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      07-01-2018, 08:52 AM   #1
HTM101
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Softening the Ride in Z4 Coupe

I've owned the car for 5 months and enjoy it.
2007 Z4 3.0si coupe
6-speed manual
factory Sport option
66k miles
tires are new Michelin Pilot SS
8.5" wheels front
9.5" wheels rear
alinement is fine

I've driven the car about 1,000 miles since purchase. Yesterday we drove 160 miles on mostly interstate roads and I've decided the very firm ride is something I'd like to change. I'm assuming I have the OE suspension, but haven't investigated for verification. I like the existing ride height and assume it is OE. I'm 65 y.o. and won't be tracking the car. I do the majority of my own work.

What are your suggestions for softening the ride?

Its not my intent to duplicate the ride of a 110" wheelbase luxury car. I'd like to eliminate some of the kidney-jarring we are experiencing.

Thanks,
David
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      07-28-2018, 08:28 AM   #2
Efthreeoh
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The OE dampers are black and will have a BMW Roundel stamped into the shock body. If the previous owner changed the suspension it would most likely be either Koni or Bilstein, which both shock bodies are yellow. The OE springs are black. Most aftermarket springs are powder coated in a color other than black.

As far as the suspension ride is concerned, BMW did offer the Z4 with a non-sports package suspension

31316785585/586 - Front non-sports strut P/N

31316785587/588 - Front M Sports strut P/N

33506777028 - Rear non-sports shock absorber

33506777029 - Rear M Sports shock absorber

Not sure about the Coupe springs part numbers.

Now, the Z4 Roadster was offered with a 2.5L I6 which was not sports oriented. I can't directly confirm it, but the struts and shocks should bolt into the Z4 Coupe suspension. Between the roadster and Coupe (non-M versions) I believe the suspension design is identical other than the bushing tuning. The front hub is the same part number for all the versions of the E85 Roadster and E86 Coupe.

The BMW part numbers for the struts and shocks are:

31316785987/988 - Front 2.5 Roadster strut P/N

33526785996 - Rear 2.5 Roadster shock absorber

But keep in mind, the Coupe is a far stiffer chassis than the Roadster, obviously because the Coupe has a steel roof. The Roadster was, in convertible-chassis terms, pretty stiff in its own right, so when the roof was added for the Coupe the chassis got very stiff. In general the stiffer the chassis the softer (i.e. more compliant) the suspension can be tuned because the chassis flexes much less, so swapping to Roadster shocks and struts may actually make the Coupe ride harsher. My advice is contact the folks at Turner Motorsports. Turner races BMWs and is well-informed on BMW suspensions. However, keep in mind most Coupe owners looking to improve the suspension are looking to stiffen it for track use. You may be asking a question that the industry sees no reason to support and may not offer parts that soften the suspension. I have the same car you have, I find the suspension is very compliant, better than my E90 sedan with either the BMW stock sports package or the recent Bilstein/H&R change I made to it. I think what you are considering as a stiff suspension is really the lack of chassis flex that the Coupe inherently has. Considering you have a Cobra clone you may already know all this and I apologize for reprising what you already may know.

Again, call Turner Motorsports and see what they recommend.

I have to ask, just because I've driven a friends Factory Five Cobra, you must tolerate your Cobra. The Z4 rides much better than the few kit cars I've ever been in.
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A manual transmission can be set to "comfort", "sport", and "track" modes simply by the technique and speed at which you shift it; it doesn't need "modes", modes are for manumatics that try to behave like a real 3-pedal manual transmission. If you can money-shift it, it's a manual transmission. "Yeah, but NO ONE puts an automatic trans shift knob on a manual transmission."
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      07-28-2018, 02:56 PM   #3
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18" or 19" on those wheels? 19s are going to be much firmer and jarring.

One option would be to replace the wheels with 17" lightweight wheels and of course new tires. Stay with the Michelins, or something else with softer sidewalls—that is, don't get a hard sidewall track optimized tire. The tire acts as a spring to some degree, and the additional rubber may do the trick. This is why, as you already have, moving from a runflat to regular tire improves the suspension compliance. Runflats are rock stiff.

If I'm running my Z on 19s vs. 18s the difference is quite noticeable in terms of harshness and vibration. I'd imagine the 17s would make an improvement.

Just one option of many. And as with the prior post apologies if you're already aware of all of the above.
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      07-28-2018, 05:35 PM   #4
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I certainly appreciate both replies. A heck of a lot of good info, Efthreeoh. I'll print and keep that on file.

After a few weeks of zero replies and before these 2 recent replies, I decided to do something easy, adjust tire pressure.

When I first installed the new tires and wheels (18") in March of this year I used the recommended pressures found on the driver door jam, 33F, 39R. So...I recently changed the pressure to 32 F & R. I actually like the ride now, so I believe I'll hold off on making any spring or shock changes. I'll measure tread wear periodically to determine uniform wear, or not.

What pressures do you guys run?

By the way, my Cobra is a firm ride as I set up the suspension for some road racing. 900 pound springs on the front, 800 pounds on the rear. Still, it's actually a rather compliant and reasonably comfortable ride. Over the many years I've had a lot of passengers remark that the car rode nicely.

I've never ridden in a Factory Five Cobra, and of the many replicas available the ride can vary greatly depending upon components used and individual car set-up. Some have IRS and quite a few have solid rear axles.

Before I reduced the air pressure in the Z4, the Cobra's ride was better than the Z.

David
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      07-29-2018, 02:59 AM   #5
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Quote:
Originally Posted by HTM101 View Post
I certainly appreciate both replies. A heck of a lot of good info, Efthreeoh. I'll print and keep that on file.

After a few weeks of zero replies and before these 2 recent replies, I decided to do something easy, adjust tire pressure.

When I first installed the new tires and wheels (18") in March of this year I used the recommended pressures found on the driver door jam, 33F, 39R. So...I recently changed the pressure to 32 F & R. I actually like the ride now, so I believe I'll hold off on making any spring or shock changes. I'll measure tread wear periodically to determine uniform wear, or not.

What pressures do you guys run?

By the way, my Cobra is a firm ride as I set up the suspension for some road racing. 900 pound springs on the front, 800 pounds on the rear. Still, it's actually a rather compliant and reasonably comfortable ride. Over the many years I've had a lot of passengers remark that the car rode nicely.

I've never ridden in a Factory Five Cobra, and of the many replicas available the ride can vary greatly depending upon components used and individual car set-up. Some have IRS and quite a few have solid rear axles.

Before I reduced the air pressure in the Z4, the Cobra's ride was better than the Z.

David
Great answer you came up with!

I run 31 front and 30 rear on my street set-up, it's a Z4M, and higher pressures aren't comfortable and the car doesn't handle to my liking. Recommended pressure in the M is different than the 3.0si, but I consider pressure a guide not a requirement.

I'm also an idiot, as I forgot that when having the car corner balanced. with new suspension (550/450 F/R linear springs, much firmer than stock) the shop set pressure to 35 psi. Even with the shocks set full soft the car was nearly undrivable. Harsh, skipping over bumps, spine jarring. I never expect anyone to monkey with my pressures, so I just thought it was the much stiffer springs and a mistake. But I routine pressure checks found the issue. Setting back to 30 psi the car is great! I should have mentioned this in my earlier post.

Glad you hit on a super effective and cheap answer! I love those!
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      07-29-2018, 10:19 AM   #6
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Quote:
Originally Posted by HTM101 View Post
I certainly appreciate both replies. A heck of a lot of good info, Efthreeoh. I'll print and keep that on file.

After a few weeks of zero replies and before these 2 recent replies, I decided to do something easy, adjust tire pressure.

When I first installed the new tires and wheels (18") in March of this year I used the recommended pressures found on the driver door jam, 33F, 39R. So...I recently changed the pressure to 32 F & R. I actually like the ride now, so I believe I'll hold off on making any spring or shock changes. I'll measure tread wear periodically to determine uniform wear, or not.

What pressures do you guys run?

By the way, my Cobra is a firm ride as I set up the suspension for some road racing. 900 pound springs on the front, 800 pounds on the rear. Still, it's actually a rather compliant and reasonably comfortable ride. Over the many years I've had a lot of passengers remark that the car rode nicely.

I've never ridden in a Factory Five Cobra, and of the many replicas available the ride can vary greatly depending upon components used and individual car set-up. Some have IRS and quite a few have solid rear axles.

Before I reduced the air pressure in the Z4, the Cobra's ride was better than the Z.

David
Good news then. I pretty much run the factory recommended tire pressures. I'm not sure why the rear is all the way up at 39 PSI. There's not much sidewall on the 18" to begin with and sidewall stiffness is much dependent upon tire brand and type (all-season, summer, etc.). I run high performance all-seasons on my car so they may be less stiff of a sidewall than what you may be running. I bought my car used at 23,000 miles and it looked like the tires were new (1st replacements). They were Pirelli Zero Run-flats, but I still found the ride not harsh and to my liking. My Z4 shares duty with my E90 as a DD, which is 175 miles roundtrip daily. I run pressures to get a balance of handling vs. wear longevity. Just as a reference point, I go though two sets of rear tires for every one set of fronts.

Glad you got it worked out.
__________________
A manual transmission can be set to "comfort", "sport", and "track" modes simply by the technique and speed at which you shift it; it doesn't need "modes", modes are for manumatics that try to behave like a real 3-pedal manual transmission. If you can money-shift it, it's a manual transmission. "Yeah, but NO ONE puts an automatic trans shift knob on a manual transmission."

Last edited by Efthreeoh; 07-29-2018 at 10:30 AM..
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