Understanding Spring Rates

[David]

I disagree, as long as a spring has been specified correctly and made from the correct materials it should not 'take set'. As long as the imposed stress is lower than the yield strength of the material the spring will fully recover its initial length when the load is removed.

Although a good point (or two) is made here: Spring Technical Information

If the spring does not have sufficient material to handle the stress (as many don't), it will take a set (as many do)

Many times, because of the long stroke requirements for certain rates of racing springs, material strength must be sacrificed to achieve significant stroke. Couple this with the fact that the ideal wire diameter is not always made and you can see why some springs have a real potential to take a set. We have seen some brands of springs lose as much as 15/16" of free height during normal operation. To eliminate any set from occurring at the race track, it is good manufacturing policy to pre-set (press to solid height) all racing springs during their manufacture.

When a spring takes a set it will normally stabilize at its new height. The rate effectively remains the same since no appreciable changes have been made to any of the three factors that determine the spring's rate. Other than creating a need to readjust the chassis (to restore the original set-up and ride heights) the spring should provide satisfactory performance. It is not uncommon for even well designed and properly manufactured springs to settle up to 1% of their free height.

Personally I think that last line is a get out but I included it for completeness. IMO if your springs are taking a set then you should look to another manufacturer.

 

[Seymour Snerd]

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?

Tim -- just to help your intuition a little with the first question about spring length: the spring rate is determined in practice by only three things:

1. The wire diameter (larger --> stiffer)
2. The coil diameter (larger --> softer)
3. The number of coils (larger --> softer)

The point is that spring length is not part of the rate equation. This means that if your 400 lb/in figure is correct for handling and ride for your car, then you can have made or buy a spring made with any length you want, within reason.

This means that for a given adjuster position you are free to choose the preload you want, and vice versa. But as you point out given those choices you need to choose a wire diameter and number of coils that does not bind at full compression. Your coil diameter is probably fixed by your shock dia. and hardware.

So if you know the spring length at full compression, and you know the preload force and length you want, you should be able to take all that to a spring winder and get something that works without altering the spring rate. The preload you choose will determine how hard it is to R&R the springs. I don't know how you should arrive at a preload figure; maybe someone else does. It's not clear to me why it shouldn't be very close to but just a little more than zero; you certainly don't want any play at full droop.

 

[Trevor Booth]

before you can do anything about springs you need to know the load at fitted length.
You can do this in two ways, measure and weigh everything and calculate it or use the HYD cylinder method described earlier. Once you know your load at fitted length you can give this information to the spring maker + compressed length ( amount of bump) + extended length (amout of droop) you will need to add some preload so that springs do not come off the seat at full droop. The amount of preload is difficult to work out initially and is best determined by experiment and how you want the car to ride and handle.

If you know what wheel rate you want to achieve then you can easily work backwards from the known wheel rate (see Eglitom spread sheet)

 

[Woody]

Hi Guys
Interesting thread. I think Rons correct, each GT has its own suspension type and geometry.And you wont find out till you drive it. A good starting point is were someones been before.
If you go into a good race car shop they always have a selection of used springs lying around because fine tuning is trial and era.
On the valve springs in your engine, At any one time there is always a couple of valves wide open and on a hig lift cam that is almost fully compressed then left for some time, Yes they will take a set or weaken.
If your not going to drive it for long periods then back off the rockers. then the springs will last longer.
Dont forget to reset them before your next Sunday drive.
Woody.

 

[EGLITOM]

before you can do anything about springs you need to know the load at fitted length.
You can do this in two ways, measure and weigh everything and calculate it or use the HYD cylinder method described earlier. Once you know your load at fitted length you can give this information to the spring maker + compressed length ( amount of bump) + extended length (amout of droop) you will need to add some preload so that springs do not come off the seat at full droop. The amount of preload is difficult to work out initially and is best determined by experiment and how you want the car to ride and handle.

If you know what wheel rate you want to achieve then you can easily work backwards from the known wheel rate (see Eglitom spread sheet)

As i said one needs to know the load per wheel = weight distribution
(Don´t know why one would need the hydraulic solution if weight cells and calculation would do it as well). But that alone is not enough to define the spring rate. The minimal rate would be the one to support the car in ride height without to much preload and still beeing strong enough to avoid a full compressed shock in extreme situations ( part of this can be controlled with the compression line of the shock as well).

Than it is a question of car behaviour with the spring rates installed.

Lets assume you have a non sway bar car and the lightest springs installed fullfilling the above mentioned requirements.
Now if your car is showing understeering you would need to stiffen the rear by increasng spring rates there. If it shows oversteering tendency than increas the front spring rate.
In car with sway bars you can control this with sway bar forces as well or combination out of spring rates and swaybar rates.
This said, this is a very simplified approach leaving out influences of toe, ackermann,camber and cast, but it should help to find a basic setting,


TOM

 

[Cliff Beer]

I disagree, as long as a spring has been specified correctly and made from the correct materials it should not 'take set'. As long as the imposed stress is lower than the yield strength of the material the spring will fully recover its initial length when the load is removed.

Although a good point (or two) is made here: Spring Technical Information







Personally I think that last line is a get out but I included it for completeness. IMO if your springs are taking a set then you should look to another manufacturer.

I tend to agree. People often misinterpret what's going on and say the spring "takes a set".......when what they're really seeing is bind in their suspension. In other words, the bind in the suspension (rubber bushings tend to produce the greatest bind) doesn't allow the spring to return to it's natural length (given the weight/load) because the bind is further reducing travel.

The front of my CAV is all rubber bushed (inboard ends of all four a arms) and there's a fair bit of bind (ie. "memory"). I plan on changing out the rubber bushings for roller bearings in the summer for a nice little weekend project. The rear is all rose joints, but even rose joints have some amount of bind.

Just my $.02.

 

[Cliff Beer]

whoops, duplicate post.

 

[Tim Kay]

As usual, you guys have turned my KISS approach upside down and are having me consider things I didn’t even think were relevant. In other words, there’s always more to it than simply addressing one thing, so many things work together to get a desired result.


Tom, thanks for the spreadsheet. Now that I’m done with your brake spreadsheets I have a new subject to study and spend countless hours playing with and trying different scenarios! <?xml:namespace prefix = o ns = "urn:schemas-microsoft-com

:thinking2:Just great!



Alan and Tom pretty much answered the simple question regarding spring length, lot’s of great info and guidance here, thanks to all you guys.


<o>

If you go into a good race car shop they always have a selection of used springs lying around because fine tuning is trial and era.

</o><o>

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

</o>
As quoted above, underlying all this technical and philosophical info is still the old fashion “trial & error ‘til you get it right” procedure. Tends to be the way I operate as well, (not by choice but as you can tell I'm not equiped with the smarts to figure it right without doing it wrong first).

HOw has your brake balance issue worked out ?

Very well Tom, thanks in big part to you, see:http://www.gt40s.com/forum/gt40-tec...ngy-pedal-new-wilwood-setup-2.html#post359066

 

[Tim Kay]

At this point I like what I'm feeling, 400# front and 375# rear. I haven't quite got all the "body roll" out I would like but I'm hoping to adjust out what's left with stiffer sway bar settings.

I had 350# on the front but by going to 400# took out the "dive" I was getting with weight transfer under heavy braking (which I can do now!!).

Only test have been on public roads, in specific areas where I can safely get aggressive. Soon I'm going to get this car on the track to see if I like the results.

 

[Trevor Booth]

It is quick easy and very accurate method of determining the actual spring load and no dismantling of the car apart from removing the spring shock unit.
These came from aircraft surplus at about $20 ea and I added a series of adjustable length rods , very simple very effective and you do not need a set of corner weight scales, you dont need to accurately measure up your suspension

The cyl is 220mm eye to eye and 1" bore

 

[EGLITOM]

It is quick easy and very accurate method of determining the actual spring load and no dismantling of the car apart from removing the spring shock unit.
These came from aircraft surplus at about $20 ea and I added a series of adjustable length rods , very simple very effective and you do not need a set of corner weight scales, you dont need to accurately measure up your suspension

The cyl is 220mm eye to eye and 1" bore

Trevor,
one would have to measure all four corners and take the average of the front values and the average of the rearvalues, by doing so one would have to watch that rideheight stays on desired value at each measurement. correct ?
Any similar smart way to adjust corner weights after you have completed the final spring/shock installation ?

THanks
TOM

 

[Mick Ridley]

Tom,
When you say

QUOTE=EGLITOM;359821] The minimal rate would be the one to support the car in ride height without to much preload and still beeing strong enough to avoid a full compressed shock in extreme situations ( part of this can be controlled with the compression line of the shock as well).

TOM[/QUOTE]

What do you mean by "extreme situations".... can you give examples ?

 

[EGLITOM]

Mick

exteme situations are situations where your suspension will be loaded much more than under standard driving conditions.
like:
exreme bumps, like permanently running over curbs on a track, like hitting a bump when you have loaded the front suspension fully anyway like the outside front wheel on fast turns or braking into fast turns, Jumps, bad roads, aso,


I still would like my suspension to have enough movement leftover to absorb this things, otherwise this loads will fully bear on every other suspension component and bent or break it if it is not strong enough. it may not happen on the first incident or not on the thenth, but who knows.

Thanks
TOM.

 

[Trevor Booth]

Tom,
I extend the cylinder about half way add the extension rods to make the cylinder eye to eye about the same as the designed shock eye to eye at ride height. Then turn rods (they are threaded) after install in car to get the ride height correct and the underside of the car parallel to the ground side to side and the desired rake front to rear. Record the ride height setting from ground at a nominated point say CL of wheels. Record the pressure on the gauge. I generally take the lowest reading and use that to work out spring load. After you have springs made and installed adjust spring base until you get the same ride height dimensions as previously recorded. Naturally when you do this the car has to have all fluids and driver. You can do front and rear separately or all 4 at the same time. It works very well and takes into account friction loads