rynberg

It will be close, I did the same thing with my HKS springs. The lowest perch will lower it ~1/2" in the rear, so....

maxcooper

But the suspension arm that the bottom of the spring sits on is also connected to the body, so the motion ratio applies.

I haven't considered this too deeply yet, but I am fairly convinced that the wrong conclusion has been reached in this thread. Someone draw some pics of the geometry with a vertical shock, double wishbones, and ride heights that result in the suspension arms being an equal amount above and below horizontal.

-Max

rynberg

It's not the wrong conclusion Max, at least not on my car....I went from the highest perch to the lowest perch at the rear with no other changes and the car dropped pretty much exactly what Koni claims -- 1/2", which is the distance between the spring perches.

maxcooper

The springs compress the same amount at any ride height because you've got the same chassis weight compressing the springs. So the length of the shock/spring assembly changes the same amount as the change in the spring perch position. And thus the motion ratio applies, and the change in perch position is multiplied into a larger change in ride height.

-Max

maxcooper

If you guys thought my photos were bad...

The picture below shows two different ride heights, set by changing the position of the lower spring perch on the shock/spring assembly. The spring will be exactly the same length at both ride heights, because the same chassis weight is sitting on it. The simple suspension is just one arm and no tire is shown (but it would be at the left side of the picture). The right side is the chassis. The only thing that changes is how much shock is sticking out below the lower spring perch, and thus what position the shock bottom is in. The change in position of the shock bottom is multiplied by the motion ratio of the suspension to produce a larger change in ride height.



Okay, here's my diagram of why the motion ratio applies when you change the spring perch position. If you think (or know ) something is wrong with this, please explain what is wrong. It seems pretty simple to me -- the motion ratio applies when you change the ride height by adjusting the spring perch. The change in perch position (say, 0.5" down the shock body) multiplied by the motion ratio (say, 1.4) equals the change in ride height (1.4 x 0.5" = 0.7").

-Max

DamonB

max, I think you're just thinking the same way I was and haven't gone full circle yet. The motion ratio is true when we are comparing wheel travel vs shock/spring travel. However, when we are discussing ride height we are talking about the chassis merely resting on top of the springs. If we imagined the chassis was a table top and the legs were the springs, then shortening the legs a given amount will lower the table top the same amount. This is the same as the car because ride height is merely how high the chassis is off the ground and the chassis weight actually rests on the spring perches.

Changing ride height by moving the perch a given amount will move the chassis about the same amount. I say about because the springs are not vertical, they lean inward slightly. Because of that the ride height change will actually be greater than the spring perch height change but not a whole lot. The leaning of the shocks/springs explains why the change in ride height on the Koni is greater than the small amount of distance the perch moves.

DamonB

You getting old and lazy?

maxcooper

But that isn't right. It isn't like a table with legs. The chassis is connected to the suspension arm, so if you move the chassis up, the suspension arm changes angle and multiplies the change. The chassis weight resting on the springs means that the distance between the lower spring perch and upper spring perch will be the same no matter what you set the ride height to. But, since moving the spring perch changes the amount of shock that is sticking out below the spring perch, you will move the suspension arm like a lever, multiplying the change in spring perch position to give a greater change in ride height.

Here's a new awesome graphic to show why the table leg analogy is false:


-Max

DamonB

You're right, it's exactly like changing the length of the leg labled as "2", but the change in length is not multiplied. The wheels and suspension don't move, they're just sitting on the ground. If we shorten "2" the chassis drops an equal amount. The suspension will move slightly because its inner pivots are bolted to the chassis, but for all intents and purposes the suspension doesn't move at all, the perch merely drops and the chassis comes down with it. Normally we think of the wheel moving in relation to the chassis, with ride height we're moving the chassis in relation to the wheel. Since the wheel and suspension is not really moving there is no lever acting between the spring and the wheel, the chassis is merely moving closer to them.

The chassis is connected to the arms but those arms really bare no weight, they just transfer the wieght of the chassis out to the wheel bearing. The chassis rests on the springs and the springs rest on the perches. Everything else can be ignored. If we were to put a half inch spacer between the upper shock mounts and the chassis you would expect the chassis to raise half an inch, right? All we're doing in that case is raising the chassis by inserting a spacer. When we remove the spacer the chassis would drop back down the same amount. Raising or lowering the perch is exactly the same thing. It confused me too at first.

maxcooper

The change is multiplied, unless your chassis is made out of Jello and you can raise the chassis without moving the suspension mount on the body. Forget about springs for a moment, and consider that if you change the length of 2 and the other arms in that picture have fixed lengths, the table will raise up off the ground MORE than the amount of change in "bar 2". This sholuld be clear -- it is the basic concept of how a lever works.

The weight of the chassis rests solely on the springs. So, the compressed length of the spring with the car at rest is not going to change no matter what you set the ride height to. If your coil-over was 12" long before, and you move the spring perch up 1", your coil-over will now be precisely 13" long with the car at rest. The compressed spring length has not changed, and you moved the perch 1", so the length of the coil-over will also change by exactly 1" with the car at rest.

Take those together, and you will see that the motion ratio does apply when moving the spring perch.

No! That would be exactly like changing "rod 2" by 1/2", which clearly translates into raising the height of the table by more than 1/2".

The chassis will drop back down 1/2" relative to the spring perch, but what is being ignored here is that by dropping the chassis relative to the spring perch, you are also changing the angles of the suspension arms, which means the height of the spring perch itself will drop (in the case of removing the spacer, or moving the spring perch down the shock body).

The motion ratios are not very large here (1.4:1 F and 1.6:1 rear, I think for the FD?), so it's effect is not going to be very large. But the effect is definitely there -- the motion ratio is in full effect for changes in the position of the spring perch on the body of the shock!

I think that may be where we are getting mixed up: change in position of spring perch on shock body VS height of spring perch relative to the ground.

For height of spring perch relative to the ground, then I agree completely that the motion ratios don't apply. The car is just resting on the springs, changing ride height doesn't change compressed spring length, and so a 1" drop of the spring perch will give a precise 1" drop in the car's ride height.

But when I change the position of my spring perches, my perspective is that of changing the position of the perch on the shock body (I even measure the change on the shock sometimes). The motion ratio does apply here. If I move the spring perch up by 0.5" on the shock body, the ride of the car will increase by 0.5" * 1.4 = 0.7" at that corner.

-Max

DassRotary2427

i hate to interupt such an intelectual conversation but i have a noob question. i have the konis and tein s-tech springs. can i lower the perch and lower the car?

maxcooper

The Konis have adjustable (3-position) lower spring perches. So yes, you can lower it, unless you are already on the lowest perch setting.

-Max

rynberg

Yes, you can lower it further using the spring perches...but I wouldn't, at least not up front. I would run the fronts at the highest perch setting and maybe lower the rear to match the front ride height. You don't want to lower the front because you run out of suspension travel and start hitting against the bump stops with the tires hitting the fender liners. Not good.

DassRotary2427

one more question... i have tein s-tech's on my car which lowers the car about 1.5 inches. will the konis drop it any lower?

maxcooper

That's the same question you asked before, isn't it?

From what I gather:

S-tech springs + stock shocks = 1.5" drop
S-tech springs + Koni shocks on highest spring perch settings = 1.5" drop
S-tech springs + Koni shocks on anything but the highest perch settings = more than 1.5" drop

-Max

Travis R

I'm with Max on this one, despite his sub-standard graphics.
You can figure it out without even including the springs and perches. Pretend they aren't even installed on the car.
The chassis is sitting on a jack at ride height, and the tires are on the ground. Measure the length of the shock. Now jack the car up one inch. Now re-measure the length of the shock. Because of the motion ratio the shock will be longer by something less than 1".
Fwew, look how short that was... and I didn't even use MSpaint. :p

maxcooper

Hey, I used the GIMP, not Paint!

-Max