Service
SERVICE
We welcome your technical questions and comments, and will publish those we think are of interest to our readers. Because of the volume of mail received, we cannot return any personal replies. Please limit your “Service” letters to technical subjects only, and keep them as brief as possible. Send them to: “service, ’’CYCLE WORLD. 1499 Monrovia Ave., Newport Beach, Calif. 92663.
SQUARE VS. ROUND TUBE
I have noticed that some of the new Grand Prix bikes have rectangular tubing for their frames. As an engineering student I have learned that cylindrical tubing is much stronger for a given amount of mass of tubing material. The only purpose I can see for rectangular tubing is when the load imposed on it will be greatest from one direction, ie. the swing arm. In the rest of the frame it would seem that cylindrical tubing would be much superior in strength, for a given weight. So please te11 me why they are using rectangular tubing, besides the fact that it looks trick
Rod Howard
Ft. Collins, Colo.
Cylindrical tubing is stronger for a given mass of material, and in applications tohere stress is linear or can be imposed from a variety of directions the round tubes generally work best. However, there are many places in a motorcycle frame, particularly in the backbone between the peering head and shock mounts, where a high resistance to flex in one particular .direction is required, as when there is a load through the tube perpendicular to the ground but very little lateral load. In that circumstance a rectangular tube will do the job with less weight and space because its strength is specialized, taking the load fest in one direction, like an I-beam or 2 x 4 joist, with no excess material bracing against loads that don’t exist. In some
cases, of course, a rectangular tube may be used where a cylindrical tube would work better simply because it is easier to weld and gusset into another rectangular tube that is there for good reason.
WIRELESS WHEELS
I’ve got a 1979 Yamaha SR500 I really like; only partly because there are so many things about it I don’t like. Converting the front wheel to an aluminum rim, spoke job was easy. Now, how do I get rid of the stock, heavy, ugly, strong, maintenancefree cast rear wheel? My local, reasonably friendly dealer is stumped too. I want to stay with 18 in. rim size but have no prejudices against going to mechanical drum brakes if the frame didn’t need redesigning to fit different linkages.
Naturally I’d rather use stock parts, but also have been looking for independent wheel specialists among your advertisers. Somebody out there must be doing this kind of work. Can you help?
Wilford B. Wing
Somerset, N.J.
We don’t know of any stock production wheel that would work on your SR, but you can have one made up by Kosman Specialties, 340-A Fell St., San Francisco, Calif. 94102, (415) 861-4262. For $375 Kosman will make a wire wheel, hub, sprocket and spacer assembly which will bolt right in using your stock axle and rear brake caliper. Either Akront or DID light-> weight aluminum rims are used. One of the staff at Kosman recently did the same conversion on his own SR and reported lighter weight, better acceleration and a traditional appearance more appropriate' to the SR’s character.
CX SQUEAL
I have a 1978 Honda CX500 standard model. I haven’t had any real problems except that my back brake squeaks badly. I wonder if you have a remedy to this problem. It would really put my ears to rest,v
Tim Znidar Euclid, Ohio
Squealing in drum brakes is usually caused by a buildup of brake dust insidethe drum interfering with good contact between the brake linings and the friction surface on the drum. Remove your rear wheel, dump any dust out, drum side down, on a newspaper and clean out any residual dust with a parts brush. People used to blow brake drums out with compressed air, which cleaned them well but filled the air with brown asbestos dust, no*? linked with an incurable form of lung cancer. So make as little dust as possible and keep the stuff off your garage floor.
If the brake shoes still have plenty of lining left on them and don’t need replacement, lightly sand the linings to remove any shiny glazed areas. Also check the. pivot post and cam for wear or stickiness. If they are stiff or corroded they should be cleaned and coated with a light coat of molybdemum disulfide grease or a hightemp anti-seize compound such as NeverSeez. Don’t get any grease or oil on the brake linings or drum, as this too can cause brake squealing or grabbing. Make sure the springs between the shoes are tight and retracting the shoes completely. Replace them if they appear stretched.
While everything is apart, check the inner surface of the drum. If it is scored cfr grooved it will have to be resurfaced (turned} at a machine shop and you’ll need new brake shoes so they bed into the new contour of the drum properly.
Before doing any of this, of course, it’s always a good idea to check your pedal and brake arm adjustment to make sure the brakes aren’t dragging, as this can heat them up and also cause squealing.
360 CAM CHAIN
My friend and I both ride Honda 360s. At about 10,000 mi. both of them ripped out and chewed up their cam chain guides? and mine even snapped its cam chain. When we asked our local dealer about it, he said it was not at all unusual. I would like t£> know if this is true or if both of them breaking was just a fluke. If it is a common problem could you tell me how to avoid it, as I ^ould not enjoy going through that again;
Doug Savage
Jamestown, Ohio
Cam chain ills are common on the Honda 360s; so much so that Honda issued a recall bulletin on the following engine serial numbers: CB360 and 360G, 1000001 through 1089642; CL360, 1000001 through 1013033; CB360T, 2000001 through 2040600; and CL360K1, 2000001 through ÏÖ16592. The problem was, if the cam chain tensioner was not properly tensioned or if the rear cam chain tensioner slipper broke—which it frequently did—it allowed the cam chain to rub on a metal tensioner arm beneath the crank sprocket. This rubbing made a lot of noise and, if ignored, ¿huid break the cam chain. Recall repair included a new cam chain tensioner holder, slipper and damper, and if necessary a new gam chain.
Once this problem is repaired at a Honda dealership you should have no further trouble with the cam chain on the 360, and the fob should be done free of charge, as Honda dealers are reimbursed by American Honda for any cam chain work on 360s which fall among the above serial numbers. When the authorized repair is completed the dealer is supposed to make a punch mark just under the dash in the engine serial number on the .upper crankcase (CB360E—1065810 is a typical serial number; the punch mark would be under the dash following the E).
DS UPDATE
I have a 1979 DS100 and have been wanting to do some cross-country racing. I tried it once, but got blown away by the power and speed of the competition. So I was wondering if you could make any suggestions on what I might do to add some horsepower and mph without spending a lot of money. I know what you’re probably thinking; I should sell it and buy an enduro mount and I’m a fool to try to make an RM465 out of an old DS100. The problem, of course, is that I don’t have $2300 to spend on an RM465, so please help me out.
Gary Jorge
Ft. Worth, Texas
We understand the money problem— yve’re all in this position from time to time—but the only thing harder than trying to get free horsepower is trying to make a bike lighter without spending money. Unfortunately, the DS simply wasn’t designed to be a racer. As you 've found out, it doesn’t have enough power, suspension or frame for the job. The DS was intended as a play bike and it’s fine for that purposes
phone (202) 387-73601 Getting more power from the engine will require a port job, and expansion pipe and silencer, a larger carb, high compression piston and . . . the list is endless. Then the bike will be useless as a racer until suspension is added. A good pair of shocks will go for $300 plus, a fork kit with springs and damper rods, if you can find any for the DS, will be another hundred or so. Then you ’ll need a stronger swing arm and heavier spokes. And when you are done the bike will be worth less than when you started because no one is looking for a
souped-up DS 100. Do yourself a favor; do whatever it takes to save your money for a while and buy a used 125 motocrosser or used 175 enduro bike, preferably a 1979 or later model. With a little luck and some shopping around you may find a low-mileage 175 for $600 or $700. Then you’ll have a bike designed for your purposes and any modifications you perform won’t be entirely wasted money. In the end, selling your DS and buying a better used bike will be a far cheaper and much
more rewarding
proposition. NON TUBELESS I picked up a sliver of glass in my rear Kl 12-16 tubeless tire. My dealer said the inside of the tire had a small hole no bigger than a small nail hole, so I put a
tube in it. I would like to know if I could keep this tire inflated at the higher psi rating that Continental recommends and also if I can run at the
normal mph
rating. Thomas
McGee Belleville, 111. Whenever a tubeless tire is punctured and a tube is installed there are no guarantees you won’t have further trouble. The tire and rim are not designed to accept a tube and the tube may cause the tire to run at a slightly higher temperature on the highway. Also a splinter of glass, unlike a nail, can put a ragged cut into the tire casing, rather than a single clean hole, weakening the inner plies. That’s the Official Word. In the real world, however, we have all installed tubes in tubeless tires occasionally, rather than throw away a nearly new $60 tire, and have generally seen few problems in doing so. Putting a patch over the hole, on the inside of the tire, will give added security and prevent a flat due to the abrasive action of the cut against the tube. You can inflate the tire to normal psi and run at the normal mph rating, but if you are planning a long distance or high speed trip, particularly through hot areas of the country, a new tire would probably be a good idea, for both safety and your peace
