The Service Department

THE SERVICE DEPARTMENT

GORDON H. JENNINGS

"WEAR INDEX"

Some motorcycles are much more durable than others. The Honda 50 has 11,520 engine revolutions per mile, and a piston travel of 2950 feet per mile, and its wear index is 339.8. The Honda 250, on the other hand (2 cylinders, 24.5 bhp) has only 6000 engine revolutions per mile and a piston travel of 2130 feet per mile, and its wear index is much lower at 128.

Why don't you include in your road tests the wear index, piston travel and engine revolutions per mile of the motorcycle? The wear index is obtained, 1 believe, by multiplying the piston travel and engine revolutions together and dividing by 100,000. If you don't know how to obtain these figures, why don't you write Road & Track magazine, 834 Production Place, Newport Beach, Calif.

It would certainly be interesting to know the wear index of the Harley-Davidson

Duo-Glide. With an axle ratio of 3.54 it's bound to last longer than a motor-

cycle with an axle ratio of 6.— something. And what about 2-cycles?

Name and address not given

The "wear index" of the Harley-Davidson Duo-Glide, when it has 3.54:1 gearing, is 52.2, this figure being derived in the manner you outlined from an engine revolutions per mile figure of 2810, and 1860 feet per mile of piston travel. As it happense, I have some familiarity with the computing of wear index, this acquired during the more than 5 years I spent as technical editor of Road & Track. Would that I had a nickel for every time I calculated wear index for some vehicle or other.

Cycle World does not give a wear index, or any near approximation, because I do not think that it is possible, by combining such straightforward factors as engine revolutions per mile and piston travel (in feet per mile). There are too many variables involved.

Let us consider the matter of cylinder wear. Given some form of effective filtration for intake air, an engine's bore(s) will wear at an almost constant rate at all engine speeds; that is to say, the rate at which metal is removed from the cylinder bore will be a function of time, and not of engine speed. This was discovered in experiments performed on test engines, and in these experiments, engineers found that the metal was removed not by the scrubbing of pistons and rings, as is commonly supposed, but by the etching effects

of acids created in the combustion process. The concentration of these acids is largely

independent of engine speed, and so is wear. Thus, from this standpoint, cylinder bore life is mostly dependent on the number of hours on an engine; not on mileage, or the number of engine revolutions per mile.

Unfortunately, there are other factors that crowd in to mar this perfect picture. For one thing, acid attack of the bores is more severe when the engine is cold. Just after starting, for example, the cylinder walls are very cool, and the acid vapors actually condense on the cylinder walls and the etching action is very rapid. The condition can persist for long periods when running in extremely cold weather, which will tend to over-cool the engine. Thus, bore wear will be more rapid in an engine used in cold climates, or in one that is subjected to start-and-stop service.

Another factor is one already mentioned: intake air filtration. Altogether too many motorcycles have ineffectual or noexistent air filters, and any air-borne dust or grit is fed right into the cylinders. Because the mixture swirls violently about as it is drawn into the cylinder, most of the dust in the air will be literally thrown against the oil-covered cylinder walls, where it becomes a marvelously effective lapping compound. Here, it becomes evident that cylinder wear will also depend to a great extent on the effectiveness of air filtration, adding one more variable to engine life.

Small displacement engines will reach the end of their useful cylinder life sooner than their bigger brothers, however, even though the wear rate does not vary too much. This is true simply because 40 thousandths of wear has a much greater effect on an an engine with a 1.5" bore than would be the case with a 3.5" bore. And, of course, when effective air filtration is absent, grit-created wear is a factor, and that type of wear is related to engine speed, or the number of revolutions per mile. In these respects, a big, slow-turning engine does offer at least the hope of better reliability and increased service life.

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One area where big engines do score well is in bearing life. Most bearings, ball, roller or plain, are good for a fixed number of cycles (of loading and unloading) and a fast-turning engine will reach the total - cycles limit much sooner. Also, a small engine is likely to be operated at full throttle, or very near it, most of the time and that creates relatively higher mechanical and thermal loadings. Of course, it is entirely possible to design for the heavier loads, and in theory at least, the bearings in a small engine can be made to last just as long as those in its larger brothers.

I would guess that in the end, reliability will depend more upon what kind of quality is built into the engine, and how hard a rider pushes it, than anything else. In all probability, even the tiny 50cc engines would be quite reliable if they were made to the same standards as the bigger engines, and if riders would not force them to run steadily at more than 1/2 to 3/4 throttle continuously. Unfortunately, this is not the case. Virtually the only reason for building such small engines is to get the price down to a minimum, and it would be unreasonable and unrealistic to expect that when minimum price is the goal, designers would concern themselves too much with specifying close tolerances and the best of materials. And, obviously, it is also unreasonable and unrealistic to expect that riders will show much restraint when traffic is whizzing by on both sides at a furious rate.

But, these are factors that cannot be incorporated into a sure-fire formula that would give a prediction of probable reliability. We cannot know how riders will treat a machine; and we cannot know what tolerances and materials have gone into its construction. In the end, we are left with one thing: reputation. If the motorcycle comes from a firm with a good name, then we have to assume that it will be reasonably reliable. If you need a figure to indicate probable reliability, use mph per 1000 rpm (in top gear) which is given in all of our road test reports.

HARLEY-DAVIDSON HANDLING

would appreciate any information that would help make my 1961 Harley-Davidson XLH handle better. I am a tool and die maker by trade, and have a well equipped machine shop to do my work in.

I have had H-D 61s, 74s, a 1952 K, a 1959 XLH, and at present a 1961 XLH, but not one of these machines would handle like my I960 Triumph Bonneville.

My present XLH has had the garbage removed, to give it that "XLCH" look. It has a 6" rise on the handlebars, Metzler tire on the front and a Grasshopper on the rear. Lots of chrome and black lacquer and plenty of Harley-Davidson power and reliability — but poor handling, which I hope you can help me overcome.

Kenneth Pike G or ham, Maine

Funny you should mention the Triumph Bonneville in connection with your Harley-Davidson's handling. As a matter of fact, the XLH and XLCH can be made to handle much better by grafting-on certain parts from the Triumph's suspension. You could use the Triumph spring/damper units at the rear wheel, mounting them vertically instead of angled forward like the stock H-D units. This will make it somewhat more difficult to fit saddlebags, but the handling will be substantially improved.

Further improvement can be had by using Triumph damper parts inside the H-D front forks. This will require fairly extensive reworking, but it is well worth the trouble. As an alternative, you might consider changing over to the entire Triumph forks. •