Deflection Figures for a GT40 Chassis

[JP]

Does anyone have any idea of how stiff their chassis is in ft/lb/degs.

I believe a McLaren F1 is 25,000. a Porsche 911 is about 10,000.

It would be very interesting to know what figures we've all got.

Regards,

J.P

[Hemipanter]

This is interesting. Anyone know anyone with a chassis in Sweden? He should be very wellcome to visit me for a torque test.
In my site http://hem.passagen.se/hemipanter/
uder "Chassis and components" I have a few words about twisting chassies, together with som Tq numbers for different cars.
Goran Malmberg

[Mark Worthington]

A search of the phrase "torsional ridgidity" brings up:
this thread called "Chassis Stiffness", and
this thread called "chassis rigidity".

I have considered measuring the torsional ridgidity of my RF and may yet do so.

[Mark Worthington]

[ QUOTE ]
This is interesting. Anyone know anyone with a chassis in Sweden? He should be very wellcome to visit me for a torque test.
In my site http://hem.passagen.se/hemipanter/
uder "Chassis and components" I have a few words about twisting chassies, together with som Tq numbers for different cars.
Goran Malmberg

[/ QUOTE ]

Goran, your web site contains much useful information. I like your car, too! I looked up your chassis stiffness information, and I'll quote it here:

[ QUOTE ]
As for references. Lamborghini Countach 1900 fp/degree. Ferrari 360 spider 6250 fp/degree. Viper gts has a "tube space frame" and 9000 fp/degree. Viper gts-R (Le Mans 24 hr) is reinforced to 13600 fp/degree. Lamborghini Murcielago also uses a high strength tube frame supported with honeycomb carbon fibre to 15000 fp/degree. It clearly shows that the Ferrari has no roof. Here we have cars with cromolly tube frames, carbon fibre, etc. Exotic material, loudly advertised as great stuff that makes those sport scars outstanding. Let me mention that the new SAAB 9-3 Sport Sedan, steel monocoque has a torsional stability of 16000 fp/degree. Showing that good engineering is more important than the use of fancy materials. Embarrassing for the SUPER cars? The Panoz racing car tub carbonfibre monocoque has a stiffness of 45000 fp /degree, but due to the front motor installation the axle to axle ratio is 30000 fp/d, at a weight of 110 pound. A street car that uses a tub monocoque is Koenigsegg . Also made of carbon fibre. This tub is said to have strength of 20500 fp/degree. As this, like the Panoz, is a tub number, the axle to axle ratio should be less. With the same reduction as the Panoz, we should land at 13600 fp/degree. This show that a monocoque is the way to go, even if made in sheet metal. The reason for using steel tube frames is the ease of production in a small numbers. A steel monocoque takes a tremendous investment in tools and engineering.

[/ QUOTE ]

Could you explain the axle-to-axle ratio? My understanding is that chassis stiffness should be measured by securing the chassis at the rear suspension mounting points and one of the front suspension mounting points while putting a load on a moment arm outboard of the other front suspension mounting point and measuring the applied torque (moment length x mass) and the resulting deflection. I imagine if you measure from points in front of the front suspension or in back of the rear suspension then the torsional ridgidity value will be less, but in my opinion the only thing that matters is the chassis stiffness between the suspension pickup points (at least for a mid-engine car).

[JP]

Interestingly an Audi A8 has a torsiional rigidity of 36,000nm. One could argue that it needs the stifness because of its weight and size. However one could also argue that a GT40 should have more torsional rigidity because of the power plants used and the handling importance.

I would hope that a GTD has more than 2,600 ft/lb per degree. That seems low in comparison to an orginal.

Regards,

J.P

[Hemipanter]

Mark
What matters is the attaching points of the coilovers. In my own test I replaced the coilovers by solid bars, attaching the twisting arms to the under A-arms. Simulating what happen on the road.
However, a "tub" monocoque is a body that usually only has the FRONT A-arms attached to it. The rear is mounted together with the motor and transaxle. Therfore the correct number must include these parts. But tub manufacturers supply numbers for the tub only.
Goran Malmberg

[Hemipanter]

Nowdays car manufacturer put a lot effort in impact safety. This also makes the car stiff at the same time. Ok, they also got softer crusch zones.

For a sportscar, we should look at the torsionally stability number as a "ratio" number, put in relation to the exposed load.

A car that is lighter and has lower CGH might then show a better value than a by number stiffer car.
It must also be put in relation to the total spring stiffness of the car. The stiffer the springs bars and schocks, the greater the torsional load on the chassies.

The picture is a bit complicated, so let us just make the car as stiff as possible.

Goran Malmberg

[Brian]

Interesting.......At the University of Auckland In 1999, under the supervision of Dr. R Halkyard, students performed a "Torsional Analysis of GT-40 Replica Chassis." Although the study is listed on the U of Auckland site, I could not find the paper, however it may have not been published.

Brian

[dave harris]

In the DRB build manual there is a copy of a page from the ADR filing that shows the torsional rigidity number, If I can remember that long (work to home - the brain usually refreshes on the commute home [img]/ubbthreads/images/graemlins/tongue.gif[/img]!) I'll look it up and post it.

[soonhopefully]

To quote the DRB brochure over 7000 Nm per degree
The roaring forties state that at over 20000 Nm of torque the deflection was 0ne degree.
Not that I've been pouring over this stuff much [img]/ubbthreads/images/graemlins/grin.gif[/img]

[killroy]

Monocoques are the way to go. I once built one for an SAE project. Our club didn't know how much carbon to use, so we decided to play it safe and use eight layers with a one inch foam core. The monocoque weighed about 35 lbs. We never tested the structure, but it was like a tank. We did crash it a few times with only superficial dammage, it had no crush zones, the driver truly was spam in a can.

Aluminum monocoques work well also. CNC punches, CNC benders, laser and water jet cutters make it possible to design a complete monocoque on a computer and make all of the pieces line up the first time. Then it's just a matter of riveting it together. It's definitely a good way to go when you need to build a fair number of chassis.

Space frames are good because they are easy to build in low quantity, they are easy to repair if dammaged, and you can build a chassis with reasonable stiffness with out too much weight penalty compared to a monocoque. This is especially true for race cars because the rules usually call for a roll bar or cage. In a space frame, those bars are a functional part of the chassis, in a monocoque, they are just extra weight, and hard to attach to the monocoque because monocoques rely on the strength of their skin and don't have very good hard points.

The important thing to remember about chassis stiffnes is that you don't need very much. More is always better, but there are race cars that work very, very well and only have about twice as much chassis stiffness in ft lbs compared to their vehicle weight.

[Trevor Booth]

Torsional rigidity (TR) is not correctly expressed as a function of the weight of the car. There are just to many factors to be considerd ie 2 seat open car, single seat, front engined, rear engined, 2 door , 4 door, the list goes on. You can compare like for like but you need to compare bare chassis weights Vs TR. A higher TR for same weight is an indication of the effective use of the material. On a tubular frame chassis the placement of tubes can make a significant difference. Bonding or close rivetting of sheet panels onto a tube frame can be of enormous benefit. In a replica tube frame GT40 7000Nm would be about the right number. But I have have tested 600kg (total)cars with a TR of 4900Nm. Reputable manufacturers of rear engined Mono tubs will apply a correction factor if tested without engine etc. A car with a high TR will handle better, can run softer springs for the same result but it is more sensitive to spring rate change. The addition of a roll bar/cage can reduce the TR as it may change the effective centre of rotation causing a higher strain energy in critical parts. If you are concerned about the TR of your chassis ask the manufacturer for the test report.

[killroy]

[ QUOTE ]
Torsional rigidity (TR) is not correctly expressed as a function of the weight of the car.

[/ QUOTE ]

I have to disagree a bit. The relation ship between chassis loads and vehicle weight is pretty straight forward. The heavier the car, the higher the load on the chassis. So one chassis will behave similarly to another with half the stiffness in a car that weighs half as much.

Of course the ratio of sprung to unsprung mass will change things a bit, wheel base and track are certainly factors, but in general it's pretty close to a linear relationship.

My point is that no one needs a 25,000 ft lb per degree chassis, 2,000 ft lbs per degree would work (drivable), and if you have more than 5,000 ft lb per degree, you won't gain very much by stiffening it further.

American road cars in the 60's had around 2-3,000 ft lbs per degree. The average car was a little bit stiffer in the 80's. Computers and crash test standards made big improvements in the 90's, now all of the manufacturers know how to make a unit body chassis with very high stiffness and crash safety.

Mure stiffness is always better, but just because a new Audi has a huge number does not mean that you car won't handle if it has less.

[Hemipanter]

Hmmm, the torsional stability is a number in Nm per degree of deflection. Just like pound per inch of a spring.

Todays shock absorbers is very sofisticated, and may load the chassies by fare higher force than the springs. If the chassies deflect it will kill the function of the shock absorbers by the same amount as the deflection of the cahassie.
To say anything about the stiffness needed, we must know qoite a bit about the car in question.

From my own experience I can say that 2-3000 fp per degree can not withstand anything more than 1,7 hz of spring stiffness on a 3000 pound car.

Goran Malmberg

[Fran Hall RCR]

I agree but you also have to take into account the durometer rating of the rubber isolation dampers mounted between subframes,front and rear, and the chassis on current vehicles ,also the deflection of the bushings in the shock mounts themselves......none of this is cut and dried and all subjective until all the relavent data is acquired.

[JP]

Its all well and good saying what you need, however what you WANT is a chassis that is as stiff as is possible within the design contraints. Then you can know how your suspension is likely to react when you change shock, spring, anti-roll bar settings.

[Fran Hall RCR]

We all know what we want but getting it sometimes takes a while...eh.... JP. [img]/ubbthreads/images/graemlins/wink.gif[/img]
I would expect that most people with any inclination towards racing would buy a chassis that combines both light weight and good structural stability...namely good useful triangulation..
I have not seen an MDA chassis but would be interested in the upgrades that have been done chassis wise..any pics JP?

[Trevor Booth]

May be I did not explain it correctly. I am talking about the relationship between chassis (and or vehicle weight) and TR, not induced loads. It was described as a "ratio number" by Goran. my reply was intended as a general reply but the new "system" directs replies to someone, intended or not. However with all that said. Your Quote "The heavier the car, the higher the load on the chassis,So one chassis will behave similarly to another with half the stiffness in a car that weighs half as much." IMHO Not necessarily so. many factors influence the behaviour of a chassis subjected to torsional conditions, the roll centre , the cg height, and the cornering loads imposed. A heavier car has a higher static load but it is the induced load that puts it into a torsional condition. Generally a heavier car will have a higher induced load but only because it would generally have a higher cg and perhaps a higher roll axis. A GT40 type vehicle with 2000 ftlb would not handle as well as one with 5000 and I would suggest that the 2000 vehicle would be unpredictable. You dont need 25000 but 5000 is a whole lot better than 2000 and 10000 is better still. You would be amazed at the difference. Gorans post re 1.7 Hz springs in a 2-3000 (TR) car is on the money, much stiffer than that and the chassis winds up during cornering, gets to a point and lets go with "flick" (for want of a better word) and sends a pulse through the chassis which goes all the way back to the wheels. What you need is a chassis TR that will resist the induced loads for the particular layout of the vehicle. I am one of those "nuts" who does this shit on computers and truly you would be amazed at the difference between a "jelly" and a "block of cheese" (relatively speaking)
Hope this helps,
Trevor

[JP]

[ QUOTE ]
I have not seen an MDA chassis but would be interested in t