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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Technical Forums > Suspension | Brakes | Chassis > Suspension Composure



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      06-30-2015, 04:36 AM   #1
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Suspension Composure

Looking for some input on a handling condition.

First a little background. I have a 2008 E92 335i with 39,500 on the clock. At about 12,000 miles I installed the BMW PS version 2 and was quite happy with the upgrade - much better feedback, better damped and more responsive. At 36,000 miles I decided to take it up a few notches and installed the front M3 control arms and tension arms, replaced the BMW PS sway bar with the E93 M3 28mm front bar and installed the M3 strut tower brace. In the rear, I installed the M3 guide rods, M3 rear sub-frame bushings, Megan Racing toe arms and Dinan upper rear shock mounts. I was going to install the E93 335i stock 15mm rear sway bar but at the last minute opted out as I have no LSD and did not want to give up traction for a only slightly flatter corning car - maybe if I do install a LSD at some point I will revisit the rear sway bar. Big thanks to CalWaterBoy for his "EXCELLENT New Year - E93 Suspension DIY "and HP Autowerks for the great sub-frame tool rental program) - really made this project a bit easier.

At the time I completed this work I was still on my winter 17" wheels with Blizzak LM25 RFT's. Had the alignment done with front right and left camber (pins still in place) at -1.0 degree, total front toe of .08 degrees. In the rear, left and right camber was set at -1.8 degrees on both sides with a total toe of .18 degrees. The car has an over all thrust angle of 0.0 degrees.

Driving impressions after the upgrades were quite good. Car handled very sharply and the oscillation in the rear that could be encountered when passing over imperfections in the road mid corner were all but a thing of the past. NVH did increase some but I expected this with the RFT's installed and as such had a set of Michelin PSS tires ready to go on the car. Now, as I noted, when all of this work was done by me I was still on the winter RFT tires and . I finally got the PSS's mounted on my 18" BMW 189 wheels this past month and got them onto the car. I run them with 36 psi front and 39 psi rear. They are quiet, smooth and offer great traction. However, there seems to be a problem (and the problem could just be me) and that is that with the installation of the PSS go-flats tires the unsettled oscillation has returned to the rear of the car when driving over bumps in the road mid-corner at speed. Not nearly as bad as when the stock sub-frame bushings were installed but present none the less. I played with air pressure some and it does get better at slightly higher pressures (have not gone above 41 psi) but there is a trade off in traction the higher the pressure.

Is this expected behavior with the non-RFT tires - especially the Michelin PSS tires? Again, traction is great but "plantedness" has suffered some. I assume this is due to the loading and unloading of the suspension caused by the undulations in the road and the associated flex / deflection in the tires sidewall when the suspension is recompressing? Can this is be minimized further? Are my BMW PS shocks / struts now under damped for my suspension configuration Do I need different dampers for the BMW PS Yellow springs (estimated Front sprints: 195 lb/in
Rear springs: 505 lb/in) - especially in the rear - something with some adjustability? OR, have I just forgotten how much "feel" there is in go-flat tires and I am chasing an issue that pays diminishing returns here?

Thanks in advance for any comments / suggestions - they are much appreciated.

Last edited by Speed_Addict; 06-30-2015 at 04:53 AM.
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      06-30-2015, 03:16 PM   #2
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Originally Posted by Speed_Addict View Post
I finally got the PSS's mounted on my 18" BMW 189 wheels this past month and got them onto the car. I run them with 36 psi front and 39 psi rear. They are quiet, smooth and offer great traction. However, there seems to be a problem (and the problem could just be me) and that is that with the installation of the PSS go-flats tires the unsettled oscillation has returned to the rear of the car when driving over bumps in the road mid-corner at speed.

Try 34# front 36# rear
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      06-30-2015, 04:21 PM   #3
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Originally Posted by CALWATERBOY View Post
Try 34# front 36# rear
Will give that a try. Had not really considered lower as I was thinking that lower PSI settings would lead to more roll in the sidewall and less responsive steering up front as a result. But sometimes what we think and what is are two different things... so, let's see what happens
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      07-03-2015, 02:00 AM   #4
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Originally Posted by CALWATERBOY View Post
Try 34# front 36# rear
Thanks Cal - great advice for my setup. One word - sublime. May play with a half pound up or down to fine tune. But really much better than the direction I was going. Taking pressure down really took a bit of "spring" out of the ride and improved ride comfort and grip. Maybe lost a smidgen of steering quickness at lower speed but higher speed all is more predictable and planted - very neutral. I had been starting to second guess walking away from the run flats with this car - there is no more of that
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      07-04-2015, 12:53 AM   #5
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Which track did you test all this out with?
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      07-06-2015, 05:17 AM   #6
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Which track did you test all this out with?
This current setup for me is not track tested... yet. However, life's circumstances willing, I hope to get out this year a couple of times at least. I am in the Seattle area so we have Pacific Raceways and the Ridge Motorsports Park. I am a member of the BMW CCA NW chapter and we have some really great instructors up here I want take full advantage of more regularly.

If it is not obvious (which I know it is) by my infant level of knowledge regarding tire pressure changes and their resultant effects on the handling characteristics of my car, I am a newbie when it comes to high performance driving. I have only ever had track experience in my car (a couple of sessions) with the BMW PS and the stock RE50 RFT tires. It was a fine setup for someone even beyond my level of experience admittedly. I was never anywhere near 9/10's (or probably even 6/10's for that matter) on the track - the setup still had plenty to give. Yes, the stock sub frame bushings are crap and can/do promote an oscillation in the rear (especially when mid corner ripples are encountered under acceleration), but even at that the car was far from ever letting go. I was actually getting somewhat comfortable with the car moving like it did - I still did not like it but, it was not limiting yet for me. However because I did not care for it and I could do something about it, I changed the sub frame bushing out to get rid of it for the simple reason that I could and it annoyed me. The M3 Guide Rods were likely overkill at this point for me but I was in there doing the work so I just went ahead and did those too. I figured they and the toe arm upgrades also would not hurt as they were all designed for the RFT tires therefore were softer and would reduce road feel / responsiveness with the go-flats. Dinan USM's were installed to improve the feedback /road feel much like the rear sub frame bushings - they also offered a bit more suspension travel as an additional benefit. Up front, I changed the tension strut and control arms to gain a little camber and improve or at least maintain steering response as I was planning on moving away from the RFT tires and all of the original components again had the softer bushings in them to keep the RFT's tolerable. I was getting some fairly significant outside tire wear at the track in the couple of sessions I did and likely at my skill level / speed that is more a result of driving skill than anything else. I don't expect camber to fix this because camber is not the entire issue - that'd be me... However, I don't believe 1 degree of camber will hurt the learning experience or do much to mask bad driving (i.e., too high of an entry speed leading to under steer and the extra wear on the outside of the tires that habit can earn). I did the front sway bar to help maintain the bit of camber I have in the corners and it happens to also have the benefit of having the car feel a bit flatter in the corners on the street. Not a huge difference over the PS kit bar but still noticeable. I left the rear bar alone - no need to mess with that as doing so only kills traction out of tighter corners without a LSD based upon advice from board members and Harold at HP Autowerks - don't want that for the sake of a "flatter feel". M3 strut bar - well, that was likely not on the "needs" list. I am sure it helps and it better than stock but its not a very noticeable difference. May be a benefit later if I ever get good enough to need camber plates or adjustable dampers just because they don't cover the top of the strut.

I know I have not exactly gone about my suspension upgrades the recommended order (e.g., driver (me), alignment, tires, springs/shocks/coils and then sway bars, etc.). The driver has a long way to go with respect to the track. But with what I wanted to achieve which was a more track capable car (tons of growth potential for me) that would be as responsive on the street as it were with the RFT's and hopefully much, much better. I made the changes all in one lump as I had a limited time in which I could do the work as a DIY and unfortunately did not see time on my horizon to do it piecemeal and learn from each change as much as I would have like to. That also was back in Dec/Jan of this year when I was still on the winter RFT tires. The improvements on the street when complete were dramatic. I have a section of highway I travel that has increasing radius turn with a bunch of ripples in it similar to a part of turn 2 at the track and it unsettled the rear even at safe and sane spirited public road speeds as I accelerated off the apex. After the upgrade that was totally gone.

So, after running on the winter RFTs until this early June (which was painful with all of the M3 bits in place), I was surprised when some of that looseness in the rear felt like it was back with the installation of the PSS tires. Some of that "feeling" was likely a result of rolling on the RFT's for the past 7 years and forgetting what "real" tires felt like... Long story short, I figured increasing pressure would increase the rigidity of the tire and get rid of the condition. I had actually started with the low-end OE pressure recommendations in the door but they seemed to slow steering response so I went with the higher speed OE pressure settings and steering response improved. At this point I had not run the highway with the rippled pavement. When I did and I got the feeling of oscillation again in the rear I figured it was due to the non-RFT's softer sidewall and so I worked the pressure up half pound at a time but it was not improving. I would not have initially thought to have brought it down in pressure as Cal suggested. At those pressures (in the middle between OE low and high speed pressures) the responsiveness / turn-in was actually better than the higher pressure (assuming improved slip angle and contact patch improvement here?) and the rear settled down (assuming less "ballooning" of the tires and letting the suspension do its work better without being burdened by an over inflated tire?). Any which way it was very much improved. So, there you have it from the newbie in training.
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      07-07-2015, 12:17 AM   #7
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I'm just going to leave this out there...

It takes me pushing the car pretty damn hard on the track, plus data acquisition instrumentation to feel 2psi's change in tire pressure. Yes it makes a difference. But not at the speeds that you can possibly feel driving on the street. And they're not usually immediate because the effect only shows up once the tires are warm (I.e. Maybe at least half a lap in).

So whatever effect you're feeling? It's all in your head.
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      07-07-2015, 08:36 AM   #8
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Quote:
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So whatever effect you're feeling? It's all in your head.
Yeah but changing tire pressures is cheaper than drugs.
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      07-07-2015, 10:23 AM   #9
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Quote:
Originally Posted by The HACK View Post
I'm just going to leave this out there...

It takes me pushing the car pretty damn hard on the track, plus data acquisition instrumentation to feel 2psi's change in tire pressure. Yes it makes a difference. But not at the speeds that you can possibly feel driving on the street. And they're not usually immediate because the effect only shows up once the tires are warm (I.e. Maybe at least half a lap in).

So whatever effect you're feeling? It's all in your head.
I feel a big difference in 2 psi. I went from 30 (it was low), to 32(door jam specs) in front, and I felt a big difference in steering response, how planted the car felt, how stiff the car was riding, everything. It was the equivalent of turning the compression up or down a click.
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      07-07-2015, 10:46 AM   #10
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I appreciate CAL throwing out a simple thing to try. There was no "parts and labor" rabbit hole to go down.
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      07-07-2015, 04:10 PM   #11
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Would have loved to see some before and after lap times with just the m3 bits installed as I plan to go that route with my street car that is inevitably becoming more focused on track performance.

My next decision is m3 control arms or camber plates up front. I'll eventually do both most likely, my last car (E92 335i) had m3 control arms but I didn't feel a HUGE difference in steering response, etc. Although that car never made to a full road course.
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