Service

SERVICE

Paul Dean

Number crunching

When Cycle World does a road test, how do you calculate the engine speed at any given rpm? For example, in the BMW K1200RS test in the August, 1997, issue, you show engine speed at 60 mph as 3393 rpm. Is there a formula I can use to figure this spec? I'd like to be able to make this calculation for my 1982 Kawasaki GPz750, which has an overall top-gear ratio of 5.66:1. I would really appreciate your help. David Maness Midlothian, Texas

Calculating this data is easy, but you first must know the circumference of the rear tire in addition to the over all ratio of the gear in question. The formula we use is: MPH = RPM ± Overall Gear Ratio x RearWheel Cir cumference (in inches) ÷ 1056. Wondering what the "1056" is all about? This formula uses revolutions per minute to calculate speed mea sured per hour, and rear-wheel cir

cumference in inches to calculate miles traveled in that hour. The num ber 1056 is simply a constant that eliminates the need to make separate minute-to-hour and inches-to-miles conversions.

Let's use your GPz's overall top gear ratio of 5.66:1 and suppose that the rear-tire circumference is exactly 84 inches, and that the engine is turn ing 4000 rpm. Divide 4000 by 5.66, multiply by 84, and divide by 1056. >

The answer: 56.2, the miles per hour your GPz travels at 4000 rpm.

Then, if you want to know what rpm the engine will turn at any given road speed, first divide the speed by the rpm in thousands; in this case, divide 56.2 by 4. You get 14.05, which means that every 1000 rpm equals 14.05 mph. So, if you want to know how fast the engine will spin at, say, 60 mph, just divide 60 by 14.05. The result is 4.27, meaning the engine will be turning 4270 rpm at 60 mph.

Shake & break

In September of 1995, I purchased a new Kawasaki ZX-11, and within a few months it developed a mild headshake. About a month before the warranty ran out, I took it to the dealer to have the shake corrected. After taking a week or so to check it out, they told me the problem was caused by the front tire. I took the bike home and changed the tire myself, but had to have a special 22mm Allen wrench made to fit the axle; worse yet, the new tire did not fix the headshake.

It took me so long to have the tool made that by the time I got the bike back to the dealer, the warranty had expired. He told me that the shake was now entirely my problem and tried to sell me a steering damper. I feel that if a steering damper is needed on this bike, it should have come from the factory with one. I also believe that steering dampers detract from a bike’s appearance. The headshake has gotten so severe that the bike once left marks on the road-and almost in my pants. I never know when or for what reason the shake will develop, but most times it occurs under acceleration. Is the dealer overlooking a reason for this problem?

Sidney Phillips Dalmatia, Pennsylvania

If the information you 've provided is accurate, the dealer has not just overlooked the cause of your ZX-11 ’s headshake; he has shirked his responsibility as a motor-vehicle retailer. When the bike was still under warranty, you took it to him for a handling problem, which he misdiagnosed. The condition persisted after you made the repair the dealer prescribed (and which he should have performed, not you), and he afterward refused to honor the warranty, even though his acknowledgement of>

the problem predated the expiration of the warranty. If I were you, I’d write the customer relations department at Kawasaki Motors and complain about this dealer’s behavior.

But to the headshake itself: Make sure the steering-head bearings are adjusted just a bit on the snug side and that there is no roughness in the bearings, especially with the front wheel in or near the straight-ahead position. Ensure that there is no perceptible freeplay in the wheel bearings or in the swingarm pivot, and that all of the front-fork pinch bolts are tight. And check for proper alignment between the rear wheel and the front.

Checking wheel alignment is a relatively easy task, but it must be done carefully. The goal is to assure that the centerline of the rear wheel is exactly in line with that of the front wheel when the chain is properly adjusted.

First, prop the bike in a vertical position with the front wheel pointed directly ahead. Locate two perfectly straight boards that are between seven and eight feet long, and place one board on each side of the rear tire, allowing the boards to extend from the rear edge of the rear tire ’s sidewall to the front edge of the front tire ’s sidewall. Position the boards as high on the tires as the chassis or exhaust system will allow.

Because the rear tire is wider than the front, neither board should touch the front tire, and the distance between each board and the side of the front tire should be exactly the same. If it is not, the rear wheel is out of alignment and will need to be adjusted.

If you don’t have access to long, straight boards, use a long piece of string instead. Loop the string around the rear edge of the rear tire ’s sidewall and, drawing it tightly, extend it in a straight line along both sides of the bike until it is, just as with the boards, parallel to the front tire on both sides.

ZX-lls have eccentric rear-wheel adjusters that normally do not require the rear axle to be loosened; you only have to loosen the Allenhead pinch bolt behind each adjuster. This ensures that both eccentrics remain tied together and rotate as a unit when either one is moved. But if the rear wheel is out of line, you must also loosen the rear axle nut enough to allow each eccentric to be moved independently.

Clutch and clunk

I have a 1993 Yamaha Seca II that I bought used with just 121 miles on the odometer. I have ridden it about 2000 miles and have had no mechanical problems, save a disconcertingly clunky shifter. Shifting down into first from neutral earns an audible “clunk,” and the entire motorcycle jolts, even though I have not released the clutch. Shifting up to second is similar, but going into third and above is very smooth. I always use the clutch and believe it is fine because the bike never tries to creep forward with the clutch in. This problem is worst when the bike is cold. Getting it hot seems to smooth out the shifts, as does allowing the engine to rev high when shifting. Per the dealer’s advice, I use Motul 3100 15w50 engine oil. Is this condition common or just present in my motorcycle? Benjamin A. Marschke Canoga Park, California

Your Seca II, just like the vast majority of other motorcycles, uses a wet, multi-disc clutch, and the clunking you describe is common with this type of design. The clutches have a dozen or so plates that, when disengaged, are pushed apart by such a tiny distance that you d have trouble inserting a paper-thin feeler gauge between any two. And because the clutch runs in oil (to lubricate the primary drive), there is enough oil on the plates to cause a slight drag when the clutch is disengaged. There ’s not sufficient drag to make the bike creep forward in gear, but it will cause the transmission input shaft to rotate at or near engine speed when the gearbox is in neutral and the clutch disengaged. And when you shift into first, the meshing of the gear dogs abruptly causes the input shaft to stop turning, resulting in a clunk.

If your Seca II gives off a similar clunking noise when you upshift from first to second, it most likely does so because you ’re failing to accurately match engine rpm to road speed. It’s my guess that if you simply make that upshift more quickly and crisply, the noise will not occur.

The magnitude of this clunking can be amplified if the clutch runs in oil that is too heavy. Yamaha specif ies either 10w30 (for cold/winter use) or 20w40 (for hot/summer conditions), not the 15w50 your dealer recommends. Try using 20w40 and the clunking may be noticeably reduced. □