Freelander 1 VCU and Tyres

[andyfreelandy]

Been puzzling this one, I am sure that someone can clarify please?

Ideally the VCU requires each wheel to be the same rolling radius so that the speed differential across the VCU is at a minimum.
Advice is to always fit new tyres to the rear.
I can see that new tyres and old tyres will increase the speed differential across the VCU but why is putting the larger diameter on the back an advantage? The VCU will see the same effect with larger on front or back so what's the science please!??

 

[GrumpyGel]

(I believe) Cos the rear axle is under geared to the front slightly by the IRD PTO.

 

[Nodge68]

(I believe) Cos the rear axle is under geared to the front slightly by the IRD PTO.

Exactly correct.

As the rear diff is under geared compared to the front, the rear half of the propshaft is turning a bit faster than the front half of the propshaft.
This rear overrunning the front effectively makes the Freelander feel like its a FWD, and also gives a slight delay in VCU activation when grip is lost.
However as the rear is already over greared, putting smaller tyres on the rear over gears it more, forcing the rear diff to drive the IRD pinion faster than the road speed needs, which causes the VCU to start taking torque, which overloads the rear diff and IRD.

 

[andyfreelandy]

Thanks gents. Worth logging the principle behind the advice we give and useful to add as a sticky I think?

Mods please can this go up with new tests to do on Freelander???