$30 for this three piece chain tension System. Cheaper than a new derailer with a stronger spring. I already have a spring, My first thought was to mount it on a derailer idler wheel and pull up to the chain stay. From the picture, I will measure the length of the derailer arm and reference it to Tensioner arm for length and angle.
Using dial calipers, the derailer arm measured 4.815 inches. the picture of the derailer measured 3.800 and the picture of the tensioner measures 4.660. Using Ratio and Proportion from my practical shop math book, 3.800 : 4.815 :: 4.660 : x 3.800x = 22.438 x = 5.900. I layed my arm out to 5. 900 inches long by .953 wide. I Measure the width of the derailer arm to have a relationship for the smaller sizes. .300 : .520 :: .550 : x x = .953.
Below: A swivel mount for the tandem bicycle roof rack, allow one end of the tandem to be lift at a time. Made from ground flat steel that I had on hand.
A litthe welding to make both hand rails
Latest project of size on my shoptask. The bending was done by hand, but the four mounting bushing were made and welded to the frame. I made a plastic pawl for my icemaker control to no pic.