Rear-wheel steering could open up design possibilities.
The general idea is to solve how to steer the rear wheel for improved handling and a tighter turning radius.
Patent applications tend to fall into one of two camps. On one side there are very detailed designs intended to solve a specific problem, and are often seen in production soon after they’re patented. On the other there are much broader brushstrokes aimed at providing manufacturers with ownership of a whole range of ideas—the latest from BMW falls very much into that second category with the hope of giving the company intellectual property rights over a whole slew of designs related to rear-wheel steering on motorcycles.
Over three separate patent applications, BMW has illustrated dozens of potential solutions to the problems posed by supplying both drive and steering to a motorcycle’s rear wheel. Although only illustrated in simplistic, outlined form, they include both electric and internal-combustion-engine bikes, with belt, chain, or shaft drive to the rear wheel and a vast array of systems to turn the rear wheel in the opposite direction to the front. A common idea runs through all the designs: They’re intended to create a greater steering effect than you’d get from turning the front wheel alone, improving both the motorcycle’s turning radius and its handling responsiveness.
It’s no accident that the template that all these designs are illustrated on is a stereotyped chopper-style custom bike, with stretched-out forks, a long wheelbase, and pulled-back bars. On any conventional bike, all those elements work against the usual requirements for lithe handling. The shallow rake means bar movements tilt the front wheel more than they turn it. The pulled-back bars make it hard to add more steering angle. The long wheelbase means you need more steering input to change direction. It’s a recipe for a motorcycle that’s about as maneuverable as an oil tanker.
BMW has used the shotgun method to get patent ideas registered.
That’s precisely where rear-wheel steering steps in to help, allowing tighter turns and more instant response without reducing the wheelbase, but in the past the complexity of turning a bike’s driven wheel means the idea hasn’t been worth pursuing.
BMW might have come up with dozens of different methods for steering the rear wheel in its new patent applications, but all of them maintain the same geometry. The rear wheel gets its own steering axis that is the same across all the illustrations, so the impact on handling should be the same. The steering solutions include electric and hydraulic ones as well as mechanical designs using a variety of shafts, belts, or chains to transfer steering inputs to the rear wheel. The different layouts mean almost any powertrain arrangement could be accommodated, but BMW’s relatively recent reentry into the cruiser market with the vast R 18 models is surely a driving force behind the idea.
In the main patent application, BMW’s designers write: “The ability to steer previously non-steered wheels in addition to one or more wheels that were already steerable would reduce the turning circle and the associated disadvantages or, with the turning circle remaining the same, allow the wheelbase to be extended so that more installation space, more comfortable high dynamics, and more lateral force reserves when cornering can be achieved. In addition, this reduces the necessary wheel steering angle on the front wheel or the wheel that has been steered conventionally up to now.”
They go on to explain that the advantages of the system could apply to high-performance bikes as well as cruisers, though, with aerodynamics in particular standing to benefit. The rear wheel’s ability to turn means the front wheel doesn’t need to be able to turn as far, opening new avenues for bodywork design. The patent says: “Motivations can be to realize a housing of the front wheel, in particular due to the lower wheel steering angle, in order to thereby create particularly advantageous aerodynamics.” Less important for road riders but mentioned in the patent nonetheless and perhaps significant for racing, is the fact that the rear wheel’s ability to turn provides a new element of control when the front wheel is in the air, with the document saying: “Rear wheel steering when riding on the rear wheel alone, with the front wheel off the road but raised in the air, would provide some or improved steerability in that riding condition.”
Rear-wheel steering isn’t a new idea. Car companies jumped on the idea back in the 1980s, bringing dozens of four-wheel-steer machines onto the market. It lives on today, with passive four-wheel-steering designed into the rear suspension of many cars and active rear-wheel-steering making something of a comeback, not least with the Hummer EV truck and SUV that even have a Crabwalk mode. It’s something that’s been attempted on bikes in the past too. Australian engineer Ian Drysdale created a fascinating two-wheel-drive, two-wheel-steer prototype back in 1990—the Dryvtech 2x2x2—using hydraulics to both drive and steer both wheels, and even before that, in the 1980s, Honda patented several ideas for rear-wheel-steered motorcycles.
Given the wide scope of the new patent applications and the fact that BMW has yet to focus in on any one design, we’re unlikely to see a two-wheel-steered bike on the market anytime soon. However, it’s clearly a topic that the company has decided it’s worth investing hard-earned R&D dollars into, with the hope it might provide an edge over the competition sometime in the future.
Alternative variations to how the rear wheel could be steered.
Alternative variations to how the rear wheel could be steered.
Alternative variations to how the rear wheel could be steered.
Alternative variations to how the rear wheel could be steered.
Keyword: BMW Motorrad Researching Two-wheel Steering