Raised Strut Spindle vs ERP Bump Steer Kit, same cost which is more useful? Options

[kdfoust]

So I'm already hemoraging money all over the suspension bits of my car. Why stop the bleeding so soon when there are so many parts to buy... (IMG:style_emoticons/default/happy11.gif)

When the car goes back together I'm lowering it as far as practical for an AX/street ride (my driveway may be the ultimate constraint). I've already experienced the near complete elimination of bump steer by installing washers under the steering rack so I never want to put up with bump steer again. So I'm looking at my options and see two. The first is to install a ERP bump steer kit (hit page down a couple of times to find it) so I can dial out the bump steer no matter where the ride height is set. The second option is to go to the Tangerine racing raised strut spindleswhich appear, looking at the digipic, to have the control arm bent to a corrected position to eliminate bump steer while resetting the ride height via the relocated spindle. I suppose the raised spindle also has the advantage of not using any of your suspension travel to achieve the lower chassis height.

Whata ya think? The price is a wash. Anybody done both or one or the other to provide and comparison/comments? Oh this all assumes that the dreaded bump steer will show up with a vengance as I crank the car lower...

Thanks,
Kevin

[TMorr]

Kevin,

Without having the specific RC heights and geometry available, I will generalize, perhaps someone on the forum has plotted the 914 geomtery and has some definitive numbers?, with change due to ride height, roll etc. If so, they would be good to pad out this thread with data - assuming the data is correct.

The pivot axis of the front and rear suspension designs are perpendicular to each other, this has a significant effect on what happens to the RC height as the car is lowered or goes into bump travel.

The front suspension, with pivot axis parallel to the car centerline, will have a more radical vertical migration of the RC height as the suspension goes through bump and droop.
The migration of the RC height will be more than the ride height change - by some significant factor. This means that in bump (same as lowering) the distance between the height of the Center of Gravity (CofG) and the front RC will increase, making the kinetic roll stiffness softer.

The rear suspension pivot axis is roughly perpendiular to the car centerline, this means that in bump and droop, the vertical migration of the rear RC is pretty equal to the change in rear ride height. This means that the distance between the CofG and the rear RC is fairly constant and kinetic rear roll stiffness is relatively constant too.

If I had to speculate, based on the assumption that Porsche Engineers did a good job of delivering a balanced car in the stock 914 - regardless of skinny, hard tires - I would say that lowering a 914 all round, with no spring changes, would lead to a fundamentally oversteering (not necessarily evil) car. This ignores the lack of suspension travel issue and many other issues, but this explanation is centered on RC effects.

The oversteer would be a function of the front roll stiffness becoming progressively softer, therefore a split of roll stiffness more to the rear and a more oversteer prone basic package in a steady state condition.

Given that the kinetic roll stiffness acts in conjunction with the roll stiffness that comes from wheel rates and ARB rates, it's a complex closed loop, as I am sure any books you are reading will illustrate.

The raised spindles are really an effort to restore the FLWB (front lower wish bone) angle to replicate a higher ride height when the chassis is lower. It has multiple advantages, including restoring some camber and camber gain (actually reduces camber loss). The RC is raised too and this is one of those invisible factors that is difficult to visualize, whereas the camber gain etc, can be witnessed by simple measurements.

I think there is sufficient empirical evidence and experince available to discover how to set up a lowered 914. I can be pretty sure the lower the 914 the more front wheel rate is desirable, primarily for roll control. If you raise the front spindles, you probably find - at equal ride heights, the front ARB and springs do not need to be as stiff (to compensate for low front RC), overall grip should be higher.

Hope this helps in a practical sense?

Regards

Hayden PTBT