Jim's Cushman Scooter Site
Choosing
the Best Sprocket Ratio
Cushman Club

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 27/8 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 teninch 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 12inch 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 VTwin. 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 lowend 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 driveout 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 driveout 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 driveout sprocket to use: Driveout
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 driveout sprockets.
Driveout 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