Lowering & suspension parameter discussion

[AtomicLabMonkey]

Originally Posted by Dean

Just remember every time you change ride height, you get to reallign the car.

Also, due to the terrible suspension geometry on the Impreza(Assuming it hasn't really gotten much better in 05) once you get below about 14" wheel center to fender, you run into excessive positive dynamic camber under cornering, and braking.

This is bad! So bad that Competitive A Stock STI Autocross cars go to the absolute stiffest front sway bar available to minimize this affect despite what one would normally consider way overstiffening the front end. And that is at stock ride height!

The net is lowering an Impreza can actually hurt cornering, so if done for cornering reasons, it needs to be done carfuly in moderation.

Pretty much any strut design will have lots of dynamic positive camber unless you start out with a lot of static negative camber. The best of them are hard pressed to gain near -1*/inch of bump, which isn't enough to keep the wheel upright as the chassis rolls. I guess what I'm saying is that with any strut at any ride height your dynamic camber is going to be largely determined by your static setting, regardless of camber gain.

[sperry]

Originally Posted by AtomicLabMonkey

Pretty much any strut design will have lots of dynamic positive camber unless you start out with a lot of static negative camber. The best of them are hard pressed to gain near -1*/inch of bump, which isn't enough to keep the wheel upright as the chassis rolls. I guess what I'm saying is that with any strut at any ride height your dynamic camber is going to be largely determined by your static setting, regardless of camber gain.

But when you start with the chassis side of the control arm already below the knuckel, the situation only gets worse as you compress the suspension, rather than at least staying neutral for the first inch or two of compression.

...course that can be combatted by simply going with extremely stiff springs, so there's is no compression.

[AtomicLabMonkey]

Originally Posted by sperry

But when you start with the chassis side of the control arm already below the knuckel, the situation only gets worse as you compress the suspension, rather than at least staying neutral for the first inch or two of compression.

That's really not how it works, it doesn't stay neutral and then go bad. The wheel angle at any given point equals the chassis roll angle minus any camber gain. As soon as the car develops any lateral force at all, there is chassis roll, simple physics. A couple inches of suspension travel will typically work out to about 3 or 4* of chassis roll. If you have camber gain that's only in the neighborhood of -0.4* to -0.9* per inch of bump, which is typical for production car-layout struts, you can do the math and see that if you start out with a static camber angle of 0, the outside wheel will always be at a positive dynamic camber angle.

My main point is that dropping the ride height way down will affect the camber gain, because it changes the angle of the lower control arm, but the camber gain is so small to begin with relative to the chassis roll and static camber setting, that it is not the main determining factor in the dynamic camber you will have when the car is loaded up in the middle of a corner.

This is not to say that the handling will stay hunkey-dorey if you slam the car to the ground! Altering the LCA angle affects lots of other things like roll center height, front view swing arm length, jacking forces, & tire lateral scrub... which in my opinion at least is usually the reason why a strut car will handle poorly if it's lowered too much - not the change in camber gain. It's somewhat of a subtle technical point, I know... but I'm just trying to shed some light on the subject.

[sperry]

Originally Posted by AtomicLabMonkey

It's somewhat of a subtle technical point, I know... but I'm just trying to shed some light on the subject.

W/o me knowing all the technical jargon, I'm gonna need a picture to understand what you're talking about.

The way I had it explained to me, as the LCA pivots, the further away from 0* horizontal it gets, the faster the suspension geometry "gets bad", i.e. if you pivot from -1* to +1*, things are better than if you pivot from 0* to +2*. Even though it's a 2* delta in both cases, +2* is worse than +1* for screwing up the geometry, which is why everyone recommends lowering the car no further than a horizontal LCA at static ride height.

Now, I don't know the details as to why things get worse, so I can't really back up what I'm talking about, but it sounds pretty good to a non-ME like me. If the dynamic camber change is really a linear function of LCA angle, and not as I described it, then it sounds like the handling issues w/ going super low aren't actually due to the dynamic camber change.

[AtomicLabMonkey]

Originally Posted by sperry

The way I had it explained to me, as the LCA pivots, the further away from 0* horizontal it gets, the faster the suspension geometry "gets bad",

This is not true. It's all just a question of application. If you have a heavy street car with a tall C.G., you might want a relatively high roll center to resist chassis roll, which would require angling the LCA's upwards towards the chassis. This then affects all those other parameters I mentioned before.

If you have a lightweight low-slung racecar, you might put more of a priority on minimizing jacking forces which requires a low roll center, so you go with a LCA angle near horizontal or even angled down towards the chassis. Which incidentally results in worse camber gain & more roll center movement, but that's not a big deal if you're mitigating it with static negative camber and stiff springs & swaybars. It's all a trade-off. Every single aspect of suspension design is a trade-off between one parameter vs. another.

Originally Posted by sperry

If the dynamic camber change is really a linear function of LCA angle, and not as I described it, then it sounds like the handling issues w/ going super low aren't actually due to the dynamic camber change.

It's not perfectly linear.. but like I said I believe it's usually the other parameters I mentioned that have a detrimental effect on handling when a street car is lowered, when they're coupled with the amount of chassis roll that street cars have.

[AtomicLabMonkey]

Originally Posted by sperry

W/o me knowing all the technical jargon, I'm gonna need a picture to understand what you're talking about.

If you want a working stick-figure model to play with instead of a picture, you can download PT's roll center calculator program here. As I recall they have some example cars you can load up and experiment with. It's a pretty limited capability program, but it shows some of the front-view geometry like instant centers, roll center, camber gain, etc...

[AtomicLabMonkey]

Also I went back and looked at the RC calculator program and it pretty much won't let you change any values to experiment with changes. Suck. There are cracks floating around for that program though, so you could have full functionality...

[sperry]

Originally Posted by AtomicLabMonkey

Also I went back and looked at the RC calculator program and it pretty much won't let you change any values to experiment with changes. Suck. There are cracks floating around for that program though, so you could have full functionality...

Or you can just edit the MUSTANG file by hand w/ the correct numbers... and open it.

All I need now are the correct numbers, but I don't exactly have a WRX on hand to measure everything.

[AtomicLabMonkey]

Originally Posted by sperry

Or you can just edit the MUSTANG file by hand w/ the correct numbers... and open it.

Keep that programmer-talk to yourself. That would take more time & effort for me than finding the crack...

[Dean]

Austin, in most cases, I would agree body roll is a bigger concern, in this case, it is just another contributer to the terrible geometry on the front of the Impreza.

The impreza dynamic camber is atrocious for all the roll reasons and amplified by the short lower arms, short overall height of the strut, arm angle at reasonable ride heights, and minimal castor availability. Oh and arm shape/connecting points suck also.

[AtomicLabMonkey]

Originally Posted by Dean

Austin, in most cases, I would agree body roll is a bigger concern, in this case, it is just another contributer to the terrible geometry on the front of the Impreza.

Whatever it is, I doubt it's worse than a 1978 Fairmont which is what I deal with every day and have formed a lot of opinions based on.

If you guys measured up all the pivot points on one of your cars I'd be happy to analyze it though, if you want to see what it's actual behavior is.

[cody]

This thread is stealing my thunder.

[Nick Koan]

Originally Posted by cody

This thread is stealing my thunder.

what thunder?

[cody]

I had never heard that the Impreza had terrible suspension geometry before. How depressing.

[Nick Koan]

Originally Posted by cody

I had never heard that the Impreza had terrible suspension geometry before. How depressing.

Its the bane of MacPherson suspensions. While it is possible to make the design workable and useful in motorsports applications (with enough money), the reality is, you are starting from an inheriently weaker design. It is an economy design, not a motorsports design.

[cody]

Ah well, I find that I'm very happy with how my car handles. I'll just compare it to the stock suspension/tires setup and remain happy. I love my Eibach/AGX combo. I can't decide if I want to pick up a Progress Sway for cheap to acompany the solid endlinks that I'm running with a 20mm SOA bar from an 02 WRX sedan.

Also this weekend I added a rear strut bar...the Whiteline quickrelase one.

So do Evo's, Miata's, S2000's, GTI's, have the same problems? I'm trying to put this in perspective. I mean if other cars in our class are just as "inferior"...

[Dean]

Lets be clear... Matt's STI on Street tires ran a faster time than a Z06 Corvette on race tires with the same driver at the last autocross, so the impreza handling doesn't suck.

All I am saying is that the front end geometry is such that some significantly unusual tuning has to be done to overcome it's limitations, and amung those is not lowering the car as much as one might like to lower the CG.

[AtomicLabMonkey]

Originally Posted by cody

So do Evo's, Miata's, S2000's, GTI's, have the same problems? I'm trying to put this in perspective. I mean if other cars in our class are just as "inferior"...

EVO's are strut suspensions as far as I remember, so yes they do. Miatas have a dual A-arm design, so no, not necessarily. I don't know about the others...

[cody]

Doesn't the Impreza suspension excel on uneven surfaces? I thought the Impreza was known for being able to hit a pothole while cornering without upsetting the vehicle?

[Nick Koan]

S2000 has double wishbone suspension front and rear. While the GTI has strut suspnesion front and semi-independent torsion beam axle rear.

I don't know nearly enough about different suspension setups, but the S2000 double wishbone front and rear is supposed to be amazing.

[M3n2c3]

Originally Posted by nKoan

what thunder?

hahahahahahaha

Sorry, just had to share my mirth.

I'm curious about this as well, particularly how to best counteract it. With the Impreza so widely regarded as a relatively impressive and inexpensive (at least to start with) AWD rally/race platform, it's disconcerting to think of its suspension as being so inferior. Not that I expect it to be perfect without heavy mods, but still. . .

[AtomicLabMonkey]

Originally Posted by cody

Doesn't the Impreza suspension excel on uneven surfaces? I thought the Impreza was known for being able to hit a pothole while cornering without upsetting the vehicle?

That will typically have a lot more to do with spring & shock rates (ride frequency) & bushing compliance design than suspension geometry like we've been talking about.

[Dean]

Originally Posted by cody

So do Evo's, Miata's, S2000's, GTI's, have the same problems? I'm trying to put this in perspective. I mean if other cars in our class are just as "inferior"...

Yes and no. The primary gotcha on the Impreza is that it is a semi-heavy front biased sedan/coupe with relatively high CG compared at least to the Miata and S2000, and it has front drive.

The Evo and GTI also have similar issues, but I am not sure to what extent as I have not studied them in detail.

[cody]

Originally Posted by AtomicLabMonkey

That will typically have a lot more to do with spring & shock rates (ride frequency) & bushing compliance design than suspension geometry like we've been talking about.

I thought it had to do with "long travel suspension" that Subaru advertises. Don't Subarus have longer suspension components with the pivot point near the center of the vehicle? Sorry I'm not familiar with suspension terminology. I'm looking for a diagram to illustrate what I'm talking about.

[cody]

This article I'm reading is pretty cool.

This pretty much echos what nKoan was saying about Macphearson supsension attributes.


The article then goes on to talk about double wish bone suspension. Yeah, I like learning.

I wonder if any rally cars use double wishbone setups (or as the article describes it, "unequal length double A-arm suspension"). More specifically, is a Macpherson or the DWB setup superior for off road use? The article claims there is no downside to the DWB setup apart from cost and space requirements.

The image shows a typical unequal length double A-arm setup. Note the difference in length between the upper and lower arms. This is what gives this suspension its ability to generate negative camber in bump.