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FORCE FOUR
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Looking for the King of Thumpers
FOR THE MOST PART, THE DEVELOPMENT OF four-stroke off-road bikes has been slower than 50-weight in February. Or at least it seems that way when you compare the relatively low glamour level of four-stroke dirtbikes to the glitter and hype that typically surround off-road two-strokes. But the truth is that while the two-strokes have been hogging the spotlight. some truly exciting four-stroke Singles have sneaked into the picture.
Shining examples of that fact are the five big Thumpers you see here. For one thing, all five dispel the age-old claim that four-stroke Singles have to be lead-weight heavy. The heftiest of this group, the electric-start ATK 604. tips the scales at just 275 pounds without fuel (a comparable kick-start model is 14 pounds lighter). And the featherweight of the bunch is the Husaberg MC501 at 250 pounds, with KTM's 600 LC4 MX just a single pound more at 251 (the cross-country model is 5 pounds heavier). The Husqvarna 510 Enduro weighs in at 268 pounds, which is 12 pounds more than the 510 MX we tested in our March, 1990 issue. And even though Honda's kickstartonly XR600, at 274 pounds, weighs just 1 pound less than the electric-start KTM. it still is not what you would consider porky.
All of these bikes have overhead-cam, four-valve-percylinder, short-stroke engines, and all produce reasonable, if not awe-inspiring, amounts of torque and horsepower. The Honda's 591cc engine is in the mildest state of tune, which helps it make smooth, linear power that's easy for novice and casual riders to manage. But expert-level riders complained that making fast time on the XR requires constant buzzing of the easy-revving engine—and even then, it never really feels all that powerful.
The ATK's 562cc Rotax engine has a user-friendly powerband similiar to the Honda's, but it cranks out a lot more power than the XR at all engine speeds. Conversely, the powerband on the KTM LC4 600 (which actually displaces only 553cc) is just the opposite of the Honda's: Gobs of power gush forth right off idle, but after that, the powerband is flat and featureless. In fact, once the KTM revs past its mid-range, all the excitement is over.
Not so on the Cagiva-built. 503cc Husqvarna 510. which offers the broadest powerband of this fivesome. Good, usable power is on tap just off of idle, and it continues all the way up to what sounds like a billion rpm. Due to its heavy crankshaft and ignition rotor, the 510 is a fairly lazy revver up to the mid-range, but after that, things get a lot more exciting. As the rpm reaches a point where you would shift a normal four-stroke Single, the Husky gets a second wind and continues revving until it’s buzzing like a 125 motocrosser.
Despite the Husky's impressive power output, though, the engine liked most by all of our test riders was the remarkably responsive Husaberg motor. Even from idle, the light-flywheeled engine revs instantaneously, seeming more like a big two-stroke than a 502cc four-stroke. Its output is impressively strong in the lower and middle rpm ranges, even though the power signs off rather early on the top end.
Such a lively engine makes the Husaberg a fun ride. It easily does all of the exciting stuff— wheelies, jumps, slides and the like—without requiring the rider to adapt to the slower throttle response typical of other four-strokes. But there's a trade-off: The rear tire breaks loose easier than with one of those slower-revving four-strokes. Consequently, when riding on rocky, muddy or other slippery terrain, a Husaberg pilot generally has to use a higherthan-normal transmission gear and play the throttle more carefully.
Four of these five bikes also put to rest another legend concerning big four-stroke Singles: hard starting. And winning the starting sweepstakes by a mile was the electric-start ATK. Every rider just loved The Button. Even hard-core four-stroke freaks, guys who never have any problem starting a big Thumper, said they would gladly add 14 pounds of weight to their normal mount to have an electric starter. Why? Because the ATK starts instantly, whether it is hot, cold or has just been righted from a crash.
Among the kick-only bikes, the Husky 510 was the most reliable starter. It always thundered to life in one or two kicks, even when prodded by riders more accustomed to starting two-strokes. The Husaberg was only slightly more difficult to fire up, even though it has a short kick lever and a kickstarter ratio that demands a hefty boot to spin the engine. The Honda also started predictably; but although its engine is easy to spin over, several rapid kicks normally are required to get the engine rumbling.
Which brings us to the KTM. When it was cold, a kick or two had it purring. But when the engine was warm, or if the bike had just tipped over, it flat refused to start. Worse yet, it often would suddenly quit running while blasting down a trail at speed. One minute, everything would be fine and the engine was making great power, and the next minute, it would be dead. We once spent more than 20 minutes flailing on the kick lever after a flame-out, only to have the engine suddenly start and run as though nothing was wrong.
Ironically, after their first ride on the $4369 KTM, most of our riders didn't care if it wouldn't start, simply because it handled so badly. The steering is heavy and slow (KTM claims a 27.5-degree steering-head angle, but ours mea sured a chopper-like 31.5 degrees); the front end con stantly hunts and refuses to hold a line on a narrow trail and the MDS air-oil (no mechanical springs) fork is in need of help. Its separate highand low-speed compres sion-damping adjusters can be dialed-in to let the front end smooth the bumps nicely; but the rebound damping is way too light, even when the rebound adjuster is turned to its maximum, so the fork constantly bobs up and down.
That's too bad. The KTM is a light, responsive, beauti ful machine intended to appeal to a wide variety of riders But until its poor handling and iffy starting are remedied, this bike isn’t on-target for anyone.
Honda’s XR600. on the other hand, has always been aimed at the casual rider instead of the racer; and for that group, its suspension and handling are in the hunt. The fork and shock are quite plush over rough ground and will bottom only if the rider starts playing racer. The steering is light, with just the right amount of quickness for tight trails, yet the XR is fairly stable at higher speeds. It can cruise jeep trails and fireroads all day without causing the rider any undue discomfort. Nevertheless, turning the XR600, priced at $3598. into a serious off-road competitor would require a sizable investment in suspension and power upgrades.
In stock form, the Husqvarna. the Husaberg and the ATK all are excellent handlers, even for competition purposes, but each has its own distinct character. The $4350 510 Husky is a big motorcycle with a wide fuel tank and thick, broad seat that stays comfortable all day. Its handling is a little slow but light; high-speed stability is excellent, and the 510 slides smooth corners almost as well as a proper TT bike. A White Power shock does an admirable job of cushioning bumps at the rear, and an inverted Marzocchi fork (with rebound damping only in one leg, and compression damping in the other) performs equally well for all but the fastest riders, who may find the springs a little too soft.
But whereas the Husky, the XR and the KTM feel like big four-stroke Singles, the Husaberg feels more like a trim, 250cc motocross two-stroke. Its seat is narrow and thin, the fuel tank is slim and its fuel is carried low on the bike. As a result, the Husaberg steers lightly and precisely, even if it is not quite as nimble as a two-stroke motocrosser. Still, it changes direction very quickly, and its trim shape, low (36.8-inch) seat and sharp handling give it the feel of a much lighter machine. This makes it a natural for tight, difficult enduro terrain. But it also has stable high-speed manners, and it can be pitched into a full-lock slide with the control of a smooth-track machine.
Part of the reason for this excellent behavior is the Husaberg's first-class suspension, the kind you'd expect on a dirtbike that costs a whopping $6495. An Öhlins rear shock provides near-perfect control and a high level of comfort at the rear. A White Power cartridge fork furnishes a smooth ride up front, although it needs to be sprung a tad more stiffly for the really fast guys.
It’s not quite as easy to describe the ATK’s handling, which depends heavily upon the rider’s size and riding style. Big guys who ride hard love ATKs, but shorter riders and novices often hate them. The White Power rear suspension is stiff at slower speeds, and the rear of the bike only settles about 2 inches with a rider aboard, thereby amplifying the seat height. Adding to this odd feel are the footpegs, which have a more rearward placement than normal.
Crossing nasty terrain under power is where the ATK— which, like the Husaberg, costs $6495—shines. Its a-trak chain-torque eliminator prevents rear suspension lock-up under hard acceleration, which, in turn, contributes to the bike’s ability to smoothly traverse extremely rough ground with the throttle pegged. The flip-side is the presence of some strange handling traits if the throttle is chopped when entering a whooped or bumpy turn. In these situations, ATK riders learn to avoid this problem by leaving the throttle partly on and applying more rear brake.
ATK 600MX Electric Start
.$6495.
Honda XR600R
$3598
Still, once a rider adapts to the ATK’s requisite riding style, he finds the big bike a suprisingly quick handler. It slides smooth roads with ease, and its precision on a tight trail is equal to that of the Husaberg.
There also are some noteworthy differences in the ATK’s brakes. A solid-caliper/floating-rotor setup is used at the front instead of the more-common off-road arrangement of a floating caliper with a fixed rotor. This design works okay, but the latter type would probably offer better braking feel in the dirt. Even more unusual is the ATK’s disc rear brake, mounted on the transmission’s countershaft just outboard of the drive sprocket. Not only does this design lack the feel of a rear-wheel-mounted disc, but our test unit’s brake tended to lock up and stall the engine at the slightest touch of the pedal. And another drawback of this system is that if the drive chain either breaks or derails, there is no rear brake.
On the KTM and the Husaberg, the braking systems are more conventional and contemporary, with single discs on both wheels. And those bikes have superb brakes that require only light effort to operate while providing excellent fade-resistance.
The Honda and the Husky rely on disc-front/drum-rear brakes for their stoppage. Both bikes’ front discs work nicely, but the tiny, 4-inch rear drum on the Honda will fade if used hard or frequently. And even though the Husky’s larger, 6.3-inch rear drum brake does an acceptable job of stopping, it falls short of a good disc.
Husaberg MC501
$6495.
Husqvarana WXE510
.$4350.
KTM600
$4369
When the time finally came for us to stop riding these five big Thumpers and rank them in order of overall preference, we were surprised to find that all our testers came up with unanimous firstand second-place choices. And even third through fifth places varied only slightly.
Everyone liked the KTM’s look and feel, and it fit a wide variety of different-size riders. But its impossible starting, terrible fork and abysmal handling earned it a unanimous last place. In fourth place is the Honda XR600—a pleasant enough motorcycle, but one that needs more power, firmer suspension and a better rear brake to be fully competitive in this crowd of serious Thumpers.
Finishing in a solid third place is the electric-start ATK 604. Its ease of starting won everyone’s heart, but its tall seat, touchy rear brake, rear-set footpegs and weird trailing-throttle handling lost it points.
Which leaves us with the Husqvarna 5 10 WXE and the Husaberg MC501, which were unanimously voted second and first, respectively. Despite its rather dated design, the Husky is a satisfying motorcycle with no glaring faults; it’s easy to start, non-intimidating to ride, and generally handles off-road situations with a high degree of competency. But the new-wave Husaberg’s low seat, narrow chassis, ultra-responsive engine, light feel and crisp handling proved to be an unbeatable combination.
All of this will change very soon, of course, as all five of these bikes are slated to undergo changes of some kind for ’91. We’re told the new KTM600 will have a hotter ignition to help alleviate starting woes, and its engine will be more powerful. Honda’s XR600 has numerous minor engine changes, better suspension and a disc rear brake. ATK’s 600 should get all-new White Power suspension and we might even see a model with the new Rotax fivevalve engine. The Husqvarna 5 10 is scheduled to get a disc rear brake, new plastic, a slimmer shape, and a new cylinder head with the waterpump driven off the end of the camshaft. Husaberg’s changes will include restyled and stronger plastic components, new brake rotors and higherquality decals. And if our info is correct, there’ll be a more-powerful Husaberg intended just for motocross.
But that will be then; this is now. And for the present, the Husaberg MC501 is the king of the four-stroke Singles. It deserves its place on center stage, with all the spotlights pointed right at it. ES
