Understanding Spring Rates

[Tim Kay]

Help me understand correctly what I feel a pretty basic question:

Say you have a 300# spring rate on a coil over shock. Does the rate change as the travel is compressed or does it maintain 300# thoughout it's travel? In other words does it start at 300# and, when fully compressed, end at 300#?

In addition, does pre-load play a factor in this as well?

Thx :stupid:

 

[Paul Thompson 'Hooligan']

Hi Tim -

A spring rated at 300lbs generally means it will compress 1" per 300lbs applied load. Apply 600lbs and it will compress 2 inches - but the rate is still 300lbs per inch.

On testing, we have found many springs to be way off the described value from the supplier but with Eibach springs being almost exactly bang on the mark. I guess you get what you pay for sometimes..

 

[Tom]

If the spring is not progressively wound, it maintains its spring rate. That is to say, on a 300lb/in spring, each increase of 300 pounds load compresses it one inch further. Pre load does not change the spring rate, it will always have that same 300lb/in rating.

The rating of a progressive spring will change as it compresses, softer with the first coils collapsing, then higher as you get to the other coils. I don't know what the load curve looks like.

The only way to change the rating of a normal spring to change the number of active coils. You can either cut off coils, which increases the spring rate, or use a spring perch which allows the spring to actually thread onto the perch, which takes coils out of action.

 

[Tim Kay]

Thanks guys,

In essence then, stiffening or softening the ride (in addition to adjusting ride height) is affected by: , pre-load, spring length and spring rate.....?

If true then merely using a 300# coil is not enough information to obtain a disired result. Seems, first the ride height needs to be determined, then the length of spring needs to be determined and finally the rate, in that order.....?

 

[Pat Buckley]

Interesting thread.

Follow up question:

What is the best advice for determining starting spring rates?

Let's assume a 2400 lb car with weight distribution of 47/53 FR.

 

[Andy Sheldon]

Pat

Its a little more complicated than that.

You need to deduct unsprung weight.

It depends on the angle of the shock absorber.

It depends on any leverage acting on the spring due to the mounting point of the shockabsorber on the wishbone or upright.

Andy

 

[Mark Clapp]

Here's a description of info needed to determine spring rate. Search on the web for spring rate charts for given weights of cars. I've heard that about 350 lbs/inch is good for GT40s.

 

[Tom]

Right.

Spring rate is not the same as wheel rate, which is your starting point. Wheel rate will depend on angle of the spring to the lever, lever arm length etc. Wheel rate can also change during the suspension stroke, depending on the geometry. Wheel rate can be higher or lower than spring rate, depending on the set up, but wheel rate is typically lower.

 

[Terry Oxandale]

Thanks guys,

In essence then, stiffening or softening the ride (in addition to adjusting ride height) is affected by: , pre-load, spring length and spring rate.....?

If true then merely using a 300# coil is not enough information to obtain a disired result. Seems, first the ride height needs to be determined, then the length of spring needs to be determined and finally the rate, in that order.....?

Once the car's weight is on the springs, the springs should not be 'pre-loaded'. If the spring is pre-loaded at full droop, fine, but after the spring is compressed past that pre-load point, it won't have any effect on the car as long as that point is NOT within the compliance range of the suspension. Ride height and pre-load can be adjusted out with the proper length spring and threaded tube legth for coil-over set-ups, so these should not factor into the decision on what spring rate you'll use.

As Andy said, to do this accurately, you'll need to know the unsprung corner weights (because this is the weight that the spring is supporting), and then the movement ratio (ratio of spindle movement verses spring perch movement). A movement ratio (incorrect terminology, but I'm sticking with it) of, for example, .5 (the spring is compressed 1/2" for every 1" that the wheel will moves) would require a spring 4 times stiffer to have the affect as the same spring with a 1.0 ratio (inversely proportional to the square of the ratio)

So in regards to the 2400 lb, and 47/53 distribution, lets assume 80 lbs of unsprung weight at each corner, and you've got a .7 movement ratio (wild guess here because I'm not overly familar with the GT suspension). Then using the above information, you'd be reasonably close to supporting 556 lbs (sorry for the lack of SI terms) at each of the rear points or corners. With a 1.0 movement ratio (which is impossible for the GT), a 275 lb spring would compress roughly 2 inches from full droop to static ride height (no pre-loading, and so-so for the track, but a little marginal for the street in my humble opinion). In reality though, the same "support" with a .7 movement ratio would require a 561 lb spring 275/(.7^2) If you wanted a little softer street ride with a 3" compression of the spring at static ride height (which is a lot in my eyes considering the car), then a 380 lb spring rate would be a better choice.
This will get you close to what you want without considering the below. I'm a little confused on the chart in the previous posting, but I believe it is for live axle suspensions which does not reflect acurately the independent suspension design of the GT, and thus indicates a fairly weak spring rate compared to the GT requirements. AGAIN, this is based on assumptions and guesses, but does illustrate what you need to know.

Something a lot of folks don't consider is the dampening. You'll need a good damper for this or higher, and you'll also want to consider the front an rear bars as well in you decision because they all work together. you'll also need to know what the stroke of the damper is in order include this in your consideration. You'll not want a really weak spring if you've got a really short damper stroke.

 

[Ross Nicol]

Tim
With coil over shocks I would suggest there are only a couple of spring lengths that will fit and work on a particular unit as in my Koni 8211 type, I have found 8" springs are good and I have had alloy spacers made to set the ride height and allow adjustment in both directions.I have no pre-load on any of the units and the springs come off the platforms when I jack the car up. I have never had a problem with this and in fact it was an advantage when calculating my wheel rates.When you think about it an unloaded wheel is not contributing to cornering power so who cares if the spring comes off the platform, even if it actually does.Anyway enough of that. I believe for the cost of a couple of sets of springs or more if your like me, it is easier to ditch a lot of the theory and fit springs you believe to be close to Ideal and then work out your wheel rates for front and rear in cycles per minute.There are guidelines for roadcars about 70-100 cpm and race cars 120-140cpm.The higher frequency for race cars indicates stiffer springs.From here you may decide to go stiffer or softer to achieve what you want. All you need to do now is work out your wheel frequency and these ingenious guys in OZ have worked out you can do it by bouncing the car.Now the only downside is you can only bounce the car if you can remove the shockers which is impossible on a coilover unit.Ok then solution 2 is a simplified formula from Punn's book 'How to make your car handle'. Like my car if your springs come off the platforms you are able to take 2 ride height measurements put them in the formula and presto you have wheel frequencies and it's so simple to do.Now you compare your wheel frequencies to the ideal for your purpose Road or Race and alter springs if necessary. I came down 100# on my front springs and the calculated cpm was 131 down from 151 and has proven to be very good for my car. I think the kiss principle has worked for me on this one, I seem to only use theory when I can't break a complex problem into smaller understandable units.

Ross:idea:

 

[Ron Earp]

I believe for the cost of a couple of sets of springs or more if your like me, it is easier to ditch a lot of the theory and fit springs you believe to be close to Ideal and then work out your wheel rates for front and rear
Ross:idea:

I agree. You can do all the calculations you want (I did) but when the rubber hits the road the laptimes and car feel will show you where you want to be. For the the feel and times turned out to be a better indication of proper spring rate. After enough seat time you'll know where you stand, or, where to go with your springs based on what you feel. Springs are cheap, easily changed, so get a few sets around your target range and try them out.

Ron

 

[Tim Kay]

A spring rated at 300lbs generally means it will compress 1" per lbs applied load. Apply 600lbs and it will compress 2 inches - but the rate is still 300lbs per inch.

Paul, simply put. Thx

Its a little more complicated than that.

Andy, I should have known:shocked:

Mark, Tom and Terry, with that information it all begins to make sense. Thx

My dear friends Ross and Ron, agree, the KISS principle is always my first option. Me not a smart as me fellow forum members here :bow:

 

[Tim Kay]

Old thread but relative questions:

1) I'm using QA1 shocks with coil overs @ 400# front. The springs measures 7" and to get the ride height correct the spanner nuts are threaded well up the shock body and have compressed the spring so much that "coil bind" is a concern if extreme compression is required.

Does it stand to reason, to keep a 7" spring, the rate needs to be increaseed thereby less spanner nut threading up the shock body (less initial compression of the spring) would be required to maintain the same static ride height?

If I increase the spring length to 8" or 9" then I have to pre-compress the spring in order to get the spanner nuts started on the shock. Is that the normal way and my 7" is simply too short?

2) Most threads I read the spring rate for our GT40's is lower in the front than the rear, but there are a couple that run higher front than rear. Would that be a street vs track preference or what?

 

[Cliff Beer]

You also need to compensate with spring rate for the amount of "bind" that's in the suspension. If you're using rubber bushings that take a strong "set" instead of monoballs or rose joints or roller bushings then you'll most likely have some bind. Some cars have a LOT of bind in them.

 

[Tim Kay]

Thanks Cliff, rose joint suspension throughout.

 

[John Fitzpatrick]

On the subject of springs, I've often heard that valve springs should not be left compressed when an engine is stored because they will weaken with age if they are. I don't understand how that happens unless the steel is past the yield point, but if it's correct, does it apply to preloaded coil over shock springs as well?

 

[Trevor Booth]

Ross,
If your springs come off the seats ( spring seat release) it would not be a good thing as your car rolls in a turn, effectively your inside wheel is doing nothing. The reason that ARB are used is so the inside wheel has some grip. To counter spring seat release as in your case you would need to be running very stiff ARB's.

A very simple method of obtaing the load supported by the spring (coil shock units) is to put a hydraulic cylinder ( 25mm bore) fitted with a pressure gauge in place of the shock and from the formula P= Load / Area you can accurately obtain the load to be supported. This generally gives a reasonable starting point.

For road use comfort is the deciding factor (generalllY) and for track use a tyre temp gauge is a valuable tool for optimising spring rates and ARB stiffness.

In any event dont skimp on cheap shocks - just does not workout.
good handling makes up for low BHP you might spend $5000 to get an extra 10 BHP BUT you may spend $2000 to get better handling and more consistent results.

However, it is the package which prodiuces the more consistent results i.e Driver and Car and some drivers like things different to others

There is no hard and fast rule with spring rates - just an amount of time -)

 

[David]

On the subject of springs, I've often heard that valve springs should not be left compressed when an engine is stored because they will weaken with age if they are. I don't understand how that happens unless the steel is past the yield point, but if it's correct, does it apply to preloaded coil over shock springs as well?

It shouldn't. If they do then its poor quality steel. Springs should never take a 'set' or weaken over time (unless due to corrosion).

 

[EGLITOM]

TIM

Allow me to express my opinion about that:
1st : Preload , does not change the springrate, it only changes the ride height. Only when the shock is full out and you add preload, this preload need to be overcome but once that is done the springrate stays the same.
2nd. Springrates is a secondary of desired wheel rates. And they are a result of geometry , spring rate and swaybar geometry and rate.
3rd. there are as much philosophies out there as drivers. One prefers a softsprung car with more role, the other one a soft sprung car with little role, the third one a hard sprung car aso.
Your suspension is predefining this to a certain extend through factors like:
max suspension travel in and out, ride height, space available for springs and shocks.

I do love to use the available suspension travel as much as possible, running by the rule to have around 50% negative and 50% positive wheel travel available starting from ride height.
You not want to come into the last 15% on each side, to have enough left for extreme situations.
Define you maximum wheeltravel without the spring, but with the shock installed.
Check if not the shock may be other parts like ball joints are blocking before the shock blocks travel. If this is the case limit the shock travel to this maximum level, otherwise something else will brake sooner or later.
To check how much your travel is used, put a cable tie around your shocks rod and see how far it is pushed up after a ride.

ON my car i tried to achieve wheelrates which are close to the front to rear weight distribution. I´m using relativly soft springs but stiff bars. Thus the car is pretty comfortably to ride and on bad roads is realy compensating a lot of the road damage, thus allowing me to be fast on such roads as well, without the car jumping all over the place ( like some of my friends GT3´s). On the other hand the stiff bars minimizing body roll to a good size.

This setup is working out pretty well on my car.

Find attached a excel sheet. In this you can put in all your values of your suspension and see where you end up. It could give you a starting point from which to workfurther on. Only put in values in the yellos fields, all other fields are calculated.

You will see that the numbers put in show that the front spring is stronger than the rear one, but the wheelspring rate is lower on the front. THis is due to the suspension geometry. So as said, spring rate mean nothing without the information about suspension geometry.

If any question feel free to ask. HOw has your brake balance issue worked out ?

TOM

 

[EGLITOM]

It shouldn't. If they do then its poor quality steel. Springs should never take a 'set' or weaken over time (unless due to corrosion).

Springs do take a set, but good spring companies are treatening them to a preset procedure. THus there setting once installed is low. If you work a spring of the designed working range, it will take set, too.

TOM