Chassis torsional stiffness 1965 and today 3

[Tmoose]

http://www.sportscars.tv/Newfiles/Chaparral2.html

About 1/3 way down the page.
"The Chaparral Frame

The basic frame of the Chaparral 2 is a most interesting and unusual piece of engineering. It was conceived jointly by Hall and Andy Green of PlasTrend, Fort Worth. The design requirements laid down by Hall called for a maximum frame weight of 150 Ib, combined with an axle-to-axle torsional rigidity of at least 3000 lb-ft deg."

3000 lb-ft/degree ~ 4040 Nm/degree

Are all modern cars so much stiffer than an early Chaparral?

http://youwheel.com/home/2016/06/20/car-body-torsional-rigidity-a-comprehensive-list/

Please say it isn't so !!

 

[GregLocock]

Yes, so far as I know. Plenty of successful race cars were down at1500. With production cars we saw a rapid increase in torsional stiffness in the 80s, and it hasn't stopped going up. At around 10000-20000 the gains in handling and S&R start to reach the point of diminishing returns but some manufacturers have a bit of a fetish about it.

Cheers

Greg Locock


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[gruntguru]

Nice link. Surprising range eg more than an order of magnitude for Lamborghini (Countach = 2,600 Nm/deg, Aventador = 35,000). Yes I know - decades apart - but still.

je suis charlie

 

[GregLocock]

Have you seen the ,um, chassis of a Count? It is a bunch of square tube that you can hang bits of fibreglass on. Mind you I stiffened the esprit up by 30% with a small bracket, let's say the 80s were a bit of a learning game, though Mercedes in particular had been chasing torsional frequencies since the 60s.

Cheers

Greg Locock


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[Tmoose]

This was what the team at Volvo initially considered sufficient in the 50s for the poor Volvo Sport.

http://www.forgottenfiberglass.com/...volvo-sport-the-volvo-p1900/#jp-carousel-3457

 

[EdStainless]

In a lot of those race cars the frame flex was part of the suspension. Ever raced 'carts? no springs just flex. now if comfort matters that is a bit different.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[SwinnyGG]

In a lot of those race cars the frame flex was part of the suspension. Ever raced 'carts? no springs just flex. now if comfort matters that is a bit different.

I think a lot of those early cars (Chapparal included) have a lot more in common with, say, a modern shifter kart, than they do with an LMP car or the like.

This bit of data is interesting, in that it shows the effect of a highly engineered car having it's roof cut off:

Bugatti Veyron 60,000
Bugatti Veyron Grand Sport 22,000

 

[Overrun]

Karts are a different animal –no suspension and a solid rear axle. By building in chassis flex it allows the inside rear wheel to lift to avoid the two rear tire turning on differing radii from fighting each other, i.e. scrub.

 

[140Airpower]

The range of chassis stiffness is amazing. Of course the stiffness vs the mass is the critical parameter, but I think crash worthiness has required more concentration on this and is probably the greatest influence on the spectacular increase in torsional stiffness over the years.
In the Chaparral's day, the unibody Chevrolet Corvair (2,400 lb vehicle) had about 6,000 lbft/deg and the unibody Lincoln Continental (5,500 lb vehicle) had about 10,000 lbft/deg.

Body-on-frame construction was very willowy. If you see crash tests of cars from the '50s and older, a common event in an accident was the popping open of doors and ejection of non-seat belted dummies. In fact, with the common door latch designs (like house doors) combined with vehicle flex, a door could pop open when the car hit a pothole or made a violent maneuver. See the Jack Nicholson movie "The Postman Always Rings Twice".

 

[dgallup]

I've had both the passenger door and the driver door open on me in late 60's Chevy pickups (2 different ones) during hard cornering. The drivers door is preferable, you can still hang onto the steering wheel. When the passenger door opened up I grabbed the handle which broke off in my hand as I exited the vehicle. Fortunately it was winter so I had a heavy coat on and I landed on a downhill slope so I slid away from the rear wheels. I didn't have a scratch on me.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[140Airpower]

Good God! Lucky you!

American auto door latches went through several design iterations between the '50s and '70s. They improved the lateral latching with ratcheting in case of incomplete door closure. But, GM did not make a longitudinal interlocking latch until late in the period. It is hard to imagine why, but they may have been reluctant to make a pry bar proof latch for the sake of extraction of passengers???

 

[NoahLKatz]

Not surprising given the Chaparral's chassis-only and overall weight, and it had no roof.

As 140Airpower alluded to, it would be more instructive to know stiffness when normalized to mass, i.e. torsional frequency.

Speaking of which, does anyone know what restraints are used when specifying frequency?

 

[GregLocock]

Typically hang the body in white from 4 bungees. You could rest it on a soft foam mattress or airbags but that is not ideal

Cheers

Greg Locock


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[NoahLKatz]

Thanks Greg, that makes sense, as resting on the suspension would muddy the waters.

But BIW doesn't tell the whole story, i.e. what's the all-up mass the chassis has to deal with.

Are any other frequency or deflection measurements done on the rolling vehicle, other than subjective road test impressions?

 

[GregLocock]

Sure, we do whole vehicle modal analysis, where the car just sits on its tires, and heaps of vibration work on roads.

Cheers

Greg Locock


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[140Airpower]

I seem to remember from back in the day they measured chassis torsional stiffness with fixtures bolted to the spring perches. Vibration modes are different, you need to account for the whole system.

 

[NoahLKatz]

> Sure, we do whole vehicle modal analysis, where the car just sits on its tires, and heaps of vibration work on roads.

Excellent; can you share what some typical whole-vehicle chassis torsional frequencies are?

 

[GregLocock]

Nope.


Cheers

Greg Locock


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[NoahLKatz]

Understood, thanks.

 

[Tmoose]

Just re-read the Ford GT SAE papers looking for somethng else. They mention the chassis was "an extremely strong unit, giving over 10,000 ft lb/degree in torsional rigidity."