I am looking for a crank shortener for my Tandem so my son can ride with me. We are doing blocks on the pedals right now but I would like crank shorteners. Everywhere I look they are $100+. The cranks are 35mm wide so I think I need wide ones but I am not sure. Is that width across or depth? I have a Kent Dual Drive Tandem Comfort Bike, do the shorteners fit all cranks or is there a chance they will not fit? Since I would have to order sight unseen I want to know they will fit. Does anyone have any they no longer need and would like to sell?