Rit Booth carries a long title: Harley-Davidson Program Manager for Rubber-Mounted Big-Twin Sport Bikes. The tall, M.I.T.-trained engineer is heart-attack serious about motorcycles, and like many of Harley’s young technical lions, is enamored with performance. One of his personal machines is an XR1000 modified almost beyond recognition.
Society of Automotive Engineers monographs on motorcycle stability bear his name, and he’s as comfortable pushing bits in computer models as he is twisting a throttle.
Yet any discussion with Booth about the project that’s filled the last few years of his professional life, the project that has become Harley’s newest Big Twin, the project that has revived the Sturgis name any such discussion revolves around one, and only one topic: achieving The Look.
The Motor Company isn’t in the business of quick, or fast, or best-handling; H-D’s in the image business. While that doesn’t mean function is treated as a bastard son, without The Look a Harley seems as bereft of purpose as George Hamilton without a tan.
After years of practice, the company has The Look down just about perfect. For proof, we offer the 1991 FXDB Sturgis, a name resurrected from Harley’s past, now affixed to its most radically changed motorcycle since the Evolution in 1984. In H-D’s lexicon, the FXDB qualifies as all-new.
Cynics may snort that this description usually means the company has simply reshuffled existing parts, bolting up a new model with as much substance as the emperor’s new clothes.
Thumbing the Sturgis’ engine to life gives the first indication that there’s more than that to this bike. As it chuffs along at idle, the quality of the vibration differs from other rubber-mounted Harleys – still pronounced, but the engine isn’t trying to jump free from its mounts, as it seems on the FXR, for example. Running through the gears confirms there’s something different about this Harley.
The vibration peaks at idle, and from there to redline it tapers off dramatically as revs climb; at highway speeds the Sturgis vibrates less than most four-cylinders. Roll off the throttle at any speed and the bike becomes eerily smooth.
Maintaining The Look, though, means that when this new bike is parked, only a dedicated Harley-watcher is likely to pick up the subtle clues to its new character. The blacked-out engine, glossy black paint, H-D orange pin-striping, and minor chrome accents are all direct lifts from the FXB Sturgis of 10 years ago, and its new namesake seems only paint-and-tape away from three or four models in the current line-up.
The sharp-eyed will notice the engine seems less cluttered, more tightly and neatly packaged. The lack of a traditional oil tank seems the only thing out of the ordinary.
Plenty of parts link the bike with the current line-up. The Sturgis uses the standard 80-cubic-inch, 45-degree V-twin, backed by a five-speed transmission, duplex-chain primary and belt final drive. Similarly, the fork, rear shocks, instruments, and many ancillary bits have seen service elsewhere in Milwaukee’s line-up.
Before the cynics can curl their lips around the words parts-bin engineering, however, remember that these components are H-D’s basic building blocks, and the company’s used these parts to fashion itself into the maker of the best-selling large-displacement bikes. Booth notes that building from scratch places inordinate demands on design and testing, and he saw little reason to reinvent the wheel in building the Sturgis. Besides, it would be stretching the point to claim the FXDB is nothing more than a parts-bin bike; nature built Michelle Pfeiffer and a garden slug out of the same DNA, but no one’s going to claim there’s little difference between the two.
Booth and Co. are entitled to a bit of hyperbole, particularly given that they’ve already got the definitive cruiser power-plant. Anyone who’s pulled the wire on a Harley V-twin knows there’s something indefinably visceral about the power delivery. The power starts at idle and is always there, regardless of rpm. The bike never drops below 55 pound-feet of torque from idle to redline, and its torque peak, 61.8 pound-feet, occurs at 3000 rpm.
At 3500 rpm, the Sturgis’ prime cruising speed, it’s making 11 pound-feet and 7 horsepower more than a ZX-11. This kind of bull elephant power delivery is the FXDB’s stock in trade. When you drop to 20 mph in top gear and then simply screw on the throttle to shudder away, there’s no denying its appeal.
Even though the basic driveline comes from the company’s FX series, it’s not a direct lift. The engine bolts to a host of new castings, including a primary case that moves the engine forward slightly, and rotates the cylinders 4 degrees back in the frame compared to the existing FXR. (Softails mount their engines so that a vertical line bisects the angle between the cylinders, FXRs mount theirs with this bisecting line canted forward 2 degrees.) The re-angled engine provides additional clearance below the steering head, which allowed the engine and transmission assembly to move forward in the frame. The additional weight on the front wheel improves high-speed stability.
In addition, the Sturgis uses the long primary case from the FXST Softail, which spans an inch farther, crank center to clutch center, than the FXR piece. The reason, Booth notes, gets back to the heart of the matter: That’s part of what gives it the length; by making it look longer you make it look lower. Longer and lower are two main components of The Look.
The added driveline length also provides room for the oil tank, which has been incorporated into a new casting below the transmission, so that it acts much like the sump on an automobile. This rids the bike of the long lines twining between the oil pump and tank, for a more sanitary appearance. The design simplifies manufacturing as well; by eliminating the oil lines running between the engine and the frame-mounted oil tank, the company can slip the complete engine assembly into the chassis pre-plumbed.
Further accentuating the length, the DB uses a chopperesque, 32-degree rake. Moreover, Booth bolted up a long fork (33.1 inches, compared to 31.6 for the short one), and grafted an additional inch into the swing arm. As a result, the Sturgis puts 65.0 inches of real estate between its axles, longer than anything else in the H-D pantheon save the Softail and Softail Custom.
Achieving The Look requires a lot of territory.
The real news, however, lies between those wheels. The FXDB takes its modified Big Twin driveline and bolts it into a thoroughly rethought chassis, and it’s this that earns the bike the right to be considered all-new. The revised chassis includes a new frame, swing arm, and reworked engine mounts.
What’s more, the Sturgis raises the functional aspects of H-D’s customs to a new level, in most ways surpassing the firm’s existing decathlon champ, the FXRS.
All this work centers on one major aspect of the FXDB: engine mounting. When the system used on other Big Twins was devised in the mid- to late-seventies, the company designed its frames around existing technology. The engine mounts Harley adapted to its own use were more or less off-the-shelf automotive items: basically a bolt surrounded by a thick rubber biscuit.
The original design involved a number of compromises, beginning with the necessity to use three rubber mounts—one in front, two in the rear to float the entire engine/swingarm/rear-wheel unit, with a pair of heim-jointed turnbuckles to help prevent torsional or side-to-side movement that would destroy wheel alignment and handling. The system allowed movement in one vertical plane only, without chain pull compromising vibration isolation. Within sharply defined parameters, the engine was free to move front to rear or up and down, but not side to side.
The design worked, but the triple-rubber-mount system made the frame relatively wide, since the rear frame rails had to reach around the rear rubber mounts, which sat outboard of the engine and swing arm. In addition, the rear rubber biscuits helped locate the rear of the powertrain package, and their side-to-side stiffness depended on how much they were preloaded—which also affected how well they absorbed vibration. The design almost guaranteed some bike-to-bike variation in vibration control.
With the new bike, Booth notes, he was looking for something simpler. We didn’t want to build the bike around the rubber mounts. [With the old FXR] we took the old mounts and packaged a bike around them. To replace the existing parts, he used a pair of laminated mounts of alternating rubber and steel. A pair of steel plates sandwich a metal block between thick rubber pads; the steel plates bolt to the engine and the center block bolts to the frame.
Helicopters use a similar system to mount the rotor blades.
Figuring the basic configuration of the mounts was only part of the solution. It remained to to determine the stiffness of the mounts, where to locate them, and even how to define what better and best meant in terms of vibration control. This was uncharted territory—so new that the existing FXR engine mounts gave no clues as to a starting point.
Booth, aided by a computer, had to sort through more than 46,000 potential variations.
The new system works because the new mounts offer stiffness that varies with direction: soft and compliant in the vertical plane to absorb vibration; stiff laterally to keep the rear wheel in line. One of the two rubber mounts snuggles under the front of the engine; the other attaches at the back, below the transmission; both reside on the bike’s centerline.
The two new mounts locate the bottom of the engine, while a single turnbuckle resides between the cylinders to control the engine’s top, and prevent any further out-of-plane movement. Compared to the FXRS, one turnbuckle and one mount is missing.
The system does exactly what it’s supposed to do for the rider: Overall, this is the smoothest Harley yet. The new mounts snub vibration differently—if not always better—than the old system. At idle, the Sturgis’ big V-twin leaps about less than an FXR engine, for a more refined if slightly sharper-edged feel.
At 35 mph to 45 mph in top gear the Sturgis passes slightly more vibration through to its rider than an FXR, though relief is only a downshift away. At higher road or engine speeds, vibration almost disappears; the view through the mirrors is clear, even though you can feel light, reassuring torque pulses from the big Milwaukee powerplant. On the existing FXR, the rubber mounts eventually lose control as the engine nears redline, but at high speed the Sturgis keeps buzziness under tight rein.
The system also allowed Harley to change the bike’s appearance, particularly the aft section. The widely splayed frame rails that loop around the swing arm on the FXRS are gone on the Sturgis, as are the exposed rails that run up to the seat. Instead, the big, stiff, rectangular-section backbone (which ends at the seatbase on the FXR) curves down, ending just above the transmission, and a pair of closely spaced round tubes snake up from there to support the seat and shocks.
Nary an inch of round tubing is visible behind the engine.
In addition, the stiffer new mounts limit engine movement. Whereas the earlier mounting system allowed for as much as an inch of engine/transmission excursion, the new arrangement cuts that approximately in half—a cut that was vital for this latest incarnation of The Look. Harley’s designers wanted to pack the engine closer to other components, as on the Softails with their rigidly mounted engines.
Movement in the existing FXR’s driveline prevents this, which gives Rubber Glides a less tidy appearance than Softails and their derivatives. The new mounts solve that problem.
Booth considers frame stiffness to be one of the current FXR’s best attributes, and wasn’t about to risk losing any of that for the sake of either vibration control or The Look. Frame construction is hefty. The rectangular-section backbone is made from 0.120-inch thick steel, and now works with with several new castings and forgings. The steering head is an investment casting, which allows the fine detail necessary to make the new steering-head lock a drop-in item.
The bridge for the swing arm is a steel sand-casting, while the engine-mount plates are both forged.
On each part we looked at the best process. We’ve really come at each one without any preconceived ideas of what each one should be. We used a mix of stamped parts, a lot of forgings, Booth notes.
Here again, the new bike is at least the equal of the old. Hard braking or cornering failed to bring out any hint of wobbling or instability. Granted, with less than 53 horsepower on tap and only a single disc up front, the FXDB isn’t going to put race-level stresses through its frame, but it still seems massively overbuilt another facet of The Look.
Within city limits, the Sturgis reigns supreme. The riding position strikes a near-perfect balance between the Softail slouch and the FXRS Sport crouch. A set of risers moves the Sport-style handlebar up 4 inches and back about 2.5 inches, and places the grips an easy reach from the saddle. The engine, wearing the Keihin CV carburetor now used on all Harley engines, starts easily, pulls strongly and smoothly from less than 2000 rpm, and delights with low-speed torque.
The new clutch that all Big Twins share this year requires less lever effort, and disengages more completely. With less clutch drag, the transmission shifts more smoothly, though it still requires a long lever throw, and gears still engage with a clunk but a softer one. Turn signals borrowed from the FL Ultra Classic touring models no longer flash only when their control buttons are held down; push once, and a signal flashes until a microprocessor decides long enough, or until you push again.
The Sturgis comes with the best stock seat ever mounted on a Harley custom it’s broad, flat, and thick enough to provide all-day comfort. Unlike previous FXR perches, it features a smooth, molded cover instead of the usual pillowy, button-tufted covers. More importantly, it doesn’t lock you into a single position, and for most riders still puts the highway pegs within easy reach.
Above 60 mph wind-blast will make you wish for a bit less rise from the handlebar, or for a duffel bag to lean against, but up to that point this is hands down the most comfortable Harley custom ever built.
Only a few detractions remain – and they’re in areas where styling and function inevitably crossed paths. The fork provides a slightly harsh ride – likely because the 32-degree rake tilts it out too far to work optimally. The rear suspension, though, is far worse.
The shocks offer an abundance of preload and a dearth of rebound damping. Rolling dips, such as suburban rain gutters, threaten to fling the rider’s butt out of the seat, and any abrupt-edged imperfections come through with the sharpness of a rifle shot.
Why? Because The Look, and Harley’s traditional customers, demand a long, low machine with a seat close to the ground – and that requires compromising either traditional wheel sizes or suspension travel. The Sturgis runs a classic 16-inch rear wheel, but allows it to move only 3.0 inches – not enough to take the edge off most bumps.
Booth and his team have mounted softish springs with substantial preload in an effort to make the best of a bad situation, but in the end that approach by making topping out more likely only makes matters worse on big bumps.
To discourage any misinterpretation of the bike’s non-sporting intent, Harley left a second front disc on the parts shelf. There’s a message to [the single disc]: This isn’t a sport bike, Booth says. But, here again, The Look and function are at odds. The Sturgis’ front brake requires an unusually firm grip, and offers unsatisfying feedback. The bike deserves better.
If it must use a single disc, it should be a bigger one clamped by a more sophisticated caliper, and with rethought leverage ratios.
Similarly, the engine, so pleasant in town or on the highway, doesn’t like being hurried. Near redline, the big V-twin loses much of its charisma, and it’s easy to bounce the low-revving powerplant off its 5200-rpm rev-limiter. The transmission also shows little enthusiasm for sport riding, becoming balky and refusing to change gears cleanly when shifts are hurried.
While the Sturgis wasn’t built with backroad sport in mind, it can handle curves surprisingly well. The locomotive-class wheelbase, stout frame, and conservative geometry keep the FXDB locked on trajectory in a corner, with nothing but the most reassuringly solid feel coming back through the bars.
Despite remarkably limited cornering clearance (kick-stand and primary case touch down on the left, front muffler on the right, when the bike has leaned only slightly more than 30 degrees) the Sturgis handles well. Lots of rake and trail slow its steering, but the 28-inch-wide handlebar keeps effort relatively low.
For normal, don’t-push-the-limits riding, this happens to be the best Harley custom yet. The Sturgis handles day-today living as competently as an FXRS Sport, yet the new chassis gives it a smoother, broader operating range. In its quest for The Look, Harley managed to enhance function as well; in the Sturgis the two are woven together as neatly and thoroughly as the strands of a rope.
Best of all, the Sturgis is only the beginning. This bike is intended as a commemorative model of sorts, marking the 50th anniversary of the Black Hills rally and the 10th anniversary of the bike’s namesake. But the new chassis design can be easily adapted to other Big Twin models, some that might have more suspension travel and a less rakish steering head angle. The Sturgis engine mounts also provide hope for someday seeing a rubber-mounted Sportster.
The tight engine-to-component spacing of the Sturgis isn’t far from XL practice, and the design can be adapted relatively easily to that chassis.
This is the most significant Harley to come out in almost a decade. The original FXRS in 1982 qualified as the most important new Harley in a generation. It showed what Milwaukee could do when it combined the company’s traditional values with modern engineering: Keep the styling without losing function.
The 1991 Sturgis does that bike one better; without compromising The Look, it takes function one step further.
Source Cycle Magazine 1991
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