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Another question about tubing
ProjectLMP - 14/1/03 at 05:39 PM

I am looking to optimize the weight of my frame by carefully selecting suitably sized tubes. Some of the diagonal bracing and pontoons I feel I could get away with lighter or smaller diameter tubing. I could either stay the same diameter and go from say 16 to 18g or reduce the diameter and stay the same thickness. What would you recommend? I am trying to get some real data on DOM tubing to compare the two approaches. This is were FEA would be really helpful in optimizing the strength of each tube.

Alan B - 14/1/03 at 08:12 PM

To be honest Paul, IMO, a plain old calculator is good enough to compare one tube to another...FEA is a real overkill.

Tube strength is proportional to cross sectional area, but stiffness is increased by going to larger diameter and thinner wall (and keeping the same CSA and weight)...there are practical limits of course, space and wall "thinness" for welding, and of course availibility.

When I used to build round tube frames many years ago we used a lot of ERW, and went down to 18G for some tubes although most were 16G. We'd save the DOM or seamless for any tubes that were bent.

Just a few thoughts and opinions.

ProjectLMP - 14/1/03 at 08:21 PM

Thanks for the info.

My comment about FEA was really more related to knowing exactly which tubes in the chassis were highly stressed and which ones you could get away with thinner/lighter tubing. The balsa model is great for getting a general idea of stiffness. However, its not so good for showing the individual loads on each tube.

I also saw this quote on a website

"As you can see, DOM tubing is nearly twice as strong in tension, and more than twice as strong in resistance to bending in comparison to the ERW tube. Also, the 4130 chromoly tube is only 20% stronger than standard DOM in tension. "

Is this true in your opinion?

[Edited on 14/1/03 by ProjectLMP]

Alan B - 14/1/03 at 08:40 PM

Paul, yes I take your point about FEA.

Regarding the other, the first bit sounds absolute bollocks, if they are comparing the same O/D and wall thickness.......in tension only 2 factors affect strength, cross sectional area and material...same OD, same wall, same material....SAME strength.
Again in resistance to bending (stiffness)the factors are material, I value (from OD and wall) and loading conditions....the fact it has a welded seam should not be relevant, if the tube is decent (normal) quality.

The last bit about 4130 may be true...it sounds about right.

The objection to ERW is really concern about the seam splitting while bending, for most applications in a spaceframe (which in theory should never see any bending) it is fine IMO.

ProjectLMP - 14/1/03 at 09:49 PM

I thought it sounded a bit fishy. Perhaps they are comparing standard mild steel ERW to something like 1020 DOM.

Alan B - 14/1/03 at 10:15 PM

Hmm...but 1020 is barely stronger than regular mild steel anyway....

Sound's like they are doing some odd comparison.....

Spyderman - 15/1/03 at 12:09 AM

Presumably you are considering thinner or smaller tubes to save weight in less structural areas.

The way I look at it is. What part of the chassis do you want to give way first when in a collision? You can't be too strong when looking at damage prevention (to occupants and structure).

The part may not be a highly stressed component, but in say a side impact I'd want the pontoons (for example) to stand upto as much as the rest of the chassis.

One of my biggest fears in a Seven style car is the side impact vulnerability. Just my paranoia!

Terry

Rorty - 15/1/03 at 04:20 AM

The materials I use are:
ERW - 250mpa
CDS "A" - 340mpa
CDS - 470mpa
CDS "20" - 540mpa (I think this is 1020)
4130N -650mpa

I agree with Alan 100%. CDS/CDW for bends, and ERW for straights and infill.
I'm a firm believer in the safety cell. I would aim to have a crumple zone all around the car, with a very strong cabin, even if that means sacrificing your pontoons.
The nerf bars on our cars are our first line of defence (although they're officially there to prevent us from locking wheels), and the wishbones are designed to come away if the car takes a really hard hit from a tree or rock. The grade of the Rose joints are selected to give way before any harm comes to the chassis (more expensive, time consuming to repair, and contains ME).

ProjectLMP - 15/1/03 at 03:57 PM

Bull,
I hear what you are saying about the pontoons. However, the theory I am using is that it is better for them to deform and absorb energy in a crash. That way the main cell has a chance of staying intact. My current thoughts are to go 16g tube exclusively with 1" tube for the pontoons with 3/4 diagonal bracing. The main cell will be made from 1 1/4" round and square with a combination of 1" and 3/4" diagonals.

Rorty,
The survival cell principle is definately something I am taking seriously. What the model doesn't show is that there will be a detachable front crash box probably made from aluminum. My home track (Mosport) is a very fast and unforgiving place and I expect down the back straight the car will hit 130 - 140 mph. I've seen some nasty accidents at race tracks and over the past three years two people have been killed at Mosport, one professional and another amateur. However, I would say that driving fast on the road is way more dangerous and the kind of multi car impacts you see are really hard to design a car to survive. The thing that amazes me is that some of the kit cars appear to have little regard for safety. Indeed some of the chassis look like they you have trouble staying together under normal road use let a lone in the event of a crash.

Rorty - 16/1/03 at 02:10 AM

LMP:

quote:

The thing that amazes me is that some of the kit cars appear to have little regard for safety. Indeed some of the chassis look like they you have trouble staying together under normal road use let a lone in the event of a crash.

This was my immediate reaction and concern when I first came upon this forum.
I'm all for freedom of expression, but there doesn't appear to be any reliable form of regulating the various permutations of the Sevenesque locosts, nor the manner in which they are built.
Australia has gone the opposite route, but that's not what I'm here to discuss.
I appreciate the huge amount of fun that can be had, not only from building a Sevenesque, but mainly from driving one.
However, having seen the standards of a few on this list, I shudder when I imagine one of them being flicked around like a hockey puck, in the event of any reasonable contact with another car or imoveable object.
The "giraffe-on-a-skateboard" occupants wouldn't fare too well.

kb58 - 18/1/03 at 02:16 AM

quote:
Originally posted by Bull
Presumably you are considering thinner or smaller tubes to save weight in less structural areas.

The way I look at it is. What part of the chassis do you want to give way first when in a collision? You can't be too strong when looking at damage prevention (to occupants and structure).

The part may not be a highly stressed component, but in say a side impact I'd want the pontoons (for example) to stand upto as much as the rest of the chassis.

One of my biggest fears in a Seven style car is the side impact vulnerability. Just my paranoia!

Terry


I respectfully disagree. If the chassis is non-deformable, where does the crash energy go? Into the driver! SOMETHING has to absorb that energy so the driver doesn't have to. The right way is to make the outermost portions of the chassis crumple first, adsorbing energy as they compress (smallest tubes). As the crush area gets closer, larger tubes begin to crumple, resisting more, and adsorbing crash energy at a greater rate. Finally is the roll cage, which is really stiff in order to avoid bodily harm.
Look at F1 cars, the very nose is designed specifically for decelleration purposes.

BTW, I recall the DOT here in the US once did a standard 30mph crash test of a stock car (NASCAR.) Damage was so slight the car could be driven away. But since the chassis was so strong, and deflected so little, the G forces were enormous and would have killed the driver outright.

[Edited on 18/1/03 by kb58]

Spyderman - 19/1/03 at 01:21 PM

I understand your point, but where does it say that the chassis is non-deformable?

Your analogy to F1 cars is not really useful, as the main body tub in F1 is made to protect the driver regardless of direction of impact.
A sevenesque car is very old in design and concept, and could not by any stretch of the imagination compare to a F1 car.

Making the outermost parts collapse is fine when talking of front or rear impacts.
However in a side impact how do you calculate how much force (without doing Crash testing) you are absorbing in crumple and how much is going to be transfered into cockpit?

As I see it the outriggers are not structural and do not carry the normal loads trough them, so therefore are not triangulated in the same way as the main chassis.
To use smaller materials is self defeating in this respect as you need more strength to decelerate an impact in a shorter space.

Frontal impact has about 4feet of space in front of drivers legs in order to decelerate/absorb impact.
Side impact has about 1foot to at best 18 inches in order to do the same (in a can am style car). And as there is no engine or Rad to help absorb impact, needs to be stronger structuraly.

Side impact forces are the hardest to design against (just ask any major car producer) as there is a lot less material between occupants and object of impact.

Sorry if this is worded strongly or a bit repetative, but it is an area that does not get enough attention in my opinion.

Terry

A car that is designed to protect a driver from an 150mph crash is bound to be too strong in a 30mph impact. Horses for courses.

[Edited on 19/1/03 by Bull]

Rorty - 20/1/03 at 05:28 AM

Bull:

quote:

I understand your point, but where does it say that the chassis is non-deformable?

Actually, a few posts back you did state:
quote:

I'd want the pontoons (for example) to stand upto as much as the rest of the chassis.
and I presume you dont plan on a wholely deformable chassis!


All that aside, if you make the pontoons from the same material as the rest of the chassis, or at least the same strength, then the pontoons will not be serving any purpose whatsoever (asuming they were designed initally for side protection). You may as well not have them.
If, on the other hand, there are even 6 inches of absorbtion built into the pontoons, then you're ahead.
Unless you're unlucky enough to get T-boned at a junction, you won't see the same impact forces on the side as you would in normal driving situations.
I have been T-boned a few times on track, and the saving grace has always been my aluminium nerf bars. When one intruding car did actually get as far as my "safety cell", the chassis didn't crumple, as I always put a floor tube from one side to the other, in line with my hips/elbows. The rollbar was also positioned to stop my head from swaying too far to the side. It still hurt though!
Just thinking out loud, but if the pontoons were filled with foam (with a thin sheet of ply/alloy on the pontoon side of the main tub), they would absorb quite a bit more.
Ahhh! Unless for course you were planning on housing rads etc in the pontoons. I should think before putting finger to keyboard!

Spyderman - 20/1/03 at 12:57 PM

Rorty,

I think you are being a bit over critical.
Agreed my wording or method of explaining may not be the best, but my intention is to make the structure as safe for the occupants as possible.

You use my quotes to try to contradict what I mean when they say nothing of the sort.

Let me spell it out for you!
"I'd want the pontoons (for example) to stand upto as much as the rest of the chassis. "
Means I'd want them made from the same materials and have the same properties. As there would be a lot less material in the side structures than in the rest of chassis in front of occupants, surely it stands to reason they will not withstand the same amount of force.

Quote! "You may as well not have them. "

Things may be ideal where you live, but in this country most driver/occupant deaths on the road resulting from collisions are due to side impacts. Either with other vehicles or objects such as trees, lamp posts etc.

Being T-boned on a race track is far less dangerous than on the streets by the simple fact that you are all traveling in the same direction. As most of us will be using our cars on the streets any extra protection is worthwhile. And yes we are more likely to be T-boned on the street in a vehicle that is harder to see than most others.

The idea of putting reinforcement into the pontoons is in my mind there to create an absorbtion zone. What other purpose could it be for if not structural or as storage.

The fact that you can use the space for other things as well is a bonus.

We are not talking about designing or building the perfect vehicle for every occasion.
The idea is not to decide who is the smartest or dumbest, but to help each other out surely.

In general if I seem over critical then I don't mind being reminded of the fact. However I do mind being shot down for having good intensions, but using the wrong terminology.

Let's keep this friendly!

Terry

kb58 - 21/1/03 at 03:47 AM

I've been giving this topic some thought. If you have a certain amount of crash energy to dissipate over a given distance, it may be that the right way is to have the G forces be roughly constant during deceleration. This seems to point to using similar sized tubing. I haven't done that, instead having the tubes become larger and stronger the closer the "intrusion" comes to me, the driver. The idea is to gradually build up the G forces rather than have a huge 10-G jump at initial contact... I still am not sure which is better...

[Edited on 21/1/03 by kb58]

Rorty - 21/1/03 at 04:23 AM

Terry, I’m sorry you think I was being too critical, and I apologise if I was taking liberties with your sense of humour. My post was light-hearted, and only intended to be cheeky, hence:
I appreciate being T-boned on track is probably less dangerous than on the street, but when a car T-bones another (at 90 degrees to it, not parallel to it!), at around 110-120kmh, the resulting damage can be immense.

quote:

we are more likely to be T-boned on the street in a vehicle that is harder to see than most others.

That’s a valid point I hadn’t considered.

All in all Terry, with the greatest respect, the impression I (and seemingly, at least one other member) drew from your posts, was that you were staunchly opposed to pontoons acting as absorption cells, hence my highlighting your quotes, albeit, tongue in cheek.
Again, Terry, with the utmost respect, I feel your ambiguous statement:
quote:

The idea of putting reinforcement into the pontoons is in my mind there to create an absorbtion zone. What other purpose could it be for if not structural or as storage.
, further muddies the waters.
Surely putting reinforcement into pontoons is at poles with making them into absorption zones. Then you suggest they should be structural. I can’t see how pontoons can be both absorption cells and structural.
Terry, I am not attempting to be a smart ar$e, shoot you down, or turn this friendly arena into a bullring. I am merely addressing things as I see them, which will help me understand them. As for the leg pulling, it seems fairly prevalent on this forum.
Let's indeed keep this civil.

Spyderman - 21/1/03 at 08:38 PM

Rorty
Well at least we are both on the same side!

As I said, my descriptive powers may not be the best.

What I meant by reinforcement into the Pontoons, was a basic framework or box section as opposed to just an outrigger to mount the body to. Something capable of absorbing an impact and not just folding.
This is assuming that the main chassis is pretty similar to a seven style chassis. And body is an enclosed type like a Can Am.

Does this make sense?

Terry

Rorty - 22/1/03 at 04:43 AM

Perfect, and I fully agree with you!