The Motorbike Archives

Dual-purpose motorcycles are a damned-if-you-do, damned-if-you-don't proposition because so many street-riding prerequisites are diametrically opposed to those of trail riding. A table of specifications guaranteed to make a motorcycle perfectly wonderful on-road might make that same bike perfectly awful off-road. A design element that might double a rider's ecstasy on a smooth, flat superhighway could triple his agony on a bumpy uphill trail. Furthermore, any street-going motorcycle must, by law, have certain equipment that is not only irrelevant to dirt riding but that detracts greatly from the bike's off-road effectiveness as well.

Fortunately, a few user-group factors have kept street/trail bikes from becoming extinct. First of all, the vast majority of the people who buy this type of motorcycle are not highly-demanding expert riders. Second, most street/trail bikes seldom, if ever, are used for any "serious" riding, on or off the road. In fact, the closest most dual-purpose bikes ever get to dirt riding is when they are wheeled onto a bare spot on the lawn for their annual wash job. The most important factor, though, is that regardless of what they do with street/trail bikes, people continue to buy them in sufficient numbers to keep the major motorcycle companies interested.

Honda has been building dual-purpose motorcycles since the early Seventies and therefore is painfully aware of the problems associated with the breed. Honda also is the first company to have tried, albeit unsuccessfully, to simultaneously market two entirely different dual-purpose bikes of the same displacement—one four-stroke (XL250) and one two-stroke (MT250).

Neither of those models were exceptional performers, but the limited public acceptance of the MT250, compared to the ongoing sales consistency of the XL250, convinced Honda that four-strokes were the direction dual-purpose bikes should take. The EPA's noise and emissions mandates for the 1980s further anchored Honda's decision, since those regulations, whether they be presently proposed or already approved, seemed more readily attainable with a four-stroke. Besides, Honda is populated with engineers who have proved by example that they would rather fiddle with cams and poppet valves than with window ports and reed valves.

And so those engineers fiddled with the old XL250's single-overhead cam and four poppet valves and with everything else on the entire motorcycle in an effort to develop a better four-stroke 250 street/trail bike. They addressed the problems of dual-purpose design more squarely than ever before and came up with some interesting new solutions. And even though one discouraging word commonly associated with dual-purpose bikes—compromise keeps popping up during the discussion of the new bike, the XL250S nevertheless is the best dual-purpose bike Honda has ever built. Possibly, it is the best 250 street/trail machine—two-stroke or four—you can buy.

One traditional drawback of a four-stroke dual-purpose bike is the engine—not necessarily its performance, but its size and weight relative to that of a two-stroke. Honda's designers responded to that challenge by fashioning an overhead-cam, four-valve, 250-cc single of comparatively small and light proportions. And the new engine even incorporates a pair of chain-driven counterbalancers which cancel out enough vibration to make the S-model XL the smoothest-running 250 street/trail motorcycle on the market.

Engine performance on a dual-purpose motorcycle is one of those things that calls for compromise. Because even though on-road motorcycles do not need extremely sharp, crisp low-rpm power, off-road machines do, especially if they are to be operated by an inexperienced or infrequent dirt rider. On the other hand, both types of riding can make good use of strong mid-range and top-end power. In addition, off-road bikes demand that the power be delivered in a tractable, nonexplosive manner.

The XL250S meets most, but not all, of those criteria. The low-end power is very good for off-road use and should be sufficient to bail the average trail tiddler out of most of the jams he'll get into. The engine lugs down much better than we expected, which, combined with a relatively low first gear, enables the XL to chug up impressively steep off-road hills at a crawling pace. We took our test bike into some long, tight, trials-like canyons, and although it did not breeze through like a Bultaco Sherpa T, the Honda did negotiate that demanding terrain without too much aggravation, which speaks well for the engine's low-end power.

The Honda does not pump out impressive horsepower numbers at any point in its very flat power curve, and so it does not storm up hillclimbs or sizzle over the trails in a manner rivaling that of an ISDT bike. But the XL does move along at a respectable speed and with relative ease. You won't get to the top of a hill first or with a continuous wheelie, but you will get there with very little wheelspin and only occasional gearshifting.

On the street, too, the XL is more at home chasing stop-sign-to-redlight on the surface streets than it is cruising the freeways and Interstates. The engine meters out its power very evenly and consistently rather than cutting it loose in quick bursts. If you're conditioned to "real" street bikes, a pass from 50 mph on the XL can be boring and rather slow, even with a downshift. But the Honda is more than a match for most cars when zipping away from stoplights, although the first few upshifts must come in rapid-fire succession to produce the fastest getaway.

On-road, the XL carburetes to near perfection. There's never a trace of bogging, lurching, hesitating, stalling or coughing of any sort. The unique pear-shaped venturi on the 28-mm Keihin carb, plus a mechanical accelerator pump, are primarily responsible for the XL's lack of mis-carburetion on the tarmac.

But the special demands of slow-speed off-road riding can often find a few soft spots in the carburetion. Sometimes, when the bike is plonking along a difficult trail at very low rpm, the engine will unexpectedly spit back through the carburetor and then stop running. Sometimes, when the ambient temperature is high and the engine is hot, the carburetion becomes so rich at some throttle openings and so lean at others that no throttle openings can produce normal power output and engine response at low revs. Not always, just sometimes.

Low-rpm carburetion on off-road four-strokes seems to be a universal problem. but we have the EPA to thank for much of the XL's mixture difficulties. Dual-purpose motorcycles must comply with specific EPA emissions regulations, so the XL250S has some wonky carburetion traits caused mostly by its lean low- and medium-speed jetting and complicated by a bit of richness on the main jet. Unless you technically break the law and modify or re-jet the carburetor, you'll have to live with this moodiness. Furthermore, the pilot jet is non-removable and the jet needle is nonadjustable. Catch-22 again.

The carburetion problem is more of a forced concession than a compromise, but compromise obviously is essential in the steering geometry of a dual-purpose motorcycle. Steering that is perfect for most off-road riding would surely be much too quick for high-speed road use, and vice-versa. Taking those facts into consideration, the Honda strikes a happy medium. The XL's steering behavior is on the slow side in the dirt, especially on the tight trails, yet it is rather quick around town and very touchy on the freeway. But the bike's steering is well within the limits of acceptability on or off the road. Occasionally, our test riders would cry out for more steering quickness in the dirt or less of it on the road, but overall they unanimously agreed that the XL's geometry is, for a dual-purpose motorcycle, close to ideal.

Street/trail suspensions also must be compromised, but not to the same extent as some other areas of the motorcycle. An ideal street suspension would not suffice, since the wheels would be bounced off the ground too often and the ride would be too harsh. Conversely, the soft, long-travel fork and shocks of a motocrosser wouldn't feel very good on the street. The bike would sit uncomfortably high and the radical geometry changes during hard, highspeed braking and fast cornering would generate some frightening twitches and wiggles.

So Honda chose to apply just a smattering of motocross technology to the XL250S by adopting a leading-axle front fork and a lay-down mounting position for the Showa gas-emulsion rear shocks. The travel at both ends is just a fraction under eight inches, which is more wheel movement than that offered by any of the XL's competitors.

The Honda's suspension may have been a design compromise, but it actual use it works amazingly well. The suspension is virtually impervious to anything normally found on or in the pavement, including potholes, paving patches, drainage bumps, reflective Bautz dots and even small curbstones. The rider can merely remain seated and bash over these obstacles with little fear of getting bounced off the saddle.

Off-road, the XL's suspension is much better than we have come to expect on a dual-purpose motorcycle. The bike must pass over the most punishing of trail-riding terrain at brisk speeds before a good rider starts feeling uneasy. More importantly. the XL stays straight when a bump or a rut succeeds in getting the chassis airborne. An abrupt, sharp-lipped little jump can cause the rear end to kick up if the rider doesn't hit it just right, but in all fairness, even the best motocrossers will do so under certain conditions.

The only real suspension complaint we had was with the compression damping in the front fork. If the front wheel would hit a fist-sized rock or a small, sharp bump while the fork was at full extension, the fork would often not compress easily and the whole front of the motorcycle would get kicked into the air. We also noticed a similar problem on the highway, where the fork would sometimes not respond to certain small irregularities in the pavement. There appears to be too much compression damping at the very beginning of the front suspension stroke, while the rest of the fork's behavior is excellent, especially on very big bumps. This is not a built-in compromise of the type necessary to a dual-purpose motorcycle. This is an engineering miscalculation.

The suspension's overall effectiveness helps the XL250S to offer a reasonably high comfort level. But despite a good ride and a low vibration level, the Honda's on-and off-road comfort quotient is affected by an annoying ergonomic feature. On-road, the seating position and the location of the controls is just fine, but the seat's shape and foam density are such that a ride of about 20 minutes or longer gets the rider squirming around to avoid numb-butt.

Off-road, a rider must move around on the bike so much during normal trail riding that the seat does not affect his comfort. But the relationship between the XL's superb handgrips and the cleated footpegs becomes bothersome. When the rider stands up, the pegs and grips are a couple of inches too close to each other for maximum comfort and control. The footpeg location was selected so as not to crank too much bend into a street rider's knees without severely penalizing a stand-up dirt rider. So once again, compromise dictated where and why one of the XL's components would be located.

Normally, one of the easiest—although not necessarily one of the best—ways to help this peg-to-grip problem on most motorcycles is to replace the stock handlebars with a set that positions the grips further from the footpegs. But unless Honda or the aftermarket people bend up some different handlebars just for the XL250S, that solution won't work, thanks to the XL's 23-inch front wheel. The greater outside diameter of the front tire meant that the steering head had to be positioned higher in relation to the rest of the motorcycle if the front wheel travel, the seat height and the center of gravity were to remain the same. So the XL has very low-rise bars that are unlike any found in accessory catalogs.

Evaluating the true worth of the XL's 23-inch front wheel is practically impossible at this point because the rest of the motorcycle is completely different than any previous XL models, and because Honda has designed special dual-purpose tires which rake their debut on the XL250S. A few of Honda's R&D engineers worked closely with Yokohama to develop the Trail Sport tread pattern, which they feel is an improvement over the old Trials Universal tread.

We agree. Off-road, the Trail Sport tires are not as good as knobbies, and they feel about the same as the old Trials Universals during straight-up acceleration and deceleration. But the new Yokohamas seem to hang on a bit more tenaciously when the bike is leaned over. On pavement, the Trail Sports are not as effective as legitimate street tires but they're appreciably better than the Trials models in every respect. The new design has a more-rounded tread surface and puts more rubber in contact with the ground.

If dual-purpose tires are just one more compromise in a long line of give-and-take propositions, so are the brakes. And once again, Honda has struck a happy medium. The XL's brakes are just powerful enough to be safe on the street, and just unpowerful enough to be safe off-road. The rear wheel often tends to lock up rather easily off-road, but the compromised rear tire tread is as much to blame as the rear brake itself. Any less braking power would be a detriment to safe street riding.

Honda's engineers apparently were just as concerned with the XL's starting as they were with its stopping. The S-model XL has a compression release which is automatically activated during the first part of the kickstarter's stroke. The device works nicely, making the task of firing up the XL a one-kick proposition—usually. One tester complained that cold starts were a five- to nine-kick affair for him. No one else ever needed more than two kicks.

When asked why the XL had a kickstarter-operated compression release instead of the usual handlebar-lever-actuated type, a Honda representative said, "We wanted people to be able to start the bike at night"—an obvious slam at Yamaha's 500-cc four-stoke singles. The starting drill for those bikes includes kicking the engine through while using the compression release until a white mark appears in a little, unilluminated window on the cylinder head—a window that is often impossible to see at night.

Too bad the designers didn't put as much foresight into a few other areas of the XL. The chain guide on the swingarm is ridiculously flimsy; the toolbox isn't big enough to carry even a spark plug once the toolkit is jammed into place; the clutch engages far too abruptly; there is quite a bit of driveline snatch evident at times; and the footpeg-attaching arrangement can allow the pegs to rotate downward after a few hard jump landings. This last problem not only lowers the pegs, but since the right peg bracket also contains the brake pedal height stop, the rear brake gets locked on as the peg moves downward.

These last complaints are a result of another type of compromise, though—a money compromise. Those undemanding people who buy dual-purpose motorcycles usually are also very demanding price-shoppers. They may appreciate quality, but they care more about cost. And with all the additional parts necessary in a four-stroke engine, Honda was hard-pressed to build a bike that is competitive with a two-stroke in price as well as performance. So a bit of cost-cutting had to be done somewhere, and the chain guide and the footpeg brackets are a couple of good examples of where it happened.

Still, the XL250S is a terrific little motorcycle, a versatile companion that can make errand-running and daily commuting almost as much fun as weekend trail riding. To boot, the XL will squeeze up to 60 road miles out of every gallon of gas, and nothing you can do short of letting the engine idle in neutral all day will yield less than 30 mpg in the dirt.

So maybe the XL250S is a compromise. So it isn't as good on the street as an XS Eleven or as good in the dirt as an RM250. That doesn't make the XL an inferior motorcycle. Dual-purpose is just another way of saying "compromise," and the XL250S executes that concept at least as well as any other street/trail 250, if not better.

The secret of understanding the XL250S is to get the right perspective on the matter of dual-purpose bikes. Don't concern yourself with what a street bike or a motocrosser can do well that the XL does only adequately; concern yourself with what the XL can do that a street bike or a motocrosser cannot do at all. When viewed from that perspective, compromise hardly seems like a dirty word.■

TECH PROBE

The true measure of a motorcycle is not how well any one part of it performs, but how effectively all the parts perform as a whole.

Nonetheless, most motorcycles have one particular feature or one specific piece that is the pivot around which the entire design revolves. Sometimes that piece is big, sometimes it's small; sometimes it's one individual part, sometimes it's a complete assembly; but whatever the case, the nature of the rest of the motorcycle is dictated by that feature.

The under-tank rear suspension unit on a Yamaha monoshocker is a good example, for it most definitely is a part around which a whole motorcycle was designed. The Vee-twin engine in a Ducati is another case in point, as is the shaft final drive on a Yamaha 750.

The engineers who design dual-purpose bikes at Honda feel strongly that their new XL250S, too, has one such key component. But few people who work outside of Honda's Osaka R&D facility would ever guess that the part singled out by those designers is, of all things, the rear counterweight in the XL's dual-counterweight dynamic balancer system.

The logic behind this odd choice is not very obvious unless you understand the problems Honda's R&D people were butting heads with during the new XL's development. They had been charged with transforming the trusty-but-obese old XL250 into a motorcycle that would remain a dual-purpose four-stroke single, yet be as light and perform as well as the dual-purpose two-strokes. This was no small undertaking, for one fact of life kept rearing its ugly head: A four-stroke engine will always be heavier than a comparable two-stroke simply because the latter has fewer parts. So even when the highest regard for light weight goes into the design of a motorcycle, using a four-stroke engine will make the bike heavier than if the same machine were powered by a two-stroke. And because overhead-cam or overhead-valve engines have much of their additional mechanisms as part of the cylinder head, vertical four-strokes are taller engines which raise the center of gravity of the entire motorcycle:

Honda's engineers were undaunted by these obstacles, though. After all, why should they back down from a mere technological challenge? In a decade and a half they had written as many chapters in the motorcycle history book as any other company had in over half a century.

The one large fly in Honda's engineering ointment was vibration. Under some conditions, previous XL250 models (which generally tipped the scales at over 300 pounds) had frame-cracking problems if engine vibration combined with certain kinds of road-induced vibration. So with that ordeal weighing heavy on their minds, Honda's designers were understandably apprehensive about the potential danger of building an even lighter version of such a bike.

This is where the balancers come in. Honda's engineers determined that by giving the new XL a pair of chain-driven, counter-rotating balance weights—one ahead of the crankshaft, one behind it—sufficient engine vibration could be canceled out to permit a very light engine/frame combination without fear of major structural problems. And while engineering a balancer system for the XL, Honda's people came up with a clever design for the rear counterweight. The rear balancer requires no separate shaft and needs no additional cavity to house it. The rear weight simply spins on the extreme left end of the main- shaft inside the transmission case as if it were a sixth gear on the five-gear shaft.

This is why Honda chose the rear balancer as the most important component on the XL250S. The counterbalance system as a whole enabled the designers to overcome the major engineering obstacles standing in the way of a truly lightweight four-stroke design; and the gearshaft-mounted rear balancer in particular allowed the engine to be made just about as compact as it would have been without any internal balancing devices.

With the threat of frame disintegration behind them, Honda's design team was able to be more daring and imaginative than ever before in four-stroke dual-purpose history. The new XL's mild-steel frame was therefore constructed with thinner-wall tubing and a thinner-gauge stamped steel backbone than before. Moreover, the frame tubes which normally run beneath the engine from the front motor mounts to the rear mounts were completely eliminated. A lightweight cylinder-head-to-backbone brace was then inserted to let the engine act as a frame-triangulating member. As a result of all this material stinginess, the new XL frame weighs 8.5 pounds less than the old XL250 frame.►

◄Inside the engine, the XL250S retains the same highly-oversquare bore/ stroke ratio introduced on the old XL, but the new engine has a 15.7-millimeter shorter connecting rod and a piston pin-to-crown measurement which is 2.6 millimeters less. Both of these new dimensions have helped Honda to build an overhead-cam four-stroke 250 single which has an unusually short top-to-bottom length.

Considerable attention was paid to size and weight in many other areas of the engine. The crankshaft-to-transmission mainshaft distance was decreased to achieve a shorter front-to-rear engine measurement. Lowering the primary drive ratio from 3.13:1 to 2.38:1 reduced the amount of torque transmitted through the clutch and gearbox, permitting the safe use of thinner, lighter transmission gears and primary drive gears. The new XL's clutch is also lighter, smaller in diameter and uses fewer plates than the old XL model's clutch.

The harvest reaped by this diet program is a street-legal, dual-purpose, four-stroke 250 that is competitively light and has a respectable center of gravity. And though the S-model has more ground clearance and much more suspension travel than previous XLs, the seat height is no greater than that of the previous XL250. For that matter, the new XL has more travel than any of its current competition in the 250 street/trail class.

Honda's R&D wizards didn't put the brakes on their creativity after successfully engineering a lightweight four-stroke motorcycle. The XL250S abounds with useful, clever innovations from stem to stern. But for provoking puzzled stares and bewildered inquiries, no part of the bike can match the dual-pipe exhaust system. Most people assume this plumber's folly to be some obscure tuning secret recently unearthed by Honda, but that's not at all the reason for the XL's two separate exhaust header pipes.

First of all, the system allowed for more even cooling of the engine's two exhaust valves. The old XL's exhaust port angled to the right while exiting the cylinder head so the large-diameter header pipe could clear the frame's single front downtube. But the angle of the port caused it to mask the flow of cooling air to the right exhaust valve, creating a hot spot around that valve.

The two separate exhaust ports on the new XL aim straight ahead as they leave the cylinder, and a cooling-air tunnel is cast into the head directly between the two ports. After air enters the tunnel it is forced around and between both exhaust ports before exiting on the left, near the spark plug recess. Thus, the hot spot is eliminated, diminishing the likelihood of engine-damaging pre-ignition.

Furthermore, the dual-pipe header makes it easier to bend the exhaust system around the frame's front down-tube. And Honda's sales people openly admit that, if for no other rea-son, they like the two-pipe idea because it makes the XL unique in all the motorcycle industry.

Uniqueness also played a major role in Honda's decision to use a 23-inch front wheel. These big rims have been kicking around the accessory business for a year or two, but Honda is the first company (other than a few small manufacturers of speedway bikes) to offer the 23-inchers as standard equipment.

A larger front wheel has a couple of advantages and a few drawbacks. The biggest and most obvious plus is a big wheel's ability to roll over large obstacles and negotiate deep ruts or holes more easily than a smaller wheel. And, by having a larger circumference, the size of the footprint the tire puts on the ground is increased by a few percentage points.

On the minus side, the larger rim size calls for longer spokes and lengthier fork tubes. Longer spokes make the wheel slightly heavier and potentially weaker, while adding more length to the fork tubes makes the front end heavier and can allow the wheel to be more easily deflected from side to side. A 23-inch tire and rim also weigh more than comparable 21-inchers, since they have a greater circumference. The new rim, for example, weighs about a pound more than the 21-inch rim it replaced.

Performance-wise, the 23-inch vs. 21-inch controversy boils down to six of one and a half-dozen of the other. So it appears that the primary reason the XL250S wears a 23 up front is. so it could be the first and only mass-production bike to have one.

Further evidence of Honda's desire to build a truly unique motorcycle can be found in the XL's absence of a replaceable or serviceable oil-filtering device, although the engine does have what the company calls a "lifetime oil sludge trap" integral with the crankshaft. The right side crank flywheel is built with a large round plate pressed into its Outer face, behind which is a huge enclosed cavity nearly as big in diameter as the flywheel itself. The right shaft of the crank is also hollow so that oil can be pumped through it.

As oil is fed through the hollow shaft, it must enter the big cavity where the centrifugal force of the rapidly spinning crankshaft flings the lubricant out to the outer walls. As the cavity fills with oil, the larger particles of metal and dirt, which are heavier than the oil, are trapped on the inside circumference of the cavity. Meanwhile, the cleansed oil continues out of the cavity, into the right end of the crankpin and on to the connecting rod big-end bearing.

Hondas of years past used a similar type of centrifugal oil filter, but it was in the form of a small, chain driven cartridge that could be easily removed and cleaned. The XL's system is neither removable nor cleanable—at least, not by the owner—but Honda claims the cavity is large enough that it should not fill up with sludge during the life of the big-end bearing.

Our greatest concern over this system is the possibility of sludge getting into the rod bearing. Since the passage that gets the oil from the cavity to the bearing is in the end of the crankpin, and since the crankpin is located very close to the outside diameter of the flywheel, the passage is very close (about 10 millimeters) to the inside diameter of the cavity. This puts the entrance to the big-end oil hole right next to the dirtiest part of the oiling system. Roller-bearing big ends don't need to be pressure-fed anyway, so we wonder why Honda went to all the trouble of designing a pressure system that puts the sludge supply between the clean oil and the bearing to be lubricated. What effect this filtering arrangement will have on the big end is a question only time and a dirty sludge cavity will answer.

The primary function of the XL250S, however, is to answer some other questions, including whether or not a four-stroke dual-purpose bike can be made remotely competitive with a two-stroke. We've felt for years that if the same amount of money and man-hours had been dumped into four-stroke off-road technology as into two-strokes, there would not be such a large performance gap between the two, Granted, the two engine styles are not and never will be totally equitable. Regardless of its performance capabilities, a four-stroke is penalized from the outset because it is more complicated and it has more parts to wear out or fail.

So maybe the four-strokes will never fully equal the popcorn-poppers when it comes to certain types of all-out racing. Maybe. But in the XL250S we see proof positive that help is on the way.■