jkrigelman

I have been back and forth with Ford a number of times due to what started out as a weird issue where my car was pulling right only when the gas as applied, it didn't matter what condition of the road or how much power was applied or at what speed. Eventually the rear-driver side tire went bald and was splitting.

I brought it in, told them, something is up. It makes no sense for a car with a LSD to have one bald tire. And they need to fix the car pulling right.

So they put on new tires and checked the alignment. Said everything was good. I take the car out. And the car now pulls left. I take it back several more times, they escalate it to Ford Hotline and one week later tell me this in the service write up:

"... this is a normal condition on acceleration for vehicles with independent rear suspension with CV drive axles and this will be particularly evident on high horsepower vehicles.... No further repair attempts should be made at this time."

First, on the phone she tells me the issue was that IRS systems have an inherent problem that the pull in a direction. I did not fully understand until I got back the paperwork. But before hand, I checked a lot of places on the internet. I do not see this draw back mentioned by anyone. I have never heard of such a thing. If you think about it. What modern day, 300+ HP, sports car does not have independent suspension at this point. It has been around a long time, and a lot of manufacturers are switching to this setup. So the write up is interesting in several ways.

1. IRS with CV drive axles... I'm unawares of another way to transfer power to the wheels besides with a CV drive axle. But I may be ignorant in this fact.
2. It is "normal condition" for a car to exit its lane, on its own, within 2 seconds of applying pressure to the accelerator? This sounds like a major safety concern and would mean that alignments are not a real thing, it is okay for your car to drift out of the lane.
3. It has been made very clear, that it did not matter how fast, or how much power is applied, the car will always exhibit this behavior.

Now, in this process I have had many bold faced lies thrown in my face. At this point my 1 year old car with <10k miles on it. It has spent over a month in the shop for a total of 6 issues with the car. They only ever fixed the one.

At this point, I think this car is a Lemon, and Ford will not openly admit that the car has an issue. So I'm curious. Am I alone in this issue? Am I unaware that this truly is a problem that is present in all new Mustangs 2015+.

I'm going to be starting the buy back process some time soon. At this point I'm pretty irritated with Ford.

jkrigelman

Some videos I made for the dealer to show some road conditions.

jkrigelman

Test drove a couple cars at the dealership. The new Mustangs do indeed all seem to share this common flaw. Only way to solve it would be to use a solid axle like the old Mustangs did.

pwt's17

Actually, most of the problems with the IRS can be corrected. . . .

In their attempt to give you a nice smooth and quiet ride, Ford used soft bushings on several IRS parts. They used soft bushings on some parts that need something firmer, NVH (Noise, Vibration, Harshness) be damned.

But, for the most part, this overly soft IRS can be corrected:
The IRS sub-frame moves around too much. There are bushings or lockouts (added to factory bushing to firm them up) available to correct/reduce unwanted movement.
The rear lower control arm main bushing is Way to soft and allows the control arm to move under changing power and/or road conditions. This results in changes to camber and toe-in and is likely the main source of your problems. The lower control arm bushing can be replaced with a spherical bearing that will not give when forces acting against it change.

The above two bushing sets are the biggest culprits that keep you guessing which way your S550 will go next. There are other parts that need help or replacement. Do Google research and search YouTube (cameras placed under S550s to view unwanted movement) and you'll find plenty of reading and viewing material.

Me - I have added sub-frame bushing lockouts and locator sleeves (holes through bushings much larger than the bolt passing through them. Sleeves locate the sub-frame and don't allow future movement). I have differential bushing lockouts ready to install (reduces wheel hop). I have springs , spherical bearings, and toe links on order. With the PP package, you already have the better toe links.

Sub-frame movement:
Differential movement:
Lower control arm movement:
More:

jkrigelman

Cool, thanks for the information. I have been looking into some of these as well. The frustrating part is they are only ever mentioned as fixing wheel hop, not my particular issues. So it would have been a short in the dark had I just started throwing parts at it. Nice to see a smaller list with someone thinking the same thing. I will look into it after Ford is done with looking at the car and continues to do nothing.

I found just messing with tire inflation went pretty far to clean it up. For some reason the dealership insists on inflating the tires to around 40 PSI despite my complaints. I'm pretty sure this is how my original tire went prematurely in the beginning of all this non-sense. At this point, I believe they at least owe me for a set of tires.

At this time, the car has been in the shop for over a month. So I may be entitled to another (2018?) car due to Lemon Law (>30 days & < 2 years & < 20k miles). Maybe I can use this information on the new car.

pwt's17

Yeah, I should've mentioned tire pressure. It's not like I haven't had similar experiences. Had a kid at a tire shop put 50 psi in all four tires. When asked why, he said "That's what the tire says." Even after explaining that the tire gives Max pressure, and my MX-5 Miata says 29 psi (I ran 31-32), he left them at 50. I had to bleed them down when I got home (didn't think I was going to make it home - the MX-5 is the most sensitive to tire pressure of any car I've had).

Hope you either get it sorted out to your satisfaction, or get a replacement!

Derf00

Yep, it's called torque steer and is the same thing you get when you apply high amounts of torque to a front wheel drive car with CV shafts. Solid rear axle cars can also exhibit this behavior but to a lesser degree because there are fewer moving bits in a solid rear axle setup.

In a front wheel drive setup they've used unequal CV shaft lengths and other suspension mods to counter the affects.

IRS RWD systems are more likely to have wheel hop and torque steer from all the moving bits of rubber. The best way to counter it is through using higher durometer rubber or even delrin (nylon) bushings where possible to counter act the effects. Yes you will inherit more NHV but that's the trade off for a more stable ride.

CroweMag55

I've been experiencing the same "Pulls to the left" in my '17 GTP- PP. Ford dealer put a new Front Tire on it, after first trying to "fix" the Toe on it and said it's fixed! (That was last night. I picked it up during horrible DFW rush hour so didn't get much of a chance to test it out, but it felt like the same at very low accelerations. This morning I had a chance to nail it a couple of times and it is still doing the same thing. I did notice that they had put 40 lbs of air in that new tire, and bumped the 2 right side tires about 2 lbs each.. Felt that was odd.

-John