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WHEELS TILTING INTO THE TURN
I've been interested in
hearing about suspension systems that allow the wheels lean into a corner much
like a motorcycle. I imagine this would give a 4-wheel vehicle superior grip
compared to a conventional suspension, although I never really seen it
put in practice. And my question is: Has any prod. car/ racing manufacturer or
person done a car that does that?
I do have one article clipping from Popular Science in the mid-90's mentioning about Michelin designing a system with an "unusual linkage" that by, "...using a separate cradle to support the
suspension arms, which
allows up/down movement of the wheels. The cradle -which is the unusual
part, is attached to the chassis w/ 4 vertical links, enabling to tilt to
the inside or the turn [while the car is leaning outside]." This is all
the info I got on it.
Plus this guy: http://www.4-mc.co.uk/
Any insight would be.... well insightful, I guess.
In general, yes, it is desirable to have the wheels tilt into the turn, at least a little. In oval track racing, where the car only has to turn left, it is normal to set static camber so the wheels have some tilt into the turn when cornering.
With car tires, there is a limit to how much inward tilt is helpful. Car tires are designed to run close to vertical. Their relatively flat treads get up on edge if they are tilted too far, and grip is hurt instead of enhanced. Short of that point, there is a range where the tire performs well, especially if inflation pressure is reduced a bit, but will wear poorly or blister.
Motorcycle tires have a rounded profile and are designed to lean a great deal: up to 50 degrees or perhaps even more. The contact patch can't be nearly as wide for a given tire width and vertical spring rate when the tire is shaped like that, but motorcycles manage to corner very well anyway.
Tire experts tell us that a tire running at a camber angle actually develops a lateral force when running straight, with no slip angle. This is called camber thrust. It is generally considered to be a more efficient way of generating cornering force than slip angle is. That is, it wears and heats the tire less, and consumes less power.
If a way could be found to make car tires lean into the turns, this would have implications for tire design, and vice versa. Tires would be developed that liked more inclination, and suspensions would be designed to suit. If no other factors limited the progression, there would be a process of evolutionary leapfrog, where the suspension designers and the tire designers would be taking turns catching up with each other. At some point, packaging would set the limit. The tires would get so wide that it would be hard to find room for them.
However, using passive suspension, this is not so easily done on a car. The problem has always been to get more than 100% camber recovery without excessive camber change in suspension roll motion (oppositional motion of right and left wheels) due to bumps. Merely providing a separate linkage to handle roll does not solve this.
There has recently been an innovation that offers some promise. It's called the Sacli suspension, after its inventor, who tells me they are going public with the invention and will have a booth at the PRI show this year. Even this system doesn't exactly discriminate between suspension roll due to cornering and due to bumps, but it does respond differently to high and low velocity suspension roll motion, and it is at least broadly true that motion due to road irregularities tends to be fast, and movement due to cornering tends to be relatively slow.
The Sacli system uses two suspension systems in series – an inboard system and an outboard system. The outboard system has geometry providing little camber change in ride, and correspondingly little camber recovery in roll. It is sprung and damped similarly to a conventional system.
Instead of being anchored to the sprung structure at its inboard pivots, the outboard suspension anchors to a second set of uprights, which are connected to the sprung structure through the inboard suspension system. The inboard suspension is rigid in ride, and can only move in roll. It has geometry that provides more than 100% camber recovery in roll.
Since both inboard and outboard suspensions can move in roll, roll motion is handled by a combination of deflections of both systems. This would appear at first glance to offer no escape from the usual dilemma of having to give up camber control on bumps to get camber control in cornering. However, the inboard suspension has very stiff high-speed damping in roll. Consequently, abrupt oppositional motion is handled with a greater amount of outboard suspension deflection, and slow oppositional motion is handled with a comparatively greater inboard suspension deflection.
The result is not perfect discrimination between bumps and cornering, but perhaps a better blend of properties than otherwise possible.
One interesting effect would be that when the car receives a very jerky steering input, it will initially roll with little camber recovery on the outboard suspension, and then gradually roll further on the inboard suspension, with improvement to camber. Reducing the inboard suspension's high-speed damping offers a reduction of this effect, at the expense of reducing the system's ability to somewhat discriminate between bumps and cornering roll. Smooth driving, or a course that doesn't require abrupt yaw accelerations, would allow the system to show at its best.
The system has so far been tried on Formula SAE cars, and so far the inventors have settled for zero camber change in steady-state cornering, but, at perhaps some penalty in some other aspects of car behavior, it appears to be possible to get more than 100% camber recovery in steady-state cornering.
There are packaging and complexity tradeoffs, of course. At the front of the car, it gets interesting to achieve zero bump steer, and still not run any rod ends or sphericals out of travel.
Sacli Suspension does not have a website at this writing. The patent can be viewed at http://www.patents.com/Sacli-Suspension-LLC/Chatsworth/CA/1571266/company/ . The inventor describes the system as a two-degree-of-freedom system. I think I'd call it a compound suspension system instead, because actually a conventional suspension has two degrees of freedom as the expression is usually understood.
Anyway, outside of this concept, the only way to make the wheels lean into both right and left turns, without having excessive camber change over some types of road irregularities, is to make the whole vehicle lean into the turn, like a motorcycle. This requires either that the suspension incorporate computerized active roll control, or that the vehicle be fairly small and light and have the rider seated astride it, so the rider can lean it like a motorcycle using body English. The suspension is then made very soft in roll. This is the approach chosen by the UK-based Four Wheeled Motorcycle Company Ltd. linked by the questioner. Their vehicle really is just what the name suggests, and it runs on motorcycle tires.
I have thought about using a similar concept for a human-powered four-wheeled vehicle, able to operate on snowy or icy roads where a bicycle is incapable of staying upright, yet narrow enough to permit same-lane passing by motorists.
With an engine, such a concept can work mechanically. I'm not sure if it would be street legal in the UK, but in the US it would not. In the US, it would legally be what we call an ATV (all-terrain vehicle), or colloquially, a four-wheeler. These are very popular for off-road use, but they can't be licensed for the street. Under US law, any road vehicle with four wheels and an engine is considered a car. As such, it is subject to automobile emission and crash safety standards, which a light four-wheeler ridden like a bike cannot possibly meet.
Even four-wheeled human-powered vehicles can be legally problematic in the US. In some states, riding such a device will get you ticketed for operating a toy on the roadway.
A three-wheeled motor vehicle, however, would be legal as a motorcycle, and that might offer some possibilities.
Frankly, I'm surprised that no major manufacturer of ATV's has tried making a narrow, roll-soft one that can be leaned like a motorcycle. The concept would seem to have promise. It would be particularly advantageous for narrow trails.
One problem with the idea is what to do for a differential. If you go to the Four Wheel Motorcycle site, they have a video of their vehicle on a wet, oily skidpad, being powerslid by the rider. No mention is made of what happens when traction is good. With no differential, a four-wheeler has a locked-axle push on a grippy surface. Conventional ATV's deal with this by having a rear suspension with near-zero compliance in roll, allowing the inside rear wheel to unload, and often lift, when cornering. If both ends of the vehicle are roll-soft, that strategy doesn't work.
For human power, my thought was to use dual freewheels, and just let the outside rear wheel coast in the turns. This will also work for a motorized vehicle, as long as it's small and light enough to do without a reverse gear. There would be no engine braking, however.
Alternatively, viscous diffs or clutch-pack ones could be used, or an adaptation of the Detroit locker. There would, of course, be a cost and complexity penalty compared to a spool.