LocostUSA.com

That is so backward to me.

Most of the weight and the forces are on the bottom rails, hence they should be the larger.

The motor & trans are mounted on the bottom. The seats are on the bottom. The gas tank is on the bottom. Most of the forces of the suspension are on the bottom A arms.

Yes, you try to spread the loads out with the other tubes but they more or less support the bottom rails.

Maybe I'm stupid for not understanding, but what you guys are saying is so counter intuitive that it makes my brain hurt.

_________________
mobilito ergo sum
I drive therefore I am

I can explain it to you,
but I can't understand it for you.

Carguy, think of the whole frame as a beam, transferring loads out to the front and rear suspension.

All of our tubes are the equivalent to what's left over after you drill or cut lightening holes in a beam that's too 'fat' for its job. Yes, the engine mounts do put local bending loads onto the lower rail, but the lower rail's predominant load is tensile, from chassis bending, in the same way that the upper rail will carry compressive loads. Similarly, the passenger loading is seen as a bending moment on the overall chassis. But in both cases (engine and passenger), after the bottom and side skins are welded or riveted on, any point loads are distributed more widely than just the attachment points of the engine mounts or the seats, and the chassis behaves like a deep beam, not a collection of little beams. Detail design still does count of course: engine mounts, for example, need to be adequately designed and constructed to avoid fatigue failure.

Of course, you could go so small (or thin) on the individual elements of the space frame that you could get local failures, but that sort of structure might challenge the welding skills of most of us. I recall reading that some competition Lotus 7s had 18 or 20 gauge chassis tubing for reduced weight, but suffered from frequent cracking at welded joints. Not a good trade-off!

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

your not alone

i prefer to base a frame on strong (thicker wall) bottom main tubes (but i`m off-road biased)
more dent/damage resistant if subjected to ground contact, overall CofG is better ,
it can take punctual loads better (like LCA mounting points or bulkheads),....

i rather go progressively lighter (thinner) on everything up top (obviously not including the rollbar)

_________________
- Stephan -

That makes a certain amount of sense, but . . . it still doesn't seem like it answers the whole question of loads. Even if that were true it still seems like the stiffer, bigger, whatever bottom rail wouldn't be overkill.

_________________
mobilito ergo sum
I drive therefore I am

I can explain it to you,
but I can't understand it for you.

thinking of are frames as a beam. OK so is that front to rear strength and torsional . I would think torsionaly a beam would not be a ideal structure . so then do you flip the car on its side and re evaluate it to improve strength torionaly.

by the way just got the soft ware to read final thesis A and B docs as well as kitcaranalysis doc. going to have to read these a couple times to get all that. then on to the next redesign.

hopefully this subject will be kicked around a little more and some research on the subject can be revealed for every buddy.



thanks Harold

Harold,

As a closing comment .....

If road rash is a concern (more for rock crawlers than sevens I think, but there are those wonderful pictures of sevens 'flying' off bumps in the road) then by all means increase the size of the bottom rail. Sure, it will 'unbalance' the extreme stresses in the 'overall' beam, but it will not be the end of the world.

With respect to your last comment about torsional stiffness, recognise that no seven chassis will be ideal in this regard because of the open-topped engine bay and cockpit. That is why you need to include as much diagonal bracing in the engine bay top as will fit around the engine. It is a supplemental benefit of roll cages in race cars.

Finally, about point loads. Text-books (Costin and Phips for example) will tell you that they should be directed at nodes to avoid applying bending loads to local elements. Your engine mounts are clearly a problem in this regard, a problem easily rectified by adding two diagonals as in the sketch below. If you look at most build logs, you will see that few of us satisfy the ideal of feeding suspension loads into nodes, but fortunately most suspension loads arrive on short, and therefore relatively stiff local elements of the chassis.

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

Ok, The latest offerings. I haven't got the 3D model dun yet but I will real soon. Until then hears what I have so far. please try and use your 3D imaginations.

Pic 1. The floor 16ga sheet with cut outs in it for the oil pan, bell housing, front U joint, rear U joint and rear diff.

Pic 2. The base of the chassis 1x1 16ga triangulated. Just realized I haven't put the motor mounts in completely yet but you can see where they go.

Pic 3. The side view of the chassis 1x1 base, 1.25x1.25 top, 1x1 uprights and braces, laid over the trans tunnel. Xsed the side to maintain load travel. May need to move the X back one more bay but still undecided. Added one more upright at forward motor mount. Stuffed a fuel cell in trying to get some idea of how much fuel it will carry.

Pic 4. The top of the frame 1.25x1.25 and the top of the trans tunnel. I think I used 1.25x1.25 on the trans here and 1x1 in the other drawing. haven't decided yet what I'm going to use but I'm lean towards 1x1. Replaced tube R in THE BOOK or tube TR5 in the other book with Y's on both sides and moved the V part of the Y back to give more room for the V8. Also tried to get some triangulation out of the trans. tunnel while keeping then perpendicular to the bottom for ease of tinning later.

So that's what I have so far. I'm going to try and convert to 3D model in the next couple of day. If any one see's any thing odd or out of whack please let me know!!!!!!!!

Thanks again, learning a lot Harold

imho with the fully welded steel floor you could skip some bracing ... the floor will be the brace in itself (stressed skin)
however, that only works with small "open" areas or you will encounter "bucking" or "crumbling" of the skin

btw
one thing that also always bugged me about the center tunnel is the waste of space by building it with tubing
i would seriously consider to replace all that tubing with a formed sheetmetal tunnel ...
would not be any heavier then all the tubing, but very low profile.

_________________
- Stephan -

I read something about no tubing just steel plate. cant remember were

So something like this out of 16ga would replace all the tubes I'm using for the tunnel.

Harold,

I like that full floor panel but it may need to be cut back a bit in way of the bell housing. Tube C (the transverse tube at the aft end of the bottom of the engine bay) is cut away in the book plans. You may not need to cut it away for an installed engine/trans unit, but you will likely need to cut away tube C (in book terminology) or tube Q (I had to cut away both) to install your engine and gearbox as a unit. (The Nissan 6-speeed on my SR20DE is very long an it is difficult to manoeuvre it all in at once.)

I started with a full floor in the transmission area and then nibbled away at it with the engine and gearbox under the shop crane. Of course, I cut away too much - you might be wise to test fit with a cardboard template in the tunnel area.

If you have those two diagonal members running down to the engine mount I am not convinced that you need the extra vertical member as well: that vertical is also transferring load into the mid-span of a member, rather than a node.

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

(Double post - deleted)

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

yes... thats basically the way i`m going to build the frame for my Vette C6 drivetrain/`54 chevy body project.
but i have some home made radius benders (~1.5 & 2.5 radius) so i`m going to make the top bends well rounded for passenger comfort.

i`m thinking to probably use 14 ga (still up in the air... i didnt start the 3D modeling yet)
for torsional rigidity i plan to bend a flange at the bottom to "box" the tunnel with a removable .095" or .125" 6061T6 aluminum bottom plate

_________________
- Stephan -

How about two hoops one over each U joint with just an aluminum cover in the middle. I haven't done any analysis of those backbone arrangements but the Lotus 30 used one and it was maybe the worst car Chapman ever made. Too Floppy. You need to maximize the section.

this would defeat the purpose ; the U joints are the widest part of the driveshaft

the idea is to shed "fat" in that area ... a slender plate tunnel could easy gain you 2" of precious seat/interior room

_________________
- Stephan -