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Standing on one Leg
Cantilevered wheels may look odd, but they make sense for many bikes. In an extract from his book, Bicycle Design, MIKE BURROWS examines the monoblade alternative.
For most cyclists the ‘monoblade' and the cantilever wheel are an anathema. This I suppose is only natural, growing up in a world of forks and stays, with the wheel always perfectly central. And woe betide any frame-builder whose wheels did not line up perfectly.
But although as cyclists we are used to seeing the wheels supported on both sides, we are of course surrounded by a world where most of the wheels are cantilever - that is, supported on one side. Indeed, there are very few vehicles other than our bicycles, their motorised cousins and wheelbarrows that still have forks. And even some of the motorised machines are abandoning forks.
Most famously, the Lambretta and Vespa scooters of the Sixties were cantilever front and rear, and carried a spare wheel under one of the side covers. Historically though it is a much older idea. I first saw the idea on a bicycle when visiting the Museum of British Road Transport, Coventry in 1985, in the form of the ‘Invincible' produced by the Surrey Machinists' Company in 1889. And even this machine had borrowed the idea from an earlier Coventry-built velocipede!
The idea did not catch on, and the first commercially successful use of the principle on a two-wheeler was the scooter.
I first consciously used the idea on the Mk2 ‘Speedy', as a way of simplifying the frame design. By mounting the single rear wheel on one side I was able to run the main 2” diameter aluminium tube right through from the bottom bracket to the rear axle, and not have to graft on a rear triangle. It looked very strange at first - although it wasn't really any different from the cantilevered front wheels – but worked extremely well. I decided that maybe a bit of asymmetry would not be a big thing. In practice this turns out to be true.
Despite having made this small mental leap in 1982, it was not until I saw the ‘Invincible' in ‘85 that I began to understand the possibilities for two wheelers. I had by then developed the original monocoque racer, which was what I was riding as part of the Rover Centenary ride when we stopped off at the museum in Coventry. On the way home on the train the pieces dropped into place. This time I did not need the cantilever for convenience but for its aerodynamics - biplanes are bad and monoplanes are good. So one big fat-but-aerofoil section blade would be a lot better that a pair of regular, or even aero-section, blades.
My first blade was machined and filed from a length of 2” square 6061 aluminium. But it was another four years and a lot more ‘triggers' to finally hang the rear wheel on one side as well. Again, it was out of line but by only 16mm this time. It was this Mk2 that was adapted by Lotus for Chris Boardman to use so successfully in the ‘92 Olympics.
[Giant's successful Halfway folding bike is single sided front and rear. It rides just like a normal bike!]
Having got this idea of cantilever wheels and monoblades as an option stuck firmly in my mind, I built a series of bikes of different types in an attempt to discover the advantages and disadvantages of the system. I had already built a short-wheelbase recumbent bike as part of the Mk2 development programme, again using the same 2” alloy tube as on the Speedy. This again had considerable wheel offset. It worked well and was eventually sold on to a friend who successfully raced it in HPV races.
I then built a shopper in answer to a request from Jim McGurn who, at the time, was editor of ‘New Cyclist' magazine. He actually only wanted me to write an article on city bikes, but I was not in writing mood. And so ‘Amsterdam' was built, brazed from largish steel tubes and with the most offset rear wheel yet – 60mm. This you could notice when you first rode it, but after 10 minutes or so it was just like a bicycle. This was followed by ‘Vienna', a carbon fibre/aluminium tube rear suspension touring bike. This was more interesting than successful.
I also played around with ATBs, adding a glass-fibre monoblade (complete with disc brake) to the front of my hardtail - very light and very strong. I also built a single leg suspension version with 110mm of travel and employing the oleo leg principle, as used on virtually all the world's aeroplanes – monoblades every one!
Not all one-sided
So what have I learned after 15 years of ‘monoculture'? ‘Swings and roundabouts' is the answer. It is a matter of understanding the reasons and advantages, but also the trade-offs.
First, and easiest to understand, are the aerodynamic reasons. If air resistance is your big problem, either on the velodrome or in a time trial, then a monoblade in place of a fork is almost a must. It will add weight, and the fact that it tends to flex sideways, rather than fore and aft, does not encourage cornering at the limit. But it will impress the man with the stopwatch.
[Cannondale's 'Urban' show bike used their production Lefty 'fork' as a starting point]
It also makes sense to have a cantilever rear wheel in these situations. This is not an add-on option, but for those intent on designing the best it's the only way to go. You can even fit gears, again as on the ‘Speedy'.
For road racing , however, it makes no sense at all. You are mostly in the bunch, so get little aero advantage - just poor handling, extra weight and a front wheel that you can't change easily if you puncture. For time trialling, of course, you have no outside help and you have to change the tube, which is easier.
Cantilever can also make sense for structural reasons, as on the rear end of the ‘Speedy'. Here the offset rear wheel is a small price to pay for a very strong simple frame.
Also, for structural reasons of a different sort, it makes sense on the front end of a mountain bike, where it is stronger and lighter than a conventional fork. This might sound strange, as I have already dismissed the idea for road bikes. But we do not do to road bikes what we do to mountain bikes. Quite simply, we abuse them. We ride them into tree stumps (collar bone!), down boulder-strewn mountain sides, not to mention the steps outside the local C&A – and we expect them to survive!
Now it is in the nature of things that, as they get bigger, they get stiffer and stronger, and that they do this faster than they get heavier. Suffice to say that, as you double the diameter of a tube, its weight doubles but its stiffness will increase some four and a half times - really nice numbers. This means that if you were to swap your two 25mm diameter fork legs for one 50mm diameter leg, you would have doubled the critical fore and aft strength without any increase in weight. You will also have increased the torsional stiffness, but that is already many times higher than necessary.
For suspension legs there can be even more advantages, as there is only one set of parts. And one big set is cheaper, and should work better, than two small ones. You will have to use disc brakes, as there is nowhere for the ‘other' canti to go. But that is no bad thing.
Despite having thought long and hard about it, and even built one, I can see no good reason for a mono rear on an ATB - and quite a few reasons not to. But I have not given up completely.
The last and probably best reason for going ‘mono' is for the sheer convenience of it. Firstly, tyre changing for punctures. Not for racers - they have quick-release wheels - but for commuter/city use, where a QR is an invitation to someone to walk off with your wheels. And anyway, it is a bit tricky if you have a chainguard, hub gear and drum brake. With a single-sided shopper you don't take anything apart. The drum brake need never be disturbed. And the chain and gears are inside the bike, running in oil rather than wiping it off on your clothes.
Big Wheels Are Faster!
Click to see:
I thought of that while riding my bike.
Einstein, on relativity