I attended the Rally in the valley this year and filmed Guy and Hal Debes pulling a Caterpillar using the drum of their Fowler ploughing engine The Chief.
They had a strain gauge to read the tonnage, which I believe, was somewhere near to 8 Tonnes (correct me if I’am wrong)
Sorry for the shaky film it was done using a compact camera.
Not sure how to link you tube clips onto TT.
Please feel free to link it
Dave Bick
http://youtu.be/mF7-a_BqueI