Snow creep c220 cdi

[cris124]

Just returned from an interesting drive to canterbury from Exeter...
On the m20 the road just froze, causing a multiple pile up some minutes before me, nobody hurt.
what was scary though apart from everything else was that on the black ice every time i put the brakes on the auto creep pushed the car harder than the brakes could stop it, with the effect of the car snow ploughing slowly on and sideways.(only way to stop was to select N quick)
After a few minutes i "cured" this by selecting winter setting. the car pulled without slipping, but more noticibly stopped without creeping!
thought others may benefit from this knowledge over the next few days.

 

[GrahamC230K]

Good point.

In my manual, I knew I was in the wrong car for the wrong situation when following the black tracks through the snow, the front wheel drive cars ahead pulling away and at 50mph, 5th gear, about 1800rpm (taking it gently gently) I could accelerate in 5th and spin the wheels!!

This was yesterday though, today the roads managed to stay clear while everywhere else the snow got deeper over night.

 

[guydewdney]

i saw a guy in a 210? amg going down a hill near me last year - doing much the same thing - i suggested he put his hadbrake on 1 / 2 clicks - -happy man as his car was slipping sideways due to the camber and bu66ring his nice shiny diamond cut alloys...

similar effect tho'

 

[Satch]

what was scary though apart from everything else was that on the black ice every time i put the brakes on the auto creep pushed the car harder than the brakes could stop it, with the effect of the car snow ploughing slowly on and sideways.(only way to stop was to select N quick)
.

Any braking system should be fully capable of stopping an auto creeping forward. Was this perhaps an effect of the ABS/Traction system trying to "help" on an icy surface?

 

[Shude]

I don't understand how the car can creep forward under power with the brakes on, it won't. Sure the car wasn't just slowly skidding? That happened to me a couple of years ago, a terrifying couple of hours going down a mile or two of totally frozen road into Keighley.

 

[GrahamC230K]

On a very slight incline, I have stopped in traffic, applied the handbrake and started to slide backwards! Luckily applying the normal brakes stopped the car.

 

[grober]

Skidding Not Creeping

A tip from the frozen wastes of Scotland. In snow when you want to stop, knock the transmission into neutral to prevent the rear wheels pushing you forwards. I have "skated" thro several halt signs before I learned this lesson. Taking your foot of the accelerator simply does not do it I,m afraid. Shude is correct the front wheels lock up despite ABS so technically you are skidding not creeping. So why do the front wheels fail to grip enough to slow you down yet the rear wheels still grip enough to push you forward. Dont know but believe me it happens. Something to do with weight transfer or inertial states

 

[prprandall51]

ABS thoughts

Pure conjecture:
Somewhere within the ABS system, something has to decide that the vehicle is stationary - otherwise ABS would stop you coming to a complete halt on a hill.

ABS presumably deals with a single or multiple locking wheels - it is looking for *differences* in the wheels' turn rates - but assumes that if all four wheels come to a rest together then the vehicle is stopping without locking its wheels.

(I wonder what happens if you are travelling at speed on a really slippery surface and bang your brakes on and cause all four wheels to arrest simultaneously - presumably ABS thinks you've come to a spectacular - but controlled - stop, meanwhile you slide elegantly off the road sideways at 60mph... This reminds me that Audi used to fit an ABS off button for very slippery conditions.)

ABS must also be assuming that the friction that the tyres can generate on the road is enough to overcome the driving force of the wheels when the engine is idling.

It would be very clever if I now drew all my thoughts into an explanation. I was about to do that when I relaised that my conclusion sucks. Therefore, I leave these thoughts for the benefit of those to come (to this post) in the hope that someone, one day, will solve the mystery.

PS: It seems sensible that once you select the Comfort setting (formerly Winter) and the car is no longer able to engage first gear, the driving torque of the idling engine is reduced - conveniently to a level where it is not sufficient to overcome the friction force that the tyres have with the road surface.

 

[concordepilot19]

ABS only works above something like 3mph. I would look in the manual but it is too cold to go outside.

This explains why the front wheels on a rear wheel drive auto can skid forward from rest while the back ones continue. I had this in Germany last week in a hire E220 CDi without winter tyres. Nightmare, it took 1 hour to do 7km on untreated roads.

 

[portzy]

Just a thought. ABS does only work above a certain (lowish) speed but does it stop working, mid operation, when you are slowing down and cross this certain speed?. I think it does not.

Lately, for obvious meteorological reasons, I have been playing with the ABS on our Polo and I have noticed that if I try to lock the wheels at say, 15 mph, the ABS does its thing but continues to occilate all the way down to standstill therefore dispelling the notion that the ABS stops mid-brake if you keep pressure on.

Is there a motion sensor?, I think there most likely is in the guise of an accelerator similar to those in old fashioned nav systems like INS. I would imagine that this is more likely to monitor the ESP.

So, creepy things?. I think it would be impossible for the engine / Gearbox, even in S to be able to overcome hard braking pressure and the liklyhood is that it was in fact a slow skid due to camber or incline albeit slight.

Just my two'penneth.

Portzy.

 

[grober]

Found this article about ABS on a CANADIAN site http://www.canadiandriver.com/articles/lh/abs.htm
and I guess they should know if anyone does. Apparently its all down to boundary layers

 

[GrahamC230K]

A mates Toyota Celica used to disable the ABS if it cycled too many times in one braking session - ie it knew it was on something too slippery to help and was indeed hindering.

 

[guydewdney]

audi had an 'off switch' to get past homologation on their ice / snow race cars...abs is a 'bad thing' (tm) on deeeeep snow.

abs works on a max decelleration rate - locking all 4 wheels at 60 will kick in abs as it knows u cant do that (unless u crash....)

 

[cris124]

creeeepy

i'm sure that i wasn't skidding.....
i could stop by engaging n, then as soon as d was engaged, even with the brake on the car would start to push forward.
This is why i engaged w setting, cause this was rather unnerving!
Once i'd done that, all okay!
c220cdi

 

[CAT220]

i'm sure that i wasn't skidding.....
i could stop by engaging n, then as soon as d was engaged, even with the brake on the car would start to push forward.
This is why i engaged w setting, cause this was rather unnerving!
Once i'd done that, all okay!
c220cdi

I know exactly what you mean, I was driving into town about a month ago, blizzard type conditions and in traffic doing 5mph max, every time I went to stop (in D) the car would slow down to about 1mph and then either push forward or move sideways, rather unnerving when you have a bus lane to your left Soon discovered if I slipped the car into N then applied the breaks this solved the problem.