Mercedes SLK World banner

Status
Not open for further replies.
1 - 20 of 26 Posts

·
Registered
Joined
·
20 Posts
Discussion Starter #1
I moved from a Subaru STI to the SLK55, and while the SLK is a hair faster/quicker, I find that it's less stable at speeds above about 130mph. The car feels like it's lifting up a bit, and the suspension isn't as sure on its feet as I'm accustomed to. Almost like the suspension is a little too loose.

Anyone who's seen an STI knows that they have an enormous wing on the back of the car, dive planes up front, a diffuser under the tail, and a front splitter. Most of this (all of it?) is missing from my SLK.

What can I do to make the car more stable at speed? Obviously, putting a spoiler on the car isn't going to happen (unless there's a piece to replace the little lip spoiler it has that does something). Is there a diffuser or underbody paneling available? How about a front splitter?

The other thing I have been thinking is that lowering the car an inch would probably make it more stable and give me the stiffer spring rates the car seems to need.

I'm not meaning to complain about the SLK. I love the car. I tracked the STI and I track the SLK -- I just bought the SLK because it was more comfortable (I broke my back in a motorcycle accident some years ago, and I couldn't take the seating position of the STI anymore).

It's very frustrating to have such a capable car start to feel "floaty" and move around a bit when you get up past 150.

thanks in advance... I did look through the forum and didn't see anything relevant; most people are interested in making the car faster. I think it's plenty fast, actually, I'm just looking to make it safer.
 

·
aka John
Joined
·
13,012 Posts
Welcome to the forum from London - I suspect part of the problem is that most of us don't have the opportunity to regularly drive 130+. I've driven mine in Germany - although I don't think I did much more than 140 - and was amazed just how stable it was based on other cars at the same speed - specifically an M3.

I suspect that you are right, and that you will achieve more with an inch of lowering that with any other mod. I know I've seen pictures of an SLK with a massive rear wing, but can't find 'em at the moment.

Right now you've got about 80% underbody paneling, and I'm not sure you could add more without impacting cooling.
 

·
Sadly Woolly has passed away
Joined
·
17,375 Posts
Welcome from NE England. I found my 350 remarkably planted (albeit in a straight line) at 155+ in Germany.

Here is a 55 lowered, with splitter and rear wing.

 

·
Administrator 2009 SLK 55 AMG/Founding Member 2006
Joined
·
98,153 Posts
wrxless,

Welcome to the World’s Premier Mercedes-Benz SLK Community from Valrico, Florida!


Please update your location/vehicle (model and year) in your profile as it helps the members answer your questions which usually are based on that information.
‘Usercp’ at the top left of page, then ‘edit your details’ on the left side, ‘additional information’ near the bottom, edit your ‘location’ and ‘vehicle’.

You might want to post an introduction in the ‘New members introduce yourself here’ section, if you haven’t already.






If you haven’t, please use the 'search' feature at the top of the page for your questions as they have probably been asked before. Remember, ‘Search’ is your friend.
In addition, if you haven’t, please post your question in the appropriate discussion forum for a quicker response. Be patient and a knowledgeable member or guest will address your question.

If you wish to PM (Private Message) someone, you need 15 posts. To get those 15 posts quickly, just go to the ‘New members introduce yourself here’ section and welcome enough new members to obtain your needed posts to PM.



Thank you,
Jeff
Founding Member/Moderator
 

·
Registered
Joined
·
20 Posts
Discussion Starter #7
Oh, one last thing -- the owner's manual says the 55 is limited to 174, electronically. I've never had the opportunity to get it up to that speed. I might, at the VIR event coming up, but 160 is plenty hairy. When I bought the SLK I had sat in the SL63 and found that I didn't like it as much. But what it did have was that electronic suspension lowering. I wonder how stable the bigger cars are at those speeds. Certainly there are SL65 black's out there that are fully capable of these speeds; somebody has to have done something to make them more stable...
 

·
aka John
Joined
·
13,012 Posts
Welcome from NE England. I found my 350 remarkably planted (albeit in a straight line) at 155+ in Germany.

Here is a 55 lowered, with splitter and rear wing.


Thanks Woolly - that's the one I was looking for! Should have known it was a Carlsson kit.
 

·
Premium Member
Joined
·
2,454 Posts
I moved from a Subaru STI to the SLK55, and while the SLK is a hair faster/quicker, I find that it's less stable at speeds above about 130mph. The car feels like it's lifting up a bit, and the suspension isn't as sure on its feet as I'm accustomed to. Almost like the suspension is a little too loose.

Anyone who's seen an STI knows that they have an enormous wing on the back of the car, dive planes up front, a diffuser under the tail, and a front splitter. Most of this (all of it?) is missing from my SLK.

What can I do to make the car more stable at speed? Obviously, putting a spoiler on the car isn't going to happen (unless there's a piece to replace the little lip spoiler it has that does something). Is there a diffuser or underbody paneling available? How about a front splitter?

The other thing I have been thinking is that lowering the car an inch would probably make it more stable and give me the stiffer spring rates the car seems to need.

I'm not meaning to complain about the SLK. I love the car. I tracked the STI and I track the SLK -- I just bought the SLK because it was more comfortable (I broke my back in a motorcycle accident some years ago, and I couldn't take the seating position of the STI anymore).

It's very frustrating to have such a capable car start to feel "floaty" and move around a bit when you get up past 150.

thanks in advance... I did look through the forum and didn't see anything relevant; most people are interested in making the car faster. I think it's plenty fast, actually, I'm just looking to make it safer.
What's up, wrxless-I used to work in Arlington, VA. I have an idea with what you mean regarding high-speed stability, even though I have a SLK350 and aftermarket lowering springs. I'd suggest looking suspension work first like KW coilovers and a rear camber kit with alignment. Maybe it's something as simple as upgrading to better tires. I would stay away from the body work unless you're interested in modifying the exterior anyway. You could get a universal custom front wind splitter from APR Performance. The Chinese diffusers that are available are probably more form than function. I'm not sure about the Piecha or Expression Motorsport diffusers-I doubt that they've been properly tested for better aerodynamics. At least the AMG rear trunk spoiler was designed to be functional.
 

·
aka John
Joined
·
13,012 Posts
Oh, one last thing -- the owner's manual says the 55 is limited to 174, electronically. I've never had the opportunity to get it up to that speed. I might, at the VIR event coming up, but 160 is plenty hairy. When I bought the SLK I had sat in the SL63 and found that I didn't like it as much. But what it did have was that electronic suspension lowering. I wonder how stable the bigger cars are at those speeds. Certainly there are SL65 black's out there that are fully capable of these speeds; somebody has to have done something to make them more stable...

Depends on the SLK, some are limited at 155, the ones with the 030 package came from the factory at 174, and some are not set at all.

I've never been over 150 in an SL, but at 150 it honestly didn't seem that much different from the SLK - other than the fact I discovered I didn't like being in a car at 150 without being behind the wheel.

SL is a bigger, heavier car, so I guess it should a least feel more stable - it's what 4300lbs? compared to 3300 for the SLK?

I'm not sure really what you are trying to accomplish, though - I've also been in a E500 at 150+, and it was a heck of a lot more comfortable than either the SL or the SLK - in a straight line - same with an M5. At those speeds there was obviously not a lot of maneuvering being done.
 

·
Registered
Joined
·
20 Posts
Discussion Starter #11
Depends on the SLK, some are limited at 155, the ones with the 030 package came from the factory at 174, and some are not set at all.
Right, I've got an 030.

SL is a bigger, heavier car, so I guess it should a least feel more stable - it's what 4300lbs? compared to 3300 for the SLK?
The only thing I'm getting at is the fundamental shape of the car may be that it lifts up a bit at speed. When you're moving through air that fast, the forces add up real quickly. I used to race old Nissan Z cars, and while you'd think they were kind of slippery-looking, they had real problems once you hit 110-120mph because they were lifting bodies. You had to put huge whale-tail spoilers on them and skirts to keep them on the ground.

I'm not sure really what you are trying to accomplish, though.
That's fair. Basically, I'd like to feel more confident that I can approach and maintain that kind of speed on the track. As it is, I'm a hundred percent confident in the car in hard turns (although the traction control kind of pisses me off) and hard braking. When I can keep it planted, it performs admirably. But when I'm cresting a hill and pushing 150 (such as at Summit Point), I really don't want to feel the car moving around much. I had a friend go into the wall there when his car got airborne. He was probably carrying 120mph over the hill.

I think the suggestions of a little more camber in the back and lowering are good ones. Stiffer springs are probably on order, but I'm walking a fine line between having a more comfortable car and having something that's welded to the ground when pushed hard. I really think that rear wing that Carlsson had would work very well -- I just have to wonder whether it's permanently affixed or if they have some fitment where I can take it off when I need to take the car to work or something. I hated the wing on the back of the STI -- when I wasn't actively using it! :)

Thanks everyone.
 

·
aka John
Joined
·
13,012 Posts
Right, I've got an 030.



The only thing I'm getting at is the fundamental shape of the car may be that it lifts up a bit at speed. When you're moving through air that fast, the forces add up real quickly. I used to race old Nissan Z cars, and while you'd think they were kind of slippery-looking, they had real problems once you hit 110-120mph because they were lifting bodies. You had to put huge whale-tail spoilers on them and skirts to keep them on the ground.



That's fair. Basically, I'd like to feel more confident that I can approach and maintain that kind of speed on the track. As it is, I'm a hundred percent confident in the car in hard turns (although the traction control kind of pisses me off) and hard braking. When I can keep it planted, it performs admirably. But when I'm cresting a hill and pushing 150 (such as at Summit Point), I really don't want to feel the car moving around much. I had a friend go into the wall there when his car got airborne. He was probably carrying 120mph over the hill.

I think the suggestions of a little more camber in the back and lowering are good ones. Stiffer springs are probably on order, but I'm walking a fine line between having a more comfortable car and having something that's welded to the ground when pushed hard. I really think that rear wing that Carlsson had would work very well -- I just have to wonder whether it's permanently affixed or if they have some fitment where I can take it off when I need to take the car to work or something. I hated the wing on the back of the STI -- when I wasn't actively using it! :)

Thanks everyone.
So one thing to keep in mind, if you have the 030 package, you also have the lower and stiffer suspension already fitted to the car. Arno (PropDr) has probably got more experience with what you can do with regard to tires and lowering than anyone on the forum - and also has the 030 package.

The Carlsson wing looks like it bolts on the rear boot face where the license plate goes, not sure how easy it would be to put on or take off - I've only seen it in pictures.

I've also noticed that the SLK Safety cars and BlackSeries all went with 19's wheels. I've got them on my SLK stateside, and it's a fine line towards "not fun on normal Louisiana roads" - and that's with stock, non-030 suspension.
 

·
Premium Member
Joined
·
3,248 Posts
Woolly is correct, the Quaife will be the biggest improvement you can make without adding aero components.
I also replaced the trailing and camber arms in the rear to get adjustability and less flex.
For high speed stability go to max toe-in front and rear, also make sure that caster and camber are the same for left and right on both ends.
One other item: to me the car feels more stable with the top down (less lift in the back?)

Been thinking about it!
Top Down = Better front to rear weight distribution and lower center of gravity.
 

·
Premium Member
Joined
·
2,454 Posts
I think the suggestions of a little more camber in the back and lowering are good ones. Stiffer springs are probably on order, but I'm walking a fine line between having a more comfortable car and having something that's welded to the ground when pushed hard. I really think that rear wing that Carlsson had would work very well -- I just have to wonder whether it's permanently affixed or if they have some fitment where I can take it off when I need to take the car to work or something. I hated the wing on the back of the STI -- when I wasn't actively using it! :)

Thanks everyone.
What tires do you currently have the on SLK55? You're not going to find the Carlsson wing anywhere-it looks like it was made for Euro plate specs. Even if you did, I'm sure it would cost an arm and a leg. I guess you could look into having a custom one made but I'm sure that option can get a little expensive as well. If you want a front strut bar, check out Wiechers Sport.


 

·
Registered
Joined
·
2,656 Posts
Woolly is correct, the Quaife will be the biggest improvement you can make without adding aero components.

Absolutely, makes a big difference in the driving feel of these cars!:burnout:
 

·
Registered
Joined
·
20 Posts
Discussion Starter #18
The Carlsson wing looks like it bolts on the rear boot face where the license plate goes, not sure how easy it would be to put on or take off - I've only seen it in pictures.
My German's not so hot, but Google did its best -- it doesn't look like Carlsson is selling that wing.

I've also noticed that the SLK Safety cars and BlackSeries all went with 19's wheels. I've got them on my SLK stateside, and it's a fine line towards "not fun on normal Louisiana roads" - and that's with stock, non-030 suspension.
The only benefit I see from the 19" wheels is I can get the Sport Cup tires on them. Seems they're missing the size I need for the fronts on my 18's. I'm hoping they correct that by the spring when I'm due for new ones.
 

·
Registered
Joined
·
20 Posts
Discussion Starter #19
One other item: to me the car feels more stable with the top down (less lift in the back?) [/FONT][/SIZE]

Been thinking about it!
Top Down = Better front to rear weight distribution and lower center of gravity.
You know, I couldn't get the car past 157mph with the top down. I think there is higher drag. Now that I think about the two (I'd have to compare the two, and driving that fast with the top down freaks me out), I think it felt more planted with the top down, but I think it's slipperier with the top up -- meaning it should reach that limited 174. At this point, I don't really want to go any faster than 160, top down or not.

You do realize that with the top down at speeds like that, even with the appropriate harnesses and helmets and stuff, we're not talking a survivable accident, right?

When I was younger, racing on the street was just something we did. As I grew up and got better, newer, more refined cars, I grew out of that phase. My next event is 11/12 at VIR. I'll see if I can get the alignment done by then. I've still got some room on my tires; I plan to be on Sport Cup's in the spring (or just Hankook slicks).
 

·
Registered
Joined
·
20 Posts
Discussion Starter #20
Absolutely, makes a big difference in the driving feel of these cars!:burnout:
I fully understand what an LSD does, and how the Quaiffe unit works (cf torsen and friends). But the rear end doesn't feel skittish at speed. The car feels "jumpy" and "lifty" and maybe a little like it's hunting -- the front doesn't seem to track straight (toe-in and caster will help here).

I've read a lot of people sayig the Quaiffe is a must-have performance upgrade, but I'm not sure how it helps in this circumstance. Can you help me understand?
 
1 - 20 of 26 Posts
Status
Not open for further replies.
Top