Pulley questions

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

For the larger pulley I’m planning to use one a with D=1.040

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"

The question is what size idler pulley I need to use so that 5.71" could fit and what is the formula to find the location that I need to install the idler pully?

By the way prior to using a pulleys and belt, I was planning to use a 2:1 ratio gears with an idle in between as seen in the picture.


#1 This is a robotics forum. Not an engine forum. So you would likely find a better answer elsewhere.

#2 An idler pulley is generally spring loaded in engine applications. The size of the pulley doesn’t matter, because the belt doesn’t wrap around the idler, the idler just pushes on the belt between two pulleys to tension the belt.

Here is an arm with a tensioner pulley on it. The arm will generally have a torsion spring on the non-pulley end that holds the pulley against the belt.

A better question is: what tension do you need on your belt. You need to pick a tensioner that will provide enough force.

As I said - take this to a car or small engine forum and you’ll have better luck.

Some idlers are fixed, others are spring loaded. An example of a fixed idler is the one on the supercharger drive on the car in my avatar.

The short answer to the original question is, it does not matter what size the idler pulley is, or where you put it. It might be best to put it on the slack side of the belt, but any location on either side will probably work ok. The idler could push the belt in, or pull it out. The idler pulley radius needs to be larger than the minimum bend radius of the belt, that’s all.

To others reading this post: Note what he says is the distance between cam and crank - one point two-nine inches. This is one very small engine.

Borna, you may get better results (and more informed responses) on a micro machining / engine hobbyist site. Steam engine folks also.

I am curious why you want to put the bigger pulley on the cam - in most 4-stroke engines the cam runs as twice the speed of the crank, not half the speed. Also, you MUST use a cogged (‘timing’) belt, since slippage will throw the timing off almost instantly. Or a chain.

As for tensioning - a belt tensioner just pushes against the belt to keep it tight, size is almost irrelevant.

No he is right, the cam runs at exactly 1/2 the speed of the crank. More importantly though unless the model ran a belt, you should run a toothed belt or chain in order to keep the timing exact. Any minor slippage or delta between the exact 2: ratio would cause all sorts of timing issues.

With that small of a center distance, you could get by with 9 and 18 tooth 25 pitch sprockets. This will give just a tiny bit of tooth clearance between the sprockets. Unfortunately going below 12 teeth gets pretty lumpy, it also won’t give you a lot of fine tuning adjustment for valve timing.

I would gear it instead of belting it personally. Gears won’t slip and there is no need for a idler. tou just need to make sure the cam and crank line up correctly. After that, I would ajust the timing off your distributor. That is the easy way to do it.

What’s this for anyway? Model car or boat, or just for fun?