Road Test: 1995 Buell S2 Thunderbolt

What the heck was Erik thinking? He's a racer, he should have known better. How many frosty cold ones had he knocked back when he came up with the idea of wedging a Harley-Davidson motor, long derided by the sportbike crowd for being heavy, under-powered and unreliable, into the raciest chassis he could imagine, a design eschewed by the laid-back, too-cool-to-go-fast Milwaukee iron mavens? Seemingly, he'd alienate both crowds and his eponymously named company would go bankrupt -- that seemed to be the path most journalists and analysts predicted for Buell. But heck, what do these pundits know? They called Ben Franklin crazy when he stood around in crappy weather with a key tied to a kite string...

Not that the Buell S2 Thunderbolt is going to have the same historical impact as the discovery and subsequent harnessing of electricity, but there's a radical new theme brewing at the Buell Motorcycle Company, one that continues with the newest model from East Troy called the Lightning: By combining the strong points of the two polar-opposites of motorcycling -- Harleys and sportbikes -- Erik Buell and the Buell Motorcycle Company hope to cross cultural boundaries and smash stereotypes. Adventure riding, their marketing suits call it. You think that a sport bike has to have a high-revving, peaky motor with little or no bottom-end grunt? Or that a Harley-Davidson has to be a slow handling heavyweight? If so, it's time you were introduced to the Buell S2 Thunderbolt.

The heart of the S2 is a 1203cc Harley-Davidson Sportster motor, straight off the Harley production line. Not quite stock, mind you: Subtle differences include modifications to the engine cases to accommodate the frame's mounting hardware, and a European-spec close ratio five-speed transmission. Notice that we didn't say anything about combustion chamber modifications -- using the stock H-D top end reputedly let Buell get past the environmental watchdogs (except in California, which was a big pain in the butt) since all the work Harley did in getting the motor to pass Environmental Protection Agency emissions tests was viable. For Buell to get the motor to pass the stringent right-wrist test, engineers designed a free-breathing two-into-one exhaust system and a similarly free-breathing intake system. These bolt-on modifications add up to an impressive 67.6 peak foot-pounds of torque at 4500 rpm (as measured on the DynoJet dyno), supposedly 18 percent more than a stock Evolution. Accordingly, the Buell is a roll-on king -- whack the throttle open in any gear and the Buell launches forward. But the catch is you can't start out at, say, 5000 rpm, because the stock Buell falls flat at 5200 rpm. This brick wall at the end of the powerband is caused by overly-retarded ignition timing -- while messing around with our long-term test Buell, we drilled the ignition cover off (it's a small, 3.125-inch cover on the right side that's pop-riveted on) and advanced the ignition a few degrees. Presto! Instant horsepower. Well, not really -- it took a few tries and several dyno runs. But once the ignition is properly advanced, a stock Buell will rev cleanly to 6000 rpm and pick up around three horsepower. Careful that you don't over-advance it, as you'll lose torque.

 Horsepower, as you can guess, isn't the Buell's strong point. Luckily most of the fixes are easy and only a hop, skip and a local Harley-Davidson dealer away. Once we got clearance from Harley-Davidson to start modifying our testbike (H-D maintains the Buell press fleet), we zapped a few e-mails to the Buell factory asking for some hotter cams and a ignition black box. The parts showed up the next day -- a set of Screamin' Eagle (SE) mild cams that retain the stock valetrain gear, including the hydraulic roller lifters and a SE black box that had 'Tilley' written on the back. Don Tilley, for those of you that don't follow road racing, tunes for multi-time 883 Twin Sports Champion Scott Zampach and does a lot of parts bin picking to get the best-performing stock (legal) parts.

Toys in hand, we headed off to Graves Motorsports -- site of the official Motorcycle Online dynamometer -- and had them prototype a slip-on muffler to replace the stock reverse-flow cannister. Once up on the dyno, we did a baseline run and then bolted on the new straight-through muffler, immediately picking up 4.0 ft-lbs or torque throughout the rev range. Next, we installed the SE cams, a ridiculously easy process: First, drain the engine oil, align the motor to top dead center (TDC) on the number one cylinder and remove the exhaust header, unbolt the ignition sensor plate that sits under the 3.125-inch plate -- taking care to etch a mark to realign it when putting the motor back together -- and pull the right side engine cover. The gear-driven cams use two bearings each, one in the engine case and one in the right cover, so you can't just yank the cover off -- gently slide it off. Next, you unbolt the rocker covers and remove the rocker arms to release tension on the hydraulic roller-lifters. That done, the cams slide right out, no chains to loop around, nothing.

Installing the cams is just as easy, in reverse -- each cam is numbered one through four and marked with punched dots for alignment, so you simply insert each cam in the appropriate slot and play connect-the-dots to line them up. Careful that you don't turn the motor over for a few minutes after re-torquing the rocker arms, as the lifters need some time to bleed down and compress. Otherwise, they'll lock open and shove the valves into the pistons. Each cylinder should be done separately and at its TDC.

With the cams installed, we bolted the stock muffler back on to see what the results were -- and we we shocked when we lost up to 8.0 ft-lbs of torque at low rpms! The SE cams, don't have much more lift, but they do have a lot more duration -- the stock cams, on the left, are noticeably narrower than the SE units. We bolted the Graves Motorsports muffler back on -- it just has a straight-through perforated tube wrapped in fiberglass, a la "glass pack" mufflers -- and peak torque shot up to plus-6.0 over stock. One of two things were happening here: The stock reverse-flow muffler, CAD-designed for this motor, was tuned for the stock cams and pulse-charging exhaust gas back towards the cylinder -- like a two-stroke that's "off the pipe" -- and the longer-duration valve timing was keeping the exhaust valve open when the wave hit, or it simply had too much back pressure to let the motor breathe. As an added bonus, the Graves slip-on isn't much louder than the stock pipe, and certainly won't wake the neighbors after a late-night ride.

 With both the cam and the new pipe back in place, we bolted the SE black box in, and immediately picked up 3.0 ft-lbs throughout the rev range -- by far the easiest power gain since the box just snaps in behind the battery. Before we started modifying our test Buell, it measured 67.6 peak ft-lbs@4500 rpm with 63.4 bhp@5000 rpm, once modified it pulled 75.8 ft-lbs@3500 rpm with 76.5 bhp at 6000 rpm. Gaining over 13 peak horsepower so easily (roughly 21 percent) is certainly impressive, but take a closer look at the dyno chart below: As modified, the torque peak shifted down 1000 rpm while the horsepower peak shifted up 1000 rpm. This broadens the usable power band since you want to start accelerating at the torque peak and shift at the horsepower peak.

Dramatic power gains from the Harley-Davidson 1200cc Evolution engine are easily accomplished -- the -S lines are stock, the -M represent a few bolt-on modifications -- so we recommend that all Buell S2 owners add, at least, an aftermarket slip-on muffler and Screamin' Eagle black box.

We never had detonation problems with the stock motor, but jetting is really lean from the factory. The upside is it nets around 60 miles per gallon on the freeway, but robs power. Once you start modifying a motor to breathe better, it actually runs richer, so the stock jetting was perfect for our "built" motor. Also, don't even think of removing the crankcase breather tube that feeds directly into the mouth of the carburetor -- the motor makes more power with it in, and if removed, the first time you roll off the throttle at high speed, you'll blow the valve cover gasket (Oops! we did it twice. Luckily, the crush gaskets for the cover are less than a buck). Fuel economy dropped to 50 mpg on the highway, 40 or less in the city.

Buell made good use of their access to Harley's parts department, using many off-the-shelf pieces. The switch gear and levers are familiar Harley fare as is the ecumenical (for Harley Davidson) belt drive. All electrics are sourced from Harley -- including the handy self-canceling turn signals.

A source of worry for Buell was the inherent vibration of a 45 degree V-twin. But this is one area where Erik and his team of engineers studied long and hard: What they needed was a way of allowing the motor to shake freely so that the evil forces of vibration wouldn't tear the bike apart, yet use the engine as an integral, strength-enhancing part of the rolling chassis. The result was the patented Uniplanar mounting design that accomplishes both tasks with efficiency and simplicity. First, the swingarm bolts directly to the motor for rigidity, and a system of three torsion bars spaced at 120 degree intervals around the motor allows the motor/swingarm assembly to shake in only one plane, in-line longitudinally with the bike and the front wheel (telescopic forks let front wheels move in one plane longitudinally, so why not let the rear wheel do it too?), and prevents movement in any other direction. Thus, the engine/swingarm unit can't "flex" transversely, so handling integrity is maintained. The torsion bars -- which don't support any of the engine's weight once the bike is assembled -- keep the Harley motor from shaking you to bits at high rpm, while a series of rubber mounts bolt between the frame and engine/swingarm, and are the final link between the front and rear wheels.

The running gear is comprised of a list of parts coveted by roadracers world-wide: White Power "upside-down" forks and fully adjustable rear shock, Marchesini wheels (Performance Machine spun aluminum wheels are a value-added option), Performance Machine six-piston front brake and Brembo rear, both with stainless-steel braided Teflon® brake lines. It even comes with a K&N air filter mounted in a carbon-fiber housing. The chassis follows along similar lines, with a triangulated chrome-moly tubing dressed in white powder coat. Wheelbase is in the middleweight sportbike range at a short 55 inches. Rake and trail numbers are racer-inspired at 25 degrees and 3.9 inches, respectively.

 The combination of these two (seemingly) polar opposite life styles is like nothing we've ever ridden. This is a good thing, but like most projects, it also has its share of teething problems. For a sport bike enthusiast, throwing a leg over the bike is like settling into a familiar chair: Low bars stretch the upper torso over the curvaceous tank and into the accustomed position. The thickly padded footpegs are mounted fairly low and directly below the rider's knees -- the peg-to-bar ratio is correct, but the gas tank is a bit too long and forces the rider to site farther back than he should.

In going a little too far to save unsprung weight, Buell opted to use one front brake rotor that's as big as possible. There is just enough room between the rotor's edge and the rim of the wheel for the caliper. Once assembled, clearances and stopping power are no problem, but try to get the wheel off for a tire change and you'll find that the caliper won't come off the rotor without first taking the rotor off the wheel. Buell's efforts to reduce unsprung weight are laudable, but merely changing to a 320mm rotor would make a common service item much easier and less costly without sacrificing much stopping power.

 The wind tunnel-inspired fairing, complete with lightweight Gustaffson tinted shield is supported by two arms that slide into the forward frame rails. This bracketry also supports the weight of the headlight and instruments and proved to be no match for the vibrational onslaught of the Harley motor at idle -- oddly, the harder the motor shakes, the better the Uniplanar system works, so you don't feel the shakes once up to speed. Fortunately Buell recognized this problem and already had an updated part before it became a problem for customers. Our modified engine has a much smoother idle than a stocker -- due to revised cam timing, as well as proper ignition and jetting -- and doesn't shake the fairing at all.

 But none of this seems to matter when the bike is in its element, lazily coursing through some serpentine road. This bike grunts out of corners with authority, and with the suspension set properly, handles the corners like a pro. Conversely, if the suspension is set up wrong the bike is evil in the twisties -- a dilemma compounded by the fact that the suspension is highly sensitive, and one click of a damper can be the difference between smooth riding and a bouncing down the road. We set the sag at 1.0 inch on the rear, with the compression damping adjuster on position "2" rebound on "6." Our front, we left the stock, non-adjustable springs alone and set compression damping at the "2" marker, turning rebound to one click shy of full "slow." Maintaining corner speed is this bike's strong point. Find a comfortable speed for the road, leave her in top gear, and straighten out the kinks in your favorite back road. You'll be rewarded with a unique and thrilling experience that, should you decide to stop for a moment, you'll be able to share with the throngs of oglers that swarm to the Thunderbolt like moths to the flame.