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Old 02-28-2002, 02:50 PM   #1
TerenceT
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Default suspension geometry question...

first of all i forgot where i read this so i am not being very technical when i throw this out...


I've heard that since WRX has the "rally breed suspension" hence long travel shock/spring, etc

wouldn't lowering this car adversely affect the handling charateristic ?

is there a certain threshold? like not good if it's more then 2 inches drop


thanks guy
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Old 03-01-2002, 01:20 AM   #2
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It depends on how you're dropping the car. If you're simply installing springs on stock dampers then it very well could hurt the car's handling, if the spring rate isn't matched properly. It could also hurt your handling if you simply cut the springs (bad idea), lowering the car without at least increasing the spring rate will cause the dampers to bottom out, and it could be very dangerous if this happened mid turn.

Choose some well matched dampers and springs, or a preassembled coil-over kit, and the handling of your car should be greatly improved, although most often affects ride quality in a negative way.

The stock suspension components seem to be no match for the car they were installed on. From what I can tell massive handling improvements can be had with aftermarket parts.
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Old 03-01-2002, 03:31 PM   #3
TerenceT
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i am talking about the angle the control arm makes

for example, it makes a 110 degree angle with struts, if you lower the car and the angle became 90 degree

does it affect the suspension geometry in a bad way
is this the rigth forum to ask a technical question like this?
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Old 03-01-2002, 04:28 PM   #4
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Quote:
is this the right forum to ask a technical question like this?
Yup.

Quote:
wouldn't lowering this car adversely affect the handling characteristic?
Mike Shields wrote up a lot of very good information about the Impreza. It can be found here:
SPD Tuning.
He speaks specifically about the ur-Impreza (GC8), but I believe the newage impreza is very similar is design.

Here is a quote from one of the tech. pages.

Quote:
The third thing is the Impreza should not be need lowered much more than an inch (25mm). If you go lower than that two things begin to happen. One, the car starts scraping the ground in driveways and such and will hit the bump stops frequently, NG (No Good). But, even more important is the front suspension geometry feature called "roll center" is upset, causing poor transition and unwanted camber change in the front end. The car looks good, feels good and is slow around corners because the front tire contact patch is "sub-optimal". (NFG)www.spdusa.com
Jim
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Old 03-01-2002, 05:58 PM   #5
TerenceT
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THAT'S WHAT I MEAN!!!!!!!!

Damn, Mike is goood... even better cuz he's close to me haahh

thanks man

so who wants a c/o that drops 2 inches?
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Old 03-01-2002, 08:59 PM   #6
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Ok since no one else has said it first that ""rally breed suspension" is pure marketing hype.

Second, many years ago we ran into this kind of thing on strut suspension on BMW 2002's. What we found was that as you lowered the car you hit a point of no gain. People were cranking in absurd amounts of static negative camber just to get back to where they started.

If you look at the front of most cars the lower control arm is angled down and makes an acute angle with the strut. As the outside wheel goes into bump as the car turns the suspension arm gets closer to II to the chassis. What this geometry does is add negative camber to the outside tire as the car goes into bump. (the lower part of the strut is pushed out away from the centerline of the car while the upper pivot point is fixed.) What this dynamic negative camber does is keep the tire upright as vs the road surface (a good thing). At the point the the lower arm goes past II the process reverses and you start to lose negative camber. Not good.

If you lower the car by shorting the spring and don't change anything else you can lose the best part of the gain curve. The problem is the amount of negative amber gain slows as you get closer to a II lower control arm and, in fact, if you go to far you will start to get positive camber as the wheel goes up. In order to combat this people were putting in lots of static negative camber so that they had some left as the car cornered.

The design of the BMW's struts were such that the ball joint was on a flange bolted to the bottom of the strut with the control arm below that. What we did was make a spacer to go between the ball joint and the strut to lower the lower pivot and regain some of the geometry we had lost.

While Mike is perfectly correct re: roll center changes we found the, at least for the BMW the big deal seemed to be the geometry change and specifically the lose of dynamic negative camber.

KCB
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Old 03-03-2002, 07:45 PM   #7
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so what about wrx?
is there a spacer need? is there one made?

any suspension set up out there that has the spacer? or is every company out there just trying to get the "dump look" and sacraficing drivability?
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Old 03-03-2002, 10:40 PM   #8
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No spacer - the Subaru doesn't have the same construction as the old BMW

I was just making the point that just lowering a car with a spring change can have negative results. I've never seen a STi Strut or any of the other aftermarket struts so I do not know if they do anything to re-establish the suspension geometry.

Car suspensions are a very complex deal, everything I've seen on this board is simplified as the subject is really to big to discuess here. (That includes my posts) If you want to know more there are a couple of on line sites and my favorite, books. Read, read read

KCB
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Old 03-04-2002, 02:12 AM   #9
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Good info, confirms some of my suspisions.
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Old 03-04-2002, 03:03 AM   #10
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Quote:
Originally posted by kbrate If you look at the front of most cars the lower control arm is angled down and makes an acute angle with the strut.... *snip*.... What we did was make a spacer to go between the ball joint and the strut to lower the lower pivot and regain some of the geometry we had lost.
Hmm.... I'd like to see how that works. Do you have a picture? I'm surprised you can get back the 1 to 2 inches you took out of the springs.. I can't imagine offsetting the subaru ball joint to make up for a 2" dropped spring set.

And as for modern automotive suspensions, yes they are very very complex. It's good though, because those 1970s cars that populated my youth had really dreadful handling. "Radial Tuned Suspension" wasn't really anything to brag about.

And KCB, I see you're from Macedon. I spent quite a bit of my above mentioned youth in Wayne country... Marion, NY to be specific.

Jim
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Old 03-04-2002, 07:12 PM   #11
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Sorry I don't have a picture - I could draw one but I'm not sure how to post it on the board.

Your right, you couldn't drop the ball joint as much as you lowered the car. I think you could get about a 1/2 to 5/8 in before you started getting close to the Wheel rim. You got to remember it was the mid-seventies and that's 25 years ago so my memory isn't perfect.

Since you've increased the spring rate (not absolutely necessary when lowering but most people do) and added a bigger bar (probably) the roll rate has gone up and therefore the car doesn't roll as much for a given force and therefore you don't compress the springs as much and therefore you don't need as much travel, just some. If you can get back enough of the curve so you can run reasonable static negative camber, i.e. you don't start going positive then that's probably enough.

The best word picture I can come up with is the bottom of the strut had a keyway on the bottom and two threaded holes with a hollow to accept the ball joint. The ball joint was on a flange that had a corresponding key and two bolt holes, the bottom of the flange was smooth. The suspension arm was a pressed steel afair with a hole in the middle and the bolt pattern of the flange in it. You placed the ball joint on the bottom of the strut with the suspension arm next and bolted up thru the suspension arm into the strut. (My details might be a bit off but you should get the idea).

We were going to make our own spacers when we found a very well known (at that time) BMW expert selling "lowering spacers". When we got them we found that they were missing the keyway and the instructions said to install between the pressed arm and the ball joint. I never could convince the guy that they did exactly nothing in that location. (I offered to prove my point with paper, a paper clip, a pin and a pencil ). We sent them back and made our own to go where they did some good, between the strut and the ball joint. This is why I don't accept anyones' word at face value.

I hope this answers your questions.

KCB
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