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Originally Posted by 135

I think it's interesting that you mentioned usable stroke of springs versus usable suspension travel. I think this is relevant to practically all suspensions makes - the one exception that I've come across is Moton (and potentially MCS, since they were born from Moton). The Moton coilovers were developed specifically for each vehicle such that (i) the maximum useable suspension travel (i.e. stroke of the strut shaft) is greater than the usable stroke of any spring combination (that could be fitted within the range of adjustment of the threaded body for the lower spring perch), meaning that the spring will always coil bind before the strut bottoms out, and (ii) they do not require bump stops, again, due to the aforementioned reason where the position at which the bump stop would normally be encountered/needed is not reached.

"Every" suspension needs a bump stop, which is to say a progressive rate spring at the limit of compression so that something does not crash (springs bottom, strut bottoms out, a suspension component contacts the chassis, or a tire contacts the chassis). The alternative is to run very stiff springs that will not be ideal under normal conditions.

"Ideal" is actually not that difficult to figure out based on undamped natural ride frequency - choose the one you want, and a spring rate drops out of that choice. With enough suspension travel, you could get away without a bump stop, but our cars don't have enough to do that. The Ohlins strut uses an internal bump stop that you can't see, but it is most definitely there. My early measurements led me to think it was a series of Belleville spring washers, but later more accurate measurement show it to be a conventional urethane bump stop.

On any given strut, you will be able to find spring (perhaps in conjunction with a helper spring) that will result in potential coil bind.

But to your point on suspension travel - that is the point. Usable spring stroke is how you get it. You need enough spring stroke to get the amount of suspension travel you need for the conditions you are operating under. It is very dicey to rely just on spring rate to avoid crashing your suspension. Without adequate suspension travel, one you will hit a big bump and break something.

Originally Posted by 135

As a side note, some Moton coilovers (such as the ones I have) come with remote reservoirs, which are nitrogen charged at 175psi, and I've been advised that the reservoir pressure can be increased to preload the springs for better transitioning.

I don't think you have that description quite right. Gas pressure results in a lifting force that will raise your car. It is equal to, in your case, 175 psi times the cross sectional area of the piston rod. Raise the pressure and you will raise the car a little bit, which actually takes load off the springs.

To be clear, when I speak of preload, I mean how much the spring is compressed when the strut is fully extended. If you are using helper springs, the preload of your main spring is zero.

Increased pressure in the strut also increases the seal drag a bit. Generally friction in a suspension is considered a bad thing, but you can also consider it to be extremely low speed damping, thus it reduces body motions under slowly changing load conditions.

Originally Posted by 135

With my Moton coilovers and Z60-178-70 60mm 7" (178mm) 392 lbs/in Swift springs (Usable/Maximum Stroke 96/115mm) set with a static spring length of 163mm (Effective Usable/Maximum Stroke 81/100), I don't experience any coil bind. Nor did I experience coil bind when the static spring length was as short as 138mm (Effective Usable/Maximum Stroke 56/75) - this was purely based on no evidence of binding on the spring coils.

Just to clarify - I would call 163 the preload length, not the static length.

The thing about a lot of preload is that as you unload the tire, as soon as the spring extends to its preloaded length the wheel suddenly picks up off the ground. This looks dramatic at the front of the car, but it results in a sudden weight transfer to the opposite wheel, which will cause an immediate understeer reaction. Ideally, you want your inside front tire to get very light at steady state limit cornering, but not come off the ground.

Originally Posted by 135

With all that being said, I would like to raise my lower spring perch to provide further clearance from my front tyres and, as a result, raise the vehicle height. My approach may be different to the majority in that my aim is not for a target ride height but, instead, for the height to be determined by the lower spring perch location (to provide wheel and tyre clearance) and the subsequent corner balancing of the car.

You need to consider the big picture. If tire clearance is the issue, this is a reasonable approach. You will be reducing usable stroke, but then your experience says that is ok.

Originally Posted by 135

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What I'm wondering initially, though, is whether the maximum usable stroke would be affected by the Helper spring. If you've considered this or have any formulae for determining this, it would be greatly appreciated if you could also attach them to this thread.

I follow your helper spring discussion but won't try to dissect it. Your calculations generally make sense.

Usable stroke is defined by the main spring and is reduced by preload. Because a helper spring lets you run zero preload on the main spring, a helper spring does not reduce usable stroke of that spring.

That said, if you are using a helper spring, that means you have selected a shorter free length main spring. By necessity, that main spring will have a shorter usable stroke than a longer spring (of the same design family) with the same rate.

Originally Posted by 135

I know you've previously recommended against a higher spring rate but I thought those comments may have been based in the limitations of an Ohlins coilovers setup and, considering the absence of coil bind in a range of a scenarios with my Moton coilovers, I would be interested to hear your thoughts on the above options, especially regarding the 6" vs 5"+Helper decision?

Based on the undamped natural frequency of the car, I would not suggest a front spring stiffer than 70 N/mm. At that, your undamped natural ride frequency at the front is about 2 Hz (when tires are considered in the calculation), which is in the high range of normal for a sedan based race car. Suspecting that your car, like mine, does dual duty, that is really the upper limit of reasonable. If the car doesn't feel stiff enough at that, I suggest that something else is actually the problem. If it is roll stiffness, that should be addressed with bars not with springs.