W164 light steering when cornering

[frewsimon]

Guys,
I am looking for some help with a strange fault with the power steering on my ML280 2008.
The steering is fine at low speed, but when trying to go round sweeping corners or roundabouts above 25mph, the car slides. This is just noticeable on the dry, but when wet, it makes the ML a death trap to drive.
All tyres are good, with Toyo’s replacing the Nexen’s (MO), with pressures as per the fuel cap.
Previously I have had track rods changed, bushings checked, 4 wheel alignment checked and even Eibach springs changed out all round, to try and solve this.
As the garages I have visited cannot find any issue, I can only assume that the steering rack is faulty, although the usual problems with a worn rack are not evident.
Has anyone had a steering rack that was given the full port effect when driving at speed.

 

[frewsimon]

Guys,
I am looking for some help with a strange fault with the power steering on my ML280 2008.
The steering is fine at low speed, but when trying to go round sweeping corners or roundabouts above 25mph, the car slides. This is just noticeable on the dry, but when wet, it makes the ML a death trap to drive.
All tyres are good, with Toyo’s replacing the Nexen’s (MO), with pressures as per the fuel cap.
Previously I have had track rods changed, bushings checked, 4 wheel alignment checked and even Eibach springs changed out all round, to try and solve this.
As the garages I have visited cannot find any issue, I can only assume that the steering rack is faulty, although the usual problems with a worn rack are not evident.
Has anyone had a steering rack that was given the full port effect when driving at speed.

Full port should read as “ full power steering”

 

[LostKiwi]

If you're running non standard wheels or spacers incorrect offset can cause odd effects.

 

[Mark A]

I binned a set of perfectly good Toyo’s from the wife’s ML as they were lethal in the wet. Replaced with Uniroyal RS3 - much more predictable.

First and last set of Toyo’s I will fit - they were that bad.

 

[Taffy7hfa]

Understeer is nasty..All diesel engine vehicles are front heavy, MLs are heavier than most due to the front differential, are the tyres the correct width and profile ? some tyres have softer sidewalls than others. the tyres you have on may just be unsuitable, stick with whatever Mercedes originally specified.

 

[frewsimon]

Thanks for the comments.
I’m running the standard wheel and tyre set up as suggested by MB.
The handling used to be fine with the nexens that MB supplied, and these were changed for the same spec of nexens. The problem started to occur when the second set of nexens had about 5mm left on them.
This is when I started to replace components.
I have only put the toyos on once the nexens wore down a bit more, as I suspected it as a tyre issue.
The same problem exists with both sets of tyres.
It definitely feels like the power steering is too responsive at speed.
No other issues when driving, either slow or quick, and no vibrations, pulling to the side etc.
Just wondering if anyone had a problem with the variable assist steering being light when driving in normal conditions.
As everyone that has looked at it can’t find anything, I might just need to replace the rack and see where it goes from there.

 

[kid-jensen]

I don't knowingly have variable assist on mine, though if I do, it's not working.....
The assistance is exactly the same at all speeds and all steering wheel angles.

If you do have variable assist (check first) is needs a STAR session to check for faults. I'd imagine if the speed sensor was disconnected it could cause over-servoing at speed.

When you say "the car slides", do you mean at the front only?

If it slides at both ends, that's not a steering problem....

 

[frewsimon]

By sliding, I mean that when there is any moisture on the round and carry out a right hand turn above 15-20 mph, such as a large roundabout, the front of the car slides to the left, and then the ABS / traction control kicks in.
It has happened a couple of times when turning left, but always happens when turning right and wet.
I have been assured that all suspension components are good, and bushes are fine, as I suspected the anti roll bar bushes as a culprit when this started.
As we all know, you know how your car “feels” and I know with mine that it’s just not right. Might be me, but I would say that it feels as if the back end sits down, with the front end being too light.
Again nobody can find anything wrong to confirm this.

 

[kid-jensen]

Just sounds like the front tyres (by far the heaviest loaded at low speed) are just running out of grip.

Not surprising when you think that the ML centre of gravity is around waist height..

Putting aside more unlikely/serious causes like a seized centre differential, my suggestion would be to pick a set of front tyres with well-reviewed wet grip, or even some "winter" tyres to use all year-round.

Obviously, if you can increase the tyre width, you will get more grip, apart from in standing water, but nothing really grips in that situation.

Just get onto one of the tyre-size websites and play around with the sizes that give you the best compromise between width, rolling radius and availability (which dictates cost).

Something like this: https://tiresize.com/tyre-size-calculator/

I went from 255/40x19 to 285/50x18 and the grip improved enormously, as did the ride and kerb protection of my precious alloys. Marginal increase in roll, and slightly slower turn-in, but not really noticeable.

At the end of the day, a well-designed SUV can still be fun to chuck around, but it will have to be at a lower speed than a Caterham. Not such a bad thing.

Don't forget, urban and suburban roundabouts are nearly always swimming with dropped diesel and other slimy contaminants. They can easily catch-out the press-on driver...

 

[LostKiwi]

Obviously, if you can increase the tyre width, you will get more grip

This is a myth. All a wider tyre does is change the contact patch shape. The contact patch size is determined by tyre pressure and tyre diameter.

 

[kid-jensen]

Yes, but you can't increase contact patch area by lowering the pressure because the sidewall has to deform more, causing overheating...

Similarly, you can't increase the diameter because it won't fit under the arches.

The contact patch shape change that you get by widening the tyre is the only practical way of getting more grip. Otherwise we'd all have ultra-low pressure bicycle tyres on...

 

[LostKiwi]

I'll say it again - changing to wider tyres only changes the shape of the contact patch and not the size of it. The only thing that determines grip is the size of the contact patch therefore changing to wider tyres will not increase grip.

 

[Uncle Benz]

I'll say it again - changing to wider tyres only changes the shape of the contact patch and not the size of it. The only thing that determines grip is the size of the contact patch therefore changing to wider tyres will not increase grip.

I think the definition of grip is part of the problem here. The way I see it, going wider tyre will change the shape of the contact patch, but not make it any bigger. You may see an improvement in lateral grip, but it will be at the detriment of longitudinal grip.
The science is actually very interesting (to me anyway). Have a look at slip angles and roll centres if anyone wants some more reading.

 

[LostKiwi]

I think the definition of grip is part of the problem here. The way I see it, going wider tyre will change the shape of the contact patch, but not make it any bigger. You may see an improvement in lateral grip, but it will be at the detriment of longitudinal grip.
The science is actually very interesting (to me anyway). Have a look at slip angles and roll centres if anyone wants some more reading.

Yes lateral grip can be improved depending on tyre deformation and contact patch changes through lateral force and turn in can be sharper due to less flex in the sidewall.

 

[kid-jensen]

This is what I'm trying to say, if you want more grip (and this is what the OP wants) you haven't got the option to increase the diameter or lower the pressure, you can ONLY increase the width of the tyres.

That's how to get increased cornering speeds. If there were any other options, manufacturers of sporty cars (which have consistently proved to be faster around any given corner year-on-year) would be staying with the 145 tyres they started with...

Alternatively, next time I bump into Adrian Newey in Waitrose, I'll mention that he's got it all wrong and point to my bike wheel...

 

[LostKiwi]

This is what I'm trying to say, if you want more grip (and this is what the OP wants) you haven't got the option to increase the diameter or lower the pressure, you can ONLY increase the width of the tyres.

That's how to get increased cornering speeds. If there were any other options, manufacturers of sporty cars (which have consistently proved to be faster around any given corner year-on-year) would be staying with the 145 tyres they started with...

Alternatively, next time I bump into Adrian Newey in Waitrose, I'll mention that he's got it all wrong and point to my bike wheel...

The point I'm making is there is more to it than just fitting wider tyres. You also have to optimise suspension to make use of the increased width. If for example camber change is too great when the vehicle rolls you can actually reduce grip by not having sufficient flex in the tyre sidewall to compensate, nett result being the contact patch deforms due to uneven loading and reduces. In the late 70s/early 80s BMW had exactly this problem in the 3 series.
A blanket "put wider tyres on to increase grip" is frankly incorrect. As you increase tyre width and decrease profile the effects of camber, caster, spring and roll rates become ever more important.
Manufacturers of sporty cars do far more development than just fitting wider tyres... thats why Mr Newey gets paid so much. If it were as easy as just fitting wider tyres anyone could do it. Its the continued development of suspension systems and tyre compounds that make cars consistently faster around corners year on year, not just fitting wider tyres.

 

[Uncle Benz]

Duplicate post. Slow forum

Doing my head in! I've removed the original now!!

Luckily LK has quoted it!

 

[LostKiwi]

^ exactly this.

I do feel we may have moved slightly away from the OP's original query though

You could be right

 

[Uncle Benz]

The point I'm making is there is more to it than just fitting wider tyres. You also have to optimise suspension to make use of the increased width. If for example camber change is too great when the vehicle rolls you can actually reduce grip by not having sufficient flex in the tyre sidewall to compensate, nett result being the contact patch deforms due to uneven loading and reduces. In the late 70s/early 80s BMW had exactly this problem in the 3 series.
A blanket "put wider tyres on to increase grip" is frankly incorrect. As you increase tyre width and decrease profile the effects of camber, caster, spring and roll rates become ever more important.
Manufacturers of sporty cars do far more development than just fitting wider tyres... thats why Mr Newey gets paid so much. If it were as easy as just fitting wider tyres anyone could do it. Its the continued development of suspension systems and tyre compounds that make cars consistently faster around corners year on year, not just fitting wider tyres.

^ exactly this.