The Service Dept
THE SERVICE DEPT
DEPARTMENTS
JODY NICHOLAS
REAR END RATIOS
In reference to Reid Sprague’s letter in your March 1972 issue concerning the advisability of swapping rear end ratios in his BMW R69S, let me say that I owned an R50S for some 40,000 miles and had similar thoughts, but in the opposite direction. I debated at length the benefits of going to the R69S rear cog ratio of 8/25 (about 3.13:1 if my memory serves me) as opposed to my 7/25.
Granted, my R50S was not identical to the R69S in power characteristics, but the differences I noticed when I finally did make the switch are worth noting. The higher (lower numerically) ratio gave a welcome reduction in vibration and noise as well as a slight increase in mileage.
If you gear your R69S down to the R50S ratio, Mr. Sprague, you will regret it on the open road (along with the eighty-odd bucks it will set you back to buy the ring and pinion gears).
To Mr. Nicholas, however, I would offer my opinion that anyone intelligent enough to calculate rear axle ratios from fractions would have no trouble installing a ring and pinion gear set in a BMW axle housing, provided he had a service manual for reference and a reasonable set of tools.
William K. Dyer Jr.
Beulah, Colo.
SADDLEBAGS OR LASH-ON
Being that your magazine publishes more stories on long distance touring than anyone else, it seems only right that I bring my problem to you. I own a shiny new Honda 750 Four, and I plan to use it for long distance touring. I weigh 130 lb. in soaking wet leathers, so my bike’s top end is a true 125 mph. My problem is whether or not I should use saddlebags or merely lash my gear to my saddle and luggage rack.
Being that most saddlebags are streamlined, would they affect my top end and handling when empty?
Also, when full of luggage, would they affect my top end and handling as much as lashing the gear (tent, sleeping bag, stove, etc.) to the saddle and rack?
(Continued on page 26)
Continued from page 24
William Genovese Whippany, N.J.
Like you say, most saddlebags are somewhat streamlined and they don’t stick out much, if any, farther than the rider’s legs. Hence, there wouldn’t be much of an increase in direct aerodynamic drag because the rider’s legs are almost directly in front of them.
The only possible exception to that would be if the rider were riding prone, which is a foolish position both from the standpoint of looks and of safety.
Most saddlebags are quite light in weight when empty and are mounted down reasonably near the axle centerline. They don’t materially affect handling when empty and aren’t that detrimental when loaded. However, placing a lot of weight up high by tying it on behind you on the seat or placing it on a luggage rack will materially affect handling.
It depends on how much you’re going to carry and how fast you’re planning on riding, as well as how much you like or dislike the idea of mounting saddlebags.
PISTON CLEARANCE
About two years ago, I bored a cylinder (Honda 90) as a casual favor for a friend. He requested I give the piston three thousandths clearance (per side), which I did, and he was pleased with the results.
On his recommendation, I secured other boring jobs and at present am boring an average of four cylinders per week.
My problem is that I don’t really know what I’m doing. I get pistons with skirts, without skirts and uneven skirts. I get round pistons, oval pistons and other shapes you would hardly believe. I get straight pistons, tapered pistons and pistons with lumps in the middle.
It is my desire to do as perfect a rebore job as possible to suit the replacement piston, but when I first started requesting information from local shops which do reboring I was treated as though I were trying to locate secret treasure. Later, having seen some of their newly rebored cylinders, I realized they knew less about it than I.
Ergo: Where can I obtain information pertaining to what size to bore individual cylinders for particular sizes and types of pistons?
T. W. Haymaker Ft. Worth, Texas (Continued on page 28)
Continued from page 26
A piston is tapered because areas adjacent to the ends of the piston pin are loaded less heavily than those areas at 90 degrees to the piston pin. In other words, on most motorcycles, the front and back sides of the piston are referenced to the rider sitting in the seat.
Pistons are tapered so that the diameter at the bottom of the piston skirt is slightly greater than the diameter above the top piston ring. This is because the top of the piston is exposed directly to the flame created by the explosion of the fuel/air mixture in the cylinder, thereby running at a higher temperature than the lower areas of the piston.
Because aluminum expands when heated, the clearance at the top must be greater than the clearance at the bottom of the piston.
There are many theories and successful methods of designing and manufacturing pistons, and each is determined by the engineers at the factory, depending on the motorcycle’s intended use and basic engine design. For example, the piston shape of the BMW R60/5 we tested in the August ’72 issue is termed convex-oval-pitched in the BMW service manual.
When boring a cylinder all you really need to know is the manufacturer’s suggested piston clearance, which in some instances might be available from a motorcycle shop in the area that has a service manual for the machine in question. Failing that, you could try writing to the service manager at the distributor or manufacturer of the motorcycle in question and requesting the information. Many addresses are listed each month in CYCLE WORLD.
One thing to remember is that if an accessory or racing piston made by another manufacturer is being used, the information on suggested piston clearance should be obtained from the manufacturer of the piston. [Ö]
