Tire Rolling Resistance on Raised TDI

Hi everyone,

Basically, one could also move this post to the "Suspension and etc." section, but since it also concerns drive components (and especially these), I'm posting it here.

I've taken care of the tires today, so I swapped the front ones for the back ones and vice versa, and I've come across something that seems a bit strange to me, and I'd like to ask how it looks for you:

I had the car with the small jack on one side, so that both wheels were no longer touching the ground and could therefore be turned freely. There was no gear engaged, and the handbrake was also loose.

At the rear, the wheels could be easily spun, although the discs were slightly worn, but this was hardly noticeable; that is, when you pushed on the wheel, it continued to turn.

On the other hand, the front was so heavy that it required considerable force to turn, and there was no room for maneuvering. The discs also did a little bit of polishing, but that wasn't enough on its own (judging by the noise).

Is that normal? I've had several other front-wheel-drive cars (although with significantly smaller gasoline engines), and in a highly raised position, the front wheels could be turned almost as easily as the rear wheels. I've never tried this on a TDI before, it's my first time...

As far as I could see (which is not completely visible), the drive shafts do not rub anywhere when the vehicle is raised and supported on jacks. (However, this is not entirely certain, and it should also not cause any noise, for example, on a Trabant, where the right shaft rests on the wishbone when raised, which hardly causes any friction). Therefore, the only remaining possibilities are the gearbox and, in particular, the differential (the other side is already blocked). But is it really that difficult?

Unfortunately, I couldn't perform the reverse test (i.e., both wheels at the top) because I didn't have a lift. It's also quite difficult to push the car to get the wheels at the top.

A bit surprised greetings,

Jan

Jan6K wrote:

I had the car with the small jack on one side, so that both wheels were no longer touching the ground and could therefore be turned freely. There was no gear engaged, and the handbrake was also loose. . . . On the other hand, the front was so heavy that it required considerable force to turn, and there was no room for maneuvering. The discs also did a little bit of polishing, but that wasn't enough on its own (judging by the noise). . . . so dass im Grunde nur Getriebe und insbesondere das Differential (the other side is, of course, blocked) remain as the cause. But is it really that difficult? . . . it's usually quite difficult to move the car.

Hi Jan

I know that too - both from my tractor and from my wife's ALH-Ibiza.
It seems that there is nothing completely unusual about the fact that TDI differentials may "weigh" a bit more than most others.
Why this is the case:

Hi,

I would like to briefly describe a phenomenon that I observed last winter with a friend and his Astra (petrol engine):
Once the car was completely cool, the vehicle would slowly and on its own roll forward when in neutral and (of course) with the clutch disengaged.
Likely due to power transmission through the solid, cold gear oil?
It was, incidentally, on a flat surface, and when the clutch was depressed, it stopped rolling.

This could also be a reason why the front wheels are more difficult to turn than the rear wheels.

'So, I haven't encountered any car that requires you to manually turn the differential and transmission to get it to work. Pushing on the pedal and letting it spin hasn't worked on any vehicle yet.'

How difficult the whole process is also depends heavily on the oil used and, of course, the gearbox. The TDI definitely has more to offer than a 55kW petrol engine with a substantial 140Nm, especially in good weather

It should be clear that gasoline-based fuels are smaller in size, and therefore offer less resistance.

When it comes to gearbox oil viscosity, you often hear about jerky shifting in VWs, which are then fixed by changing to low-viscosity oils. Unfortunately, I don't know what kind of oil is usually put in the cars from the factory. When there's a loss, SAE75 is put in (which is very rare), and my cars need ATF

@jan

'Just get the mill really hot and try again'

Cu Gremlin

Hi,

...but that was also a very small gearbox, which didn't need to transmit much torque.

Regarding the issue with the heat, the car had already been stationary for approximately four hours at the relevant time, which meant the gearbox oil was cold.

Best regards,

Jan

Hit me, but but, a Trabant usually didn't have a free-running differential, right? I seem to recall something like that being described in the 'Schlauer Trabant' book. Of course, it's probably 50 kilometers away...

Cu Gremlin

Hi Gremlin,

not universally. The Trabant has one, but only in 4th gear, so that it doesn't matter when the wheels are turning in neutral - the free-running mechanism must then somehow be connected to the 4th gear, and the other three gears are "normal", which is not unimportant, because otherwise one could not restart the car without using the starter if, for example, it ran out of fuel while driving (since there was almost no fuel gauge until the very last drop, this was the normal way to determine when to switch to reserve).

Also, auch das Freilaufen im 4. Gang war wichtig, da es keine gute Idee war, die Motorbremse für längere Zeit zu benutzen, da ein Zweitaktmotor die Schmierung durch den Kraftstoff erhält, und gerade davon gibt es im Schubbetrieb nicht genug (Ich glaube, er hatte keine Schubabschaltung, aber die Ölmenge im Leerlauf ist nicht genug für höhere Drehzahlen). Therefore, one had to really push down on the pedals or take their foot completely off the pedal (allowing for free movement), but not to run at a high intensity.

Best regards,

Jan