Spring Pre-Load

[BHerscher]

1. What are the determining factors in establishing spring
pre-load?
2. What is the equation/formula that gives the number spring
perch turns/quantity of spring compression?

[Bob Wright]

Oh boy- this should be a good discussion...the easy one first:

1 inch of spring compression should be about 12 turns on a modern shock.

The hard one: the amount of preload you put in a shock has a lot to do with how the shock is valved, set up of the car and track conditions. Some people use no preload, for others, its an integral part of setup. In a properly set up car, I have been able to cure a push changing one or two turns of preload only leaving everything else the same. Someone else might be able to do the same thing changing rake a degree or two and have no preload at all.

Talk to whoever services your shocks first. If you don't know anyone- Call Joe Stimola at 516-671-9715

[BHerscher]

Thanks Bob. I have a general understanding of shocks, valving, preload, spring rate, etc. It seems to me that there must be a formula used to determine preload. Not just using what someone else says to use. With all of the 'scienific methods' used for everything else on our cars these days there must be a 'scientific method' to determine pre-load!

[Charles Warner]

If you compress a 400# spring by .250" with the spring seat you will have 100# of preload, no? You will have to exert more than 100# of force on that spring before the shock/spring will move. At least that's the way I understand it. As far as I can tell pre load is mainly used to maintain a desired aero platform and does little for mechanical grip. I'm sure others' mileages will vary.

[Mike Scanlan]

Bob's right, there will be a great many opinions on this. The way it was explained to me the pre-load affects the roll rate of the spring. My Penske triples compress 1" with 10 turns of the platform. So a 300 lb spring will have 300 lbs of pre-load with 10 turns on the platform. So if I want to change spring rate but not the roll rate of the spring, I can install a 375 lb spring with 8 turns of pre-load (37.5 lb/in x 8 turns = 300) and retain the same roll rate on the front of the car. I made up a simple spreadsheet for quick reference at the track since springs get changed all the time. As Bob said above, call Joe Stimola, he will spend the time explain it all a whole lot better.

[Steve Demeter]

But won't the roll rate change as soon as the car rolls at all, since the basic spring rate is stiffer??

[Douglas Kniffin]

The scientific way to figure out proper amounts of preload is test..test..test. The cars and drivers are too different to turn it into a formula. Play with it...I'm a fan but there are lots of guys who aren't. I second the advice to call Joe Stimola.

[R. Pare]

Once more on preload:

Pre-loading does not change roll resistant rates (rate from the springs and anti-roll bars). It changes the degree of jacking, and therefore the 'anti-roll' "stiffness" contribution of that effect.

For those who are unfamiliar with jacking effects, think of it as being the same thing as anti-dive, except lateral, not longitudinal - ie - a self-stiffening of the suspension in reaction to the lateral loads being fed thru the a-arms.

When finding that amount of preload that works for your car, you are actually looking for the amount of force left on the spring ( and weight left on the tire) for the particular lateral g level at which the push becomes noticable. It is at lateral loads above that level that you are changing the effect of jacking by actually reducing it a bit.

When the shock tops out, the ACTUAL roll center shifts to (nearly) the center of the inside tire contact patch - for any further roll, the car now pivots about that point. The only way it can pivot is to lower itself, and it is in that lowering that the jacking is reduced.

Roll rate from the springs and bar are not affected at all, as for every degree of roll, the distance traveled (for that remaining spring and bar link) is doubled, and the rate per degree stays the same.

[BHerscher]

So four people know something about pre-load but can't or won't answer my questions. It MUST be a very well guarded secret!

[Bob Wright]

Its not a secret; its a personal preference and one of a number of ways of achieving a particular car setup for a given condition. Mr. Pare gave what I think is a very good description of how it works.

I don't think there's a general, catch-all formula, at least none that I've heard of-unless you're specifically talking about the spring rates as you compress the spring the first inch- and that's from some people who really do know what they're doing in the setup business.

[R. Pare]

Your question HAS been answered - the answer is that there is NO formula that you can just plug numers into to get an easy solution. YOU HAVE TO TEST, and to make the tests effective, you first need to understand what it is that you are working with and what your changes are actually doing. Until you UNDERSTAND what you are changing, you are working blind!

[Charles Warner]

1. What are the determining factors in establishing spring
pre-load?
2. What is the equation/formula that gives the number spring
perch turns/quantity of spring compression?

Question #1 has been answered fairly well by Richard et al. Some cars may have a known pre-load starting point. I.e. the 1986 Ralt RT-4 worked well with around 200# of preload on the front so that was a good place to start testing. Maybe some issue is with setting ride height/corner weights? The ride heights were set/matched with the front push/pull rods and all corner weight changes were made with the rear spring perches - at least on the Ralts.

If you are considering pre-load on the rears as well then check with Richard.

Question #2:

(spring rate in #/") (# of turns of spring compression/x) = amount of pre-load

(where x = threads per inch on shock body)

to determine wheel pre-load you then make all of the normal calculations regarding motion ratios, tire spring rate, etc.

[RobLav]

On my RF-96 (front monoshock), the pro recommendation was a lot of pre-load on the front spring and none at the rears. I tested and found that the car handled more to my liking, especially on the oval section at NHIS, with about 7-8 turns of pre-load at the rears. It let me run flat out around turns 1 and 2, which shaved about 1.5 seconds off my time. So yes - you need to methodically test.

[skt]

Alright, maybe I'm dumb but are we talking about changing the neutral position of the rocker arm for the suspension (moving spring perch and changing pushrod length to maintain ride height)? The preload will change a little dependant on where the rocker is as there is some change in mechanical advantage between spring and wheel? Is this what we're referring to as preload or am I missing something?

If it were we would be talking about some rate changes dependant on how the motion ratio changes through the suspension movement. The more efficient the motion ratio the stiffer the wheel rate with the associated results. Of course this would be fine tuning after the spring rate is chosen.

Or somebody tell me I'm way off here??

Steve

[John Robinson II]

You cant run preload unless your shocks are droop limiting as well. So the biggest thing you are changing, as I understand it, is the amount of force required to compress the springs further from full droop. right? Been a few years, Richard, have I forgotten to much?

John

[Purple Frog]

Steve,
You are sort of wandering off into another subject. You are getting into a discussion as to where the bellcrank best should be when the car is in it's static ride position.
What this thread started on was simply 'preload'.

Preload:
Take the unloaded shock with spring installed.
Tighten the spring perches until the spring is just barely in contact with it's top and bottom perches.
Then tighten the perch "X" amount to get a desired preload.
At this point the shock is fully extended.
Install the shock back on the car.
Set the car back down on earth.
At that point with weight on the suspension, the shock may or may not be fully extended (droop limited). The amount that the shock might have compressed is dependent on how much weight is on that corner, spring rate, motion ratio, suspension friction, and how much preload was dialed in.

Note: on older rocker arm cars, preload is a bit more difficult to accomplish, but not impossible. It usually requires the use of custom shock lengths paired to certain ride heights, or the famous/infamous droop limiting device.

OBTW, I knew if I skipped one year of ICP schools I would forget all that I never learned in the first two years. It took me 3 days this week to finally figure out the explanation that Richard posted above (reply #8). Finally this morning the light flickered.

[R. Pare]

Finally this morning the light flickered.

Yea, right! We all know your elements burnt out years ago!

[BHerscher]

Thank you all for your responses.

[MarkM]

Please answer slowly and clearly, this is all new to me. LOL

I drive an F/SCCA. One of the few adjustments available to us is spring preload. GENERALLY, will more or less front preload TEND to result in less understeer. I realize the need for testing and tuning, I just would like a direction to go in.

[alanbryan]

GENERALLY, will more or less front preload TEND to result in less understeer. I realize the need for testing and tuning, I just would like a direction to go in.

I found this old thread and didn't see this answered so thought I'd see if we can get an answer here as I'm wondering the same thing.

Thanks.

[Brian]

I copied the tome of information that Mr Pare posted the last time I asked about this...

Within that (which should be searchable) are these nuggets concerning a push condition...

Droop limiting can combat a bad push, sometimes caused by soft springs and/or above ground geometric roll centers. The jacking effect from the above ground roll center creates a self-stiffening effect, overloads the outside front tire, and presto a bad push.

When the inside front corner goes into droop limiting, the roll center shifts to near the center of the inside tire contact patch, the car lowers if it continues to roll about the new roll center, the height of the geometric roll center drops, the self-stiffeneing effect is reduced.

So maybe, in some cases, droop limiting will help combat a bad push.

Brian

[mousecatcher]

my brain hurts.

[supertiga2]

I know!

I find it helps to use a rolling pin on brain! Roll it thin!
I find its easier to wrap around as opposed to stuffing ideas in.
(practicing comedy to fall back on when I fail as race car driver)

The first thing I do at my point in pushrodbellcrankpreloadiffuseritus learning curve is to set the fully extended damper lengths to equalize the bell crank positions(side to side)

This should also give equal droop(limit) side to side.

Now to wander in thought
Pre load will also allow lower needed ride hight with same springs?

It has the spring in the usable range of the spring therefore slowing down(stiffer) initial movement of chassis?

When the inside front corner goes into droop limiting, the roll center PARTLY(?) shifts to near the center of the inside tire contact patch depending on preload and where droop stop occurs?

Any one set up with zero droop movement from ride hight?

Would droop travel from static ride hight to the point of limit record/info be of use in base line set up of preload?(ride hight at droop limit)

Would a 5th spring be easier for dealing with down force loads.

Time for asprin

David

[DaveW]

1st round of questions:

Q: Please help me understand something. A spring is a fairly linear mechanical piece (aside from the ends) I understand the concept of preload and use it quite often.
If you have a 400# spring and compress it one inch you have 400# of preload. I understand this.
Q: A statement that I hear over and over, but don't agree with:
With 400# of preload it takes a force greater than 400# of load before it will begin to compress the spring.
A: You already have 400# of force built into the spring with having 1" of preload. Therefore an additional force of 400# will compress the spring an additional inch.
Q: Correct?
A: Whether your last statement is correct depends on how I read what you said. If you mean you add 400lb to the original 400lb, giving a total of 800 lb, the spring will deflect an additional inch, giving 800lb load at a total of 2in deflection. If that does not make it clear yet, read the following step-by-step explanation.
The key to understanding this is that the initial 400lb compressed the spring the 1st inch, and the spring is pushing back with an equal 400lb force against whatever is holding the spring in the 400lb preloaded position (say the damper is holding it preloaded by being fully extended). Now, if you clamp the damper body in a vise and push on the extended shaft, it initially will not move because it is held fully extended by the 400lb spring force. As you add force (say 125lb) to the damper shaft, you reduce the tension on the damper shaft by 125lb, but as long as that force is less than the 400-lb preload, nothing will move, because the net force (now 275lb) on the damper shaft is still holding it fully extended. As soon as you get to 400lb, the damper shaft the extra force will be 400-400=0lb, and the damper shaft will start to move and the spring will be operating as if no preload existed, i.e., the force on the end of the shaft that you are applying is now equal to the deflection x the spring rate. Since the spring was initially compressed 1in, the spring deflection from its non-loaded position is 1in + whatever deflection you add. And the force you need to compress the spring more than its preload is now the preloaded 400lb + any additional deflection x the spring rate.

So, yes, the spring will not move until the preload force is overcome. Try what I described yourself if it is still confusing.
-------------------------------------------------------------------
Second round of questions:

a) So, by increasing the preload on any given spring does it also dynamically add rebound force?
a1) Could you then essentially take away most of all your rebound damping because the preloaded spring will always try to return to its static preloaded position and then you could dynamically control the oscillation with bump?
b) So, perhaps running "some" preload in the front would make sense to help instantaniously help load the tire at turn in without allowing the car to roll?
c)Wouldn't then a heavily preloaded soft spring end up being stiffer than higher rate spring with little to no preload?

Answers:
a) In the sense that it puts a step in the rebound force plot, yes, but, what it really does is change the possible suspension travel in rebound, limiting it to whatever you have set.
a1) IMO, no. You still need to control whatever suspension motion remains. Also, since the rebound is limited, you do not want to hit the rebound limit with a thud. So, IMO, you still need rebound damping, pretty much as you would have w/o rebound limiting, or maybe even more to prevent the sudden stop at the limiter.
b) Yes, possibly. Preload also changes the way the car reacts during roll, since once the limit is reached on the inside suspension, further roll occurs by compressing the outside suspension, lowering the car and the roll center. As I said in one of my posts, whether this will overcome the instantaneous doubling of the springs' effect on roll stiffness is a matter that needs testing to evaluate.
c) Yes - the preloaded spring is essentially infinitely stiff until the preload is removed. However, other things, such as the tire, the damper internal rebound limiter, etc., still allow some compliance, so the total system is not infinitely stiff.

All of this complexity and non-linearity is why, IMO, preloading should be used only if other tuning methods have proven unsuccessful in removing understeer. Preloading means to me that you should test to optimize it specifically for every track you run, depending on the corner speeds, track roughness, etc. Of course, testing is always a good thing, if you can afford the time and expense, even w/o rebound limiting.

[supertiga2]

I received this car 95VD with out any setup info and have no way of knowing if these dampers(triple penske) are set up for droop limiting.

Is this a valving or structural thing?
Structural = can damper deal with the droop limit load.
They shouldn't be very high loads?

The car when received had no preload ,the ride hight was real high.2.5+ inch
Camber settings were goofy.
large amounts of bump steer on all but right front and it was out some.
toe was no where near reasonable.

Do wish someone would post decent starting set up for the 94/95VD.
The info from Budrox was iffy possibly from typeOs.

Currently running 3 rounds of preload on front and rear.
The front is well planted.
The rear is loose during transition from brakes to throttle.

Some questions are off topic possibly--trying to learn.

David

[S Lathrop]

David

You will need to get some idea of how your dampers have been valved. If your dampers have a Stimola type setup then you will have to use some amount of preload. Penske does make shocks with top out bumpers just for this aplication. The part is for off road shock applications and is not common in road racing shocks.

I did work with 2 drivers with this vintage VD. We were able to get really fast setups without preload-- pole at a Mid Ohio run offs. I did do the shock valving. At the same time there were plenty of fast cars that used a lot of preload.

As DW pointed out, using preload makes the dynamics of the suspension very complex and subject to sudden changes.

The biggest advantage to a no-preload setup is that it reduces stress on the tires. If you are using a relative hard tire, such as is required in various series, then you may want a high tire stress setup to generate heat in the tires quickly.

I have never used preload at both ends of a car. But I have used preload at one or the other end. I also try to get a setup without preload first.

Again as DW suggested, you really need to test and determine how much preload to use. I would suggest that you keep track of your proload in terms of pounds of preload vs. turns. When you change springs, your setup will be closer. Remember you are managing the mass of the car and pounds of preload is more closely related to the mass being supported than the spring being installed.

[supertiga2]

Thanks DaveW---more to digest. A good thing.


S (Steve?)


Have no clue on identifying Stimola type damper setup.
Received zero info with car.

This is my first car with other than Bilsteens.
First car with any damper tuning!

I did go through the dampers to clean and change fluid.
Do not remember if there were any
Is it an O ring?

There are O rings on outside acting as bump rubbers.
not sure if this is enough for bump loads.

Lot to learn here

The tires do seem over worked
Hoosier R35s
Go greasy on about lap 6 0r so.
Was planing on trying Goodyear's next race.
The local FM guys say the Hoosier come in quick the go away.
the goodyears come in slow and stay.
Ive been out to long to know.

This leaves me with our last race of the season next month.3 days on track.
Biggest worry at the moment is damaging the damper with possibility of not having top out bumpers and using preload.
Would like to make this race(April) before starting over on set up development.
The car is really good now.

Advice Ideas and Jokes all welcome!

David

[Charles Warner]

IMO, preloading should be used only if other tuning methods have proven unsuccessful in removing understeer.

THAT is what all of us should take away from this thread!