What's the most economical way of shortening the stoker cranks on the tandem I just won on Ebay? It has Truative 170mm cranks. It's going to be brilliant fun, but my 7 year old girl with seat right down (incredibly flexible though she is).. is tending to get thrown off when I start it spinning fast. Her little Islabike must have got ~130 mm cranks and is perfect.
Crank shorteners, a few I've come across seem to be expensive. Are there any DIY solutions which have worked?

