Underpan Design!

[Urbalizer]

I have been wondering for some time about the clip release system for the underpan, Lee is a great chassis designer, and i feel he did it with 200 screws at 50mm centres for a reason,"Rigidity".
I am an engineer and space frame chassis design is something i understand very well and can calculate how loads get transfered.

The rear underpan acts as a "diaphram", and believe it or not, even with 1mm thick material with the close screw spacing, can add great rigidity to the chassis.
The materials we use to clad a build are only 1.3mm thick but again are fixed at close centres so again act as a diaphragm, adding massive rigidty to the building, if 1.3mm thk steel works structurally as a diaphragm for a massive aircraft hanger, then imagine how much ridigity it offers to a small space frame chassis.

So all i would say is from a point of view of somebody who designs space frames, truss and braced steel frames, it would seem quick release on the underpan does decrease rigidity.
Put quick release clips on the rear diffuser no problem as it is not integral to the main chassis, but the material thickness will need upgrading to combat sag.

Think about it, Lee did not put 200 screws in for the fun of it, and i am pretty sure he weighed up the pros and cons of increasing the material thickness to decrease bolt spacings.

If it were tested on a machine and you put a horizontal load on the chassis, with and without the diaphram, the one with diaphram would be able to take 30% more load, that is just fact, that is why on buildings with lots of glazing we have to upgrade the steel frame strength by 30% because of the lack of a diaphragm.

So the question is "did Lee design the chassis with diaphragm or without?
My instincts tell me he did, as it is allows you to have use less weight on the space frame, or all the space frame chassis members may have needed upgrading.
If he didn't design it as a diaphragm then on his next car he should come to me because i could save him loads of weight on the chassis design and maintain the same rigidity. To any engineer it just would not make sense to not design the underpan as composite to the chassis, it is the basics of lightweight steel frame design.

To put it in laymans terms, take some small hollow section and make a 1m square, brace it up, yes its strong, now clad it in 1mm thick material on one side, fasten it at 50mm centres, the frame is now 30% more rigid.

I see lots of great improvements on engineering design carried out by people on these cars, but this is one i would advise against.
People who want quick release are usually the ones who want quick access due to track racing, if you are into racing, then the extra rigidity is all important, bit of a catch 22.

[ThatPhilBrettGuy]

Screws are cheaper. There's your answer...

[AMG Merc]

Even so, a pity they weren't stainless.

[Urbalizer]

ThatPhilBrettGuy said:

Screws are cheaper. There's your answer...

So he definately did not design the underpan composite to the chassis?
Then he must have accidentally without knowing created a composite diaphragm design, strengthening the chassis by 30% because he wanted to save money on screws, lol

Seriously though, if anybody Knows Lee personally can they ask him if he purposefully designed the underpan composite to the chassis, because at the moment that is actually what it is, a composite structural diaphragm.

Edited by Urbalizer on Thursday 15th October 12:24

[peterguk M500]

Whilst i am no structural engineer, i looked at the chassis and the "cube frame" part that surrounds the engine/transmission.

Then i considered the extra rigidity that the engine pan would offer with all the screws. Decided that the extra rigidity that would be gained in a single plane would be much more than the "3D" nature of the chassis in that area. In other words the screws would offer a very small improvement. If any.

Then i thought about the aggro that removing all those screws involves.

Then i thought about the sheer pleasure and ease involved in 1/4 turning a dozen or so Dzus fasteners.

After a long deliberation, my coffee had brewed and i went with the Dzus fasteners.

[Urbalizer]

peterguk M500 said:

Whilst i am no structural engineer, i looked at the chassis and the "cube frame" part that surrounds the engine/transmission.

Then i considered the extra rigidity that the engine pan would offer with all the screws. Decided that the extra rigidity that would be gained in a single plane would be much more than the "3D" nature of the chassis in that area. In other words the screws would offer a very small improvement. If any.

Then i thought about the aggro that removing all those screws involves.

Then i thought about the sheer pleasure and ease involved in 1/4 turning a dozen or so Dzus fasteners.

After a long deliberation, my coffee had brewed and i went with the Dzus fasteners.

I can totally understand why you went for the easy release Dzus fasteners and it is because you are a friend and i look to you on advice about electrical and mechanical, i thought i would return the favour and impart my knowledge of steel frame design on you

When you look at a frame in 3D you have to split that into 2D planes to calculate the chassis design and how loads are transfered, i.e. any vertical members do not offer rigidty to the horizontal members the vertical braces only transfer loads to the bottom members, this bottom frame is responsible for taking all horizontal loads.

Lee may have designed the chassis rigidity isolated from the composite diaphragm, that is why i want to see if anybody knows him personally enough to ask the question, if he says yes it is a composite diaphragm, then that means the space frame chassis is inadequate without the diaphragm. But until we know how Lee designed it, we will never know the real answer.

Like i say if he did not design the underpan as composite, then i can save him a lot of weight on the space frame design on future projects. This is what i do on a daily basis, it is only horizontal loading that will be affected, i.e. when cornering at high speed, more stress will be put on the individual strut joints rather than evenly distributing the load over the entire frame, the vertical loading will not be affected whatsoever.
Movement is very slight, but fully fixed connection are not meant to move at all, so they become stressed, so without a diaphragm, each individual welded connections is getting stressed trying to move, eventually stress cracks appear always near the weld, where the steel is more brittle due to the heat affected zone, the weld itself is always stronger than the steel local to it, so what a diaphragm does, it shares out the load to the entire frame, evenly distributing the load across the entire chassis, rather than just stessing the space frame connections which is where the loads automatically get transfered where a diaphragm is not present, it could take a long time for stress cracks to appear under extreme cornering.

The only reason i am worried is that as it was originally, it does act as a structural diaphragm, offering 30% more rigidity in the horizontal plane, with Lee's expertize on lightweight space frame design i would find it hard to believe he created a composite structural diaphragm accidentally.

Edited by Urbalizer on Thursday 15th October 14:31

[nigelw]

Possibly , or maybe he just didnt want it flapping around or vibrating / buzzing ... I suppose only he knows for sure what was in his mind at the time , but it's hard to believe that he was looking for strength in that area / plane when you look at the triangulated box that surrounds the engine .

[Urbalizer]

nigelw said:

Possibly , or maybe he just didnt want it flapping around or vibrating / buzzing ... I suppose only he knows for sure what was in his mind at the time , but it's hard to believe that he was looking for strength in that area / plane when you look at the triangulated box that surrounds the engine .

Not hard to believe at all it is the difference between stressing individual joints or evenly distributing loads.
Would you find it hard to believe on all steel buildings you see with all the roof bracing that the actual roof sheet lining panels at 1.3mm thick add 30% more rigidity?
It happens that the same fixing centres that stop it from sagging is also the same point at which it becomes actively composite as a structural diaphragm.
Steel frames can be designed two ways composite or non composite to the diaphragm, non composite steel frames are always heavier, therefore i would find it hard to believe he did not design it compositely.

[nigelw]

only because he never put the car in a wind tunnel , didnt even have a stab at an aero section for the rear diffuser , so find it hard to believe that he stressed the car so tightly that he would rely on a flat section , in the wrong axis for chassis rigidity ..... I agree it probably does have that effect , but I doubt that it was intended to , or that it is required for chassis integrity , but it's really only my opinion , and probably not of much value , sorry ! I'll shut up now !

[Urbalizer]

nigelw said:

only because he never put the car in a wind tunnel , didnt even have a stab at an aero section for the rear diffuser , so find it hard to believe that he stressed the car so tightly that he would rely on a flat section , in the wrong axis for chassis rigidity ..... I agree it probably does have that effect , but I doubt that it was intended to , or that it is required for chassis integrity , but it's really only my opinion , and probably not of much value , sorry ! I'll shut up now !

No your quite right, its whether he intended to or not, thats the question, if it was unintentional then, it is absolutely 100% ok to quick release the underpan, as the the chassis was designed non compositely.
But even for cars calculations have to be provided as proof of strength, it is all down to what was issued on those calculation on the chassis design, i assume there is a motor authority to check production cars, make sure they are safe for the road, if so they would want to see chassis design calculations, showing how loads transfer through the members, proving that the hollow sections were adequate to take those loads.

Edited by Urbalizer on Thursday 15th October 17:50

[andygtt]

your talking chasis strength against chasis ridgity... the strength of the Noble chasis going to be uneffected if the alloy panels are removed therefore it is safe to run around without them fitted... in fact I did for about 2k miles and the handling was still fine.

The alloy panels will add to the chasis ridgity though and lee without doubt knows this, however I very much doubt he had a ridgity target figure in mind and just designed the chasis as stiff as he could and then took the gain from fitting the alloy panels.

[joust]

I'll ask him when I next see him if you want?

[paulcundy]

Dear All,
What you need to answer this are two Nobles and a loss adjuster; one Noble with the ally floor in place, one Noble without and the loss adjuster can have his clipboard. Then crash them both. If you are right the one with the floor will have a greater scrap value. Job done.
Regards
Paul C


apologies but its been a long day and its late.......................

[Urbalizer]

andygtt said:

your talking chasis strength against chasis ridgity... the strength of the Noble chasis going to be uneffected if the alloy panels are removed therefore it is safe to run around without them fitted... in fact I did for about 2k miles and the handling was still fine.

The alloy panels will add to the chasis ridgity though and lee without doubt knows this, however I very much doubt he had a ridgity target figure in mind and just designed the chasis as stiff as he could and then took the gain from fitting the alloy panels.

Contrary to what people might think the horizontal loads on a chassis are far greater than the vertical loads, the car tries to twist and fold on itself on heavy corners and the loads get transfered from corner to corner thats why horizontal rigidity is so important, notice why all the bottom chassis members are laid flat with the strong axis horizontal, rigidity is what creates strength, they come hand in hand, the chassis is designed on the theory of plasticity not elasticity, the difference is strength comes from rigidity, where as with elasticity, you allow joints to be flexible in order to transfer loads to a certain destination.

Notice there are no diagonal members in the cockpit floor and the underpan is thicker, this is a clear sign that it has been designed as a composite structural diaphragm as the loads can only pass diagonally through the underpan, so Lee has used this design method in one area of the chassis design for certain, so don't think he doesn't know about these design methods, it is a basic principle of lightweight steel frame design.

The fact he has diagonally braced the rear chassis, does not automatically eliminate that this are may also have been design composite to the underpan, there may be greater horizontal loads passing through the rear part of the chassis.

To test the theory conclusively a car would have to be tracked for 10k miles with underpan off, then some non destructive testing carried out to show up any hair line fractures or change in the metal grain local to the welds, mild steel is quite prone to ductile fracture which is invisible to the naked eye, but still the properties of the steel will have changed making it weaker and weaker until eventually it goes, you may have caused damage to the steel grain structure of your chassis without even knowing it.

Like i said before the chassis will still be strong without the diaphragm, but it does not mean the joints are not being over stressed, so although the car felt fine, you do not know if it was weakening the chassis gradually, just like bending a paper clip, you can bend it back and forth twist in into knots, and the metal still stays in one piece but it has been weakened and if you keep bending it, eventually it breaks, same principal with the chassis but just on a bigger scale.

Without long term proper testing and then carrying out NDT, or somebody asking Lee, we will never know conclusively.

But not knowing, I would say to be safe and for the longevity of the chassis, keep the underpan acting as a structural diaphragm.

[Urbalizer]

joust said:

I'll ask him when I next see him if you want?

If you could that would be great, he definately designed the cockpit underpan as a structural diaphragm, just not sure about the rear.

[peterguk M500]

My cockpit floor was 2mm thick, my engine pan 1.2mm thick.

[andygtt]

I was only refering to the engine bay floor pan, the cockpit is bonded in and should remain so... I also base my assumption on knowing other chasis that Lee has designed that followed exactly the same principal and the Noble is the only one with a paneled engine bay.

You can have a very strong chasis that is not rigid (old hot rod chasis) and equally a rigid chasis thats not strong (badly designed composite)... I have a chasis here for a mid engined car thats massivelly strong but because its not well designed its not rigid at all... I made a chasis to replace it thats the same weight and yet around 10x a rigid.

[Urbalizer]

andygtt said:

I was only refering to the engine bay floor pan, the cockpit is bonded in and should remain so... I also base my assumption on knowing other chasis that Lee has designed that followed exactly the same principal and the Noble is the only one with a paneled engine bay.

You can have a very strong chasis that is not rigid (old hot rod chasis) and equally a rigid chasis thats not strong (badly designed composite)... I have a chasis here for a mid engined car thats massivelly strong but because its not well designed its not rigid at all... I made a chasis to replace it thats the same weight and yet around 10x a rigid.

I am only referring to the engine bay floor pan also, strength comes from rigidity, that is the design principles af plasticity, which is what a space frame is with fixed connections. We create rigidity by transfering loads to joints that do not move, that is what makes the frame rigid as a whole, if a frame has been designed composite to a diaphragm then the diaphragm is also responsible for making sure the joints do not move, hence integral to the rigidity, which gives the strength by resisting torsional load being applied to the fixed joints and over stressing them.

I am not saying for certain that Lee designed the rear chassis composite to the diaphragm, i was just pointing out that he has definately used this design principle on the floor pan, as Peter says it is thicker, this is because it is doing more work to transfer loads diagonally through the diaphragm.

I hope Lee comes back and says the underpan is not composite, but at least we will know one way or the other for certain, because at the moment it is just guess work.

If i were to guess i would say due to the additional lattice of ties and struts in the rear end of the chassis, that it was to compensate for lack of structural diaphragm, which would agree with what you said, but as i am not allowed the luxury of guessing in my line of work and without having the chassis design and calculations in front of me, the only way we can eliminate assumption is by asking Lee.

I am not knocking your chassis design skills or knowledge whatsoever, we seem to be on the same page in that we looked at the lattice under the engine bay and thought that looks well braced, therefore rigid and strong, but again that is just best guessing, if Lee comes back and says it is not intentionally designed composite to the chassis, then please understand i never said it was, i am only saying there is a possibility it could have been.

It is at the moment acting as a structural diaphragm, intentionally or not, that is the only reason i am thinking why the hell did Lee not just use a quick release system in the first place, Which will take us right back to the beginning of the thread where Philbrett was correct, Lee did it because screws were cheaper and he could get away with using thinner materail.
But this is not an econonical design, he could have saved weight on the chassis by designing the underpan as a composite diaphragm.

[david.mt]

You guys need to get out and play with your nobs more

[nigelw]

Ha ha ! ..just about to do it !