The Wheel

In this extract from his book MIKE BURROWS asks; How much of wheel lore is science and how much myth? Do spoking patterns make a difference? And do disc wheels give a hard ride?

The wheels are the most important part of your bicycle - at least that is what any racing cyclist will tell you. And with good reason, for the wheels are a large and crucial part of what a bicycle is. Indeed, around the turn of the century the Americans referred to racing cycles simply as ‘wheels'.

Evolution of the Cycle Wheel

The wheel started out just like the frame to which it was attached. It was built from wood. And it was built really quite well, for wheelwrights had been perfecting their skills for many years. But like the wooden frame it had its limitations. For not only was wood not the ideal material, it could only be used in compression. That is, where the spokes at the bottom hold up the hub.

With the arrival of steel, engineers soon realised that they could, as it were, ‘hang' the hub from the top of the rim by using fine steel rods that could be tensioned individually. This meant not only could the wheel be adjusted to stay circular (not an easy thing with wooden structures) but that a large part of the structure was now in tension rather than compression - a much more efficient way of using material. Just imagine how much weight you could hang from a single spoke, then think of the sort of structure you would need to place below that same weight to support it.

The new form of construction meant that early cycle wheels quickly became very good. This was just as well, as the bicycle was evolving into the ‘high wheeler'. In this design the front wheel was the largest part of the structure, its diameter being dictated by the rider's inside leg measurement. However, these early wheels had one drawback. For although they supported the rider's weight adequately, they did not transmit the power from pedal to rim so well. This was because they were all built radially. That is, with the spoke going directly from the centre line of the hub to the rim opposite. However much tension you put in a radial spoke, there will always be a degree of ‘wind-up' as you apply a twisting force.

The problem was eventually solved, as were many others, by James Starley with his tangential or crossed-spoke design. The wheel as we know it was with us. However, like the diamond frame, the detail has continued to improve. So we now have a selection of types from track racing to trade bike. And since the late 1980s, there has been a whole new area of development with the re-introduction of the ‘solid' disc and semi-solid aero wheels. I say ‘re-introduction' as they are illustrated in Sharp's book, albeit in sheet steel! Much nonsense is talked about these new style wheels - and not a little about the more traditional variety.

Traditional Tension Spoke

These are built with anything from 24 to 48 spokes[1] and a shallow rim that can vary from heavy steel on the cheaper utility types to hollow rolled alloy on most of the better models. Spoking patterns vary from radial to cross four. These non-aero designs are the strongest and lightest types of wheel, keeping as they do the amount of material in compression (that is, the rim) to a minimum and concentrating it in the tensioned spokes. This also produces a very tough wheel. It is far better at coping with abuse than a moulded wheel, which may appear to be very strong but will actually be relatively brittle.

I do not, though, believe that riders can feel the difference in ride quality even of these radically different types of wheel, let alone the difference that spoke patterns can make. I was once sent some wheels to review for a magazine that had been built with ‘wavy' spokes. The idea was that the waves would act like springs to give a more forgiving ride. Being an engineer I tested the wheels not by putting them on my bike but on a test rig. I loaded the rim to simulate a load some three times normal. Measuring this with a dial gauge and not with my backside showed a rim deflection of just 0.4mm. However, the 32mm cross-section tyres, that had been inflated to 500 kPa (about 75 psi), had deflected 16mm.

This did not seem unduly ‘springy' for a wheel. I therefore checked a selection of others, from radial race wheels that had a deflection of 0.35mm to a tired cross three tourer that gave 0.5mm. The conclusion must be that deflections of this magnitude, let alone the small variations, are beyond the range of any part of the human anatomy to detect. This being so, the most logical spoked wheel is one with radial spokes - provided that it is not transmitting power or braking torque, and that you use an appropriate hub, as the radial spoke loads are high. This applies equally for track, road, touring and ATB wheels.

For the rear wheel, or if using hub brakes, a cross three pattern would seem the best. Any more crossing and you start to lose lateral stiffness. Any less and there is still some wind-up. For racing wheels, where weight is important, there are some very nice 15/17 gauge spokes available. For bomb-proof touring, try to find some 13/14 gauge single-butted. These are expensive but virtually unbreakable.

Titanium should make sense for spokes but current ones do not seem to be very reliable. Aero section steel spokes are another option, but really these should be used in conjunction with an aero rim.

Deep Rim with Tension Spokes

Although closely related to the traditional wheel, this is the most recent style of wheel to emerge. There were aero-ish rims earlier, but they were no deeper than they were wide - and thus not very aero. The true deep rim was pioneered by Steve Hed in the USA and Hed wheels still make some of the best available.

Deep rims are typically 30-75mm in depth, with 12 to 24 spokes usually of aero or aero-ish cross section. Rims can be rolled or extruded aluminium, or moulded composite with or without a metal sub-rim - essential if you need to stop! These wheels will always have a small weight penalty compared to their shallow-rimmed cousins, but any difference in ride quality is, as I said, imagined. They may well feel different but this is due to the aerodynamics of the deep rim, which can be detected even in calm conditions. For this reason the very deep (60mm or more) rims are not a good idea for road racing, though the shallower ones are ideal. The deeper sections are perfect for time trial and track, being manageable in winds that would make a full tri-spoke a problem.

Moulded 3, 4 & 5 Spoke

I find it curious that, although only very small numbers of bad bicycle frames are bought, vast numbers of quite awful wheels are sold to unsuspecting cyclists. One reason for this could be that cyclists do not know anything about aerodynamics, and the people who produce the wheels are equally ignorant about the mysteries of airflow - not to mention basic engineering.

The original tri-spoke and five-armed aero-spoke were good examples of a lack of even the most basic understanding of the problems involved. The rim was hardly deep enough to be streamlined and the spokes came straight from a wheelbarrow. The best example of a good wheel is the Specialized, which was designed and built by Du Pont and uses proper aerofoil sections. It is also beautifully moulded using RTM technology. Wheels like the Specialized and the new Mavic are, in still air, probably the most streamlined available. If conditions are not too windy, they are also satisfactory on the front for time trial use.

My current least favourite wheel is the Spinergy, which appears to be weak, non-aerodynamic, ugly - and a best seller!

Discs

The original aero wheel and still the best - or at least, the most aerodynamic, if well done. However, mostly they are not so good, being flat sandwiches of foam and composite around 20mm thick. This is not very good structurely and, although better than a spoked wheel, not very aerodynamic.

Better for both reasons is a conical design. The extra width of the hub adds stiffness and improves the aerodynamic shape. It also hides the hub, which is a not very aero bit that we tend to forget about. Most of these designs are still moulded compression structures, but at least one design (I think from Russia) uses an Aramid skin under tension. Very neat!

Best of all aerodynamically is the lenticular or curved side design. But again, it has to be done properly. The only good one I have seen is by FES of Germany. Even that had a rather narrow section rim. It is always better for the smooth flow of air if the rim is at least as wide as, or even a shade wider than, the tyre.

Many organisations now allow you to use add-on discs, either clipped or bonded to conventional wheels. This is just as aero as the more expensive alternative and has been the norm with HPV builders for years. It is all I have ever used.

Disc wheels are only for use on the rear except on indoor tracks, as the effect on the handling of a bicycle can be quite dramatically negative. Contrary to popular belief though, they are even better (on the back) in a cross-wind.

For off-road use the regular shallow rim design works well. Deep rims are particularly pointless, as there is no airflow behind a 2" knobbly tyre! And the extra rim stiffness is of no advantage. I would like to see the opposite - an extra shallow tough but flexible rim, maybe of composites, that would absorb major impacts. This should reduce rim, frame and tyre damage. For the same reason moulded wheels would seem to be especially silly for off-road use.

Recumbents can generally use a regular wheel with ‘added' discs. As the wheels are usually smaller you may even be able to run a disc on the front. Three-wheelers can create a lot of side force, which is not something cycle wheels are designed for. Therefore keep the diameter down to 20" maximum for the main pair of wheels. A trailing single rear wheel is less heavily loaded and I have found 700C fine here.