The chassis con
Discussion
Avocet,
You’re quite right to bring wishbones and bushes into play when looking at cornering forces which apply different (and lesser) forces into the chassis. However when considering torsional strength
then it is correct IMHO to test the chassis at the point at which torsional forces would be applied.
If one were testing a table one would test it from the perspective of its 4 legs. The ‘legs’ of a car are it’s wheels which are supported on its springs which locate onto the chassis.
The point at which they fix to the chassis IS the correct place to apply the measurement IMHO.
You make the astute observation
that there are other attributes (than chassis stiffness) against which a vehicle design has to be assessed
. However, the chassis is the foundation upon which everything else hangs
, so is IMHO the most fundamental to get right. Like you say there’s plenty of scope to spoil things further down the line
, but as they say, that’s another story.
I was thinking that the issues raised in this thread might make a good article
. However, I reckon to do it justice a 
book would be required.
A whole chapter could be taken up with round verses square tubes before getting into all the other stuff.
Den
You’re quite right to bring wishbones and bushes into play when looking at cornering forces which apply different (and lesser) forces into the chassis. However when considering torsional strength then it is correct IMHO to test the chassis at the point at which torsional forces would be applied. If one were testing a table one would test it from the perspective of its 4 legs. The ‘legs’ of a car are it’s wheels which are supported on its springs which locate onto the chassis.
The point at which they fix to the chassis IS the correct place to apply the measurement IMHO. You make the astute observation
that there are other attributes (than chassis stiffness) against which a vehicle design has to be assessed. However, the chassis is the foundation upon which everything else hangs, so is IMHO the most fundamental to get right. Like you say there’s plenty of scope to spoil things further down the line, but as they say, that’s another story. I was thinking that the issues raised in this thread might make a good article. However, I reckon to do it justice a book would be required. A whole chapter could be taken up with round verses square tubes before getting into all the other stuff.
Den
kitcarman said:
I was thinking that the issues raised in this thread might make a good article
A whole chapter could be taken up with round verses square tubes before getting into all the other stuff.
Den
Why don't you ask "detail" Dean for his opinion?
(light the blue touchpaper and retire)
kitcarman said:
I was thinking that the issues raised in this thread might make a good article
A whole chapter could be taken up with round verses square tubes before getting into all the other stuff.
Den
Haven't I mentioned this to you ? ? ?
John suggested this to me offline. Looks like it'd be a hell of a lot of work to get a definitive answer...
As for where to test the load. I'd agree with Den that it should be tested at the suspension mounts as we're talking about chassis rigidity.
When you then start talking about the road holding and ride quality etc, this will depend very much on the suspension. Say you have two identical chassis, one with fully rose-jointed suspension, another with nylon bushes, the handling and ride *could* be substantially different.
But, rose-jointing suspension to a floppy chassis will never result in a stiff chassis, so testing the chassis in isolation allows you to determine how good a starting point you have.
As for where to test the load. I'd agree with Den that it should be tested at the suspension mounts as we're talking about chassis rigidity.
When you then start talking about the road holding and ride quality etc, this will depend very much on the suspension. Say you have two identical chassis, one with fully rose-jointed suspension, another with nylon bushes, the handling and ride *could* be substantially different.
But, rose-jointing suspension to a floppy chassis will never result in a stiff chassis, so testing the chassis in isolation allows you to determine how good a starting point you have.
Ex-Biker said:
Haven't I mentioned this to you ? ? ?
You did indeed, Mark
LexSport said:
Looks like it'd be a hell of a lot of work to get a definitive answer...
It would represent an awful lot of work, but some it could be interesting and highly informative if illustrated with copious pictures and examples. I’m going to give it careful consideration, because it could provide part of Kit Car’s answer to the ‘detail’ Dean advantage currently enjoyed by a certain other magazine of ‘integrity’
. Den
>> Edited by kitcarman on Wednesday 3rd December 16:57
LexSport said:...
...But, rose-jointing suspension to a floppy chassis will never result in a stiff chassis, so testing the chassis in isolation allows you to determine how good a starting point you have.
True, but putting squidgy suspension bushes and bendy wishbones on a stiff chassis will negate all the advantages of the stiff chassis! I think it's important not to loose sight of what we're ultimately trying to achieve - a car that feels tight and handles nicely. The chassis is just one element of this package and, as can be seen in a number of cars, you don't necessarily need a stiff chassis for a car to go round corners well (although the best cars at going round corners seem to have the stiffest chassis).
I think we should also clarify the terms "strength" and "stiffness" here as they seem to be used interchangeably in some posts. "Strength", as I understand it, is a measure of the amount of load that can be applied to an object before it breaks. "Stiffness" is a measure of the amount of force that can be applied to an object before it starts to deform. Rubber is very strong, but not in the least bit stiff. Chalk is very stiff, but not strong.
I seem to vaguely remember (if anyone wants a good book on this) that there was one by Costin and Phipps (I think the Costin went on to be the "Cos" in "Cosworth") called "Racing and sports car chassis design" - or something similar. It is probably out of print and I've never read it but I've heard it highly spoken of. Also the series of books called "**** to Win" by Carol someone-or-other are supposed to be good.
Finally, just to put my two penn'orth in on the subject of round vs. square tubes, I prefer round. They're more hard work to profile (although there's an excellent free computer programme to help with this) but it means you get a bigger weld area for a given tube diameter and because the joint isn't all on one face of the tube, you reduce the risk of fracturing in the Heat Affected Zone - and you feed the loads closer to the neutral axis.
There is a lot of discussion about the terminology being used. Without the correct terminology we would find it extremely difficult to describe anything!
Just try and describe a chassis without using any technical terms and it becomes very confusing.
Admittedly some terms are more commonplace in the use of our language than others, but this does not justify dismissing the lesser used terms in order to describe something.
To say that a backbone chassis is a spaceframe backbone chassis is incorrect. It is either a spaceframe chassis or a backbone chassis, regardless of how the backbone is constructed! The TVR chassis in discussion should be called a "backbone chassis" that is constructed of spaceframes.
Let's not confuse chassis and frame!
The loading points for torsional stiffness do not go through the suspension mounts, but through the Spring mounts (in a standard outboard shocker setup).
Therefore the statement that the loads should be put through the uprights with locked up springs is more correct. The other implies or gives the impression that the loads should be applied through the wishbone mounts!
The foundation of this whole thread is misleading as we each have our own ideas of what is being discussed. Very often argueing over what we think is implied as opposed to what may actually be implied, but not quite clarified.
To state that a ladder chassis is "better" than a lot of spaceframe chassis is not necessarily right nor wrong. The question is "what" ladder chassis is better than "what" spaceframe chassis?
Cymtriks uses the Locost chassis as an example of a "not quite as good as it could be" chassis, but does not give an example of a "good ladder" chassis for direct comparison (in 7 clones).
I also disagree with the direct comparisons between one type of chassis with another without all of the bodywork and other such parafernalia.
The ladder chassis will need a lot more structure building onto the chassis to accomodate body etc, which may or may not add to it's stiffness. It will definately add significantly to it's weight.
The spaceframe (whether good or bad) will need less adding to it, but the body panels can only add to it's stiffness (when new). Both these examples are purely of the 7 clone style.
Therefore to state that a good ladder chassis is better than a poor spaceframe is still misleading.
To say that car "A" using a ladder chassis is stiffer than car "B" using a spaceframe would however give a better idea, but car styles and intended purposes must also be taken into consideration. Agreed though that a stiffer structure would benefit all vehicles.
Hope I've not added to the confusion!
Terry
Just try and describe a chassis without using any technical terms and it becomes very confusing.
Admittedly some terms are more commonplace in the use of our language than others, but this does not justify dismissing the lesser used terms in order to describe something.
To say that a backbone chassis is a spaceframe backbone chassis is incorrect. It is either a spaceframe chassis or a backbone chassis, regardless of how the backbone is constructed! The TVR chassis in discussion should be called a "backbone chassis" that is constructed of spaceframes.
Let's not confuse chassis and frame!
The loading points for torsional stiffness do not go through the suspension mounts, but through the Spring mounts (in a standard outboard shocker setup).
Therefore the statement that the loads should be put through the uprights with locked up springs is more correct. The other implies or gives the impression that the loads should be applied through the wishbone mounts!
The foundation of this whole thread is misleading as we each have our own ideas of what is being discussed. Very often argueing over what we think is implied as opposed to what may actually be implied, but not quite clarified.
To state that a ladder chassis is "better" than a lot of spaceframe chassis is not necessarily right nor wrong. The question is "what" ladder chassis is better than "what" spaceframe chassis?
Cymtriks uses the Locost chassis as an example of a "not quite as good as it could be" chassis, but does not give an example of a "good ladder" chassis for direct comparison (in 7 clones).
I also disagree with the direct comparisons between one type of chassis with another without all of the bodywork and other such parafernalia.
The ladder chassis will need a lot more structure building onto the chassis to accomodate body etc, which may or may not add to it's stiffness. It will definately add significantly to it's weight.
The spaceframe (whether good or bad) will need less adding to it, but the body panels can only add to it's stiffness (when new). Both these examples are purely of the 7 clone style.
Therefore to state that a good ladder chassis is better than a poor spaceframe is still misleading.
To say that car "A" using a ladder chassis is stiffer than car "B" using a spaceframe would however give a better idea, but car styles and intended purposes must also be taken into consideration. Agreed though that a stiffer structure would benefit all vehicles.
Hope I've not added to the confusion!
Terry
Oh, lots of replies!
Dear Cymticks, Yes, I am in FE, as a muture student undergrad, I also own an ancient TVR.
I lack the skills to get as far as you have in the analysis, but I'd like to know how the various chassis designs cope with a moment applied in the other two axes, e.g how the chassis copes with a side moment (like if you are t-boned) or the case where you get a moment in the vertical plane, such as when you go over a speed bump, or have a heavy passenger load. I could see a low torsion in these other two planes leading to all sorts of chassis twisting, and vibrating.
I'd like to know what stiffness my 280i TVR is rated at.
-Jim
Dear Cymticks, Yes, I am in FE, as a muture student undergrad, I also own an ancient TVR.
I lack the skills to get as far as you have in the analysis, but I'd like to know how the various chassis designs cope with a moment applied in the other two axes, e.g how the chassis copes with a side moment (like if you are t-boned) or the case where you get a moment in the vertical plane, such as when you go over a speed bump, or have a heavy passenger load. I could see a low torsion in these other two planes leading to all sorts of chassis twisting, and vibrating.
I'd like to know what stiffness my 280i TVR is rated at.
-Jim
I think most chassis work quite well in bending (speed hump) loading. The easy, practical way to assess it would be to park it on level ground and load it up with sandbags while periodicaly opening the doors and cecking the shut gaps at the top and bottom. That will tell you if the chassis is bending. Similarly, jack it up in the middle so that it's see-sawing (if there's a convenient strong point to do it on) and see what the door gaps do. When I used to jack my last car (a big Citroen) on its forward jacking point (under the front wing trailing edge - quite near the centre of gravity of the car) the doors were noticeably less easy to latch!
I think the "T" bone loading is a bit irrelevant for a TVR. The backbone is the only bit that does anything useful and by the time whatever has hit you is loading the backbone up, you'll probably be past caring! the outriggers are useless for a "T" bone with a car because they are way below its bumper and just give the wheel something to "climb" up. They're useful in a side impact with a lamp post though! Also, when you get hit in the side by another car, the rear wheel often takes most of the load and puts it straight into the rear suspension / diff cradle. I think the 280i was the last car TVR sold in the USA in any numbers so it has side impact bars that used to meed the old US Federal side impact requirements.
I think the "T" bone loading is a bit irrelevant for a TVR. The backbone is the only bit that does anything useful and by the time whatever has hit you is loading the backbone up, you'll probably be past caring! the outriggers are useless for a "T" bone with a car because they are way below its bumper and just give the wheel something to "climb" up. They're useful in a side impact with a lamp post though! Also, when you get hit in the side by another car, the rear wheel often takes most of the load and puts it straight into the rear suspension / diff cradle. I think the 280i was the last car TVR sold in the USA in any numbers so it has side impact bars that used to meed the old US Federal side impact requirements.
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