The Wheel Deal
Words and graphics: Andy Hornsby
Photos: Performance Machine/Taylormade Engineering

Wheels make a bike special. They contribute enormously to the Fat Boy and the V-Rod, and they defined the Disc Glides of several generations, but while some people use a variety of wheels to give their bike its distinctive look, just as many – if not more – clean them and think no more about it. But whether you’re trying to improve your looks, handling or choice of tyre sizes, there is more to wheels than meets the eye.

Since the first enterprising anonymous cave dweller created a circular shape and realised it rolled, the wheel has undergone developments to make them easier to produce, lighter, stronger, more attractive and rounder. Despite AmV’s tendency to start at the very beginning, we’re not going back that far here because we don’t care that much about wooden wheels. Okay, so there’ll be a bit because there is some heritage leading back to spoked wooden wheels and we wouldn’t be where we are now without them, but don’t worry, I’ll keep it brief.

In their earliest days, wheels were a cross section of a tree made more accurately circular by working the wood. Handy things are trees, but sadly they don’t often come in large diameters, and those that do would have been a bugger to cut down with primitive tools so create their own difficulties. But why would you want a larger diameter wheel? Because wheels with a larger diameter are less affected by small holes because they can’t drop into them, and they smooth out small bumps by almost forming ramps in front and behind them, which is why they are still the popular choice for the front of off-road motorcycles. They reduce the need for suspension which was lucky because early wheeled things hadn’t really got any. A simple experiment illustrates how nicely: roll a ping-pong ball across a sheet of corrugated iron, then repeat the same process with a football.

The simple way to get bigger wheels was to make planks out of the wood, bolt those planks together to give the effect of a single piece of wood and then cut a disk out of that. Made a lot of sense and it worked quite well, except for the wood’s grain, knots and various other weak points. We got bigger wheels, but we also got structural failures – oh, and they were heavy because the sheer amount of wood used to make them strong enough.

Wood is incredibly strong in compression, in the direction of its trunk, boughs and branches, but as soon as you cut it onto planks, and especially when you cut those planks into other shapes, you lose a lot of the original strength and load it in ways that are less robust. It wasn’t the solution, so the next stage of the evolution was to use smaller, more highly selected bits of wood chosen for the direction of their grain, and make them into sections of the wheel’s circumference, and then support them from a central hub by strong spokes turned down from branches, using their inherent strength. Each piece of wood, whether for use as a spoke or a section of the rim, was selected for its grain, and used its strength to produce a very large, very strong and relatively light wheel.

It wasn’t a tough job for early spokes because all they had to do was hold up the weight of their burden. Braking was taken care of by applying a wooden block to the circumference of the wheel – an early rim brake, if you like – so that wasn’t an issue. Only with the advent of powered wheels, first by pedals and then by motor, did the spoke need to deal with rotational forces. Under power, the hub desperately wanted to turn, while the rim was quite happy to stay in the same place and the spokes weren’t really geared up to cope with the forces created. Low initial power didn’t trouble spokes radiating out at ninety degrees to the hub especially, but the advent of hub-based brakes generating the same forces in reverse – compounded by weight and momentum – changed all that.

All of a sudden, it was more serious. Wooden wheels couldn’t take the rotational stresses of power and braking, and as power and weight increased, the gauge of the spokes needed to be increased to handle the role.

It’s a trend that we’ve only just started to see reversed with the rim-mounted front disk of the Buell XB-series, and anyone who has ever picked up a fully trimmed front wheel from an XB model will affirm that removing the braking stresses from the front wheel allows a far lighter construction. It makes you wonder how long it will be before we get a rim mounted pulley matched to a rear rim brake and lose the weight in that wheel too … which isn’t as daft an idea as it first sounds. Perhaps I’d best get down the patent office before Erik Buell.

By the time we get involved, iron and steel were around, and the starting point of the Safety Bicycle was established: the bicycle having had a twenty-year start on the motorcycle. Steel had already replaced wood for frames, and steel wheels appeared after sewing machine manufacturer and mechanical genius, James Starley, invented the "lever tension" wheel in the 1880s.

These steel spoked wheels became the obvious replacement for spoked wooden wheels, although they worked in diametrically opposite ways. On a wooden wheel, the spoke provides its strength through compression and stops the rim collapsing onto the hub, but they need to be thick spokes. On a spoked steel wheel, the hub hangs from the top of the rim using spokes in tension. Steel has massive tensile strength so significantly thinner spokes could be used – all you need is a means to adjust their length to determine the position of the hub, but he’d already though of that which is what the "lever" bit is about.

Steel spokes were initially set out in a conventional radial pattern, along the same style as wooden spokes, with the spokes lining up at ninety-degrees to the hub and rim, but it was also Starley who came up with the idea to lace the wheels in what was called the "tangential spoking" method in 1876, which was so good that we still use it today – and marvellous for motorcycles, and cars, as it affords a better rotational strength to the wheel. There’s nothing complicated about the tangential spoke pattern once you’ve spotted that the spokes leave the hub at a tangent rather than ninety degrees, and alternately face forwards and backwards to best transfer the rotational forces to and from the hub. Spokes facing forwards are transferring the braking action, and those facing backwards, the drive. In each case, the spoke is pulling the rim along, or is being pulled back by the rim, using the tensile strength of the steel.

In case you were wondering what was so safe about a "Safety Bicycle", it was nothing more than a term to differentiate it from the "Standard" bicycle, which we now know as a Penny Farthing, invented by … oh, James Starley. Standard bicycles had a poor safety record and prove that he was only human, but they were still a major improvement over the preceding hobby-horse. In the case of a standard bicycle, the big wheels were used as much for their gearing as their road smoothing properties, but while they might be less susceptible to road imperfections, if a bump was big enough, the whole bike could rotate round the front axle and the rider would nose-dive into the road.

The Rover Safety Bicycle – and yes, that Rover – was developed by James Starley’s nephew, John in 1885. It had two steel spoked wheels of roughly the same size with a solid rubber tyre wrapped round each, the rear wheel was driven by a chain on different sized sprockets, solving the gearing problem. The rubber tyres gave the slippery metal a better grip on the road surface, and protected it from damage in the same way that steel tyres protected wooden wheels from the rudimentary roads of the time. Pneumatic tyres came along sometime after 1888 and offered a level of comfort that was positively luxurious at the time.

You didn’t want to know that Starley also invented the open differential long before the car needed it, did you? Well, tough because I’m going to tell you, in only because it answered the weird question of why anyone would invent a differential before the car needed it. Seems James liked to go cycling on a bicycle made for two with his nephew, and the differential was developed to allow the younger John to pedal more energetically without condemning his aging uncle to keep pace.

We’ll cut through the rest of history, with just a brief stop off at the first motorcycles: tall 26 or 28-inch wheels with steel rims, laced tangentially, wearing rubber pneumatic tyres. And an engine where the pedal crank previously lived: literally a motorised bicycle.

But for a range of diameters, widths, materials and spoke patterns production motorcycle wheels remained largely unchanged until the arrival of cast aluminium wheels in the 1960s. Okay, so there were a few pressed steel wheels in utility vehicles, but they are the exception rather than the rule and tended to be on the back of trikes.

Cast wheels for our purposes started off in racing where they were typically lighter than spoked wheels by virtue of the materials used: aluminium being lighter than steel. Racing types had already discovered that aluminium alloy rims were lighter than steel ones when laced to their quick-release hubs, and are always keen to keep weight to a minimum. The performance case for lightweight wheels is that less weight corresponds to better acceleration, better braking and a lesser gyroscopic effect, making steering easier.

The weight thing is an easy thing to illustrate to anyone who used to turn their bicycle upside-down to clean it. Human nature dictates that once you’ve done that, you are inevitably going to turn the pedals and get the wheel spinning as fast as you can, and likewise to don a pair of gloves and try to stop it without using the brakes. It’s dead easy with a bicycle. Light wheels, light tyres. Now imagine doing that with a motorcycle wheel – a 3x19-inch should do it, and complete with the tyre, obviously. Getting the wheel moving would be harder, making it accellerate would be harder too but one it was moving it would spin for a long, long time. If you attempted to stop it in one grasp of a gloved hand, you’d be as likely to end up with a broken wrist as a stationary wheel. In truth, a wheel is little more than a flywheel. It just happens that it runs along the ground and has a tyre on it, so is therefore seen more as a wheel.

Alloy rims for laced wheels are generally extruded – a billet of alloy is pushed under very high pressure through a shape that corresponds to the profile of the rim, before being formed into a circle and the two ends welded together – or spun, but cast wheels came from moulds and then machine finished. Cast wheels have the in-built advantage for motorcycle manufacturers in that they are relatively cheap and very much more simple than laced wheels, which is why they are so prevalent on modern motorcycles but they are not a new phenomenon.

People have been casting wheels for almost as long as they’ve been casting metal, but it was only when they became more commonly used on cars, and especially racing, then sporting and custom cars, they gained a new kudos.

It’s worth mentioning here that racing wheels are actually magnesium alloy, hence "mags", and therefore even lighter still, but magnesium is both highly flammable and brittle, so it doesn’t cut it on the road. People conveniently forget about that and wrongly assume that all cast wheels are extremely light, which wasn’t necessarily the case in their early days.

In two respects cast represent a backward step. They return to the compression-loaded spokes that we walked away from when we walked away from wooden wheels – although with a significantly stronger substrate than wood to handle the forces – and what you’re given is what you’ve got. There is no potential to change a cast wheel beyond throwing it away and buying another, or more likely another matching pair. It goes without saying that you can change a spoked wheel for relatively little money by attaching different rims in different widths and heights, in different materials using different spokes. Not so with a cast wheel. In exchange for that flexibility we got the freedom to cast weird and wonderful shapes, use tubeless tyres and got something else to paint … and lighter? Well, not by much in the early years, if we’re being honest here.

As things have evolved, casting processes have been developed to make things as light as possible and to use the minimum amount of material, saving money and weight, with the result that hollow pressure cast wheels are common on the current generation of Super Sports bikes. This new generation of wheel is increasingly finely tuned for the weight it is expected to carry, and you would be well advised to check the load capacity of any wheel from such a source if you are planning to stick it in your heavyweight Harley – the days of over-engineering are gone, as our oriental friends search for that last spare ounce of weight. Bear in mind that a Jap sports bike will tip the scales at around the 200kg mark, and compare that to your current generation "lightweight" Super Glide’s 300kg, or even a Sportster’s 240kg.

Cast wheels are machine finished to give the clean polished surfaces and are typically lacquered before we get them to keep those surfaces cleaner for longer. That lacquer is usually the first thing to go, lifted by the aluminium oxidising beneath its protective skin courtesy of an imperfection in the original casting, or a ham-fisted tyre fitter breaking the surface and seal. It is typified by white powdery deposits and once that happens get some paint stripper and clean off the lacquer, and either polish the bare metal or re-lacquer it.

Nipping back a few generations, pressed steel might not be a terribly practical or elegant solution for a motorcycle wheel, but it was used extensively in car wheels, and still is. Car manufacturers needed metal pressing hardware so it made sense to use it’s full potential and they pressed two steel half rims and welded them to a wheel centre, which is then bolted to a hub fixed to the car. The two half rims were generally of different widths with a shallow outer rim to hold a hubcap and a deeper inner one to keep the brakes and steering reasonable clean, and it didn’t take too long before enterprising gear-heads of their day twigged that by fitting the two wider rims either side of a wheel centre, you could have a wider wheel. And a not unattractive deep dish outside too. They were equally quick to notice that the wheel centre was relatively easy to produce and so made them for the purpose in a variety of styles and materials, generally to save weight. Once you start on the weight-saving kick, aluminium rears its head quickly, and the next obvious step was to get aluminium rims either spun or extruded, and in a wide variety of sizes.

The big name on the street used to be Compomotive: builders of composite wheels and providers of split rims, but they specialise in car wheels, hence use car rims, which have a different profile for a different kind of tyre fitment. Still, the principles are the same.

Starting with two halves of a rim, you bolt them either side of a wheel centre that makes the body of the wheel. If you have two 11/2-inch rims and a 1/2-inch centre, you’ve got a 31/2-inch rim: simple elementary maths but with extraordinary potential.

The wheel centre can be cast, machined from a billet of aluminium alloy or cold forged from pure aluminium for even greater strength with incredible lightness, and then machined to create just about any shape you can imagine – and a few that you probably can’t. And it’s not just aluminium, or even just metal – few will have missed the stunning bullet-proof glass centred wheels on Charlie’s Harley when it burst onto the scene last year.

The next generation of wheels will undoubtedly involve carbon fibre for even less weight, and a whole new production process, but even now the choices are manifold and the overall effects very different. We are in the enviable position that the technology is there to be used, according to the style of the bike you want to run, and providing you recognise the trade-offs you can stick whatever you want into your bike.

Spoked wheels have had something of a renaissance in recent times, with the boom in retro-style machines, and are beloved of traditionalists, but cast wheels have been with us for long enough now for them to reach mainstream acceptance – and some can be used tubeless, but not all: check before you assume.
Composite wheels, whether bolted or welded together have opened up new avenues of opportunity for show customs, and are filtering down quickly into the world of road bikes, where their easy to clean finish and tubeless tyres have been welcomed.

It’s not as straightforward with cast wheels, because you have to live with what you can find but if cast wheels can be found to match your style, and in the right sizes, they can make a cheap alternative if you have the engineering skills to mate everything up, but be wary of overloading newer wheels.

So, why would you want to change your wheels? To make your bike more distinctive? Yes, but there’s more, there always is, and it goes back to why people started changing them in the first place, which was performance related. More traction from a bigger surface area for the rubber on the road plays its part, and with heavy cars it is good to spread the load a little.

It is a style that has stuck for cars and bikes, and it is good as long as you realise that the same weight spread over a larger area will actually make your bike lighter on the road: great for burnouts but questionable for heavy braking in wet weather where you pray the increased contact patch will provide the grip that you’ve lost from the weight.

There must be a calculation somewhere that will tell you, and account for the characteristics of weight distribution at speed, but I’ve not got it.

Quite apart from spreading the weight though, it can make your bike a lot easier to ride: a lighter wheel gives better handling, suspension and braking. Heavier wheels give rock solid stability, but you can get a flywheel effect if you push it too far.

Most of the rest of it is cosmetic, and wholly subjective … but it’s probably a good idea nowadays to steer clear of wood.

Wheel Types

Laced Wheels

Comprising of a hub and rim, and the spokes to join the two together.

In an ideal world, the hub is specific to the bike and everything therefore lines up with the rest of the bits on the bike. It acts as the carrier for the wheel bearings, the brake disks and the drive sprocket or pulley. Using the original hub is a good idea if you don’t have an engineering background, or an open cheque-book, because it means that the brake disks will be aligned with the callipers, the wheel spindle will slot straight into the forks or swinging arm without a problem, and the final drive will line up. There really isn’t a lot of point in trying to get another hub from another bike to fit because there would be no major advantage, but there are increasing numbers of laced hubs being made for specific applications.

Harley-Davidson hubs tend to be generic, which is to say that the same hubs will fit many bikes over many years: it’s one thing to reinvent the wheel, but providing the bearings are up to the job and it has drillings to offer a home for the right number of disk rotors, what more does it need to do?

Check wheel part numbers for your model and compare those from other models to work out which hubs you can use, or indeed which wheels. Hub selection comes down more to whether you want a single or twin disk set-up than much else.

Once you’ve got your hub, you can then work out what sort of rim you wish to lace to it to – and this is the major fundamental freedom of the laced wheel. The standard widths of Harley rims are 3-inch at the back and either a 3-inch (FL 16-inch diameter) 2.5-inch (FX/XLH 19-inch diameter) or 2.15-inch (FX/XL 21-inch diameter) at the front, but that is only your starting point. You can adjust the width of the rim, and the height, and to make it fit your bike, you can offset the rim in relation to the hub – moving it to the left or right to clear belts, swing-arms or whatever – just by adjusting the spokes.

And then there are materials.

The stock wheels are chrome-plated, rolled-steel rims connected to aluminium hubs using steel spokes, but unless you keep on top of your cleaning tasks, and take care with tyre levers, the chrome will eventually pit or flake and the steel beneath will rust when exposed to the elements.

Stainless steel is a popular replacement for rims because it doesn’t rust, but there is no weight saving over the standard and if you’re going to go as far as re-lacing your wheels, you might as well get the full benefit of a low unsprung weight if you have any intention to ride the bike reasonably hard.

If you want to save weight, you can use an extruded or spun alloy rim, and it makes sense to use stainless spokes because they won’t rust – they don’t make alloy spokes because it hasn’t got the same tensile strength as steel, and it wouldn’t hold a thread as well where the nipple meets it.

Having gone this far, you might be tempted to create different effects by playing with spoke patterns – but unless you’re going for something radical, like radial spokes, no-one else would notice but you so why bother. If you are tempted by radial spokes, be aware that they have to work a lot harder under radial stresses than tangentially laced spokes so will need to be appreciably stronger, and therefore heavier: not a good idea for a rear wheel.

The only major things that concern us regarding spokes are the gauge (thickness) of the spokes, and whether they are nail or hook type: a nail spoke has a straight head where it is laced through the hub, and hook has a ninety-degree bend at the head where it passes through the vertical surface of the hub. There are also spokes available that are twisted, or flat, or both to further enhance the appearance of the finished wheel, but they don’t make cleaning any easier and the one criticism that is always levelled at laced wheels is the cleaning effort required.

As standard, Harleys arrive with forty-spoke hubs but if you want to get into serious customs you can get over-spoked hubs and rims. Sticking twice as many spokes in to get an eighty, or three times as many for a hundred and twenty-spoke is possible, but you should consider the practical implications if you intend to ride the bike afterwards. The over-spoked rims and hubs are the same weight – if not slightly lighter as there are more holes in them – the weight of the additional spokes has an impact and the hundred and twenty-spoke items are seriously heavy. Next time you visit your neighbourhood wheel-builder, ask to pick up a bag of forty spokes, then two bags, then three. No doubt about it, they look stunning, but you wouldn’t want to clean them after a wet weekend on a rally site, and you’d be advised to improve your brakes, and get used to a different braking technique with greater emphasis on the rear just to slow your reinvented flywheel down.

Sounds ideal. Why would we want anything else than a spoked wheel?

Because they are extremely complicated in terms of their surfaces and because they need inner-tubes to hold the air in – well, you’ve got between forty and a hundred and twenty holes perforating the air chamber so something’s got to hold it in – and inner tubes puncture, perish and piss you off.

Tubeless laced wheels are available in two forms: one seals the holes in the rim with a high quality sealant, and the second is a radically different solution from BMW, but I’ve not seen it used on a custom, except their own R1200C ... yet. The sealant option is an obvious one, but just how well the forty, eighty, or hundred and twenty holes will be sealed over time is something I’d prefer not to think about, and bear in mind that most spoked rims have been built with the assumption that they’ll have a tube, so might not provide as good a seal against the tyre as you might like. Check with your supplier before making any rash assumptions.

In case you’re tempted to get tricky, the BMW design has the spokes meeting the rim at its edge, rather than its centre, and therefore outside the sealed air chamber formed by rim and tyre. It would require a wholly different hub because it all works the opposite way round to convention, with the nipples being located at the hub and the fixed, and exposed head at the rim. Alternatively, you could redesign a spoke. If you need a nudge in that direction, you might like to know that the BMW uses a forty-hole pattern too, but after that you’re on your own.

Cast Wheels

Comprising a wheel – or two, if you want your bike to look right.

Couldn’t be simpler: a single piece of cast aluminium that is either designed for your motorcycle or someone else’s. The casting process is to get your head around, complicated to get right and so is not a good reason to go and build a smelting works in your shed. Molten aluminium alloy is poured or, more recently, forced under pressure into a mould and then cooled under controlled conditions.

Gravity cast wheels use the natural gravity of the planet to draw the molten metal into the mould and is the simplest form, but least efficient as they can have problems with porosity, shrinkage. Combine that with high unit density – or weight, to you and me – to achieve the desired strength and they become increasingly less attractive. They are also more prone to imperfections with air bubbles and localised cooling, so they tend to be over-engineered to build in a safety factor.

The resulting metal is fairly coarse – snap a piece of cast alloy and the surface of the break will be very rough, but with a uniform grain, so is equally strong in all directions. The resulting wheel is fairly soft, and a good clobbering will leave a dent in the wheel rather than a fracture, and specialist companies now are able to repair such damage.

Low pressure casting gives gravity a helping hand, ensures a quicker and better fill of the mould, fewer localised cooling problems and a higher density of metal to provide a finer grain, and offers greater strength at relatively light weight.

Harley’s 13-spoke cast wheels have been around for years, replacing the 16-spoke ones that first arrived on the Electras and saw duty in the first FXS, and apart from occasionally playing with paints – colouring the centres in either black or gold according to the model and celebration – and drilling additional holes for dual disks, they’ve generally not messed about with them.

If you have laced wheels in your stock Harley and you want a set of cast Harley wheels there is a very good chance you’ll find a pair that fit. Check to make sure your spoked wheels are fitted to the same model as the bike that will be the donor for your cast wheels, and get your spanners out if they match. If not the same specific model, then the same generic forks, or swing-arm.

The exceptions to the Harley spoked wheels are the sixteen-inch front and back wheels from the Fat Boy – including the chromed and slotted one fitted to the XL1200C and the blacked-out Night Train’s version – Disk Glide wheels from any generation (although strictly speaking at least one of them purported to have a spun aluminium wheel), and the Deuce’s seventeen-inch rear wheel. Disk wheels are especially popular on just about anything else, and look good on just about anything when matched to a spoked front
or another disk.

Any other cast wheel spoke pattern will be an aftermarket item, although the aftermarket is favouring billet and forged nowadays, or will have come from a Japanese performance bike. The most common cast wheel on a custom bike these days is a three-spoke wheel from any of the big four Japanese factories, and is usually accompanied by its braking system, and sometimes its forks – although they’re a little short these days as they have a relatively low headstock.

It is seldom a simple case of bolting such a Japanese wheel into your Harley forks because the interface between the two, the wheel spindle, is not common to both. The spindle from the Jap bike will not match the Harley, and the Harley spindle won’t fit the Jap wheel bearings. You can either machine the spindle, the forks or change the bearings to suit your requirement and then work out the spacers to get the wheel centrally within the forks or swinging arm ... and then the spacers to get the disks aligned with the callipers ... and the spacer to align the final drive. BUT before you do any of that, find out what the maximum loading of the wheel is and make sure it is actually physically strong enough to take the weight of the rest of the bike, because American-made motorcycles are more, errr ... substantially built than the bike that those wheels will have come out of.

On the bright side, having verified the loading and made the spacers, you’ve now got rims that will take modern rubber, and that generally means wider one, and radials.

Sounds ideal?

It’s a cosmetic thing as much as anything else. Harley cast wheels are well established enough for a lot of people to accept them for their ease of cleaning, but they are restricted in the wheel sizes and lack flexibility. With the Deuce, there is a seventeen-inch option in the stable but that’s your lot.

Jap wheels are not bad within their constraints, but they lend an appearance to a bike that you either like or don’t. You’re also generally stuck with a front wheel to match the back whether it’s the right size for your vision of beauty or not, because mismatched cast wheels look iffy, and cast spoked wheels don’t sit too well, visually, with laced wheels either. That means you can’t stick a nineteen or twenty-one inch wheel up front because those wheel sizes are very much yesterday’s news for the Jap market.

Billet Wheels

Comprising of a pair of hubs and two spun aluminium half rims, with a wheel centre that sits between the rims, joining them, and mating them to the hub halves.

It’s easy to think of a billet wheel being nibbled from a single block of billet aluminium, but in reality the wastage would be extraordinary. You could scoop up the waste and resell it for recasting but the scrap value isn’t high. In reality, pre-made rims are produced separately as blanks and are then built up into wheels by engineers who determine the quality and appearance of the wheel centre and hub.

A common misconception is that billet aluminium is a grade of aluminium, but it is nothing more than the form in which it comes. There are a number of grades of aluminium, which are determined by how much aluminium is in the alloy and what other metals comprise the final mix – which can be copper, zinc, nickel, iron and others – and the different grades have quite different properties.

There may yet be a space for a billet tech in the future, but suffice to say here that you’ll find billet wheels are made from a stable, workable alloy that has high tensile strength and accepts a decent finish. You’d think that would be all anyone would want but you’d be wrong because different grades that use the same basic ingredients in different quantities can produce anything from a very soft alloy, which is very good for bits that need a very high finish but with little tensile strength, to one that adds nickel to a different mix to give a very high tensile strength but with a difficult finish and few applications.

An aluminium billet is not so much cast as wrought: pressed into shape so avoiding the issues of a complicated mould, and with all of the cooling problems avoided. The resulting lump, the billet, is usually a simple shape, and the metal produced is of a consistent density with a consistent blend of the composite metals. It can either be machined in its original form, or else heated to red hot and rolled into different shapes and then machined, or else extruded under extreme pressure, and all without introducing inconsistencies or changes in density, so giving it an intrinsic advantage over cast alloy.

Stylistically, the world is your oyster as just about any shape can be carved into, or out of the wheel centre using modern CNC engineering kit. It also hasn’t gone unnoticed that a drive pulley, or floating disk brake’s carrier can also be matched to the style and shape of the wheel centre, as it is little more than a scaled down version of the same. You wouldn’t tend to have a billet disk rotor surface because it is not the best material to use in practice, though coatings can be applied which will make it more suitable.

Wheel rim diameters and widths determine what your size options are, but they are wide and varied, and can incorporate an offset by using non-identical left and right hand rims. It’s easy to give an example than to explain it.

A typical wheel centre would be about 1/2-inch thick, so if you wanted a 9-inch wide wheel, you’d subtract that half-inch from the nine and divide the remainder in half, so you’d want a pair of 41/2-inch rims. If you wanted a one-inch offset, you could have one 31/2-inch and one 51/2-inch remembering to account for that at the hub.

Sounds like a job you could do at home, except that wheels have to be round – perfectly round – as well as it being useful that they’re balanced, and the chance of your getting the whole thing aligned correctly on your Black & Decker Workmate is so small that you’ll understand why the rim manufacturers don’t supply the public: you talk to the professionals.

The professionals take the rims – either pre-drilled of not – design and produce the centre and the hub, and bolt the whole lot together into something that will bolt straight into the bike: all you’ve got to do is put a tubeless tyre onto it. There is no reason why they cannot be welded together to produce a one-piece rim, but a number of manufacturers choose to bolt them because it offers the opportunity to repair, or modify the wheel after the original build.

Forged Wheels

Nothing to do with etching a famous brand name onto a poor copy, forged wheels have little to do with a traditional forge either. A forge is usually associated with a big hammer and red hot metal – well it is if you know any blacksmiths – but in this case it little to do with big hammers, and everything to do with big presses. We’re back to billets of aluminium, pure aluminium this time, which are cold pressed into the right basic shape, and then machined to create the desired finished product. Remember "Isothermal" to impress your mates when you explain that the lack of heat treatment keeps the metal more malleable and less brittle: sounds dead good, but just means that it was forged at a single temperature.

How much pressure do you need to reform aluminium without heat?

Between 35,000 and 85,000 tons.

You use no less aluminium, in fact you use appreciably more and that aluminium is significantly more tightly packed together and therefore substantially denser, and correspondingly stronger. If you snap a piece of forged aluminium and compare the surface of the break with that bit of cast aluminium you snapped earlier, it will be considerable less coarse. What do you mean you threw it away? Being pure aluminium the finished wheels are generally lighter than billet ones.

The pressing process endows the finished metal with a radial grain at the centre of the wheel, which is to the advantage of the finished wheel, and increases its strength.

It is possible to forge a wheel as a single piece and machine-finish it but a lot of forged wheels are composite wheels, utilising the same spun aluminium rims as billets wheels: composite can refer to both types. Forged wheels tend to be welded rather than bolted together, but I have absolutely no idea why.

As you might guess, there aren’t massive numbers of back street engineers knocking out forged wheel blanks for the aftermarket to machine into their intricate shapes. In fact there’s not very many at all: probably no more than a couple in Europe dealing with the relatively small motorcycle market, and the cost of diversifying to make bike wheels means that there’s not likely to be a major increase in them. This means that the forged wheel you’ve found will be engineered rather than manufactured by the company on the label, and it therefore largely comes down to the business of aesthetics.

Spun Wheels

I’ve left this to the end because I didn’t really understand it properly until an hour or so ago, when I opened Steve Taylor’s parcel. Spinning aluminium is akin to sticking the aluminium original in the chuck of a lathe and then working on the metal as it spins round. Unlike a lathe, it is not about cutting but about moving the metal to where you want it, or forming, and the application of pressure on one side, pushes the metal away, forming a bowl in its simplest form, or a complicated rim with more sophisticated formers.

Regular spun wheels are made in two halves and welded or bolted together together – welded in the case of the optional PM wheels in your Buell, bolted if you’ve got a set of Astralites in something.

The other route to a spun aluminium wheel is for a groove to be cut by a Parting Off Tyre Tool round the circumference of the wheel, from the outside to the centre – see the Illustration above – and the remaining aluminium either side of the groove is spun to form the rims. These are known as "Split and Spun" and have the advantage of being formed from a single piece of aluminium.