Sloppy rear end

[gadget73]

The frame cracking thing is more a concern with boxed arms and poly bushings. The geometry in the back is such that something has to be able to flex. If you make the arms and the bushings unable to move, it will eventually rip the mounts right out of the frame. Its not related to power levels, its more about suspension travel from turns. The stock configuration arms have enough flex that they will let the frame mounts stay where they belong.

[slack]

That looks something like what I experienced from a big gust of wind while stopped at a red light before changing my rear control arms and Watts link. I couldn't resist going out and giving my car a few pushes just now. You can definitely see the tires flexing, but my car's ass end has a high frequency, low amplitude 'giggly' motion rather than the low frequency, high amplitude wavy motion in the video. I would have liked to post a video but I'm not a video person at the moment.

Yeah, what you described is more what I was expecting to see.

After finally climbing underneath the car while it was being shaken around, nothing down there seems loose. The whole car is rocking side to side on the side walls of the tires. The rear axle, frame, and body are all moving as one unit. The bushings may still be shot (they do look a little dry rotted) but they didn't seem outwardly sloppy when I checked.

I think it has to do with have much larger tires on the back of the car than the front. (Both on 15'' turbine rims, I don't remember exactly what they are but it's something like 225/65r15 in the front and 255/65r15 in the back)

I bumped up the tire pressure in the back to 55psi and it seemed to firm up the rear end a little bit. I think when I can, I'll switch the rears to the same size as the front so the side wall flex is even from front to back.


I found this post on a random forum while trying to Google the issue further:

I think the root of your problem is the size tires you have selected. When Porsche (or pretty much any manufacturer) puts larger tires on the rear, they use a lower profile tire. Your front tires have a "shorter" side wall than your rear tires. This is because they have the same aspect ratio (60) and the front is smaller so it has a shorter sidewall. When you load the car into a turn the sidewalls flex. The taller sidewall flexes further than the shorter sidewall. You now have a greater slip angle in the rear than in the front which gives you the feeling you are describing. This is also dangerous because it causes oversteer which can be difficult to control. You can try to use much higher pressure in the rear than in the front so the tire doesn't flex as much, but that's about all you can do unless you change either the front or rear to a correct ratio.

[gadget73]

Those are rather large tires. Lots of roll on the sidewall will make it more mushy than it ought to be. I run 225/60R16 on mine with H rated tires. Much better than the stock jelly rolls.