Join Date: Mar 2003
Location: Grand Prairie, TX
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If you have the sort of child stoker kit with which I am familiar, they come in (clamp) sizes appropriate to fit specific seat tube diameters. If yours isn't a perfect fit for your seat tube, it is a pretty good bet that you have the wrong child stoker kit. Perhaps you have a steel Trek, and the stoker kit was intended to fit a larger-diameter aluminum seat tube. You should be able to notice that the diameters of the inside of the clamp and the outside of the seat tube are not a perfect match.
I suspect the best solution to the child stoker kit cranks contacting the FD will be to lengthen the chain between the child stoker kit chainring, and the child stoker kit timing ring on the left side of the rear bottom bracket. Then you can run the child stoker kit up the stoker seat tube, run the saddle up, and the problem will be solved.
Twice I have seen short cranks on the back of a tandem, and some sort of wood block arrangement used to get the pedal circle up high enough so that a child can reach from the saddle on top of a regular tandem seat tube. That might be an intermediate solution as the child grows.