Car changes trajectory under acceleration / deacceleration

Rostami · Post by **Rostami** » 31 Jan 2006, 16:01

Hello to all,

Here is a problem which is related to a Citroen AX.

While driving on a straight line, as soon as I bang the accelerator pedal to the floor, the car deviates from its trajectory, that is, it gives a nodge to the right, and then continues on a straight line, but parallel to the original expected trajectory. The nodge is of about 15 cm / 6 inches. The opposite happens when I suddenly lift my foot from the accelerator pedal. The car is really scary!!!

I feel no slack on the steering wheel.

This is a general problem, and I guess can happen on most cars. Any ideias what can cause this? Could it be the whishbones? Or could it be that one of the engine mounts has packed up and the engine moves more freely hence changing the car´s center of mass under acceleration or deacceleration?

All and any inputs are truely welcome, as I don´t have a clue as to where to start.

jeremy · Post by **jeremy** » 31 Jan 2006, 17:45

This sounds a bit extreme and its worth having a look at the front suspension. i'm assuming it doesn't make any strange noises which would indicate play somewhere - so have a look at the lower wishbone/strut swivels, track rod ends, steering rack fixings, inner wishbone bushes etc.

Engine mounts might affect it but would normally either be obvious as the thing could be felt moving on aceleration or would be felt as harshness and vibration. its worth inspecting them as well.

having said that if the car has unequal length driveshafts this can give rise to a pull. the reason is this - imagine you are running steadily then acelerate. the driveshafts will twist a bit but the longer one is going to twist a bit more - so the side with the shorter one is going to be ahead of the longer one - so that wheel will want to lead.

This problem is supposed to be cured or at least lessened by an additional driveshaft joint and a short driveshaft as fitted to ZX, BX, Xantia and so on. Quite why this should help I cannot imagine as I'd have thought the critical thing was the length for the driven portion of the shaft - ie from the differential - but they are commonly used.

davehughes · Post by **davehughes** » 31 Jan 2006, 17:48

I used to have a Lancia Delta Turbo that did this. It was a knackered steering column Universal Joint.

I have known engine mounts (depending on which one) to cause this problem as well

rossnunn · Post by **rossnunn** » 31 Jan 2006, 18:30

could be torque steer, oh hang on its a AX!

fastandfurryous · Post by **fastandfurryous** » 31 Jan 2006, 19:23

jeremy wrote:imagine you are running steadily then acelerate. the driveshafts will twist a bit but the longer one is going to twist a bit more - so the side with the shorter one is going to be ahead of the longer one

I have a feeling that while very true, the effect of this will be really small.

I've had cars that do this in the past... they turn left under acceleration and right under deceleration (or vise-versa)

Almost always it's been due to something being worn or broken. The first candidate would be wishbone rubber mounts, allowing the wheel to move fore-and-aft in the wheelarch, thus putting the suspension geometry out, but variably, relating to torque.

after that, look at stiff CV joints, or maybe a stiff damper on one side of the car.

Also, are the front tyres matched?

alan s · Post by **alan s** » 31 Jan 2006, 19:51

Check your lower control arms on the front suspension.
Totally buggered rear arm bearings will cause it on BXs but I'm not sure of the type they use on your car, but to me, the lower control arms could be your culprits.

Alan S

andmcit · Post by **andmcit** » 31 Jan 2006, 20:36

alan s wrote:Check your lower control arms on the front suspension.
Totally buggered rear arm bearings will cause it on BXs but I'm not sure of the type they use on your car, but to me, the lower control arms could be your culprits.

Alan S

I think this is more likely the culprit as Alan says. The Ax's are still rear trailing arm aren't they?

Had this on a CX GTi turbo...

made hard acceleration all the more interesting mind...

Andrew

oscarloco · Post by **oscarloco** » 31 Jan 2006, 23:53

Check the lower balljoints in the wheel end of the lower arms. I had one worn in my ZX and gave me the same problem. If I accelerated hard, the car would go left, when I braked hard, the car would go right.

Post by **Peter.N.** » 01 Feb 2006, 01:04

First of all check your front tyres, if they are sufficiently unevenly worn one could have a smaller diameter, in effect putting more power down on that side.

Allanxantia · Post by **Allanxantia** » 03 Feb 2006, 11:02

I would even go with the tyres check. Even different brands of tyres can cause a significant pull