Hello Gentlemen,

The discussions about strength of the chassis is very interesting, but can anyone describe the practical importance of this?

I have made most of the earlier described reinforcements of the chassis, but will I notice it in practice?

From a technical point of view I can see that stronger is probably better, but how important is it?

If my chassis is twice as strong as "the nextt guy“s", how will the difference show up?

Better feeling? Faster lap times on track? Something else?

Regards
/Hasse

On a simplistic level, the handling will be better.

There's no point designing accurate suspension geometry if the mounting points are going to move around!

Dampers can suffer 'stiction' instead of linear compression, the car may wander under braking, the list is endless.

As to whether you'd notice any difference, that depends how hard you're pushing. Push hard and your sloppy chassis will show up. I know this as I've just borrowed a 1997 Astra. Dreadful chassis flex and handling that lurches from understeer to oversteer and back to [lots of] understeer again. <>

I've wondered about this issue myself. Some kit car chassis are fairly floppy things and surprisingly spaceframes are at the botom of the heap. They are also at the top of the heap, the difference being in the triangulation.

The lowcost chassis has about 1200 ftlbs per degree of twist but as the book has a few options in it anything from 1100 to 1400 is entirely possible. For comparision I know someone who did a completely independent analysis of a westfield chassis and he got 1500. A Caterham is very dependent on the panelling but something in the region of 2400 to 3000 looks probable.

There are a few small mid engined car chassis on the kit scene that have only a fraction of the lowcost stiffness (400 to 800 ftlbs per degree) and they get good reviews. I'm a bit reluctant to name names here.

The really important bits to get right are-
1) the chassis sides must look like a network of triangles or welded in panels from the rear suspension region to the front. V, Y and X braces and panels with reasonably sized holes in are OK.
2) the front suspension region should be completely triangulated. On the lowcost this can be done by extra side diagonals, a welded in bottom panel, a V brace across the front and the book K brace across the top.
3) a properly triangulated engine bay either by 1 or preferably 2 tubes as the book tube R or a Y brace as on the lotus 23. I'm assuming here that the bottom and sides of the engine bay are already reasonably braced as this seems to be the case with even otherwise useless designs.
4) mid engined cars benefit from a properly triangulated box around the rear suspension region. A stressed pierced bulkhead as on the back of the lotus 23 is Ok but a simple tubular hoop around the transaxle is not actually that good.

Any chassis that hasn't got point 1 is hopeless. Most spaceframes miss out on a decent attempt at 2 or 3 which is why a decent ladder frame would beat them for the same overall weight. Hardly any middy kits make use of 4 which is surprising as it makes a big difference.

I'm sure a lot of the spaceframe issues are psychological. The end user is fooled into thinking the product is better than it actually is.

I'd be very interested to know if anyone has any information about what stiffness is actually required.

I once found a rule of thumb that the stiffness in ftlbs should match the cars weight in lbs for a road car with that value being doubled for a race car. Most kit cars fail this though.

Cymtriks, what do you think of the Avon chassis? Reply in private if needed!

Thanks

My personal opinion is that the Avon rear end looks like a bit of a lash up in order to accomodate the IRS in a way that's quick and easy to produce. But I've only seen one or two photos and could of course be well wrong.

I'd be very interested to know how the Avon rear performs in the stiffness area (or stiffness/weight). Cymtriks?

Liam

I have had a look at the picture on the tiger website and can't see enough to make a judgement. Does anyone have a picture like the one on page 40 of the lowcost book?

From what I can see, which isn't much, the cockpit sides appear to be better triangulated though that does not mean it will definitely be better than the lowcost approach of using a welded in panel towards the cockpit rear.

There appears to be two longitudinal tubes running along each seat base in one picture I've found and these add nothing structurally that welding in a steel floor could not do for, probably, no extra weight.

If there is a big difference it is likely to be around the front. As I keep saying, this is where nearly every small kit car spaceframe could be improved by adding a few extra diagonals.

Reading some threads on the Avon section makes me think that this chassis isn't as highly thought of as some others, mainly due to build quality and book errors. Is the Avon book any worse, or better, than the lowcost book?

A good feature of the Avon chassis, IMO, is that it uses standard rear suspension parts. I know someone who is building a Fisher Fury and he is having nightmares with his modified driveshaft lengths.

quote:

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Originally posted by cymtriks
... Does anyone have a picture like the one on page 40 of the lowcost book?

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I'd answer the original question with another question, "How important is suspension tuning to you?"

If the answer is, "What's suspension tuning?," then I'd say you'll be happy no matter what car you buy.

stiffness and handling primarily relates to 'is it safe' which can be defined as

1. is it gonna fall to bits in normal driving

and

2. is it gonna be safe the way you drive it.



the first , if it aint, will hopefully be fouind out in sva so you can have a longer life.


the second depends on exactly how yer gonna drive it.

The Astra scenario - being a mid range boring vauxhall that goes shopping, to work, and on holiday most its life, doesnt have a big demand on ulimate handling. A ferrari would.

Id classify as follows :

1. My old metro turbo. For whatever erason it didnt feel safe with the power it had. kinda wobbly. Not tolerable cos you would drive it normally such that it would catch you out.

2. average slidy poor grip family saloon - would put my wifes megane in this one. predictable. If you want to throw around a 1 tone car on 13 inch 165 wide tyres, then yer gets what you expect.

3. A really well set up car. Kinda thing you might take to a track day or race.



I believe that really well set up cars that encourage chucking about on the road is a dangerous thing. Having been to silverstone in a formula first car, it was really suprising how you got to the limit - one lap it was sh^$%e to a blanket, next it was spinning.

do that on a road and its curtains.

If my car is 'ok' or vauxhall like, id consider it a result.

if I was racing it id feel a bit pissed.

its all relative I recon.

atb

steve

Slightly off thr topic here, but its OK having a stiff chassis, but if it is too strong (there is a difference), in an accident, ALL the loads will be absorbedby YOU!

I have done most of my triangulation in 13mm ERW, both round and square section. It is almost as strong in tension as bigger section, but in an accident will tear out the welds after absorbing most of the energy. I have left the TR tubes as per book because the will compress and bend outwards deflecting the front of the car towards the energy centre and deflect the car around (better have whiplash than being crushed in a cage of ERW!). The chassis is stiffer than book and a little heavier, but more crash worthy.

I used to think that it would never hapen to me, but I see an average of 8 accidents per working day - I am a little more aware now

Mark

quote:

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The Astra scenario - being a mid range boring vauxhall that goes shopping, to work, and on holiday most its life, doesnt have a big demand on ulimate handling. A ferrari would.

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quote:

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Originally posted by Mark Allanson
Slightly off thr topic here, but its OK having a stiff chassis, but if it is too strong (there is a difference), in an accident, ALL the loads will be absorbedby YOU!

I have done most of my triangulation in 13mm ERW, both round and square section. It is almost as strong in tension as bigger section, but in an accident will tear out the welds after absorbing most of the energy. I have left the TR tubes as per book because the will compress and bend outwards deflecting the front of the car towards the energy centre and deflect the car around (better have whiplash than being crushed in a cage of ERW!). The chassis is stiffer than book and a little heavier, but more crash worthy.

I used to think that it would never hapen to me, but I see an average of 8 accidents per working day - I am a little more aware now

Mark

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Terry,

Unless the car absorbs most of the energy before it gets to you, you aint going to recover from the whiplash. You want the car to be stiff when being loaded with normal driving loads, but when impact loads are applied you have got to know what is going to give fist and work out a progression of deformation. remember an impact load on the suspension or chassis will be massivly greater than any loading you can put on it when driving, these driving loads cause plastic deformation on the chassis, which the chassis instantly recovers from, in an accident you get permanent deformation, totally different.

You can have a perfectly rigid chassis which can collapse in a predictable way, protecting you in the process.

Mark

PS The shot of the cars broken in half on the M1 were after the fire chaps had done their bit

In an accident, it is apparently quite normal in extreme collisions, that the heart can be torn from its vessels due to its sudden rush forwards. I think thats what happend to Diana.

In Mika Hakkinens crash, his belts stretched and allowed his head to hit the wheel, nearly killing him.

It said in the press that if the belts didnt have a degree of stretch (and car belts are not exactly elastic!) then the heart bursting vessels due to rapid forward motion due to lack of chassis compression occurs.

Problem is, how to build a kit car, without extensive crash testing - that is the best of both worlds. Do you end up like pate or whole on the outside but not on the inside....I hope I dont have to do a real life test........

The Smart car is advertidsed as being short cos it has a front end that 'exploits the crumple zone of the other car'. Right then......just dont hit a land rover or a volvo....or another Smart car!

atb

Steve

[Edited on 3/3/03 by stephen_gusterson]

..........or a tree!

that would do it!

But i did when I was 19

the tree won - i spent 9 days in hospital

atb

steve

Mark,

I agree with what you are saying, but at what point do you say it will absorb this amount of force and not that much?

In an ideal situation the chassis would stretch as far to either side of the driver as it does in front in order to deform and absorb the energy.
In the real world it doesn't work like that.
What you might consider as the average accident, I might not!
The biggest threat to a locost driver (or most drivers for that matter) is not from front or rear collisions but side impacts.
Sorry if I seem to be labouring this point, but it is my main concern in a severn style vehicle. Saying that I would not miss an opportunity to ride in one!

Terry

Terry,

In a side impact in a Locost, lets face it, you wouldn't know anything about it - just hope it never happens (I have only done 4 fully side impacts in 10 years of doing 30 car a week)

In its favour, the locost is so light that it might be shoved out of the way easier, the rear axle may take the shunt before you, the engine has got enough grunt to zap you out of the way, a few straws to clutch at!

Think a defensive driving course may be in order, or those of us who are ex bikers go back into bike mode!

Mark

Hello all,

Tank“s for the answers, but they have moved a little bit away from what I hoped you could help me answer.

My question in the first place was about the importance of the stiffness in normal/racing use.

This is all an interesting discussion, but personally I do not expect to survive a serious crash in this type of car.

I have been involved in real crash testing a few times for my company, and when you have seen it "live", you better understand the forces involved.

They are massive!

In this car I hope to be able to push hard for the joy of it, but I will sure try to stay out of larger cars and solid objects.

/Hasse