Calculating Top Speed, 4 & 2 Stroke Engines

[Deleted Member 4613]

Here's how I do the calculation: (for 26 inch wheel):

Top Speed = top rpm/ gearbox ratio x wheel sprocket/drive sprocket x wheel circumference x 60, all divided by 5280

Four stroke: Using a 44 tooth rear sprocket, a 9 tooth drive sprocket, and 4/1 gearbox:
7500 rpm x 7feet x 60, all divided by 5280 = 30.5 mph
4 x 44/9
Using a 48 tooth sprocket:
7500 rpm x 7 ft. x 60, divided by 5,280 = 27.9 mph
4 x 48/9

Two stroke, assuming a 4.1/1 gearbox:
Using a 44 tooth sprocket, 10 tooth drive sprocket, and top rpm of 9000 rpm:
9000 x 7 x 60, divided by 5280 = 39.68 mph
4.1 x 44/10
Using a 36 tooth rear sprocket:
9000 x 7 x 60, divided by 5280 = 48.5 mph
4.1 x 36/10

 

[Chainlube]

I always tell my speed by how hard a bug hits my eye. If it's real hard, I'm doing 40. If I can still see, I'm doing 30.

 

[Deleted Member 4613]

These numbers are theoretical under no load. The
actual speed may drop a mile or two.

 

[Street Ryderz]

Here's how I do the calculation: (for 26 inch wheel):

Top Speed = top rpm/ gearbox ratio x wheel sprocket/drive sprocket x wheel circumference x 60, all divided by 5280

Four stroke: Using a 44 tooth rear sprocket, a 9 tooth drive sprocket, and 4/1 gearbox:
7500 rpm x 7feet x 60, all divided by 5280 = 30.5 mph
4 x 44/9
Using a 48 tooth sprocket:
7500 rpm x 7 ft. x 60, divided by 5,280 = 27.9 mph
4 x 48/9

Two stroke, assuming a 4/1 gearbox:
Using a 44 tooth sprocket, 10 tooth drive sprocket, and top rpm of 9000 rpm:
9000 x 7 x 60, divided by 5280 = 40.6 mph
4 x 44/10
Using a 36 tooth rear sprocket:
9000 x 7 x 60, divided by 5280 = 49.7 mph
4 x 36/10

Sorry but this is wrong,a 26" wheel is 81" circumference not 7 feet and 9k with a 44t is 38 mph not 40,and a 36 t gear at 9k is 46 mph not 49 ,the internal ratio is 4.1:1 not 4:1.

 

[Wrench]

I measured the Circumference of my 26" wheel (tire tread) and it come out to 83 1/2" Circumference Measure with a string
But If I just go 26 x 3.14 = 81.64" Circumference

 

[Deleted Member 4613]

On the circumference, I measured the radius of my 26 inch wheel, including
the tread, so yours may differ depending on your tire. On the 2 stroke
gear ratio, I assumed it is about 4/1, but I couldn't find the exact
ratio online. Again, these numbers are only theoretical and do not
consider the power of the engine and the weight of the rider. In
practice, I would substract 2 mph to adjust for load. But they're
fairly close to what one can expect, and should help is selecting the
size of the rear wheel sprocket and the drive sprocket.

 

[Street Ryderz]

I measured the Circumference of my 26" wheel (tire tread) and it come out to 83 1/2" Circumference (Measure with a string)
But if I go by the size printed on the tire 26.2125 x 3.14 (pie) it comes out to 82.30" circumference
But If I just go 26 x 3.14 = 81.64"

Yeah it varies a bit with different tire but you also have to take into account that the tire compresses some when you sit on the bike so using 81" is the safe all around figure to use.

 

[Street Ryderz]

On the circumference, I measured the radius of my 26 inch wheel, including
the tread, so yours may differ depending on your tire. On the 2 stroke
gear ratio, I assumed it is about 4/1, but I couldn't find the exact
ratio online. Again, these numbers are only theoretical and do not
consider the power of the engine and the weight of the rider. In
practice, I would substract 2 mph to adjust for load. But they're
fairly close to what one can expect, and should help is selecting the
size of the rear wheel sprocket and the drive sprocket.

Weight nor power have anything to do with this, if you have a tachometer and the engine spins 9k then the speed from the ratio used is a given.

 

[Deleted Member 4613]

I adjusted my numbers to reflect a gear ratio of 4.1/1
rather than a 4/1 on the 2 strokes. Thanks for the
info. I wasn't sure about this.

 

[Street Ryderz]

My tire is 26.35 and x pie =82.7 but when i measure the radius the tire is 27.1" which would be 85" but the speeds reflect 82" figure's most accurately.