Somehow that doesn't seem like a correct description of what happens with a damper's preload adjuster in relation to the whole vehicle though. All the forces you describe are within the same reference system (for lack of a better term). So in essence what you are describing would be this:
1. Screw the collar to the corresponding setting for 100lbs causing 1 inch compression.
2. This preload absorbs the first part of any subsequent load. So if I screw the collar to add 50lbs of compression nothing will happen.
Which, as you've pointed out, is wrong.
What you have is a spring forcing your suspension arm down with 100lbs. If you want that suspension arm to move you have to counter those 100lbs first. The spring is in the car. The force you are applying to move the suspension is external to the car.
I think I'm getting the picture now, but I think I need to do that test I said I would...
So it's wrong to consider the spring loads seperately, and ignore the fact that it's in a damper connected to a pushrod. The first 100lb of car weight isn't additive to the preload, but replaces the preload (the preload being applied by the dampers internal bump-stop, which of course isn't in use when you actually compress the whole spring/damper unit.
But after that initial load cancels out the preload, the spring moves with the original rate (be it 100lb/in). It doesn't make a 100lb spring behave like a 150lb spring.
I always want at least SOME preload to stop the spring rattling on full droop, which would be bad.
What effect would more preload cause, other than altering the ride height (which would be returned by shortening the pushrod) and moving the bellcrank, thus changing the rising rate used.
Hope you understand me. This is why I like the LFS Forum - clever people, and an ability to explain things in such a way that makes sense! Thank you all so far. Now let's try and make it useful in the real world.
True. But going through that thread helped me realize that I don't 100% understand the issue and made me go a bit deeper. I've come up with something now, hope it's right.
First of all I think it's important to be clear about what exactly we are doing when we say we are adding preload. As far as I can tell there are four main scenarios:
1) You turn the preload nut (see shotglass' pic) but not further than the point where when the wheels are on the ground, carrying the weight of the car, the damper travel just is maxed out but not more.
2) Same as 1) but afterwards you change the length of the pushrod to make sure the static ride height stays the same as it was before the adjustment. This is something you would definitely do with a single seater racing car.
3) Continue adding preload after this point.
4) Again same as 3) but with with ride height compensation.
I found that each of these four cases reacts slightly differently to adding preload.
1)
-static ride height increases
-maximum ride height stays the same
-suspension acts linear (no minimum force needed to make it move)
2)
-identical effect to a mechanical rebound limiter
-maximum ride height decreased (up to zero droop possible)
-suspension acts linear (no minimum force needed to make it move)
3)
-no change to max ride height
-no further change to static ride height (compared to 1))
-minimum force needed before suspension will move
4)
-no further change to max ride height (compared to 2))
-minimum force needed before suspension will move
So in summary preload can be used for a variety of effects. You can change ride height, limit rebound travel and add an offset to the displacement vs force function.
But how to use it? I say have a test, see what it feels like and what the stop watch says. Beats making your head hurt by analyzing it anyway.
Testing is great, but we can't test frequently at all. So we have to most of the analysis on paper, and only try stuff that works on paper.
Okay, so if we preload a spring a small amount (1/8"), and put the weight of the car on it, the spring will reach the same final length as it would if we had lots of preload (but not quite as much as the load of the car on the spring). Hurrah! But too much preload therefore eats into the droop travel available, which we don't want - I don't think it's possible to ever have too much droop. The spring rate remains the same at all times. However, we can vary the wheel rate (both the static and dynamic) by using the preload to choose where the bellcranks operate on their curves. Once that is defined we can set the ride height on the pushrods.
Great guys - I now understand this better than ever.
So, if someone came up to you and said that running less preload was good in the wet (ignoring rising rate curves for now), what would you say [assume ride height remains the same via pushrod adjustment]?
I wouldn't know really apart from following the logic that apparently less preload makes the suspension more "active/sensitive" which would be good for a less twitchy car I suppose.
Have you searched online for any MatLab simulations of this by any chance? (suspensions and dampers - not the "wet road = less preload" scenario)
Well for the wet your pulling significantly less lateral G's. If it was me I would want to use that little bit of extra "soft" springrate which would make the car "float" a bit more when under a strait line? Maybe? I would think it would.
focusing just on the preload, if you had a spring preloaded at 100lbs and had one at none, the one with none would vary between 0-100, with the load changes while the preloaded one would prevent the suspension from moving, thus giving the tire the job of absorbing all those forces.
I don't know if it would make it handle better though
With one inch preload the car already takes up the preload statically (and more so with downforce and cornering load transfer). In other words because the preload is already overcome just supporting the car (even if we use lots of preload) I don't think that's an issue. But there would be a slight reduction in wheel rate - i.e. the car is effectively slightly softer sprung. I'd need to do some maths to work out how much it varies, but I'd be shocked if it was lots.
We run with 1/4" preload on 325lb springs front and 250lb springs rear. I cannot see any point in running 1" preload on the front (other than it making the wheel rate higher, but I can use springs for that without limiting droop) or 0" preload on the rear (which gives a lower wheel rate and slightly more droop travel).
In other words I am confident that I have the car set up quite well (keep in mind that we aren't a 'specific track setups' kind of people - there are whole seconds to be had in my driving alone before we worry about tuning the car to a particular track).
Either way you are still easily in case 2) conditions in my earlier post. I think with single seaters even preloading until the point where the suspension doesn't move downwards when you lift the car off the ground isn't considered a lot of preload.
We set the preload with the dampers off the car. Turn the spring platforms until they just touch the spring, then a further 1/4" worth of turning. You can't set the preload on the car with weight on it, because preload has to be unladed. And even a single seater needs droop movement.
Then we fit them to the car, lower the car to the ground on it's wheels at the hot inflation pressure (20psi), and set the ride height using the pushrods. Then we set the corner weights using the rear pushrods only, and ensure the front corner weights are equal. The rear corner weights will be what they will be (and being a symetrical car they'll be fairly close - within 10lbs in our experience).
Apologies for people not used to the imperial unit system, but it's an English car, using English tools (our corner weight gauge is in lbs, and our springs are imperial (as in 6"/2.25"), so it makes sense to keep consistency. The engine is obviously metric (being a Toyota 3SGE), and there are a few annoying nuts and bolts that have been replaced with metric, which is when I curse and get the metric spanners from the toolbox.
You're right about that. I'll rephrase my post. Zero droop is indeed considered normal for many single seaters as you can see in both the AtlasF1 and FSAE discussions I linked to.
I don't know why. Aero stability springs to mind but in the FSAE discussion they are talking about FFords.
But if you have zero droop (and a stiff chassis), then as soon as diagonal load transfer occurs it'll lift the inside rear wheel (or the inside front when accelerating and turning) wouldn't it? And that isn't desirable in something as slidey as a FFord!
Wow...lots has happened in 24 hours! Been busy at work, then went to do some PC repair for a friend so no time for posting
Anyway, it seems like the issue has been more or less sorted. Going back to my diagram (end of page 1), obviously the analysis only applies if the damper rod has reached its full extension and the elastic modulus of the rod can be considered as a part of the system. It's true that in that specific case, you'd have to add a force equal to the preload force to get the damper off its end stop and get the spring moving. However, once it's moving, the spring rate is exactly equal to the spring rate of the spring, with or without preload.
Obviously when the suspension is fitted in a car you'd hope never to reach the end stops of the damper (in droop or bump), so this analysis isn't representing the correct situation. However, if you imagine hanging the car in the air with the suspension at full droop, then lowering it gently, I think my equations work. Once the damper is clear of the end stop, however, I think the preload only acts to reduce the ride height by the preload distance (multiplied by the bellcrank ratio).
I'm not sure I agree on the 'slight reduction in wheel rate'...the preload only seems to affect the offset, rather than the slope, of the load/displacement line. So, assuming no change in the bellcrank position, where does the reduction in wheel rate come from?
No, the slight reduction comes from the ballcrank movement. If I added preload, and corrected the ride height again on pushrod lengths, the bellcrank would move to a different position, thus changing the wheel rate slightly. At this stage I think it is slight, and not worth thinking about until I can lap consistently within a tenth of my best lap over and over again - at the moment I rarely ever get my best sectors all on the same lap, e.g. at Silverstone in the race I never improved my Brooklands/Luffield time after lap 3, did my best lap on something like lap 7, and had 'best sectors' on several other laps. So there is more to come from me than the car.
It's like an LFS newbie - won't get much benefit from a really good setup until they are pretty much on the pace with a standard set. I don't know how many racing laps I've done now, but I would still very much be in the newbie stage if it were on LFS.
No, you're not on my ignore list. I just don't always read every post, especially if there are lots to read. Sorry if I missed it But I think we got it sorted now anyway. I'll read your posts and see what you had to say
The misunderstanding that I had (and I think Stewart had at first) was considering the spring on it's own, and trying to work out why the preload on the spring somehow magically knew not to move until that figure was reached, meaning that preload + load gave a displacement of just preload (when preload = load).
But of course it's not like that. The spring and damper unit have to be considered as an assembly. The preload is an internal force created by the internal bump stops of the damper, whereas the load is external. The external load has to first move the damper off it's bump stops before movement will occur, hence the preload does, in fact, 'soak up' the first bit of load up until the preload value.
It would seem that you (and probably one or two other people) got this quite quickly, but we didn't grasp the concept immediately. Sorry for that - the difficulties of text-based communication with static images, rather than a real time chat in person with sketches on paper.
You think I should just believe everything you say? Where's the fun in that?
No but seriously. I went back to reread your posts and realize that you had pretty much said the same things that I came up with later, just in a more compressed form. The question I started out with was whether my claim that preload is nothing but droop limiting was true or false. I guess I could have derived the answer from your posts but I didn't notice this when I read them the first time.
Anyway, loved this thread. This is the sort of thing I come here for.
Springs don't have a soft bit to start with. It takes the same extra force to compress it 1mm no matter how much it's already compressed. So the ride isn't altered at all (although again this ignores the effect the rising rate bellcrank has on the suspension, which might make the ride worse or better with increasing preload).
