I don't understand the 911 torsion bars
 

Being from the 944 world I am familiar with torsion bars.

However, I've been looking at 911's and have some questions about the torsion bar setup.

In the 944 world it's best to set up your front springs and rear torsion bars to a close to equal rate for a good front and rear balance...since the car is 50/50 weight ratio (yes this is over simplified).

What I notice in 911's is that they all have 19mm front torsion bars (regardless of model) and the rear bars start at 24mm (I think) and go up to 26mm for turbo models. But the fronts always stay the same.

My question is...with all the oversteeriness (not a word) already in the 911, why would you have such a difference in the rear bars. Wouldn't just increasing the spring rate in the rear add to the problem?

The diameter of the bar is not always directly related to stiffness / spring rate

The 911 has far more weight on the rear than the front.

Very different car than the 944. I have one of each.

Matt,

In general higher spring rates go where the mass concentration is greatest. That is true for stock 924s/944s/968s as well.

Front engine == stiffer front springs.

Rear engine == stiffer rearsprings.

The "oversteeriness" (I like that word) is because of the weight of the engine way back behind the rear axle.

The 944 is an easy car to drive quickly. The 911 has some quirks that, once used to it's advantage, makes the it an incredibly capable car. Difficult for the uninitiated (some say "evil"), but pretty much unstoppable in the hands of an experienced driver.

Throttle on, the 911 understeers. Throttle off, you get oversteeriness.

With larger displacement engines and crap like cruise control and air conditioning, the rear of 911s got progressively heavier than the front. Porsche felt the need to increase the torsion bar diamter to compensate.

And, FWIW, the diameter of the bar is *always* directly related to the spring rate. Diameter, length and modulus of rigidity are the three factors determining rate.

Mike

To add to the good observations above; don't confuse spring weight and "wheel rate". The wheel rate is a function of the spring weight and the suspension geometry, it factors the length of the lever arm acting on the spring.

I guess it confuses me because by increasing the rear spring rate you increase oversteer. I would think you want stiffer front springs and thicker sway bars in the front to counter the engine weight being in the rear.

Also, I saw a post that had a link to a rennlist page that showed the spring rates. It looked like the 911 spring rates were around 120#. That seems very low...the stock 944's came with 123# springs front and rear. I would have expected the spring rate to be higher since everyone says it's a harsher ride?

I think what asphaltgambler is referring to is comparisons between solid/hollow bars. That does make a difference.

Tim K

Chalk and cheese. The quoted 944 springs rate is a "spring rate". The 911 torsion bar equivelant, properly converted, is a "wheel rate".

The two can be properly compared, but your data isn't doing that.

Not necessarily. In general, less stiff springs will give more grip, but only within limits. If the springs are not heavy enough for the job, then excessive suspension deflection will limit grip to an even greater degree.

ianc

But... P AG did adjust the relative sizes/rates of teh F & R torsion bars over time. I've always assumed (w/o checking) that it was to get more understeer. The rear wt. didn't go up all that much rel. to the F.

Matt, if you are asking why the bars are not the same size, it should be obvious that the rear weight bias necessitates a larger rear bar to get to neutral handling. Once at neutral, you are correct, that increasing the bar size in the rear theoretically increases the tendency to oversteer.
David

Yes...that does make sense now.