What's up with this fishtailing?

A few years back I had an issue "fixed" of an issue that occured on the highway.

Every time I would drive over an expansion joint in the highway the rear would fishtail enough to make me feel a bit nervous.

This only occurs in the rear and then evens out. I'm hoping to plan for a fix and would like to know if anyone could tell me what could be wrong.


Has anyone else had a similar problem?

No idea what might cause that, but i WISH my riv would slide out. It take a serious corner for my back end to slide at all. Normally the front breaks loose and i have to correct.

Does your rear suspension have any parts replaced? How is the rear camber?

trade rear end bits?

Mine slides out rear first, but just slightly. When I'm on the throttle coming out of a turn, she slides neutral. I like it.

There are lots of ways to make this happen, and it can absolutely be corrected. First question is, where you braking when it occurred? If so, it probably means the front suspension is a bit soft, maybe in need of new struts or springs. When you brake, load is lifted from the rear tires, and the rear can slide out. Or, the rear shocks may be too firm. If you just replaced them, maybe that's it.

Next question: do you have lots of stuff in the trunk? A stereo system? Added rear weight makes your car do exactly what you're saying.

Another thing it could be is incorrect tire size, tread, or air pressure. If the pair of tires on the rear are different than the front set, it can cause your issue. If your rears are worn more than the fronts, it will cause your issue. If the tire pressure in rear is too low, it can cause the issue. Likewise, if front pressure is too high, it can cause the issue.

How's the rear ride level? Too high and the rear will slide out.

If your front sway bar or links are broken, it will cause the rear to slide out.

Wheel misalignment can cause your issue.

Lastly, if your front brakes are not biting as they should, it could cause oversteer. Unlikely, but if your brakes feel weak, it could be the rears are locking up first, which causes a slide.

AA wrote:

If the tire pressure in rear is too low, it can cause the issue. Likewise, if front pressure is too high, it can cause the issue.

Not sure i follow that one. How would those cause the rear end to want to slide out?

Because of a thing called slip angle. When the car is in a corner, the direction of the treads at the contact patch and the actual direction of the wheels are slightly different. This is due in part to tire sidewall flex. In other words, the contact patch distorts as it moves around the corner, pointing in a slightly different direction than the wheels - in effect the contact patch is twisting in relation to the wheels and car. The difference between the direction of the tire tread and the car's pointed direction is measured as angle of slip.



There is an optimum slip angle for best lateral grip. If you have too much, the tires will loose grip. Lowering tire pressure decreases the side walls' support, which increases slip angle. Although lower tire pressures increase grip in a straight line, in a corner the increased slip angle causes the wheel to slide out, especially on heavier cars. If the pressure is lower in rear, the rear slip angle is increased, and the rear end wants to slide out. This is why I run 45 PSI front/40 PSI rear. It helps with oversteer.

You are correct that technically an extremely high tire pressure would also result in losing grip in the turns, for the opposite reason - too little slip angle, and a smaller contact patch. But this would require a very high pressure, maybe 60 PSI before it would happen. So there's a range of pressure (and slip angle) that lets the tires grip optimally in a turn. I believe it's easier and safer to use increased slip angle for getting oversteer than trying for decreased slip angle with higher pressures.

More info: http://www.mgf.ultimatemg.com/group2/suspension/chassis_and_handling/slip_angle.htm

AA wrote:

Another thing it could be is incorrect tire size, tread, or air pressure.

Ah-ha! one of my rear tires is notceably low. I'm going pump it up. I'm going to see my girl tonight so highway driving is in the immediate future. I'll let you know how it rides.

Andysdorm wrote:

AA wrote:

Another thing it could be is incorrect tire size, tread, or air pressure.

Ah-ha! one of my rear tires is notceably low. I'm going pump it up. I'm going to see my girl tonight so highway driving is in the immediate future. I'll let you know how it rides.

Also be absolutely sure to check your rear shocks for bulging/rusted case (seeping oil) or wear-out. My experience is the OE shocks are done after 80 to 120 thou miles.

I filled up the tire and it made all the difference. Besides visually determining the condition of the airs in your tires, this has got to be a major clue for those wondering.

When I did my break job this summer I did notice one of the rear shocks was seeping oil. Compared to the bigger issues I had at the time with the MAF and TPS, I had to put the shock on hold.

Andysdorm wrote:

I filled up the tire and it made all the difference. Besides visually determining the condition of the airs in your tires, this has got to be a major clue for those wondering.

When I did my break job this summer I did notice one of the rear shocks was seeping oil. Compared to the bigger issues I had at the time with the MAF and TPS, I had to put the shock on hold.

Rear shocks for our cars are REALLY cheap. no reason not to replace them if they are not holding air. They even have some on rockauto that are conversion shocks with springs on them.

Glad you found your issue, i think im going to try and find some poly sway bar bushings (not link bushings) and maybe fab up a rear STB.

Andysdorm wrote:

I filled up the tire and it made all the difference. Besides visually determining the condition of the airs in your tires, this has got to be a major clue for those wondering.

When I did my break job this summer I did notice one of the rear shocks was seeping oil. Compared to the bigger issues I had at the time with the MAF and TPS, I had to put the shock on hold.

So maybe now you can replace the rear shocks... for monroes check out rockauto.com and for KYBs look at (I think) summitracing.com... and oh by the way you also need to check the rear lateral arms and the sway bar links. The OE links can snap and you won't see it because of that goofy plastic sheath over the link. check is quick though... With the car parked and on the ground, lay on the ground next to it and push up on the bar, each side, by the link. If you have a busted link you will be able to push the bar up on that side of the car.

It might seem backward, but the rear shocks and rear sway bar links won't cause the rear of the car to slide out. However, these are important to the overall handling of the car. Glad to hear the tire pressure helped your issue.

AA wrote:

It might seem backward, but the rear shocks and rear sway bar links won't cause the rear of the car to slide out. However, these are important to the overall handling of the car. Glad to hear the tire pressure helped your issue.

Correct, worn shocks and busted swaybar links won't cause the rear end of the car to slide out, but they will contribute to the sensation that it is doing so. The badly underinflated tire is probably the proximal cause of the swinging rear. However, OP did confess his shocks were leaking oil -- after I mentioned checking -- and my guess still is that at the age of his vehicle if he has not checked the swaybar endlinks it's high time to do so now. And if they've already been replaced with say Moogs or others that are not shrouded in plastic, he won't need to go shove on the bar to check.

Albertj