I have run through some variations on bracing the engine bay. Here are the results for chassis stiffness in ftlbs.

book chassis:- 1155

my modified chassis design as described in earlier posts:- 2505

Both the above have standard tube R layout.

book chassis with two shorter R tubes, one on each side of the engine, to allow wide engines to be fitted:- 907

my mods but with two short R tubes as above:- 1898

Fitting two short R tubes reduces the stiffness by about 25%

book chassis but with two Y braces, one on each side of the engine, from where the H tubes meet tube Q to the tops of FU1/2:- 1215

my earlier suggestion for replacing the engine bay portion of J1/2 with 4x2 14g RHS braced to the footwell ends:- 1604

These results are from my new model which includes a better representation of the rear suspension mounts and the framework of the luggage area.

I can't believe how much 'R' affects the chassis strength.

Seems disproportionally so really.

Just purely out of interest, is 'R' therefore the single most important rail to chassis stiffness? I'm guessing that'd be the 'J' rails.


I don't know if this'd work but could those attempting to fit a wide engine replace 'R' with a slightly longer one running from the bottom of 'H' up to 'S' and 'T' as opposed to the top of 'H'?
Not sure if this would fit but just an idea I had!
Cymtriks could say whether this has any FEA benefit or not!

Cheers,

James

Cymtriks,
Interesting stuff as usual and I'll need to figure out how best to use this info for my V8 wide-body design. Howwver, from talking to a fellow V8 wide-body builder, he expressed concern over another strength issue which I'm not sure if you are covering here.

He said that with his engine fitted and the front wheels on the ground, if he jacked up the rear of the car and put it on axle stands, he could visibly see the engine bay "banana-ing". Hopefully you can understand what I mean by this but basically tubes J1 and J2 were exceeding compression capacity and starting to buckle a little bit.

Does this make any sense? Is the extra weight of the V8 (Rover V8 btw) too much for J1 and J2?

I'd appreciate you comments on this because it seems to me that chassis "twist" is very important but bending loads resulting from basic static gravitational forces are perhaps even more so. Note when I say bending I am thinking chassis "big picture" as I know that there should be no bending loads in a spaceframe... (Is this naive?)

Cheers,
Craig.

Cymtricks,

I'm hoping to get an MK Indy, a variation of the locost, will the chassis mods in one of your earlier posts provide similar gains to this chassis (someone gave a more detailed description here(post at bottom of page))

Another thought I had, would the closeness of riveting of the ali on the side panels/floor change the tortional rigidity of the chassis or not noticably. I have read most of your posts, sorry if I've missed something.

Ned.

The most important tubes are probably TR1/2 as without these the front of the book chassis is seriously short of stiffness

The engine bay diagonals (Tubes R, G1/2 and TR1/2) are highly stressed so removing any of them may have a drastic effect

regarding V8 weight it is probably a case of tubes TR1/2 being too long as much as anything else. Tube length is often more important than cross section where long thin tubes are concerned. I suggest replacing TR1/2 with three tubes on each side:-

1) from where Q meets J1/2 to the engine mount base.
2) from where FU1/2 meets J1/2 to the engine mount base.
3) from the engine mount base straight up to join J1/2.

All the TR tubes should be either 16g 1 inch square or 14g 1 inch round.

Replace the mount plate with a tube connecting F2 and G2 and, if it will fit, add a tube from the inner end of this tube to J1/2.

In the book chassis J1 and 2 are long enough to be Euler columns which normally means too long in general engineering.

A further improvement, probably important, is to increase the depth of tubes G1/2 to at least 1-1/2 inches deep.
My suggested mods should work on any seven type chassis which hasn't already got them.

Riveted floor pans are not safe with a book chassis! I've heard of them falling out. The twist of the book chassis over bumps can work the rivets out of their holes over time. A high stiffnes chassis or a welded in floor solve the problem.

Cymtriks,
Good stuff as usual and ironically I am just in from the garage having built my engine mountings for my V8. I ended up with a design which sounds exactly like your latest recommendations. I have put in an extra vertical tube between F1/2 and J1/2 in line with the engine mount and then welded in a tube from half way up this new tube across to G1/2. This tube has two crush tubes welded in which bolt to my engine mounting plate. I am using the standard Rover SD1 mounts and they have two holes through which bolts hold them to the crossmember. Effectively I have made a crossmember similar to that found in many RWD cars for engine mounting. Here is a sketch, I'll upload a picture when complete. I intend to replace TR1/2 with multiple sections as per your description.

Thanks again,
Craig.

Here's a picture of the engine mounting arrangement. Note that I've not put in the triangulation yet and FU1/2 are not fitted yet. I think that everyone should add the extra vertical tube rather than have very long TR1/2 tubes.




Btw, my top tube LC isn't bent, it's just due to me taking the picture so close and getting some distortion. There are more photo's of this on my website if anyone is interested.

Cheers,
Craig.

I thought that using tube as the main side diagonals was a bit whimpy for my V6 cologne.

So, I used inch RHS formed into X sections instead. The engine mounting rails were also doubled up, being 2 ins wide and 1 ins thick - 2 1 inch sections welded together.

The front RHS side sections cross over in front and behing each other, with a 'filler' piece welded between them to complete the X.

I wonder what affect this has? At present I have one 'short' R tube on passenger side....another could be added later on driver side.

atb

steve
Rescued attachment braces.jpg