Floating / diving at speed

[Redman]

At high speeds the car starts to float and dive all over the place if go over even small undulations in the road surface (for example on motorways). Quite alarming! Before I start attacking anything, any suggestions as to what us most likely to be causing it? Tyres are at 32 lbs all round; suspension rubbers were all replaced 2 years ago; but I'm uncertain as to when tracking etc was last checked. Air getting under front end and causing loss of grip? Suspension issues? Tracking? What might it be?

[mgcjag]

Sounds like ypu need a thorough safety check over...frames/crackes/ connected to body...all suspension/steering rack...wheel bearings..tyres. brakes.
And alignment...Steve

[DWW]

How old are the tyres on the car?

[madjack4]

Sounds to me like tracking is out run your hand across the front tyres if it feels rough one way and smooth the other (like a hacksaw blade sharp one way and smooth the other) the tracking is out

[abowie]

Of the number of possibilities, I agree that incorrect front wheel alignment is the most likely cause.

Check toe in; if this is incorrect it would cause your symptoms.

Check castor; if this is wrong it will interfere with self centring and make the car feel unstable at speed.

Once you have corrected these problems you can move on to the other things mentioned above.

[Redman]

Thanks all. Will get on with checks (simplest ones first!) and report back idc.

[lowact]

First thing I would check would be the front shock absorbers ...

[FRE444K]

It could be all the things mentioned so far but I notice you are running tyre pressures at 32 p.s.i all round whereas the Jaguar recommended pressures are 24 p.s.i front and 28 p.s.i rear which is good enough for normal speed upto 120 m.p.h. I'm not saying this will cure your problem but it might help.

[Redman]

Thanks for advice on tyre pressures. But when I first had the car it was on various pressures in the 20s and was all over the place and terrible to drive. Lifting the pressures to 32 all round has improved things markedly, but of course the high speed weaving issue remains. This forum seems to recommend anything from 30 to 38. Anyone care to add to this aspect for a S3?

G

[chrisfell]

I will not run radial ply tyres on an E-Type at 24psi under any circumstances. Lower than 32psi will result in uneven wear. 32 to 34 all round is the sweet spot on my bag of nails.

[caveman]

32 all round (min).

[Heuer]

If the tyre pressures are OK (suggest 32psi all round) then the weaving is most likely due to incorrect toe in/out at the front. A wheel alignment check will confirm but you cannot use a laser alignment on the E-Type. Gunson make a gauge for home use if you fancy tackling the job yourself.

[bitsobrits]

The type and manufacturer of the tires can have an impact on recommended pressures, and if you are not running the exact same tire as Jaguar specified back in the day, you would do well to follow the tires manufacturers/supplier recommendation.

Note the variances in these charts from Longstone:

https://www.longstone.com/e-type-jaguar-tire-pressures

[caveman]

Here’s more variants for you. In the day you also had to decide pressures for normal use up to 120mph and sustained speeds in excess of 120mph (From S3 Manual E165 Issue2);


Then there were winter tyres to choose from (from S3 Service Bulletins);


Once you’ve read enough about tyres, you could also check your anti roll bar bushes and steering rack mounts whilst attending to the wheel track/alignment.

[deano1968]

Had this problem with my Series 3 too. Checked the Geometery and tracking and all was okay. In the end it was the age of the tyres as they were not gripping the road. Renewed all the tyres and floating stopped straightaway. As a rule of thumb our tyres should be changed under 10 years.

[caveman]

[Redman]

Advice on getting new tyres noted. Thanks.