Rear End Sway- Problem?

[TRCKWMD]

Howdy all. I picked up a 2017 GT350R a few weeks ago and man, its so good. After having multiple high performance cars (Porsche's mainly) the rear end of the car, under load through a corner, feels very wrong and unstable. Not a loosing traction and going into oversteer type scenario, almost like the rear end of the car is swaying and/or walking. I haven't gotten it out to the track yet, but on the road (highway interchanges, and some, not all corners) the rear end of the car feels very unsettled, in a dangerous way. Its to the point where I've pulled over and made sure that there isn't a loose wheel or suspension component. I understand that these cars tramline pretty hard, and with the track alignment on the car, I get that can be exaggerated on anything less than perfect pavement, but has anyone else experienced this? I'm on PS4's right now 305/315 staggered. The car was set up by the previous owner so not sure of alignment spec's but definitely in the -3 range up front (unsure of rear). It does have camber plates but after looking over the car, all other suspension components are stock. Is it grooved pavement or expansion cracks that could be causing this sensation? All components (bushings/bolts) are fine and torqued as needed. Am I going crazy? As mentioned, I can take some corners and this thing is like a go cart, but others its just extremely unsettled and feels dangerous. Thoughts?

Sponsored

 

[cmxPPL219]

Congrats on the purchase! I saw your other thread on it. A lot of folks will rightly point to rear suspension sub-frame bushings and all the alignment kits and inserts you can buy from the aftermarket like Steeda etc., and that might do some good to clean up some of the slop these cars have stock. Lot's on the forum on these upgrades.

However, assuming your (a) tire pressures are within spec (b) later get the car in the air and confirm no suspension components are loose, things like end links not broken, (c) alignment not too out of whack, get this checked (d) no other issues [collision repair, etc.] as a result from previous owner, as car was pre-owned - similar comments have popped up before on these cars (not necessarily GT350) and it can come back to tires.

As you may know, GT350Rs come with Michelin Pilot Sport Cup 2s as OE equipment. These tires are less forgiving on the street, but have higher limit grip and handling characteristics. And with the GT350Rs positioned for track duty, it made sense tires of this class came as OE equipment. Also critically, the sidewalls are much stiffer than the MPS4S you currently have; not sure if installed by you or the previous owner. Just noting tires as a possible avenue to check. I have a GT pp2 that I daily and kept the factory MPSC2 tires (identical to those on factory GT350R) because I value immediacy of steering angle inputs, rear end rotation and the planted feel and grip when I get heat into these and find opportunities to "responsibly hoon..." plus whenever I get to take it to track. Tramlining and relatively low tread wear and harshness on street are trade-offs I gladly pay for personally.

 

[WItoTX]

Lot of topics started on this forum addressing this issue. A good spot to start your research would be the suspension thread.

https://www.mustang6g.com/forums/forums/suspension-brakes-chassis.64/

If it's only been a couple weeks, I suspect you just don't have enough seat time. I've seen guys run a stock R round P-cars, BMW's, whatever else on the track. It's always the driver experience/skill, rarely is it the car.

Good luck!

 

[TRCKWMD]

Congrats on the purchase! I saw your other thread on it. A lot of folks will rightly point to rear suspension sub-frame bushings and all the alignment kits and inserts you can buy from the aftermarket like Steeda etc., and that might do some good to clean up some of the slop these cars have stock. Lot's on the forum on these upgrades.

However, assuming your (a) tire pressures are within spec (b) later get the car in the air and confirm no suspension components are loose, things like end links not broken, (c) alignment not too out of whack, get this checked (d) no other issues [collision repair, etc.] as a result from previous owner, as car was pre-owned - similar comments have popped up before on these cars (not necessarily GT350) and it can come back to tires.

As you may know, GT350Rs come with Michelin Pilot Sport Cup 2s as OE equipment. These tires are less forgiving on the street, but have higher limit grip and handling characteristics. And with the GT350Rs positioned for track duty, it made sense tires of this class came as OE equipment. Also critically, the sidewalls are much stiffer than the MPS4S you currently have; not sure if installed by you or the previous owner. Just noting tires as a possible avenue to check. I have a GT pp2 that I daily and kept the factory MPSC2 tires (identical to those on factory GT350R) because I value immediacy of steering angle inputs, rear end rotation and the planted feel and grip when I get heat into these and find opportunities to "responsibly hoon..." plus whenever I get to take it to track. Tramlining and relatively low tread wear and harshness on street are trade-offs I gladly pay for personally.

Tires were brand new upon purchase, they have about 400miles on them. They were at 39psi when I last checked so dropped that down to 32, haven't driven since. No accidents, etc, it was a no sins car. I think getting it on an alignment rack and maybe diving deeper than I did in my garage might be the next step.

 

[cmxPPL219]

Tires were brand new upon purchase, they have about 400miles on them. They were at 39psi when I last checked so dropped that down to 32, haven't driven since. No accidents, etc, it was a no sins car. I think getting it on an alignment rack and maybe diving deeper than I did in my garage might be the next step.

Not sure what temps have been in Denver lately, but obviously cool ambients and new tires that may still have the mold release coating on them due to still being new, don't equal good traction. However, your issue didn't sound like lack of traction, more an issue of it feeling unsettled and feeling of sway/roll. Be interested to know what you find out.

 

[TRCKWMD]

Not sure what temps have been in Denver lately, but obviously cool ambients and new tires that may still have the mold release coating on them due to still being new, don't equal good traction. However, your issue didn't sound like lack of traction, more an issue of it feeling unsettled and feeling of sway/roll. Be interested to know what you find out.

If its not above 50 F I don't drive the car. Not the grip you'll get at warmer temps, but its not a traction issue. Its a disconnected, walking feeling in the rear end of the car. It does feel like a tramlining sensation, but just in the rear of the car; the front is planted. Maybe its just the road and expansions cracks/surface since this isn't an every corner occurrence. I think I have some more digging to do but i'll report back.

 

[Andy13186]

When my tires are on different grip level surfaces and I am turning and powering out of a corner, I do get a kind of weird feeling from the rear. I think it may be the torsen differential doing work

 

[Jobodizo]

People on the forum refer to that as the "rubber band effect." What you're probably feeling is lateral movement in the big rubber bushings that isolate the rear IRS subframe. Does it tend to happen more over corners when there is a slight bump or crowning that would unload the weight on the springs? Like @cmxPPL219 mentioned, grab yourself some of these and some of these and that'll probably fix you up.

 

[WD Pro]

People on the forum refer to that as the "rubber band effect." What you're probably feeling is lateral movement in the big rubber bushings that isolate the rear IRS subframe. Does it tend to happen more over corners when there is a slight bump or crowning that would unload the weight on the springs? Like @cmxPPL219 mentioned, grab yourself some of these and some of these and that'll probably fix you up.

This is the kit for a 350 (two pieces less so a bit cheaper) :

https://www.steeda.com/steeda-555-4449-gt350-irs-bushing-support-system

WD

 

[Champale]

OP - that does not seem normal. Your car should feel very locked down through hard cornering once you have heat in the tires.

In saying that, these cars feel a lot different than a car with a mid or rear engine. I would get it up on a lift and check everything to make sure nothing is loose or damaged and then verify the alignment. A well set up 350R is absolute magic through corners.

 

[Champale]

Also, 39psi is way too high. 30-32 much better.

 

[Aless_andro00]

Congrats on the purchase!
But the way the previous owner set up the alignment, with a lot of negative camber up front, could be making the instability worse. You might want to get the alignment checked and adjusted to make it more stable when you're driving on regular roads.

 

[mavisky]

Get an alignment and pay close attention to the rear camber adjustment bolts. I had a similar scenario where my car was having issues retaining it's camber adjustment stable in the rear. The problem is that a rear camber change also affects toe in a pretty serious way which leads to active toe/camber change at speed and outside of your control. I'd grab a set of solid adjustable rear camber arms and hopefully that locks the issue down for you.

Of course, once you have those I'd go for a solid 4 wheel alignment too. My car is rock steady in the rear after resolving this issue.

 

[Gearz]

I very much recommend you check the rear wheel hub bearings for slop. They are way undersized on these cars especially when tracked or autocrossed. I've replaced mine twice and consider them a consumable item. Make sure the parking brake is not set when you check them.

 

[NightmareMoon]

The main thing is check the rear wheel bearings for play by getting the tire up off the ground and trying to shift the wheel with a big bear hug and see if it moves at all. The rear hubs are wear items and that causes some really sketchy instability.

After that, eyeball all the rubber bushings in the rear suspension, especially the big lower rear control arm monster. Those can tear.

Then check alignment and rear toe.

After that look at replacing some of the rubber in the rear suspension changing to sphericals, and possibly bracing. Specifically that formentioned lower rear control arm bushing is a good one to swap to a spherical. Noticeable improvement in how smoothly it powerslides.

Sponsored