22 replies to this topic

[mcerqueira]

Been searching around trying to compile a list of the types of chassis used on cars. There's come conflicting info, but here's what I compiled:

- Ladder Chassis
- Backbone
- Tubular Space Frame
- Monocoque

Being that the monocoque can be divided in several sub-types such as unitary, carbon, etc?

Feedback much appreciated

[Greg Locock]

Been searching around trying to compile a list of the types of chassis used on cars. There's come conflicting info, but here's what I compiled:

- Ladder Chassis
- Backbone
- Tubular Space Frame
- Monocoque

Being that the monocoque can be divided in several sub-types such as unitary, carbon, etc?

Feedback much appreciated

Don't see many true monocoques around. (deliberate attempt to move off-topic)

OK, you missed unibody, but that's all I can think of.

[mcerqueira]

Don't see many true monocoques around. (deliberate attempt to move off-topic)

OK, you missed unibody, but that's all I can think of.

Isn´t unibody a type of monocoque?

[ray b]

Isn´t unibody a type of monocoque?

unibody is a formed sheet with connecting braces
skin can be unstressed or stressed

monocoque is stressed skin construction like aircraft with bulkheads and ribs

so most F-1 cars recently are more unibody with some unstressed skin panels

[Greg Locock]

It is very hard to design a car that really uses stressed skins to carry most of the loads, although these days it is probably easier than before.

To be honest the only vehicles I can think of that really resemble monocoques are airships, balloons and submarines. Rigid airships cheated by using stringers and ring bulkheads. If you allow that and then say just that the skin must provide some shear capacity then it gets easier, ships, boats, and aircraft would qualify. And some cars.

The problem with cars is that loads are very concentrated, so your structure starts out by being very strong locally. Feeding those loads into a stressed skin takes a lot of thought. In a modern production car probably the only skin panels that are important structurally are the front and rear windscreens(I think) and the floorpan. The other problem is that the skins are not continuous because of doors and so on.

GM used to have a design philosophy that they scarcely ever used called membrane theory. This was used to shape the panel so that all the loads were taken in tension in the plane of the curved panel. I've only seen it done for a spare wheel well. If you were to do the same for the external form of a car then it would look very odd.

So you end up with a problem. The reason you'd go with a monocoque is light weight, and design elegance. But if you shape the skin to take the loads, it would no longer look very good, and would not be aerodynamic. If you make it the shape you want for aero and looks, then you need a lot of bridging structure between the hardpoints and the stressed skin, so it gets heavier.

If you have access to a shape optimisation program it would be quite funny to set this sort of problem up and let it rip. If you ignore crash I bet you end up with something that looks like a ring bulkhead at each axle. I don't know what you'd get between the two bulkheads.

Edited by Greg Locock, 24 October 2009 - 23:07.

[Tony Matthews]

It is very hard to design a car that really uses stressed skins to carry most of the loads, although these days it is probably easier than before.

Fascinating, Greg, thanks.

[NeilR]

Twin tube chassis? Ala the FSAE car that used twin carbon tubes.

[mcerqueira]

Whoa! Excellent, although you left me stranded long back! :-)

What I'm trying to do is establish the types of chassis that one can (broadly )apply to cars along history, say:

- Ladder: Land Rover...
- Tubular Space Frame: Porsche 917...
- Monocoque: Porsche Carrera GT?...
- Unibody: most modern cars?

etc, so if I wanted to use a list to apply, this would be complete?

- Ladder Chassis
- Backbone
- Tubular Space Frame
- Monocoque
- Unibody

BTW, in what group would the Lotus Elise "tub" aluminum chassis be? Monocoque?

[Greg Locock]

I reckon twin tube is a ladder chassis with a stressed body... which is pretty much a unibody.

Ohh, Elise is a good one... unibody I reckon.

The torque transfer in a normal unibody is done via the tunnel rails and the rockers. I don't think either of those examples really evade that description.

it's all shades of grey, admittedly.

[Cirrus]

it's all shades of grey, admittedly

It could be argued that the Lotus 25 "monocoque" was in fact a twin-tube ladder chassis, with the driver sitting between two big aluminium tubes

[J. Edlund]

It could be argued that the Lotus 25 "monocoque" was in fact a twin-tube ladder chassis, with the driver sitting between two big aluminium tubes

Lotus Elise is similar to that. Two rectangular aluminum tubes with the driver and passenger placed between them.

[mariner]

Two thoughts

1) A lot depends on how you provide for the varying required levels of strength from the hard points etc. If you use something like glass fibre composites you build up the strength with homogenous layers. That goes right back 50 years to the Lotus Elite or the Chaparral 2. Same with a modern F1 car. If however the material is non homogenous e.g aluminium you can't so easily build up layers so you go to stringers etc. to get the stiffness required at various points.. That is how aeroplanes were built for 70 years+ since alumimium was the best all round material.

So if you look at a Boeing 767 - alumium and a 787 ( when it flies !) - composite you could say the 787 is a monocoque and the 767 is not but I do't really think they are very different since both provide strength via an extremely thin skin with some hard points. BTW in the London Science museum they used to have a cross section of a 747 which really made you realise just HOW thin the skin really is when youn are sitting inside it at 35,000 feet.

2) For those who like different things railway(road) coaches are also interesting as they have to cope with some pretty big end impact standards plus heavy tractive loads from the motor boigies.so are an interesting comparision to a car shell with somewhat similar requirements. They are sort of half way between cars and aircraft in terms of two of the key determinants , aperture size versus skin area and point loads.

[Greg Locock]

Open cockpit cars are heavily compromised by the hole in the top. can anyone think of a succesful open cockpit car that really didn't use torque tubes (maybe Birdcage is one). Costin+Phipps probably has some lovely drawings showing this I'll have a look tonight.

[kikiturbo2]

Open cockpit cars are heavily compromised by the hole in the top. can anyone think of a succesful open cockpit car that really didn't use torque tubes (maybe Birdcage is one). Costin+Phipps probably has some lovely drawings showing this I'll have a look tonight.

gordon murray's LCC Rocket has a real nice structure around the cockpit... I'll try to find a pic...

on my project I used a much simpler method of regaining the stiffness lost by the opening... but it works..

[Tony Matthews]

on my project I used a much simpler method of regaining the stiffness lost by the opening... but it works..

You mean the driver acts as a structural member? Cool!

[kikiturbo2]

No, driver is used as a crash test dummy..

[Bill S]

If you have access to a shape optimisation program it would be quite funny to set this sort of problem up and let it rip. If you ignore crash I bet you end up with something that looks like a ring bulkhead at each axle. I don't know what you'd get between the two bulkheads.

An utterly daft question, if I may, Greg.

Could you make a successful car that had very little chassis torsional rigidity? I mean, basically make each end work as well as it could but the chassis in the middle ... flexible in torsion/roll but not in longitudinal flexing?

[carlt]

An utterly daft question, if I may, Greg.

Could you make a successful car that had very little chassis torsional rigidity? I mean, basically make each end work as well as it could but the chassis in the middle ... flexible in torsion/roll but not in longitudinal flexing?

Morgan +8

there are some extreemely quick hillclimb Morgans with very fat slicks [ porker turbo 911 , tvr tuscan , westfield 8 etc competitive] , they handle really well with a very flexible chassis and rear live axle [cart springs ? ]

Edited by Pascal, 06 January 2011 - 09:57.

[Greg Locock]

Not daft at all. We built a Lotus proto with rubber mounts between the front subframe and the spine chassis and the rear subframe and the spine chassis (SID). I don't think it ever ran with soft mounts in those locations, pity really. I left just after it first ran, didn't get time to play with it. I vaguely remember the Chev Indy showcar used the same idea, but it was still bloody noisy. What we were doing was looking at how to mount 4wd powertrains in mid engine cars, and playing about with suspension isolation at the same time. So the engine was mounted fairly firmly into the spine, and then the spine was supposed to be isolated from everything else (hah). Dave W or Richard Hurdwell can probably describe the practical result of this better than me, I was mostly concerned with noise not handling at the time.

So in practical terms what would happen is that roll is no longer used to transmit information along the car, so the rear end will be much 'slower' to react to jacking of the body by the front suspension. and some other stuff no doubt.

[Bill S]

Not daft at all. We built a Lotus proto with rubber mounts between the front subframe and the spine chassis and the rear subframe and the spine chassis (SID). I don't think it ever ran with soft mounts in those locations, pity really. I left just after it first ran, didn't get time to play with it. I vaguely remember the Chev Indy showcar used the same idea, but it was still bloody noisy. What we were doing was looking at how to mount 4wd powertrains in mid engine cars, and playing about with suspension isolation at the same time. So the engine was mounted fairly firmly into the spine, and then the spine was supposed to be isolated from everything else (hah). Dave W or Richard Hurdwell can probably describe the practical result of this better than me, I was mostly concerned with noise not handling at the time.

So in practical terms what would happen is that roll is no longer used to transmit information along the car, so the rear end will be much 'slower' to react to jacking of the body by the front suspension. and some other stuff no doubt.

Interesting .....
Many years ago I came up with the idea of making a very lightweight Clubbie (Now Sports 1300 in Aus) by making the chassis just three long tubes, two along the bottom and one on the top so it'd be stiff enough longitudinally but not very good in twist. The ends were all braced-up to takes the loads from the suspension but in between there was only the three tubes.
Got poo-poo'd by everyone so I didn't go any further than the wood model I made.

[cheapracer]

Got poo-poo'd by everyone so I didn't go any further than the wood model I made.

Which I'm sure is why today we haven't saved the planet, got to have a thick skin to be an innovator.

Many an older race car/sedan does well in historic comp setting very respectable modern lap times with a twisty chassis.

[Bill S]

Which I'm sure is why today we haven't saved the planet, got to have a thick skin to be an innovator.

Many an older race car/sedan does well in historic comp setting very respectable modern lap times with a twisty chassis.

Yes, a while back I decided to incorporate the television method of science, where I keep finding new engineers, until I find one that agrees me, then I am right!

[gruntguru]

Which I'm sure is why today we haven't saved the planet, got to have a thick skin to be an innovator.

We can save the planet with the technology we already have - it only needs a change of attitudes. Got to have a thick skin to be a reformer.

Edited by gruntguru, 27 October 2009 - 01:33.