Front bushing deflection, neg camber

[911st]

Check out this cool digram that shows what happens to the front strut bushings and tire in a corner.

http://www.pelicanparts.com/catalog/images/ermp/frontcomponentscamberanimated.gif

[burgermeister]

What the diagram doesn't tell is that the absolute maximum deflection possible at the lower bushing is around 3.9mm (that would bottom out the torsion bar against the A-arm tube). Actual deflection must be less since it is possible to run 20 or 21mm front bars with factory rubber bushings. (36mm OD, 4.6mm wall thickness, 19mm torsion bar = 3.9mm free space per side)

And 4mm deflection at the lower bushing over a 560-ish mm long strut gives a camber loss of 0.4 deg. Actual loss is more likely to be in the 0.2 to 0.3 deg range ... add to that whatever the top strut bushing does (another 0.8 mm or so according to some coarse measurements), and you get 0.3 to 0.4 degrees.

This does suggest if you are very concerned about camber the A-arm bushings are a good place to pick up 0.3 degrees ...

[Zeke]

Well said. Rubber bushings have their drawbacks, but huge deflection is not probably at the top of the list.

[surflvr911sc]

That's from Elephant Racing. Chuck has lots of cool diagrams, tech articles, etc.

http://www.elephantracing.com/

[Chuck Moreland]

I did create that animation. The motion is exaggerated to make clear the behavior.

Not illustrated but also important is the toe change that happens due to bushing deformation. The control arm moves with bushing deformation, but the tie rod does not. When cornering, the loaded wheel wants to toe in. When braking, both wheels want to toe in.

A similar change happens to caster when cornering and braking.

The point of eliminating the compressible rubber is less about achieving a maximal camber value, and more about keeping the toe, camber, and caster from wandering.

Wandering alignment settings make the car a handful to drive as the driver has to constantly correct for the changes.

Naturally this is more important in cars that corner and brake hard because the loads presented to the bushings are greater, and the need for predictable/ precise handling behavior is similarly greater.

For top performance you want to eliminate the rubber and resulting imprecision. Rubber is still a fine bushing for regular street driving. And porsche did an excellent job controlling the compressibility of the rubber, delivering impressive performance for a production car.

On the control arms, this was accomplished by tightly compressing the bushing into the mount such that it is pre-loaded. We've done testing to measure the deflection and deformation under load and find that OEM spec bushings in good condition perform very well.

[RWebb]

so, if a rubber bushing is used, then it needs to be under compression to get the best handling?

[burgermeister]

Originally Posted by Chuck Moreland

Not illustrated but also important is the toe change that happens due to bushing deformation. The control arm moves with bushing deformation, but the tie rod does not. When cornering, the loaded wheel wants to toe in.

But the ride steer curve (steer caused by vertical suspension movement) wants to toe the loaded wheel out. I suspect the ride steer curve wins (or at least ends up even) - toe in during cornering is an oversteer effect, and I can't imagine Porsche working to add oversteer to the 911.

That would explain the substantial ride steer in the 911 front suspension.

Of course, raising the rack or significantly stiffening the front suspension would lower the amount of ride steer, and then the bushing compliance steer might win.

[911st]

Originally Posted by Chuck Moreland

We've done testing to measure the deflection and deformation under load and find that OEM spec bushings in good condition perform very well.

Can you give us any numbers on how much deflection w OEM's allow and or how much neg camber is lost?

We are assuming that it is about .5 deg, maybe including strut tower flex.

Thank you!

[safe]

Originally Posted by 911st

Can you give us any numbers on how much deflection w OEM's allow and or how much neg camber is lost?

We are assuming that it is about .5 deg, maybe including strut tower flex.

Thank you!

Have you seen Chucks video?
Rubber bushing installation video

[Chuck Moreland]

I'm in the movies. You should see my Christian Bale imitation, off camera

[RWebb]

I'm curious about how the absolute levels of deflection translate into subjective feel for the driver....

After all, even solid metal will deflect some tiny amount from a force. I assume the polybronze deflects a tad more than steel...

[Bill Verburg]

Chuck, Have you got a Shore hardeness on your buses vs stock?

when are you going to have some stiffer than stock for 964/993?? There is a market.

[burgermeister]

Thanks for posting the deflection test, Chuck!

Also about as good a commercial as I've seen for a product. I'm sold when I need to replace my stock pieces - and thanks for making factory rubber available as an option.

0.6mm deflection is pretty darn good.

[911st]

I think 1 deg of camber is about .3" and one half about .15".

Per Chuck's video a stock equivalent rubber lower bushing moves about .025".

If we have been assuming there is .5 deg loss from top bushing, bottom bushing, and tower deflection.

Dose this still seem a reasonable assumption?

[burgermeister]

I'd say more like 0.15 to 0.2 deg (for the bushing and strut tower together) based on Chuck's measurement.

[Chuck Moreland]

Bill, we already have replacement control arm bushings for the 993/964 in both stock (60A)and sport (85A)hardness.

We don't have them on the website yet, but have had them for awhile.