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Motorhead Memo: Twist and shout

By Kip Woodring

Motorhead-web

OK, since there are at least 900 things that can cause your bike to handle poorly, we’re gonna have to narrow it down. To do that, the presumption is that you have enough tread on proper tires that are correctly inflated, working shocks with spring adjustments that work for the intended load and dampen in both directions, forks (set like shocks) filled accurately with the correct fluid, triple trees torqued, bearings all great (especially in the swingarm), payload distributed correctly—shall I go on?

There are some things to keep in mind:

1. Where do chassis loads come from?

2. Where do these loads go?

3. What happens when they get there?

4. Rubber-mount motorcycles are a special case.

No matter how stiff the frame or swingarm, the one area where torsional deflection or lateral flex is most likely to occur is the front fork. All right, to be perfectly clear, the other flexible parts of the chassis are the swingarm pivot, rear shocks and the axles. These dangers to the true trajectory of a two-wheeler interact constantly on any motorcycle. Add rubber and a design that isolates the engine, transmission, swingarm and rear wheel from said frame and you’ve added a whole new layer of complexity to the handling dynamics. Truth is, trouble often appears, not in the form of loads from cornering, transferred by grippy tires, but surprisingly—mainly—pull from the drive belt (or chain), which can bend the rear axle into an “S” shape. If the wheels, shocks, swingarm and its pivot are stiff enough (which they usually are these days) rear wheel deflection is held to less than one degree. No problem, as far as it goes. But, with our four points in mind, let’s go further.

The 1970’s were the last Ice Age in motorcycle development terms. No radial tires, no fuel injection, only Honda Gold Wings and Suzuki GT750 “Water Buffaloes” sported water jackets, nothing for sale to the public made 100 hp, engines were wide, tires were narrow and slippery, and chassis were from the 50’s. Mostly, though, we suffered a “glacial” 55 mph speed limit and computers that were barely out of the Stone Age as design tools (let alone on-board nannies). Only a handful of motorcycles were considered good “handlers” 40 years ago: none were Japanese; none had fairings—except Harley’s FL Big Twins.

One influential machine on the “handler” list was the Norton Commando. Unique at the time for its patented, rubber-mounted “Isolastic” method of vibration control, it was considered as good or better in the twisties than its benchmark predecessor, the renowned “Featherbed.” As long as the Isolastics were kept in shape and properly adjusted, that is!

The front rubber doughnut, as most are aware, has been changed from a Barry 28043 with protruding spacer to a Lord J-21920 with a flat (more stable) top. The later one (H-D #16207-79D) is also considered to be “stiffer” in the vertical plane by most who have tried both back to back. Some say too stiff! Be that as it may, unless you are willing to experiment with aftermarket versions, like Drag Specialties #DS-243515 (which is urethane), Glide Pro or Velva Ride, etc., this is what the factory considers appropriate—and all they offer. No matter, because with a bonded bush such as this, the “failure” is usually at the bond of rubber to center spacer and most always appears first as “cracks and splits” normally visible from the bottom of the mount, not the top.

The front rubber doughnut, as most are aware, has been changed from a Barry 28043 with protruding spacer to a Lord J-21920 with a flat (more stable) top. The later one (H-D #16207-79D) is also considered to be “stiffer” in the vertical plane by most who have tried both back to back. Some say too stiff! Be that as it may, unless you are willing to experiment with aftermarket versions, like Drag Specialties #DS-243515 (which is urethane), Glide Pro or Velva Ride, etc., this is what the factory considers appropriate—and all they offer. No matter, because with a bonded bush such as this, the “failure” is usually at the bond of rubber to center spacer and most always appears first as “cracks and splits” normally visible from the bottom of the mount, not the top.

When it came time for The Motor Company to move its products into the 80’s, they took a real hard look at the Commando system. To avoid patent infringement issues, changes from the Norton system were few but key. Instead of five rubber mounts (using shims to control lateral and torsional movements) Harley used three non-adjustable mounts and two lateral ball-end type “stabilizer” links to hold the vibes in one vertical plane. After nearly a decade of stop-start development, the FLT was finally released in 1980. Although the FLT chassis was as close to an all-new Harley as the factory had gotten since the 60’s, most of the design features were achieved by men without the aid of computer modeling.

These men (Erik Buell, and his three-link patent, not among them), on the way to a vibration-controlled touring chassis, created a five-speed transmission with a swingarm mount incorporated, a new backbone frame and rectangular swingarm with shock mounts located close to the axles to avoid flex, a short primary in order to bolt the engine and tranny together to make them into a stiffer “unit,” and much more. Arguably, the most praised of these additional innovations was (and is) the reversed triple trees for the front forks. In Harley speak: “balanced” forks that make steering a big machine with a heavy fairing perched right on it effortless.

To use these forks, as well as the fairing integrated into the overall design, the FLT frame has a steering stem several inches away from the junction of the backbone and the down tubes. Nicely boxed in and gusseted due to extensive testing, it is still a lot more laterally limber than, say, an FXR steering head. This “feature” wasn’t altered in the struggle to make a batwing fairing work on a frame it was never designed for, either. The fact is, dealers started sticking batwings on FLTs before the factory ever did. The predictable result, at first, was really crappy handling! (A clue!) The Motor Company also had a hell of a time coming up with the FLHT because of this. Truth is, they never did get handling back to FLT levels, because those forks have a decided pendulum effect when you bolt a non-aerodynamic weight to them. Would you also be interested in the science that demonstrates most weaves and wobbles actually emanate from the front end of the bike, even if you think it’s coming from the back? Well, we can save that for later or you can look it up. I think you get the point.

These "three-point" bagger frames are decades apart in age but almost identical in design. The backbone, while not ideally straight, is torsionally stiff and the cross braces underneath as well as the boxing and gusseting around the steering head make for a frame that, under any usage it was designed for, will not readily shape shift. Yet, the swingarm and its bushings/bearings have been made increasingly rigid over the years. Less flex in the arm and axles means more load fed from tarmac to that “stretched-out” steering head. Going around corners in itself isn’t likely to show defects in the arrangement. Chopping the throttle going in, or nailing it coming out of an apex, just might.

These “three-point” bagger frames are decades apart in age but almost identical in design. The backbone, while not ideally straight, is torsionally stiff and the cross braces underneath as well as the boxing and gusseting around the steering head make for a frame that, under any usage it was designed for, will not readily shape shift. Yet, the swingarm and its bushings/bearings have been made increasingly rigid over the years. Less flex in the arm and axles means more load fed from tarmac to that “stretched-out” steering head. Going around corners in itself isn’t likely to show defects in the arrangement. Chopping the throttle going in, or nailing it coming out of an apex, just might.

The virtually unchanged swingarm doughnuts very rarely show visible damage or wear externally. Sometimes even if you take them off and look inside, you can’t easily tell if they are shot. (Yet, this should be one of the first things checked after a close encounter with a pothole.) The one you see here failed completely, and is the exception, as the damage clearly shows. Simple replacement makes it sensible practice to renew these things at least every three years or 30,000 miles.

The virtually unchanged swingarm doughnuts very rarely show visible damage or wear externally. Sometimes even if you take them off and look inside, you can’t easily tell if they are shot. (Yet, this should be one of the first things checked after a close encounter with a pothole.) The one you see here failed completely, and is the exception, as the damage clearly shows. Simple replacement makes it sensible practice to renew these things at least every three years or 30,000 miles.

Unless it has completely collapsed like this one! I believe the purpose of the change in these 1980-2008 front doughnuts was more to prevent this than any other reason. With the larger surface area on top, the new “Lord/-79D” mount is (pardon the expression) “bound” to be more durable and less likely to flex sideways.

Unless it has completely collapsed like this one! I believe the purpose of the change in these 1980-2008 front doughnuts was more to prevent this than any other reason. With the larger surface area on top, the new “Lord/-79D” mount is (pardon the expression) “bound” to be more durable and less likely to flex sideways.

Harder to comprehend was the continued use of silent block/cleve block bushings in the swingarm. Although much maligned of late, these things are actually pretty tricky parts. First off, they are not rubber, and therefore don’t have much in common with those bushings used almost universally in automobile suspensions systems. What they are, technically, is a silicon-filled, elastomeric tube mount, compressed in an interference fit, as a specially-designed automatically compensating component of the original swingarm. (Well, kiss my urethane and rock my sphericals!)

This suggests cleve blocks were an engineering “insurance policy” against excessive loading and/or damage to the aluminum tranny mount, as well as a vibration cushion to keep both twist and shakes away from the drive line, let alone the person in the saddle. The compressed silicon filling and the durometer rating (range of Hertz damping) means cleve blocks are easier on the entire drive train and plenty stiff enough for spirited handling when in good shape. Make no mistake; they do not last forever, but while they last, they aren’t nearly as bad as some would have you believe.

When H-D designed the 20-percent stiffer FLT frame in 2009, it featured four rubber mounts, strategically placed in the corners of the chassis—almost the same arrangement as the Norton had back in the 60’s! The single lateral link is no longer adjustable either and has been placed on the top of the front rocker box. So, we have a much stiffer frame, surrounding a less “lever-able” driveline, bottom line. But consider this: the ’09 and later models also have “softer” primary drive compensators and “cush drives” in the rear wheel.

Spherical bearings, stronger swingarms, bigger pivot shafts and axles are part of the package, as engineered, for later model and/or big-inch FLTs, because of the massive increases in torque since 1980.

It is a given that polyurethane bushings, spherical bearings, added links and many other aftermarket “improvements” are stiffer. That said, these components also make for a much stronger (and longer) “lever” that can be applied like a crowbar to the frame by the driveline via your right wrist. So, regarding 1980–2008 models the real question is, overall, are they better?

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Since the older three-point, two-link setup was lacking any real “cush” in its driveline, most of the shock loads from the driveline (or the road) are taken up and absorbed by cleve blocks. This older driveline design, which served pretty well for over two decades, may not take kindly to stiffening in one area (the pivot) and neglecting the other areas, especially in the long run.

Harley manuals all mention “check” or “inspect” schedules for links and rubber mounts as part of routine maintenance. What they don’t tell you is exactly what to do. They never mention recommended replacement intervals for any of these things either! Considering that mileage, age and exposure to the elements are major factors for deterioration in handling and vibration in the rubber cushions (as it is for tires), seems like they would give some damn details! The factory wants the stabilizer links checked every 5,000 miles. But with no guidelines, that could mean anything from making sure they are still there, to simply tightening the bolts, to removal in order to find out if the ball ends are seized or about to fall out. Most of us never touch them! As critical as they are, this is nuts!

The factory rod ends have a PTFE (Teflon) “liner” between the ball and socket on the business ends of all stabilizers (adjustable or rigid) and they are indeed tough and durable, but suppose—just suppose—there’s some wear/play/stick issues that you never noticed until the driveline tries to lever the front mount and/or link, via the fulcrum of the swingarm pivot! Since the doughnut has a near 1/2″ hole in it where the 3/8″ bolt goes, a crappy link, loose bolt, and/or a tired front doughnut could then let the whole driveline flex sideways… in a bad way! Think an “event” like that could make for quite a wicked weave and/or wobble? With older models, this scenario is actually more likely than flex in the swingarm pivot. Three points define a plane, but if any of the points move, well, that’s the definition of a “plane” crash! Ensuring the cleve blocks, stabilizers and rubber mounts are in good shape is the best insurance against getting out of shape!

On the new four-point system, most of what could result would be less abrupt, held mostly in check via one (or more) of the four corner mounts—if the mounts are in good shape and the softer driveline cush. Not that it’s fool-proof!

(To be continued…)

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