Are lowered cars to be avoided?

Also sprach "JohnR" :-

You're confusing stress with strain. If you apply the same /force/ to your string of rubber bands the stress on each remains the same. As does the strain, actually. It's only if they're paralelled up that there's a difference.

Two bands side by side individually experience half the stress for a given load on the overall system. They also obviously experience half the strain.

Yeah but you're actually just shortening the travel, the material left doesn't suddenly become stiffer because a coil or two are missing. What's actually happenning is that the coils left are being deformed much more than they would normally be with the other coils there. As the spring gets compressed the forces needed to squash it increase in a non linear way so at the extremes a very large additional force is needed to compress a very small additional travel. By lopping off coils you're just pushing the spring into the extremes where the deformations are smaller but at the expense of much increased (outside the design envelope I would guess) stress and strain on the remaining coils.

NO they won't, if you hang 1 1b on 1 elastic band it will stretch 1 unit, assume we choose an elastic band that won't snap & will stretch 1", if you hang it on 4 elastic bands each band will stretch 1", for a total of 4", take 1 away & they'll stretch a total of 3", the tress in each band is the same. For the best primer on the subject try

Yes it does, that's the whole point of Hookes law

my understanding is that strain is a measure of distortion, either elastic or plastic. Stress is a physical function of external force and the material on which it acts.

I knew it was a crap analogy - trying to make the point that by removing a component (if a coil in a spring can be called that) the same energy still has to be absorbed by what's left.

Also sprach "JohnR" :-

Quite so. But for the ordinary chav, cutting a lump off the end of a spring will make it stiffer. The reason I wrote what I did was 'cos there are people out there who don't believe cutting a lump off will change the rate.

Also sprach "JohnR" :-

Energy isn't absorbed by springs - that's what the "shock absorbers", properly called dampers do.

It's the relevant analogy but the coils in a spring don't act side by side, they act end on end, if you took a leaf out of a leaf spring you'd end up with more stress & strain in the remaining leafs.

hehehe - i'm busy today, let me have a think and i'll try explaining it in easier terms - might just be crossed wires.

Of course it is - energy in the form of a force applied to a spring squashes it - that deformation is absorbing (using) energy. The dampers act as a device to reduce sudden shock by adding extra resistance to force (energy) spikes, the springs still do the bulk of the job. I have got to leave it there for today.

Also sprach "JohnR" :-

Oh dear - you ain't got the hang of this, have you.

Springs /store/ energy. The don't "use" it[1]. Springs are there to provide some compliance in the chassis without which the whole chassis would have to rise and fall to every bump or pothole in the road. The dampers are there to absorb any excess energy which is not returned by the spring to the forward kinetic energy of the vehicle.

When a wheel hits a bump it tried to push the car upwards. Because of the geometry of a wheel hitting a bump there's also a retarding force pushing backwards against the wheel. This force, multiplied by the distance the wheel has to rise give you the energy of the event. This energy is stored in the spring and on the down side of the bump is returned because as the spring forces the wheel back down it also pushes the car forward. Because the dampers are active all the time they absorb some of this energy on the up-stroke and again on the down stroke.

How successful all this is depends on many factors - principaly the fundamental frequency of the spring/mass pair of the suspension. If it happens to be exactly right for the bump you've hit then you'll get most of the energy of the event back, though clearly the dampers will always take some out. If, for example, the down side of the bump doesn't come in time then the springs will be pushing the car up instead of the wheel down - which is wasteful but unavoidable.

Dampers are not there to stiffen the spring or cope with overload situations - at worst the bump-stops are there to do that. Dampers stop the spring/mass pair of the car continuing to bounce for ages after the bump. Drive a car without dampers (or just bounce one up and down) and you'll see that it's no easier (well, marginally because of the energy taken out by the damper) to bottom it on a single bounce than one with dampers. The trouble starts if the undamped car's oscillating wildly and hits another bump at just the wrong moment, adding energy to a system which already has too much.

its the language i'm using - should just stf up really - when i say absorb i mean store.

Also sprach "JohnR" :-

Ah.

Quite

It was somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember DuncanWood saying something like:

The easiest way to visualise a coil spring is to think of a torsion bar

- which is just a straightened out coil. Iow, a coil is just a curled up torsion bar.

You can twist a torsion bar a set amount with a set load. Cut a piece off the torsion bar and use the same load - you'll find the deflection (twist) is less. The natural ability of the material to deform is the same, but there's less of it to deform with.

Do it too much, of course, and the material will crack.

Also sprach Grimly Curmudgeon :-

D'yer know - I spotted that when I was about 12 and had enormous trouble getting my physics teacher to believe it. 'Cos I'd thought it out for myself and not got it from a book so couldn't give him a referrence he went all huffy.

Still, he was a silly arse, happily replaced by one who made the subject interesting and knew what he was talking about.

It was somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember Guy King saying something like:

Heh. That's often the case. I found college lecturers who'd had a career in industry before teaching made better teachers than any I knew at school. They would pepper their instructional material with anecdotes relating to the subject. Tales of explosion and disaster and sometimes just humour made the subject much more interesting.

: In other words the reduced coils means the ones left are : having to cope with the additional stresses and strains that the coils : you've removed would have done.

No. The stresses will be exactly the same.

: Get three or four equally sized elastic : bands together... Each coil in a spring : is similar to each elastic band in the example above.

No it's not. The coils of the spring are in series: they see the same force/stress and all contribute to the total displacement/strain. The elastic bands are in parallel: they all see the same displacement and contribute to the lower stress.

Ian

: I knew it was a crap analogy - trying to make the point that by removing a : component (if a coil in a spring can be called that) the same energy still : has to be absorbed by what's left.

Not quite. Springs deal with forces, rather than with energy - use a stiffer spring and you store less energy for the same force (energy =

Ian