Yamaha’s SpinForged wheels save weight, improve handling and don’t cost a packet
Excess weight has always been a bike designer’s nemesis, but while overall reductions are welcome, there are disproportionate benefits from cutting the mass of a motorcycle’s wheels.
That’s where Yamaha’s SpinForged items – initially introduced on the 2021 MT-09 and now spreading to other models – come to the fore.
Wheels, and their weight, have a particularly complex relationship with not only the outright performance of a bike but also its ride and handling characteristics.
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They’re a rotating mass, which means every time you brake or accelerate, you’re not only changing the speed of your bike but also the speed with which the wheels are spinning.
Heavy wheels, with a high moment of inertia, are harder to get moving in the first place and to accelerate up to speed; even stationary, running in gear on a dyno or paddock stand, a machine with with lighter wheels will rev more quickly than an identical one using heavier rims. The same principle applies when braking, so lightweight wheels help there, too.
When wheels turn, they create a gyroscopic effect and that increases with the weight of the wheel. So, when you turn the bars or lean the bike into a corner, the spinning mass of the wheels is working against you. So reducing that mass helps handling as well as performance.
To reduce the inertia and gyroscopic effect of a wheel, where the mass is removed matters as much as the amount. Taking mass away from the rim – the fastest-moving part when the wheel is spinning – will have a much greater impact than reducing it near the hub.
Wheels are part of the bike’s unsprung mass. Most of a motorcycle’s weight is sprung, ie carried on top of the suspension, so it remains steady over bumps.
The unsprung parts – wheels, brake discs, calipers, axles, tyres etc – all have to move rapidly up and down over uneven surfaces. The greater their mass, the harder it is for those unsprung parts to change direction, so when they reach the top of a bump and need to travel down the other side to keep the tyre in contact with the road, anything that reduces unsprung mass is a benefit.
When it comes to wheels, the usual choices are cast or forged alloys. Cast wheels are cheaper: molten aluminium alloy is poured into a mould, creating the right shape. It’s quick and easy, but castings aren’t as strong as forgings, so need to be thicker and heavier.
For a forged wheel, billet aluminium is heated and pressed into a blank wheel shape using a vast amount of pressure, then machined into the final component.
The result is stronger, so the spokes and rims can be thinner and lighter, but it’s a labour-intensive and expensive process.
Yamaha’s SpinForged wheels use a third route – called flow forming – which strikes a better balance between light weight, strength and manufacturing cost.
The result is a wheel with much of the strength of a forging, allowing rims just 2mm thick instead of 3.5mm, and on the MT-09 saving 700g from the two wheels’ combined mass.
Yamaha wheel technology explored:
- Start with a casting Despite the 'SpinForged' name, flow-formed wheels start as an undersized casting, already including the details of the spokes and shape, ready to be stretched into the finished wheel.
- Get them spinning The initial casting is mounted on a mandrel which spins it as though it were mounted on a lathe. A burner fires at the spinning wheel, heating it to make it pliable.
- Getting the right alloy Yamaha had to formulate a special type of aluminium alloy that would respond well to the new process.
- Stretch them out A roller moves on the inside of the rim, just like a potter’s hand forming a vase, to stretch and form it into the final shape. This process compresses the grain structure of the alloy to make it strong like a forged wheel.
- Final machining There’s a final machining process to complete the wheels (as there is for cast alloys as well) but it’s far less complex than on a forged wheel, helping make Spin Forged wheels cheaper to manufacture.