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Rear axle steering. The future?

Rear axle steering. The future?

Rear axle steering. The future?

Hi :) I am new in the forum, I really enjoy reading so much knowledge engaged to all the topics in here.
I really admired the handling of the Porsche 918 Spyder, it kind of show us that the rear axle steering really works, but since I saw that technology in the new 911 Turbo S, I stared wondering if that technology will be in most cars in the future, what are the disadvantages of this system from the driving experience and from the lifetime of the components (tires, axle steering system, etc)? do we really need this technology in our everyday driving? since ever, most cars just come out of the factory understeering for security reasons, will this technology end up that by correcting any possible understeering or oversteering?

RE: Rear axle steering. The future?

It only "works" as a supplement to front wheel steering.

It's expensive, and that reason alone means it won't be in "most cars".

Given that "most cars" intended to be driven by "most people" don't have such a system and work well enough without it means you don't "need" it. The two cars that you mention are hardly indicative of "most cars". The 911 in particular needs help with stability due to the rear engine. "Most cars" don't have that situation.

Supplementary rear wheel steering systems have been tried long ago in more mass-market applications, and never caught on. Honda tried it in the Prelude in the 1990s. GM tried it in pickup trucks ("Quadrasteer") some years ago, and that's an appplication that one would think could really benefit (parking!) but people didn't buy it, so it ain't around any more.

RE: Rear axle steering. The future?

Renault reintroduced it in 2008, seems like a waste of $500 per car to me (rough estimate). I don't notice any great rush to copy them. With the exception of the parking thing it doesn't really solve issues that we can't solve with boring things like geometry and compliances and tire pressures and springs, bars and shocks. I can't say I've driven a 918, what did you like about the handling particularly?.


Greg Locock

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RE: Rear axle steering. The future?

Thanks for your knowledge and point of view of both BrianPetersen and GregLocock,I think both of the answers hit the point at the biggest disadvantages with is the cost for such a few benefits. thank you a lot. And answering your question GregLocock I have not driven either but particularly in the nurburgring lap record video you can easily see the way maybe because of the AWD and the rear axle steering the oversteering and understerring it is almost unseen. Link

RE: Rear axle steering. The future?

Back in 2013 Don Sherman described driving the 918 like this - "We found the 918 surprisingly easy to drive very quickly on the challenging Leipzig circuit. The car provides clear feedback through the electrically assisted quick-ratio steering, manageable understeer in the middle of sweeping bends, and a fast reaction to countersteering when the rear wheels drift wide in response to an early, enthusiastic jab of the throttle. "


RE: Rear axle steering. The future?

Rear steering (actually front steer with rear steer augmentation) has nothing to do with the understeer/oversteer characteristics of any vehicle. Rear steering does not and can not affect the max lateral g capability of a vehicle either. Rear steering ONLY affects the steering gain of the vehcle (gs per 100 deg at a designated speed is the common way to represent it. Since it's usually electronically conrolled, manufacturers can easily make the steering gain speed sensitive to 'quicken' a 'slow' car at low speed and to attenuate the steering gain at high speed. This is usually done for cars and trucks which inherently have low understeer to begin with, and are beasts to drive (and park) at low speed and are outright scary critters at high speed because of the quadradically increasing steering gain of the base car.

All this is easily verified by anyone with a car, test equipment and a place to do the ISO test manuevers. Of course, a proper simulation will tell you the same thing. The key element is that the rear steering is an input control parameter and NOT driven (forced) by lateral acceleration, as are all the other understeering/oversteering effects recognized in the vehicle's suspension and steering recipe.

Future vehicles with electrical overhead are likely to be shunning rear steer for all the previously mentioned reasons unless its mechanically architechted. Range anxiety.

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