Why is the front axle final reduction gear ratio 3.900 yet the rear axle is 3.545 (from Subaru UK website). Surely the wheels don't revolve at different speeds ?

Kevin.

 

This may be to do with torque distribution (i.e. the ~ 60/40 rear/front torque split).

Moray

 

Hi,

Hmmm... I've always wondered too.

Assuming that the front and rear axles turn at the same rate, then the output from the center diff must be differently geared for the front and rear axles.

Maybe the long propshaft to the rear turns more slowly to reduce rotational inertia and vibration. That is if I have the reduction ratio the right way around, which I probably haven't.

Cheers,

Alex

 

Just checked the SIDC faq - 3.545 is incorrect and the actual ratio is 3.9 !

Kevin.

 

If the ratios are different then on a grippy surface the centre diff is doing a lot of work. On my last car (a Range Rover) the diffs had the same ratio, so presumably the centre diff did less work than on the scooby.

IMHO it would seem that the purpose of having different ratios on the Scooby is to bias the drive more towards one set of wheels, presumably the fronts.

I assume that the centre diff is of the "Ferguson" type and therefore the work involved is in shearing the oil between the plates and not actually direct wear on the mechanicals themselves.

If the centre diff takes up this difference are you really putting much strain on the centre diff by having part worn tyres on one end and new tyres on the other? The difference in rolling radius would have a negligible effect compared with the effect of the different ratios.

 

Are the axles, drive shafts or whatever ( ) of equal diameter?

Stef.
(Who knows nothing about it whatsoever!)

 

If an engine produces a fixed amount of torque at a given RPM then the only way to increase this torque is to reduce the RPM's.

Hence the reason that we have gearbox's now to generate a torque split from a single drive shaft that is not 50:50 then you require different gear ratios so that if the shafts where rotating in free air then the wheels would be rotating at different speeds.

In the Ford Focus WRC they actually increase the speed of the drive shafts so that they can use lighter weight components and reduce the inertial masses and then return the shaft output to normal speed at the final diff.

I have a full right up of the way the focus drivetrain works somewhere but it takes about an A4 side just to explain it!

 

I think I understand now.
The wheels would rotate at different speeds if the car was jacked up, but when driving, they must be rotating at the same speed (going in a straight line). Therefore, the centre differential must be compensating which results in the 60/40 torque split ?
So maybe the front and rear diffs do have different ratios after all ?
Kevin
I think I'll dig out the old Meccano set and make a working model ! (or maybe not)

 

<fx: sound of penny dropping>

Ahhh, that explains why locking the centre diff on a Type-R is such a bad idea on dry tarmac. Presumably on slippery surfaces the differing rotation speeds of the front and rear wheels doesn't matter, as the "faster" set of wheels can just spin to some extent.

On a dry surface, there would be tremendous strain on the diffs as they try to overcome the grip of the tyres. Presumably you can feel the transmission straining even on the straightahead on a dry tarmac road. Am I on the right track here Darren?

Presumably the fact that the center diff is always "working" helps account for the relatively high transmission losses.

-= mike =-

 

Yes you can here the transmission working overtime if you lock the diffs up but not fantastically!

The strangest thing is if you lock them up on a loose surface then the car actually becomes quite hard to steer because it always wants to go straight ahead! I think this is mainly becuase the back end is pushing the car straight on and you are trying to divert it from its planned course.

One of the things that is not very clear at all is what sort of center diff the Type R has because it seems to be some sort of Viscous coupled one that you can adjust by altering the Electromagnet but it is very uncertain.

With the Non Type R's then you have to send you power through 2 Viscous coupled diffs one in the center and one in the rear axle which will sap even more power!

Darren