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So by lowering the car you are lower the centre of gravity, but you have also lowered the roll centre.
By then raising the roll centre back up you make the most of the lower CoG
Audi CQ, yes, moving the inner pivot points of the wishbones needs some serous modifications. Have been thinking that and it's next to impossible with the stock subframes.
Ryss, keeping the control arm horizontal does you nothing good really, that is not the point. The point is to move the lower balljoint down, so the wishbone is tilted down the way that the balljoint is lower than the inside pivot points. That does two things: when cornering, the suspension of the outside of the curve compresses, and the inner pivot points of the wishbone move down. If the wishbone was tilted downwards at the first place, it starts moving towards horizontal thus making the effective length of the wishbone longer -> it pushes the lower end of the hub carrier out and thereby increases your negative camber. When cornering, the increasing negative camber of course gives you more grip. If the wishbone is horizontal in the starting situation, in cornering it starts to pull the carrier in -> decreases negative camber -> less grip.
The other thing which is beneficial, is that when the wishbone angle is altered, it changes the place of the car's roll center. Roll center is the imaginary point around which the car's chassis is rolled during cornering. The thing with the RC is, that the farther away from the actual mass centerpoint, or center line of the car it is, the more "leverage" is involved in the rolling situation. So the longer distance between RC and mass center point -> the more the car chassis tends to roll and vice versa. IF the RC would be situated exactly in the same point as the car's mass centerpoint, the chassis would not roll at all! But that brings other worries, like wheel jacking effect.
So, when you lower your car it's a complicated thing. Lowering will move your mass center point lower, which is good because it will reduce the weight transfer in the corners. But at the same time the roll center goes in all the wrong places and also your geometry with the wishbones pointing to the sky.
Here's some pics I made with Solidworks of the situation:
4th pic, no extension, but moved the wishbone inner points up 1,5" = 38mm = about the same lengt what the extender brings the lower balljoint down. RC @ 117mm so about the same as the extender.
The rest of the pics are with more and more tilted wishbone, but you get the idea.
About the camber gain, I have made some graphs:
It's in Finnish so most of you propably don't understand much of it. But in the graph the vertical axis numbers represent the suspension compression in mm. The horizontal axis represent negative camber in degrees. Red curve is with standard geometry. Green is std caster with the extended balljoint and the blue curve is 9 degrees of caster & balljoint extender. In all curves, the wheels have been turned 5 degrees, so to simulate cornering situation.
I guess it wasn't quite simple explanation but I hope it was of some use...
First two pieces of 42crmo4 extenders are born. In that picture there are bolt holes missing, but now they are ready and awaiting to be mounted in my car. Today I'll be doing the mounting & tracking if I have the time. The first "summer" trackday is to be in three weeks, and we only have half a meter of snow here in Finland...
When you modeled the negative camber versus wheel deflection did you account for the roll of the car? As the body rolls during cornering the strut top is pushed outward introducing positive camber. This is the reason macpherson strut systems really need the wishbone to be pointing down through their working arc. Modeling against wheel deflection alone give a false impression of negative camber, also the roll center is a function of both sides of the axle - to gain a true log of roll center position you must include compression on the outboard side, extention on the inboard side and body roll.
Also on roll axis, the reason that the quattro (pictured previously) lifts its inside wheel is the axis of roll. The front axle roll centre is lower the the rear - that is the roll axis is tilted forward. This helps to kill understeer. With a RWD car the axis is tilted the other way, which is why you see a MK2 escort cornering hard with its inside front wheel off the ground. In the case of RWD this leaves the driving wheels on the ground and providing drive while also reducing the available front axle grip therefore reducing oversteer. Exactly the opposite applies to a FWD car, rear wheel up driven wheels down and countering the inherent understeer. Quattros are AWD but understeer like a bitch so it stands to reason they would have engineered the roll axis down infront like a FWD car to encourage the back to develop a greater slip angle.
Next time your bored stick a knitting needle through a cereal box and try out roll axis and its effect on cornering attitude.
Donnie, I agree with everything you said. And no, the chassis roll wasn't taken into account when making the model. That would indeed make the situation even worse with the standard geometry. As well as every other geometry.
I have been planning to do that, even tried once to construct a model which takes into account spring rates etc. but it failed. :/ It could be done of course, if I had some more time. But then there is the question of the location off the mass center point/line, which will affect the modeling significantly. It could be calculated if I had wheel scales and lots of time but unfortunately I don't.
Other way would be to just simulate different degrees of body roll and draw the graphs. But without the mass axis & spring rates taken into account it would be still far from realistic as it would be difficult to say how the suspension moves in relation to chassis and the whole system in relation to the ground.
I've also seen how the rwd & fwd are tuned differently in the way which wheel is lifted during cornering. In 4wd it's a bit of a problem if you don't have lsd's. The quattro is an interesting thing to play in that sense. With some modifications, you could change the behaviour to be more biased to the fwd or rwd. In any case, all wheels should stay on ground to prevent torque leaking all over the place.
With regards to modeling chassis roll, I'd just run the simulation with 2, 4 then 6 degrees of chassis inclination as a start point. If I remember correctly the rally cars did without a middle diff so you'd still have drive with a wheel elevated. Hard to say if the reduction in rear axle grip caused by lifting an inside wheel would be more than the reduction front axle grip caused by transmitting all that lovely 5cyl torque.
As always the elephant in the corner with quattros is the sledgehammer like weight distribution.
Yes, well if we simplify and assume that the chassis roll does not roll the whole system, 2 degrees of roll relative to the wishbone will shift the balljoint up 14mm. 4deg 28mm and six degrees 42mm respectively. So then you just look at the curves at those points and up.
If we assume the chassis roll rolls the whole system, just add 2, 4 or 6 degrees _positive_ camber to the starting values of those curves.
In reality the situation is a combination of those both. If the car has any springs installed I'd say the 6deg should not really happen.
About the sledgehammershness, from what I have learned from modifying the geometries, the main culprit for the massive understeer is the overly safe suspension geometry. As the caster is almost nonexistent, the car will push and eat the side wall of the tyre during cornering due to the chassis roll and positive cambergain. When countered with lots of caster, the feeling of the car is transformed significantly. Had that "wow"-experience last year.
Of course, if you compare t89/85 to some mid-engined sportscar, they are way different. Just saying that fiddling with the geometry is well worthwhile, and we shouldn't hang ourselves to the lump of cast iron hanging in the front.
Your right in what you say but..... a car with 70% (+/-) of its 1600 kg over the front will never corner like a honda integra. By no means interpret what I say as "leave it alone its pointless" because thats not what I mean at all. I think my s2 is fantastic, all things considered its the best compromise of torque, useability, toughness and handling for any money. Tuning suspension is as much of an art as tuning an engine.
As for the roll induced positive camber, you could set the static negative so the wheel never pushes into positive but I believe a compromise is required to keep the wheels as vertical as possible without compromising braking (dive) and without making the thing twitchy in a straight line. Macpherson systems are a compromise derived from packaging needs - maybe someone could develop an unequal length double wishbone system or at least an upper control arm system. - Maybe one day in my dreams.
Even the weight distribution is not that impossible. Ok, my car is not an S2 but a 90q which I have lightened for ~40kg. Anyway it has 20vt and it weighs 1321kg full tank, no driver. Weight distribution 816kg front & 505kg rear. Percentages being 62/38, which (this is my favourite argument ) according to the Internet is the ~same as Mitsu Evo IX.
But yes, the static camber must be quite aggressive. I have run 888's with front static camber -2,5 degrees and it feels ok. Stable in the straights and brakings. That needs some more experiments though. For this season I will also use those extenders at least in the front and swap much stiffer springs. I now have kw v3 standard springs which are rated 70N/mm front and 60N/mm rear.
Hopefully I have time to set the alignment & go for a drive soon. The only concern I have is that if the extender will try to twist itself around the axle stub. The only thing stopping it from doing that is the clamping force of the hub carrier's joint. Of course the pinch bolt will eventually stop the twisting motion, but it's not good if it's rotating there back and forth.
To be honest I am a bit worried about the effective leverage that is being imposed on the hub carrier ball joint fitting. This however is based on my logic .. not any engineering background. Greatest forces will be imposed on braking and cornering. Where as with a ball joint direct to the hub carrier the effective leverage is very short. with the extension is is much longer and therefore that much more leverage.
The fact that you have shaped the ball joint extension to match the bottom of the hub carrier and has a nice big matting surface should help a lot, in stabilizing the extension for lateral forces imposed on it.
Only thing I would like to suggest is to make the upper pin a bit longer and thread the excess, so that a large dia.low profile, fine threaded nut could be added to pull the extension up against the hub carrier, taking some of the load off the pinch bolt. But there is no space to do that.
I was always uncomfortable with the Type 85 type 2 bolt ball joint under heavy braking, on my track car. I see your discs are huge.
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