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RichardD

3,449 posts

133 months

[news] 
Monday 8th October 2012 quote quote all
500dread said:
What are these limitations in torsional stiffness inherent in a tubular backbone chassis? In particular, compared to what when you speak of modern GT cars?
Have a look at a TVR chassis from the front or the back. One with no suspension on it. Compare this to the body shell of a mass produced metal box car. The latter is BIG, the TVR is tiny. It it this lack of cross sectional size that means they have a tougher time resisting twist.

Anyway, whatever increase in stiffness for the chassis as per the title of this topic wasn't the main goal and will be just a bonus ...

500dread

195 posts

31 months

[news] 
Monday 8th October 2012 quote quote all
RichardD said:
Have a look at a TVR chassis from the front or the back. One with no suspension on it. Compare this to the body shell of a mass produced metal box car. The latter is BIG, the TVR is tiny. It it this lack of cross sectional size that means they have a tougher time resisting twist.
Big is thin and does not make for stiffness in itself over and above the design of a tube frame chassis. If you wanted to I'm pretty sure with the correct choice of materials and design you'd always end up with a stiffer tube frame than a tin box if other factors were not in play such as ease of build, cost of materials, weight, collision strength, etc.



RichardD said:
Anyway, whatever increase in stiffness for the chassis as per the title of this topic wasn't the main goal and will be just a bonus ...
Thought the op was asking the benefits of experience, which no one yet has, so I believe my points are beneficial to any consideration of chassis. Be it a Sportmotive upgrade, modified from original, or just a good old Griff chassis in fine form with a fresh coat of rust preventer and a top coat and better modern shocks, springs and bushes.

jwoffshore

341 posts

142 months

[news] 
Monday 8th October 2012 quote quote all
Dear Mr Dread, clearly this improved chassis is not for you. However, a lot of thought, effort and engineering has gone into it because many other people do wish for something better/different. I think we should leave it at that rather then getting drawn in to pointless theoretical arguments.


RichardD

3,449 posts

133 months

[news] 
Monday 8th October 2012 quote quote all
500dread said:
Big is thin and does not make for stiffness in itself over and above the design of a tube frame chassis....
An ultra stiff tube frame chassis is a cage! They are ultra stiff relative to just a backbone because they are cross sectionally big and there is more distance for the twisting forces to be absorbed...

Also "thin" doesn't matter when the forces you are dealing with are compressive.


Graham

15,426 posts

172 months

[news] 
Monday 8th October 2012 quote quote all
You can certainly feel the std griff/chim chassis flexing on the road, and even a decent ( i.e. roll centre) roll bar makes a big difference.


There was also some testing done by either tvr or rollcentre on a chassis dynometer testing various changes. the stock chassis has fairly low rigidity, much lower than your average tin box..

the metal tray under the gearbox on the chim/ griff makes a big difference, as does triangulation over teh engine, and a full cage.

Even the tuscan challenge chassis if fairly floppy.

A stiffer chassis does though need the suspension to be tuned with it to make use of the stiffness..
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500dread

195 posts

31 months

[news] 
Monday 8th October 2012 quote quote all
Oh sorry if I ruffled some feathers. I thought we were discussing the necessity or otherwise of a modified chassis.

I thought it pertinent to debate, not argue, what is the need for such a chassis where the Griff is concerned seeing as you with such knowledge it seems would have me believe a Griff chassis is woefully lacking in torsional stiffness.

All you have pointed out is a bunch of higher numbers of poundage per degree for other cars but you make no solid case as to why these numbers are better for these cars which seems to have you suggest a higher poundage rate would suit a Griff. Whilst it might you make no case as to what is wrong with the Griff chassis other than to say it has a low number. I'm sure this isn't really what you believe is reason enough and I'd expect there'd be a benefit for such a modified chassis for higher powered cars offering bags of torque, but all else being equal it sounds like a lot of overkill to me.

But if you can't be bothered to explain what is wrong with the Griff chassis I should consider improving it, then good luck with your endeavours to better yours for your own reasons. I'm confident mine will never be beyond my fantastic driving abilities I'd need to improve it.


500dread

195 posts

31 months

[news] 
Monday 8th October 2012 quote quote all
RichardD said:
An ultra stiff tube frame chassis is a cage! They are ultra stiff relative to just a backbone because they are cross sectionally big and there is more distance for the twisting forces to be absorbed...

Also "thin" doesn't matter when the forces you are dealing with are compressive.
I was talking about a tube frame 'backbone' as in Griff.

Thin does matter otherwise we'd be building bridges like tunnels from tin plate or high rise structures by stacking boxes of tin. When did you last see a fat crane?

RichardD

3,449 posts

133 months

[news] 
Monday 8th October 2012 quote quote all
Last last post on this topic which has gone to the usual PH tangent!

One of the most significant things regarding the chassis here (roll centres) was mentioned on here by JW and not picked up on (probably because it counts as voodoo or whatever).

This is the document as mentioned, looks like the r&d that was done for the Cerb ...

www.mscsoftware.com/support/library/conf/wuc96/07b...


500dread

195 posts

31 months

[news] 
Monday 8th October 2012 quote quote all
Graham said:
You can certainly feel the std griff/chim chassis flexing on the road, and even a decent ( i.e. roll centre) roll bar makes a big difference.
Please tellme what I am feeling as I've not felt my griff twisting. In fact it tends to roll or lift or squat rather than any form of twisting be obvious to driving it.




Graham said:
There was also some testing done by either tvr or rollcentre on a chassis dynometer testing various changes. the stock chassis has fairly low rigidity, much lower than your average tin box..
Does not explain a need for any more rigidity.



Graham said:
the metal tray under the gearbox on the chim/ griff makes a big difference, as does triangulation over teh engine, and a full cage.

Even the tuscan challenge chassis if fairly floppy.
I wouldn't know having no experience of these add on's though I find it odd that in all the years I've been around TVR's chassis flexing has not been a talking point by way of concern.

Obviously a roll cage will stiffen the chassis and the stiffness here may well be felt, I couldn't say from experience. But this does not mean the chassis therefore flexes any more than anything else and similar additional stiffness will apply to any vehicle which a rollcage is fitted to.



Graham said:
A stiffer chassis does though need the suspension to be tuned with it to make use of the stiffness..
And in that you might just build a horrible car to drive on the road - but maybe worth the sacrifice to some for other reasons. The question would be will this stiffer car be beneficial as a road car or are we delving in to track car special.

As you were chaps!

RichardD

3,449 posts

133 months

[news] 
Monday 8th October 2012 quote quote all
500dread said:
I was talking about a tube frame 'backbone' as in Griff.

Thin does matter otherwise we'd be building bridges like tunnels from tin plate or high rise structures by stacking boxes of tin. When did you last see a fat crane?
It has already been said about "twisting" forces, so why all these different tangents to give examples of tubes with structures who's main purpose is to resist load in one direction (mass/gravity).

But one comment I can't resist - a crane or bridge made from solid box - that'd be great in gusty winds wouldn't it wink !?

Since I'm a nice person I'm going to suggest you read that PDF and then decide if you want to revisit your comments (as it helps visualise torsional ridity in terms of a car chassis)!


RichardD

3,449 posts

133 months

[news] 
Monday 8th October 2012 quote quote all
500dread said:
Oh sorry if I ruffled some feathers. I thought we were discussing the necessity or otherwise of a modified chassis.
...
You haven't ruffled feathers, you have flown off on a tangent in a "my car is fine, why change anything" mindset.....

If a chassis is being made to replace an old rusty chassis, and the opportunity exists for minimal effort to add in a few more tubes here and there (to increase rigidity), then why not.


Chilliman

Original Poster:

5,458 posts

49 months

[news] 
Monday 8th October 2012 quote quote all
Jeez guys, I was only wondering what the Sportmotive chassis was like confused

Thanks for all the replies and the discussion on stiffness though, it's all interesting stuff smile

900T-R

19,549 posts

145 months

[news] 
Monday 8th October 2012 quote quote all
A few points.

TVR did marginally strengthen the Griff/Chim chassis (at least according to the SH Griff/Chim Bible) when developing the 500 versions with their higher and more brutally produced torque levels (this chassis including the 260/273 mm brakes later became standard for all Griffs and Chims). This (and the Cranfield University study)to me indicates that chassis stiffness was indeed a concern at the time.

Chassis flex might have contibuted to the unpredictability of the 500-engined variants at the limit on anything but a smooth surface. However I would say that the main factors are insufficient suspension control (especially rear damping), not helped by a less than favourable sprung/unsprung masses ratio, first; and suspension geometry second.

It looks like the Evolution chassis deals with both latter factors as well as aiding torsional stiffness. It has to be said that without things like structural sills, floors and bulkhead, the effects will be limited and by no means you should expect 'modern' alloy spaceframe construction-like values.

While its true that the double decker tubular backbone construction is massively strong compared to a steel monocoque derived convertible (you won't likely find convertible TVRs folded or broken in halves after an accident) and this is big part of the percieved 'solidity' of the construction, it's not necessarily torsionally stiff. The body mounting points at the outriggers have a fair bit of leverage at the relatively narrow central spine.
However, as the pivots of the wishbones are directly attached to said central spine it is from a geometry POV perhaps unfair to compare the torsional stiffness numbers posted with those of monocoque construction cars where part of the suspension loads are working against pretty much the outside of the body (strut towers/spring platforms).

Having said all the above, even if you made the stiffest chassis in the world, grip levels and how quickly the car is pointing the wrong way if you exceed them, are pretty much limited by the modest footprint of the cars (wheelbase is the same as a Citroën AX's, and I'd imagine those would be a bit lively at the limit if you stuck a 300 bhp/lbs.ft engine in them!).

Given the above I can see both points of view. It's perfectly possible to make a very well-balanced & satisfying overall package out of a Griff/Chim (easier done with a 430/450 as a starting point than a 500 IMO) keeping close to the original specification. Modest spring rates (to avoid overworking the chassis), the best & most sophisticated damping you can afford, keep unsprung masses to a minimum and enough compliance in the tyres (which will ideally be of a well-developed 'road' compound/constuction rather than some very stiff, ultra-grippy trackday special). On the engine side of things, concentrate on a smooth torque curve building to decent top end power rather than (even more) thumping low/midrange and immediate/accurate throttle response so you can very accurately control power delivery near the limits of adhesion.
It will always be an 'old-school' experience keeping close to the great classic sports cars to which the cars were meant to be the natural successors anyway, more about feel and involvement than outright grip and pace. Let's call it the 'anti-GTR'.

Should you wish for a more 'modern' experience, something to scare 21st century supercar drivers with - more -and more consistent- grip right up to the limit, an iron-fisted composure and loads of grunt - from the classic V8 TVRs, well that's where developments like Sportmotive's come in. If you are developing the car as far away from the original as that, I'd suggest you'd start with a proper foundation - wehich is what the Evolution chassis aims to provide. Personally, I'm quite excited about developments like this. I would imagine it is aimed straight at folks who would normally have bought a T-car if it weren't for them preferring a traditional V8 and/or the Chim/Griff aesthetics, which is a perfectly valid stance from where I stand. smile

The worst of both worlds IMO in terms of creating something that handles well overall irrespective of personal taste/direction is to take a tired old chassis with the std Ford hubs and front uprights, and burden it with LSx/big Ford/supercharged 5.x litres RV8 engines, huge brakes & wheels with rubberband tyres, et cetera. Of course, these will work in certain sets of conditions and meet certain owner expectations and if you fit that profile, more power to you. smile The result, however, will more than likely be something that I wouldn't want to drive along a country road with a lot of commitment...

RichardD

3,449 posts

133 months

[news] 
Monday 8th October 2012 quote quote all
Chilliman said:
Jeez
More on topic, it has been mentioned by JW regarding anti roll bars but not specifically that the T-cars have +3mm thicker anti roll bars than the standard Griff/Chim, so the mounts on the SM chassis will be designed to allow the thicker bars. A minor feature to give less body roll without having to go stiffer on the suspension.....

Quietlybonkers

9,286 posts

32 months

[news] 
Monday 8th October 2012 quote quote all
This is so fascinating I am just posting so that it comes up on "My Stuff", so i can continue to follow it every time someone posts. Sorry for being lazy,.....nothing to contribute

RichardD

3,449 posts

133 months

[news] 
Monday 8th October 2012 quote quote all
900T-R said:
A few points....
Most excellent points they were too imho biggrin

For reference your epic "discussions" on suspension with Sam_68 from the past have all been taken into account on the design here! Those alloy uprights as per earlier have been subject to computer analysis including camber and roll centre movement! Whether the latter will actually make any difference in the real world... who knows!

900T-R

19,549 posts

145 months

[news] 
Monday 8th October 2012 quote quote all
Personally, I'd like to use as little ARB as I can get away with, adding the necessary roll stiffness through higher spring rates if at all possible (of course, this assuming the structure is sufficiantly stiff and ride quality an secondary objective at best, neither of which might apply to our TVRs wink ).

What an ARB basically does is transfer loads from the inner to the outer wheel (which already carries the highest load), limiting ultimate grip.

At the front, you'll be balancing turn-in response with understeer in steady-state conditions.

At the rear, a stiffer ARB might be desirable for limiting roll-oversteer, again at the expense of ultimate grip.

900T-R

19,549 posts

145 months

[news] 
Monday 8th October 2012 quote quote all
RichardD said:
Most excellent points they were too imho biggrin

For reference your epic "discussions" on suspension with Sam_68 from the past have all been taken into account on the design here! Those alloy uprights as per earlier have been subject to computer analysis including camber and roll centre movement! Whether the latter will actually make any difference in the real world... who knows!
I'd love to try a set that would work with the OE wishbones and 4-bolt hubs!

In fact, I'd already had one or two exploratory conversations at trade shows with engineering firms about designing and fabricating some, only to come to the inevitable conclusion that it would be, at the very least, a long term project given cost/volume and possible litigation consequences when offering them for road use.

RichardD

3,449 posts

133 months

[news] 
Monday 8th October 2012 quote quote all
900T-R said:
Personally, I'd like to use as little ARB as I can get away with, adding the necessary roll stiffness through higher spring rates if at all possible (of course, this assuming the structure is sufficiantly stiff and ride quality an secondary objective at best, neither of which might apply to our TVRs wink ).

What an ARB basically does is transfer loads from the inner to the outer wheel (which already carries the highest load), limiting ultimate grip.

At the front, you'll be balancing turn-in response with understeer in steady-state conditions.

At the rear, a stiffer ARB might be desirable for limiting roll-oversteer, again at the expense of ultimate grip.
In theory I agree, but for a road car unfortunately the Griff/Chim is too narrow and the weight is too high up (relative to a small clutched, dry sumped racing car) for that level of idealistic-ness imho.

The logic on anti roll bars is simple - the t-cars work very well as road cars, copy features that work for them.

I did once think of seeing if any Citroen Xantia Activa (or is that a yogurt?) dampers would be available from a scrapyard hehe

500dread

195 posts

31 months

[news] 
Monday 8th October 2012 quote quote all
900T-R said:
A few points.

......stuff......




The result, however, will more than likely be something that I wouldn't want to drive along a country road with a lot of commitment...
Thank you. Kinda puts my points across without the flying birds laugh

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