Optimizing Suspension Performance: Understanding Preload for Progressive Springs

[Mark Tamblyn]

Hello I have a question on coil-over preload for Progressive springs. I understand the theory on spring rates, in that setting pre-load does not alter spring rate (Hookes Law), it just compresses the spring, adding load, which does raise the height of the car. Setting pre-load is necessary so to load the spring slightly so as to not allow it to flop or rattle when fully extended (if you know what I mean). Conversely, setting too much pre-load has its problems such as reduced spring and damper travel.

Thats all well and good for a linear spring, BUT what about a progressive spring? As you know a progressive spring is just that...the rate is lower at the bottom for normal driving, and progressively the rate increase as the spring is compressed, well I think that's how it works!

My question is...by setting more pre-load, despite popular belief, would this be altering (increasing) spring rate?

thanks :)

 

[Ranger Mike]

yes it increases overall spring rate. A slight amount of " load" is necessary to retain position of spring in pocket or on the coil over but not excessive. Crank the collar but don't go crazy unless you know exactly what spring rate you want.

 

[Mark Tamblyn]

Thanks Ranger Mike. Just to confirm...this is only for a progressive rate spring you refer to, right? I am under the impression setting pre-load does not alter rate for a linear spring (Hookes Law), and many other forums suggest this too. I am referring to specifically a progressive rate spring.

Thanks

 

[Ranger Mike]

Pre-Load is how much the springs have been pre-loaded with zero pre-load being the point at which the springs, slider, spring plate, and coil nut adjusters are touching with no play. This applies to all coil springs, linear and progressive. Your particular set up could have pre-load from zero to 1 to two inch and up but this defeats the purpose of the coil over set up. That purpose is to deal with cornering momentum ( many call weight transfer ..incorrectly) interaction of the tire contact patch. You want to go as light on the spring package as you can and still keep handling control. Pre-load adds spring rate.

 

[Mark Tamblyn]

Thanks for your reply Mike, but I must disagree with you, preload on a linear spring does not alter the spring rate...it only alters spring (and damper) travel distance. Its not to be confused with load of the spring, which is essentially what preload is. For example adding weight in the boot - the load increases (more weight ans suspension drops), but the rate doesn't (it can't its set by the manufacturer).

I do understand what the purpose of preload is (to take up slack basically). However, on a progressive spring whereby the rate of first few coils is less than the top remainder coils, adding preload, or more of it, would affect the overall summed rate, by 'taking' up that lower rate.

Taken from Silvia.net forum - Linear springs are the same spring rate no matter how much they are compressed (preloaded). With progressive springs, preloading the spring does all the same things as linear springs, but it also increases the initial rate of the spring. So say you have a spring with a 200lb/in initial rate, and a 400lb/in max rate. (all progressive springs are rated this way.) Preloading 200lb's will compress the spring just under 1". Since the rate goes up with a function, it will have increased to probably somewhere around 250lb/in by the time you reach a 200lb preload. So now your initial rate is 250lb/in, with a max 0f 400lb/in.

So I've kinda answered my own question really, lol ! But if you or others have anything to add...

 

[Ranger Mike]

I may not have been clear on this. a 200# spring is rated at 200# and this will not change. What will change is the springs resistance with compression. (per-load is compression btw)
ifin a linear spring is rated 200 # per inch and you compress it one inch , what does the spring tester read? 200# ..right? now compress it another inch..what does it read?
A. 200
B. 400
C. 100

 

[jack action]

Just to show you how a linear spring works:

Note that coil spacing between each coil is changing at the same rate at any moment. When the spring reach its solid length, then it becomes as stiff as a solid rod, i.e. you have no spring anymore.

One thing you should know is that the less coils you have, the higher is the spring rate.

Knowing that, you can make a dual-rate spring (high and low spring rates):

The upper part of the spring has a smaller coil spacing, thus they will close the gap before the lower part does. At this point the upper part becomes a solid beam and the spring has less active coils, thus a higher spring rate.

To make a progressive spring, you just need to increase the coil spacing slightly between each coil, This way, each coil will close the gap one after the other, thus increasing progressively the spring rate:

 

[Mark Tamblyn]

Thanks Ranger Mike and thanks Jack Action for your comments.

Jack Action - so for my progressive rate springs by applying more pre-load, will in fact change overall spring rate.

The springs are rated at 850lb/cm, but I'm not sure of the lower rate, and would this 850lb rate be the overall rate or the highest rate? I might have to find this out by the manufacturer (KING Springs). BTW Jack I notice image 1 is from UWA - you in Perth?

 

[Randy Beikmann]

When adjusting pre-loads with a coil-over setup, the goals are usually to 1) achieve a desired ride height, while 2) equalizing the suspension pre-loads as much as possible (i.e., adjusting "corner weights"). If the CG is along the centerline of the car, the spring pre-loads on an axle would then be equal side to side.

With a given car and weight, adjusting the ride height at an axle evenly side-to-side does not change the pre-loads, so even with nonlinear springs, their rate wouldn't change. Of course changing the weight of the car would put the springs at a different rate.

 

[jack action]

The springs are rated at 850lb/cm, but I'm not sure of the lower rate, and would this 850lb rate be the overall rate or the highest rate?

Looking at http://kingsprings.com.au/product/progressive-rate, it is probably their highest rate. (I hope it doesn't go higher than that! )

The reasoning behind this design is to put a really stiff spring at ride height, but that will go to a lower rate as it is in full droop. This will favor ride comfort, shock absorber control and spring entrapment. So, in essence, it is more like the dual rate I showed you in my previous post, but the upper portion is of the progressive type.

BTW Jack I notice image 1 is from UWA - you in Perth?

Nope, just look for an animated image on Google.

 

[Mark Tamblyn]

Thanks Randy, but I am under the impression though that it is not good practice to set ride height using pre-load adjustment, but yes it can be used, and racers do do it ;) I plan on setting my corner weights this weekend, and will do a CG test aswell.

Hi Jack, yes assume it is their highest rate, lol. but it would be good to get the springs load/rate tested to know the 'true' rate as I assume the lower rate influences the higher rate. That's what I meant about if that 850lb is the overall rate, or just the highest rate, or doesn't it matter, in that the two rates are clearly defined.

 

[Mark Tamblyn]

sorry I made a booboo - unit should lb/in not lb/cm. Gotta get my head around the imperial vs metric. Personally I favour metric :)

 

[Randy Beikmann]

Thanks Randy, but I am under the impression though that it is not good practice to set ride height using pre-load adjustment, but yes it can be used, and racers do do it ;) I plan on setting my corner weights this weekend, and will do a CG test as well.

Hi Mark. Adjusting the spring seat height on a single spring actually does several things. If you raise the seat, the spring must shorten, but as it does, the load goes up on that tire. This can't happen without load increasing on the tire diagonal to it, and being reduced on the other two (racers call this "wedge"). So you have already "adjusted pre-loads" on all four tires by "adjusting spring seat height" on one. The lengths of the other springs also changed accordingly as the vehicle tilted.

My point was that if you raise or lower the ride height by adjusting the spring seat height equally on the rear tires (or front), you're not actually adjusting pre-loads at all! Why? Because the spring lengths will be the same before and after - the vehicle height changes only from the spring seat height.

So "adjusting spring seat height" and "adjusting pre-loads" are two different, but very interrelated, things.

Are you doing a tilt test to measure the CG height? Or using corner weights to measure the fore-aft location?

 

[Ranger Mike]

Your question about the progressive spring rates -
“Progressive rate springs” are generally classified into two sub-types: 1) constantly increasing rate springs and 2) a “dual-rate” spring with two linear rates connected with a rate-transition range.

The first type (constantly increasing rate) of springs are most often used as “load-compensating springs”. They are identified most easily by continually varied spacing between the coils. Check out a dirt track modified using a pull-bar on the right rear axel. This constantly increasing spring rate, is used to control rear axle wrap during acceleration. These springs are not suspension main springs, but are supplemental springs.

The second type of progressive rate suspension springs, the dual-rate spring with two linear rates connected with a rate transition range is a much more sophisticated suspension spring. The design is much more focused on the specific use for the spring. These types of springs are used primarily in road racing and high performance street and GT applications where the vehicle trim package will stay in a very predictable range. These springs are easily identified by having a few closely wound coils at one end and then wider, equal spaced coils at the other end. They have rates described as 200/425lb/in. This means that the spring has an initial rate of 225lb/in through some range of deflection and then the rate transitions to 425lb/in through a deflection range of 1”-1.5.” The big advantage of these springs is that they can provide “roll control” in addition to roll control provided by ARB (sway bars).

Let’s assume we are working with a progressive 200/425lb/in spring. At ride height the car will be setting on the low end of the rate transition range, about 210lb/in. As we enter a high speed turn, the outside of the vehicle will roll into the 425lb/in spring rate and the inside of the vehicle will be pushed up with a much lighter 200lb/in rate. At the rate transition point, the closely wound coils will be closed almost to the solid point.
The advantage of these springs is that they control body roll where you have a requirement to keep the body at some steady state.
For race cars turning left only, not the hot set up as you need to put the body roll to good use to turn the car. Linear springs are used in racing because they are easier to dial in for any given track versus trying to figure a dual rate spring adjustment to compensate for load transfer due to momentum ( weight transfer to the ill informed) during cornering.
I re-read your original post and I think your question is really about setting proper ride height. Please permit me to venture into this topic as it may be relevant.
Ride height is usually dictated by the ruling organization and you pretty much have to meet this at Tech. From that point it becomes a working backwards drill.

The actual Ride Height of the race car is a function of the load at height of the springs and the adjustments of the coil overs and not the spring rate. If you install solid steel bars in place of the coil overs, you will not effect the corner weights. In other words, if we swapped out the coil overs and replaced them with bars having same center to center mount points ,set the car with ride height and rake, dummy drive weight, full fuel and tires at race specs, and scaled the car, the result should be the same as when we had the coil overs fitted. That is the ideal way to do it. There are a number of ways to do this without making a major project out of it but you get the idea.
A few things that must be done is knowing the % left side and rear % weight and know your % cross weight. Other things to figure is Center of Gravity and polar moment. All of this results in figuring the corner weight of each wheel. hope it helps.
rm

 

[OldYat47]

Progressive springs have no "overall spring rate". They change rate as they compress. Adding preload changes the "starting point", if you will. Past that preload the spring will follow the compression load chart. So if your preload is 10 mm then the spring will exhibit that amount of force. At 20 mm it will exhibit the same force as if you started with an unloaded spring and compressed it 20 mm.

 

[Mark Tamblyn]

Thanks Randy, Mike and OldYat.

Randy, yes I'll do a CG test and also doing corner weighting.

Mike, I have the type1) you explained, and just in the rears. Pretty standard for rears for most US/Aussie rear wheel drive IRS sedans. However, it looks to be they are not "true" progressive rate springs, in that the progression is not uniform. My springs have smaller diameter at the bottom, for them to sit on the perch, then they 'wind' out to probably double diameter. See photo of whole assy. My original question isn't so much about ride height, rather setting preload for progressive rate springs, in particular 'preloading the softer rate out' and asked whether I had the concept right.

OldYat - good simple answer thanks mate, and pretty much answers my question straight off the bat. My springs are rated at 850lb/in, so wonder how they have measured that? - first 1inch, which wouldn't work right, or fully compressed, then averaged out, or it's the highest spring rate. I might see if they would give me the compression load chart, good one.

I'm understanding progressive rate springs more and more thanks to you guys, and I think I was on the right track, but your guys confirmed it for me.

So, what I want to do from here is preload the springs to past the starting point (thanks OldYat), as it is for a track car and want the springs to be within the main highest rate, if you get my drift (no pun intended, and no I don't drift). Without the comp chart this may be difficult, but was thinking go by feel, in that I tighten (load the spring) the preload perches until I feel high resistance. Obviously this will be well past a normal preload point. The chart would be good as I could measure it. The c/overs are in the car and couldn't be bothered talking them out yet again, so will do it in the car, but that's not a biggy.

So does anyone see a problem with what I am about to do? The only problem I can see is reduced spring travel, and it'll be interesting to see how much preload I take up. I'm not worried about ride quality so much.

 

[Randy Beikmann]

So, what I want to do from here is preload the springs to past the starting point (thanks OldYat), as it is for a track car and want the springs to be within the main highest rate, if you get my drift (no pun intended, and no I don't drift). Without the comp chart this may be difficult, but was thinking go by feel, in that I tighten (load the spring) the preload perches until I feel high resistance. Obviously this will be well past a normal preload point. The chart would be good as I could measure it. The c/overs are in the car and couldn't be bothered talking them out yet again, so will do it in the car, but that's not a biggy.

There is still a fundamental issue what you're suggesting. You can't really "tighten" the springs, by adjusting the spring seats to a certain point, and operate there. Imagine you have all the corner weights equal and raise all the spring seats by turning them an equal number of turns. The springs won't shorten, and thus they won't "tighten." Instead, the car will rise up by sliding up the shock rod. Why? Because the load supported by each tire is still the same as before (determined by the weight and weight distribution of the car), and so is the force produced by each spring.

By the way, those don't look very progressive, if at all. The wire diameter looks constant (easily measured), and the spacing fairly even. If they are progressive, it seems like it would have to compress a lot before changing rate. If they are as linear as they look, then it doesn't matter what their preload is anyway.

 

[Mark Tamblyn]

Hi Randy, yes you raise good points. My car weighs about 400kg (880lbs) at each corner (im yet to do corner weighting). Wire dia, I think is 16mm but that may be for the 760lb one.

I get what you are saying, in that setting preload to a certain point won't achieve anything, nor possible, as load past preload has been reached once car is grounded. And, looking at JackActions diagram of a 'true' progressive spring, mine may not be progressive at all. mmmmmmmmm :/

Thanks Randy, you have made me re-think. I might just pre-load to 5mm and be done with it. Just a another one, is it normal for preloads to differ a bit between each c/over? these differ about 5mm from 0

 

[Mark Tamblyn]

...The only thing is, I don't know what the lowest rate is, and may well be over the 400lb, in which case preloading out this is possible, but like you said these springs are not truly progressive.

I might try tonight and preload by feel and suck it and see.

 

[Ranger Mike]

I advise against this action. Pre-load like you intend to do will not improve handling and will regrade it. I think it is a mistake to do it. But will not damage the car unless you wreck it. so have at it an use this as a learning point.

 

[Randy Beikmann]

I might just pre-load to 5mm and be done with it. Just a another one, is it normal for preloads to differ a bit between each c/over? these differ about 5mm from 0

Mark, I don't understand what you are saying by "pre-load to 5 mm." What is it that will be 5 mm? Maybe putting the seat 5 mm from bottom?

Here's how I set mine up for the track, with the idea being to avoid any diagonal wedging:
1) With the old springs, and on a flat surface, measure the rocker heights front and rear on both sides of the car. You can also measure the heights to the top of the wheel arches, but this is less consistent.
2) Decide what you want your rocker heights to be once the coil-overs are in. I dropped it 1 inch.
3) Install the coil-overs with the spring seats about the same distance from the bottom (same number of threads exposed) side to-side.
4) Adjust the spring seats up or down to get the desired rocker heights, averaged for the front and for the rear. Turn the seats the same amount of turns on both sides of each axle at each step.This will take a few tries.
5) If one side is higher than the other (likely), adjust the side-to side heights by turning one side up, the other down. At this point your car should be at the desired attitude, with the corner loads nearly equal. Not exactly, because the car's build geometry isn't exactly symmetric side-to-side.

Now to adjust corner weights to make sure that one diagonal pair of tires isn't carrying more weight than the other. This can be done with corner scales, but I did it by adjusting each axle assuming that if weight is distributed correctly, the car will remain level if raised at one end exactly in line with the side-to-side location of the CG (darn near in the middle).
1) I strapped a level across the car, and shimmed it until it read level.
2) Lifted the front of the car with a knife-edge (made from sawing a 4X4 piece of wood at a 45 degree angle) on a jack at the exact center of the front cross-member. I determined center by measuring across from the inside of certain ribs on the tires.
3) Adjusted the seats on the rear coils to where the car was again level, turning the right seat an equal but opposite number of turns as the left (say, one turn up on the left, one turn down on the right).
4) Lifted the rear of the car and adjusted the front coils the same way.
5) Checked to make sure the rockers were still at the right heights. They were, but I would have adjusted them again if necessary.

If you do this, the car should be where you want it, and the weight on each tire (its pre-load) should be nearly equal side-to-to (except for any side-to-side CG bias). It should turn right or left with the same understeer/oversteer characteristics.

There are a few forums on the internet that describe procedures similar to what I did.

 

[Ranger Mike]

Racing is all about tire contact. The better you keep the tire contact patch the better you handle. The stock car you are flogging is a balance between comfort, gas mileage and mild performance. When you change any of the geometry you ruin millions of dollars of engineering effort to keep a good balance. Cranking on the coil over spring height changes the overall geometry, camber curve and roll center location. Not good since it is neutral from the factory. When you monkey with it without knowing the end result to the geometry you will have a worse handling car. You are effectively adding spring rate when you adjust the spring height and this takes away the spring doing its job scrubbing off the momentum force and makes the tire work much harder to compensate for the lost spring function. It will make the steering feel more responsive. You end up under steering because of this.
When you rotate your CG around your roll center the force you're controlling looking front/rear is different than the force you're controlling from side-to-side. ARB or Sway bar make up the difference in wheel rate required to control those forces. If you jack up the spring rates to control the side-to-side motions you're likely to be over sprung for the front/rear motions.

I suspect you are racing Autocross. Autocross has a lot of transitions where you want to control/limit the roll because it's likely you're about to head back the other direction (i.e.. slalom). Road racing typically doesn't have as harsh transitions so you can afford the body roll in an attempt to keep more contact patch without upsetting the weight transfer... meaning on a road course you usually have more time between left/right transitions than on the autocross.
I recommend you buy the OEM or aftermarket spring /ARB package for the vehicle. This way the instant centers, Roll Centers, camber curve will be optimum and thus you get maximum tire contact patch.

 

[jack action]

Mark, I don't want to insult anyone, but I don't think you understand the concept behind setting preload to get the proper ride height and the proper corner weights. http://www.elephantracing.com/techtopic/cornerbalance.htm is very short and explains it in a simpler manner. This one is more intense to read but goes in depth about the subject. This last article is a commented, step-by-step, corner weighting of an actual car, which can be very helpful to visualize the whole thing.

Most of what has been said by others in this thread is in those articles, but having it nicely wrapped up in one article will probably be more helpful.

 

[JBA]

Mark,

Other than saying that you want to "improve handling" you have not indicated what element of handling you want to address. As Randy stated "you cannot increase the spring load because that is determined by the car's weight" all you can change with your adjustments is the cars ride height and this has a direct effect on the suspension geometry which is designed to operate at its optimum when the ride height is that designated by the car designer.

If you want to lower the car then you must install shorter springs; or, if you want to reduce wheel travel then you must install higher rate springs. At the same toime yoiu need to keep in mind that the shock absorber's are designed to operate in concert with the suspension spring rates and stiffer springs will increase the damping load on the shock absorbers and potentially result in fluid heating and deterioration of the shock absorbers' performance.

Alternatively, if you want to reduce lateral body roll to aid keeping the tires as near optimum contact camber then you should either install an anti-roll bar (in old time terms) on either the front or rear( or both) of the cars suspension(s); or, if you already have these items then you will have to replace the current one(s) with stiffer units that have larder diameter x-bars that are stiffer than the current ones; or, If you want improve tire contact angle during the car's roll in turns then there are accessory "adjustable camber plates" to address that issue.

I recommend that before you start any suspension tuning you spent a bit of time researching and reading reference materials on suspension design and even more importantly discussing the subject with those with experience in the type of racing you want to, or are participating, into see what works best under those racing conditions.