Chassis torsional stiffness model by model

[red vtec]

If you look at the post pics 88-90 it states that the composite body is so strong it doubles the torsional strength of the chassis.

[MrDangerUS]

Here is the method, if anyone wants to try:

First, place car/chassis on axle stands at the points corresponding to the suspension attachment points; wheels off the ground.

1. Level it on 4 stands.

2. Keep it on axle-stands; load the rr corners with ballast to keep it from jumping off.

If you’re testing fully dressed car the weight of the engine and transaxle will keep it pinned down.

3. Remove one (front driver sd) stand and a wheel and measure the distance from the fixed chassis/suspension point to the ground.

If you’re to use a dial indicator, set it up and zero it.

4. Put known ballast on one corner that is not supported**, [F].

5. Now, measure how much the level on this corner drops down for a given ballast weight [D]

6. Measure the distance to the opposite side stand (closest to the measuring point), [L].

The torsional rigidity can be calculated by finding the torque applied to the frame and dividing by the

angular deflection.

K^2= FxL/tan(D/L)

This method of frame testing is relatively straightforward and the advantage is the frame stiffness can be determined without including the suspension components. The primary disadvantage is the artificially created load paths do not load the frame in the same manner as on the track. Also, the choice of what rear nodes to fix and what front nodes to apply the load can affect the results significantly. For this reason a whole car chassis torsion test is the preferred method for capturing the true vehicle stiffness.

Chassis testing, however, has very high non-linearity in the early stages. For small forces gaps in the suspension and compression of various bearing elements occurs. As these gaps are closed and bearing friction is overcome the slope of the load deflection curve becomes linear.

** For this reason, it is necessary to map the force displacement characteristic of the structure, rather than finding one stiffness value. To get better data small steps of load should be applied, and the corresponding displacement measured. It is also interesting to note that the force deflection curve has some hysteresis. To accurately gauge this characteristic, it is helpful to add or remove the load in finite steps and record the deflection. This will build a load deflection “path” that rises and then falls again. At high loads the deflection is linear. This represents the deformation of the elastic frame and suspension members after gaps are closed. Therefore one should start at loads higher than 40-50 lbs. Then, load it in 10 lbs increments. Plot the results and draw the load-deflection line.

Please, post the numbers.

To determine the stifness contribution of bulkhead attachments, I may test it with bolts removed, first. Then, I'll test it with anchor plates of my own improved design and AN-grade M10 bolts, (OE are just M8-s). It would be nice to find out how much the OE plates contribute, but my car originally did not have them.

Recommended reading:

Determining chassis stiffness: http://www.theoryinpracticeengineering.com/resources/fsae/cornell%20design%20anal%20testing%20fsae.pdf

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.25.8734&rep=rep1&type=pdf

Edited November 9, 2013 by MrDangerUS

[Bibs]

That's not going to help with the horizontal lozenging measurements is it?

[MrDangerUS]

Very little. Welding web plates at the forward tub corners would do more. 

[ramjet]

I wouldn't expect that lozenging is a big issue with the esprit chassis by virtue of it not being a mechanical rectangle?

When you hit a bump with a front wheel, you would get rearward deflection of the front cross member on that side, but it is essentially isolated from the rear axle line and would dissipate via a bending moment at the front of the backbone member. Wouldn't it?

[MrDangerUS]

Could anyone tell how much stiffer the chassis gets after implementing 1/8" bolt on cover plate over the shifter opening, something similar to what Mike Fennell did?

http://www.exit109.c.../lotus/chassis/

also see post #48 here http://www.thelotusforums.com/forums/topic/47670-s300-no-25-restoration-resumed/page-3

Edited November 18, 2013 by MrDangerUS

[benjamincalleja]

Did anyone get anywhere with this id still love to find out an actual stiffness number in Newton meters.

[MrDangerUS]

+1

 

Still looking for someone doing a full restoration.

Bare chassis stiffness number is pretty academical,

but

chassis+body (fully dressed, w/engine and guts) could be more relevant/useful, see: .http://www.racingsoftware.com/roll_center_geometry.htm

 

Apparently, not a new question: http://www.thelotusforums.com/forums/topic/5573-increase-the-torsenstiffnes-in-the-esprit/

 

 

Check this thread from time to time for updates:

http://www.germancarforum.com/community/threads/the-list-torsional-rigidity.12334/page-5

Edited July 20, 2014 by MrDangerUS

[Neanto777]

hi guys,

im restoring my 1990 Esprit SE and once i found this post i kept thinking about it. so, i spoke with some friends (civil engineers) and we came up with the following idea. just check the pics below.

i havent welded anything on the chassis. just reinforced it with 3 main bars (6mm thickness) and braced them around the chassis. i already fitted the body and it seems really fine.

once i give it a drive, i will let you know about the results.

 

[Escape]

I fear you're mostly adding weight. An open frame like that without triangulation wont do much for torsion (or any other loads).

[Barrykearley]

Looks somewhat familiar to me

[ramjet]

I feel that it will be adding more weight to the car than benefit as well. 6mm flat bar is not light and with them amount that you have there, that is a lot of weight. Civil engineers can build big heavy dams and  bridges. Just not sure how much they know about cars though and the dynamics of them?

[Neanto777]

Added around 27kg. But i already removed around 43kg, so its fine regarding the added weight.

indeed civil engineers are for big heavy stuff, but also know about torsional stresses etc.

if its not working, i will remove it🤢

[Escape]

2 hours ago, Barrykearley said:

Looks somewhat familiar to me

No need to go into the details Barry! 🤢

 

1 hour ago, Neanto777 said:

indeed civil engineers are for big heavy stuff

Not all of us are into heavy stuff, some want to keep things light and simple. 😉

[TAR]

I'm not sure where you feel the twisting motion will come from?  What are you trying to overcome?  

[Barrykearley]

The sport300 had a chassis brace which went over the engine - they did that for a reason.

[NG5]

Neanto,

Sorry, I'm an ex structural engineer and don't think you are going to like what I'm going to say.

Looking closely, all you seem to have done is add weight without increasing the torsional stiffness at all.

To increase the torsional stiffness you need to make the central spine harder to twist. Your straps to next to nothing in that regard. If you remove them, attach one end rigidly to a bench and then try to twist the other end, that will be how much extra torsional stiffness you are adding, essentially nothing. I'd bet you could twist it by hand 100mm without difficulty.

The main torsional rigidity for any lotus with a backbone chassis comes from the body, they have been designed that way from the Elan. You would gain far more by finding extra ways to attach the body to the chassis and/or increasing the bodies torsional stiffness (which is what a roll cage does). As an example the difference in stiffness between the S1 Europa which had the body bonded to the chassis and the S2 Europa where it was bolted on, is huge, with the S1 far stiffer (it did make repairs of the S1 VERY difficult!)

I spent a long time quite a few years ago trying to work out an effective way of increasing the stiffness of the backbone in my Europa, which is very similar to the Esprits. I failed and couldn't find a way that worked, even making a new chassis with thicker metal made almost no difference. The closest I got in theory was to use diagonal bracing wires from opposite corners (front bottom left to back top right etc) within the box, but even using "superstrand" stressing wires to resist stretch, the tension required in the wires to have any meaningful effect would have crushed the backbone.

Despite looking like a chassis, in reality it's a front and rear subframe with the backbone conveniently locating them in place. The backbone mainly acts in bending it's torsional strength is a useful bonus for the bodies torsional strength but secondary.

[Neanto777]

Thanks NG5 for the info… guess i wasted my time building that…

9 hours ago, NG5 said:

The closest I got in theory was to use diagonal bracing wires from opposite corners (front bottom left to back top right etc)

I was thinking the same thing but instead of  wires, i would use thick metal plates.

any other ideas??

[jonwat]

9 hours ago, NG5 said:

To increase the torsional stiffness you need to make the central spine harder to twist.

As an ex civil engineer they were my thoughts exactly, plus I'm surprised the body would fit over over that bracing. I'd have expected it to be a much tighter clearance. 🙄

[TAR]

58 minutes ago, Neanto777 said:

any other ideas??

 

On 17/02/2022 at 22:29, TAR said:

What are you trying to overcome?  

 

[Barrykearley]

1 hour ago, Neanto777 said:

any other ideas??

Weigh in the scrap and enjoy as lotus intended 👍

[Neanto777]

On 18/02/2022 at 14:36, NG5 said:

To increase the torsional stiffness you need to make the central spine harder to twist.

the idea came from the following link.

i thought, instead of using stiffeners inside the beam, i could brace the beam and hold the braces with the main plates.

https://engineeringlibrary.org/reference/beam-torsion-air-force-stress-manual

1.5.2.2.4 Effect of Stiffeners on Noncircular Closed Beams in Torsion

 

 

[Barrykearley]

Looks like fantastic reference material in case you were building something like a tower. I bet it doesn’t once mention add lightness. Frankly that’s the point it’s missing.

[Escape]

Actually, it's from the US Air Force, so not about towers and does take weight into consideration. Which is why the emphasis is on internal stiffening as those have a direct effect on stiffness, rather than adding heavy pieces on the outside with few or no tie-in points.

Very interesting read! I'll be skimming through it for some other builds, like a rack for engines I'm planning and a winch craddle.

[NG5]

Neanto, 

The manual you quote is of course 100% correct and adding top hat stiffners as shown inside or outside will help. But to work properly they need to be ridgedly fixed to the main box with NO relative movement, so need to be fully welded to the main box. Secondly the main benefit comes from the depth of the stiffner from the main box which in the Esprit is limited by the space between the chassis and body. That is one reason why if you attach the body at more points to the body, particularly near the centre tunnel overall stiffness is increased, the tunnel walls are starting to act as stiffners for the chassis and vice versa. 

It's the overall stiffness of the composite chassis/body that counts, not just the chassis. You could argue that the Esprit chassis is far stiffer in torson than any modern porsche because the porsche doesn't have a chassis it's a unitary body so it's separate chassis torsional strength is zero! Silly argument true but it highlights you need to compare the whole car stiffness not just part of it. Btw the closed doors and windscreen also add significantly to the overall torsional stiffness. 

The other factor of course is that the heaver the car the higher the loads, the stiffer the car needs to be for the same amount of twist.

As i said before you have more to gain and it's easier to increase the stiffness of the body than the chassis.