The Service Dept
THE SERVICE DEPT
JODY NICHOLAS
LEAVE THE TESTING TO US
In connection with your specification figures on various machines, I have a question to ask. I have sometimes checked them and found a considerable disagreement as to your computed top speeds at a given rpm. For instance, in your February 1971 issue for the Yamaha RT-1B on page 41, computed top speed in gears at 6500 rpm is: 5th, 73; 4th, 57; 3rd, 44; 2nd, 32; and 1st, 22. I get for these, 5th, 79.5; 4th, 61.1; 3rd, 46.8; 2nd, 34.1; and 1st, 24.2. In addition, you quote an actual top speed at 7400 rpm of 83.72 in 5th, but 92.7 would be produced, as I figure it. With a locked up drive there is no slippage, and the computed figures should be the same as the actual, if the tach and speedometer or other device is accurate.
This is a matter of simple mathematics, and the only place / can see where a difference might occur and my figures be in error would be in the actual diameter of the wheel, including tire, but that should make only a slight difference and not account for as much as occurred here. As to wheel diameter, I used your lS-in. wheel and 4-in. tire which give, if the tire is round, IS plus S, or 26 in., times 3 and 1 ¡7th, which produces a distance per revolution oj 6.S ft. / would appreciate very much vour explaining this to me.
/ would also like to know why some of the engine graphs show a road speed increasing with an rpm higher than that at which maximum horsepower is developed. Horsepower is the rate of doing work and a moving machine is, of course, doing more work at a greater road speed. This is a contradiction in terms unless something else is meant, which / don V understand. I suppose if horsepower were read directly off the engine drive shaft it might turn faster without putting out any more horsepower, but the method of testing horsepower is to put a brake dynamometer on the shaft, which puts a load on it so, therefore, calls for the production of more work with more rpm, and more work means more horsepower.
Terrel! Marshall Tittle Rock, Ark.
From the aspect of practicality, C YCLE WORLD road tests are the most complete and valid in the world. Obviously, it is not possible to test each and every machine under conditions of an exact nature, “exact” here meaning conditions of the same ambient temperature, relative humidity, barometric pressure and wind speed, to say nothing of the altitude of the test location. It is also not possible to test the various machines using the same rider in each case. Even the same rider may ride a certain machine on two consecutive days and get different test results. The reasons for this should be obvious.
What we do, however, is report the conditions under which we test each machine, including the temperature, relative humidity, test weight, etc., and present these figures to the reader in graphic form. That’s more than you can say for any other motorcycle magazine published in America! It’s the most objective way we know, and 1 think most readers will agree with what I’ve said.
But getting back to your letter. Your first mistake was in measuring the tire. Just because a tire is designated 4.00-18 (on the rear) doesn’t necessarily mean that the tire is 4 in. “high.” The “4.00” means that the effective tread area is 4 in. wide. The tire on a Yamaha RT-1B we have in the shop, which has 500 miles on the speedometer, has a measured distance from the outer edge of the rim to the edge of the tire of 3 3/4 in. If you measured the tire from the bottom of the bead, the reading would be something like 4 1/4 in.
(Continued on paye 34)
Continued from page 32
You’ve queried “why some of the engine graphs show a road speed increasing with an rpm higher than that at which maximum horsepower is developed.” That’s quite simple to answer, really. Some engines are capable of turning more rpm than that at which maximum power is developed, and many will continue revving (if no load is imposed) until they destroy themselves. Many manufacturers specify a maximum safe rpm for a particular engine, but it is difficult to know just when that rpm figure is reached if no tachometer is fitted. I’ll wager that not many people could tell just by feeling the vibration in the handlebars, footpegs or the seat of his pants!
The Yamaha RT-1 B we tested did, in fact, have a tachometer, but because of the final gear ratio, it would easily exceed not only the maximum horsepower rpm of 6000, and the red-line speed of 6500 (at which engine speed the computed speed in gears is calculated), but would go all the way to 7400 rpm. We wouldn’t recommend that the average owner run his RT-1B that high, but we’re in a little different situation. Some machines we’ve tested were so overgeared that they wouldn’t even reach the maximum power rpm, much less the maximum safe engine rpm. Had Yamaha decided to gear the RT-1 B with a total reduction ratio of, say, 4.0:1 in top gear, I don’t think it would have been capable of reaching 6500 rpm anywhere, short of going down a 25 percent grade! (That’s 112.5 mph!) In fact, most road machines are geared a little on the high side on purpose to protect the engine from being overrevved in top gear.
