what does spring preload do?

[DaveW]

Not sure of the effect, but if I raise the rear ride height on the Lola by .250" the front weights change quite a bit, and I can notice a definite difference in point and turn in with three turns up on the rear spring perches as a quick fix at the track.

What you are changing that makes a handling difference is the rear roll center height. Back in my Z-16 days, that was my main tuning tool. For dry conditions, we'd keep raising the rear until I started to get wheelspin, and then back off 1/2 turn on the rear perches. That method was never wrong for that car. In the wet we'd run it as low as we could w/o scraping the ground in the rear.

Why your front weights would change much raising the rear 1/4" is not apparent to me. The calculations of how much the CG moves and affects front-rear weight distribution don't support that.

[Charles Warner]

Thanks Dave. I'll have to check my notes when I get back to the US but I remember at least a 40# change (total) was possible with spring perch adjustments. Remember, that is in the S2 with the OLDS motor, which is heavy and high, and I sit quite high with the tall man rollbar. My CG is not that low so it may make a slight difference.

On edit: I found my data in the laptop. So much for memory

A one half turn spring perch change to both rears made a 6# increase in front weight. So , with 18tpi (again from memory), a .250" change would be 27#.

[R. Pare]

That sort of weight shift can come about only with an extremely high CG - possible on an S2 because of the mass of the bodywork and a tall driver, expecially the 'glass bodywork of older cars before they allowed carbon. You won't see that on the typical formula car.

The main change in handling will still be from the change in roll center height and the attendant change in the jacking effect.

[HNR Engineer]

Just some things to keep in mind.

When you use the preload to stiffen the rear you also jack the rear for instance so you not only change the roll centre of the rear suspension but the angle of the roll axis. The roll axis is probably the more important one in this case. The CG with respect to the roll centres and roll axis gives you all the weight transfer while braking and cornering so if this geometry gets out of line some strange things might begin to happen that are worse than the original problem you tried to solve with preload.

So when you measure the static weight of the 4 wheels you probably will not see much difference (due to jacking or liftign the rear) but the dynamic (at speed) difference will be more noticeable. When the rear roll centre is lifted the front to rear roll axis increases which in turn will bias weight distribution more to the front wheels in the turn or under braking.

So if you look back at my previous post you will see that adjsutign preload to much will solve one problem and add others in some cases.

It is always best to get the jacking correct with the use of correct spring weights and not preload. If you are using preload to solve complex suspension problems the chances are you might get very frustrated if you dont understand all the in and out to suspension.

Suspension tuning can be a vicious circle and no matter what you do make good notes that you can refer back to later just incase you need to go back to a setting that worked better. Even track and ambient temp are good things to note on the day.

[supertiga2]

Yes, but it's somewhat screwed up. Can you not download Adobe reader? It's free for the taking...

I sent you an email with the files.

Dave
It is something in my registry or ????????who knows!???
All I get when doing pdf is error messages.
Even with successful down loads of pdf program(adobe)
Time to reformat/reload this computer.

Thanks for email.

David

[RussMcB]

Reviving this old thread. It was one of the ones I found after searching for "preload" and "rocker arms".

My Swift DB-3 FF has rocker arms and a droop limiter.

Question: When I read in set up sheets that I should use "16-18 turns" of preload in the front, does that mean I start with the springs fully extended (but touching at both ends), then screw the spring perches up an additional 16 - 18 full turns?

Do I need to do that with the spring/shock mounted in the chassis? I'm guessing yes, because they will be be extended under too much pressure for me to easily compress for installation.

Thanks for any insight. I bought new springs and will be swapping them soon.

[mousecatcher]

Question: When I read in set up sheets that I should use "16-18 turns" of preload in the front, does that mean I start with the springs fully extended (but touching at both ends), then screw the spring perches up an additional 16 - 18 full turns?

That's going to be specific to your setup sheet, as well as the spring rate and motion ratio. It probably does mean, starting from the point where the spring perch contacts the spring. But you can imagine the difference between "16-18 turns" on a 400# spring vs an 800# spring.

[PCalhoun]

Question: When I read in set up sheets that I should use "16-18 turns" of preload in the front, does that mean I start with the springs fully extended (but touching at both ends), then screw the spring perches up an additional 16 - 18 full turns?

16-18 turns of pre-load sounds excessive, but your methodology to adjust is correct. On my DB1 w/ internal droop limiters you are generally talking 6-8 turns based on Stimola & Gib's recommendation and I use a fairly light spring.

[Paul LeCain]

The 'number of turns' thing assumes something about the threads-per-inch (tpi) on the shock body.

Newer shock have Acme threads that are 10 (I think) tpi. Older ones might have 16 or 20 tpi.

[RussMcB]

Thanks for the responses. My front springs are 250 lb, so maybe that explains why the amount of turns seem high?

[PCalhoun]

Thanks for the responses. My front springs are 250 lb, so maybe that explains why the amount of turns seem high?

Paul's point about the thread pitch of the shock body is a good one; what brand of shock are you using?

I have run 250# and the pre-load was still 6-8 turns on a Fox shock.

[RussMcB]

My car has Fox shocks.

They were serviced by Shock Shop in 2009. I'll probably call them to see if my conversion from FC to FF might mean changes are needed or recommended. The FC springs were 425F, 650R, and I'm changing to 250F, 425R.

[PCalhoun]

My car has Fox shocks.

They were serviced by Shock Shop in 2009. I'll probably call them to see if my conversion from FC to FF might mean changes are needed or recommended. The FC springs were 425F, 650R, and I'm changing to 250F, 425R.

FYI- SRP, i.e. Stimola, & Anze have all the NOS Fox shock auto parts, plus the SRP proprietary updates w/ Penske style pistons and valve stacks. I would call James Stimola as no one has more experience servicing and updating Fox shocks than SRP.

SRP 516.671.9715
srpracing1 at aol.com

[Dick R.]

All the spring "preload" does in a rocker car is set the ride height. Since the perches are adjustable you don't need a spring compressor with relatively soft springs like you would with a normal street car with fixed perches and stock springs. If you know approximately how much the spring will need to compress at ride height you can do a least some of the adjustment out of the car after assembling the shock/spring combo. Assuming you can "hold" the assembly out of the car while adjusting the perch. FYI if you decide to move to much stiffer springs you probably will need shorter spring free length to keep the final perch location in a convenient range for access or clearance in the car.

Dick
85 VD (aka DB1 copy)

[Purple Frog]

It is interesting to me that making a 9% reduction in car weight causes you to go 40% softer in springs. I'm thinking you will have to raise the ride height a bunch to compensate.

Even if you figure 200# of downforce on the FC (1400#), the FF at 1100# is only about 20% lighter.

Those new numbers seem to be real soft, even for a rocker car. But, I didn't sleep at the Holiday Inn Express last night.

It appears with the 450# springs as a FC, you had ~450# of preload. I can believe that, since my rocker Reynard is using 850# front springs with no preload.

I guess what I'm getting at, have you ever thought of using stiffer springs and less preload?

But then... Having taken two ICP courses, if i learned anything, it was, "Stiffer is faster." So, i'm biased in that direction.

[RussMcB]

Yea, I was a little surprised too, but I've seen many references for DB-1 set ups with springs in this range. Maybe it's a case where the rocker arms flex too much with stiffer springs.

I haven't driven this car yet (or any FF in 30 years), so I'm just aiming for a decent starting point.

[Dick R.]

To clarify "preload" with a rocker car is really just the amount the spring compresses to support the unsprung weight of that corner of the car (after all that motion ratio stuff ). The spring will "compress" the same amount no matter where the perch happens to be . . . assuming the car hasn't hit the ground first) Stiffer springs will compress less, of course, than softer springs.

I think "preload" as racers normally talk about it requires pushrod/pull rod suspensions which can use the rod to adjust ride height in addition to spring perch location.

Dick
CM 85

[Dick R.]

Thinking about your apparently large number of "preload" turns. It MAY be that the notes simply refer to the number of turns from the "bottom" of the threads on the shock to the bottom of the spring perch rather than to the number of turns after contact with the extended spring. I tend to do that in my personal notes. That number will vary depending on spring rate and length as well as the assumed ride height. Also the dimensions of the shock body and threaded sleeve.

Dick

[BWC54]

Do I need to do that with the spring/shock mounted in the chassis? I'm guessing yes, because they will be be extended under too much pressure for me to easily compress for installation.

I don't understand your concern about having to compress the shock to install it. You set your preload with the shocks out of the car in a vise on a bench. You put in about 1 inch preload. 1 inch is easy on a 250 lb spring. 10 turns if your shock is 10 tpi and 16 if your shock is 16 tpi. You install the shocks in the car with the rocker links disconnected and the car on stands. Then you just pick up the suspension until the rocker arm lines up with the end link and slide the bolt through. No compressing of the shock is required.

[RussMcB]

I don't understand your concern about having to compress the shock to install it. You set your preload with the shocks out of the car in a vise on a bench. You put in about 1 inch preload. 1 inch is easy on a 250 lb spring. 10 turns if your shock is 10 tpi and 16 if your shock is 16 tpi. You install the shocks in the car with the rocker links disconnected and the car on stands. Then you just pick up the suspension until the rocker arm lines up with the end link and slide the bolt through. No compressing of the shock is required.

Thanks, Bruce. That almost makes sense, but since my car has an external droop limiter (not built into the shock), I think the procedure might be different for me.

I could be mistaken, but I think I need to do almost all of the alignment (ride height, caster, camber, toe, etc.) first, and then set the droop limiter at my ride height. Once the droop limiter is in place to prevent the ride height from increasing, only then I can turn spring perches to create preload.

I'll post a picture of my droop limiter later tonight. I am interested to know if the process is different if the droop is controlled by the shock or by an external limiter.

[Charles Warner]

Russ,

I think you may be muddling things with preload and droop limiting. I would PM Richard or Steve and get their input. AFAIK none of the shocks we can afford should be used to limit droop. Shocks should never be used to restrict suspension movement. Preload is a function of the location of the spring seat on the particular shock/spring combination. As such, you do set the desired preload on the bench, by doing the maths and coming up with a TPI number that will give you the desired amount of preload. Then you set the ride height with other means.

[Dick R.]

Russ,

I was ignorant of the concept of locking the ride height with droop limiters and then raising the spring perch to compress the spring further with a rocker arm suspension. Hence my apparently incorrect statement above. Learn something new almost every day. I previously used some home made front droop limiters on my 85VD but just used them to keep the suspension from rising in droop.

Now that means I need to find the old posts on why preload and droop limit can be a "good thing" in some circumstances.

Dick
VD RF-85

[Charles Warner]

but just used them to keep the suspension from rising in droop.

I'm afraid you'll have to explain that remark a bit more clearly.

[Dick R.]

I'm afraid you'll have to explain that remark a bit more clearly.

I meant that the droop limiters kept the springs from extending when weight transferred off that corner(s). I set the limiters to contact their stops when the car was at ride height with driver. Ride height was set with the spring perches. I had no clue that I could turn the spring perches up to preload the springs.

Dick

[RussMcB]

Here is a picture of my droop limiter. I don't know if this type is common or not. It's the first one I've ever seen.

The large chunk of aluminum is bolted to the frame and is a hard stop. As the inner end of the rocker arm rises (as the suspension droops), it eventually hits it. It appears to have shims. I assume that's to set the ride height first, then shim the limiter against it.

[sccadsr31]

[FONT=Verdana]This is how I droop limited my DSR, it was easy to have preload and ride height adjustability, it does look a little agricultural though. I'm converting the car to push rods to accommodate tunnels and plan on using a similar scheme (where the red line is) on the push rod. I have found this method to be very effective and from my point of view better for the shocks. [/FONT]

[FONT=Verdana]David[/FONT]

[rickb99]

....It appears with the 450# springs as a FC, you had ~450# of preload. I can believe that, since my rocker Reynard is using 850# front springs with no preload.

How do you set the ride height on your Reynard, Frog?

We had to whip some preload into the 90SF to set ride height. Or, do you have a whole lotta spring sets in 1/8" length increments?

[Charles Warner]

As someone said earlier in this thread, this can be a circular excercise with tail-chasing the main activity. We are dealing with three issues here, as I see it: droop limiting, preload, ride height adjustment.

IMO, leave droop limiting out of the equation. If you decide you want to use it at a later date then make that choice based on the way the car handles. Proper sway bar and spring choices may mean you don't need it.

Ride height adjustment is set differently depending on the suspension configuration. On push rod cars the ride heights are usually set with pushrod adjustment. On rocker cars the ride height is set by spring perch adjustment. If you can not set the ride height on a rocker car without compressing the spring then get a shorter spring. Use the shortest springs available and ally spring spacers to get the proper spring length. It's quite OK that the springs rattle when the car is lifted.

Preload is merely a spring/shock combination that is preset with a rebounding force when installed. Again, you may or may not want this in your given car.

Preload should be (IMO) stated in terms of "pounds". I.e, how many pounds of preload do you want for a given car. Not how many turns of the spring. The amount of preload is computed by turns x TPI x spring rate. The number of turns will change given the amount of preload desired or spring rate change. How much preload is desired? Maybe none. Some cars like some preload, others don't. Early rocker RT-4/5s used none, front or rear. Later RT-4/5s with pushrod front suspensions used 200# on the front, regardless of spring rate chosen. Front ride height was set with pushrod length, and the corner weights and rear ride heights were set with the rear spring perches only - with no preload.

IMO preload and droop limiting are close bretheren. Do you really need them? Use the KISS principle. Set the car up without preload and droop limiting, unless you have some verifiable data from a known expert in your car. Then, using testing, decide if you really want to add these functions. Maximize the spring, shock, sway bar combinations before you muddy the waters with too many variables.

YMMV

[RussMcB]

Charles, I like that you point out that it's a good idea to keep those three topics separate where possible. That will certainly help my mind get a clearer picture of why different things should be done and in which order.

I do want to point out that my car's ride height can be set with either spring perches or by altering the link lengths (see picture above in post #46). Also (to muddy the waters further), I think ride height can also be determined (or affected) by either preload or droop limiting. So, it's hard to completely separate those three topics.

I do understand that testing is the best way to determine an optimum set up. However, I am at the very beginning of the process (never driven the car, making major changes due to FC --> FF conversion), and really only want to copy someone's known decent set up. I do have very good suggestions about set up parameters (rd ht, camber, springs, etc.), but still unsure about the order of steps.

The Swift documentation (http://www.apexspeed.com/forums/show...42&postcount=3) suggests an order of things to do, BUT it does not include anything about when in the process you adjust/introduce preload or droop limiting.

Since I haven't driven this car (or any FF in 25 years), it is going to feel foreign for the first few outings. I wont have a clue if droop limiting or pre load is helping or hurting at first.

PS. I also agree that it's better to talk preload amount in terms of lbs. rather than turns.

[Charles Warner]

Russ,

You mention the data you have from the factory does not mention droop limiting or preload. I wonder why? Could it be that this car was not designed with those concepts in mind? I would suggest setting the car up as per the factory specs - maybe a bit stiffer given the newer tires we have now - removing all droop devices and no preload, and getting a baseline. Chassis flex or, as you pointed out earlier, suspension arm flex, may disguise any real effects preload/droop limiting might have.

Follow the suggestions from the factory. Setting up suspension is always a circular exercise in that after you set something you will change something you have already set up. I personally set suspension arm lengths first, then ride height, then camber, then bump steer . . . . and then start all over again.

IMO preload should never be used to set ride height. Only use it to affect the handling in a specific way. If you can not achieve the desired ride height because of spring contact, get shorter springs.

[Brandon Dixon]

Russ,

...
I would suggest setting the car up as per the factory specs - maybe a bit stiffer given the newer tires we have now - removing all droop devices and no preload, and getting a baseline.
...

+1

It's not like the guys at Swift were idiots.

When there is soemthing about the behavior that you want to try to fix or, perhaps, mask a little bit, then evaluate droop limiting and preload.

[BWC54]

+1

It's not like the guys at Swift were idiots.

Yes, however, that set up sheet was done 26 years ago. 26 years experience racing a car can lead to set up developments. You can debate whether it's correct or not but droop limiting and running preload on front has been common on a DB-1 for a long time. Probably because of Joe Stimola's packages right or wrong.

[RussMcB]

Good points, but I assumed the Swift literature was from 2 or 3 decades ago, back when the DB-1 was new, and maybe lots of people have discovered that preload and DL was an improvement.

Did you follow that link to view it? My guess it was done on a Selectric and mimeographed a few dozen times. Probably by someone with big hair and shoulder pads listening to MC Hammer. :-).

(Oops. I see Bruce beat me to it.)

[Mike Scanlan]

Spring pre-load, as it was explained to me by Joe Stimola, controls the rate of roll between each spring. I've attached a chart that is a good reference for a particular length spring and it makes it easy if you want to change springs and still keep the roll rate betwen them the same. Another thing Joe used to tell me was that by the time you got to around 18 turns of pre-load it was time to go to the next spring. Basically, a 300 lb spring is always a 300 lb. spring no matter how many turns of preload it has, what changes is the roll rate of the spring.
UNfortunately, I am unable to upload this chart since it's a Works file (.xlr), but if someone can tell me how to change the file extension I'll be glad to post it, or, I'd be happy to e-mail it around and let someone else take a crack at uploading it.

[john f]

From what I see, first you can not use spring pre-load without droop limiters of some sorts. If you have no droop limiter, the ride height would just rise. As you add pre-load you are changing the point at which the suspension will start to move when a load is applied to it. Once the wheel starts to move, the wheel rate will be based on the spring rate that is being used times all the stuff to give a particular wheel rate. Until the suspension starts to move, you theoretically have a wheel rate that is infinite. If the suspension is rigid, you have a go cart set up: wheel rate is equal to the tire rate. Once you transfer enough weight to the corner in question and over come the pre load you now transfer to a corner with a lower spring rate assuming the load transfer stays there to over come the pre load. Kind of like having a very stiff sway bar or shock for initial turn in and then transforming to normal setup. Just my $0.02.

john f

[RussMcB]

Thanks, John. That is exactly how I've been imagining it.

So, with that in mind, preload can't be set before mounting the shocks on the car, right? Because they need to bump up against the droop limiter before any preload can be started.

That's why I think droop limiter adjustment and preload are very near the LAST steps in set up. For instance, after setting ride height, caster, camber, toe, etc., then (once everything is pretty perfect), finish up with droop limiter, preload, corner weights, attach ARB...

[kea]

Russ,
Corner weight before droop limiter.

[Charles Warner]

Another thing Joe used to tell me was that by the time you got to around 18 turns of pre-load it was time to go to the next spring.

What that means, assuming 18 tpi, is that when the preload equals the spring rate, and you want more preload, you have to go up in spring rate. Makes total sense if you think about it. You certainly would not want 350# of preload with a 300# spring.

[RussMcB]

Russ,
Corner weight before droop limiter.

Right. Thanks. What I had in mind when I typed that was doing the corner weights again at the end to make sure I didn't throw them off after droop limiter, preload and connecting the ARB's. More of a check than actually setting them.

Thanks for the clarification.

[john f]

What that means, assuming 18 tpi, is that when the preload equals the spring rate, and you want more preload, you have to go up in spring rate. Makes total sense if you think about it. You certainly would not want 350# of preload with a 300# spring.

I would disagree with your last statement. If you had 300 lb/in springs pre loaded to 350 lbs in your car with a 1:1 motion ratio you would not see the wheel move until 350 lbs was applied to it. At a load of 350 lbs the wheel is on the verge of moving. any additional movement will be at a rate of 300 lb/in. if the load goes up to 650# the wheel would move 1". With out any pre load a 650 lb load would move the wheel 2.166" (650/300).

To accuratly set pre load you need to know the exact rate of the spring to set it via so many turns of the spring perch. Another way that does not require the rate to be known would be to measure the length of the spring for a given load. Kind of the same thing as rate but when installing them you do not have to try to find the zero point of the perch touching the spring. Just collapse the spring to a given length and you know the pre load. Just my $0.02.

john f