Tale of three RSR struts

[66S]

I have three RSR struts and have noticed each has the steering arm at a different angle. This certainly makes replacing one a little complicated.

[smedley]

It also looks as if the spindles are raised at different heights.....the strut with the most radically bent arm has the greatest spindle distance from the bottom of the strut....or is it an optical thing from the photo?

[Flieger]

If you raise the spindle to get a lower ride height while maintaining the original suspension geometry, then you need to adjust the steering arm height back down to match where it used to be since the rack hasn't moved. There are several ways to do this- one being to heat and bend the steering arm as appears to have been done here.

[gled49]

RSR struts generally only raise the spindle 11mm, and now no adjustments to the steering arm are made. I think in the old days the factory did bend the steering arms. Here is a pix of 935 struts I bought new, the spindles are raised 20mm the ball joint mount is moved off the center of the shock insert and the steering arms were bent way down from the factory. These carried Weisach labels instead of Bilstein labels. Gordon

[66S]

All three are at the same height, I hadnt noticed the offset ball joint mounts on the green ones, must be from Weisach, got them for under $200 a few months ago, will go on my track car. Thanks for the ID.

[chris_seven]

I believe that the 935 Arm was bent to introduce Ackermann Effect to the steering geometry which wasn't a feature of early 911 which needed camber change and some degree of bump steer to aid 'turn-in'.

[gled49]

The lowered steering arms on my RSR and 935 struts accomplish bump steer correction in conjunction with rack spacers, something rack spacers couldn't accomplish alone. Gordon

[Flieger]

I believe that the 935 Arm was bent to introduce Ackermann Effect to the steering geometry which wasn't a feature of early 911 which needed camber change and some degree of bump steer to aid 'turn-in'.

Hard to tell from that photo as the wheel is turned, but I would think that the 935s, being built for high-speed "power" circuits like Le Mans where designs are optimised for high speed, large radius turns, would not need positive ackermann. In fact, I would think they would have a bit of reverse ackermann which is common on such types of cars. Tires reach their maximum lateral force at a lower slip angle when they have less vertical load on them. This means that with the lateral load transfer during a turn the inside tire should be turned less than the outside. The cost of this is lots of scrubbing in tight radius turns of course.

If you meant reverse ackermann instead of positive ackermann then sorry for my mistake.

[gled49]

More pix. RSR struts, I lowered the steering arms, copying fact struts. The 935 struts again, you can see offset in ball joint, changes scrub angle I think. Also no taper in the tie rod hole

[chris_seven]

Max,

I think it is always best to use the term Ackermann in a generic manner. Positive and Negative Ackermann can then be discussed with reference to parallel or Zero Ackermann geometry.

The early 911s uses a combination of Bump Steer, Camber change and Caster to achieve their turn in behaviour and the detail of these settings has a significant impact on the behaviour of the chassis.

If we then consider the impact of the ‘spool’ that we fitted to the RSRs used at Lemans the detailed geometry used on these cars is unlikely to translate into the most suitable set up for a road car.

Tyre slip angles are also a major variable and do need to be considered with road car tyres being significantly different to race tyres.

We also seem to have become obsessed with the elimination of bump steer and consider this aspect of our car’s steering geometry in splendid isolation.

I do believe that the picture is quite complex and needs a better understanding.

I have no idea of how a 935 used Ackermann positive or negative and there seem to be as many theories as there are opinions, many of which appear contradictory.

The vast majority of F1 cars now employ Positive Ackermann and the reason this is used is due to the extremely stiff suspension demanded by the aerodynamics influencing inner wheel behaviour.

I think to assume positive Ackermann is never a benefit on high speed circuits could be argued in the same way that negative Ackermann may not always be good for Autocross.

The link below may be useful.

http://www.me.ua.edu/me364/PDF/Steering_Ackerman.pdf