911 Torsional rigidity

[m110]

The latest Excellence quotes torsional rigidity of the new 918 at 40,000 Nm/degree.

I was wondering what our cars are. A cursory internet search gives us a list here:

beautifully engineered • Torsional Rigidity: A Compiled List of Known Specs

The Porsches:

Porsche 911 Turbo (2000) 13,500 Nm/deg
Porsche 959 12,900 Nm/deg
Porsche Carrera GT - 26,000Nm/degree
Porsche 911 Turbo 996: 27,000 Nm/deg
Porsche 911 Turbo 996 Convertible: 11,600 Nm/deg
Porsche 911 Carrera Type 997: 33,000 Nm/deg

Some other data points:
Lamborghini Countach 2,600 Nm/deg
Mazda RX8 - 30,000 Nm/degree

A terrific old thread:

Torsional Stiffness of 1988 911 coupe

In that thread Buergermeister calculates 5300 ftlb/deg or 7185 Nm/deg for a 1988 (and Mr Verburg shows use where the 964 has extra strong sheetmetal).

The question of how this makes a car handle or ride is certainly a whole can of worms.

[proffighter]

As you already named, rigidity does not make a ride better in general, could be even worse.

Just look at MotoGP, how they are testing frames, forks etc... softer, more flex there, less here etc... At the end, science does not help much and you have to sample expiriences with different parts.

[javadog]

Originally Posted by proffighter

As you already named, rigidity does not make a ride better in general, could be even worse.

Just look at MotoGP, how they are testing frames, forks etc... softer, more flex there, less here etc... At the end, science does not help much and you have to sample expiriences with different parts.

Actually, it does make it better in a car. A stiffer frame allows for a softer suspension, if desired. Motorcycles are completely different. The problem there is that a MotoGP bike corners at a bank angle of 60 degrees, so bump absorption is more from lateral flex than normal suspension deflection. Cars don't have this problem.

JR

[proffighter]

Maybe I should say it different. A stiffer chassis is of course better in general, but there are some reasons (not for a formula 1 car maybe) to not have an ultra stiff chassis.

Grip limit range is much smaller and therefore less controllable by driver if too stiff. I know some of the DTM cars removed a couple of tubes of the rollcage to get more flex, wich made the car much more driveable. Albert Motorsport for example leaves the rear inner banana bearing of their 934 stock for better control.

Tyre wear is also increased due stiffer chassis, but thats maybe not that important...

[Driven97]

Maybe I'm just dense but I can't think of situation in which undamped chassis flex would be more beneficial than having everything controlled by the suspension, except maybe an articulated chassis off road hauler or something.

Could you explain why a more rigid body leads to more tire wear?

[proffighter]

Well the thing with the tyre wear I've heard from some Kart guys and read in a motorsport magazine about when rally cars moved on track... (and from MotoGP as well) Ok, not bullet-proofed I have to say.

Concerning stiffening I read in a book about Porsche sportscars, this one:



Here is something I found in English:

Chassis Tech - Rigidity & Flex - Suspension Setup - Circle Track

This is pretty much what I mean. I do not wanted to say you should not make your chassis stiffer, I only wanted to say, that maximum stiffness has some downsides, which are not worth to reach regarding driveability. I also not meant torsional stiffness (what you talked about), but stiffness overall on a chassis.


This is of course not reached, when re-inforce shock towers, suspension mounting points etc...

[rusnak]

I always wondered why Porsche didn't install anti roll bars on the 914-6. I guess it's because it can't really use them.

[Captain Ahab Jr]

On a race car stiffness is everything until parts start getting too heavy.

[Tilikum Turbo]

Originally Posted by proffighter

As you already named, rigidity does not make a ride better in general, could be even worse.

Just look at MotoGP, how they are testing frames, forks etc... softer, more flex there, less here etc... At the end, science does not help much and you have to sample expiriences with different parts.

Doing real research, real engineers don't 'guess' like layman which direction to go(as in trial and error).

Example is Thomas Edison(brilliant inventor, but bad engineer). In making a light bulb, he tried running an electrical current thru ANYTHING(such as cow dung, or knitting thread, etc until he decided to try a wire). Hence his quote of 90% sweat, 10% inspiration.

[Flieger]

Originally Posted by Driven97

Maybe I'm just dense but I can't think of situation in which undamped chassis flex would be more beneficial than having everything controlled by the suspension, except maybe an articulated chassis off road hauler or something.

Could you explain why a more rigid body leads to more tire wear?

Originally Posted by Captain Ahab Jr

On a race car stiffness is everything until parts start getting too heavy.

This.

Motorcycles (when leaned way over) and karts have 2 things in common: their frames are their suspension. They do that because it would be too heavy on the kart (it would be really stiff anyway on the kart) and too complicated/heavy on the bike. They still need some extra bump compliance, however.

The reason race cars went from steel ladder frames to steel tube frames to aluminum monocoques to carbon fiber tubs is less about weight and all about stiffness. There will be a point of diminishing returns where you are just adding weight for no appreciable difference in handling, but you want your suspension to be the thing controlling the loads on the tires. That is why you spend all the money on fancy dampers.

Similarly with low-profile tires.