donch@tekirl.UUCP (Don Chitwood) (07/09/86)
The front forks on my l974 Moto Guzzi 850T are getting to me. I've decided on a rebuild as opposed to replacement, at least in part. A bit of history. When I got the bike new, I had arranged with the dealer to help uncrate it so that I KNEW this was a virgin machine. One of the first things I saw was rust on the left fork tube immediately above the dust cover on the fork slider. I knew this would cause problems, but the dealer was intractible, I was naive and a pushover. Several years later, when the corrosion had gone further and fork action was noticably sticky, I tried to get the dealer to exchange it as a factory problem. I even talked to the importer himself who told me that anything with chrome on it was considered cosmetic and specifically NOT covered in warranty (had I been on my toes, I would have reminded him of the chrome cylinder bores on all current Guzzis, but.....) So, I replaced the fork tube, only to discover that chrome had become inbedded in the aluminum slider and was actively scouring my new fork tube. That was over ten years ago. I decided to rebuild and improve the forks by getting new fork tubes and putting a teflon/bronze bushing in the sliders for super slipperyness. Garlock Bearings makes a sleeve bushing called "DU". It is a steel backed shell with teflon imbedded in a lead/bronze porous matrix. The teflon transfers somewhat to the opposing shaft so that an extremely low friction interface results (we're talking microns or less of material). The bearings are very long lived and can be run without lubrication, although life is greatly enhanced with oil. Since the teflon is backed by the metallic matrix and steel shell, the entire bushing has virtually no flex or stretch under normal stress unlike a solid plastic bushing. DU bushings are available in inch or metric sizes, so getting the 35mm inner diameter to match my fork tubes was no problem. The heart of the rebuild is accurately boring the fork sliders to accept the DU bushings with the correct interference fit. I'll be using three 2-inch long bushings in each slider: one right at the top of the slider, and two pushed together near the bottom so that the fork tube is always supported adequately, even at full extension. The Guzzi forks have a separate internal shock unit that looks like one of those screen door closers. The body of the unit mounts inside the fork tube up within the triple clamps with the piston rod extending down through long coil springs and fastening at the base of the slider. Thus, changing fork oil viscosity has no major effect since it is only there as a lubricant. These internal shock units vary from one to the other in terms of damping characteristics. The italians don't seem to understand quality control very well. Also, every one I've ever bought has had scratches and dings on the piston rod, resulting in rather rapid failure of the shock (the rod and its seal are at the bottom of the unit, so oil quickly drains out past the damaged seal.) The virtue of the design is to reduce unsprung weight. Period. I've yet to do the machine work (or have it done). I'm anxious to see how this rebuild works in reality. Don Chitwood Tek Labs Tektronix, Inc.