15 replies to this topic

[mariner]

I'm not sure how accurate this list is but it's nice to see somebody putting some technical stuff on a car website

 

http://youwheel.com/...rehensive-list/

 

I hadn't seen the site before but it seems good. I found it via a piece in Autocar on-line which asked an interesting question “how much torsional rigidity does a car lose over time?"

 

I don't know the answer but I do remember a rather scary demo from the 1970's which showed how much crash worthiness cars lost with rust - in those rust prone days a LOT .

 

I imagine rust would reduce rigidity a lot but modern cars rust less so do the spot welds go etc?

 

The older aluminium racing monocoques definitely get weaker as the rivet holes fret but I have no idea if carbon fibre tubs weaken with age.

 

Also the bushes definitely go so everything gets soggy.

 

The point the article was making BTW is that you can't judge an old cars ride/handing versus a modern one properly unless the bushes etc are replaced. I seem to recall somebody took the original XJ6 Jaguar and fixed the wear out bits in the suspension etc. and the ride was up to modern standards at least

 

 

There is a very informative link in teh comprehensive list for the  recent Mondeo

 

http://blogs.youwhee...ion_pp.1-30.pdf

 

Over to you, Greg!!

Edited by mariner, 28 August 2016 - 09:17.

[Bloggsworth]

How much does it lose when you take the windscreen out? Wasn't the Austin 1800 phenomenally rigid?

[Greg Locock]

That's a nice list. Countach! Mind you when you see the Countach chassis that number is believable. Mercedes had always been serious about torsional rigidity even in the 50s.The other German firms sometimes bothered, and sometimes didn't (911) (Z1) (lots of convertibles).

 

Once you get over 5000 then you can start to tune around the soft chassis (for a road car) and once its over 15000 Nm/deg frankly you are just playing specmanship or have other (possibly useful)motives.

 

Bear in mind you really want to normalise it by vehicle inertia and wheelbase, and consider tire radial stiffness.

 

The most obvious reason to go for a high number (say 20000 ish) is that you are running low profile tires and don't want your car to sound like a bag of nails after 5 years. Although it is hard to measure, a stiff body is so much nicer for squeaks and rattles than a floppy one. In the 1990s if you ran a Falcon over body twist (large slow humps on alternate wheels), and jammed your finger in the door gap, you'd lose your fingertip if you weren't quick. In 2000 the bodyshell upgrade got rid of that entertaining demonstration.

 

Take the numbers with a pinch of salt, test methodology varies - obviously you need the windscreens in but what about the doors? 

[munks]

That's a nice list. Countach! Mind you when you see the Countach chassis that number is believable.

 

LOL, if my calculations are right, the body would twist by almost 2 degrees if you simply jacked one of the rear wheels off the ground.

[mariner]

I guess once you have normalised by wheelbase you can divide by the BIW weigh to get an idea of how efficient the body engineers were on weight vs stiffness. I'm not sure if the data is good enough for that though. Also as bigger cars are usually wider and a bit taller they have an advantage when normalised to wheelbase as the torsional stifness goes up,  I think, by the 4th square of tube diameter.

 

With respect to tyre profile I doubt the Citroen 2CV or Renault 4 had much torsional stiffness given how light they were but they had very long travel, low rate suspensions too. So The chassis seldom took the pounding from the tyres that modern ones do.

 

I well remember my first ride in a R4. We floated along then the driver put two wheels on the grass verge and the ride, whilst worse, was as good as many UK cars  entirely on tarmac. Of course you paid a price in mega roll angles under cornering.

Edited by mariner, 29 August 2016 - 18:13.

[Fat Boy]

In all fairness, I think the Countach that they measured had come from Miami in the 80's. It had a bunch of holes cut in the frame rails so you could pack about 100 kilos of coke in the car for all the women you're gonna pull out of the clubs...    

 

Horses for courses, folks.

[Greg Locock]

The way round it is to tune each end of the car to look after itself. F1 cars from the fifties and sixties were light rather than stiff.

 

Munks - off roaders have a test called ramp travel index, you are right 1 or 2 degrees of body twist under their own weight is common.

 

http://www.offroader...-corner/rti.htm

[Fat Boy]

The way round it is to tune each end of the car to look after itself. F1 cars from the fifties and sixties were light rather than stiff.

 

 

That's a little easier to do with 50's and 60's level of downforce and ride rates an order of magnitude softer. It's funny when you see 80's vintage cars. They had good downforce, but often wet-noodle chassis. ARB's are often little more than a 'beam axle conversion kit'.

Edited by Fat Boy, 02 September 2016 - 18:32.

[Charlieman]

With respect to tyre profile I doubt the Citroen 2CV or Renault 4 had much torsional stiffness given how light they were but they had very long travel, low rate suspensions too. So The chassis seldom took the pounding from the tyres that modern ones do.

Those two chassis were everywhere in France when Michelin designed radial belt tyres -- I presume that the original cars were made for cross ply belt tyres. So Michelin had to make a tyre for the old motors which would work with future motors.

[Greg Locock]

Bear in mind the 2CV didn't have shock absorbers as such on the rear, and that's where a lot of the peak forces come from. They used inertial dampers, which are a mass suspended on a spring/damper system, bolted to the unsprung mass. Real Citreon engineering (a compliment).

[MatsNorway]

Surely these numbers needs to be looked at with the weight factored in?

Edited by MatsNorway, 04 September 2016 - 15:35.

[kikiturbo2]

LOL, if my calculations are right, the body would twist by almost 2 degrees if you simply jacked one of the rear wheels off the ground.

 

I used to have a lancia delta integrale... if you stepped on the brakes and turned the steering wheel left-right you could clearly see the bonnet going left-right in its opening.. and if you jacked up one wheel you could forget about opening the drives door.. Ever since then I have a strong dislike for cars with low torsional rigidity, including most convertibles...

[Fat Boy]

Surely these numbers needs to be looked at with the weight factored in?

 

That means looking at resonant frequencies. Ya, you could do it, but it wouldn't make much of a difference. A shitty stiffness number is a shitty stiffness number whether or not they used a lot of material to arrive at that shitty number.

[Kelpiecross]

I used to have a lancia delta integrale... if you stepped on the brakes and turned the steering wheel left-right you could clearly see the bonnet going left-right in its opening.. and if you jacked up one wheel you could forget about opening the drives door.. Ever since then I have a strong dislike for cars with low torsional rigidity, including most convertibles...

You should try jacking up an open top MGB - very flexible to put it mildly.

[Fat Boy]

You should try jacking up an open top MGB - very flexible to put it mildly.

 

I've never done it, but I've heard that a TR7 will rip a convertible top if jacked from 1 corner. That seems unlikely, but at the same time believable.

[Nathan]

McLaren F1 13,500?