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[BlitZ]

When you cut linear springs it increases the rate. When you cut progressive springs generally it increases the static rate. But dynamically ( when moving) the spring itself will still only move within its limits because you have not altered the stiffer rate , only the "comfort" rate (whic usually sits above as the closer coils)

yeah thats if u cut a progrssive coil... progressive coils bound under pressure ... but if they dont bound the spring rate will increase.. even though it might not be much

[JohnL]

i get what you mean about shorter bars requiring more torque to bend, but it isnt really any stiffer/stronger than it was originally designed to be, it is only the illusion that it is more stiffer (i cant really explain what i mean too well), in the end, it is still of the same compound and of the same concentration. its a one of those paradoxical examples where less mass can produce more inertia and thus a greater resistance to flex. but yeah, i can see where you're coming from there.

with regards to the reheating of springs, i was referring to something i heard that people were doing, where they heat up the spring to reshape and make it shorter but thicker to increase the rigidity/stiffness.

With due respect, I doubt you really are getting what I mean or see where I’m coming from or you wouldn’t be saying what you’re saying. I’ll try again!

Just to be clear, I don’t mean bend. A torsion bar is designed to twist, though it would usually tolerate some bending (anti roll bars usually bend a little bit as well as twisting, especially more complexly shaped ARBs or those with less than ideal linkage geometry, but the purer the twisting action the better). Even a well designed coil spring may at some points within the spring have a very small element of wire bending with flexure (more likely near the spring ends, or in any area where coil pitch might change), but it should be very slight if it exists.

The torque force required to twist a torsion bar is, as far as the wire in the spring is concerned, exactly the same as the ‘weight’ of the car, i.e. the wire in the coil doesn’t ‘see’ any difference between a torsional force and car weight because the force (torsion or car weight) acts into and is resisted by the wire in exactly the same way, whether the wire is wound into a coil or not.

There is no paradox here, and no Voodoo! If you shorten the spring by cutting it then there it is not just a subjective illusion of stiffness increase, it really is objectively stiffer (though it may possibly be, with some drivers, that there could be an illusion that the stiffness increase is greater than it actually is in reality, but this may simply be a placebo effect, i.e. they expect a stiffness increase so imagine one greater than is actually the case).

On the other side of the coin, possibly some people might even subjectively perceive it to be a little softer if as a result the suspension bottoms out a lot more due to the lower ride height, keeping in mind that many / most drivers aren’t all that sensitive to subtle dynamic changes and that the rate increase from cutting is unlikely to be all that great unless the cut is pretty drastic!

I’ve slightly raised the rear ride height on my CB7 to lessen bottoming out (especially with a load of rear seat passengers) and the occasional chassis scrape on road humps, and (at first) it certainly subjectively felt as if it were just a little stiffer (and it does help decrease under-steer slightly by increasing rear weight transfer), even though I know it isn’t and I’d like to think of myself as being quite sensitive to subtle handling nuances! (after all I was club champion at my local kart club, which impresses me if no-one else!!).

The stiffness increase of a cut spring will be directly related to the % decrease in wire length and is most probably a very linear relationship, i.e. if you make the wire length say 10% shorter then the stiffness will increase by 10% (I think, meaning I don’t think that there is any logarithmic progression or whatever involved, as there is with a wire diameter increase, but I could be wrong!).

The spring is of course still the same “compound” (I think you mean alloy), and the same “concentration” (less certain what you mean here), but regardless of your meaning it’s for all intents and purposes irrelevant to this discussion. I’ll reiterate; alloy and state of heat treatment have at most little (according to ‘Young’s Modulus’ - less than 1%) or nothing to do with how stiff the spring is (only how long it will last), but any change in physical dimension (i.e. wire length and / or diameter) WILL affect spring rate.

Note that the dimension of free length has no affect on rate (nor coil diameter for that matter, unless greater or lesser length or diameter represents a change in wire length), only an affect on ride height. This is the case unless the free length is so long that the resulting pre-load is so great that when the car weight is lowered onto the spring the spring cannot compress at all (i.e. the preloaded ‘tension’ exceeds the car weight). So long as it is not so great that static car weight cannot compress the spring, ‘pre-load’ created by compressing the spring tighter between the upper and lower spring seats is also a non issue re rate, as preloading a spring to a greater or lesser degree will in no way stiffen or soften the spring, only alter the ride height.

Pre-load can be increased by either increasing spring free length or by further compressing an existing spring via the seat mounts, and the affect is identical (all else being equal), i.e. ride height increase. Excessive ride height is of course a bad idea, not just because you may excessively increase CG and / or roll centre heights, but also because you might significantly reduce suspension droop travel. The effect of running out of droop travel is similar to running out of bump travel, i.e. abrupt weight transfer from inside to outside, which may be OK in some circumstances and not in others. Decreasing pre-load will simply lower ride height.

A mild steel spring will be just as stiff (within its elastic range) as any other spring of similar physical dimension made from properly tempered spring steel, though the mild steel spring may not last more than 5 seconds driving because mild steel has a very low elastic limit (might not even manage to support the chassis at designed ride height before the spring sags, depending on car weight etc. etc.). With any steel we’ll actually find in any non hypothetical spring on real cars there will be no difference in rate between them if the springs are of the same physical dimension, the only difference being that the spring with better steel will last longer.

You mention “mass” and “inertia”, your meaning is unclear, but I suspect you are getting confused. Regardless of what you think you mean, greater mass will ALWAYS involve greater inertia than less mass. At any rate, while the mass of the spring is real, for purposes of this discussion it is irrelevant. When discussing chassis dynamics, mass and inertia of mass are only of concern when we are considering the affect mass and inertia will have on weight transfer (of sprung mass, and to a lesser degree unsprung mass), and the ratio of sprung to unsprung mass and the affect this might have on required damper rates etc (less unsprung mass always being very highly desirable, for both ride quality and handling).

Having said that, any difference in mass between a heavier or lighter spring (that could be fitted to a particular suspension) will in reality be so small as a component of overall unsprung mass as to be of no practical consequence, unless we were considering the design requirements of a very serious racing car, where every gram is of consequence!

The only way to make a spring “thicker” is by making it from thicker wire. It might be OK to heat a (compressed) spring to shorten its free length if you know exactly what you’re doing and have complete control of the process (temperature etc), but this will only affect the free length and thus the cars ride height, not the spring rate. I would be cautious of using such methods because of the danger of ‘over-shortening’ the spring and / or heating it to a degree that starts to interfere with the original temper of the steel, which is not to say it will…if you know what you’re doing! If the temper is affected the rate will be unaffected, but the spring may ‘sag’ prematurely.

A good spring can last effectively forever. The OE springs on my CB7 are still identical lengths on each axle line (LF = RF and LR = RR), despite being 16 years old! This is because Honda designs the spring well for its intended purpose, uses high quality materials, good technique and tight tolerances in manufacture (good old Honda!). However, if you cut even a very good spring you may shorten its useful lifespan, maybe a little… maybe a lot?

The design and intended purpose of a spring can also affect spring life. Springs made from shorter wire length will be stiffer (all else being equal), but will also be more prone to premature sagging (or possibly breaking, though I think this much less likely) compared to a spring of identical stiffness made from a longer wire (more coils) of thicker dimension. This is because for X suspension deflection the spring with the shorter wire length has to twist the wire more per cm of wire length than the spring with the longer wire length, each deflection causing the wire steel to more closely approach its elastic limit potentially leading to earlier fatigue.

Some racing cars use springs made from a relatively short wire of relatively small diameter, which can result in a stiff spring (or less stiff, if you want that) with relatively few coils of wire of relatively small diameter. This is done mostly for the desirable purpose of reducing unsprung mass, but, such springs can have a very short lifespan and must be replaced regularly for fear of spring failure (sag or break), i.e. the spring becomes a consumable item. Such a spring is not what you want on a road car even if it may perform very well, for a short while! A spring that has been cut has been moved at least a step toward more closely resembling this kind of spring, which may or may not be a problem depending on the specifics.