I still like my white 1993 VFR "Snow White", don't get me wrong. The arrival of the white livery for the 2006 model (among other updates the VFR series has enjoyed over the years) gave me pause to reconsider, but eventually I decided that freshening up Snow White would cost a lot less, rejuvenate my interest, and be a lot easier on the bank account in better proportion to the reduced saddle time I get these last few years since the conversion from being a D.I.N.K. to a parent.
Mindful of an article I read in a British bike magazine a few years ago in which one of the famous racers (Niall Mackenzie? Neil Hodgson?) talked about how much more bang for the buck one gets with suspension than with motor, and having a local shop well-versed in suspension tuning, I took the plunge to have the front end reworked while the bike was due for a fork oil change anyway. Residual enthusiasm for suspension tuning carrying over from an OntVFR suspension clinic early in the year didn't hurt either.
A basic plan would call for new springs, and revalving of the rebound damping "valve" stacks to retune suspension behaviour. But I was interested in even more. One intriguing add-on was the concept of a mid-valve. And I had it in mind to scavenge a set of CBR600 forks to try to make use of the adjustable rebound damping they offer, unlike the VFR which has preload as the only adjustability. Luck was with me and I came up with a set of 1998 CBR600F3 forks that were quite bent and quite inexpensive (read, free!) thanks to benefactors who couldn't use them.
But what was the point of all this? Why mess with it when it was working? Well, it just felt like the suspension was a bit tired, not as planted and well-behaved as it was when I first got the bike, despite servicing the fork oil. I was also a bit bothered by the amount of fork dive, and mindful of the frequent criticism of the '90-'93 series for having overly soft forks. I'm a very slender fellow, so I assumed that they wouldn't be as bad for me as for 200 lb American magazine columnists who know their way around a race track reasonably well, but still. And of course, there's always the latent desire to tweak and improve even if things seem okay.
With these things in mind, I discussed the project with James Hunt and the crew at Cycle Improvements in Waterloo. First things first: new springs with a stiffer rate selected for the weight of me and the bike. Then an MX-Tech kit to replace the stock compression assembly and facilitate custom revalving. A custom mid-valve would provide even better control of compression damping behaviour. And finally, use of selected parts from the CBR forks would give me adjustable rebound damping atop the forks. New seals and fresh oil would set things up for smooth operation.
Straight rate or progressively wound? Each has its proponents. Hunt is a believer in straight rate for the following reasons.
Overall, a progressive spring is desirable so that the force required to compress the fork increases as the fork compresses. But there's already one there: the "air spring" that is created from the air column above the fork oil in the fork is very progressive in nature. That is why adjusting the oil level, which changes the volume of the air space, has such a dramatic effect in the last half of the fork travel. Adding a progressively wound spring on top of this will make the fork too stiff in the last half of travel.
The other problem with progressive springs is that they are very hard to wind, so consistency as well as the rate varies dramatically.
Hunt's preferred supplier of high quality springs wasn't able to deliver in a reasonable amount of time in this case, so we ended up going to Race Tech for a set of .80 kg/mm springs (PC203724, $139.95).
These were not as stiff as Race Tech's automated calculator recommended, but one of my main worries in this project was that the bike would end up being too stiff and uncomfortable to ride for long distances. I emphasized the desire to retain plushness and comfort for sport-touring rather than trying to turn the VFR into a track bike, so springing (and valving) was chosen accordingly.
For comparison, the stock '93 VFR springs are supposedly only .63 kg/mm and Race Tech was recommending .86 kg/mm for my bike and weight when aiming for standard stiffness for a rider of intermediate skill level. I measured the free length of the stock springs afterwards, and they were still right on spec, not bad for almost 70,000 km of use.
Since the CBR600F3 forks are the same diameter as the VFR's, I was hoping that the fork cap bolts and some internals would be usable in order to provide adjustable rebound. Adjustable rebound works by raising or lowering a slender tapered needle down in the cartridge when you turn the adjuster on top of the fork cap bolts, thereby affecting the size of an oil passage. The VFR's rods could not be used because they're not hollow all the way down; fortunately, the CBR rods were salvageable after a bit of straightening and lengthening, the CBR fork being shorter than the VFR fork. So, we used the CBR cap bolts and rods (and perhaps a couple of other little parts associated with attaching the caps to the rods), and the VFR parts for everything else (cartridges, tubes and sliders).
Voila -- adjustable rebound, though this really affects only low-speed rebound. It took a couple of hours of labour to incorporate the CBR parts, partly because my freebies were a bit bent.
When it comes to suspension control, this is where the magic, if there is any, really happens. The compression and rebound piston bodies and their "valve" or "shim" stacks control the damping characteristics of the fork by varying the flow of the fork oil through variable orifices.
The size, location, and pocketing of the openings in the piston body, and the selection of quantity, thicknesses and diameters of the shims, determine the effective size of the orifice that is created in response to low- and high-speed suspension movements. Low force means the shims deflect a little, passing a little oil. High force deflects the shims more, passing more oil.
It's the spring's job to absorb bumps by compressing; this stores energy in the spring. It's the job of the piston and valve assembles to control both the compression and the release of this stored energy in a manner that does not upset the chassis. This is done by by varying the flow of fork oil through the cartridge in response to the motion of the fork. Moderating the speed with which the spring absorbs a bump is compression damping. This is done by the compression valve at the bottom of the fork leg. Moderating the speed with which the spring returns to normal after a bump is rebound damping. This is done by the rebound valve on the end of the damper rod.
The speed we're talking about here is not road speed -- it is the speed with which the suspension is moving. Sharp bumps cause high-speed fork motion. Gentle bumps, steering, or normal braking cause low-speed fork motion.
So, what's wrong with stock valve assemblies? They work, but they're not custom tailored to you. Also, the design of aftermarket kits suggests that the stock piston bodies may sometimes be of poor design with simple single-stage valving which can lead to poor low speed control and harsh response due to hydraulic lock under severe suspension inputs. One popular aftermarket design uses a piston body with extremely large openings, ensuring that it will not restrict fluid flow.
Cycle Improvements is the Canadian distributor for MX-Tech's piston/valve kits ($199.95, SWMXTV20 for the '93 VFR). The piston body has what MX-Tech considers to be "right size" openings, large enough to avoid restricting flow at maximum suspension movement speed, but no larger. This is intended to provide a higher pressure at the port or seat of the piston, causing a greater deflection of the shims. This results in a larger variance in the orifice size which gives much better control over the amount of damping created. Better control of the orifice size means better control over the damping.
In a standard revalve job, the stock compression piston/valve assembly is replaced by the aftermarket kit, and the tuner selects an assortment of shims to aim towards the desired damping characteristics over the whole range of low- and high-speed suspension movement. The shims on the rebound piston/valve assembly may also be tuned. This alone provides improvements over stock. But I wanted to go beyond that, and try the mid-valve idea.
On typical street bikes with cartridge forks, the rebound valve assembly is located on the end of the moving damping rod, and the compression or base valve assembly is stationary on the bottom of the fork leg. Each has a seal or check-valve that stays closed in one direction of fork oil flow, and opens wide in the other direction. This forces the fork oil through the compression valve as the descending rod displaces oil in the cartridge, and through the rebound valve as the rod rises again.
The concept of a mid-valve borrows from common practice in the motocross world. Just like the others, it too consists of a piston and a stack of shims. The check-valve is removed from the rebound piston assembly, and the mid-valve is installed directly on the back of (i.e. above) the rebound valve. This allows the mid-valve to act as an additional compression damping valve.
The compression valve's shim stack can now be set up to focus on damping of low-speed and small-volume fork motion. The mid-valve, with very large openings in its piston to handle larger flow, is set up to "float" open enough to not affect low-speed low-volume damping but takes over under mid- and high-speed and high-volume motion to better control compression in those conditions. This can include not only severe bumps, but also more gradual motion that still produces a large flow of fluid, such as braking and back-and-forth cornering esses. This appealed to me as a way to reduce fork dive under braking. Further, it seems that bike reviews in magazines often relate compression behaviour to perceived ride comfort, so the mid-valve seemed like a smart idea to offer better compression control well-tuned for a variety of conditions.
One could build a mid-valve onto the back of a stock rebound valve and leave it at that, but this would not be as effective because of the restrictive nature of a typical compression valve. An aftermarket compression valve, with its larger flow capability, is a better bet for use in conjunction with a mid-valve.
A combination of the predictability of standard fork internal dimensions and the tuner's experience with revalving lots of bikes lets the tuner select a valve stack that will achieve the characteristics the rider desires across the range.
With everything apart, and the bike due for a fork oil change anyway, it made sense to install new seals while we were at it to ensure that everything fit snugly and moved smoothly. (Honda fork and dust seal kit HPS1490-MN8-305 41-54-11.0, two sets at $29.95 each).
Okay, enough tech talk. Does it work? Well, there's good news, and there's better news.
First, the good news. My fear, as previously mentioned, was that stiffer springs and increased damping might go too far and make the ride too harsh for long days in the saddle. But the folks at Cycle Improvements heard my concern and succeeded in choosing springs and damping that, while noticeably firmer right off the bat, still yield an acceptably comfortable ride.
Next, the even better news. My first trips covered various roads around Muskoka and later in the Marten River area. I was developing the sense that the front end was providing much more detailed feedback about what was happening on the road, tracking very smoothly without upsetting the chassis. In one high-speed sweeper, I hit a rough patch that I'm sure would have provoked a head shake or at least a good jolt through the bars, but the reworked forks handled it with deft stability (leaving just the stock rear shock to spank me on the way through). Fork dive under braking has not been eliminated of course, but it has been reduced.
An annual club rally in the Quebec Laurentians over Labour Day weekend offered a wide variety of testing grounds, from bumpy cracked cottage country roads to fresh smooth sweepers around scenic lakes. The customized forks acquitted themselves marvelously in both situations. For one afternoon ride, I traded bikes with an experienced friend who used to have a 1991 VFR with Fox rear suspension and Hyperpro front end. His current mount is a '98-01 series VFR, again with Hyperpro springs and carefully sorted preload. He also brings experience as an amateur racer on an NSR400 triple (or as we call it, a V3). He loved the front end on my VFR, pronouncing it supple and beautifully well-controlled.
So, overall, colour this project successful. It has not totally transformed the bike -- it still feels like my familiar VFR -- but it sure has reinvigorated the handling and my enjoyment of the ride. For lots less money than an aftermarket fork, I have stock-fitting, stock-looking forks with excellent spring, compression, and rebound characteristics, and, for what it's worth, adjustable rebound. Stock fit matters a lot: no need for expensive additional changes to triple clamps, handlebars, brake calipers, rotors, or wheel. I guess the only drawback is that it does leave me wanting some work on the rear shock next to go with it!
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