911 torsion bar vs. 964 struts?

[sugarwood]

I don't know much about suspensions, but would like to learn some basics.

I saw the vintage Porsche ad someone posted a few weeks back that stated torsion bars took up less space and weighed less.

What is the story behind Porsche ditching the torsion bar for the 964 and going with the more common McPherson strut/spring combination?

What are the pros and cons of each setup ?

[wayner]

My limited understanding is this (and I am going to over simplify)

A coil spring is simply a torsion bar coiled up.

A torsion bar was better than the leaf spring that was common on competitors cars back in the 50s. Porsches of that era used torsion bars, and they continued to with the new car they launched in the 60s (901/911).

Torsion bars (like flat engines) keep the weight low in the car for superb handling. The Porsche Pensky spiders used torsion bars in ALMS a few years back from what I understand.

Torsion bars are good, but at certain spring rates, the bar has to be so darned big that packaging within the confined of a vehicle becomes difficult. Coil springs were an evolution of torsion bars for ease of packaging (like stereo speakers that use a folded horn design instead of a big long horn).

Shocks with coil over springs were used on the 1973 2.8 RSR in order to increase spring rates beyond what was practical with the torsion bar setup. Meanwhile the street cars continued with torsion bars since spring weight was adequate for a street car. Later street designs like the 964 incorporated coil springs when the suspension was redesigned

A secondary evolution of the coil spring includes the ability to vary the spring rate with tighter or looser windings. varying the windings within the same spring make for a progressive spring that is more compliant at low loads but stiff at high loads.

Cons?
Proponents of torsion bars assert that coil springs raise the weight distribution point in the chassis to the top of the spring rather than down at the floor with torsion bars.

Back to the pros
A third evolution is not simply using a coil spring, but a coils in conjunction with a shock with advanced dampening that varies depending on the forces on the shock. A modern coil over suspension system used this and can include separate adjustable rebound and compression dampening. One method used in this sort of shock design is that instead of using a dampening rod with fixed size holes for the fluid to pass through to provide dampening, some use essentially a stack of flexible washers that deflect under higher loads allowing more fluid to pass (sharp bump) version remaining tighter under other conditions (slow rolling hill or dip). Some use a magnetically energized fluid that becomes thicker when the road sensors tell it to.

All of the improvements in easier packaging allows the engineers to redesign the suspension arms so that the geometry of the wheel remains more constant rather than travelling through an arc, thus maintaining constant camber versus constantly changing camber.

That is my oversimplified explanation

[Flieger]

911s simply don't have big enough splines for large enough torsion bars, and the A-arms are too flexible themselves to reach the spring rates that coils can since they are acting almost directly on the wheels.

Most F1 cars and most top LMP cars use torsion bars for packaging reasons mainly. When you combine them with a rocker-pushrod system they can reach the required spring rates.

Coils can do progressive rates which can be nice in some cases but for racing a stiff linear rate is usually used.

[Neilnaz]

Isn't it true that a torsion bar cannot be linear as the torque is constant but the mechanics result in a change in effective spring rate through the travel of the suspension? Therefore, if carefully designed, a torsion bar can make use of the non-linear forces required to compress the suspension whereas a standard spring must be designed progressively or use some revised geometry to gain the same effect.

Sorry rambling.....

Cheers, Neil

[Flieger]

Originally Posted by Neilnaz

Isn't it true that a torsion bar cannot be linear as the torque is constant but the mechanics result in a change in effective spring rate through the travel of the suspension? Therefore, if carefully designed, a torsion bar can make use of the non-linear forces required to compress the suspension whereas a standard spring must be designed progressively or use some revised geometry to gain the same effect.

Sorry rambling.....

Cheers, Neil

Well you didn't state it exactly correctly but I know what you meant. In almost every suspension there will be a change in motion ratio throughout the travel. The change in motion ratio will change the effective spring rate. But for sports cars the suspension travel we look at is rather small such that the motion ratio doesn't change much. On an offroad car it would be a bigger deal.

If you had a rocker arm and pushrod system like on an F1 car you would be able to make more use of the change in motion ratio by having a very short rocker.

But generally you want the motion ratio to change very little.

[winders]

Originally Posted by wayner

Back to the pros
A third evolution is not simply using a coil spring, but a coils in conjunction with a shock with advanced dampening that varies depending on the forces on the shock. A modern coil over suspension system used this and can include separate adjustable rebound and compression dampening. One method used in this sort of shock design is that instead of using a dampening rod with fixed size holes for the fluid to pass through to provide dampening, some use essentially a stack of flexible washers that deflect under higher loads allowing more fluid to pass (sharp bump) version remaining tighter under other conditions (slow rolling hill or dip). Some use a magnetically energized fluid that becomes thicker when the road sensors tell it to.

This all has nothing to do with the spring type so these "pros" are not pros for this discussion. You can get the same shock (damper) package with torsion bars or coil springs.

[wayner]

You win

Wayner out...

[winders]

Originally Posted by wayner

You win

Wayner out...

It's not about winning...it's about the facts. You can put the same style damper package on a torsion bar car as you can on a coilover car.