4 wheel drive '“ does it work?

[Mark Elder]

Now I know it's not intended for using like a Land Rover, but...

About 7cms of snow around at 5:30am Tuesday morning when I was setting off for airport. I have to drive up a slope of about 20 degree (approx) to get out of my drive.

No problem, I think, the green ABD light will flash a bit.

Rear wheels spin, no green flashing light. Fair enough, the fronts won't get any power until the viscous coupling has heated up. So I gently spin the wheels some more, expecting some front wheel action. I did not try for long (4 or 5 seconds).

Gave up and went for a shovel and dug myself a channel to get a grip.

Once car was moving I was fine, green light came on and I could hear the ABD pump running once I was up to about 5 mph "" and it stayed on for 30 seconds or so as I made my way up the road.

Anyone any idea how quick the viscous coupling should lock up? Can't believe it's any use except for the Paris to Dakar if it takes more than 5 seconds to get some power to the front wheels.

 

[944s2]

Maybe it needed movement/rotation in fronts before it locked?

 

[RSGulp]

In snow it sometimes works to try pulling away in 2nd gear to get better traction and stop the wheels spinning (in all cars not necessarily C4 specific).

 

[Pickled Piper]

Heres a description of how the system works that came from Autocar at the time the C4 was launched;

"For the new Carrera 4, Porsche uses a viscous clutch in combination with a mechanical rear differential lock and Porsche's automatic brake differential (ABD), instead of the old setup that employed electrically controlled and lockable centre and rear differentials. The viscous clutch is located just in front of the gearbox on an extension to the main gearbox shaft. It distributes torque between the two axles, constantly and automatically varying the drive to the front wheels from a minimum 3 per cent to 41 per cent, though in normal motoring it settles at about 30 pert cent.

The system relies upon any difference in speed between the wheels to apportion the torque and is clever enough to distinguish between a slippery surface and hard acceleration. Boot the Carrera and the drive to the front wheels drops to about 10 per cent, thus decreasing any tendency to understeer. If the rear wheels continually break traction, as on snow and ice , the torque spread goes in the opposite direction, sending between 30 to 41 per cent to the front axle.

The diff lock maintains a 25 per cent locking effect between the rear wheels under power, but a 40 per cent lock up on the over run. Lift off abruptly mid-corner and it has a stabilising effect to reduce any hint that the tail might whip out. The upshot is that, in these conditions, the lock creates a yaw force that almost doubles the understeer."

pp

 

[clyde]

Mine cuts in very quickly, even when pulling away in the wet sometimes (normally at roundabouts for some reason). In snow or ice it doesn't make a lot of difference when I have the summer tyres on though - it just means that all four wheels spin instead of just two .

Winter tyres help a lot [8|].

 

[Mark Elder]

Thanks for all the replies.

The frustrating thing is that, now it's Saturday and I have time to play, all the snow has gone!

Problem being that I don't really know what was happening. It is possible that I did have all 4 wheels spinning and just did not realise. I need snow, time, and a helper to watch - all at same time.

I've read the Autocar description before (or something very similar) and it's a good description, but given what it says, I'm just surprised that I got the wheel spin and lack of traction.

More snow please.

 

[IndischRot]

ORIGINAL: Mark Elder

I've read the Autocar description before (or something very similar) and it's a good description, but given what it says, I'm just surprised that I got the wheel spin and lack of traction.

Summer tyres will be absolute pants in the snow - you might well have had all 4 wheels spinning.

 

[RichardLW]

Both my 964C4 and 994C4S were totally useless on summer tyres in snow - pulling away was a complete work of art but once you were going, so was stopping. I got the impression that if there was no traction to be had, the wheels wouldn't spin but just sit there and do nothing, or all four would spin for a second and then nothing.

 

[pse_SC]

You could save money by changing your sig to 993C2 ..

Seconded.[]

 

[sawood12]

Could be wrong but I didn't thing that the 4wd system was capable of sending 100% of the torque to the rear wheels, so you should have had some drive to the fronts whether it was needed or not.

 

[Mark Elder]

ORIGINAL: phelix

ORIGINAL: Mark Elder

I've read the Autocar description before (or something very similar) and it's a good description, but given what it says, I'm just surprised that I got the wheel spin and lack of traction.

Summer tyres will be absolute pants in the snow - you might well have had all 4 wheels spinning.

Any recommendations on "Winter Tyres"? I'm running Mich PS2s
And how many of you actually change tyres for the winter?

I've read the article on the latest Porsche AG testing of tyres for 993s (https://www.porscheclubgb.com/legacy/clientFtp/Register/993/0802_Classic_recommended_tyres.pdf)and the PS2s are not as good as some others in the wet - bt no specific mention of "winter".

 

[burrow01]

I put some Michelin winter tyres onto our Alhambra every October / November, and take them off in March

Makes an astonishing difference in snow and wet conditions

We bought them a few years ago after spinning on a German autobahn (overtaking a snowplough...) everyone else was able to waltz past, as they had winter tyres ( compulsory in Bavaria )

After they were fitted, we drove down to a ski resort, and were able to drive straight up a steep snowy hill to our hotel.

They were also excellent on wet motorways with standing water, much more secure

They do get defeated by deep snow (6-8 inches) but the normal stuff we get in the UK does not present a problem.

I think a lot of the accidents and traffic chaos we get in the UK would be reduced if Winter tyres were used by more people

rgds

Pete

 

[IndischRot]

I run winter tyres on my BMW for all the reasons above. The rubber in summer tyres has greatly reduced wet weather grip below about 3 degrees C and summer tyres have a tread pattern that isn't "self cleaning" so in any snow they clog up and your interface to the snow is... more snow! Not good at all for grip. Any AWD just means you have starting grip but still the same poor braking as a FWD/RWD car.

I bought them from www.camskill.co.uk and had them fitted to a set of smaller diameter wheels bought off ebay. Winter tyres do wear a bit quicker and ought not to be worn down as low as summer tyres but I reckon the actual net cost over a few years is mostly the cost of the wheels as I will be wearing tyres one way or the other. The added peace of mind is worth it to me. If you regularly drive where you'll encounter snow it's a no-brainer, IMHO.

 

[clyde]

I have a spare set of sixteen inch cups with Khumo M&S tyres. It's much easier than faffing about getting tyres swopped over twice a year. I normally swop over around October/November and take them off February/March. I paid around £60 a corner last year for them and as there's no Porsche approved snow tyre (as far as I know) they're as good as the unapproved Dunlop M&S's I had before.

I'm sure I read somewhere that it's actually below around 6 or 7 degrees that the rubber in summer tyres begins to become less effective.

 

[Mark Elder]

ORIGINAL: clyde

I have a spare set of sixteen inch cups ...

Yes, I'm thinking similar. A spare set of complete wheels/tyres use no more space than a spare set of tyres and are a lot less hassle.

 

[burrow01]

ORIGINAL: clyde

I have a spare set of sixteen inch cups with Khumo M&S tyres. It's much easier than faffing about getting tyres swopped over twice a year. I normally swop over around October/November and take them off February/March. I paid around £60 a corner last year for them and as there's no Porsche approved snow tyre (as far as I know) they're as good as the unapproved Dunlop M&S's I had before.

I'm sure I read somewhere that it's actually below around 6 or 7 degrees that the rubber in summer tyres begins to become less effective.

Thats actually what I do with the Alhambra, have a spare set of steel wheels with the winter tyres on

Been thinking about 16" Cups for the 993, but was not sure that winter tyres would be available - do you have details on the sizes etc you have?

Current Porsche winter tyre list is here:

http://www.porsche.com/filestore.aspx/default.pdf?pool=usa&type=download&id=documents-wintertires-actual&lang=none&filetype=default

Continental and Pirelli, plus Nokian for actual snow use - presumably for Sweden etc

Would be interesting to know what tyres they use for Camp 4 in Finland?

Pete

 

[burrow01]

ORIGINAL: RichardLW

Both my 964C4 and 994C4S were totally useless on summer tyres in snow - .

Don't remember you ever driving that very rare 994 in the snow Richard .....[]

Pete

 

[clyde]

Pete

They are 205/55's on the front and 225/50's on the back, although they are only H rated but I figure that I wont be going faster than 130 mph during the winter [][]. Having said that when I brought it back from Germany in December 2000 it had snow tyres on and I was motoring along the Autobahn at 150 before I remembered [][]. Despite the fact that I had the four summer wheels/tyres in the cabin beside me.

I'm sure I had a bit of a choice of makes when I bought the Kumhos, including, I think Dunlops. I have been quite impressed with them and seem they better than the Dunlops I had on before.

My only issue with the 16 inch wheels is that they have a 70 offset at the rear so do tend to sit into the arches a bit. Not sure if they came with the 55 offset but I would have thought so.

 

[burrow01]

Hi

Thanks for the info - are they the correct sizes for 16" wheels? just thinking about speedo accuracy etc

I knew someone in the 80's who had a 944 and used some older style (tractor like...) winter tyres, driving back from a skiing holiday he forgot the 80mph limit they had and wondered why the tread blocks started falling off.....

This put me off of the idea of winter tyres for a long while, until I looked at the more modern versions available now - as you say, H rating should be OK for the conditions they are designed for []

My early 17" wheels also have a 70 offset, so would imagine 16" wheels would be available in both offsets - did yours come with the car, or did you buy them seperately?

Cheers

Pete

 

[Porker993]

Mark I would not automatically assume your 4wd is not working from what you have said.

I know it's a very different set up, but by way of illustration, On our LandRover Defender we have diffs in the front and rear axles, and a lockable centre diff.

1. If the centre diff is not locked and one wheel spins, you don't go anywhere.

2. If the centre diff is locked and two diagonally opposite wheels spin, you don't go anywhere.

3. If you have ARB air lockers fitted front and rear, and three wheels spin, one wheel will still grip. This puts huge strain on the drive train, obviously.

Hope you are with me so far.

So, I'm thinking, as you do not have a centre diff lock (unlike my old 964 C4), it may be, that if the 993 C4 loses traction on only one wheel, all drive is lost.

Hope that helps.

Mark