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Is it Me, or the Car? Tracking and Stability

Discussion in 'SVT Shelby GT500' started by ShelbyGT5HUN, Aug 17, 2018.

  1. ShelbyGT5HUN

    ShelbyGT5HUN Well-Known Member Established Member

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    Hello,

    I have owned two GT500s. Both had the same suspension mods, Eibachs, camber bolts (yes I know...), the same aftermarket 20" rims, I kept the 20" rims from my first GT500, and put them on my current GT500, LCAs, and a full real deal chassis shop alignment. The problem I'm having is neither GT500 felt "planted", or wanted to track straight, at any speeds above 50mph. Even on smooth, rut free roads, the front of the car wants to dart either left or right to some small degree.

    I'll post my conclusion here first, then fill in the details:

    I think I'm just not used to the feel and drive of a solid axle car, or my 20" rims are throwing the tracking off.

    ---------------------------------------------------------------------------------------
    Why I think this...

    I never owned a solid rear axle car until my first GT500. I have always had German or Japanese sports cars, with fully independent suspension. I know these solid axle cars, give a different type of ride. I can actually feel like the car sits on "three" points, the two front wheels, and from the UCA in the back. It's a little unsettling of a feeling when driven, from my perspective. Perhaps its just a learning curve, I need to experience more seat time? These cars are supposed to be 155mph/200mph capable cars, but I couldn't imagine getting that fast in mine.

    My daily driver is a new Nissan Altima, and I take that at speeds of 100mph on occasion, and even on back roads, that car feels PLANTED at speed, even over bumps and dips. If I take my GT500 on the same roads, by 70mph, I'm feeling very unsafe, and "darty". The fastest I've ever taken the GT500 is to 120mph on a flat Interstate, once or twice, and it was just too damn twitchy for my tastes.

    Next possible clue. I have aftermarket 20" Foose rims on the car. 20x8.5 front 35mm offset, and 20x10 40mm offset in the back.

    I know they aren't the best made rim, and actually joke they are my "cars and coffee" rims, for show, and not "go". I would never try a top speed run on them. When I look down the side of the car, from rear to front, it "appears" the front rims are a little bit further out from the axle hub, than the rears. It's just barely noticeable with the naked eye. It's not cambered out, just that the front wheels sit squarely out, as if they had a 10-20mm spacer plate. I have been getting very normal tire wear. I do know about the brake disc clips, they were removed prior to the rims being installed. So I don't know the exact terminology for that dimension, so I apologize in advance.

    This is how it would look, in a "not to scale" sketch:


    wheel alignment gt500.jpg


    If the front and rear rims, aren't on the same centerline, would this be an issue?

    The car was aligned with the 19" rims, as per the previous owner, done at a reputable chassis shop. I put the 20" rims on after I bought the car from him. He gave me the stock 19" rims, but the tires are shot, and they need to be replaced. I have thought about buying a set of tires for the 19" rims, just as a hunch, thinking the 20" rims, due to their offset, or some other dimension, just wasn't letting the car track straight. Would this be worth a shot? If a car is aligned with 19" rims, and you put 20" rims, should the car be re-aligned?

    There is no damage or worn out components, car has 9,000 miles on it. My first GT500 was bought new, and like I said had the same "issue", as my current one.

    Thanks for any insight.
     
  2. Catmonkey

    Catmonkey I Void Warranties! Premium Member Established Member

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    Are you running the asymmetrical Goodyear's, if not what tires? What you're describing sounds like tramlining. Read this and see if what they describe might be your issue.
     
  3. RedVenom48

    RedVenom48 Ghost Editor-in-Chief Premium Member Established Member Beer Money Bros.

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    #1 super important: make DAMN sure the disc brake installation washers are REMOVED from the brake rotors. These little washers are installed during assembly to keep the rotors on the hub. They are NOT removed from the car after production. All Ford OEM wheels have a machined out section around each lug hole to accommodate them. Aftermarket rims do NOT. Take to a tire shop immediately, have them pull the wheels and inspect each rotor to see if they are on. Take all of them off. Since you have had low mileage GT500s, the rotors may not have ever needed to be removed, and a tire shop may not know to remove them for aftermarket wheels.

    If they are left on and an after market wheel is installed, the wheel will bolt on, and stay torqued, but will always have a small wobble. this could upset the car like how youre feeling.

    Lets assume that the washers are not on the car. I spoke with a seasoned technician I work with, hes not a fan of the centerlines not matching up like in your diagram. Fine for drag racing wheel setups, but on the road the centerlines should be closer to in line than not.

    A proper alignment machine *should* be able to compensate for minor differences in wheel size. That said, I have a few different tire size options I need to input on LS, IS and GS Lexus cars when I align them.

    Thrust angle is still displayed on alignment readouts. But here's a tip: if your rear axle flanges arent DEADNUTS straight, the alignment machine will pick them up as either camber or toe, depending on how the axle is rotated. There is no spec to what is an allowable on the machine, but perhaps in the manual. That wobble could cause an unstable sensation in a low profile tire.


    Id bet that its the combo of 20" Foose wheels and the tires that are on there, and possible rotor washers left on.

    Id get those Fooses off asap and put the factory ones on with some good street rubber (Michelin, Bridgestone etc). Even with factory wheels going back on, get any rotor washers off!
     
    1Kona_Venom likes this.
  4. ShelbyGT5HUN

    ShelbyGT5HUN Well-Known Member Established Member

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    "I do know about the brake disc clips, they were removed prior to the rims being installed."

    I did state that in the first post....

    The brake clips were removed, personally by me. The car has an adjustable pan hard bar, forgot to mention that. Tires are Nitto Invos.

    Lexustech, what's your opinion on the whole solid axle thing?

    Here is the spec sheet from the alignment:

    Alignment Specs.jpg
     
    Last edited: Aug 17, 2018
  5. ShelbyGT5HUN

    ShelbyGT5HUN Well-Known Member Established Member

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    If tramlining is ONLY dependent on the grooves on the road, I can assure you its not that. Mainly none of our roads here have them. The secondary roads are perfectly smooth blacktop. The car feels the same on the highway as it does on these smooth secondary roads.

    I'm more interested in your buddy's comments about the centerlines, as I think this is the problem. You just can't take a finely designed car, and change the "stance" of it with different offset wheels, where the centerlines don't line up. That just doesn't sound right on any level. I would want the centerlines equal front to back.
     
  6. Black Cobra '99

    Black Cobra '99 Active Member Established Member

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    As far as the solid axle goes, it should not matter if you're going in a straight line and the road is smooth and flat. The problem with the solid axle is, when equipped with a pan-hard bar, it will move in an arc during vertical movement. Which gives the car a "boaty" feeling and the car will have a "delayed" response between hitting a bump and responding to it.

    The GT500 will not feel planted, unless its built properly. Maybe your expectations are exaggerating the problem. If possible, drive another GT500 or at least another solid axle car to know for sure.
    Since you said the problem persist on highway and back roads, you can also try the 19" wheels, even if the tires are worn just drive around town to know if the problem is from the 20 wheels or not.
     
    ShelbyGT5HUN and shelbygt500hoss like this.
  7. RedVenom48

    RedVenom48 Ghost Editor-in-Chief Premium Member Established Member Beer Money Bros.

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    Shit, i definitely missed that clip statement by you, i do apologize. Ok, so we know clips arent the issue.

    Your alignment results look good save for passenger front caster. Ironically enough, my passenger front camber is off like yours, more negative than the left front.

    Now, the magic number is .4 degrees. Above .4 degree's difference of caster or camber side to side it will begin to pull.

    .4 and under, and the difference shouldnt cause a pull, but .3 and under is ideal.

    Can you take pictures of the tires? Turn your steering wheel full left or right and take a pic of the treads. Id be interested to see how they look.

    Also, like Black Cobra said, you might really want a Watts link for your rear end. Definitely allows for more articulation with less chassis disruption over bumps.
     
  8. Catmonkey

    Catmonkey I Void Warranties! Premium Member Established Member

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    If you suspect the wheels, you should be able to check them for run-out. Do you still have the OEM rims and tires. What's the drive like with those? What year car is this?
     
  9. RedVenom48

    RedVenom48 Ghost Editor-in-Chief Premium Member Established Member Beer Money Bros.

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    Ok, looking at the rear you may have some things to look at. -.05 for the right rear toe is ok. The left rear camber at -.2 is something to have checked out.

    Get it on a lift and raise about 2 feet distance from bottom of tires to ground, disable traction and skid control and let the car idle in 2nd gear. Get out and visually inspect runout on the rear wheels/flanges. .2 seems like a lot for a solid axle rear end.

    Count me as a fan of having factory wheels with good name brand tires installed too.

    Are the hubs on the wheels specific to Mustang? Are you using hub spacer rings if the wheel's hubs are larger than the Mustang hubs?
     
  10. ShelbyGT5HUN

    ShelbyGT5HUN Well-Known Member Established Member

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    Car is a 2010. The wheels are a direct bolt on, no spacers or rings. The car does not pull, shimmy, or shake.

    Drives straight, but I'm always micro-correcting the track, almost as if the front wheels were both "toed out", or some other alignment fault that decreased the car's natural straight line stability. I'm working on those pictures lexustech wanted.

    The front 19" tires, are completey smoked, on the inside of the tire. Previous owner said he got the camber bolts after he noticed this. Current wear on my 20" Invos, looks very normal, after a few thousand miles on this car.

    Thanks for the replies so far.
     
  11. ShelbyGT5HUN

    ShelbyGT5HUN Well-Known Member Established Member

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    You raise a good point here. I'm interested in your opinion of the GT500 compared to other cars you have owned. I'm thinking, if everything checks out, that I just need more seat time.

    I am going to try to put the 19''s back on, even though the fronts are completely bald on the inner edge.
     

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