Infinite Stiffness

[xpensive]

A coil spring is actually a long torsion bar if you think about it, the stiffness comes from the area inertia of the cross section of the thread, but as long as said cross section inertia, spring diameter and pitch is constant, so is stiffness in terms of Newton per compression distance.

The one in you image seems to have different pitch, which I guess makes it different.

[rjsa]

Just to crack a number so that we have a vague idea of what we're talking about here, a half meter long
aluminum pull-rod with a 20 mm diameter, loaded with 10000 N (1000 kg) will stretch some 0.023 mm.

If we are using nubers we'll need many more.

[Jersey Tom]

I really have no idea what the discussion is about. Infinite stiffness? Of what? In what context?

[DaveW]

Forgive me, JT, for interrupting your point (which I agree with), but there is something to clarify here.

A coil spring is actually a long torsion bar if you think about it, the stiffness comes from the area inertia of the cross section of the thread, but as long as said cross section inertia, spring diameter and pitch is constant, so is stiffness in terms of Newton per compression distance.

The one in you image seems to have different pitch, which I guess makes it different.

I don't think the varying pitch makes it different, particularly. Difficult, perhaps, but not different. Even if the bar has a fixed section & the windings are even, I think your first paragraph is an approximation.

Stiffness is actually a difficult concept...

In fact, I think stiffness is a very simple concept, it is calculating its value that can be difficult.... If I were being unkind, I would suggest that your last few posts have "put the cart before the horse", so to speak.

[xpensive]

Forgive me, JT, for interrupting your point (which I agree with), but there is something to clarify here.

A coil spring is actually a long torsion bar if you think about it, the stiffness comes from the area inertia of the cross section of the thread, but as long as said cross section inertia, spring diameter and pitch is constant, so is stiffness in terms of Newton per compression distance.

The one in you image seems to have different pitch, which I guess makes it different.

I don't think the varying pitch makes it different, particularly. Difficult, perhaps, but not different. Even if the bar has a fixed section & the windings are even, I think your first paragraph is an approximation.

Stiffness is actually a difficult concept...

In fact, I think stiffness is a very simple concept, it is calculating its value that can be difficult.... If I were being unkind, I would suggest that your last few posts have "put the cart before the horse", so to speak.

Apparently not so simple, why it might be useful with a little spring theory, to put the horse before the cart so to speak;

A coil spring is indeed a type of torsion spring;
http://en.wikipedia.org/wiki/Coil_spring

Here you can see the parameters involved, wire-dia, coil-dia, free length and number of coils, knock yourself out;
http://www.planetspring.com/pages/compr ... ulator.php

[DaveW]

I am aware of that, & also that a coiled torsion bar both rotates & bends....

[xpensive]

I am aware of that, & also that a coiled torsion bar both rotates & bends....

I'm surprised that a member of your cred should make such a statement.

http://www.planetspring.com/spring-know ... on-springs

[DaveW]

There you go.... Not that it matters much, but I think you might study your last reference. Quote:

"Please bear in mind that effective coil spring design can only be accomplished by using a computer program capable of running hundreds of simultaneous calculations."

[rjsa]

While most of the effect from a coil spring comes from torque, there will always be shear and bending involved. Values could or could not be disregarded, but they will be there.

http://www.codecogs.com/reference/engin ... prings.php

[xpensive]

Computer aided calculations are surely a must when it comes to understand dynamic properties and the effect of hysteresis at high frequencies, but that doesn't mean that the thread itself bends other than at the ends perhaps.
How would that be possible, if so it should make for a funny looking coil, If you put your mind to it I'm sure you agree?

What was your position with Lotus during those active suspension years anyway?

Btw, thanks for that most supportive link rjsa.

[marcush.]

the original question was not really reduced to springs but suspension systems per se.
I´m surprised how Formula 1 still to these days is inventing suspension systems with ever more parts instead of trying to reduce parts count when everyone is claiming Aero is all that counts.
Coming back to the stiffness issue ..I´m sure the calcs really suffer considering the chains of parts adding up to a considerable slack or give (the opposite of infinite stiffness) ...
With CF technology I had the idea one could build 1 piece suspensions with NO friction at all plus stiffness numbers farbeyond what is achievable with bolts and spherical bearings let alone needle rollers etc or plain bushings..

[DaveW]

What was your position with Lotus during those active suspension years anyway?

I was never employed by Lotus. My day job was running the Flight Systems & Measurement Laboratories at Cranfield. Active suspension was an interesting diversion for us.

[DaveW]

I probably should follow Marcush's example and respond to the original question, if I may.

With my rather limited view of the world, I think that there are several compliances that matter. My list would be

1. Torsional compliance
2. Heave compliance.
3. "installation" compliance.
4. ARB compliance.
5. Wheel alignment compliance.

I'm sure that others will be able to add to that list. Tolerable compliance will depend on the vehicle (of course), its suspension set-up, and (in particular) its tyres.

I'd like to leave out the first two, if I may, because whilst I have some ideas, they are immature.

I spend much time worrying about number 3, largely because it can have a significant affect on suspension set-up. I model it as a spring fitted in series with the normal spring and damper, to represent Marcush's "chain". For open wheeled aero cars the rear installation stiffness is usually the more important, and it is also usually the more compliant. Across the range of such vehicles, and omitting damper complaince, I have estimated values ranging from around 0.16 KN/mm to 3 KN/mm. In my experience, F1 vehicles are among the most compliant. However, they do vary substantially - I have observed that certain designers seem to leave a legacy that has occasionally enhanced the reputation of their successors (but I couldn't possibly comment further). Any value greater than 2 KN/mm is unlikely to have many problems. Any value less than 0.5 KN/mm will likely affect performance significantly, and values between 0.5 & 2 KN/mm will, ideally, require some compromises to be made.

ARB's sometimes do not work precisely as their designers intended. Often this is caused by compliance & backlash, but sometimes it is because their geometry allows them to bend as well as twist (apologies, X). I struggle to put some context to it, but in one case, a bar with a rotating blade adjuster read X at 0 degrees, X+Y at 45 degrees & X again at 90 degrees.

Because I usually don't see (in a measureable sense) the affect of wheel alignment on a rig (although I have identified the odd problem by elimination), I leave others to supply more information.

[marcush.]

I think most guys in racing completely ignore the effects of slack and and also don´t understand that things don´t necessarily behave like they were intended .
Very often you see ARBs mounted in hard bushings whne in effect ther eis not only torsion but bendinf as well so you will very likely come into bind and stick slip effects .
Quite often you can transform a twitchy car into somthing sweet just by removing the wrongs in the equation.
I think the spring analogy for a soft chassis is a good one and it must at least be a partial answer to the active suspension question elsewhere if you need a stiff chassis to make active work...you cannot work with too many springs in series as those add up softening thre total spring stiffness no matter how much force you apply .

[Jersey Tom]

I think most guys in racing completely ignore the effects of slack and and also don´t understand that things don´t necessarily behave like they were intended .

I dunno about that. Certainly not at the pro level. But if you say "most guys in racing" which includes Joe Autocrosser and all the feeder series, then maybe.