Clutch

By: Ron Milam

 

  I am now pretty sure that brakes are the most universal problem area on a trials bike. I think the next one might be the clutch. There are great ones and bad ones. I think it would be interesting to first discuss the factors that affect the way a clutch works.

   First would be the master cylinder ( for you hydraulically challenged  guys with cable actuated clutches this would just be the clutch lever ). These mostly look the same on the outside, but can have varying piston diameters. Second is the slave cylinder. Since these are often made by the bike manufacturer and not purchased, they vary widely in design and piston diameter. Just like on your brakes, the ratio of the 2 piston areas is very important because it affects the sensitivity of the clutch engagement. ’94 Betas had a very quick clutch that was difficult for most riders to use. In ’95 I think it was, they increased the slave cylinder diameter to make the clutch engagement more gradual. The same thing happens on a cable actuated clutch when the actuating arm is lengthened. This is normally done to lighten the lever pull however, not to soften the engagement.

   For the average rider there is not much you can do to modify these items. But you can optimize them. If your clutch is too quick, you can position the lever farther from the bar end, so that you are working it near the end. This means your finger has to move farther to engage the clutch, so the movement doesn’t have to be so precise. This has the same effect as what Beta did with their modification in ’95. If you want quicker action, you can just do the opposite. Moving the lever assembly closer to the bar end will place your finger closer to the pivot point and give trigger like engagement. ( It will also cause more bent levers since they will be more vulnerable in a crash. ). Lever free play is also important because your finger has less accuracy when it is stretched out than when it is closer to the grip. Set up your free play so that your finger is in a comfortable position when the clutch is at that critical engagement point. There is a trade off here though. Obviously if you set it with too much play, the clutch may not completely disengage. ( Can you say “5” ? ). I am talking about the adjustment screw where the lever pushes the plunger into the master cylinder. Many levers have another adjustment screw that you can use to tighten up the play that you induced and keep the lever close at hand. If this doesn’t make sense, I’m sorry. I don’t have any pictures, only a thousand words. Look at your lever. You’ll know what I mean.

   The third item is the cluch unit itself consisting of friction plates ( faced ), driven plates ( metal ), and springs. This seems simple enough. Let’s quickly mention springs and get them out of the way. If your clutch isn’t slipping, your springs are probably OK. If it is, you should remove them and measure their length. Call your dealer and find out how long they oughta be. If’n they’re too short get some new ones. If’n they’re not, the problem is with the plates. ( Be sure the actuating mechanism is not preventing full engagement ). Hardly anyone complains about slipping. Mostly everybody complains about dragging. Next to engine stalling, hard cranking, brakes failing, fenders breaking, ah heck, forget it. Lets just say most of us experience clutch drag at some time.

   The easiest thing to try is to change the oil. Transmission oil is critical to a trials bike clutch. Many people resort to ATF to reduce drag. The result is often reduced drag, but often is also very grabby engagement. When you are trying to finesse your way around a turn by slipping the clutch, and have your front tire lift slightly and wash out, you will know what I mean. Smooth engagement is just as important as full disengagement. ATF helped reduce the clutch drag on my Beta, but after a while I noticed my turning ability was really bad. Not that it has ever been any good anyway, but it was even more badder than usual. I switched my transmission oil to Elf Moto T 10W30 and the result was much smoother engagement. The stuff costs a lot and maybe there is a cheaper substitute, but a transmission doesn’t hold that much and a few dollars worth of something that works is way better than a few cents of something that doesn’t. Along with the oil change, I also had to deglaze my friction plates. Next month I will get into how to do that, and disclose a few other secrets.

 

Last month, as you may recall, we started a discussion of clutches and transmissions. We talked about adjustments and eliminating drag by changing transmission oils. There are a few things more that can be done to further reduce drag. This involves removing the clutch cover and poking around at the innards. If you have a newer bike with a separate cover plate for the clutch, this is extremely quick and simple to do. If you have an older bike, you may have to remove the entire side case. In either case, you are not getting into the actual transmission. If you are the least bit mechanically inclined, you should have no problem.

  Before you begin, be sure to clean the bike or at least the area you will be operating on. You can usually complete the job without draining the oil if you lay the bike on its side. I will dispense with the obvious, but I do have a small tip that will help prevent damage to the seals on those bikes that have a one piece side cover. Simply wrap some thin tape around the shaft splines before removing the cover. This will prevent any sharp edges on the spline from cutting the seal.

   Before you dissasemble your clutch, but after you remove its cover, pull the lever a few times and watch the pressure plate move out and in. The amount of travel may vary between brands, but I think a good number would be about 2mm. If it isn’t moving this much or close to it, you may need to bleed the hydraulic system. The pressure required to disengage the clutch is very low compared to what is required to clamp your front brake. Consequently, you can’t feel it when air is trapped in the syatem. But any trapped air will compress and that will reduce the clutch throw out. If  the travel is OK, pull out the plates. I’ll bet you they were stuck together in there. This is because the oil layer between them causes a suction that must be broken before they will separate. If your clutch is dragging, this is where a low cost mod will make a huge improvement. Send your metal plates to somebody that knows how to knurl them. One such person is Bob Ginder, but other folks can do it too. The result of the knurling process is a multitude of small depressions that reduce plate sticking. If you ever saw the dimpled plates in a Japanese clutch you will know what I’m talking about. Along with this and maybe even before you send out your plates for knurling, you should deglaze the friction plates. You can do this easily by placing a piece of 150 grit sand paper on a sheet of glass or other flat surface and lightly rubbing each side of the friction plates in a figure 8 pattern on it. Be sure to clean off excess oil first. You should make a few strokes and then reposition your grip on the plate. This technique will help ensure that the plate surface remains flat and parallel to the one on the other side. You need not worry about prematurely wearing out the plates. You are only removing a tiny amount of material. You should make every effort to do this evenly. Check the plate surface often and change hand position often. You only want to take the shine off. Look to see that the surface is becoming roughened all over the plate. If you have areas that are not cleaning up as soon as adjacent areas, check the plate on your flat surface and look for warpage. If they are warped, they should be replaced. You should also lay the metal plates on the flat surface and check for warping. They should be flat within a few thousands of an inch. If they are warped, they too should be replaced. Do both sides of all friction plates and reassemble, oiling each part as you go.  I have the metal plates in my Beta knurled and I am running stock friction plates. I find that every 3 months or so, my clutch will start dragging. When it does, I do the deglazing routine and change oil. The whole thing takes about 30 minutes. I can now find neutral while waiting at a section with the engine running. This may not sound like such a big thing to some of you, but if you have one of those bikes that just jump from first to second and second to first, but refuse to stop at neutral (unless you are in a section ) you know how nice it is.

    My son Chris has a ’95 Fantic that has Sureflex brand friction plates in it. The metal plates have not been knurled. That clutch works beautifully. It has a super smooth engagement and no drag. This leads me to believe that if you have good friction plates to begin with, knurling may not be necessary. A set of  Sureflex plates cost about $ 80. If  playing with oils and deglazing do not solve your problem, I think the Sureflex plates would be a good next step. If you’re still not satisfied, go with the knurling.

    In conclusion, I would say that a cluch that does not drag should be your minimum expectation. You should keep working at it until you also have smooth engagement. This  will result in lower scores and much less frustration.