88 replies to this topic

[24gerrard]

From what I remember Colin had little problem with stiffness.

[bigleagueslider]

cheapracer,

If you don't mind a bit of nitpicking, here's my two-cents on the pictures you posted of a tube chassis front end structure.

First, there is the basic issue of load paths and triangulation in tube truss structures. Ideally, for optimum structural efficiency, all of the tubes should be arranged to transfer loads purely in tension/compression. Having the upper A-arms attached to a tube mid-span puts that tube in bending. There is also an issue with the upper spring/dampener attachment, the structure behind it is not fully triangulated.

Second, I don't believe the upper A-arm structure is properly constrained, since there is a rotational joint located mid span in the front leg.

Third, while it's hard to tell from the pictures, it looks like there might be some bump steer in the steering linkage geometry, due to the location of the inner tie rod joints.

slider

[cheapracer]

cheapracer,

If you don't mind a bit of nitpicking, here's my two-cents on the pictures you posted of a tube chassis front end structure.

First, there is the basic issue of load paths and triangulation in tube truss structures. Ideally, for optimum structural efficiency, all of the tubes should be arranged to transfer loads purely in tension/compression. Having the upper A-arms attached to a tube mid-span puts that tube in bending. There is also an issue with the upper spring/dampener attachment, the structure behind it is not fully triangulated.

Second, I don't believe the upper A-arm structure is properly constrained, since there is a rotational joint located mid span in the front leg.

Third, while it's hard to tell from the pictures, it looks like there might be some bump steer in the steering linkage geometry, due to the location of the inner tie rod joints.

slider

You do understand it's not mine?

1/ There's no doubt that feeding loads into nodes where it can be distributed appropriately is the best way to go but there's also a case for loads that are fed into tubes that are appropriate for the load size. Yes that could be arranged better and yes I would do the whole lot differently - but I still like the concept of the top A-arm support as it is.

2/ This is very common to get caster and camber adjustment, very common and thousands of race cars use this setup. You may be missing that it is a triangular structure and can not move. http://www.speedwaym...ieds,29430.html

3/ Wouldn't have a clue, would need more than one POV and a little math to know and even then would still come down to physical testing.

[cheapracer]

Costin's remark that Lotus Mk 8 spaceframe* was nearly perfect example of spaceframe in terms of stiffness and lightness (and best up-to-date)... I understand it had stressed floorboard and skin panels,

It may well have been at the time but of course these days after many years of development and FEA it looks a little lacking in some areas especially the open top of the engine box amongst others.

Stress skinning is not as good as bracing - of course a stress skinned tube (aeroplane, cigar racing car etc) is a different thing again.

[bigleagueslider]

You do understand it's not mine?

1/ There's no doubt that feeding loads into nodes where it can be distributed appropriately is the best way to go but there's also a case for loads that are fed into tubes that are appropriate for the load size. Yes that could be arranged better and yes I would do the whole lot differently - but I still like the concept of the top A-arm support as it is.

2/ This is very common to get caster and camber adjustment, very common and thousands of race cars use this setup. You may be missing that it is a triangular structure and can not move. http://www.speedwaym...ieds,29430.html

3/ Wouldn't have a clue, would need more than one POV and a little math to know and even then would still come down to physical testing.

cheapracer,

Yes, I understood the design was not yours. My reply was addressed to you simply because you posted the pics.

There is a small, but very important difference between the upper A-arm design in your pictures and the design of the Speedway Motors A-arm you compared it to. As I noted in my post, the upper A-arm in your pictures is not properly constrained and has excessive kinematic degrees-of-freedom, due to the presence of a spherical joint at the outboard end of the tie rod. The Speedway Motors A-arm instead has a simple clevis joint at this point, so it is properly constrained.

Bump steer with double A-arm suspension:



regards,
slider

[carlt]

cheapracer,



Bump steer with double A-arm suspension:



regards,
slider

your diagram is showing a very poorly set up steering rack

although difficult to see in the pics the rack in cheapies initial post looks far more likely to have been set up to eliminate bump steer [except for large degrees of suspension deflection]



Regarding the suspension in the opening post -
I assume the reason to hang the top wishbones 'out on a limb', so to speak , is to attain a required ratio between top and bottom wishbones for a prerequired degree of camber gain etc
So why not keep the top and bottom wishbone length ratio the same , but lengthen them both so that the top wishbone is attached to the chassis and the lower wishbone attached towards the centre line of the chassis bottom

Edited by carlt, 28 December 2011 - 14:42.

[MatsNorway]

I assume the reason to hang the top wishbones 'out on a limb', so to speak , is to attain a required ratio between top and bottom wishbones for a prerequired degree of camber gain etc
So why not keep the top and bottom wishbone length ratio the same , but lengthen them both so that the top wishbone is attached to the top chassis rail and the lower wishbone attached towards the centre line of the chassis bottom

A mix of need for space for the feet, and if done like you ask it would alter the roll center as the arms would be need to be relocated, moving the arms would aso give less camber gain per unit wheel movement and less chassie stiffness from side to side.


I personally believe that there is rarely any gains in having a stiff chassie over a light chassie. IF there is a weight limit ofc you want it as stiff as you can within that weight limit but im guessing chapman found it faster to go lighter rather than stiffer and heavier.

Disclaimer: i have no idea what im talking about, i am actually not even a person but a simple robot that communicates to keep you guys busy with engineering discussions.

[cheapracer]

the upper A-arm in your pictures is not properly constrained and has excessive kinematic degrees-of-freedom, due to the presence of a spherical joint at the outboard end of the tie rod.

FWIW I necessarily have a similar 2 piece linkage arrangement currently on my own car's top Aarm because as I get a caster gain my short lateral links from the top of my upright down to my beam have to be able to move - you can grab my upper radius rod and rotate it a few degrees on the spherical joints with the lateral link moving up and down a bit but in reality it doesn't change the geometry at all, not one appreciable little bit - I have to think this is the "unconstrained" bit you might be refering to?

I am quite experienced with eliminating bump steer and more so than ever over the last year sorting out an ideal beam setup.

I think I got the HP Book's "How To Make Your Car Handle" when I was 15 or 16 and it all started from there ...

[Wolf]

It may well have been at the time but of course these days after many years of development and FEA it looks a little lacking in some areas especially the open top of the engine box amongst others.

Stress skinning is not as good as bracing - of course a stress skinned tube (aeroplane, cigar racing car etc) is a different thing again.

Now that you mentioned FEA analysis, here is something I've been wondering about... Somehow, I distrust the FEA regarding frame analysis- as a layman in the field, I find it hard to believe that it takes into account the buckling stresses, which should be quite important aspect of the frame design. FEA may tell you that the stress in a tube is xy MPa, but it will not tell you how it relates to 'load capacity' of that particular tube (which depends on its length, all other things being equal). Besides, it's hardly appropriate in early stages of the design- having to model the whole frame, even with modern parametric modelling,

Recently, I've stumbled on a small (and free!) program which I find ever more appropriate- it does a 'classical' analysis of a frame, calculating the tube loads and taking into account buckling... I don't think it works with gussets, but once the tubes needing them are identified by it it's rather elementary to see whether the problem can ber aliviated by using them. Here's the link if anybody wants to take a look: http://www.cuylaerts.net/ (but be warned, it's whopping 550kb large ).

[cheapracer]

Recently, I've stumbled on a small (and free!) program which I find ever more appropriate- it does a 'classical' analysis of a frame, calculating the tube loads and taking into account buckling... I don't think it works with gussets, but once the tubes needing them are identified by it it's rather elementary to see whether the problem can ber aliviated by using them.

Will try, might even be better than "Bridge Builder"!

[Greg Locock]

Classical linear FEA takes no account of softening due to applied moments etc, or buckling. I'd be a bit surprised if a car's spaceframe was operating at high enough stresses for this to matter, and still be worried about stiffness. Still, it is nice to see spaceframe analysis improving, thanks for the linky. Maybe somebody could dig up a non linear FEA of a spaceframe, or even (shock horror) a physical test showing this.

here's an FSAE report, linear range only sadly. Swinburne have got FSAE-A reporting down to a fine art:

http://users.telenet.....E Chassis.pdf

But I know my customers would not believe an FEA model that was 150% stiffer than test.

Here's a non linear FEA of spaceframes, nice and curvy, but no test results

http://kuiraq.com/li.....they maky.pdf

Yeah, bridgebuilder aka pontifex was surprisingly popular for a while. the 2d one is freeware now.

[Wolf]

Greg, your post has given me a nudge to dig out a trusty old engineering handbook (well, I'd like to get the feel for numbers, for my own sake). It seems that if we take e.g. 3/4" 17 gauge 4130 tube, free length of 22" will half the allowable stress (that would be around 37ksi)...

Edited by Wolf, 03 January 2012 - 19:43.

[rachael]

Classical linear FEA takes no account of softening due to applied moments etc, or buckling. I'd be a bit surprised if a car's spaceframe was operating at high enough stresses for this to matter, and still be worried about stiffness. Still, it is nice to see spaceframe analysis improving, thanks for the linky. Maybe somebody could dig up a non linear FEA of a spaceframe, or even (shock horror) a physical test showing this.

here's an FSAE report, linear range only sadly. Swinburne have got FSAE-A reporting down to a fine art:

http://users.telenet.....E Chassis.pdf

But I know my customers would not believe an FEA model that was 150% stiffer than test.

Here's a non linear FEA of spaceframes, nice and curvy, but no test results

http://kuiraq.com/li.....they maky.pdf

Yeah, bridgebuilder aka pontifex was surprisingly popular for a while. the 2d one is freeware now.

It's true that a linear static solution will not predict buckling but most fea codes can do a linear buckling analysis that predicts the elastic buckling load - nastran sol 105 for instance. I'd suggest that most of the struts in a typical spaceframe are too short to buckle elastically and will only buckle plastically under impact loads - Properly bent!

[Tony Matthews]

Blimey! Without knowing the severity of the impact it's hard to tell whether that is a good result or a horrendous one - it looks worse than it might, I suppose, as several tubes have been chopped out by the rescue team.

[cheapracer]

Blimey! Without knowing the severity of the impact it's hard to tell whether that is a good result or a horrendous one - it looks worse than it might, I suppose, as several tubes have been chopped out by the rescue team.

Broken legs to the driver I believe but otherwise ok.

[Tony Matthews]

Broken legs to the driver I believe but otherwise ok.

I'm slightly ashamed to admit that I was refering to the chassis and not the driver...

[Kelpiecross]

I'm slightly ashamed to admit that I was refering to the chassis and not the driver...

I think it is surprising that anyone survived this accident at all - it must say something for the progressive crumpling of the chassis tubes.

[cheapracer]

I think it is surprising that anyone survived this accident at all - it must say something for the progressive crumpling of the chassis tubes.

I agree with that, the collapsing curved tubes would have dissipated the forces slowing down the deceleration.

But bear in mind the occupants would also have been wearing 4 point harnesses which are a huge help.

[Tony Matthews]

I don't see how you can say that without knowing the force involved. If it was a 15 mph collision it doesn't look too good, but if it was 70 mph...

[cheapracer]

I don't see how you can say that without knowing the force involved. If it was a 15 mph collision it doesn't look too good, but if it was 70 mph...

It was reputed to be 100 mph, I avoided saying it as I don't believe it for a moment.

Those are 4 x 50mm tubes well braced and check out the front suspension box up there as well, so that kind of compression and damage does point towards a very hefty impact.

[rachael]

It was reputed to be 100 mph, I avoided saying it as I don't believe it for a moment.

Those are 4 x 50mm tubes well braced and check out the front suspension box up there as well, so that kind of compression and damage does point towards a very hefty impact.

I'd say that was a poor design from the aspect of frontal impact - the top tubes have failed plastically in a localised area which will have absorbed minimal energy - the plasticity should ideally be spread over a large area commonly called a crumple zone. There is a massive amount of intrusion into the footwell and the steering wheel doesn't look the right shape, Tony's right the speed of impact is the key to how good the design really is.

[Kelpiecross]

It was reputed to be 100 mph, I avoided saying it as I don't believe it for a moment.

Those are 4 x 50mm tubes well braced and check out the front suspension box up there as well, so that kind of compression and damage does point towards a very hefty impact.

This is the first time I have seen a clear picture of the Atom chassis (I presume that it is an Atom). Why are those bottom main frame members curved? It would seem to me that making them straight and at the bottom of the chassis would be a better and simpler idea.
I get the impression that they are curved just to make the car look distinctive. Maybe FEA says that curved is better? (Sad attempt at humour).
For that matter - why are the top rails curved? - there would seem to be no advantage in this.

[Wolf]

This is the first time I have seen a clear picture of the Atom chassis (I presume that it is an Atom). Why are those bottom main frame members curved? It would seem to me that making them straight and at the bottom of the chassis would be a better and simpler idea.
I get the impression that they are curved just to make the car look distinctive. Maybe FEA says that curved is better? (Sad attempt at humour).
For that matter - why are the top rails curved? - there would seem to be no advantage in this.

I don't recall aynone saying that curved is better (when it comes to chaassis tubes, at least ;)). Cooper of late '50ies (current designation of T45 springs to my failing mind), despite the objections of expert analysists, might have gotten away with it on account of other contributing factors (rear engined, and suspenion good enoug to be allegedly nicked by latter Ferraris of early '60ies vintage)... Atom, and latter KTM Bow seem to pick up on the 'trend', but my cynical point of view says it's got more to do with styling than structural considerations.

Mind you, as luddite as I get, I still might be tempted to think that curved tubes in key areas might improve impact safety, on account of dispersing the energy in more progressive way- but structurally the construction would be better off with straight tubes. Curved beams would IMHO promote less abrupt impac absorption- they get to bend whilst absorbing the impact, whereas more conventional frame with purely tenson/compression stressed tubes, would absorb impact more stiffly and then collapse in more 'abrupt' or catastrophic manner.

In a sense of impact g-loading, I do feel (albeit with no evidence save a 'gut feeling') my reasonoing is that even though straight tube my provide less average decceleration in case of impact, curved tubes will provide lower peak deccelartion- on account of failing in less abruppt, and spectacular, way (eve n though failing at slightly lower impacts).

[cheapracer]

I don't see how you can say that without knowing the force involved. If it was a 15 mph collision it doesn't look too good, but if it was 70 mph...

The owner had accelerated flat through the first 3 gears and was approaching the 'T' intersection at 90 to 100mph. When he lifted the engine didn't, jammed throttle and he applied the brakes but only the fronts locked with the engine overpowering the rears and straight into the wall they went.

[kikiturbo2]

This is the first time I have seen a clear picture of the Atom chassis (I presume that it is an Atom). Why are those bottom main frame members curved? It would seem to me that making them straight and at the bottom of the chassis would be a better and simpler idea.
I get the impression that they are curved just to make the car look distinctive. Maybe FEA says that curved is better? (Sad attempt at humour).
For that matter - why are the top rails curved? - there would seem to be no advantage in this.

curved tubes are a design feature... Truth be told, this whole car, as much as I like it, has been mostly designed then engineered.. The floor section is a joke IMHO, but it does look good..

[MatsNorway]

I don`t see no link with pics of a crashed atom..

And curved tubes makes it easier to predict what way they will go during a crash.

[Kelpiecross]

curved tubes are a design feature... Truth be told, this whole car, as much as I like it, has been mostly designed then engineered.. The floor section is a joke IMHO, but it does look good..

I don't think it even looks good.

[cheapracer]

The Atom crash we were talking about above happened in Belgium and a Belgium Journo has just been killed in one and the driver's not too good apparently...

http://jalopnik.com/...est-drive-crash

[Tony Matthews]

The owner had accelerated flat through the first 3 gears and was approaching the 'T' intersection at 90 to 100mph. When he lifted the engine didn't, jammed throttle and he applied the brakes but only the fronts locked with the engine overpowering the rears and straight into the wall they went.

Well in that case I reckon the chassis did a pretty good job. However, I think a chassis, in fact any sub-assembly or part should look good enough to hang on a wall or put in a display cabinet. The Atom chassis on its own is, in my opinion, aesthetically grim, however tough.

[Grumbles]

The Atom crash we were talking about above happened in Belgium and a Belgium Journo has just been killed in one and the driver's not too good apparently...

http://jalopnik.com/...est-drive-crash

I don't want to sound unsympathetic - I'm not - but modern performance cars and especially bikes are just way too quick to use at anything near their full potential on public roads. It's crazy to be testing any vehicle built primarily for performance on anything but a race track. I'm not saying they shouldn't be road legal but if you are going to drive or ride anything on a public road you have an obligation to do it like a grown-up. I've certainly done my share of silly driving/riding on the road but eventually you have to realise that you're pushing your luck and that it's best left for track days.

[kikiturbo2]

that last crash looks like side impact into steel structure beside the road...

I will second your opinion grumbles... below 100 mph, an atom is just mind blowingly, stupidly fast on a open road..

I had a chance to sample the atom and some fast "sevens" and it is on the open road that you realise just how fragile you feel with other traffic on the road..

[cheapracer]

I don't want to sound unsympathetic - I'm not - but modern performance cars and especially bikes are just way too quick to use at anything near their full potential on public roads.

Yup, we had recent discussions about bikes and I believe there is no place for Hyperbikes anywhere.

I was blown away by Greg recently when he said "We don't make a family car that does less than 150mph" - it's just gotten insane.

Kiki, never liked the Seven's lack of protection ..

[Wolf]

Yup, we had recent discussions about bikes and I believe there is no place for Hyperbikes anywhere.

I was blown away by Greg recently when he said "We don't make a family car that does less than 150mph" - it's just gotten insane.

Kiki, never liked the Seven's lack of protection ..

What's a Hyperbike??? As I'm given to understand Germans (BMW) have opened a can of worms by thinking they should break 200BHP barrier with 1l bikes. I'm told that Japanese have for years had 'gentleman's agreenent' about 1000cc bikes- not to go beyond 185-ish BHP, and now that Bavarians have thrown them a gauntlet, presumably all bets will be off... Judging by the performance of (one and only) 750 GSXR, which wasn't much down on power compared to 1000cc bikes, Japanese will soon be around 220-230BHP without much trouble...

Lack of protection on 7, Cheapy? I was surprised to hear Isignosis' quote when asked on safety of Mini- something like "I've given them good power, good brakes and good handling- if they get into accient anyway, it's their own fault" (another of his classics is "I make cars to be driven, not to be wrecked", or something to that effect)...

(as an aside, I'm still intending to make my own car-I'd hate to hear what you'd think of that one in terms of safety , but I don't think you'd be unkind to the design*)

* it will be GPL suicide pod- 2 seater with basic Lotus 25 layout in GRP

Edited by Wolf, 06 January 2012 - 04:54.

[Kelpiecross]

* it will be GPL suicide pod- 2 seater with basic Lotus 25 layout in GRP

Sounds interesting - have you got a sketch or drawing you can post? What's a "suicide pod"?

[cheapracer]

What's a Hyperbike???

Well lets just say I believe that a 600cc single is quite enough for anybody in any circumstance.

My 3 last bikes were a GSX-R 750, Valkerie 1500 and VX800 so I'm not exactly anti-bike.

The first 2 were useless btw as road bikes although the GSX-R certainly helped fund my State Government.

[cheapracer]

Sounds interesting - have you got a sketch or drawing you can post? What's a "suicide pod"?

[Kelpiecross]

Aptly named you would have to say. So a 2-seater "suicide pod" would be? A pod each side of the middle bit - or a double pod to one side?

[kikiturbo2]

Lack of protection on 7, Cheapy?

I loved the seven until I had to drive one at night on the old road from Rijeka to Zagreb, ... with trucks coming the other way..

[Tony Matthews]

I saw a 7 at Delamare Road, Cheshunt, that the driver had tried to force between a vehicle he was overtaking, and one coming in the opposite direction. It had parallelogramed and looked like two parked motorbikes, one slightly behind the other.