[F32x15]

I wanted to ask you guys for suggestion. The car (F32 420d, 190hp) feels wobbly/floaty (swaying from side to side, like I'm driving in windy conditions) at high speed (110mph and above, in sport mode), feels as its not really planted, wheel alignment is done, tyre pressure is around 38psi (2.6 bar) all around, new tyres Michelin PS4. Any suggestions? thinking of dropping the pressure to 35psi? Anyways I have the sport suspension.

I know those are high speeds (done on a safe road, not putting anyone else in danger), but that car should do 150mph on autobahn all day long.

[Wires]

How many miles on the car, outside temp when this happens?

[F32x15]

Originally Posted by Wires

How many miles on the car, outside temp when this happens?

110K miles, outside temp 59F/15*C

Don't believe temperature could influence, I've driven much less aerodynamic cars at those speed at different temperatures (winter/summer) and they felt more stable than the F32, which doesn't make any sense, because that should not be the case.

[HighlandPete]

Originally Posted by F32x15

110K miles, outside temp 59F/15*C

Don't believe temperature could influence, I've driven much less aerodynamic cars at those speed at different temperatures (winter/summer) and they felt more stable than the F32, which doesn't make any sense, because that should not be the case.

Could be some wear in the suspension bushes and dampers. Depends on what sort of life the car has had through the mileage, and what may have already been changed, if anything.

Certainly experiment with pressures.

[Orion4]

With 110K miles, it sounds like it might be suspension/bushings related. Shocks may be losing their dampening effectiveness.

[F32x15]

Originally Posted by Orion4

With 110K miles, it sounds like it might be suspension/bushings related. Shocks may be losing their dampening effectiveness.

Milage is a bit high, but even a less effective shock shouldn't have those results in my opinion. Car felt as there was lots of wind pushing it left and right (no fast and furious kind of scenes) but making the car unsettle and feel as it's not planted on the road.

[Wires]

On my E90 (with 55k miles), the rear suspension needed work, making it was BRUTAL on slippery roads, even with new winter tires doing 30 MPH. It did exactly what you are describing. Almost sideways drifts for no reason.

I'm betting your bushings are tired, and then need a replacement.

[B58 _gran_coupe_]

Does the car still have the engine undertray in place? These can be left off after work and cause crazy front end lift vagueness at higher speeds

[COKen]

You may want to jack up each wheel and check it for looseness both side to side and top to bottom.

[dmanb2b]

38psi is rather high for go flat mp4s. Anytime I run them that high I get premature center wear. Try targeting 32psi front and 34 rear and see how it feels. MP4s do have a tendency to give more instant feedback as well, so just make sure toe on alignment is relatively close to zero.

[F32x15]

Originally Posted by dmanb2b

38psi is rather high for go flat mp4s. Anytime I run them that high I get premature center wear. Try targeting 32psi front and 34 rear and see how it feels. MP4s do have a tendency to give more instant feedback as well, so just make sure toe on alignment is relatively close to zero.

Yup, just came back home from a 100 mile trip and wanted to update you guys. I lowered the psi this morning, did a 140mph run and problem seems to be solved, hopefully stays that way, so if anyone has a similar problem try the same before jumping to suspension costs, cause it might get expensive without solution.
I lowered to 2.2 bar (32psi) front and 2.4 (34.8psi) back as suggested on the door sticker on my 420d, did a 140mph run, felt much better and stable. I'm going to leave it as it is for now, might increase by 0.1 bar for all around (32psi front seems to be a bit low, ps4 sides feel pretty soft -91W- and I wouldn't want premature wear but hey if it works, not going to change it).

My vulcaniser set all four to 2.7 bar (39psi), don't actually know why, as it seemed high in my opinion, but I was not sure so I left it as I believed he might know better.

TYRES setup in case anyone is interested 225/45/18 square, Michelin PS4 225/45/18 91W
ALLOYS Double Spoke 397 18* square

Going to leave an update if it happens again, but I don't believe it should.

[thejeremyman9]

This is interesting, because my door sticker specifies a higher PSI for both front and rear if you are going to be traveling at 100+ mph. But i also run about 32/35 F/R on PS4S cold psi. I forget the 100+mph pressure specs, but im pretty sure rears is over 40psi.

Also a different car through (2015 335i), so different aero etc, so not apples to apples.

[Wires]

Originally Posted by thejeremyman9

This is interesting, because my door sticker specifies a higher PSI for both front and rear if you are going to be traveling at 100+ mph. But i also run about 32/35 F/R on PS4S cold psi. I forget the 100+mph pressure specs, but im pretty sure rears is over 40psi.

Also a different car through (2015 335i), so different aero etc, so not apples to apples.

I know the faster they spin, then the centers start to pull away from the rim to to centrifugal force. Higher pressure prevents that from happening.

Door pressure would be for whatever the car was delivered with though. If it's not the same BMW tires, then who knows.

[HighlandPete]

Originally Posted by Wires

Door pressure would be for whatever the car was delivered with though. If it's not the same BMW tires, then who knows.

Also remember on a vehicle with over 100k miles, tire pressures can be more sensitive, as tires may become a more noticeable suspension component. The 'softer' tire may be reducing some of the bounce, due to absorbing more energy, and less going into the worn suspension system.

Normally on a tight suspension, a soft tire at high speed would show as a negative, more pressure improving the handling precision.

[F32x15]

Originally Posted by HighlandPete

Also remember on a vehicle with over 100k miles, tire pressures can be more sensitive, as tires may become a more noticeable suspension component. The 'softer' tire may be reducing some of the bounce, due to absorbing more energy, and less going into the worn suspension system.

Normally on a tight suspension, a soft tire at high speed would show as a negative, more pressure improving the handling precision.

Sorry but I can't agree, its like saying the more pressure your tire has, the better a car will drive which is not true. Overinflation is as bad as under inflation, the tyre will not perform well. 32-35 psi might be a bit low, but 40 is too much in my opinion.
Yes, 100K is not low, but I've driven cars with 200K with original suspension and drove better then some 50K miles cars, all depends of how the car was driven.

[HighlandPete]

Originally Posted by F32x15

Sorry but I can't agree, its like saying the more pressure your tire has, the better a car will drive which is not true. Overinflation is as bad as under inflation, the tyre will not perform well. 32-35 psi might be a bit low, but 40 is too much in my opinion.
Yes, 100K is not low, but I've driven cars with 200K with original suspension and drove better then some 50K miles cars, all depends of how the car was driven.

You may not agree, I can understand that, but with years of experience with BMW (40+) and an engineering background, where I spent a lot of years developing damping systems, I don't write without knowing a bit about the subject.

This thread along with your other thread on the Nexen's, do give clues that the suspension may well be involved. Agree on mileage, do you know the history, whether the car had an easy life or not? Dampers may well be past their best, particularly if running RFTs for a good part of that mileage.

Starting with basics, tires may be viewed as the number-one ride control component. Tire size, construction, compound and inflation are very important to the ride. Keep in mind the air pressure determines the spring rate of the tire.

Now add the vehicle's spring rate and run a soft or worn damper and we can get bounce and float, change the damper or tighten down (if adjustable) and the chassis motion is immediately reduced.

Higher tire pressures (giving a higher tire spring rate), along with the vehicle spring can overwhelm worn dampers. Resulting in more motion than the lower pressure, where more energy is absorbed by the tire, reducing the suspension's motion and work load.

You may be getting a good result, but may still be masking a suspension past its best.

BTW, 40psi is not high on the rear of a BMW, for high speed driving.

[thejeremyman9]

I just checked my door sticker and the OEM recommendation for 18s on a 2015 335i with msport is 32/38 F/R cold. It says only use those up to 100mph. Then the yellow sticker says 39/46 F/R cold for over 100mph. That's for 18s, my car actually came special order with 19s, and i generally run 32/35 cold. I did inflate higher once for top speed runs, but didnt notice a difference, and i hit a peak speed of 177mph.