Man’s greatest scientific discovery has evolved, but the wheel is still the motorcycle’s key component. – By Glynn Kerr
Wheels are arguably the most important component on a motorcycle, without them, you don’t even have a bicycle. While wheel designs are extremely visually important to cars, motorcycles have forks, swingarms, chain cases and brakes that frequently obscure the view. Whether you favor the 1869 Roper steam velocipede or the 1885 Daimler Reitwagen as being the first motorized two-wheeler, the first wheels were made of wood, with heat-shrunk iron ‘tires’ in the fashion of horse drawn carriages up to that point.
All-metal spoked (or laced) wheels with rubber tires quickly replaced wood and iron, as seen on the first ‘production’ motorcycle, the 1894 Hildebrand & Wolfmüller. Due to the unique design of this vehicle, the rear wheel was solid, as it also acted as the flywheel, crankshaft and camshaft via connecting rods to the pistons. Thin radial painted spokes were added as an attempt at visual unity with the considerably larger front wheel, so the effect carried little conviction. The imbalance wasn’t helped by the rear mudguard, which invited a wheel the same size as the front. The Germans may not have invented the first motorcycle, but they can certainly be credited with creating the first ugly one.
Metal spoked wheels were the mainstay of the motorcycle industry for the next 75 years, and continue in current traditionally styled models. Cheap, strong and simple to produce, by bending formed flat sections or extruded ones into a circle and welding the joint, wire wheels are nonetheless labor intensive to assemble.
In use, they can require periodic maintenance to ensure even tension on the spokes. Repairing rim damage is easier though, and not beyond the abilities of the home mechanic with a block of wood and a mallet. Steel has mostly been the material of choice, although aluminum rims with steel spokes have also been popular, due to lighter unsprung weight.
Spoked wheels long dictated the need for inner tubes, due to air leakage around each spoke nipple. BMW got around that issue by locating the spokes around the edge of the rim, beyond the pressurized area, as introduced on the 1994 R1100 R and GS models. Other manufacturers developed an alternative solution, with the spokes attached to the side of a wider central flange. In fact, Honda beat BMW to market by a whole decade on its Paris-Dakar inspired XL600L and R models.
Several current bikes also utilize this feature, including Suzuki V-Strom, Triumph Tiger 1200 and Yamaha Super Ténéré. It is most visible on the MV
Agusta Dragster 800RR, which exposes the rear wheel in its entirety, thanks to a single-sided swingarm. Coloring draws additional attention to the detail. Cast aluminum wheels arrived in the mid-1920s with the curious Czech Böhmerland, no doubt for added strength on a motorcycle that could accommodate up to four people. These were replaced by steel versions immediately before WW2, after which cast wheels didn’t make a return until Freidel Münch introduced his Mammut at the 1966 Cologne IFMA.
It was fitted with a magnesium alloy casting on the rear, after strengthened wire spokes on the prototype had proven prone to breaking, due to the high output of the engine.
Alloy castings in the 1970s were initially developed for road racing, due to their strength and lightness, whilst the fitting of tubeless tires was a further incentive. The absence of tubes offered additional weight savings, while also reducing heat inside the tire. Castings allow much more freedom than pressings in creating the shape of the section, permitting a deeper outer flange onto which the tire’s bead can seal itself, and essential in tubeless applications. Given the clear advantages, cast wheels were soon introduced on some higher end sportbikes, and became standard on many models by the 1980s, including mopeds. Tubeless tires were mostly fitted, as these provided additional advantages for street use.
When punctured, tubeless tires usually deflate more slowly than an inner tube and are easier to repair temporarily using a plug, without the need to remove the wheel and tire. Aluminum has usually been the material of choice for castings, although magnesium alloys from companies such as Marchesini offer lighter weight, at a price. Not all alloy wheels are lighter than their wire counterparts though. The cast wheels employed on the 1976 Yamaha RD400 were slightly heavier than its wire predecessors.
Early castings had some issues with porosity, especially sand-cast variants. Newer technology, including pressure or rotation casting, has resolved porosity problems. Due in part to porosity concerns, and to retain a certain amount of radial flex, Honda developed an alternative solution using either steel or aluminum stamped centers riveted or bolted to extruded aluminum rims. The Comstar wheel and its successor, the Comcast, became a regular feature on models from the 49cc MB5 up to the 6-cylinder CBX and GL1100 Interstate.
Easy cleaning was one big advantage with the pressed versions. Around the same time Honda introduced the Comstar, UK-based Astralite introduced a similar principle for its aftermarket wheel, using pressed aluminum centers riveted onto a cast rim with a Kevlar layer in-between. There were some stories of electrolytic corrosion caused by using steel rivets on an aluminum wheel, but the brand developed quite a following among sportbike enthusiasts. Astralites were a standard fitting on the short-lived Hesketh brand. Bimota built a similar product using aluminum rivets, which were featured on models such as the HB2 and DB1 in the 1980s. Over the years, wheel diameters have varied with understanding, fashion, technology and the quality of road surfaces. Since the 1980s, alloy casting has become the industry standard for serious street bikes, although wire wheels persist in certain categories.
True off-road bikes need the flex of spoked wheels to help absorb the impact of jumps, as do street bikes in many developing markets or rural areas with poor road surfaces. Larger wheels are better at traversing potholes and rough terrain, which is why most off- road models still employ 21-inch front rims. For purely aesthetic reasons, traditionally styled models still look right with spokes, and they are nearly obligatory on retro scramblers and cafe racers. However, larger diameter rims are less stable torsionally, as a simple proportion of height to width, and 17 inches has be- come standard street-bike specification, almost across the board. Alloy wheels offer a stiffness that is desirable in sport riding or racing, especially given the extreme width of many modern tires.
For the ultimate in strength and lightness, carbon fiber wheels have also been developed, with a considerably higher price. Also higher in cost, CNC milling offers individual design freedom from billet, and is especially popular for one-off custom bikes. For flames, skulls or other personalized imagery, CNC will create whatever the data tells it, and will never judge on taste, only on your wallet. Surely 3D printed wheels will also be coming soon, even if it’s just the center bolted onto a regular rim. As wheel widths grew, the increasing size of the spokes to maintain rigidity in cast wheels demanded creative solutions, to avoid an equivalent increase in weight.
Yamaha went with hollow spokes, which required expensive mold inserts that could be removed or drained once the cast was set. Honda preferred a solid, but asymmetric design, with the desired relief on one side being countered by the negative result on the other. In design circles, this was referred to as good side/bad side. By the time brakes, forks and other componentry was added, the difference wasn’t as evident as it sounds. Wheel proportions are fundamental to motorcycle design, and to whatever function or rider category we perceive those designs to portray. Both rim size and rim density are also used as a styling ploy, to adjust the effect on the visual weight of a motorcycle. Although sportbikes mostly use 17-inch rims at both ends, the front wheel is packed with brakes, forks and a mudguard, adding visual weight to the front. These are often combined with large scoops on naked bikes, or low side panels on faired models, to further link the front wheel to the main mass of the motorcycle.
The rear wheel, by contrast, has plenty of lightness, thanks to a smaller brake and chain, and a higher mudguard that leaves plenty of air space around the tire. Visual weight is further emphasized toward the front. Custom bikes are often exactly the opposite, with visual weight bias clearly toward the rear. The front wheel is of large diameter, with a thin tire and small brake, giving lightness to the front of the bike. Moreover, the front wheel is pushed away from the central mass by long forks. The rear wheel is small in diameter, and sometimes even solid, with a thick tire, the enveloping rear mudguard leaving no air space, and fusing the rear wheel visually with the central mass. There are also wheels with no centers at all.
The empty rims rotate on outer bearings, either regular (hugely increasing the speed of rotation) or shell-type used on jet engines and internal combustion crankcases for decades) located on a peripheral frame. Power and braking forces are then applied directly to the rim. Centerless wheels have appeared on a few custom show bikes in recent years, no doubt inspired by the Light Cycles in Disney’s 1982 movie, “Tron.” Oddball Swiss car builder Franco Sbarro displayed two models at the 1989 Geneva
Automobile Show, both of which destroyed the whole point of air-light rims by having heavy, slab-sided bodies. Despite claiming to be the inventor, hubless wheels existed decades before Sbarro, in the form of powered mono-wheels, in which the rider and engine were located inside a huge single revolving wheel. Sbarro’s concepts were followed in the early 1990s by a proposal called the Apache, from Ohio-based Next World Design, predicting a futuristic American V-twin. As with all the other hubless designs, it remained a one-off. Despite the high novelty value, more practical considerations seem to have won out in the market.
After decades of silver paint, chrome or polished metal, black currently seems to have become the mandatory wheel color choice on most vehicles. However, black makes the wheel visually merge with the tire, which also makes it look heavier, and negates many of the styling tricks previously mentioned. Color shows more detail, so after two decades of black wheels, surely it’s time for a change. One alternative is colored tires, which vied for a comeback a few years ago, but went nowhere. There are arguably some reasons why black is compulsory for tires, but there are plenty of white ones at vintage bike shows, and red tires are commonly seen on the 1928 Opel Moto-club. Colored tires might look gimmicky, but it would be refreshing to see more design possibilities. Perhaps the final step will be airless tires, as pioneered by Michelin. The tire is a matrix of flexible rubber walls, with a small metal rim at the center, about the size of a current disk brake. On a motorcycle, the wheel itself would be all but invisible, leaving us only the tire to color or style. The idea has been in progress for some years, but I have yet to see a motorcycle application.
Don’t hold your breath for a major revolution to motorcycle wheels and tires in the immediate future.