wheel shake at 70- 72mph

stu12345_2

 i will do your test asap, any comment on why car drops to gear 3  from top gear 4 on its own accord when going down a hill whilst I'm braking.

Goudy

It's to stop you cooking your brakes.

In a manual car going down a steep hill, you would normally change down to a lower gear and that holds the car back.
If done properly, it stops you braking heavily and for a prolonged period which would otherwise cause you to over heat the brakes to the point the no longer work.

Once your brakes reach a certain temperature, they effectively become useless.
The heat boils the brake fluid which is uncompressible to a gas which is now compressible and the pads start producing a layer of gas between the pad and disc.

Now you have the inability to push the pads out on to the disc due to the boiling brake fluid and if you could, the pads and discs no longer have any friction as the pads are burning and smoking.

With an auto you don't normally get to select a gear so they have different ways of overcoming the down hill problem.

Some autos have the ability to select and lock into a lower gear or even 2 or 3 different lower gears.
These tend to be older autos with a torque convertor gearbox. You have normal selector options of P, R, N, D plus 3, 2 and 1. (though it might have a L for lower gear)
These 3, 2 and 1 (or L) options let you lock it into those particular gears and it won't change until you reselect a different option.
Later torque convertor autos tend to work the same but it's not always left to you to decide. Some will detect a sleep hill via a level sensor, then notice you braking before deciding it's time to change down by it's self.

CVT gearboxes, the sort in lots of hybrid cars have tend to have "B" setting on the selector.
This has a similar effect as it's creates extra drag on the drivetrain to help with steep down hills.

Dual clutch autos usually have a  manual override, either via the selector or paddles behind the steering wheel which allows the driver to override what it wants to select but they also tend to sense the incline and as the driver starts to brake it will automatically select a lower gear, just like yours does.

I once saw a nasty accident happen in front of me on the A635 coming off the moors past Dovestone.
Luckily the woman walked away but she said the car kept running away from her.
I noticed she was continually speeding up, braking and swerving on the way down as one brake was grabbing and the other obviously not due to overheating.
When I looking into her wreck of a car after, it was still in "D" and it all became clear.

born_again

stu12345_2 said:

it has been doing this for 3 years, it has never failed or had reports of drive shaft problems etc in the MOT.
I have never had the car tracked since I've owned it.
 the discs are smooth and perfect, there is no rumble from wheel bearings.
I think this as I said happens at a certain resonance at precisely 70 to 72 and not a mile an hour above or below that speed.
I rarely go on dual carriageways at speed, I'm in a rural county,  hence I don't experience the shake often  through the steering wheel. I aver 3000 miles a year, the car is used daily .

it changes gear beautifully  no bangs or thuds, note the only odd thing I get is when I'm going down a steep hill say at 60 and I touch the brakes the car shifts into gear 3 by itself until the hill levels off then it goes back into gear 4 , top gear in the auto. 
thought it was just  a safety gear breaking characteristic of a torque converter auto astra

no other auto cars I've ever owned does this.

that's the only odd thing it does apart from steering wheel shake at 70 to 72

I would suggest you find a forum that deals with your make/model & ask there.

If it has been doing it for 3 years, then what changed before it started, or has it done it since you got the car??? Which is???

stu12345_2

started when got car, the two recent punctures  and new tyres got me thinking 

darrensurrey

stu12345_2 said:

surely these speed signs are accurate if they are using lasers. what would be the point of them installed then.

anyway , so it's alignment check then

The ones in Richmond (London) all show 2mph over what my satnav reads. The ones in Kingston (next door) all show what my satnav reads. Maybe the satellite coverage is poor for the entirety of Richmond borough. 😆

facade

Goudy said:

 <snip>

I think one driveshaft on these Astra's, (the offside) is longer than the other and has a carrier or support bearing half way along, that can be prone to wearing out and again allow the shaft to run out of true and the car to judder/shudder.

<snip>

I think that some years of astra auto have a dynamic balancer in the middle of the longer shaft.

This is a ring of metal that corrodes virtually instantly and falls away leaving just the rubber core- and a driveshaft that will vibrate at one particular speed......

Aftermarket driveshafts don't come with them either, but the OP reckons they haven't been changed.

I'd have a look at the driveshafts for the presence & condition of a dynamic balancer in the middle.

Also, engine speed might make a difference, try knocking it out of overdrive* when it starts- it should do 72 in 3rd and the increased engine speed might stop the resonance as it is driving it at a higher frequency.

*I assume we are dealing with a 3 speed + overdrive 4th TC auto. Itshould only lock up in 3rd & 4th, so I'd expect resonances in those 2 gears only, at the same engine speed, but different road speeds.