I'm on the drivetrain team this year and was trying to set up the packaging for the rear of the car. We are changing around the chassis this year and taking away the rear bay on the car. This forces me to move the drive sprocket closer to the engine sprocket to keep our half shaft angles low. I've been searching in a few mech engineering books and on the internet but haven't been able to find a difinitive way to determine the minimum sprocket center distance. I found the following equations.

(N+n)/6 + 1

(2N+n)/6

with N being the number of teeth on the large sprocket and n the number of teeth on the engine sprocket. Either of them correct?

Can anyone lead me in the right direction?

The minimum distance is the major radius of the drive sprocket plus the major radius of the driven sprocket. How much greater than the minimum you have them is more a consideration based on sprocket strength and amount of wrap on the drive sprocket. Calculation of wrap gven a centre distance and diameter is very simple, and calculation of force per tooth should be easy if you have torque curves, then add a safety factor for shock loading. You can of course increase the amount of wrap using idlers/chain guides to shorten the distance if you need

There are a few factors to consider when determining sprocket centre distances.

Here are some questions to get you to logically go through the steps.

1. What is your final drive ratio?

This will determine your sprocket sizes. By now you would have also set the chain pitch size to correspond to the sprocket.

2. What method of chain tensioning will you use?

There are several ways of achieving this and I'll leave this up to you and your team. It will affect packaging, serviceability, chain wrap etc

3. What amount of chain wrap can you live with?

Again, this is part of the fun of analysing the situation and determining what is suitable for FSAE. Hint: 1/4 - 1/3 of wrap around the front sprocket is a good place to start.

4. Depending on your decisions above, the rest becomes a geometry exercise as you may have found out.

That formula doesn't mean much to me at the moment, but you may or may not have to take into account the following

- Sprocket tooth numbers
- Chain pitch length
- Number of links in the chain (generally a number divisible by 2)

I used CAD because I was lazy and worked out what the closest the sprockets could be before physically colliding. But I had to derive at specific centre to centre distances because of our chain tensioning method (eccentrically adjustable).

Now if you're completely lost by the time you've read this post, feel free to reply with your e-mail and I'll get in touch.

Regards
Fred

Thanks for the posts, both were very helpful. I know the final drive ratio and the tensioning method.

This should be a good basis for me to start. Thanks a lot.

I don't think ours was as close as UWA's last year, but our 2007 car had a center distance of 193mm with a 11t front and 48t rear sprockets. We increased wrap with a 11t idler sprocket on the slack side. Had no issues at all with this setup with 30hrs of testing over 3 months.

I believe ours was 164mm with 8mm adjustment.

Regards
Fred