Pulley and idler question

[borna]

Hello all,
I need Help with selecting a correct size pulley and bet for a model engine I'm building.
The engine is a 4 cycle 2 cylinder In line.
I'm panning to select a correct Pulleys and belt between the camshaft and the crankshaft with a ratio of 2:1
The larger pulley goes on the camshaft and smaller on the crankshaft.

The center distance between camshaft and crankshaft is 1.29"

To calculate the belt length I used this formula

L = 2C + 1.57(D+d) + (D-d)^2 / 4C
L = Pitch length of belt
D = Pitch Diameter of Larger pulley
d = Pitch Diameter of smaller pulley

For the smaller pulley I'm planning to use a one with d=.520,
.0816(40DP) pitch, 20 teeth

For the larger pulley I'm planning to use one a with D=1.040,
.0816(40DP) pitch, 40 teeth

based on the calculation above the length of the belt came out to around 5.034" long
the closest to that size that I could find is 5.71",
.0816(40 D.P)pitch, 75 teeth,

The question is what size idler pulley I need to use to create a correct tension for the 5.71" belt?

For the idler pulley, I selected something like this one
P.D: .780, .0816(40 D.P) pitch, 30 teeth.
Is that work?

Also is there a formula that I can use to find a correct size idler?

Thanks
Borna

 

[diskullman]

Idler diameter is not critical, has no effect on ratio. Whatever physically fits into the application. Plan your idler around a good bearing.

Russell Giuliano