Loose Suspension when Accelerating

[ATL_Canes]

I was able to find some threads on this subject from back in 2013, but couldn’t find anything more recent. Over the past two days, I’ve realized the car is doing something odd. When accelerating on the highway, the car moves slightly to the left and then slightly to the right. It’s a similar sensation to driving on a really wind day where you feel Iike your car is being pushed slightly to the side. It feels like it’s the rear of the car that is “loose.” Tire pressure is fine. My car is a 2016 S90D (pre-facelift) with SAS and PUP and is running 2017.42 a88c8d5.

I’ll call the service center, but was just curious to see if anyone has dealt with something similar recently. Thanks in advance for any input/experiences.

 

[The Duke]

Is this under HARD acceleration? Son following on his motorcycle has seen Nicki light up all four tires at 45 mph going up an on ramp. Not for long, but enough to make me think something is amiss. Traction control is very good, but not perfect so you get some slip now and again. If one front wheel slips a little it will pull you off to the side.

 

[ATL_Canes]

It’s not only when accelerating hard - it’s really any acceleration.

 

[AmpedRealtor]

If it's what I think it is, this is normal ride quality for a Model S. I've brought this up with service a number of times with my P85 and after multiple detailed inspections of my suspension, there is "nothing wrong". Basically the car feels like it's being hit by a gust of wind and feels like it dives a little bit to the left, then the right. Googling the issue brings up links to something called "camber thrust", which is something cars with steep rear camber experience. Basically, with heavy camber, the right rear wheel pushes to the left and the left rear wheel pushes to the right. This can lead to a "wobbly" sensation of the car feeling like it's constantly being pushed from left to right. This makes it challenging to keep it in lane on a windy day, for instance, or makes you want to over correct for what feels like the car drifting when it isn't. Tramlining is a bitch, too, as the vehicle will follow even the tiniest groove in the road even when the steering wheel is aimed straight. At the end of the day, it feels like a wobbly rear end.

Totally normal per Tesla. I've had my rear half shafts replaced, my drive unit replaced, my rims inspected for cracks, all suspension components thoroughly inspected (my bushings are "like new", they said), and had all four air struts replaced.

 

[mongo]

I was able to find some threads on this subject from back in 2013, but couldn’t find anything more recent. Over the past two days, I’ve realized the car is doing something odd. When accelerating on the highway, the car moves slightly to the left and then slightly to the right. It’s a similar sensation to driving on a really wind day where you feel Iike your car is being pushed slightly to the side. It feels like it’s the rear of the car that is “loose.” Tire pressure is fine. My car is a 2016 S90D (pre-facelift) with SAS and PUP and is running 2017.42 a88c8d5.

I’ll call the service center, but was just curious to see if anyone has dealt with something similar recently. Thanks in advance for any input/experiences.

Do you mean left then right while accelerating, or left while accelerating and right while decelerating/ returning to cruise?
If the direction changes based on acceleration type, then it may be a loose suspension component that is shifting based on wheel force. Steering tie rods, or really anything that controls front/back movement of the wheels could be the cause.

 

[ATL_Canes]

If it's what I think it is, this is normal ride quality for a Model S. I've brought this up with service a number of times with my P85 and after multiple detailed inspections of my suspension, there is "nothing wrong". Basically the car feels like it's being hit by a gust of wind and feels like it dives a little bit to the left, then the right. Googling the issue brings up links to something called "camber thrust", which is something cars with steep rear camber experience. Basically, with heavy camber, the right rear wheel pushes to the left and the left rear wheel pushes to the right. This can lead to a "wobbly" sensation of the car feeling like it's constantly being pushed from left to right. This makes it challenging to keep it in lane on a windy day, for instance, or makes you want to over correct for what feels like the car drifting when it isn't. Tramlining is a bitch, too, as the vehicle will follow even the tiniest groove in the road even when the steering wheel is aimed straight. At the end of the day, it feels like a wobbly rear end.

Totally normal per Tesla. I've had my rear half shafts replaced, my drive unit replaced, my rims inspected for cracks, all suspension components thoroughly inspected (my bushings are "like new", they said), and had all four air struts replaced.

Thanks for sharing - that's interesting. I've had the car since June and have never noticed this sensation at all until this week. Nothing notable happened this week that would cause a change - no fender benders, no curbs, no software updates, etc. I have an appointment tomorrow morning so we'll see what the SC says.

 

[ATL_Canes]

Do you mean left then right while accelerating, or left while accelerating and right while decelerating/ returning to cruise?
If the direction changes based on acceleration type, then it may be a loose suspension component that is shifting based on wheel force. Steering tie rods, or really anything that controls front/back movement of the wheels could be the cause.

I haven't yet been able to figure out a consistent pattern. To me it feels inconsistent - sometimes pulls left, sometimes pulls right on acceleration. But I'll pay closer attention to it on my drive home today to see if the pull to the right is actually upon me letting up on the accelerator. We'll see what the SC has to say tomorrow as well.

 

[The Duke]

Have you tested at different ride heights? When leaving home (set to high for curbs) our on ramp is 1/4 mile away. If I go slow the car is still on high suspension and that first turn does feel loose.
Once Nicky gets down she is as stable as well as any BMW or AMG I have driven. Better than most in winds.
My alignment was verified by another shop at low suspension setting.

 

[Joshib]

I had a similar issue in my 1999 Mercedes ML 430. It was diagnosed to be rear tie rod / lateral arm bushing where it connected to the rear wheel. The service shop replaced the arm as you couldn’t get the bushing on its own and it stopped dancing on the road as you describe. Was able to move the wheel back and forth when the rear was jacked up.

 

[evp]

I had a similar issue in my 1999 Mercedes ML 430. It was diagnosed to be rear tie rod / lateral arm bushing where it connected to the rear wheel. The service shop replaced the arm as you couldn’t get the bushing on its own and it stopped dancing on the road as you describe. Was able to move the wheel back and forth when the rear was jacked up.

I had this same thing happen on my S85 -- some pulling to the side under acceleration, some rattling. They replaced a tie-rod, showed me the bushing that had been cracked and loose.

 

[AmpedRealtor]

Have you tested at different ride heights? When leaving home (set to high for curbs) our on ramp is 1/4 mile away. If I go slow the car is still on high suspension and that first turn does feel loose.
Once Nicky gets down she is as stable as well as any BMW or AMG I have driven. Better than most in winds.
My alignment was verified by another shop at low suspension setting.

The higher the suspension, the stiffer it is. Not looser.

 

[mongo]

The higher the suspension, the stiffer it is. Not looser.

How's that? The vehicle weight is the same, so the pressure is the same and the difference between heights is just the volume of air in the shock. Higher suspension is more air at the same pressure, that means the same amount of suspension travel produces less of a change in pressure. Conversely, the same cornering force will require a larger displacement to counter. So higher would be looser.

Edit: higher also raises the center of gravity which increases body roll.

 

[The Duke]

I think he means the spring rate is effectivly stiffer when the suspension is set to high. While I meant the handling is looser due to alignment changes when the car is at high suspension setting. I believe there was mention in another thread some SC did not set ride height correctly when aligning.

Sears checked mine at low ride height with me in the car and verified alignment was correct.

 

[mongo]

I think he means the spring rate is effectivly stiffer when the suspension is set to high. While I meant the handling is looser due to alignment changes when the car is at high suspension setting. I believe there was mention in another thread some SC did not set ride height correctly when aligning.

Sears checked mine at low ride height with me in the car and verified alignment was correct.

I could be wrong. I looked around for a curawat view, but came up short. Do you know of an internal diagram for the SAS unit? It seems like it would need dual air chambers or a counter spring to achieve a stiffer response at a higher setting. Either that or adjustable shock damping based on height.
Still seems like it would be stiffer (higher spring rate) at lower heights due to less available travel.

 

[The Duke]

Air suspension usually has "stiffer" springs at higher load settings or in our case higher ride height but not always.
For further study:
Smart Air Suspension- Dampening

 

[mongo]

Air suspension usually has "stiffer" springs at higher load settings or in our case higher ride height but not always.
For further study:
Smart Air Suspension- Dampening

Thanks for the link.
It seems like the thread said:
For the same height, air suspension is stiffer at higher loads which keeps the frequency the same.
At very high settings, the suspension could hit the stops (guessing it must be on rebound?).
But I don't see anything that indicates a higher setting at the same load is stiffer, not that it really matters, just causes me cognitive dissonance with my current knowledge base.

It could be that the shock has a progressive rate so it is stiffer at the ends vs middle. So higher could be stiffer due the shock vs the spring. That seems logical .

Stiffer as measured by ride quality vs stiffer based on displacement per unit load?

 

[Xenoilphobe]

check the torque on your lug nuts.