I was just wondering if anyone knows the factors that determine a car's high speed stability. I'm driving an 84 Z28, and all wind noise and rattles aside, the car starts to feel a little unsafe after about 70 mph. I've driven plenty of newer cars that feel just fine at the same speed, and I have a feeling it's not the few hundred extra pounds of leather and carpet that they're stuffed full of. Any information would be helpful, like if it's spring rates, shocks, chassis parts, whatever ideas anyone can come up with. Worrying about your car going over 70 mph starts to suck pretty quick and I'd like to do something about it.

 

well one of the biggest things that will get rid of the rattles is subframe connectors. My car used to rattle like crazy above 80mph but once I installed subframe connectors almost all of the rattling was gone. Doing mods that stiffen the frame are good. Things such as strut tower bar, wonder bar, rear bar (custom job) stuff like that really gets rid of them all. Another thing is also good tires that are well balanced. Also a set of new springs weather theyre stock or aftermarket (good aftermarket dont buy dropzones, intrax, etc). And ofcourse a good set of struts/shocks will help. But yea anyways one of the first things you should do is SFC's they'll help with most of your problems.

Side Note: With only subframe connectors, eibach prokit springs, balanced Z rated tires, and new stock replacement struts/shocks my car was pretty stable up to about 155mph O_o after that it was mostly me being unstable cause our freeways here arent super long and have alot of twists and turns. haha

 

Thanks for the help man, you gave just the answer I was hoping for. I had already picked up a set of the Hotchkis SFCs, their coil springs, and the Bilstien shocks to match them, I was afraid that after I bought all that, I was going to hear that I had to replace something really involved like the control arms. Speaking of the Hotchkis SFCs, I got them from the Scratch and Dent section of the website for like $115, I checked these things over like 30 times when they came, and the only thing that's wrong with them is that the Hotchkis logo is chipped a little, and a little black primer covered that up pretty well. Anyone looking for a bit of a deal, check them out.

 

I totally agree with the suggestions that revlimit gave you. Especially good tires that are balanced well. But before doing any mods I'd go through the entire front end and check for worn parts like ball joints, tie rods ends, idler arm, center link, a-arm bushings. And check the play in the steering box and adust to specs from a factory manual. Then you need to go to the back of the car and completely check the rear suspension out.

You can do all the mods in the world to the car, but if you have worn componets nothing will help until replace the worn parts.

Good luck with your project.

 

Thanks a lot for the info. I'm with you on the front end stuff, I had the tie rods, centerlink, idler arm, and ball joints all done about 8 months ago, I had almost forgotten what it was like to drive a car with an actual alignment. When I replace the springs and shocks I'll most likely replace the panhard rod and the control arms in the rear also, having the wheels centered under the car never hurt anyone, last time I checked...so hopefully I'll be in good shape soon enough. Thanks again, both of you.

 

i can`t find that scratch and dent section on the hotchkins website any link? i really want sub- connectors from them

 

You know something, I just looked at the website now and it doesn't look like the link's in the same place anymore, I can't find it myself. Maybe give them a call and see if they still offer it.

 

alloy hit the nail on the head.. you car should feel just fine crusing at 70... somthing is worn out and thats why it doesnt feel right..


i would suggest replacing all of your bushings and your struts as a good start.. you already said you did the steering stuff, so that part should be taken care of...


i replaced my struts and shocks on my 91 last month... huge diff in turning.. in the first 4-6" of travel on the strut the resistance was nonexsitant.. i could move it quickly by hand with the strut in my arms...

it really narrowed the turning radius of the car.. and it kept it much flatter.. no more tossing to each side.

 

One other strange little detail here, once I hit 70 mph like I was saying before, the car's interior console starts to shake back and forth like crazy, enough so that I can barely read the radio display sometimes. The first thing I thought was the driveshaft needed balancing, but this doesn't happen all the time, only sometimes. Last time I checked, once a driveshaft needs balancing, it doesn't change it's mind and fix itself too often. I'm not even sure if the driveline is even shaking, but the console sure is. Anyone else have this happen before?

 

Sounds like you have a broken tranny mount and or broken motor mounts!

 

Thanks, the motor mount maybe. I did the starter in the car back in February and the trans mount looked fine. Heh, plus my friend's 2.8 Firebird had a bad trans mount, and the shifter not only sank into the console about an inch, every time he did a burnout in it, the whole console would twist in the direction of the transmission rotation. Quite a spectacle, I think I'd notice if mine was doing the same thing.