The Service Department

THE SERVICE DEPARTMENT

GORDON H. JENNINGS

QUESTIONS, QUESTIONS!

There are a few questions that I have after reading the recent issues of CYCLE WORLD:

First, how does the cornering power of a motorcycle compare to its four-wheel counterpart — especially the racing variety?

Second, why do motorcycle engines always have exhaust ejected forward and then routed 180-degrees to the rear while the intake is behind the engine? It seems more logical to have the intake in front to use the ram effect of the air and the exhaust ejected straight backward.

Finally, I would like to know why the front and rear brakes are applied independently? Also, in auto racing, disc brakes have almost competely replaced drums. Are brakes a problem on racing bikes and have disc brakes been used? John Riordan Brooklyn, New York

These questions come up from time to time, and I will try to supply answers, although there are no incontrovertible answers for any of them.

Generally speaking, the cornering power of a motorcycle is slightly inferior to that of a comparable automobile. That is to say, the sports/touring bike will not corner quite as fast as the sports/touring car and the same is true of the pure racing machines. However, it is my opinion that the average bike is slightly quicker than the average car, if one considers that the “average” car is apt to be a family-type sedan. In the racing field, the margin is clearly with the four-wheeler: times taken on bikes and cars around the same corner on the same day establish this fact beyond any doubt. The bike has a couple of things working against it: first, the tires must be designed to grip when presented to the road surface at a wide range of angles, which tends to limit the contact-patch area and stability — the tire has to be rounded in section, and never can offer the broad, flat gripping area of the automobile tire; second, the human operator must exercise considerably more caution on a bike than is necessary in a car. A driver can overestimate the situation going into a corner, and then slideoff enough speed to recover. Also, he can go around the corner on the ragged edge of adhesion, making adjustments to prevent a slide from going too far. The bike rider, on the other hand, must not tread too near the edge, for there is no recovery after a slide begins. On relatively slow corners, a very good rider may balance the machine with one foot and do a bit of sliding, but it seems unlikely that this technique would be successful at, say, 100 mph. There is simply too much danger that the situation will go beyond any hope of recovery.

Motorcycle engines have their exhaust pipes emerging from the front of the cylinder head because they are air-cooled; the exhaust port is the hottest part of the engine, and it is necessary to have this very hot area up in front where the cool airstream can do the most good. With a water cooled engine, the water can be lead around to the exhaust port no matter where it is located, but that is not true of air, which must be supplied in much greater quantities. We would say that there is no hope for the rear-facing exhaust port but for the contradictory evidence supplied by a certain Americanmade V-twin, which has the exhaust port on the rear cylinder facing aft, and apparently this creates no problem. Even so, it seems to be advisable, whenever possible, to get that hot port area up front and in the direct airstream.

We have discussed the matter of independently-applied front and rear brakes before, and came to the conclusion that it is done that way largely because that is the easiest way to cover all situations and because there has been no outcry for anything else. Still, if a satisfactory brakebalancing device could be developed, the single-pedal control could become popular (see the Feb. 1962 issue, CYCLE WORLD).

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Disc brakes have become a sine qua non in racing cars, and there is every reason to expect that the same will eventually be true of bikes — not only racing machines, but those intended for touring as well. The motorcycle, in any of its forms, does not present as difficult a braking problem as an automobile. Its brakes are not limited in size, in practical terms, by the size of the wheels, and the wheels do not shroud the brake to the extent that cooling-air flow is impeded. Hence, except in the most extreme cases, it is relatively easy to supply brakes that will handle what is asked of them. In the extremes, the disc brake has a lot to offer and we may expect that there will be progress in that direction. Indeed, that is already being done; note the disc-brake conversion being made by AÍ Gunter (reported on in the July issue, CYCLE WORLD).

BIBLIOGRAPHIES

ƒ have been reading your magazine for several months and recently entered a oneyear subscription. In common with most magazine subscribers, 1 now regard myself as a sort of minor stockholder in your enterprise, and thereby entitled to advance all kinds of comments, carping criticisms, etc. This letter is an exercise of this Fundamental Right.

Of your choice of material, and manner of presentation, 1 generally approve. Technical series are of special interest, as are the road tests of various machines.

The only suggestion 1 might offer for the improvement of technical articles is the inclusion of a bibliography for the benefit of those interested in further reading on the subject.

Rex V. Allen San Francisco, California

Thank you for your kind comments; please do feel free to regard yourself as a stockholder (and not minor) in this enterprise, as we would be in a pretty fix without a readership. Also feel free to write and offer any comment or criticism, carping or otherwise, that comes into your mind. I cannot promise a personal answer; the sheer volume of letters fairly well rules that out. However, I like to know what is of particular and immediate interest, for that interest — relayed to me by your letters — gives the technical department and the magazine staff generally a sense of direction.

As for the bibliography; I know that it would be a Good Thing to include, but much of the material comes right off the top of my head, from sources I do not always recall, and to compile a listing of reference material would take more time than the writing of the article. Still, the need for such a listing is there; I have received several requests such as yours, and it appears that I shall have to do something about it. Probably the best way is to list the material in my technical library, at least that part of it that would he available generally. Look for it in this column, soon.

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RATIO CALCULATING

1 own a Vincent 500cc Comet, and it is geared too high. How can I go about figuring the gear ratio? The countershaft sprocket has 18 teeth; the final sprocket 48 teeth.

J. E. King Grand Prairie, Texas

Calculating gear ratios is easy, if you only want to know what the ratio is, and not what it should be. Just divide the number of teeth on the drive sprocket into the number of teeth on the driven sprocket: 18 into 48, or 2.66:1. This is, of course, not the overall ratio. To get that figure, you must do the same thing with the primary drive, and then multiply the primary ratio, probably around 2.2:1, by the final drive ratio: 2.2 times 2.66, or 5.85:1. Easy, isn’t it?

Getting the right ratio is another matter, one that deserves, and will get, a full article explaining the details.

ENGINE SWAP

/ want to make an engine swap in my 1951 Harley-Davidson Super 10. The engine 1 have in there now is very small and sluggish going up hills and the acceleration isn’t too fantastic.

I would like to put the 22 hp HarleyDavidson engine in mine. I would like to know if this is a good swap and just how you would go about doing this. I would like to have your recommendation and advice.

David Jobs Bristol, Connecticut

I assume that, when you say a 22 horsepower Harley-Davidson engine, you mean the engine (in one form or another) that is used in the Sprint. If that is the case, you might be well advised to forget the whole thing. The Sprint engine would never fit into the Super-10 frame without a great deal of modification (the front down-tube would have to be removed entirely). In the end, it seems very likely that you could simply buy a new Sprint, and have less money invested in the finished result. And by following this course, you would also get the Sprint’s vastly better brakes and suspension.

ANOTHER FROM THE PAST

It was a pleasant surprise to be leafing through your December issue and on page 28 observe the interesting article about the make and model that I cut my motorcycling teeth on over 45 years ago. My first solo ride was on this particular motorcycle.

Incidentally, this machine does have a clutch. The left side lever Mr. Bigsby called the shift lever is the clutch lever. An examination of the right side will expose the two-speed foot shift lever, which was a rocker heel-toe arrangement.

Note the slotted friction device for keeping the clutch lever in any desired position or amount of engagement, permitting the replacement of the left hand back on the handlebar when needed.

It is understandable how, after so many years, the control setup could have been forgotten — unless one had put in a good many miles on the model in question.

Les Myers Wichita Falls, Texas We bow to the superior knowledge of a man who obviously knows whereof he speaks. In studying the photos available, it does indeed appear that the lever in question is a clutch. This is just one more reason why we try so hard (not always successfully) to maintain a high standard of accuracy throughout our magazine: every time we slip, one or more of our readers will write to let us know about it.

Mr. Myers is, incidentally (his letterhead tells us this), the Harley-Davidson dealer in Wichita Falls. Three cheers for a man who has stuck with two-wheelers well past the passing-fancy stage. •