Car steps to either side when going over cracks in the road

[rcorona]

Hey everyone,

I have a 2018 Stinger GT2 AWD with Eibach springs & sway bars, aftermarket wheels and tires (F 255/35/R19, R 275/35/R19) as well as Megan Racing camber & toe arms in the rear. Typically run 36psi up front and 34psi in the rear.

I've noticed some instability in my car recently, not while on perfect roads, but whenever I drive over an imperfection (cracks, bridge joints, small/large potholes, manhole covers, etc). When I drive over these imperfections the rear of my car will sort of 'step' to whichever side ran over the imperfection. If the imperfection is on the right side of the lane and I drive over it with the right side of my car the rear of the car will step out to the right and then return to driving straight. No sounds of tires screeching but it feels like a big ogre just kicked the rear of my car to either side.

As you can imagine this is unnerving. I just had the car aligned back in February 2023, perhaps something slipped. The car really just doesn't feel planted and its frustrating. I'm wondering if anyone's experienced this before and any input is appreciated!

Thank you

 

[JSolo]

How are your sway bars set? If set too stiff, you'll get a sort of kick back sensation going over imperfections.

 

[socalvn]

Almost sounds like your tire is tracking the groove

 

[rcorona]

How are your sway bars set? If set too stiff, you'll get a sort of kick back sensation going over imperfections.

Both front and rear set at their softest settings

 

[rcorona]

Almost sounds like your tire is tracking the groove

I’m thinking it may need an alignment like the toe is off or something

 

[StingertimeNC]

i'd start there. Guessing it was all behaving normally for a while then started acting up, so something may be off.

 

[Rodtheviking]

Mine does this, even after a good alignment.

 

[Volfy]

What are your wheel offset? Spacers?

I noticed your tire sizes are off front and rear by a good bit. May or may not be related to what you are describing, but for an AWD Stinger, that is not a good thing.

 

[rcorona]

Mine does this, even after a good alignment.

What are your wheel offset? Spacers?

I noticed your tire sizes are off front and rear by a good bit. May or may not be related to what you are describing, but for an AWD Stinger, that is not a good thing.

No spacers. Wheels are staggered, Front 19x8.5 +30 and Rear 19x9.5 +35. That thought had crossed my mind before, that I should go back to a square setup.

 

[Rodtheviking]

I would say it has more to do with the AWD system (rear biased) and and open diff than anything else.

 

[Volfy]

No spacers. Wheels are staggered, Front 19x8.5 +30 and Rear 19x9.5 +35. That thought had crossed my mind before, that I should go back to a square setup.

I suspect your problem stems from insufficient suspension compliance.

For the tires to stay in constant contact with the road surface - especially over rough roads - what you'd want is for the suspension at each corner to impose a downforce perfectly in synch with the road undulations. Refer to my post in this thread regarding suspension tuning: Bent Rim…stronger and lighter rims? I need advice But here is the key diagram that explains what are the suspension components involved:

Ideally, you'd want the unsprung mass Mus (wheel, tire, brake rotors/caliper, wheel hub, etc.) to weight next to nothing. Then the suspension spring and shock absorber would have a very easy job of reacting to even large road imperfections and keeping the wheel/tire pressed against the road surface - providing optimum traction. Obviously, there is no such thing as weightless wheel/tire/brakes/etc., but you can certainly imagine the lighter the whole assembly weighs, the better your suspension would be able to do the best job it can. Conversely, the heavier your wheel/tire is, the worse a given set of suspension spring and shocks would be able to do a good job keeping the tire glued to the road.

This is where going with heavy oversized wheels and super wide tire might not yield the best traction - especially over rough roads. Say your wheel/tire hit a bump on the road. It is going to react by deflecting upward to conform to that road bump. Once the wheel/tire gets moving, the more mass it has, the more it wants to stay moving up. When the road bump is gone, your spring has to fight the inertia of your heavy wheel/tire to force it back down. Your Eibach springs are about 15% stiffer than stock, so that helps to some degree. However, concurrently, the shock absorber will also try to slow down that upward deflection, by absorbing some of that energy. The heavier your wheel/tires are, the more energy the shocks will have to absorb. Unless you have changed your shocks, the OEM shocks are rather underdamped, even for dealing with OEM wheel/tires. Throw larger heavier wheel/tires on, and the shocks become even more underdamped and less able to absorb the impact energy and to control suspension movement.

All this means that over rough roads, your larger heavier wheel/tire will likely be less controlled and bounce up and down more, lessening contact with the road surface. When traction is compromised at one wheel, it will slip. Without an LSD, more torque will be sent there, worsening the slip... until either (1) traction control senses the slippage and activate the brakes in that corner to fight it, or (2) your wheel/tire finally comes back down to terra firma, bite into the tarmac, and regain traction... quite possibly causing the other side to loose traction. Either way, over a rough road, this slippage will happen constantly and might swap sides randomly, leading to the "wiggle" you feel.

Another problem is with the very low profile 35-seires tires. Sure they look great and make the car look like a race car. Except... race cars get to run on race tracks that are, for the most part, well-maintained and smooth, so they they work well. In fact, my next set of track tires on my dedicated track wheels are gonna be 255/35R18s. However, out on public roads, higher-profile tires with more sidewall will have more built-in compliance to keep the tire tread in contact over rough road surface. Narrower tires will also catch less bumps and deflect less.

So... this is why my very first suspension mod on my Stinger was to a set of 18x8.5 Enkei wheels that weigh right at 20 lbs each, which is some 10-15 lbs less than OEM wheels, depending which ones came with your Stinger/G70.

LIGHT makes RIGHT. And sometimes, less is more.

 

[D.J.]

No spacers. Wheels are staggered, Front 19x8.5 +30 and Rear 19x9.5 +35. That thought had crossed my mind before, that I should go back to a square setup.

did this issue begin when you installed the wheels?

 

[rcorona]

I suspect your problem stems from insufficient suspension compliance.

For the tires to stay in constant contact with the road surface - especially over rough roads - what you'd want is for the suspension at each corner to impose a downforce perfectly in synch with the road undulations. Refer to my post in this thread regarding suspension tuning: Bent Rim…stronger and lighter rims? I need advice But here is the key diagram that explains what are the suspension components involved:

Ideally, you'd want the unsprung mass Mus (wheel, tire, brake rotors/caliper, wheel hub, etc.) to weight next to nothing. Then the suspension spring and shock absorber would have a very easy job of reacting to even large road imperfections and keeping the wheel/tire pressed against the road surface - providing optimum traction. Obviously, there is no such thing as weightless wheel/tire/brakes/etc., but you can certainly imagine the lighter the whole assembly weighs, the better your suspension would be able to do the best job it can. Conversely, the heavier your wheel/tire is, the worse a given set of suspension spring and shocks would be able to do a good job keeping the tire glued to the road.

This is where going with heavy oversized wheels and super wide tire might not yield the best traction - especially over rough roads. Say your wheel/tire hit a bump on the road. It is going to react by deflecting upward to conform to that road bump. Once the wheel/tire gets moving, the more mass it has, the more it wants to stay moving up. When the road bump is gone, your spring has to fight the inertia of your heavy wheel/tire to force it back down. Your Eibach springs are about 15% stiffer than stock, so that helps to some degree. However, concurrently, the shock absorber will also try to slow down that upward deflection, by absorbing some of that energy. The heavier your wheel/tires are, the more energy the shocks will have to absorb. Unless you have changed your shocks, the OEM shocks are rather underdamped, even for dealing with OEM wheel/tires. Throw larger heavier wheel/tires on, and the shocks become even more underdamped and less able to absorb the impact energy and to control suspension movement.

All this means that over rough roads, your larger heavier wheel/tire will likely be less controlled and bounce up and down more, lessening contact with the road surface. When traction is compromised at one wheel, it will slip. Without an LSD (Limited Slip Differential), more torque will be sent there, worsening the slip... until either (1) traction control senses the slippage and activate the brakes in that corner to fight it, or (2) your wheel/tire finally comes back down to terra firma, bite into the tarmac, and regain traction... quite possibly causing the other side to loose traction. Either way, over a rough road, this slippage will happen constantly and might swap sides randomly, leading to the "wiggle" you feel.

Another problem is with the very low profile 35-seires tires. Sure they look great and make the car look like a race car. Except... race cars get to run on race tracks that are, for the most part, well-maintained and smooth, so they they work well. In fact, my next set of track tires on my dedicated track wheels are gonna be 255/35R18s. However, out on public roads, higher-profile tires with more sidewall will have more built-in compliance to keep the tire tread in contact over rough road surface. Narrower tires will also catch less bumps and deflect less.

So... this is why my very first suspension mod on my Stinger was to a set of 18x8.5 Enkei wheels that weigh right at 20 lbs each, which is some 10-15 lbs less than OEM wheels, depending which ones came with your Stinger/G70.

LIGHT makes RIGHT. And sometimes, less is more.

Thank you for the explanation, I appreciate it! I'm thinking I may need to swap to different wheels and tired and sell my current set. Square setup all around like yours with the 8.5. I have a feeling that the suspension module you can buy would also help. What size tires do you run on your Enkei's?

 

[rcorona]

did this issue begin when you installed the wheels?

I installed the wheels almost exactly a year ago and honestly I can't remember experiencing this issue.

 

[Volfy]

Thank you for the explanation, I appreciate it! I'm thinking I may need to swap to different wheels and tired and sell my current set. Square setup all around like yours with the 8.5. I have a feeling that the suspension module you can buy would also help. What size tires do you run on your Enkei's?

We have two sets of Enkei TFR 18x8.5 ET38. One set with 255/40R18 Dunlop A/S is currently on our G70 for everyday driving. IMO, that is a better setup for 3.3T to handle the torque it can put down, and that's why the OEM rears are 255. However, my oldest drives that car and he prefers the fatter tire look. It's about 1 lb heavier than I'd prefer, but going with 18s (smallest that would fit over the Brembos) means the tires have moderate profile and excellent compliance. Lighter weight wheels mean less unsprung mass for better suspension control, and it also reduces rotational inertia for faster accel/decel.

On my 2.0T Stinger for everyday driving, I actually run on 235/45R18 A/S tires on my old Genesis sedan 18x7.5 rims, which weigh a surprisingly light 23 lbs each. For regular commuting, I have no need for street heroics and so much prefer to tilt towards comfort and "real-world" traction. Handles plenty good enough for the occasion cornering fun, but boy... do they ride supremely smooth. The car maintains composure even over rough pavement. I did also swap in an LSD from a 6MT G70, so that helps too.

The other TFR set is our dedicated track set currently with 245/40R18 Kumho V730, which is about 1/2 way through its service life and will likely be replace with 245/35 or 255/35R18 when the time comes. Rolling diameter will be a bit smaller compared to stock, but on the track, speedo accuracy is a don't-care, but we would appreciate the faster spool up and less rotational inertia. As I mentioned above, at the track is where the low profile tire's sharper turn-in really shines. We run them with 5mm hub-centric spacers up front to correct scrub radius to within 1mm of stock 34mm. Rears go on as is.

 

[rcorona]

**UPDATE**

Wanted to jump back on this thread and post an update hoping that it will give people some info and perhaps help others that may have a similar issue in the future.

I removed the Camber & Toe arms and the ride is back to normal, perfect. Feels planted again and no wishy washy when going over imperfections. So now as far as suspension/handling mods go I just have the Eibach springs & sway bars.

I find it odd that other people run a similar setup with the megan racing control arms but I haven't seen anyone express the same issue. Perhaps the shop that installed them initially didn't do it correctly? Torqued stuff down while the car was in the air still? Not entirely sure, but glad its fixed now.

 

[socalvn]

**UPDATE**

Wanted to jump back on this thread and post an update hoping that it will give people some info and perhaps help others that may have a similar issue in the future.

I removed the Camber & Toe arms and the ride is back to normal, perfect. Feels planted again and no wishy washy when going over imperfections. So now as far as suspension/handling mods go I just have the Eibach springs & sway bars.

I find it odd that other people run a similar setup with the megan racing control arms but I haven't seen anyone express the same issue. Perhaps the shop that installed them initially didn't do it correctly? Torqued stuff down while the car was in the air still? Not entirely sure, but glad its fixed now.

Interesting I run both those arms with no issues. The stock flimsy pressed medal ones you think would give more issues.
Driving home last night I barely had one finger on the wheel. The car didn't follow any lines. Perfectly straight

 

[D.J.]

Perhaps the shop that installed them initially didn't do it correctly? Torqued stuff down while the car was in the air still? Not entirely sure, but glad its fixed now.

I would guess it all comes down to the alignment, once installed

 

[D.J.]

Interesting I run both those arms with no issues. The stock flimsy pressed medal ones you think would give more issues.
Driving home last night I barely had one finger on the wheel. The car didn't follow any lines. Perfectly straight

Yours is probably aligned well

 

[Volfy]

...Perhaps the shop that installed them initially didn't do it correctly? Torqued stuff down while the car was in the air still? Not entirely sure, but glad its fixed now.

^this would be my guess.