Lance's "Kraffty" MWB

Lance Oberg showed his nearly complete MWB (Medium Wheel Base) bike. Built with Dave Krafft, it's based on a very slick pattern that Dave has now used five times for himself and others. It uses an unusually large piece from a donor MTB frame including all of the rear triangle, about half of the seat tube, and most of the down tube. The old seat stays are un-brazed and then put back on at a lower position. This is a very simple design that requires very little new material and doesn't require nearly as much tricky jigging as building from scratch. A straight piece of large O.D. tubing is added from the old seat tube, past the end of the sawed off down tube, and on to the new head tube. Another new tube then goes forward from the head tube to act as the crank boom. The fork is a donor piece that had its blades cut in two and then shortened and plugged back together. The seat is a Rans unit. The odd configuration at the front is to allow the lowest possible main tube, yet have a high BB. Like others of it's ilk, this bike may eventually go inside a shell and a low stepover height is helpful for getting in and out. .
We did some comparisons of this bike to Dave's number one bike from this design, that also happened to be at the meeting. Note Lance's bike (front) has a lower step-over height, and much more tiller for a "Praying hamster" position. Mark thinks this is a poor setup, from a control standpoint, but Lance likes it. (Probably because his handlebar position is similar to that on his Rans Screamer tandem. M.S.)


Mark also compared his orange LWB bike with Dave's bike. Despite their being apparently quite dissimilar, they have some important ergonomic similarities. Backrest angle and distance from the rider's shoulders to the steering axis are the same, and both have bars that droop downward for good steering ergonomics. Mark has wider bars, less reach and more tiller. Dave's bike, designed to fit in a fairing, has narrower bars, more reach and less tiller. Neither has enough trail to cause wheelflop. Mark thinks these bikes are 2 of the best handling he's ridden. With less reach, the weight of Mark's arms doesn't pull back on the bars with as much force. But with the lever of wider bars and more tiller, it translates into similar self-centering force. Dave's narrower bars allow him to make equally quick steering motions despite his arms being much more extended.

A couple of Mark's conclusions from this (and many other observations and tests) are

1. If - When you extend your hands and arms straight forward at handlebar height, the steering axis intersects your hands somewhere within the length of your fingers.
Then - You have a bike that, given the right bars, positioned and oriented correctly, can be made to handle very well.

2. If - The above criteria is met.
Then - Within a range of 4-10" of tiller, proper bar width, (center of grip to center of grip) should be approximately twice tiller.


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