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The final alternative is to dispense with the frame altogether and make the tunnel from 16ga sheet only welded in place. This is reputed to be just as stiff as, if not more than, a tubular frame with riveted skin. Its what I intend to do.
Bruce

Well firstly it doesn't make sense to say a box is stronger after you take away not only it's supporting frame, but also it's corner gusseting (and that offers 100% gusetting in this case), it just doesn't pan out regardless of what a computer program says.

A box* can be as strong as a skinned frame but the box will rapidly deteriorate in strength once deformation occurs whereas a frame doesn't suffer anywhere near the same deterioration.

*BTW, that's refering to a box with structural pressings such as ribs or 'X's, a flat sided box will not compete and there are plenty of real world examples starting with a shipping container.

FWIW, I designed a 6m x 2.7m x 2.7m coal transport bin earlier this year and it had to be as light as possible, be manipulated by wharf crane without spreader with 35 tons onboard and a skinned frame was the only way.

This is such an interesting issue. For myself, I'm a "tunnel believer." That is, I think just a simple sheet metal piece is not adequate and that a tube&skin or RHS&skin structure is a benefit. I don't have any proof of that, it's just based on my personal experiences and beliefs. I'm prepared to change my mind if I see good evidence to the contrary.

I've actually had the opportunity to look through an original Lotus 7 with the tunnel of sheet metal, which had the original low-powered I4 in it. It carried the transmission mount plus some degree of structural loads. The car is owned by a local aircraft mechanic and he had reinforced the tunnel himself. Interestingly, he built the car from a kit as a teenager while he was working for a Los Angeles area Lotus dealer. I took that as 'prima facie' evidence it wasn't adequate even in that early design. I looked for my digital photos of his car this morning, but couldn't find them.

From what I know at the moment, FEA analysis of thin structures including primarily sheet metal require some particular kinds of FE meshes, very carefully arranged in three layers. Most FEA programs do not do this by default and very few (just the top-tier ones) give you the freedom to do it on your own in the required way. And, you have to know what you're doing to get accurate results. So, I'm somewhat doubtful of the results I've seen from amateurs in terms of FEA analysis.

I think a good sheet metal structure CAN be developed that's every bit as good as a tube&skin one, but I don't think just simple, 2D sheet metal bent in the form of an inverted U with simple flanges at the open end is that piece. Someday I hope to be smart enough with FEA to prove the point, but I can't today.

Cheers,

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886

...this was the basis of the Centaur back in the 70/80's.... it is monocoque construction with no frame at all....I'm assuming the tunnel is a major strength factor....it is welded to the floor/firewall/rear cockpit panel..... certainly a solid structure.
The side panels of the engine compartment have the steel sheet bent into a square tube along the top (hood line) and I assume the bottom...that gives the strength to that area......

Seems like a pretty good design....

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gblawson (Gordon)
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http://gblandco.com/gb/cent/cent.html

With respect Cheapracer a coal bin is hardly a fair comparison as you are dealing with a whole heap of different loadings, but since you mentioned it I understand completly why you would have needed a frame or ribs. You would have to admit though that the most important part of the frame would have been the part around the top or opening, you would have needed some vertical components but the bottom corners would have needed little if any frame. Turn it upside down and weld the opening to a frame that is in the car and we have a frame where it matters most. My thoughts are at this stage are, to facilitate driveshaft removal, that there will be some bolt in cross bracing across the bottom of the tunnel. From this point of view the tunnel becomes a large tube and I feel that it should be just as stiff as if it had a frame inside it. The top will be curved so should be stiff enough on its own, the sides (being flat) may benefit from some extra support and as room will not be that tight there is no reason why some X ribs could not be stich welded inside. This reasoning is mostly intuitive on my part so would be prepared to listen to any good arguments to the contrary and also change my design to suit.
Bruce

Since you're so nice I have passed it over to a Naval Architect mate of mine to run the FEA numbers properly for you.

Can I sneak in here?

A bolted in tunnel formed from sheet or plate is potentially as strong or stronger (or weaker if the thickness is inadequate) but the temptation would be to have this tunnel removable for access. In such cases, fastening will become critical and examination of (old) riveted ship structures or riveted aircraft structures intuitively suggests that we might need more fasteners than we want to have for convenient removal.

When I compare my boxy tube-framed tunnel to the (useless) removable round-top GRP tunnel on my friend's Birkin, I love the the appearance and lack of bruising edges on the Birkin variety. I would be inclined to use a formed steel tunnel in a future life but would probably weld it in - but assembly and maintenance of mechanical components would then be a stinker.

I suspect that I'm not adding anything new to this argument.

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Warren
Isuzu Pickup/SR20DE, +401 COLD frame
Build Log: viewtopic.php?f=35&t=11601

I don't think there's any issue about a sheet metal structure being the equal of a tube & skin structure when designed and built by the right party. My favorite example to prove the point is the GT-40 chassis structure. It was done ~40 years ago in thin mild steel (22 & 20 gauge?) and yet is stronger in many respects than modern chassis built of carbon fiber and/or high strength aluminum. We'll have to leave crash-worthiness or impact resistance out here in fairness because those things were not real considerations in a 60s race car.

However, I haven't seen a Locost (meaning a Champion or Haynes chassis) yet with something I consider convincing in terms of a sheet metal tunnel. FEA would be the first step in getting to a plausible design and then building and testing real examples would be the proof of the pudding. I know people talk about the later Caterhams as examples of the minimalist tunnel, but that is a different animal all together and designed by professional engineers using sophisticated software and built (the chassis anyway) professionally. It's not a fair or appropriate comparison IMHO.

Without some real proof, I think this argument can easily devolve into, and it has in the past, the equivalent of 12 year old kids arguing about whether Godzilla could really beat King Kong in a fight. Eventually, someone may take on this challenge for real.

Cheers,

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886

Yo, Lonnie-
I'll take on the challenge for real. I think I could whip Godzilla's arse on one of my good days. Not sure about King Kong, but I'll give it a try.


JDK

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JD, father of Quinn, Son of a... Build Log
Quinn the Slotus:Ford 302 Powered, Mallock-Inspired, Tube Frame, Hillclimb Special
"Gonzo and friends: Last night must have been quite a night. Camelot moments, mechanical marvels, Rustoleum launches, flying squirrels, fru-fru tea cuppers, V8 envy, Ensure catch cans -- and it wasn't even a full moon." -- SeattleTom

My take on this is try and look at the bigger picture then just the tunnel. I think you a re trying to also. So among all the things you would like to accomplish I would prioritize occupant safety, which pushes me to put more material on the outside of the car. A simple thought experiment here is that if you remove one piece of 1" square tube from the tunnel, that's enough material to increase the size of the chassis rails by 25%. That would increase their stiffness and strength by a factor of 2.5X.

You can see where this is going. Using material to increase the diameter of chassis rails swamps any other effort. There is enough material in the tunnel tubes to both increase the diameter and the section of the chassis rails. Also to raise the upper rail a bit.

Protection from the driveshaft can be provided by 1"x1/4" hoops with an attractive bend on their top. These hoops make a nice way to bolt in a sheet metal cover welded to them. Less mounting hardware because the sheet can be well attached with welds to the hoop and the hoops are strong enough for fewer bolts.

That's thinking outside the tunnel!

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Marcus Barrow - Car9 an open design community supported sports car for home builders!
SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
Car9 Roadster information - models, drawings, resources etc.

Yup - Go out and lift up the carpet of your everyday car or look carefully around the engine bay, car manufacturers are the experts at manipulating sheet for the least weight, cost and maximum strength and you won't see a flat panel anywhere.

Problem is a Locost structure'less flat panel tunnel wasn't designed by the "right party", something like this would be a better design with 'X's (or ribs) stamped or beaten into the sidewalls. ..

That's good, JD. Of course I meant the challenge of designing a sheet metal tunnel and comparing it with a tube & skin. But, that was good. I'm laughing.

OK, so now, what if Godzilla, was still in a gang with Rodan and Mothra? Think you'd be talking arse-whipping, Godzilla trash then? I don't think so.

Cheers,

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886

That's a very interesting point of view, Marcus. That's a very interesting figure, the 2.5X one, that I've never seen before. It's pretty dramatic and I'm going to assume you've done your homework and accept it. I see lots of people obsessing about weight. I understand the argument and if you're building a race car 30 pounds can make a big difference, all other things being equal, with respect to your competitors. Most of us are building street cars and I think strength and safety are the first concerns and an extra 30 pounds (what I added in steel including size and gauge of outside rails) is not a big issue in that context.

First, we could all lose 15 pounds and get half back immediately and be healthier too. Second, we can probably all get 15 HP by careful and minor work on the engines and still be ahead in terms of the most basic indicator of performance.

Case 1) 1600 lb/ 150 HP = 10.67 lb/HP

Case 2) 1600 + 30 -15 = 1615 lb/(150 + 15) = 165 HP = 9.79 lb/HP

My other "back woods" reasoning is that with a better tunnel (compared to a simple, quasi-structural sheet metal one) you are essentially taking the worst performing area of the Locost chassis, the passenger compartment, and turning it into two improved "cells" that will be obliged to give you better over-all chassis performance no matter what the particular numbers in bending and torsion prove to be. I'm assuming you do something sensible and not totally silly with the tunnel.

I also accept that someone could come up with a nice, sheet metal structure that would accomplish the same thing. I just haven't seen one yet on a Locost.

Cheers,

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886

When you change the thickness of something, it gets stiffer by the cube of it's thickness. So a 2"x8" piece of wood should be 8x stiffer then a 2x4. If you go from a 1" square tube to a 1.25" square tube it is not only thicker in section it is also wider. So it turns out to be ( 1.25" / 1" ) ** 4.

I'm not really arguing for minimum weight here, I agree it doesn't really matter. If you haven't welded in your upper chassis rail yet though, you might consider picking up a little 1.25" tube for that.

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Marcus Barrow - Car9 an open design community supported sports car for home builders!
SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
Car9 Roadster information - models, drawings, resources etc.

IF you had the room inside the locost tunnel im still thinking a torque tube inside the tunnel
or a 1/8'' plate rolled into a tube long enough to cover the front universal and front part of drive shaft and the same at the rear only oval to allow for suspension movement..If its not gonna be a track car the extra weight shouldnt matter..it would give you the ultimate protection if a uni joint ever exploded or disconnected from the driveshaft

PS i think if JD and i were to join forces as the new super hero team.. slocostman and goatboy..we could beat both Godzilla and king kong

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I USE TO HAVE AN OPEN MIND ..BUT MY BRAINS KEPT FALLING OUT