High Speed Stability

[03SonicBlueGT]

I'm looking for pointers. I've got an SRA swap, and the rear end isn't stable at all. It's a stock 28 spline unit out of a 98 GT. My car is noticeably unstable in the rear at highway speeds, especially under throttle.

Right now, it's got Maximum Motorsports LCA's and new stock replacement UCA's and new rubber bushings on the axle side.

My car is noticeably unstable in the rear at highway speeds, especially under throttle. Feels a bit like "rear steer". Power is 741/678. I've read that a panhard bar would help, but want to make sure I do my due diligence before spending money. Any other pointers? It's 99.9% street driven. I only drive it when its nice out, so NVH doesn't really bother me too much, as it sees less than 2,000 miles per year.

TIA!

 

[Dave.O]

I had a maximum Motorsports pan hard bar on my 03 gt. Night and day difference, worth the $. Mostly noticeable on cornering and turns over uneven pavement


Sent from my iPhone using svtperformance.com

 

[03SonicBlueGT]

I had a maximum Motorsports pan hard bar on my 03 gt. Night and day difference, worth the $. Mostly noticeable on cornering and turns over uneven pavement


Sent from my iPhone using svtperformance.com

That's what I've read, keeps the rear square and relative to the chassis. Not sure if it'd make a difference for keeping things stable at higher speeds though.

 

[01yellercobra]

What tires are you running?

 

[Ferris Bueller]

Could it be because of a 1998 axle on a 99+ stock front suspension? 1979-93 Mustangs use a 57 inch wide rear axle, 1994-98 use a 58.5 inch, and 1999-04 use a 59.875 inch. Front tracks for those years are different as well. Not sure what having a narrower rear track does for stability.

 

[Bullitt1448]

I would take a good look at the tires first before changing anything else, you could also ditch the rubber bushings in favour of urethane to start with. These cars should not be unstable at highway speeds.

 

[hotcobra03]

This may sound crazy.

Something I found about driving in rain.

Mine got to a point 40 mph you had your life in your hands.
Even with new tires. No puddles just light rain

Only thing I touched were the front wheel bearings,

Problem went away. 80mph Puddle was nothing..

 

[03SonicBlueGT]

What tires are you running?

315/35/18 Toyo TQ's. Ran them at 20 psi but just put them back to 32 psi to see if that helped. Same thing still.

 

[03SonicBlueGT]

I would take a good look at the tires first before changing anything else, you could also ditch the rubber bushings in favour of urethane to start with. These cars should not be unstable at highway speeds.

MM says to be careful with urethane on the upper control arms because if you've got urethane lowers and uppers, the axle can't articulate enough and it could cause damage.

 

[patc84]

what width rim?

 

[2003RedfireVert]

Are the LCAs adjustable? What was your pinion angle?

Any specific reason why you went 98 and not 99-04?

Anytime I had weird behavior in my 01 GT in regards to the rear end it was due to bushings.

Stupid question…did you torque the control arms while the suspension was unloaded? I assume you torqued them under load and at ride height…don’t think I’m implying you’re dumb.

 

[BlckBox04]

the alignment could have something to do with it also. do you notice the front end pulling?

 

[01yellercobra]

315/35/18 Toyo TQ's. Ran them at 20 psi but just put them back to 32 psi to see if that helped. Same thing still.

I'm running those in 315/35/17. I don't have the feeling of instability, but I do have a vibration at higher speeds that wasn't there when I had the R888R's on. I do remember the car felt weird when I first put the TQ's on going down the road. I found out the shop had aired them up to 32psi. Aired them down and it got a lot better.

The only time I've felt like the rear end was wandering was when one of my hub bearings went out. That was a wild ride. Could the axle bearing have the same affect?

I've heard the same thing about the rubber bushings, but I think they gear that more towards the guys that turn a lot. The rubber might be distorting under hard load. Maybe you need to look into the aluminum bushings with the ball joint to make things more solid.

 

[03SonicBlueGT]

what width rim?

10" wheel

 

[03SonicBlueGT]

Are the LCAs adjustable? What was your pinion angle?

Any specific reason why you went 98 and not 99-04?

Anytime I had weird behavior in my 01 GT in regards to the rear end it was due to bushings.

Stupid question…did you torque the control arms while the suspension was unloaded? I assume you torqued them under load and at ride height…don’t think I’m implying you’re dumb.

LCA's aren't adjustable. Will be replacing them with heavier duty ones that are adjustable this winter. I was mainly just looking to swap my IRS for an SRA, no specific reason why I went with the 98. The guy was local to me at the time. And yeah I made sure to have the rear loaded before I torqued anything.

 

[03SonicBlueGT]

the alignment could have something to do with it also. do you notice the front end pulling?

No, I make sure to align my front end within ford's spec. Front tires have perfect wear on them too.

 

[03SonicBlueGT]

I'm running those in 315/35/17. I don't have the feeling of instability, but I do have a vibration at higher speeds that wasn't there when I had the R888R's on. I do remember the car felt weird when I first put the TQ's on going down the road. I found out the shop had aired them up to 32psi. Aired them down and it got a lot better.

The only time I've felt like the rear end was wandering was when one of my hub bearings went out. That was a wild ride. Could the axle bearing have the same affect?

I've heard the same thing about the rubber bushings, but I think they gear that more towards the guys that turn a lot. The rubber might be distorting under hard load. Maybe you need to look into the aluminum bushings with the ball joint to make things more solid.

I've looked into those aluminum bushings too, to help with deflection. The car mainly does 2-3 pulls but anything past the top of 3rd feels too sketchy right now.

 

[BlckBox04]

is it possible you're just roasting the tires because of the power

 

[01yellercobra]

I wonder if strapping it down on a dyno and having them do a couple pulls would show anything? I remember seeing videos of big power IRS cars bending the stock toe links on dynos.

 

[03SonicBlueGT]

is it possible you're just roasting the tires because of the power

Last week my wife was behind me after we'd met for dinner, and I gave it the beans. She said I left marks on the road through the whole 2-3 pull. But I remember when I let off the throttle to coast, it still seems the rear wants to wander. Maybe my tires are too wide for my wheels? theyre measured at 12.5" per the spec sheet and my wheels are 10" wide.