Spring size criteria

[hardingfv32]

Given two springs of equal rate, one small and one large in size, what design features come into play when choosing one or the other. I appreciate the benefits of less weight and a smaller package. Just wondering if the spring can get too small for either a coil spring or torsion bar.

Brian

[Sonic59]

If they are made from equal material then to be equal rate their dimensions would differ. One would have smaller diameter, but longer. Another would be shorter but larger diameter. To choose between them depends on your design capabilities.

[WilO]

I had thought that the ability of a spring to store energy was a direct function of its mass, though maybe I'm wrong....

[DaveW]

A broad, perhaps even an inadequately defined, question. It would, in my opinion, be necessary to define load range, required displacement, duty cycle, life, & acceptable rate variation. A good starting point for research might be here.

[olefud]

Two parameters determine spring rate for a spring of given length; wire diameter and pitch, i.e. turns per unit length. Greater wire diameter and fewer turns increase the spring rate –the former increases weight and the latter increases stress. Thus the thinnest wire with the fewest turns that yields an appropriate stress rating for your spring material, with a safety factor, is usually the best bet for a light spring.

[DaveW]

Two parameters determine spring rate for a spring of given length; wire diameter and pitch, i.e. turns per unit length. Greater wire diameter and fewer turns increase the spring rate –the former increases weight and the latter increases stress. Thus the thinnest wire with the fewest turns that yields an appropriate stress rating for your spring material, with a safety factor, is usually the best bet for a light spring.

Good answer, except that you didn't mention the constraint imposed by spring buckling... & you assumed that torsional stiffness would be the major compliance. Belleville washers are used by some (e.g. Ferrari rear third spring)...

[hardingfv32]

The context of my question is in regard to this spring ad for dirt track racing.

"Some coil spring manufacturers build their springs lighter with
short lengths or few coil winds. This creates a very abrupt frequency that
does not let the tire follow the surface of the race track. Coil springs are
essentially a torsion bar wound into a cylinder shape. Coil springs with a
small number of coils are the same as a short torsion bar. Racers learned
years ago that cars with short torsion bars will hop across the bumps."

Brian

[riff_raff]

Given two springs of equal rate, one small and one large in size, what design features come into play when choosing one or the other. I appreciate the benefits of less weight and a smaller package. Just wondering if the spring can get too small for either a coil spring or torsion bar.Brian

"Some coil spring manufacturers build their springs lighter with
short lengths or few coil winds. This creates a very abrupt frequency that
does not let the tire follow the surface of the race track. Coil springs are
essentially a torsion bar wound into a cylinder shape. Coil springs with a
small number of coils are the same as a short torsion bar. Racers learned
years ago that cars with short torsion bars will hop across the bumps."

hardingfv32,

That last quote is a bit inaccurate. A helical coil spring is not "essentially a torsion bar wound into a cylinder". While it's true that both torsion bars and helical coil spring wires are both primarily stressed in torsion, there is a big difference in the mechanics of how the load is applied and reacted. The torsion bar requires the load to be applied as a moment, while the coil spring requires the load to be applied as a linear force. The spring rate of both simple torsion and coil springs is mostly linear within their elastic limits, and their inherent structural vibration modes (frequencies) do not change or "become abrupt".

The decision as to whether a torsion bar or coil spring is best for a particular suspension application basically boils down to packaging. The torsion spring requires a suspension linkage that will twist it, and the coil spring requires a linkage that will compress it.

Torsion springs are much easier to manufacture, since they require less specialized manufacturing equipment. But helical coil springs tend to have a more consistent spring rate, due to the fact that the torsional strain in the wire for a given load takes place over a much longer effective length.

Whether you use torsion or coil springs, having a suspension linkage that maximizes movement in the spring and dampener will always make adjusting/setting up the suspension easier. Suspensions with limited motion and/or high spring and dampener rates, results in greater sensitivity to small changes. If a wheel is "hopping across bumps", then that sounds like a rebound dampening problem and not specifically a spring problem. The accuracy of the dampener can be improved by increasing the fluid volume flow across the valving. If the dampener stroke travel is limited, you would need to increase the cylinder diameter.

Good luck.

[DaveW]

"Some coil spring manufacturers build their springs lighter with
short lengths or few coil winds. This creates a very abrupt frequency that
does not let the tire follow the surface of the race track.

I think that the second of the above statements is nonsense (if I may say so), although I do understand the logic (if that is the right word). Extreme or poorly manufactured coil-over springs can generate large damper shaft bending moments which can (could), in turn, increase damper friction significantly. Excessive damper friction could certainly cause the stated effect.

[olefud]

[quote="hardingfv32 wrote:
"Some coil spring manufacturers build their springs lighter with
short lengths or few coil winds. This creates a very abrupt frequency that
does not let the tire follow the surface of the race track.[/quote]

Taken literally, this may make some sense. Fewer coils of the same wire diameter would be lighter but with a higher rate. A stiffer spring will change the unsprung/sprung weight response and compromise tire contact. Of course this is outside of the original “same rate” premise and would cause other serious mischief.

To get back to the same rate, a spring with greater coil spacing would require a smaller wire diameter which, in turn, would increase stress. A premium spring alloy with costly molybdenum, vanadium etc alloying elements would also be needed to cope with the stress.

let the tire follow the surface of the race track.[/quote]

[Belatti]

Going directly to the grain, to reply your answer in an "easy" way, my first select criteria would be in line with the suspension geometry and travel the car you gonna put the spring has to cope with.

Once that is defined, I would check the rest of Dave´s list: load range, required displacement, duty cycle, life, & acceptable rate variation, constraint imposed by spring buckling etc.