Ceriani Forks
CERIANI FORKS
PROVEN BEST BY RACING'S TEST.
THOSE ITALIAN-MADE "Ceriani" forks have been cropping up everywhere in recent months. In big-time International scrambling and road racing (especially the former) it seems as though at least half the bikes are now equipped with these distinctive-looking forks. Obviously, the Ceriani forks must be doing the job.
Until quite recently, there has been no opportunity for us to get much information on these forks except for the correct (Italian-style) pronunciation of the name, Ceriani: "Cherry-ahny." Now, with a company in our area (Steens, Inc.) importing these forks in quantity, it has been possible for us to borrow a set just to disassemble for study, which is precisely what we have done.
No sooner had we lifted the forks from the shipping box than we discovered the primary reason for the Ceriani's popularity: lots of travel. There is a full 6.5-inches of available travel. About 33Á" of this is upward movement from the static loaded position; the remaining 234" is downward travel.
Long travel is of limited usefulness without proper damping, and this aspect is handled by a patented piston-type system. The damping system appears to have been devised by (in effect) combining the old Norton "Roadholder" forks' tapered damping rod and the new Norton's oneway disc valve. In so doing, Ceriani has combined the best features of both, obtaining a damping action that stiffens as the forks move toward the ends of their travel, but with a heavier damping action on rebound than bounce. That would appear to be the most logical damping action to have, for either scrambling or road racing, and to the best of our knowledge, the Ceriani forks are unique in having such characteristics.
In general, the construction of the Ceriani forks is much like that of other teiescopics, but done with a trifle more precision. The fork tubes, for example, have a mirror finish on their outer diameter, and there is a similar finish inside the cast aluminum-alloy "sliders." This ultra-smooth finish is necessary in the Ceriani design. Unlike most forks, in which the slider makes contact with the fork tube only through a couple of bearing collars, the Ceriani forks' sliders provide a bearing surface, internally, through virtually their entire length. There is one bearing collar: a cast-iron insert, located right at the top of the slider. The collar serves a dual purpose: as a bearing at a point where loads are very high; and as a hard-metal insert in an area sure to collect any grit that finds its way past the seals.
Actually, not much dirt is likely to get past the seals. A tight-fitting, double-lip neoprene seal at the top of the slider retains oil in the fork and helps exclude grit and moisture. Usually telescoping dust covers, or concertina-type rubber boots protect this seal. Unfortunately, both these types of covers tend to pump air in and out as the forks move, and inevitably some dust finds its way in. Ceriani's solution to the problem is a conical rubber sleeve, clamped to the slider and a tight slip-fit on the fork tube. The shape of the seal's lip is such that the fork tube is continuously being wiped clean, so precious few abrasive particles will ever find their way to the oil seal.
We have already described the Ceriani forks' damping characteristics; it is interesting to spread the damper system's parts around and see how it is all accomplished. Like virtually all others, Ceriani gets the motion-damping effect by pumping oil back and forth through a restricted orifice (actually, orifices). However, while reasonably good damping may be had by simply restricting oil-flow through a hole (a device all too often used by cost-conscious manufacturers) there is a better way of doing things. Damping that acts more strongly on rebound is highly desirable, and so is damping that increases with fork travel; neither can be had with a simple, drilled hole.
In the Ceriani forks, the damping action is provided by oil being pushed through a variable orifice by the damper piston. This piston slides inside the fork tube and is fixed to the slider through a tapered "hourglass" column. A snap-ring holds the piston on the tapered column. Clearance between column and piston allows the latter to align itself inside the fork tube. Without this, there would be an inconveniently close-tolerance concentricity requirement, so the damper piston is allowed about .050" of side-float.
Below the damper piston, space is divided into two chambers, and as the fork slider moves, oil transfers between chambers. There is a slight change of volume due to the bottom of the fork tube displacing oil in the slider, but this is accommodated by allowing a free passage of oil through the hollow center of the tapered column to the "resevoir" space above the damper piston. No such free passage is available between the upper and lower chambers of the damper. Oil moving between these chambers must pass through a restricted annular orifice (formed by the tapered column and a close-fitting ring).
Because the primary damping is provided by a tapered rod, which has its minimum diameter right where the valve ring settles when the forks are at their static loaded position, resistance to motion in either direction becomes greater in direct proportion to the deflection. At the ends of the rod, the taper terminates, and from that point onward the valve ring is a close slipfit, with the oil virtually trapped. Obviously, this will reduce the possibility of the forks slamming against the end of the travel (either up or down) to almost nil.
That valve ring we have mentioned serves more than one function. It is, of course, the outer surface of the annular damper-orifice; it is also part of a one-way check-valve. The ring-valve fits inside an aluminum sleeve that is secured within the mouth of the fork tube by a snap-ring. The ring rests on a shoulder inside the sleeve, and has a spring-washer above it to hold it in position. And, above the springwasher is a retaining ring, with a center bore substantially larger than the maximum diameter of the tapered rod. It has, moreover, a row of drilled holes, the combination providing virtually a free flow for the oil once it gets past the valve. It should be apparent that the ring-valve will pass more oil one way than the other. On the bounce stroke, the ring lifts from its seat due to pressure from below, and the oil can flow around its sides (in addition to moving through the annular orifice) into the upper chamber relatively unrestricted. But, when the slider moves downward, the ring drops back to its seat and virtually all flow is forced between the inside of the ring and the tapered rod. Thus, the desired progressive damping, with a fairly strong bias toward the rebound stroke, is obtained.
A pair of small holes near the top of the tapered rod also flow some oil, but they cannot have much effect on the damping properties. These forks are filled not with the usual thin "spindle" oil, but with heavy, multi-viscosity 20/40 motor-oil. The use of a multi-viscosity oil is very sensible, as it gives more or less uniform damping characteristics over a broad temperature range.
Ceriani has no exclusive American distributor, but Cosmopolitan Motors has been a supplier for some time and now Steens, Inc. is importing a stock. Steens will have these forks available in a variety of sizes, with steering stems and spacers that will adapt them to the popular range of motorcycles. Eventually, the adapter kits will cover just about everything being sold in quantity in this country. And, you can get the Ceriani forks in either road racing or scrambler pattern.
One of John Steen's employees has a Greeves Challenger fitted with Ceriani forks, and this gentleman loaned us the bike for evaluation. We were somewhat surprised to find that these forks transformed the Greeves' handling. That is not to say that the Challenger was tremendously improved — but it was surely different. Having springs intended for a heavyweight, the Ceriani forks could not show to best advantage on the Greeves, but they did work extremely well. They do not, quite frankly, appear to have the Greeves forks' phenomenal ability to climb over things. However, they do seem to make the steerin a bit more precise. In any case, they must offer an overall advantage in scrambles racing, for we have just received an announcement stating that all of the "works" Greeves will have Ceriani forks. More important (to Average Rider), Greeves has announced a new model, the "MX3," which will have the Ceriani forks as optional equipment, at $56 extra (in England). There will also be an "official" Greeves conversion-kit, priced at $126 exworks, for those who have a Challenger and would like it fitted with Ceriani forks.
From all the evidence, it seems as though Filippo Ceriani's forks will soon be holding up the front-end of every racing motorcycle in the world. Either that, or (most likely) others will be investing an equal amount of effort in their forks. Telasco, in Spain, now builds a very good Ceriani-type fork; others are sure to follow.
