I am certainly no expert on recumbents, but I do appreciate seeing interesting recumbent designs from time to time and I am always interested in learning more about them. When Arnold Ligtvoet from RaptoBike sent me some information about his latest bike, the fact that it was front wheel drive with an innovative way of adjusting the frame length is what immediately caught my attention. I asked Arnold to elaborate on the Mid Racer’s ‘splitting-frame’ feature, and that is what I want to share with you in this post.
First, I will mention that most recumbents have a ‘noseboom’ to allow for length adjustable to accommodate riders of different heights. The noseboom is generally a tube sticking of the front, which holds the bottom bracket and slides in and out of the main frame tube. When you slide the noseboom to vary the length of the front beam, the chain length needs to be adjusted as well (and most recumbents have pretty long chains). The Mid Racer gets around the traditional noseboom construction by dividing in the middle (under the seat), so that as the wheelbase changes, the front layout remains the same.
Arnold can do a much better job explaining his design than I can, so read what he had to say about the Mid Racer below.
“After my first bike (the Low Racer) I had some further ideas. It occurred to me that if I made the length adjustment of the bike behind the drive train, this would mean that I could develop a bike with a fixed drive train and even importantly a fixed geometry at the front.
The fixed drive train combined with the splitting frame has the advantage of being able to quickly adjust the bike to the length of the rider, without having to change the length of the chain. It gives an added bonus in the sense that the bike can be taken apart in two sections for transport for instance. It’s not a folding bike, nor was it designed to be that. The drawback of making the bike change it’s length by splitting it in the middle is that, as the wheelbase is shortened or made longer, the handling of the bike changes. To counter this we have designed the main tube to slope. The amount of sloping needed to counteract the change in size of the wheelbase has been calculated with an application called JBike6. JBike6 is a computer program that determines the stability of bicycles and was developed by Delft University of Technology & Cornell University. As far as I can tell now it is the first commercial bike that has been partly developed using this application.
The added bonus of the fixed front geometry is that we could design the bike to fit a large portion of potential riders. Some recumbents, mainly the faster ones, have an issue where the bike, when set to shorter riders, does not allow for enough room between the cranks and the front wheel. Shorter riders either can’t ride the bike or forcibly have to use short cranks. This is never the case with the Mid Racer as the whole setup never changes, so shorter riders can ride the bike without this problem.
The bike has become quite a versatile machine. It can be fitted with most any type of brake (vbrake, disc brakes and race calipers) and can be delivered with 2×26″ for touring and commuting. People wanting to race the bike can opt for 2x700c as well as the bike is shown in the picture.”