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Transformative Suspension for Motorcycle Forks with Damping Rods

Transformative Suspension for Motorcycle Forks with Damping Rods

Posted by Adam Devine, CEO & Avid Motorcycle Rider on Jul 23rd 2019

Proper chassis and suspension setup is transformative. It will transform your ride, it will transform you. It will make you a better person. Don’t scoff. This is not some wild claim. Hear me out. In first world countries like the U.S. motorcycles generally aren’t basic transportation, one step up from a donkey or an ox cart. Most of us own them for pleasure. If we do something for pleasure, wouldn’t it follow that increasing that level of pleasure would increase our enjoyment of the ride, and thus our overall sense of happiness and fulfillment? Would an increased level of happiness have a positive effect on our outlook, and our relations with other people? Voila, you’re a better person. Not buying it?

You know what else we do for pleasure? Sex. So answer me this: if you could, without a series of awkward or hurtful conversations, somehow tune your sex partner in to your every desire in bed, transforming her/him into the willing sex partner of your dreams, would you do it? I mean, without the associated guilt of slavery by whatever method? I think you would.

Competition riders in all motorsports understand well that good chassis and suspension setup is one of the keys to winning. But maybe you’re not that guy. Maybe you can freely admit that you are unsophisticated in the finer points of suspension. That’s ok. Most people are. Vehicle manufacturers of all types count on it.

The suspension components that go on most passenger vehicles and entry to mid-level motorcycles are cheaply made items just good enough to pass muster in a world of unsophisticated consumers. You will only find the good stuff on cream of the crop sport bikes and luxury/sport cars. Pay a lot of money for a slick looking cruiser? Sorry, you did not get the good stuff.

Maybe you’re thinking, hey, I don’t go in for crotch rockets, and I’m not an aggressive rider, so suspension upgrades are of little benefit to me. This is a very common assumption, and it could not be more wrong. Proper chassis and suspension setup is Transformative. This is true of every bike, even the high dollar race replicas. But the transformation is truly profound on motorcycles that are poorly equipped to begin with. I can guarantee you that once you’ve ridden a motorcycle properly set up for you, you will never willingly stoop to anything less. No matter what kind of rider you are, or what type of bike you ride.

There are 4 Cs in suspension. That is, there are 4 metrics by which we evaluate it, and they all start with C. Comfort. Confidence. Control. Contact (between tire and earth). The 4 Cs are inseparable, and as a rule they battle for dominance, for there is no one solution in which all are supreme. That's why we named our suspension kits C4.

The difficulty for motorcycle makers is that bikes are light vehicles, often 500 lbs. or less, and riders make up a high percentage of the bike’s rolling weight. American adult riders might be anywhere from 100 to 300 pounds, which means they can be anywhere from 20% to 60% of the bike’s weight. Even if we clip the extremes and put the average between 150 - 250 lbs., there is no suspension on a 500 lb. vehicle that will work well for all riders.

Add to this the fact that two of the four Cs have a high degree of subjectivity, and the OEMs are in a real pickle. What is comfortable to a 170 lb. commuter may be a horrific lack of control to the 170 lb. canyon carver.

The final hurdle for OEMs is that motorcycles, like every other consumer product, must be built to a price. Manufacturers understand that consumers—even of mid-level bikes—are going to have fairly high expectations regarding style, fit and finish, and engine performance. One of the areas in which OEMs consistently pinch pennies is—you guessed it—suspension.

What this means is that if you recently paid less than $10k for a new street bike, you almost certainly got bargain bin suspension. If your front forks lack any external adjustment, then you are probably riding on the worst of it—damping rods.

What are damping rods? They are the crudest method of damping. The technology has been around longer than most humans have been alive, and still soldiers on in plenty of new bikes today. Damping rods look like big, hollow nails and are about as sophisticated. In simplest terms, the compression of the telescopic forks forces oil through holes into the center of the hollow nail, and as the fork rebounds against its spring, oil is forced back outside the nail through another small hole, thus “damping” the action of the spring, and keeping your ride from pogoing uncontrollably.

You don’t have to understand hydraulics very well to realize that fixed orifice damping is severely limited in its ability to cope with the demands of widely varying rider weights, vehicle speeds, and terrain features. Even for a fixed vehicle and rider weight, it is not possible to accommodate varying wheel speeds (ie, vertical wheel speeds) with fixed orifices. Small orifices will afford reasonably good pitch control at the cost of harshness over bumps. Larger holes will afford more comfort over bumps at the cost of poor pitch control (eg front end dive during braking, or wallowing in dips).

As a simple illustration of how fixed orifice damping works, try this easy experiment. First, take a swig of soda through one of those big gulp-style straws that you get at a lot of fast food places and convenience stores. You can fill your mouth almost instantly, with very little suction effort. Now use the same straw to pull some milkshake into your mouth. Much slower fill, with much more effort, right? That is the effect of increased fluid viscosity. Now repeat the above procedures with one of those tiny coffee stir straws. This is the effect of a decrease in orifice diameter. Pretty basic stuff right? These are the limitations of damping rod suspension.

More than two decades ago, a much more sophisticated type of front fork damping was already commonplace on sport motorcycles. These “cartridge” style dampers use the same technology that most rear shock units do: a pair of two way valves (one for compression, one for rebound) have large orifices covered by a series of precise spring steel shims (like super thin washers). As vertical wheel speed increases, raising oil pressure, these shims bend to open the valve, allowing increased oil flow. The bigger the bump, the more the valve opens. By arranging the stack of shims with varying thicknesses and diameters, the valve can offer excellent damping characteristics for all wheel speeds, avoiding the harsh maximum flow rates encountered with fixed orifice damping rods.

When I was a motorcycle suspension specialist in the early 2000's I felt sure that damping rods would be completely phased out in favor of cartridges within a few years as the cost of the newer technology came down. But I was wrong. Here we are in 2019, and a great many new motorcycles still suffer from damping rod affliction.

There are a couple pieces of good news though. The first is that despite 20 yrs of inflation, today you can get most of the performance and technology of a Y2k superbike for the same amount of money you would have paid then. That’s remarkable. Oh, and don’t kid yourself, a 20yr old superbike is still better than 95% of riders. Moreover, there is an even greater selection of bike styles to choose from, particularly in the mid-price range, and many of these are now coming standard with features like ABS, an uncommon bit of tech in Y2k. Point is, $7-10k gets you a heck of a good bike these days, it just won’t get you top shelf suspension.

The other bit of good news is that in choosing a mid-level bike, you saved yourself thousands—congratulations!--and you don’t have to be stuck with that damping rod fork arrangement. For a fraction of what you saved, you can upgrade your front fork suspension, which typically is far worse than the rear shocks they’re paired with.