Sunday, November 2, 2008

Designing for the belt drive system...

Laying out the frame tends to be done differently by the diverse group we call frame builders; everything from full sized blueprints that you can literally lay the actual frame upon to those that just "know" what they need and build from the cuff. I tend to fall somewhere in between.

I never use prints anymore, instead using the information I get from my info sheets and converstations from the customer to formulate a plan, then jot my numbers down on a sheet of paper, tape it to the fixture and go.

However, I will lay out the rear end of the bike if it causes me to pause or diverts from the norm in some way. The belt drive components required this.

If you take a look at the components, the sizes of the chainwheel and the rear "cog" are much larger than a typical chain driven single speed set up. Part of the reason for this is that the belt gains much of it's strength from the increased surface area of the larger tooth interface and wrap as evidenced below.

The issue for my consideration is that I like to build my single speed rigs with tight geometry for the technical trail work that is found in the North East...that means short stays for quick maneuvering and stuck to the earth traction for climbing. With the larger chainwheel that is 4 times as wide as the typical chain toothed jobby, 650b wheel, and the round bendy chainstay, it'll take some unconventional fabrication to fit it all in.

So...I began by laying out the rear end on the ground work table. Bottom bracket on the top, dropouts at the bottom, arc for the tire, all drawn to size.

Once the rear end is all mapped out, the next order of business is to determine exactly where the chainwheel will sit. I busted out the Shimano Tech book to look at the distance from the center of the bottom bracket laterally and towards the rear wheel, plotting the intersect line where the inside edge of the chainwheel will sit once installed.

The point is diagramed in the lower right side of the photo below, just to the right of the tire arc.

and looking down at the chainwheel as it sits on the table diagram...
Now that I know where the chainwheel will reside, I lay the stays back down on the diagram to determine how much interferance there will be...let's keep out fingers crossed.
Dang, the heavens did not align this time :( Looks like we'll have to get creative.

As you can see, we need more space than a mere dimple will provide, so off to the mill I go. Through the magic of the internet, I've milled out a section of the stay, cut out a curved piece of tubing to fit in, and welded it in place. A much stronger and elegant solution than just crimping in the tubing.
So, here's a pic of the stay mocked up on the jig. The concave piece will allow the chainwheel to pass easily by without any interferance.
And the stay with the concave mod and the coupler in place...

Lot's to think about when building up some bike love...til tomorrow,

No comments: