Because for anything other than commuting, the strength of the rear triangle would be seriously compromised. Think of the increase in the effective length of the moment arm transfering any lateral force from the rear wheel into the frame.
It could be possible with _really_ beefy dropouts on a reinforced seatstay, but then I think you might run into the same issue at the bottom bracket. It's hard to see that design not having alot of flex (steel, Ti), or being susceptible to breaking under heavy load/impact.
At least, that would be my take on the issue. I really like the idea of belt drive, but I think it needs a little time to become refined, and for competition/free market forces to bring the price down out of the rafters.
I built up this elevated chainstay bike as a quick way to try out a belt drive on an MTB seeing as I had just ruined a brand new conventional frame trying to do the same thing.
To get a Magic Ratio without any adjustment takes a bit of calculating and it required just a light dressing with a file on the dropout to get this to fit.
I didn't expect great things from the frame because the elevated chainstay design is not noted for stiffness which is essential for a belt. So Sunset1123 is dead right, and as it stands this setup does not like any heavy pressure.
The frame is one I bought years ago for some of my early experiments on belt drive, but I never used it before. Once I had the belt fitted I then had the brainwave of building it up with some of the dud parts I have cluttering my attic. Unfortunately I could not get my Look Fournales forks to fit
I named the bike The Penance Quasimodo Superniche
The good points are that the belt drive is nice and smooth, and the bike was light.
The next bright idea was to ride it in the SSUK09...
So how did the belt perform? Couldn't really tell the difference apart from the silence and smoothness.
The flexing of the frame meant I had to jump off on the tougher parts and run because the belt would jump, but that is really the frame not the belt. An interesting side effect is that I felt fresher at the end of the race than I normally would.
BTW using these tyres on a course with slippery rocky downhills is highly recommended as an alternative to casting yourself off high bridges when depressed. (But I knew this before the start, so maybe I was feeling depressed)
The SSUK had a brilliant course, lots of exciting stuff, and funnily enough riding this bike made it even more fun.
Meanwhile the belt drive conversion on my cross bike is peforming well.
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