Hasse
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| posted on 28/2/03 at 09:22 AM |
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Practical importance off stiff chassis?
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
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Noodle
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| posted on 28/2/03 at 01:14 PM |
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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.
<>
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cymtriks
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| posted on 28/2/03 at 05:34 PM |
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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.
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Northy
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| posted on 28/2/03 at 06:26 PM |
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Cymtriks, what do you think of the Avon chassis? Reply in private if needed!
Thanks
Graham
Website under construction. Help greatfully received as I don't really know what I'm doing! 
"If a man says something in the woods and there are no women there, is he still wrong?"
Built 2L 8 Valve Vx Powered Avon
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Liam
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| posted on 28/2/03 at 06:49 PM |
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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
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cymtriks
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| posted on 1/3/03 at 09:23 PM |
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I need a picture!
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.
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jcduroc
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| posted on 1/3/03 at 10:08 PM |
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quote:
Originally posted by cymtriks
... Does anyone have a picture like the one on page 40 of the lowcost book?
Cymtriks
Do you mean the figure in page 42 (2nd ed.) - 1st page of Chapter 4?
João
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kb58
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| posted on 1/3/03 at 10:54 PM |
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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.
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stephen_gusterson
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| posted on 2/3/03 at 12:26 PM |
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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
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Mark Allanson
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| posted on 2/3/03 at 01:14 PM |
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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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Noodle
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| posted on 3/3/03 at 09:27 AM |
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quote:
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.
Agreed, however most manufacturers now attempt to attain a reasonable degree of stiffness. The new Astra is a completley different fish-boiler, as is
virtually any modern mass-manufacturer mobile.
A couple of the previous Astra models I've driven, I'd actually class as dangerous at normal motorway/dual carriageway speeds. (Or I'm a crap
driver, perhaps more likely) 
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Spyderman
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| posted on 3/3/03 at 05:25 PM |
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quote:
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
Again as Steve has already said, it is relative!
What sort of crash are you protecting against? A similar sized vehicle going in same direction would need less protection than if going in the
opposite direction. Also a collision with a larger vehicle would need more stiffness/protection again!
The speed you travel at will also have an effect and so on! As will angle of impact, whether you spin into an object or get T-boned by a bloody great
4x4!
Personally I'd rather it was too strong and I could recover from the whiplash. The opposite doesn't bare thinking about!
Going further off track, did anyone see the tv prog on channel 2 on sunday night?
The early footage of the M1 motorway and accidents? It looked like nearly every car that had been in an accident was broken into two parts, front and
back.
Terry
Spyderman
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Mark Allanson
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| posted on 3/3/03 at 08:36 PM |
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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
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stephen_gusterson
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| posted on 3/3/03 at 10:44 PM |
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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]
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Mark Allanson
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| posted on 3/3/03 at 11:24 PM |
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..........or a tree!
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stephen_gusterson
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| posted on 3/3/03 at 11:55 PM |
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that would do it!
But i did when I was 19
the tree won - i spent 9 days in hospital
atb
steve
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Spyderman
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| posted on 4/3/03 at 04:16 PM |
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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
Spyderman
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Mark Allanson
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| posted on 4/3/03 at 08:41 PM |
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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
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Hasse
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| posted on 5/3/03 at 07:22 AM |
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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
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