Unstable car over 70mph

[TheDeuce]

nm36.hw said:

Thanks all - very helpful!

I don't think it's the lane assistance as when that kicks in, it beeps and turns the car (quite abruptly) back to the centre of the lane.

Instead, this feels exactly as when there is wind and the car gets "light" and starts to zig-zag. It only happens at speed (e.g. above 65-70 mph). I also had a few instances of aquaplaning/hydroplaning during the weekend, which seemed excessive for the amount of water on the ground.. and I am wondering if that could be also related.

Can it really be the tyre pressure? I agree 2.5/2.6 might be slightly low when the car is fully loaded but still within the range.. With my previous cars I never adjusted the pressure that often (definitely not if I was driving alone or with other 3 people)..

Thanks again!!

I think it should be fairly obvious if it's LKA, and you seem confident it's not.

Regards tyre pressures, they are already within the range advised by the manufacturer and already slightly higher at the rear. It might be true that they're not 'optimum' for the car when fully laden as it was, but I wouldn't expect what is at worst a minor deviation from ideal to result in handling traits that bothered you enough to post about...

If we imagine it isn't a tyre pressure problem, if you increase the tyre pressures anyway then all you're going to do is increase the dynamic forces acting upon the suspension elements, one of which could have an issue which is causing the problem.

If it's not LKA and the tyre pressures are within design spec range and the car is pulling around at high speed for some other reason... You really should just ask a dealer to take a quick look. Or an independent who will very likely take a look for free. If it's something like a worn bush or tie rod joint, it would feel exactly as you describe as they wheel on one corner can 'wander' and will occasionally attempt to dictate the direction of travel. It's always worth ruling that sort of issue out because it's a fault that can very quickly become worse. Generally a loose or worn suspension element is considered a reason to not drive the vehicle until resolved.

[samoht]

TheDeuce said:

Regards tyre pressures, they are already within the range advised by the manufacturer and already slightly higher at the rear. It might be true that they're not 'optimum' for the car when fully laden as it was, but I wouldn't expect what is at worst a minor deviation from ideal to result in handling traits that bothered you enough to post about...

There is no 'range', at 2.6 bar the rear tyres are too low vs the fronts to maintain stability when fully laden at speed.

The recommended fully laden pressures from https://www.mg.co.uk/sites/default/files/2022-09/M... appear to be 2.5 bar front, 2.8 bar rear. So rear tyres 0.3 bar or 12% higher than the fronts.

Imagine there is some small lateral disturbance pushing the car sideways at 70mph (eg going through a slight curve)
The rear of the laden car weighs 10% more than the front, so there is more lateral force on the rear tyres.
If the tyres are identical type and pressure front and rear, the rear tyres will deflect more than the fronts.
This deflection means that the rear of the car will track outwards more than the front (slight oversteer).
So the car will rotate into the turn, increasing lateral load further.
This will then require a steering correction to compensate for the yaw.

Now increase the rear tyre pressures by 12%. For the same load, the sideways deflection is 12% less.
For a load 10% higher than the front, the 12% higher rear tyre pressure means the rear wheels will deflect less than the fronts in response to side load
So the front tends to understeer, yawing away from the lateral disturbance and thus damping the yaw movement.
In this condition the car is naturally stable, slight disturbances will be damped out and not require the driver to make steering corrections.


I remember my parents commenting returning from on a family holiday in France. Six up plus luggage, autoroute speeds plus the requisite multiple cases of wine in the boot led to the family Previa starting to need steering corrections to stay on course.

[nm36.hw]

And would it be a problem to drive around with 2.5 and 2.8 even if it's just me driving? I would just find it a bit annoying to constantly change the tyre pressure depending on the n. of people with me..

[samoht]

Yeah, it wouldn't be a problem to leave the rear tyres at the higher pressure as it's just slightly more stable that way.

I agree it's not that convenient to keep changing tyre pressures!

[nm36.hw]

thank you very much! I have booked a slot at the car dealer for next week but will try pumping up the rear tyre in the meantime...

[Pica-Pica]

samoht said:

Yeah, it wouldn't be a problem to leave the rear tyres at the higher pressure as it's just slightly more stable that way.

I agree it's not that convenient to keep changing tyre pressures!

That’s were staggered tyres help - the low load and full load pressures are closer together than with a square set-up.

[TheDeuce]

samoht said:

TheDeuce said:

Regards tyre pressures, they are already within the range advised by the manufacturer and already slightly higher at the rear. It might be true that they're not 'optimum' for the car when fully laden as it was, but I wouldn't expect what is at worst a minor deviation from ideal to result in handling traits that bothered you enough to post about...

There is no 'range', at 2.6 bar the rear tyres are too low vs the fronts to maintain stability when fully laden at speed.

The recommended fully laden pressures from https://www.mg.co.uk/sites/default/files/2022-09/M... appear to be 2.5 bar front, 2.8 bar rear. So rear tyres 0.3 bar or 12% higher than the fronts.

Imagine there is some small lateral disturbance pushing the car sideways at 70mph (eg going through a slight curve)
The rear of the laden car weighs 10% more than the front, so there is more lateral force on the rear tyres.
If the tyres are identical type and pressure front and rear, the rear tyres will deflect more than the fronts.
This deflection means that the rear of the car will track outwards more than the front (slight oversteer).
So the car will rotate into the turn, increasing lateral load further.
This will then require a steering correction to compensate for the yaw.

Now increase the rear tyre pressures by 12%. For the same load, the sideways deflection is 12% less.
For a load 10% higher than the front, the 12% higher rear tyre pressure means the rear wheels will deflect less than the fronts in response to side load
So the front tends to understeer, yawing away from the lateral disturbance and thus damping the yaw movement.
In this condition the car is naturally stable, slight disturbances will be damped out and not require the driver to make steering corrections.


I remember my parents commenting returning from on a family holiday in France. Six up plus luggage, autoroute speeds plus the requisite multiple cases of wine in the boot led to the family Previa starting to need steering corrections to stay on course.

I understand what you're saying and I don't disagree - but the rear tyres in this instance are already inflated to a higher pressure than the front, just neither are quite as high as ideal. But its not as if they're miles out or the pressure is biased higher at the front... Probably half the cars on the road today have incorrect tyre pressures to a similar degree - but they would have to be severely outside of normal pressures to have the sort of effect the OP is talking about, especially at just 70 mph.

If the OP was contemplating a sustained high speed run I would very much advise they get their tyre pressures spot on. But for driving a family around at motorway speeds, I certainly wouldn't expect tyres being slightly under pressured would give cause for concern and make the car feel difficult to control. Most people's tyres end up similarly out in terms of pressure at points through the year when the ambient air pressure changes.

[georgeyboy12345]

100% it’s the lane keep assist and you hadn’t disabled it.

[samoht]

TheDeuce said:

I understand what you're saying and I don't disagree - but the rear tyres in this instance are already inflated to a higher pressure than the front, just neither are quite as high as ideal. But its not as if they're miles out or the pressure is biased higher at the front... Probably half the cars on the road today have incorrect tyre pressures to a similar degree - but they would have to be severely outside of normal pressures to have the sort of effect the OP is talking about, especially at just 70 mph.

If the OP was contemplating a sustained high speed run I would very much advise they get their tyre pressures spot on. But for driving a family around at motorway speeds, I certainly wouldn't expect tyres being slightly under pressured would give cause for concern and make the car feel difficult to control. Most people's tyres end up similarly out in terms of pressure at points through the year when the ambient air pressure changes.

I certainly agree with you that most motorists aren't radioing back to their pit crews to take half a psi out of the fronts to settle the balance through Eau Rouge. Probably a majority of cars on the road have tyres more than 1 psi off the 'correct' pressures.

However, most family cars on the road, of which the OP's previous Alfa is one, have an internal combustion engine mounted between the front wheels, and as a result have 55-60% front weight bias. As such they have an inherent nose-led stability as the front tyres take more than half the lateral load. So doing the tyres to the same pressure all round and/or letting them drift through the seasons is unlikely to lead to instability - they have a lot more stability margin built-in.

The one exception I can think of would be BMWs with 50/50 weight distribution, many (though not all) of which come with a staggered tyre setup.

The MG4 is a family car with 49/51 weight distribution unladen, and more rear biased when loaded up, running on equal dimension tyres all round. As such it would seem on paper to be much more likely to experience marginal stability at speed than most front-engined ICE cars or front-drive and thus front-weighted EVs. The fact that the OP has experienced this in reality, in specifically the kind of higher speed fully laden condition that theory would predict, makes the tyre pressure difference from recommended pressures the prime candidate in my eyes.

Note it's the difference in pressures front to rear that affects yaw stability, not the absolute pressures. If they were inflated to 2.5/2.8 and then lost ~10% all around due to colder weather, the relative lateral stiffness would still be in the desired ratio for stability, as long as the under or over inflation is similar at both ends.

[TheDeuce]

samoht said:

TheDeuce said:

I understand what you're saying and I don't disagree - but the rear tyres in this instance are already inflated to a higher pressure than the front, just neither are quite as high as ideal. But its not as if they're miles out or the pressure is biased higher at the front... Probably half the cars on the road today have incorrect tyre pressures to a similar degree - but they would have to be severely outside of normal pressures to have the sort of effect the OP is talking about, especially at just 70 mph.

If the OP was contemplating a sustained high speed run I would very much advise they get their tyre pressures spot on. But for driving a family around at motorway speeds, I certainly wouldn't expect tyres being slightly under pressured would give cause for concern and make the car feel difficult to control. Most people's tyres end up similarly out in terms of pressure at points through the year when the ambient air pressure changes.

I certainly agree with you that most motorists aren't radioing back to their pit crews to take half a psi out of the fronts to settle the balance through Eau Rouge. Probably a majority of cars on the road have tyres more than 1 psi off the 'correct' pressures.

However, most family cars on the road, of which the OP's previous Alfa is one, have an internal combustion engine mounted between the front wheels, and as a result have 55-60% front weight bias. As such they have an inherent nose-led stability as the front tyres take more than half the lateral load. So doing the tyres to the same pressure all round and/or letting them drift through the seasons is unlikely to lead to instability - they have a lot more stability margin built-in.

The one exception I can think of would be BMWs with 50/50 weight distribution, many (though not all) of which come with a staggered tyre setup.

The MG4 is a family car with 49/51 weight distribution unladen, and more rear biased when loaded up, running on equal dimension tyres all round. As such it would seem on paper to be much more likely to experience marginal stability at speed than most front-engined ICE cars or front-drive and thus front-weighted EVs. The fact that the OP has experienced this in reality, in specifically the kind of higher speed fully laden condition that theory would predict, makes the tyre pressure difference from recommended pressures the prime candidate in my eyes.

Note it's the difference in pressures front to rear that affects yaw stability, not the absolute pressures. If they were inflated to 2.5/2.8 and then lost ~10% all around due to colder weather, the relative lateral stiffness would still be in the desired ratio for stability, as long as the under or over inflation is similar at both ends.

That's a point I just made - they relative rear over front bias is still in place... And you and I could debate what is 'slightly out' or 'too far out' all day long, but I think we both know that many cars, including EV's, frequently have tyres that aren't ideally inflated at certain times of the year, but it's not normal to experience anything like what the OP is explaining.

I just checked my own tyres...


You can see mine are low too, just like the OP's. I'm going to do nothing about it however, it's just a cold day today, tomorrow is far milder, they'll be back up to 'correct' or near enough then. My car has not begun choosing it's own direction travel as a result this..

[AW111]

TheDeuce said:

That's a point I just made - they relative rear over front bias is still in place... And you and I could debate what is 'slightly out' or 'too far out' all day long, but I think we both know that many cars, including EV's, frequently have tyres that aren't ideally inflated at certain times of the year, but it's not normal to experience anything like what the OP is explaining.

I just checked my own tyres...


You can see mine are low too, just like the OP's. I'm going to do nothing about it however, it's just a cold day today, tomorrow is far milder, they'll be back up to 'correct' or near enough then. My car has not begun choosing it's own direction travel as a result this..

Just did a short road trip in my '98 Outback.

Front tyres - 29 psi (200 kPa)
Rear tyres - 28 psi (190 kPa) unloaded, 36 psi (250 kPa) fully loaded.

So that's a big difference, and you can really feel it at highway speeds.

[samoht]

TheDeuce said:

That's a point I just made - they relative rear over front bias is still in place... And you and I could debate what is 'slightly out' or 'too far out' all day long, but I think we both know that many cars, including EV's, frequently have tyres that aren't ideally inflated at certain times of the year, but it's not normal to experience anything like what the OP is explaining.

I just checked my own tyres...


You can see mine are low too, just like the OP's. I'm going to do nothing about it however, it's just a cold day today, tomorrow is far milder, they'll be back up to 'correct' or near enough then. My car has not begun choosing it's own direction travel as a result this..

For your car, the manufacturer recommended pressures are 2.5/2.7 bar, so you're recommended to inflate the rears to 8% more than the fronts for stability.
As of now, your actual pressures are 2.3 / 2.55 bar, so the rears are ~11% higher pressure than the fronts. So you have more stability margin than recommended.

For the OP, the recommended pressures are 2.5/2.8 so rears 12% higher. However when he checked he found he had 2.5/2.6, so rears only 4% higher pressure than the fronts.

If the OP's car was say 10% heavier at the rear when 4-up with luggage, then the difference between 4% and 12% more pressure in the rear tyres than the fronts would seem significant to me in terms of stability.

[TheDeuce]

samoht said:

TheDeuce said:

That's a point I just made - they relative rear over front bias is still in place... And you and I could debate what is 'slightly out' or 'too far out' all day long, but I think we both know that many cars, including EV's, frequently have tyres that aren't ideally inflated at certain times of the year, but it's not normal to experience anything like what the OP is explaining.

I just checked my own tyres...


You can see mine are low too, just like the OP's. I'm going to do nothing about it however, it's just a cold day today, tomorrow is far milder, they'll be back up to 'correct' or near enough then. My car has not begun choosing it's own direction travel as a result this..

For your car, the manufacturer recommended pressures are 2.5/2.7 bar, so you're recommended to inflate the rears to 8% more than the fronts for stability.
As of now, your actual pressures are 2.3 / 2.55 bar, so the rears are ~11% higher pressure than the fronts. So you have more stability margin than recommended.

For the OP, the recommended pressures are 2.5/2.8 so rears 12% higher. However when he checked he found he had 2.5/2.6, so rears only 4% higher pressure than the fronts.

If the OP's car was say 10% heavier at the rear when 4-up with luggage, then the difference between 4% and 12% more pressure in the rear tyres than the fronts would seem significant to me in terms of stability.

I still think at that level it's going to make next to no difference, to most drivers not even a perceptible difference, other than the car obviously feeling heavier in general - not unless the speed was far higher and as a result any lateral movement became more abrupt.

Whatever the reason, I think if the OP is being accurate when they said 'if felt like the car being pushed off course by a gust of wind', the best advice now that it's established the tyres aren't massively low on air, is to ramp the car and check for play in the suspension components.

I'm not disagreeing with your point about tyre inflation bias, that's all spot on. I don't agree however that in this instance the tyre pressures are anything like bad enough to account for what has been described. I'm tempted to suggest the OP corrects the pressures and 'tries again', but given its free and quick to check there isn't something more significant amiss, that has to be the advice; go get it checked out.

[Evanivitch]

Caddyshack said:

Have a read on here:

https://www.mgevs.com/forums/mg4-ev-forum.38/

This. It's a dodgy prop shaft, known issue on MG4.