Paioli RSU front forks, as used on Bimotas in the nineties.
Here we have a collection of scanned manuals for the different types of Paioli RSU font forks. In addtions, we have some drawings of the tools used to maintain these forks. The tools can be made your favourite machinist.
RSU41: used on Bimota db2 (Note! 12.9 mb)
RSU43: used on bb1, Mantra, db4 and yb9 (not yet available).
RSU46: used on SB6, SB6R, SB7, Vdue (4.1 mb)
RSU51: used on YB11. (soon to appear here)
The tool in the image is the tool used to loosen the cartridge from the fork leg on an RSU46. The RSU41 and RSU43 tools look similar in appearence (according to the books, I have never seen them in the flesh), but the dimensions are most likely different. In Paioli's toolkit, there were also other tools, but I think this is the most important one. For seating bushings and seals, the Motion Pro Fork Seal Driver of correct dimension could be a suitable replacement for the original, I presume(?) I have never tried the MP tool on the Paiolis, but I use them on other brands of forks.
RSU41: (no measurements or drawings available).
RSU43: (no measurements or drawings available).
RSU46: as measured by Peter Nisgaard Brink of the Bimota mailing list.
RSU51: (no measurements or drawings available).
Comments on the RSU family of forks:
The Paioli RSU forks has been used on the nineties era Bimotas, and other italian bikes of the same era. The manuals shown above are for the Bimota variants. These forks are a result of Kayaba's cooperation and ownership in Paioli Meccanica.
The former norwegian Bimota importer Jon Oftedahl writes the following abouth these forks, and the spares situation today:
Paioli made 4* different RSU style forks
for Bimota between 1993 and 2000, namely the RSU 41, 43 ,46 and 51. The numbers
indicate the dimension of the innertube. These forks shares a basic design, but
as there are some slight differences in the assembly as well as diffent volumes
of fluid and part numbers Paioli and Bimota came up with a separate workshop
manual for each version. You can use any of these manuals as a guidance when
working on your forks once you
know the differences. It is the RSU 41 that differs most as this fork has compression and rebound in separate legs. Also the cartridges (damping assembly) are fixed by an allen bolt from the outside while the others have internal
Paioli made specific tool kits for each of these forks. The tools used for removal of the cartridges and installing the upper bushings and oil seals are specific for each fork. As the cartridges are of a KYB/Kayaba design most shops dealing in Japanese brands should be able to help. You do not have to undo the cartridge to replace oil seals and bushings, but removing
the cartridge simplifies working on the forks. Also the cartridges should be periodically removed from the forks for a good cleaning. Just pumping oil through them while still fitted in the forks is not sufficient. The internals of any forks will wear. Also oil will age and water, teflon and metal particles will form a sludge over time and affect performance. We are not talking about old style damping rod forks here, but advanced cartridge forks with shims, tiny orifices and adjustable valves.
I believe that the RSU forks are quite good. Bimota have won several championships with these forks and what is good enough for racers like Risitano, Malatesta and Gallaso is good enough for you and me. These Paioli RSU forks were not cheap to manufacture. The modular design with a lower leg machined from billet, a separate (cast alloy) axle clamp, an aluminum
cartridge and the oil seals carried in a "floating" cup shows that little expence has been spared. But these forks will not perform if neglected. I firmly believe they are up to the job, especially when taking into account that they were designed almost 15 years ago. They may not be in the same league as an modern Öhlins unit. There also have been some batches (early 46 and 51 units) with production tolerances a bit on the large side, but a well maintained and correctly set up Paioli RSU fork will carry you safely both on road and track. It really upsets me when owners neglect regular maintenance (nor know how to set them up to suit their riding style) and than moan and groan about the forks.
Most Bimota owners don't ride their bikes much. You can not expect oil seals and scrapers to be up to their job after 10 years even if the odometer shows only 10000 kilometers. Most owners will forget about the forks untill they start leaking, but that is to late. When they fail to impress it is cause people think to believe that forks does not wear nor require any particular maintenance. In fact they are very wrong. If you have ever seen the internals of a fork after 20-30 thousand kilometers you'll be amazed by the wear and tear. Most forks will require a total rebuild if you want original performace retained, a fact that fork tuners know only to well about. Ask any specialist and he will tell you that forks should be maintained on a regular basis. If you are using your bike on a track or pushing it hard on the road the forks may require a rebuilt every 10000 kilometers. Bimota did
not take lightly on fork maintenance. That is why they made those overhaul kits. It is a shame that many owners and dealers did not pay attention.
Regarding oil seals for the RSU 51 (YB11) fork there are no alternatives except the original (51/10/63) ones made by Kayaba/KYB. Freudenberg/Simrit (the largest manufacturer in the world of industrial oil seals) even checked with their competitors. I do not consider quality problems with the OE oil seals to be the reason for premature oik leakages, but rather a combination of a batch of RSU 51 forks being manufactured with large internal tolerances and little use of these bikes. Silicon grease like DOW 111 works wonders on these seals and help them not leak first thing in the spring. For those of you that are looking for alternativ oil seals and think that those for the Marzocchi 50 forks can be used (they are 50/63/10) my advice is not to bother. They will leak after a few miles. Also there are no alternatives except original parts for other wear parts like DU bushings and scrapers.
For the RSU 41, 43 and 46 forks parts for the 4 Japanese brands, KTM, MuZ, Triumph etc can be used. However, the cartridges are spesific for Bimota.
* Paioli made 3 versions of the RSU 46 forks (the last one being a hybrid design made for the final version of the Vdue) and 2 versions of the RSU 51 not counting identical forks except for the spring rate, lenght of the innertube or colour of the fork legs for Bimota.
(former Bimota Norway)