Suspension Options for Light Vehicle: Help for Building Off-Road Vehicle

[fatcat]

Can anyone help me? I want to make a light vehicle (4 wheel, steering column not quad-type) capable of going on and off-road. I am confused.
Quad bike people are getting excited because apparently they have only just heard of independent suspension, it being the big new 'feature' that is available. Road users would be surprised at anything but. Yet off-road 4WDs/SUVs seem to be predominantly solid axles with leaf springs and archaic ladder chassis. Toy off-roaders have independent suspension (Freelanders et al), real off-roaders use solid axles.
(a) Why do some vehicles persist in the use of solid axles? I presume there must be a reason...? IRS must give some advantage somewhere along the line, or it never would have been adopted? Have I missed something elemental? I naturally thought it would need IRS, am I wrong?
(b) Why are off-road vehicles undesirous of the chassis developments seen as essential in other vehicles? Ie the ladder chassis, apparently as solid as a sheet of margarine, seems to be alive, well and positively brushing off newcomers. Is the chassis flexing like mad seen as a good thing? Do I forget the nifty spaceframe backbone chassis I've been doodling, and just order a big rectangle from an agricultural engineering co.?
(c) If so, do I hang the bodywork from the chassis on bungees and hope for the best? Engines don't bend very well either - does the engine hold the thing together? I'll be needing some bigger engine mounts (on the engine) then!
(d) Is it obligatory that the 'traditional' chassis weighs 2 tons?
(e) What is the verdict on the leaf-spring v coil-over debate? I presume the coil-over wins, but you never know. Toughness with articulation is what I'm after - is IRS just too weak when it comes down to it? Vehicle weight or just the stress of hitting rocks?

I have no problem either way - I like simplicity, and it fits in with the ethos of my idea perfectly. I am just confused as to how I decide which to go for. I am not building a hot-rod, I am not going for prettiness first. Latest technology is no good if there is no requirement for it. Which system? I have a target max kerb weight of approx 400kg unladen, engine power options from 50-120 bhp. Must be able to roll/slide/drop 20ft and brush it off, be capable of towing its own weight, max off-road speed 60mph. 9'x5' outside dimensions.

This is an idiotically simple problem, yet it affects the whole chassis design, obviously. I can see the wheels, the seats, load facilities, everything. But the undercarriage - at first I had no idea, now I know about several potential candidates suspension-wise I am even further from an answer.

If anyone can give me any pointers I would be most grateful. This is beginning to drive me nuts. I had an idea, have spent a long time looking into all the options, have driven my family mad with it. I just have to decide the underpinnings to sit beneath the concept I want to sit on top of it SO ONE CAN ACTUALLY BE BUILT AND I CAN RELAX!

Help? This could really send me to the wall soon. Thanks.

 

[wimms]

I'm not exactly qualified, but I'll give my opinion.
Off-road vehicles are meant to last.
a) solid joints are stonger. Less moving parts, less things to brake apart. When climbing over rocks, quite some stress on axles to tear them apart. But if you build it, you can rebuild it, so performance might be your focus.
b) not sure, but guess that (re)flexible stuff is less prone to crack, spreads loading more evenly over whole frame, thus avoids hotspots of unbearable stress. If you have solid chassis, more work to suspension.
c) its not individual feature, imo. Its all about balance. tensegrity comes to mind. Engine obviously must not face any of the body stress.
d) no, its exploiting of stupid consumers. Confidence that it rusts longer due to sheer mass.
e) coil wins performance, imo, but its not that needed for slow off-road. There might be more benefit in simple things, that are possibly "fixable" by handy stuff when you manage to brake it.
Independent suspension has one issue - when you hit smth at speed with one wheel, its prone to turn over the chassis. Not good. Completely independant suspension should be damn smart (active?), well over target of traditional offroaders.

There is a lot of science in it all, so google more. From what I've seen, I liked Hummer design, and also saw a funny vehicle on tv made by McPherson for Dakar, they had quite fancy suspension. Look at buggys.