Aerodynamics, Lift, Tires/Grip, or ?

[ShadowImg]

Alright folks, not quite sure where to toss this, so general ahoy. One of the final things I'm investigating about the 2.5GT prior to purchase is in regards to aerodynamics on the highway, specifically in windy conditions.

 

My 2.5RS at 70-75 feels like it gets significant amounts of lift on the front end when running on stock wheels and stock size tires. Oddly, this feeling doesn't exist when running on the 225/45R17s on the Rota Tarmac IIs. When reading up on it back then, it was agreed that the bodystyle did have significant front end lift, and Prodrive produced their aero kit specifically to combat it.

 

Now, my problem is I don't know that it even is lift, but assuming it is -- does the 05-07 2.5GT have a similar problem at these speeds?

 

Assuming it's not, anyone have an idea of what gives that feeling? Is it just soft springs and downforce at high speeds? Lack of grip in tires? Heavier wheel weight on the S03s/Tarmac IIs keeping them planted more firmly?

 

I've test driven the LGT, but it's hard to make the determination since the suspension/steering was so radically different it was hard to come up with exactly what all I was feeling differently.

 

Hopefully some of you suspension/grip/dynamics gurus can tell me what I can expect or what I need to do to fix this if it does happen with the LGT as well.

 

Note: I've only ever observed this at 70+, or in *extremely* heavy tail winds at 45+, which is why I suspected it to be an aero thing.

[douglas751]

These are regular cars, not track cars. If you do not like how it drives after you test drive it, then you need to find another car. Planning to modify a car before you buy it to correct an Aero Dynamic problem that you think it may have is not a good idea. As for Aerodynamic modifications, that is not really a factor unless we are talking about Autobahn speeds, and since you are living in Missouri, I highly doubt that you will be able to drive that fast on a regular basis.

 

If you like how the wheels and aero kit look, just do it. You cannot justify the minor change in Aerodynamics and most likely they will have a negative effect not a positive effect. Do you have a wind tunnel that you will be able to use?

 

Heavier wheel weight is not ever a benefit. Heavy wheels have a negative impact on ride, accelration, fuel mileage and how the car handles. The wider wheels will make the car tramline and follow ruts in the road, and to me that will be a bigger negative than high speed wind stability as it will effect you all the time.

 

My Spec B feels great up to 100 mph. In Michigan, we cruise at 90 mph on the freeway every day. The car is not as stable in cross winds as was my BMW 545i, but it is certainly not unstable or dangerously so.

[ebpda9]

my problem got solved when i ditched the re92's i never had a problem this whole summer riding on the pzeros and i had the car in triple digits speeds

[ShadowImg]

These are regular cars, not track cars. If you do not like how it drives after you test drive it, then you need to find another car. Planning to modify a car before you buy it to correct an Aero Dynamic problem that you think it may have is not a good idea. As for Aerodynamic modifications, that is not really a factor unless we are talking about Autobahn speeds, and since you are living in Missouri, I highly doubt that you will be able to drive that fast on a regular basis.

 

If you like how the wheels and aero kit look, just do it. You cannot justify the minor change in Aerodynamics and most likely they will have a negative effect not a positive effect. Do you have a wind tunnel that you will be able to use?

 

Heavier wheel weight is not ever a benefit. Heavy wheels have a negative impact on ride, accelration, fuel mileage and how the car handles. The wider wheels will make the car tramline and follow ruts in the road, and to me that will be a bigger negative than high speed wind stability as it will effect you all the time.

 

My Spec B feels great up to 100 mph. In Michigan, we cruise at 90 mph on the freeway every day. The car is not as stable in cross winds as was my BMW 545i, but it is certainly not unstable or dangerously so.

Actually, I didn't notice it on the LGT -- but I do notice it on the 2.5RS -- again, it's largely a question to whether anyone else notices it, or if I'm diagnosing what I'm feeling incorrectly.

 

I don't like how aero kits look, and would prefer to avoid it, your last sentence is helpful though. That's really what I'm after.

 

Given the nature of these boards, I think your statement about modifications to a vehicle is pretty silly though. I fully intend to replace the RE92s because I don't like how they ride/feel, and because they serve no real purpose for me. The same may very well be true for other aspects of the suspension.

 

This forum wouldn't really exist if a lot of people here only bought a car that was 100% perfect from the factory and didn't want to change a thing...

[CMull123]

These are regular cars, not track cars. If you do not like how it drives after you test drive it, then you need to find another car. Planning to modify a car before you buy it to correct an Aero Dynamic problem that you think it may have is not a good idea......

 

......In Michigan, we cruise at 90 mph on the freeway every day. The car is not as stable in cross winds as was my BMW 545i, but it is certainly not unstable or dangerously so.

 

I completely agree with you about planning aero mods b4 you even buy the car. Any aftermarket kit you buy for everyday use is 99% just for looks. Now if you can get an SCCA kit or sumthin like that, which make the car look like crap, then you'll feel a marked difference.

 

And douglas751, whereabouts in MI do you live? I live next to South Bend in IN.

[Guest subiecity]

ahh i have the 2.5i i never had too much of a problem with lift at hi speeds but once in the triple digits natrually i did feel sum lift...i dont reccomned this at all but i lowered the car an inch, tightened up the suspension hardcore, installed oem factory spoiler, along with front lip spoiler and thicker tires 18x7.5, not to mention front and rear strut tower bars for those hard turns at higher speeds....and as of now i have absolutely no body roll and no lift whatsoever and tha car handles like an absloute dream

[Scotty]

Or get the wagon. The wagons do have more drag, but generate less lift.

[forestir]

I've test driven the LGT, but it's hard to make the determination since the suspension/steering was so radically different it was hard to come up with exactly what all I was feeling differently.

 

Hopefully some of you suspension/grip/dynamics gurus can tell me what I can expect or what I need to do to fix this if it does happen with the LGT as well.

 

Note: I've only ever observed this at 70+, or in *extremely* heavy tail winds at 45+, which is why I suspected it to be an aero thing.

 

My Outback wagon has been stable in 70+, with and without heavy winds. If anything, a jacked up wagon would exhibit the worst aero problems, so I don't think you have anything to worry about there.

 

I would suspect alignment issues if you felt anything strange.

[ShadowImg]

Thanks folks, I've heard alignment from the mwsoc folks as well, and that was actually my original thought (I've had two alignments done since then, one by the shop that did my wheels/tires, and then one by the dealership shop).

 

It was pointed out that the 02-03 impreza body did have a lot of lift problems at high speeds, which is why I was wondering if this is what I was feeling.

 

Another suggestion is that it's the soft sidewalls vs stiff sidewalls that I'm noticing the feel difference in.

 

Unfortunately this being the only car I've had it makes it tough for me to compare these feels -- if anyone has any suggestions on how to identify the dynamics differences of tire softness vs spring softness vs alignment problem vs aerodynamics, I'd love to hear it.

[forestir]

Poke around at Whiteline's website. They do lots of Subaru suspension stuff, based in Australia. Their site's chock full of good info on Impreza suspension.

 

You probably have worn bushings on your front control arm. They cite that as a problem area with Subaru suspensions.

 

Thanks folks, I've heard alignment from the mwsoc folks as well, and that was actually my original thought (I've had two alignments done since then, one by the shop that did my wheels/tires, and then one by the dealership shop).

 

It was pointed out that the 02-03 impreza body did have a lot of lift problems at high speeds, which is why I was wondering if this is what I was feeling.

 

Another suggestion is that it's the soft sidewalls vs stiff sidewalls that I'm noticing the feel difference in.

 

Unfortunately this being the only car I've had it makes it tough for me to compare these feels -- if anyone has any suggestions on how to identify the dynamics differences of tire softness vs spring softness vs alignment problem vs aerodynamics, I'd love to hear it.

[Mr.EMan]

Now, my problem is I don't know that it even is lift, but assuming it is -- does the 05-07 2.5GT have a similar problem at these speeds?

 

What are you defining as "lift". What response are you feeling on the car? How fast are you driving?

 

Most typical production cars have lift. I've never noticed anything under triple digits in most cars I've driven, including rentals. I would look to suspension issues first rather than lift.

 

I have an LGT wagon, and other than crosswind, I haven't noticed any issues at 125+ mph.

[ShadowImg]

What are you defining as "lift". What response are you feeling on the car? How fast are you driving?

 

Most typical production cars have lift. I've never noticed anything under triple digits in most cars I've driven, including rentals. I would look to suspension issues first rather than lift.

 

I have an LGT wagon, and other than crosswind, I haven't noticed any issues at 125+ mph.

Around 70mph on the freeway, road feel/response seems to drift off in steering. The tramlining and bump response both fade out as speed increases, and general feeling from steering correction/turning is the front tires aren't planted firmly against the ground any longer (like the front end is generating too much lift).

 

Crosswind also gets significantly worse on the front, but it doesn't seem like the same effects are felt in the rear of the car.

 

It sounds similar to what is described around Whiteline's ALK (as pointed to by forestir above) -- though I'm a bit confused since the ALK actually increases wheel deflection, though I suppose if the chassis is lifting adding deflection to keep the wheels down would be a good thing.

 

Suspension is a complicated beast.

[Mr.EMan]

Have you had the alignment checked?

[ShadowImg]

Thanks folks, I've heard alignment from the mwsoc folks as well, and that was actually my original thought (I've had two alignments done since then, one by the shop that did my wheels/tires, and then one by the dealership shop).

 

Ayup.

[Mr.EMan]

Ayup.

 

I missed that bit, and about the speed, too. So much for reading in my advancing age.

 

When I thought I had lift, it was pretty disconcerting. It was on the autobahn in a rental, doing 100+ mph. The steering felt light, and the car's directional stability made me queasy.

 

At 70 mph, I wouldn't expect lift in most cars, unless there's something wrong with it, like the spring in the back are sagging.

 

You mention windy conditions. If it's crosswind, then lift isn't your problem. If it's against your direction of travel, then it could be, since 70 mph into 20 mph wind is like 90 mph.

 

My wagon is sensitive to crosswinds, as it has a large surface area from the side.

 

One last thing-- how old (time-wise) are the tires, and what inflation are you running?