How do I determine what springs I need ?

[Corpulent Tosser]

Sorry if this has been asked many times before, but I am asking anyway.

I suspect the springs on my Striker are too firm, I don't know what they are as they came with the car.

Anyway assuming a weight of 600lbs front and 700lbs rear with the driver in place, is there a calculation or rule of thumb regarding what springs to fit ?

Thanks
CT

[GreenV8S]

I think your best approach is to work out what bump rate you want from the suspension as a whole (i.e. at the wheel) and once you have that, use the suspension geometry to work out the spring rate that gives you that. A usefull way to compare suspension rates between disimilar vehicles is to look at them in terms of static deflection i.e. weight on the wheel divided by suspension bump rate. As a very rough guide, you might start by aiming for an average static deflection for the car as a whole of around 4" and then work up or down from there to get your desired compromise between comfort, ride height/travel and handling. Once you know roughly how stiff you want the vehicle as a whole you also need to decide how you want to distribute this bump rate between the front and rear, and what roll stiffnesses you want. That is not trivial to work out, but if you only want a rough idea whether your car is massively oversprung for its weight you may not want to get into that much detail at this stage.

[Corpulent Tosser]

Thanks Green, not that I understood all your post but it has given me something to think about.

The car handles well IMO, but watching video of it compared wityh similar 7 style cars it looks firmer and I certainly get "all shook up" on bumpier tracks.

Thanks Again
CT

[Sam_68]

Make sure it's the springs and not the damper settings that are causing you to get 'all shook up'!!

The Gospel according to Colin Chapman was always to use relatively soft springs with firm damping, but many people with 'Seven' style lightweight kit cars assume that 'firm' means they need to set the dampers rock solid! The Sylva is quite a light car and with 'normal' adjustable dampers intended for road use (eg. Spax), which are usually valved to give a range of adjustment capable of working with much heavier cars, they often only need to be set a couple of clicks up from the softest setting (with fronts usually being set a click or two stiffer than the rears).

For what it's worth, I usually run my Sylva Phoenix on 180 lb/in springs on the front and 130 lb/in rear, with the Spax dampers at +1 click rear and +3 clicks front, but mine is on outboard front suspension rather than the rocker arms that I assume your car will be fitted with, so the leverages will be different. Try talking to Jeremy Philips (Sylva),RAW or Fisher Sportscars to see what they recommend.

Unless you are unhappy with the front/rear balance, I'd tend to stick to similar ratios of spring stiffness front:rear (eg, mine is 180/130= 1.38:1, so if I wanted to go stiffer, I might for example try swapping the 180lb springs onto the back and using 250lb springs on the front... 250/180 = 1.38:1, still).

Sylvas were originally designed to run without anti-roll bars 'cos Jeremy doesn't like them (in principle, I agree with him - they are used as a crutch by lazy and incompetent suspension designers when often they aren't required), but there is no doubt that they can be a powerful tuning tool if you do decide to go for softer springs, so you might want to consider a fairly light, adjustable front ARB to allow you to adjust the car's balance and to keep body roll in check.

Beyond that, it all gets a bit complicated, I'm afraid...there are ways of calculating spring rates, if you know your corner weights and unsprung weights, but it is quite involved and will in any case take a bit of final tuning to arive at the best set-up to suit a particular car ands driver's preferences. As others have said, unless you want to get really deeply into suspension set-up, your best bet is to base you selection on the advice of others who have experimented before you.

[Corpulent Tosser]

Thanks Sam

Some interesting points, when I got the car the dampers were set at their softest setting, I tried five clicks from softest (20 from softest to hardest) and that seemsed OK, I tried mid position, 10 clicks and the car had much less grip so I went back to 5 from softests and have experimented from there a few clicks either way.

From mid season on I started having understeer problems, this was around the same time I got the engine management working right so was probably getting more push mid to late corner, at the final event I decided to disconnect the anti-roll bar for my final run and there was a big improvement.

Maybe all I need to do is soften up the dampers both ends and leave the arb off.

Thanks
CT

[the gazman]

You could try the following to combat understeer.
1) back off the front dampers or stiffen up the rears
2) lift the back of the car a bit to throw the weight forward.

I increased the rear damping on a recent trackday to combat understeer every session until I provoked oversteer and then took it back.

[GreenV8S]

The dampers play a large part in the transient weight transfer that occurs on the way into or out of corners, and springs (road springs and ARBs) play a large part in the steady state weight transfer during steady state cornering. (Dampers also play a small part in steady state cornering.)