REAR-AXLE STEER­ING

In­tro­duced for the 991-gen­er­a­tion Turbo and GT cars, To­tal 911 ex­plains how rear-wheel steer­ing works and how it isn’t such a new tech­nol­ogy

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Porsche first ex­per­i­mented with pas­sive steer­ing of the rear wheels on the 928, de­vel­op­ing the clever, toe-com­pen­sat­ing ‘Weis­sach steer­ing’ sus­pen­sion setup. How­ever, it wasn’t un­til the re­lease of the 918 Spy­der that a full ac­tive rear-wheel steer­ing sys­tem made its way to a Zuf­fen­hausen sports car. Since then the sys­tem has also been rolled out on all but the base 911 Car­rera.

With its rear-en­gined lay­out the 911 has a ten­dency to un­der­steer due to a lack of weight over the front wheels. In or­der to rec­tify this on the 991 gen­er­a­tion, the wheel­base has been length­ened more than the body, help­ing to ef­fec­tively trans­fer more load to the front wheels com­pared to the 997. This, though, has brought its own prob­lem. The longer the wheel­base, the less re­spon­sive the car is when it comes to han­dling.

This is where the rear-axle steer­ing sys­tem comes in. At speeds below 31mph the rear wheels are steered up to 2.8 de­grees in the op­po­site direc­tion of the front wheels. This ef­fec­tively re­duces the car’s wheel­base, mak­ing it more ma­noeu­vrable at low speeds. As a com­par­i­son, 2.8 de­grees of turn on the front wheels would equate to 45 de­grees of steer­ing lock.

When the car is trav­el­ling over 50mph the rear-axle steer­ing turns the rear wheels up to 1.5 de­grees in the same direc­tion as the front. This ef­fec­tively length­ens the wheel­base, mak­ing the car more sta­ble in high-speed cor­ners.

It also en­ables the rear wheels to load up faster, im­prov­ing the 991

GT3 and Turbo’s abil­ity to change direc­tion. The whole sys­tem uses two electro­mechan­i­cal ac­tu­a­tors bolted on to ei­ther side of the chas­sis just fore of the top wish­bone. Steer­ing arms con­nect to the top of the rear up­rights. The ac­tu­a­tors are con­nected to the car’s ECU be­fore send­ing a sig­nal that causes elec­tri­cal mo­tors to ei­ther ‘push’ or ‘pull’ the steer­ing arms to cre­ate the re­quired an­gle and direc­tion of rear-wheel steer­ing.

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