Front Springs

[happycat]

Interesting problem this....
.....the OP states in his first post "she sits quite evenly" which I presume to mean that at rest the car is level and sits parallel to the ground, sills evenly levelled front to back. This would indicate that the front springs are not weak and unable to support the weight of the front of the car (at rest).
Hence I'm wondering since the problem is bottoming out on undulating roads whether the shocks are failing to damp the oscillations induced by the road surface?
I guess the springs could weaken and be level at rest but insufficient to carry the extra load from road undulations?

TonyS

Hi Tony what I mean by evenly is she is not lop sided,ie not lower on one side but she does look low on the front, so looking side on sloping towards the front, hope that makes sense Cliff

[greenlane]

Hi cliff
Just looking at the old Haynes manual from 1980. It gives the ride height for a V12 saloon as 611mm to centre of headlights and XJS as 6in (152mm) from ground to front subframe. Guess other engine models would be the same.
Hope this helps.
Regards,
Simon.

[ptjs1]

Cliff,

Jaguar quote the bottom of front subframe to ground at 153mm.

As I said, if the subframe is sitting low, you've got corrosion or subframe mounting problems. However if the body is sitting low in relation to the wheel centre then, assuming you have no corrosion problem, it's a worn spring.

Good luck

Paul

[bokapita]

As I said, if the subframe is sitting low, you've got corrosion or subframe mounting problems. However if the body is sitting low in relation to the wheel centre then, assuming you have no corrosion problem, it's a worn spring.

Could you explain this to me, please? I had thought that if the subframe bottom was lower then the prescribed height, it could be accounted for by weak springs. I had thought this because, unless the wishbone is sitting on its bump stop, wouldn't a soft spring lower both the body panels and the subframe towards the ground? The subframe is essentially fixed to the body, after all.

I am quite willing to beieve I have not understood your points properly!

[ptjs1]

Bokapita,

Apologies! You're absolutely right, I re-read my words and I've not been at all clear!

As you say, the subframe is mounted to the body so if everything is sound, a weak spring will cause the body to sit low in relation to the wheel and the subframe will sit correspondingly low in relation to the ground. Corrosion in the lower spring pan will tend to cause both subframe and body - wheel height to drop together.

Where one can get the body sitting low in relation to the wheel but the subframe sitting at its normal height to the ground, is where corrosion has set in on the suspension turret top or in the chassis rail where the subframe mounts to the body. The more usual of these is the suspension turret top.

With the passage of time, variances in floor surface, minor dents etc in the subframe from jacking, layers of paint and underseal, it's sometimes difficult to accurately measure subframe height. Even the different tyre sizes now fitted to many XJSs can cause confusion on the gap between tyre and wheelarch lip. The body to wheel centre measure is a better indication of problems. If the car is in overall very good condition, I would still suspect springs are the problem.

Bokapita, tks again for correcting me on a sloppy post!

Paul

[bokapita]

Thank you PJ for clarifying things for me.