what does spring preload do?

[John Robinson II]

The other way to think of preload is that it now takes x pounds of force before that spring starts to compress any more, but once it achieves that force it will roll more based on the spring rate. Example, 300# spring with 350# of preload will take 350# of force to start to move and then compress at the rate of 300# per inch of spring travel. Whereas a 350# spring would require more weight transfer for the same amount of roll.

Droop limiting increases the amount of weight transferred per degree of roll once you hit the limiter. The inside spring no longer is able to assist in raising that side of the chassis and you now have to lift the whole suspension.

There are different types of limiters as well. I do it with a spacer inside the shock, but also have a cable set up that allows the car to still roll to allow the roll rate to remain consistent. I used to run an external limiter but for whatever reason it would never stay consistent.

Keep in mind this is on a pushrod car not a rocker.

I think the best advice has already been given, KISS. Set the car up with no droop. Use the spring perches to set the ride height and drive it. Once you feel you have maxed out that set up, try the limiter and see what the result is. Trying to engineer the car before it even hits the track is a sure fire way to get it all screwed up. Both preload and droop limiting are tuning tools that have both pros and cons. You as the driver have to be able to judge if the benefit out weighs negative effect.

John

[Dick R.]

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.

Mike, Can your computer "print to pdf?" using one of the many free pdf conversion programs or full Adobe (not just the reader) or simply print and scan to pdf if you have scanner. If so, attach the pdf file (I assume the site support pdf).

Dick

[Purple Frog]

Not to get too far away from Russ' issue, but Rick99 did ask about Reynards...

At one time i owned an 87-90 Reynard with droop-limiting built into the shocks. It was a ROYAL PIA. You had to disassemble the shock and change packers to be able to change ride heights. Or carry multiple sets of front shocks. oh fun...not

Almost immediately I started ordering shorter springs, and using Averill's external droop-limiter if I needed one. Far, far simpler to do set-ups. Set the ride height with the spring perches, set the corner weights, then just before you put the bodywork back on, screw the external droop-limiter down until it touches the top of the shock.
So easy.


Droop limit is the last thing i set, just before I take the dummy weight out of the car, and roll it off the set-up pad.

Since I'm from the ICP School of Stiffer is Better, I don't use preload... unless Gibby tells me to.

Trust me, if you run 1200# spings on the front of a rocker Reynard, you don't need preload, nor droop limiters. You tune the suspension based on the flex of different rockers you have in the trailer.

[swiftdrivr]

Okay, I have read the preload thread, and it all seems to apply to pushrod cars, not a Rocker-arm DB-1. There are comments that seem to indicate preload doesn't apply to rockers, which seems suspiciously like saying "physics doesn't apply here" . I have managed to stuff the car into a tire wall, rear corner only, affected repairs, and had my shocks redone. I find I now need more turns of the spring perches up front to achieve ride height I ran before. [ no front-end changes and my nitrogen pressure is the same,]

So, if I understand it correctly, I have gone from six turns to nine, so a significant change I'd think. This should cause more corner entry understeer and heat up my front tires a bit more than before. If this is problematic, adjusting rake would be a reasonable way of managing it.

Anyone know if this is correct? Other thoughts?

[RussMcB]

I think, Jim (and hopefully people way smarter than me will chime in), you do not have the ability to "pre-load" a rocker arm car. All you can do is screw the spring perches up and down, affecting your ride height and corner weights.

I'll do a lame attempt to explain ... To use preload you need to crank the spring perch way up (causing the coil over springs to be compressed a bit - the shock is extended as far as it will go), and then you adjust the lengths of the pull/push rods to affect ride height and corner weights.

When you crank up your coil spring perches (on your rocker arm suspension), you are just raising that corner of the car, until you run out of threads.

I think the main reason designers went to pull/push rods is because rocker arms were not strong enough for some of the high spring rates people wanted to start using. Rocker arms would flex too much.

[swiftdrivr]

Russ,

Thanks for the reply. Sometimes I have to ask the question before I realize some critical bit that makes it all make sense [I think]. The reason preload wouldn't affect a rocker car is that it really doesn't compress the spring, unless you are totally out of shock extension. It "uses up" some of your shock extension, but that would only have effect if you top out. As it is, it doesn't compress the spring, it just pushes down on the spring, which extends the shock a bit as the shock moves away rather than the spring shortening.

Does that sound right?.

[RussMcB]

I think so. Maybe. :-)

I don't quite follow your last sentence.

Let's hang out here a bit until one of the knowledgeable people improve on (or correct) my explanation, and/or confirm your reply.

[DaveW]

...The reason preload wouldn't affect a rocker car is that it really doesn't compress the spring, unless you are totally out of shock extension. It "uses up" some of your shock extension, but that would only have effect if you top out. As it is, it doesn't compress the spring, it just pushes down on the spring, which extends the shock a bit as the shock moves away rather than the spring shortening.

Does that sound right?.

You have that correct. But once shock travel is used up by the perch adjustment, you have transitioned into preload which would function the same as preload on a pushrod or other suspension. The issue is that at that point you have no independent ride-height adjustment. So the only way to independently adjust ride height while preloading with a rocker or outboard shock placement would be to limit the actual shock extension, either by internal bumpers or an adjustable rebound-limiting device in parallel with the shocks, maybe attached between the inner ends of the rockers and the frame.

It seems to me that I've seen or heard of someone using frame-mounted adjustable stops hitting the inner ends of the rockers at the height wanted.

IIRC, some FVees use something like that in parallel with the shock on their zero-roll-stiffness single-shock rear suspensions.

[problemchild]

You can certainly preload the springs on a rocker arm car. Virtually anyone who went fast in a Reynard FF or FC had the front pinned down with preload and or droop limiters, which are essentially doing the exact same thing, just in slightly different ways. High spring rates were the enemy of rocker arms because the rocker arms flexed, but also because they became bent, so that having a consistent setup was all but impossible.

The typical club racer or vintage racer who is driving their car 2-10 seconds per lap off its potential, should be avoiding aggressive setups using lots of preload, high spring rates, high cambers, etc. In a complete oversimplification, to benefit from aggressive setups, the driver needs to be driving the car hard, with committed inputs. Most of us will progress better, and build pace, with a setup that the "hotshots" would call a "rain setup". The aggressive setup may help you get from 2 seconds off to 1/2 a second off, or the last increments, but I believe it impairs most people's ability to drop from 5 seconds off to 3 seconds off.

I used adjustable shock eyelets on my Reynard FF to sidestep many of the bent rocker issues.

[DaveW]

...The reason preload wouldn't affect a rocker car is that it really doesn't compress the spring, unless you are totally out of shock extension. It "uses up" some of your shock extension, but that would only have effect if you top out. As it is, it doesn't compress the spring, it just pushes down on the spring, which extends the shock a bit as the shock moves away rather than the spring shortening.

Does that sound right?.

You have that correct. But once shock travel is used up by the perch adjustment, you have transitioned into preload which would function the same as preload on a pushrod or other suspension. The issue is that at that point you have no independent ride-height adjustment. So the only way to independently adjust ride height while preloading with a rocker or outboard shock placement would be to limit the actual shock extension, either by internal bumpers or an adjustable rebound-limiting device in parallel with the shocks, maybe attached between the inner ends of the rockers and the frame.

I think I remember someone using frame-mounted screw-adjustable stops limiting rebound travel of the inner ends of the rockers.

IIRC, some FVees use something like that in parallel with the shock on their zero-roll-stiffness single-shock rear suspensions.

Greg said basically the same thing more clearly and with experience to back it up a few minutes before I posted...

[veeten]

Since It came up in this thread. I just sent rear shock off FV zero roll system to be service.
I sent it as race with spring preloaded around 2.5 inches.
Has anyone done a zero roll without the large preload and did it work?
My first zero roll came with a 12 inch spring and 10 inch shock from lathrop
and it worked great so i have never tried anything different.

[swiftdrivr]

Unfortunately, I am not talented enough to use the super-stiff hot-shoe setup. I just want to keep the nose off the ground and the tail out of the tire-wall ! If I have it correctly, preload gives a delayed response in bump, as you must overcome the preload. It looks like it produces zero droop also, as the shock is fully extended by the spring already. I have droop limiters inside the shock, if I understood Angelo correctly. I am not sure why I want zero or limited droop, but it seems I have it by design. but it also seems I am not out of shock travel, so I expect my handling won't have changed markedly. I still hope to feel it out at Roebling, which is a more forgiving track than VIR or Road Atlanta, just in case.
Thanks for all the help and the explanations!

[RussMcB]

Great info and corrections by the non-Russ responders. I've certainly learned a bit more.

<snip> ... It seems to me that I've seen or heard of someone using frame-mounted adjustable stops hitting the inner ends of the rockers at the height wanted. <snip>

I wonder if this refers to what I had on my DB3 (and was probably on many Swifts), I was told it was a droop limiter. I think it was an aftermarket kit from someone like Fast Forward.

[Chuckles]

I think, Jim (and hopefully people way smarter than me will chime in), you do not have the ability to "pre-load" a rocker arm car. All you can do is screw the spring perches up and down, affecting your ride height and corner weights.

I'll do a lame attempt to explain ... To use preload you need to crank the spring perch way up (causing the coil over springs to be compressed a bit - the shock is extended as far as it will go), and then you adjust the lengths of the pull/push rods to affect ride height and corner weights.

When you crank up your coil spring perches (on your rocker arm suspension), you are just raising that corner of the car, until you run out of threads.

I think the main reason designers went to pull/push rods is because rocker arms were not strong enough for some of the high spring rates people wanted to start using. Rocker arms would flex too much.

The front shocks on the swift can be easily preloaded. There is a pull rod between the inner end of the rocker and the bell crank which is adjustable. You don't have to move the spring perches to adjust ride height or corner weight just the pull rods.

[swiftdrivr]

The front shocks on the swift can be easily preloaded. There is a pull rod between the inner end of the rocker and the bell crank which is adjustable. You don't have to move the spring perches to adjust ride height or corner weight just the pull rods.

I adjust corner weight with the links, but never used them for ride height per se. Good thought.
BTW: I am glad to see an open wheel racer in Summerville! I pass through there occasionally.

[924RACR]

Hmmm, seems to me (having the same goofy rocker front suspension on my SE1-based P2) that using those end links to play with corner weight up front is pretty iffy, since you can so easily start cranking in preload on that annoying front ARB/stab bar design, no?

In our case, we've been trying to migrate to just doing that adjustment at the back, which has a more modern, fully-independant design with which we can dial out the ARB preload.

Though also, with the original Swift rear suspension (going on memory here since I pitched mine long ago) I seem to recall the rear shocks connected directly to the rockers, without a pushrod or pullrod, allowing no adjustment for height without touching the spring perch - getting back to the issue of needing to do droop limiting elsewhere, be it within the shock or an external add-on?

[swiftdrivr]

I thought leaving the front ARB disconnected until after the corner weights are done pretty much eliminated the "preloading the ARB" issue.

[R. Pare]

It does, but you have to remember ti readjust its links so that it isn't preloaded when reinstalled.

[924RACR]

I thought leaving the front ARB disconnected until after the corner weights are done pretty much eliminated the "preloading the ARB" issue.

You must have a nicer, more modern version than my car. I have the (perhaps older?) style that is incorporated into the pivots of the front bellcranks. You can take it out, but still have to get it back in. And it's not even a spline, but a hex. So it's slaved to the front shocks and pullrods - there's no ARB-specific end link that can be adjusted to match once everything's where you want it.

Not the ideal design IMO, and would love to get rid of it, but I'm learning to live with it (until such day as we rip that all off and convert to a modern pushrod design)...

[swiftdrivr]

You must have a nicer, more modern version than my car. ...

.

My car has the ?Fast Forward? mods, which allow you to set it after all is done. I will send a photo when I get back to the shop

[swiftdrivr]

I don't know whose system this is, or if it's still available, but it does allow a no-preload setup.
I don't know how to rotate the picture, but basically, loosen the nuts, back off the allen heads away from the cam, then once done. snug the allens and lock in place by tightening the nuts

[stonebridge20]

I don't know whose system this is, or if it's still available, but it does allow a no-preload setup.
I don't know how to rotate the picture, but basically, loosen the nuts, back off the allen heads away from the cam, then once done. snug the allens and lock in place by tightening the nuts

Protec ?

[John LaRue]

Protec ?

Yes I think it was Ray Germaine in Arizona who fabricated those. If I recall correctly he listed all of his fixtures for sale on APEX a few years ago.

[problemchild]

Unfortunately, some people don't understand the front suspension on the Swift. but it is anything but prehistoric or flawed. Those links are a very clever way to fix the flaws of rocker arm suspension (like the turnbuckles on my Reynard shocks). You can set spring preload on the shocks, and adjust irregularities in the rockers, while setting ride height with those links The Protech FARB mods are just a way to zero the front bar after you have finished setting all your corner weights. It is also compensating for irregular rockers but also allows the tuner to use setups with intentional cross-weight. Certainly with the Protech mods you can do almost anything that a pushrod car will do, but you will have less consistency due the tweeked rockers (which means more frequent trips to the pad and/or less curb hopping). With the softer spring rates in use with the spec tires, the flexing of the rockers during normal use should not be a major concern. Often priced as in the discount bin, the DB1/3 are awesome buys capable of winning races today.

[swiftdrivr]

Often priced as in the discount bin, the DB1/3 are awesome buys capable of winning races today.

Someone once kidded me about owning the same car for 20 years. I told him I'd step up as soon as I felt that the car was holding me back. I figure I'll be buried in the Swift!

[S Lathrop]

For give me if I repeat stuff that has been covered before in this thread..

I have used spring preload a lot over many decades and on cars from various Indy Lights to F2000. All of those cars have been winged cars which means that the corner speed changes the ride height of the car a bunch.

I know that droop limiting is common at the front end but I have never droop limited the front of a car. So I can not speak about what that does or why one would use it.

In general, I use droop limiting to solve mid corner push issues in slow speed corners. Think corners like the Key Hole and Carousel at Mid Ohio and corner 5 at Elkhart lake. I also use droop limiting on the 1 mile ovals in Indy Lights cars. Think Milwaukee and Phoenix.

I found that it took a very precise amount of preload to get the results that I wanted and I tuned the setup with one flat adjustments, about 15 pounds of preload on the spring. I also found that the amount of preload was very driver dependent. One driver might be comfortable with 200 pounds of preload where another wanted 225. But the driver that was happy with 200 could not handle the 225 setting.

Bottom line, preload is a valuable tuning tool but it has to be done with each car make and model, and it is best done after a lot of testing. But it is very car, track and driver dependent.

[924RACR]

Thanks for the input, Greg... as mentioned, even without the Protech setup we're learning to make it work for us... though we're also through the roof with spring rates because massive aero the chassis wasn't originally designed for (migrating from FF to P2 trim!)... add in the fact that even spare rockers aren't really available because we have the goofy Euro setup for the taller radials, there's much in the way of limitations/compromises we're working around to keep it out there.

That being so, as noted above - it's still not holding us back, yet, and not likely to tear it up until it is.

Back on topic - sounds like some preload testing will be well indicated for this summer; after some changes last year to my front end, I've had to/been able to cut my rear spring rates in half - now at a more appropriate balance of ride rates front/rear (about 4 and 4.25Hz, FWIW) - but now I seem to find the rear wing/downforce overpowering my front end, to the extend that (with half the spring/wheel rate at the back) I suspect it's trimming out all my rake in the chassis, and I'm struggling to make any significant front downforce.

I've run a little front preload in the past (though not lately), but haven't tried any in the rear... sounds like it's going to be time to try some of that out, in addition to simply dialing out some rear wing... a third spring setup would be ideal, but I think we've gotten ourselves too clever with packaging back there and implementing such a setup may simply not be possible without serious design heroics... and the car probably can't carry enough downforce/drag efficiently to justify that level of effort...

Have implemented rear droop limiters, but still haven't gotten far enough with chassis redevelopment to investigate: spent all last summer chasing fuel system problems instead of tuning the chassis.