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Mercedes GLC "tyre hop"

Mercedes GLC "tyre hop"

Mercedes GLC "tyre hop"

(OP)
Gentlemen,

This is a fair bit out there on this subject on the net already, but much of it low level and questionable.

In discussion with a colleague yesterday regarding his just-purchased 2019 Mercedes GLC. Something like 10 months old, less than 8000 miles. Former owner, the Mercedes dealer (ex-demo / staff car presumably). It's the 220d (probably irrelevant) but with AMG package, so big wheels with low profile tyres, which probably is relevant. It's on the original factory fit tyres. They are in pretty fair condition but do have some odd wear patterns on the outer tread blocks.

He's unhappy with it because at low speed (<25 mph) and large steering angles (but not necessarily full lock) the front end "hops" as the tyres stick/slip. This is quite violent and while worse at low speed and very tight angles (his driveway particularly apparently), it also does it during town driving when turning into/out of junctions, on smaller roundabouts and even (around here) on some of the tighter bends in the country lanes. An expensive, near-new prestige luxury car, but definitely not a luxury driving or passenger experience.

Short video showing the problem (our own example is more violent than this)
https://www.youtube.com/watch?v=VTT0U8fciR0
Or if you enjoy a good rant, the John Cadogan version....
https://www.youtube.com/watch?v=BwR9YQjQriQ

The dealer has told him "they all do that sir, it's a feature, not a fault". Though they are going to try and "improve" it after he threatened to reject it (which remains an option).

From my internet researches so far
- It is a pretty common issue for this model, at least in RHD form. Main complaints seem to be from UK and Australia, but I did find one or two reports of it in LHD as well.
- Afflicts vehicles with big wheels/low profile tyres more seriously (no surprise there).
- Supposedly worse when using summer tyres in low temperatures. This may be true to an extent, but on this vehicle it was doing it markedly at 12ÂșC ambient, having just completed 250 mile drive, so the tyres were thoroughly warmed through.
- Mercedes themselves suggest "winter tyres" as a cure, though they rarely offer to pay for them.
- Opinions vary as to whether alternate summer tyres and suspension alignment work helps or not. Seems it does for some and not others.

I realise this is something that affects some other vehicles, 2WD and 4WD alike but usually expensive German ones with big wheels and low profile tyres, but I suspect only in more extreme conditions. This GLC seems like a pretty extreme example.

I wonder what the underlying cause is and why the apparent disparity between RHD and LHD. Ackermann angle? RHD having different steering geometry? Possible fixes? Mercedes general reluctance to do anything much about it suggests a proper engineering fix is quite involved and expensive.

Any thoughts?

Thanks

Nick

RE: Mercedes GLC "tyre hop"

To me, in the first video, it looks like the wheel is being dragged sideways and then goes stick/slip. That the wheel is being dragged sideways at such low speed means the ackerman geometry is off, and that's something built into the design of the vehicle and not something a dealer repair shop would be able to fix.

No idea of any steering geometry differences between LHD and RHD. There shouldn't be.

Smaller diameter rims with higher profile tires should cover up the issue.

RE: Mercedes GLC "tyre hop"

Well that's a new one on me. The way production steering geometry is set, the inner wheel cut angle is held at some maximum for packaging reasons or the wheel end CV joint reaches its maximum articulation angle. Then typically you set the 'steering error', ie inner steer-outer steer to about 80% of the nominal Ackerman setting. This tends to give the best turning circle. That Ackerman then gets baked into the hardware.

But when AMG get their sticky little hands on the car they probably drop the ride heights, changing the steering error, and the camber of the wheels, and fitting wide tires.

One possibility, if you have infinite time, is to adjust the static camber to give more negative camber (just to reduce the contact patch area), and do a swing on the static toe setting. Static toe is fake ackerman, that is, it controls the steering error.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Mercedes GLC "tyre hop"

That car has <<100% Ackerman, (insufficient toe-out in turns) so it would need some static "toe-out" to improve it. Of course that's not a fix (and I doubt you would get more than a slight improvement from changes to static toe) its just a band-aid. I reckon he's stuck with it unless he's prepared to modify the car (or reject it).

je suis charlie

RE: Mercedes GLC "tyre hop"

Nice one Tmoose. Using oil as the fluid? Soapy water?

je suis charlie

RE: Mercedes GLC "tyre hop"

A couple of off the wall thought's here. There are actually quite a few videos on this.
1) Yes the Ackermann geometry topic makes a lot of points. But before you bust your knuckles, consider: The wide wheel with a grippy tire and a low limit may still chirp because of the static Mz and Fx components.

2) Try out a set of front tires off of another Merc. model of the same size but greater profile and narrower wheel. The wide wheel plus short sidewall plus low grip level of these tires allows the Mx to "Snap Thru" as we say. You get your money's worth when it does this at 200 kph. [ caused by limited slip differential mis-tune ]'

3) Back off the front caster. Caster is a "camber by steer" function as well as sideforce and static alignment causes). With less camber change at high steer angles, the tire Mx may be able to hold on.

4) BTW: because the outer suspension bits seems to be angry about this and the inside stuff is quiet snug (did I get this right?) , I suspect that there are some soft suspension, or steering gear, or chassis guts that had to be softened. They would do this because the fat meats on wide wheels and carry over geometry leaves the car at a VERY low understeer level. So, wouldn't you guess?, "They" had to sponge it up in order to be 'safe', OK to drive with a soft spare tire (The one that may not even be there, or the one that everybody forgot about and now 7 years later you have a flat spare).

5) If your car has a V8 motor, then I know that the propeller heads had to screw around with the steering shaft geometry, but used the same I-shaft. Since your AMG Wizz wanted a different phase angle on the I-Shaft than the vanilla version [but didn't get it] , you have a fairly large amount of "lumpy steering" from the I-shaft's U-joint(s), that your wheels get a bit schizo at that magic, large steer angle. More proof that these cars are more Tupperware than engineering marvels, IMHO. Slap a set of them there tars on a Hyundai and get the feeling your beast grabbed 1st gear instead of 3rd.

Cheapest route is a new set of tires on a less 'thick' wheel... Heck, just do the fronts only. If the rear is decent, you will LOVE the car.

RE: Mercedes GLC "tyre hop"

(OP)
Thanks to all for your thoughts. It was supposed to be with the dealer today for attention - not sure whether that actually happened though. Things are strange just now...... I will find out.

I did find some text online, allegedly of MB origin, suggesting that the front diff position meant that the RHD steering rack had to be slightly re-positioned to miss it. Seems possible that this may further compromise Ackermann angles and be the reason why RHD are worse affected than LHD.

Agree that as it is likely Ackermann related, and that (as Greg so nicely put it) is "baked in", it is unlikely that a complete cure is possible - though it may be possible to mitigate the symptoms. Moral of the story? Your test drives should always include some tight maneuvering......

I will speak with my colleague and see if anything was done and whether it worked.

Thanks

Nick

RE: Mercedes GLC "tyre hop"

Hmm, a RHD version would sure have the steering box/pinion/shaft pretty close to that diff!

Pushing the rack forward with front-steer spindles would reduce ackerman, would it not?

Looks like someone at the design stage evidently didn't account for RHD + all wheel drive.

There's probably no way to fix this properly. Tire and rim choice might mitigate it. Narrower rims, narrower and higher profile tires. Won't be cheap, unless maybe you can find someone with a lesser model with smaller rims and narrower tires who wants to "upgrade" to your wheels and tires.

RE: Mercedes GLC "tyre hop"

So one of those has droplink to the spindle, which gives a direct steer effect, and has a massive potential for 'returnability' at low speeds (the original X5 for example didn't take advantage of this, so the steering would tend to go to full lock when parking). The other one is droplink to the lower control arm which has a smaller effect on steering in general, but if your lower arm bushes are soft still has some effect.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Mercedes GLC "tyre hop"

(OP)
Well..... thanks Cibachrome. Those pics explain alot I reckon. (unforeseen)Packaging problem leading to geometry compromise.

The car did get to the dealership this week and they changed the "steering knuckles", which I'd call track-rod ends. Apparently this has all but cured the problem without any changes to wheels/tyres. I imagine they will have checked the toe/camber etc and adjusted as necessary too - which likely also had an influence.

Would have liked to see the old and new rod ends side by side. Some offset added somewhere I suspect.

Cheers

Nick

RE: Mercedes GLC "tyre hop"

And I imagined the Atlantic Ocean turned into lemonade, the Pope was Catholic, and the Weather-Person guessed correctly what yesterday's weather was like.

It sounds like you now own a Mercedes 'Bends'. Ask to see a the two sets of parts side by side. Tell them your vintage AMC AMX doesn't have this problem.

By some chance is your minimum turn circle LARGER ? (As in they restricted rack travel with some altered knuckle stops.

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