4WD Concern

[sidned]

I've just returned from a successful MOT, but I had to yell at the tester who started to test the brakes using the 2 wheel rolling road. My car is a Carrera 4 and obviously shouldn't be tested like this. The front wheels were spinning independantly of the back for about 20 seconds, and upon driving the car home I could not here any unusual noises.
Should I be concerned about this and have the 4WD system checked out by an OPC or similar, or do you think that 20 seconds is okay for the rear wheels to be stationary and the fronts rotating slowly? I don't want to here a grinding noise in 6 months time and find I have no claim against the MOT station by this time!

 

[Dave Wilkinson]

It's not a problem at all. The 4WD system on a 993 is a viscous coupled arrangement that can take any amount of slippage F/R. This type of MOT treatment isn't even a problem for the 964 with a mechanical F/R diff since the test is only done at slow speed.

Even if you take the car to an OPC they will subcontract the MOT test to a local tester who will just do the same anyway.

I've queried this with J Z Machtech in the past and they have said exactly the same - it's not a problem.

Regards


Dave

 

[sidned]

Thanks for the reassurance, Dave.

 

[IndischRot]

ORIGINAL: Computamedic

The 4WD system on a 993 is a viscous coupled arrangement that can take any amount of slippage F/R.

There isn't an issue with something as short as an MoT test but the above statement implies a 993 could, for example, be put on a 2wd rolling road without disconnecting the front prop shaft and I think there's lots of evidence that contradicts this.

 

[Shane Hodges]

This was my question on the 9M dyno day, as the recommendation is certainly to disconnect the prop at the front to get a true reading and, as I have been told "to prevent any diffs being damaged or exploding."

 

[Mark Elder]

You have to disconnect the front drive.

I'm not saying an MOT test would do damage, but the whole idea of the viscous couping is that, if the back wheels turn without the front ones the fluid heats up very quickly and this heat stiffens the viscousity which means the front wheels turn with the back wheels (a type of temporary dif-lock).

Therefore if you put a 993C4 on a single-axle rolling road something must go wrong - even if it's only the smell of burning rubber as your front wheels start to spin (hope car is tied down!).

If this is wrong then the 4WD would be fairly useless - which it might, of course, be....I've never actually seen any sign of the front wheels of my car ever helping the traction....

 

[Dave Wilkinson]

ORIGINAL: phelix

There isn't an issue with something as short as an MoT test but the above statement implies a 993 could, for example, be put on a 2wd rolling road without disconnecting the front prop shaft and I think there's lots of evidence that contradicts this.

I have to admit that it was probably a bad choice of words on my part when I made the original reply. My intention was to relate it only to the MOT brake test and I certainly wouldn't knowingly suggest something as severe as using a 993 C4 on a 2wd dyno.

As a matter of interest, even on a 4wd dyno it seems to be "de rigeur" to disconnect the front drive shaft since the viscous coupling seems to have an adverse effect on the readout. Conversely, the 964 AWD won't work at all if you disconnect the front!!! Can't even get the car revered off the lift!!!

Regards

Dave

 

[Gary C]

They are meant to use a Tapley meter (basically a G-meter) for 4WD cars while test driving them.


Last time I had an MOT on the EVO, they tried it on the rollers but the active diffs would not let it work properly, took one look at the Brembo's and decided it would probably stop ok anyway.

 

[dparslow]

ORIGINAL: Gary C

They are meant to use a Tapley meter (basically a G-meter) for 4WD cars while test driving them.

This is true for permanent 4WD systems, like L/Rovers and R/Rovers, Audi`s, Subaru`s and Evo`s. The 993 is not a permanent 4WD and as long as you don`t leave the brake rollers running too long all will be fine, as it only takes a few seconds to check the brake efficiency on each wheel.

 

[Pickled Piper]

ORIGINAL: dparslow

ORIGINAL: Gary C

They are meant to use a Tapley meter (basically a G-meter) for 4WD cars while test driving them.

This is true for permanent 4WD systems, like L/Rovers and R/Rovers, Audi`s, Subaru`s and Evo`s. The 993 is not a permanent 4WD and as long as you don`t leave the brake rollers running too long all will be fine, as it only takes a few seconds to check the brake efficiency on each wheel.

That's interesting. Two of my last three MOTs were completed with a brake test on the rollers. However, the last time the tester took the car out with the 'G' meter in the footwell . It's always been the same testing station. Perhaps the tester just fancied a drive in my car this time.

pp

 

[Mark Elder]

ORIGINAL: dparslow


This is true for permanent 4WD systems, like L/Rovers and R/Rovers, Audi`s, Subaru`s and Evo`s. The 993 is not a permanent 4WD and as long as you don`t leave the brake rollers running too long all will be fine, as it only takes a few seconds to check the brake efficiency on each wheel.

In what way is the 993 not "permanent" 4WD?

L/Rovers have a level to let you select 2 or 4WD - that is non-permanent.

I guess you are referring to the time it takes the viscous coupling to "tighten up" - anyone any idea how long this is? I'd assumed it had to be fractions of a second (to make it any use) - but maybe it's only intended to be useful on the Paris to Dakar...

 

[Dave Wilkinson]

This thread gave me some inspiration to look for some background information on differentials in general and viscous couplings in particular. Amongst the varied stuff I found there was this write-up which explains the workings of the various types of differentials.

The significant fact I established was that the viscous coupling provides no torque split at all until there is some slippage. The technicalities are very much down to the specific design implemented by Porsche so it's difficult to get any meaningful numbers from general literature on the subject.

Regards

Dave

 

[Gary C]

You should have a look at an EVO rear diff, facinating bit of kit (had an old one in bits)

Its basically an open diff with one half shaft having two concentric hollow shafts running at +10% and -10% speed.
An electronically controlled clutch pack then either progressively speeds up or slows down one half shaft which gives it a fully active diff as well as active yaw control. Coupled with an active center diff, the stability is awsome.

As to other diffs, there are so many types, most road ones only give 5:1 torque transfer so are fine on the rollers. The Suretrac is a nice torque sensing diff. Track work really needs plated diffs which are adjustable and can give total lockup (964RS ?)

 

[willstatt]

Landrovers ARE permanent 4wd. The lever you refer to is to change from low box to high box (both of which are in 4wd, and the same lever can also engage the central diff lock, but no matter what setting, a LRover is always in 4wd.

Will

 

[colt]

I cant believe that nobody has mentioned the fact that there is a limit on the traction able to be transmitted by the viscous center diff fitted to 993's.Yes the fluid heats up and the coupling stiffens,but it also has a shear point at which it can transmitt no torque/drive at all. All porsche factory litriture states brake testing is ok full stop as long as it is carried out at slow speed. I agree chassis dyno testing must always be conducted with the front prop disconnected unless it is a 4wd test rig.

 

[IndischRot]

Good point Colt; high powered 993tts that see track work usually kill their viscous coupler from overheating it due to too much rear wheel spin resulting in trying to put more torque through the front wheels than the coupler can cope with.

 

[Mark Elder]

ORIGINAL: willstatt

Landrovers ARE permanent 4wd. The lever you refer to is to change from low box to high box (both of which are in 4wd, and the same lever can also engage the central diff lock, but no matter what setting, a LRover is always in 4wd.

Will

Well, OK, I have to admit I have no recent experience: So the "modern" ones may well be permanent - but the 1977 model I drove all summer in 1977 (model = whatever they called a "defender" in those days) had two coloured levers (red and yellow I think) -plus the black gear stick.

One of the levers was the 2/4wd switch and one was the switch to low ratio box.

You had to stop to be able to make all changes except switching out of 4wd back to 2wd.

It also had a hand throttle, which added to the games you could play.

What a summer that was. 7am to 7pm 7 days a week, working with a bunch of Irish guys all day and drinking with them all night. Did not sleep for 3 months.[]