EDIT:help needed in calculating speed.

Hello friends,

My Apologies. I had amade a mistake in the earlier post. This is the
corrected post.

I have a Honda 4 stroke motorcycle. I wish to know the steps required
in calculating the actual ground speed (in kmph) based on the
following factors:

a] Engine RPM ==> known from rpm meter on console
b] Primary reduction ==> known from user manual
c] Gear number and ratio ==> known from user manual
d] Final drive reduction ratio ==> known from user manual
e] Tyre circumference ==> radius known from user manual. Circumference
derived using 2.PI.radius.

I have done the follwing.

a] My manual says the rear tyre is 3.00 - 18 57P. So i know that it is
an 18 inch radius tyre. I then convert 18 inches to metres and got
0.4572 metres. Thus, my rear tyre has a radius of 0.4572m.

b] My Primary reduction is 3.350.

c] My first gear ratio is 3.076, and

d] My Final drive reduction is 2.800.

Now, if i engage first gear and move off normally (that is without
wheel "slip") and i maintain engine RPM of 2500 on the RPM metre, i
want to know how many revolutions my tyre will make per kilometre in
the first gear.

I have managed to calculate the RPM of the wheel in *first* gear to be
as follows:

RPM at rear wheel = Engine RPM / Primary reduction / First Gear Ratio
/ Final Drive Reduction.

To illustrate:

RPM @ rear wheel in Gear 1 = 2500 / 3.350 / 3.076 / 2.8

Which gives me: 86.6464 RPM at the rear wheel.

Now using this and the circumference of the wheel (calculated above)
how do i determine the NUMBER OF REVOLUTIONS of the wheel per
kilometre *in the first gear*? I also want to extend this to calculate
the same quantity for
any gear ratio on my vehicle.

Thank you for your response in advance.
Have a great day,
Kaizer.

 

The tire diameter isn't the actual diameter of the tire, but of the wheel
rim. All calcs will be flawed if you use 18".

Les

 

The number of revolutions of the rear wheel per kilometre is the same
for all gears, as the wheel's circumference is the same in each gear.

 

*NO* you have a *WHEEL* that is 18 inches in *DIAMETER*. Radius of the *rim*
is thus 9 inches. To get the rolling radius of the *tyre* you have to add to
this the profile height of the tyre. That tyre size of yours will be at
least an 80% aspect ratio which would give a profile height of 2.4 inches
and hence a total rolling radius of 11.24 inches for that size tyre on that
size rim.

You divide 1000 by the circumference of the tyre in metres.

It's the same number! You wheel/tyre don't change size as you change gear


SteveM

 

suggestion: MEASURE the radius of the rear tire. you'll be surprised.
check from the axle centerline with the bike standing up and you in the
saddle.
good luck, sammmm

 

Start your bike. Go into the flat and soak a sponge in a viscous fluid
like olive oil or syrup. Grab a tape measure. Come out and slap the
sponge down on the tyre crossways and press. Toss the sponge and ride
down the street for 10 or 15 meters, or until the fluid on the tyre stop
s making marks. Use the tape measure to record the distance from the
center of the first patch to the center of the last patch you can see.
Take the total number of patches minus one, and divide that by the
distance in meters. You now have revolutions per meter. Multiply by
1000 to get revolutions per km.

 

IMHO, you're all going about this wrong. A minor measurement error will be
multiplied to yeild a poor estimation. Further, friction and mechanical
slippage will have an impact on your real world results. Why don't you
start out with a known value, like a 1 KM straight test strech, and time how
long it takes to cover it at known RPMs in different gears. Once you've got
a handle on this you'll be able to easily calculate (and probably remember)
the applicable factor for each gear. Alternatively, you could pace a
confederate on a quiet strch of road and probably figure the whole thing
accurately out in 5 minutes.

 

Thank you *all* for your most valuable inputs. I still have one
nagging doubt. One of you have mentioned that the number of
revolutions per kilometre is independant of engine speed.

My argument is thus... In first gear, my wheel will rotate fewer
number of times per minute (let us say 60 RPM) due to the fact that
the first gear has a certain ratio. On the other hand if i am in fifth
gear, the rear wheel will rotate more number of times per minute (say
120 RPM) due to the fact that the gear ratio in fifth is smaller
(overdrive) and so it will turn more number of times.

If this is true, then a kilometre done in fifth gear would be quicker
vis-a-vis a kilometre done in first gear due to the fact that the RPM
of the rear wheel is more. As a consequence i deduce that the number
of revolutions per kilometre in fifth gear would be more vis-a-vis
number of revolutions per kilometre in first gear, is it not?

Or am i missing something here??

Its just that i am stuck with the notion that the number of
revolutions that my rear wheel does per kilometre is inextricably
dependant to the Gear number i am in.

Please help me see the light.

Have a great day, people.
Kaizer.

 

What they are all trying to say (correctly) is the aside from the effects of
skidding, a tyre of a given size will make the same # of revolutions over a
given distance, even if you take it off the bike and roll it by hand. The
rpms at the engine get transferred to the drive wheel and are reduced or
increased depending on the gearing. 4500 rpms in first gear might have you
going the same exact speed (ergo revolutions of the drive wheel) as 2200
rpms in 2nd for example. I still say the easiest way to check it out on a
course or with a pace car.

 

The gear you're in won't affect how many revolutions of the wheel per
kilometer, just how many revolutions of the engine.

Les

 

Thanks LJ for your inputs. I now understand what it's all about.

Have a great day,
Kaizer.

 

Pot, kettle, yadda yadda. What does time have to do with it? Why would
you bother doing this in more than one gear? "Cover it at known
RPMs"??? You must have some smooth fuel injection or carburetion or
throttle hand or tachometer or all of the above if you think someone's
going to hold a constant engine speed for a kilometer.

 

Try removing the wheel from the bike and roll it for a kilometre,
counting the revolutions.

Ted

 

No. Roll it 10 km. Walk the first five and run the second five. Count the
revolutions each km. Repeat as often as necessary to see the light.