The fork-travel measurement...
The fork-travel measurement was taken with a linear potentiometer. The Drack data-acquisition system is capable of data rates that allowed us to parse the suspension's movements quite finely.
When you bought that cruiser, you knew you wanted a motorcycle that was long and low—not only for the stylish profile, but for the luxurious, comfortable-looking style that goes with it. You're obviously not the only cruiser buyer who feels that way, and the manufacturers have paid attention. That is the way it's supposed to work, isn't it? Yet sometimes consumer demand speaks louder than logic. So it is with cruiser suspension design and technology. Overruling almost all engineering needs is the requirement to make the bike long and low, a look that doesn't work as well as it might appear.
The requirement to be long is not that difficult to accommodate. To get there you need to build a longer frame, allow the engine designers some latitude and stretch the bodywork to match. It's the low part that's create problems. Keeping the seat low requires clever frame configurations and forces the engine designers to make compromises in the V angle and cylinder height so that it all fits into what has become a long, low box.
Perhaps the hardest function hit by this fetish for long and low is the suspension. Here's what we mean: Hit a nasty bump on the road and a shock reverberates through the grips and pounds your backside. Lean a cruiser into a moderate-speed corner and feel the wheels take on a life of their own, with the chassis bobbing and pitching, either end acting as if it would rather be somewhere else. Load your long, low cruiser with a passenger and luggage and you might find it riding perilously close to the bottom of the suspension travel, even hitting bottom with a loud and painful thunk on occasion.
And it's not as if manufacturers don't know better. They spend a tremendous amount of time developing and fine-tuning cruiser suspension. This is, we know for a fact, not a slap-it-on-and-go approach, but a methodical, organized trial-and-error program in which experienced test riders evaluate several options and make considered compromises thought to appeal to the widest audience.
Accepting the fact that the manufacturers' test riders aren't stupid and that leading OEM suspension companies such as Kayaba, Showa and Soqi are capable of great things, we arrive at the central question: Why isn't cruiser suspension better? Given enough time and money, is it possible to make a heavyweight cruiser's suspension genuinely good? We decided to find out.
Meet The Mule
We used a Drack data-acquisition...
We used a Drack data-acquisition system to measure suspension movement. The small battery-powered computer was mounted to a plate where the passenger saddle used to be.
We kicked around several performance cruiser platforms for this experiment, but came quickly to the Honda VTX1800C. First, it's one of our favorites, with a great engine, comfortable ergonomics and generally sophisticated airs. Second, you've heard us clamor on enough about the Mean Streak. Third, while the Yamaha Warrior, with its aluminum swingarm (working through a linkage to a high-quality single shock) and inverted fork, might have made for an interesting experiment, it hardly represents the EveryCruiser.
We started riding the VTX, reacquainting ourselves with its gregarious power and —oh, yes, now we remember—limited suspension travel and cornering clearance. To stave off insanity and backstop our subjective impressions, we fitted the bike with our Drack data-acquisition system. Using a small computer powered by the bike's battery, the Drack system can capture up to 180 minutes of high-resolution data that can, in turn, be pulled up in a sophisticated data-reduction program for analysis. On our mule, we elected to measure front and rear suspension travel through a pair of linear potentiometers. Every time the suspension compresses or extends, the Drack system takes note, with as many as 20 readings per second. We also recorded vehicle speed through a rear-wheel sensor and experimented with a small accelerometer mounted below the main saddle.
For the sake of consistency, one tester made every measured run over a predetermined course that included city streets, a few twists and turns and, most illustrative for tuning purposes, a section of concrete-slab freeway. In fact, this segment of road is nearly famous. Some 20 years ago, Honda took cast samples of it for reproduction at its test facility in Japan. Bikes that seem to have pleasant, compliant suspension everywhere else in the world often turn into bucking broncos atop the infamous Harbor Freeway. Later, we gathered the staff for back-to-back riding with the (nearly) completed bike and another VTX graciously loaned to us by Honda.
Baseline Thoughts
The wheel-speed sensor helped...
The wheel-speed sensor helped us keep track of vehicle speed and also place the bike on the same piece of road—or close to it—to help compare one setup to another.
First subjective impressions place the VTX's suspension in the middle of the big-cruiser spectrum. Firm in midstroke—where the suspension spends most of its time&151but prone to bottoming on really large bumps, the VTX seems at once a bit harsh and kind of floppy. Transitions from one concrete block to another on the freeway don't seem to move the front suspension at all. (This is an illusion; the data shows the fork moving constantly, just not very deeply.)
With the baseline readings stored away, we promptly tore into the bike. Plan A was to send both ends to Traxxion Dynamics, a suspension-tuning firm near Atlanta. Traxxion is more accustomed to sportbikes (and does a sizable business supplying the racing industry), but owner Max McAllister is almost deranged in his willingness to experiment. He took on our little science project without hesitation.
McAllister was more animated than usual after he'd been through the fork legs. "You won't believe this," he said. "There's damping on one side only. The other leg is just spring and some oil. Amazing." At least the other leg had a proper cartridge damper, something he could revalve with some hope of success.
The rear travel sensor was...
The rear travel sensor was mounted vertically to help measure actual wheel travel.
McAllister disassembled the fork and considered what he had to work with: a heavy motorcycle with sufficient power. Using his experience with sport- and racebikes, he chose a spring rate (using straight-rate springs, which is the preferred method among high-end tuners) and damper settings appropriate for the VTX's weight. Simply put, McAllister turned up the spring rates and dramatically increased low-speed damping—in part to help bring the chassis under control, in part because one damping circuit would have to do the work of two. Traxxion's Axxion damper pistons are designed to have more maximum flow than the standard items, which dramatically blows off (relieves) high-speed compression damping over the sharpest bumps.
Fork work is well within Traxxion's purview, but we were stymied on the shocks. The original plan was to Frankenstein up something from hlins or Penske that could work on the VTX. Alas, McAllister could not find the right combination of body, spring and eyelets that would do the deed on the mighty Honda. Dang it.
The off-the-shelf choices amount to Progressive Suspension items or custom-valved Works Performance shocks. Eager to try Progressive's new 440 shock with the patented Inertia Active System (I.A.S.) valve, that was our choice. (We have good feedback on Works' stuff as well, and it has one advantage—the shocks can be ordered for a specific rider weight.) For good measure, we ordered 12.5-inch Progressive 440s half an inch longer than stock.
On The Road Again
This graph (click for a larger...
This graph (click for a larger view) shows suspension travel over time, before and after the first round of modifications. With increased damping and spring rates, the modified fork moves less than the stock item, but the Progressive shocks react much more quickly and vigorously to bumps. It looks busy, but the ride is better.
The modified VTX was, at first, an unhappy camper. The shocks seemed fine, but the front was far too stiff. (We'll get back to the shocks shortly, but it's worth understanding that suspension ills can be deceptive. Sometimes a problem with spring or damping rates in the shocks can make the fork feel wrong, and vice versa.) Check out the travel data in the accompanying chart. The fork hardly used any of its travel, and responded poorly to both small and large bumps. The good news was that with the front end raised, the VTX now had much more cornering clearance. Hmmm. What's up here?
Lesson number one in tuning a cruiser's suspension: Fork angle makes a big difference. Accustomed to the sharp rake angles of modern sportbikes, McAllister and the guys applied their weight-to-spring-rate theories to our VTX. The math called for stiff springs 1.15kg/mm items. That's a long way from the stocker's progressive-wound, 0.45kg/mm starting rate.
What's happening is that bumps are trying to bend the fork as much as compress it. It makes sense: The sliders can only move along the major fork axis, but bumps act on the suspension in a near-vertical plane. The greater the rake, the more pronounced the effect. Rather than slide smoothly up into the legs, the fork lower is pressed hard against the bushings inside that carry the load, increasing friction (sometimes called stiction) and adding harshness. You can move the bushings far apart by increasing stanchion overlap—as Honda has done with the VTX fork—but you can never fully overcome this rake effect.
Aha. The light went on. McAllister promptly sent us a second set of springs rated at 0.9kg/mm. We first tried replacing just one—giving us one 1.15 spring and one 0.9 spring—for a total rate of 1.025kg/mm. Better, much better. We can see in the data that the fork stroked more easily at the expense of riding lower in the travel.
Although losing some ride height up front hurt cornering clearance, the longer shocks out back afforded a bit more height at the rear, which provided an overall improvement. At this point, we called Honda to get a second, totally stock VTX for some back-to-back comparisons. In a nutshell, we liked the modified bike better, mainly because the chassis felt a lot more composed and less willing to pitch back and forth during hard acceleration and braking. The VTX's mild shaft effect—the tendency for the rear wheel to extend as you get on the gas—was nearly eliminated with the Progressive shocks.
In fact, we have nothing but praise for the Progressive 440 shocks. The IAS system uses a floating valve that alters rebound damping according to rear-wheel movement. Sit on the bike and the shocks feel almost wildly overdamped. The rear end moves slowly and predictably but much less freely than stock. But look at the data. The Progressives use more of their travel more freely than the stock shocks by a wide margin. This increased wheel speed—the ability of the wheel to quickly follow the bump and then just as quickly return to the road without upsetting the chassis—is the Holy Grail of suspension compliance.
The main benefit of the IAS valve is that it dramatically relieves rebound damping after a hard hit, allowing the wheel to plunge back toward the ground where it's needed to maintain traction. With the stock shocks, the wheel could not respond as quickly.
The Final Round
Though our efforts improved...
Though our efforts improved the ride compliance and control of the fork, it remained limited by the angle of the legs and the weight of the wheel.
Inveterate tinkerers are we. So with the end of the project looming, we tried one last iteration. Replacing the remaining stiff spring with the other 0.9kg/mm fork spring and substituting the 7-weight fork oil for some 5-weight, we reduced overall spring and damping rates significantly.
This was the best configuration yet. The fork was more supple—though still not as good as the best sportbike or sport-touring suspensions—yet still more controlled than stock. The additional low-speed damping showed up in a more stable, predictable chassis. Still, the Harbor Freeway could get the fork bobbling along, seeming like it was locked solid. By increasing the static oil level in the spring-only leg, we forced the front end to be more progressive at the end of the travel, thereby reducing bottoming out while still permitting comparatively soft spring rates. It's all good.
And yet we're slightly unfulfilled. Despite a lot of effort and plenty of tinkering, the VTX's ride is still not as good as we'd like it. Better, much better than stock; but still not the combination of creamy smooth and perfectly controlled we'd hoped for.
Here are a few reasons why. As mentioned, the fork's rake effect makes it difficult to get truly luxurious action. You can improve it, but there's only so much you can do to fool physics. Add to that the typical cruiser's rearward weight bias, which keeps the front end more lightly loaded—as a percentage of overall weight—than on a sportbike, and slightly harder to tune as a result.
Moreover, the VTX—and bikes like it—have heavy wheels. With tires and brake discs, the VTX's wheels weigh 18.1 pounds front and 28.3 rear. That's a long, long way from a typical sportbike's lightweight hoops (11 pounds front and 17 rear, typically). A heavy wheel is harder to control —more resistant to the initial deflection of bumps but also harder to stop once it does get going. And those heavy wheels are often fitted with tall-sidewall tires, which will flex more than their short-sidewall counterparts on sportbikes. The effective spring action of a tire is basically undamped, so it's hard to control.
Finally, all these shortcomings are heaped on top of short suspension travel. Simply put, you make the suspension's job much harder when you try to cram six inches of quality travel into three and a half.
What This Means To You
If we had to do this all over again, we'd shop first for the shocks. By a wide margin, they made the greatest difference in the VTX's ride quality. We suspect it's because you can't do a whole lot with the fork, but more importantly that shocks do a lot to make you feel better. After all, the riding position puts most of your weight right over the shocks. A small change there is more obvious than a big one up front. We have no hesitation recommending the 12.5-inch shock for the VTXC, but even the stock length is likely to be an improvement. The Progressive Suspension 440 shocks may be the best $535 you could spend on your VTX. No kidding.
For the truly fussy, slightly stiffer-than-stock fork springs will improve the bike's handling. However, for those whose cruisers come with old-tech damping-rod forks, we strongly recommend Race Tech's cartridge emulator, an add-on device that makes this kind of fork act more like a sophisticated cartridge system.
Ultimately, we worked long and hard to improve the VTX's suspension, gaining less than we'd hoped, but more than expected. Science is like that sometimes.
Resources
Progressive Suspension
Traxxion Dynamics
Works Performance
Related Articles
How Front Forks Work
Installing Race Tech Cartridge Emulators in a Fork
Set Up Your Bike to Suit YOU
All About Wire Wheels
How to Replace Fork Seals
How to Service Steering-Head Bearings
For more articles on how to maintain and modify your motorcycle, see the Tech section of MotorcycleCruiser.com.