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            Choosing the Best Sprocket Ratio


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For any motor vehicle there is an optimum rear end gear ratio that will give the best performance for any chosen objective.  For this article we will refer to the overall rear end ratio as simply “ratio”.  The ratio chosen is always a compromise since different objectives require different gear ratios. The Ratio is determined by dividing the RPM of the engine by the RPM of the driven wheel. For this part of our discussion we will assume the vehicle is left in high gear. The Cushman transmission is a 1:1 ratio when in high gear so it is does not have to be considered when calculating the ratio. Shifting to low gear will greatly increase the overall gear ratio and provide increased acceleration and lower top speed. We will discuss the transmission later in this article. 

As previously stated, a Cushman scooter, or any motor vehicle, will perform the best when the overall rear end ratio is selected to match the desired objective.  The objective may absolute maximum top speed for racing, lower top speed with better acceleration, or fantastic hill climbing ability with a very low top speed. Once the objective is selected several other factors must be considered. They are: the tire size, the horsepower of the engine, the weight of the vehicle, the weight of the driver, and the type terrain likely to be encountered. The engine torque curve is also a factor but that is beyond the scope of this article.

Cushman chose a ratio about 4.46:1 for the Springer Eagle and the earlier tubular fork Eagles that used the 2-7/8 inch bore 5 HP engine, and lowered it to about 4.28:1 for the later 3.0 inch bore 8 HP Eagles. Some of the early Pacemakers had ratios above 5.0:1 because of the smaller engine used and the fact that no transmission was present to increase performance at lower speeds. Trailsters have ratios much higher than 5:1 and have the ability to climbing steep grades since high top speed is not required.

In my opinion Cushman choose the correct ratio for average usage. The scooters are quick to accelerate for the power available and the top speed is reasonable.  However, many people like to lower the ratio a little. Lowering the ratio allows the engine to run slower at a given ground speed and usually provides a little more top speed.  How much to lower the ratio depends upon the factors listed above and each will be discussed. You should write down each factor that applies in your case to help you decide how low to go. If you go too far the engine will not be able to pull the increased load and you will actually loose top speed, not to mention loss of acceleration.

There has been a lot of interest lately in using larger tires on our Cushman Scooters and this is the first consideration.  Keep in mind what counts is how far the scooter travels with one rotation of the tire and rim size is not an indication of this distance.  I have two scooters that use ten-inch rims and the tires on one measure about 55 inches in circumference and the others measure over 58 inches. The Cushman 100 tire is about 52.5 inches in circumference and the Cushman 200 is close to this figure. Keep in mind that the circumference of these tires may have varied slightly over the years. If your tire circumference is more than 52.5 inches you must increase your gear ratio by the same percentage as your new tire circumference is over the standard tire circumference. The correction percentage you calculate for your specific tire must be applied to any ratios suggested later in this article. Another factor to consider is that the circumference of any tire will increase slightly with increasing air pressure. This is usually minor and can be ignored in most cases.  However, increasing the air pressure in the tire will tend to reduce the rolling friction at high speed. The modern 10 and 12-inch motorcycle tires reduce rolling friction even more. A very knowledgeable person told me to expect as much as a 4 MPH increase in speed with a motorcycle tires running at maximum recommended air pressure.

If you are using a hot engine you can lower the ratio proportionately and still have the same performance as a stock scooter but with a greatly increased top speed. Some super hot engines like the Lloyd Lohon job on my 1959 Eagle and the one I modified for my Springer Eagle Street Rod will handle ratios down to 3.5:1 or lower, a ratio that would almost stall out a stock Cushman engine. With a super hot engine using a Mikuni carburetor you can expect to see an honest 65 MPH. Both of mine are almost as fast as my 16 HP Vanguard V-Twin. The difference is that the Vanguard is just loafing along at 65 MPH and the Cast Iron engine is simply screaming. And in a drag race the Vanguard just simply walks away and leaves the Cast Iron engine scooters in the dust because the modifications to the Cast Iron do not increase the low-end performance much.

If you are “gravity challenged” you should generally use a little higher ratio and you skinny guys can go a little lower. Any weight addition or reduction to the scooter itself must also be considered. You should also consider the terrain that you will encounter on your ride. If you expect steep grades you should keep this in mind and use a little higher ratio.

One factor that you cannot control is the altitude at your location. Higher elevations reduce the air density and an internal combustion engine will not produce its rated horsepower. A little higher ratio might be indicated.

There is a wide difference of opinion on how low to go with the ratio. It mostly comes down to an individual choice about how much acceleration to give up to obtain more top speed. Also, Some like to let their engine really wind up, others like to limit the RPM’s a little. Where a person lives and the terrain may also affect the recommendations we see from time to time. The recommendations that follow are strictly my own opinion. You should work out your optimum ratio and obtain a couple different drive-out sprockets in that range and determine which is right for you.  Keep I mind that Cushman made 53, 60, and 65 tooth large sprockets. When someone recommends a drive-out sprocket is meaningless unless he also tells you which tooth large sprocket is being used.

For a standard unmodified 8 HP engine I would suggest going as low as about 3.9:1 if you are of average weight, say 180 pounds, and have no steep hills to climb. Depending upon your weight, your top speed will be perhaps 2 to 4 MPH higher and you will notice some loss of acceleration.  For the earlier 5 HP unmodified Springer Eagle and early tubular fork Eagles I would not go much below 4.2:1. I have a Springer Eagle that uses a ratio of about 4.2:1 and it will exceed 54 MPH and has excellent acceleration. It does have the new higher lift 8 HP camshaft and a milled head. My hot engines use a ratio of 3.7 and I think that is just right. I have tried lowering it to 3.54 and I gained a couple miles per hour top speed but the acceleration was noticeably slower.  Lloyd Lohon uses a ratio of 3.2 in his own personal “super souped up” engine, but you should never go that low. Just for comparison, my Vanguard Eagle has a ratio of 2.5:1, which means the engine is just reaching 3600 RPM at about 70 MPH.  The engine is running so slow at 55 MPH that it feels like it is idling. 

A word about low gear is in order. All of the above discussion was based upon performance with the transmission at 1:1 ratio in high gear. The transmission is somewhere near 1.7:1 from input to output when in low gear, and this figure varies between the different transmissions.  If you have the standard ratio of 4.28:1 and you shift into low gear your new ratio is about 7.27:1 (4.28 times 1.7).  I think that with a lower ratio gearing, low gear is even better than before, acceleration is good, and the point at which you shift increases by several miles per hour. With my hot Eagles I do not usually shift before reaching 30 to 35 MPH. When you hit high gear at 35 MPH the engine will be turning fast enough to provide acceptable acceleration.  If you continue to shift down around 20 to 25, as with the original setup, the scooter will be very sluggish when you first hit high gear.

The formula used to calculate ground speed is: Speed = (RPM * Distance traveled in one tire rotation  /  3.14) divided by (336 * Ratio).  As an example, an engine turning 4400 RPM using a 4.28 ratio and a standard Cushman tire goes like this:  Speed = (4400 * 52.5 / 3.14) divided by (336 * 4.28), or about 51MPH. 

Restated to determine the ratio, the formula becomes: Ratio = (RPM * distance traveled in one rotation) divided by (MPH * 336 * 3.14). As an example, Ratio = (4400 RPM * 52.5) divided by (51MPH * 336 * 3.14), or about 4.28:1.

To sum up, you should choose a ratio somewhere between 3.5 and 4.28 Depending upon your desire for speed or acceleration, your engine horsepower, your weight, plus any added bike weight, the terrain in your area, and your elevation above sea level.  Now you must correct the chosen ratio for your specific tire size if it is other than the standard Cushman tire.

Once you have chosen a specific ratio apply your tire correction factor and then use the following formula to determine the drive-out sprocket to use: Drive-out Sprocket teeth = (Big sprocket teeth * D) divided by (desired gear ratio), where D = 1.2 for the larger engine pulley on later eagles or 1.56 for the smaller engine pulley on early eagles.

Also remember that you may not be able to obtain an exact ratio that you want because you are limited by the ratios created by the number of teeth on your large sprocket and the number of teeth available on the smaller drive-out sprockets. Drive-out sprockets are available in a wide range of teeth. Sometimes only the standard sprockets that Cushman used are listed in a catalog’s main section. You can usually find many more choices in the racing or specialty section of the catalog.

If you are algebra challenged just send me an email with all the variables and I will do all the computations for you. Also be sure to see the Cushman sprocket ratio tables at this web site.  

Jim Frederick