for hard core motocross use. Since every ATK was virtually a custom built machine, you could also request ATK set up your 604 for cross country racing. Either way you were guaranteed excellent suspension performance.

Even though most of the bike was made up of European sourced components; each ATK proudly bore the ?Made in USA? label.

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One of the unique traits of all ATK?s was this countershaft mounted disc.? It was used to reduce unsprung weight and lessen brake torque on the rear suspension.? Unfortunately, braking power was less than stellar and it was prone to fading. Because of the 604?s massive engine braking, this was less of an issue than on its two stroke stalemates.

?Handling on the 604 was excellent for its size and weight. The bike was very tall and wide with its 4.2 gallon fuel tank. Combined with the 262 pound weight the ATK was no bike for a small rider. It required a good deal of skill and strength just to muscle the big machine around. Here the power characteristics of the Rotax thumper worked against the bike. The massive power pulses only served to magnify the feeling of mass when riding the machine. In spite of the heavy feeling, cornering was excellent and the ATK was rock solid at speed. As long as you were strong enough to handle the bike it was a deadly race weapon.

In the eighties a Supertrapp megaphone exhaust like this was considered high performance. You could adjust the number of discs in the end cap to tune the power and sound output.? It is a far cry from the slick carbon fiber and titanium pipes we see on thumpers these days.

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On the ATK,?White Power handled the front forks and a combination of Magura, Grimeca and KTM components made up the front brake.? Fork performance was excellent while braking was less stellar. The brakes were decent but not in the same league as the class leading CR500R.

?One of the strangest things about the 604 was its unique rear brake system. The rear disc was completely different than anything else on the market with a backwards facing pedal and countershaft mounted rotor. The reservoir was mounted under the tank and connected to the caliper by braided steel lines. There were a couple of reasons for ATK?s innovative brake placement, both related to improving suspension action.? By removing the rotor from the rear wheel, ATK lowered the unsprung weight and reduced the brake torque?s effect on the rear suspension.? On the downside, the smaller brake received less air and was more prone to fading than conventional designs. This was less of a problem on the 604 than on ATK?s two-strokes that lacked its compression braking (This was such an issue in fact, that at one point ATK actually added a second rear disc to its 406 two-stroke. This meant the bike actually had THREE disc brakes!). The front brake on the 604 used Magura, Grimeca and KTM components and it worked well if not spectacularly. Again the bikes increased weight hurt it in this area.

With only 200 hand-built 604?s being produced annually, the bike could be special ordered in dozens of different configurations.? Everything from the color of the tank to suspension setting could be altered to suit the taste of the buyer.? At nearly twice the cost of a new Japanese 500, the ATK was a premium bike for a premium customer.

?The ATK 604 was not a machine for everyone. It was a bike for more well-heeledriders. Riders with the experience to make use of its awesome potential and the bank account to handle its jaw-droppingprice (I know $6950 does not sound bad now, but that was a fortune in ?89). It was exclusive, unique and one of the best Open class race bikes made in ?89.?

Source: http://www.pulpmx.com/stories/look-back-old-moto-mags/gps-classic-steel/gp-classic-steel-1989-atk-604e

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