Different types of diffs?

[Crispin]

I've often wondered why a diff on a Subaru is so much smaller than say, a Landcruiser. It looks as though there is very little in it. Power can torque through a Subaru is not exactly low. I noticed on a Honda CRV, which also has independent rear suspension that it too has a really small diff.

Is it a different design to the ones we have? If so, why the difference?

edit:
so having a look at some pics, it seems it is a normal diff. Just looks a lot smaller.

 

[TonyP]

Bored Or looking to upgrade your rear diff to move away from ATRAC,....

 

[Andrew Prince]

Mostly to do with ratios - the higher revving petrol engines and smaller tyres mean that the differential between engine/prop-shaft speed and half-shaft speed is not as great as a diesel LC with big tyres.
So you don't need such a big crown wheel presumably as the gear ration is lower, which means a smaller casing - less metal means less cost. Lower weight and less drag means improved fuel economy. These cars are generally less laden than an LC and are lighter duty (i.e. not slogging through mud or over rough terrain) so the drive-train does not need to be so beefy.

Also with independent suspension the diff is not quite as integral to the load structure as the diff on a solid axle, so the casing doesn't need to be quite as massive, all else being equal.

Interesting question!

 

[Crispin]

Tony - no comment Like an ugly wife, I will always love ATRAC (until there is better model)

I had not thought about the load thing Andrew, guessing the solid axle will have a much harder live with the weight, shocks etc than one with independent suspension.

As for stress, a large wheel with a torquy engine trying to turn it through gloop, one can see why it needs to be strong. Would a Subaru's be under less stress then (higher speed but less torque) than low speed / lots of torque? I'm thinking about the rally stuff, surely that must take a hammering then. (or not?)

 

[Chris]

Stick some 33"s on a Scooby, rev it up and pop the clutch. Now that's what I'd call music.

When you get torque AND power together, now that's a combination.


Chris

 

[Andrew Prince]

Short answer to your torque question is that the amount of torque transferred is equal to the resistance - i.e. a spinning tyre has negligible torque passing through it. And rally cars spin their tyres a lot. You are correct in that rally cars probably having a fair amount of torque with their turbo-charged engines. But the smaller tyres (remember the moment is a factor - a bigger tyre requires more torque to turn) and lower mass means the resistance that the rally car generates is lower than your 3t LC.

The rally car only has to last a race or 2 - your LC is designed to have a service life of 300,000 miles, with the designers factoring in that some of this life may be on rough terrain, overloaded etc etc. All means lots of torque shock-loading that requires beefy diff gears and half-shafts etc. Just compare an LR diff and axle to an LC. Or compare an 80 axle to a Nissan Patrol and you'll see why Patrol's are reputed to have tougher axles and have fewer diff problems than an 80. Even 60 series had big diffs and axles but there is a weight penalty, which costs in fuel but also in terms of handling and performance (unsprung weight) and wear & tear on the suspension mountings, radius arms, panhards etc. Not to mention cost of manufacturing bigger, heavier components.

Lots of factors to consider

 

[Crispin]

Is an LR diff smaller then or a different type? Are there different types or is there only one.

Suddenly curious about diffs

 

[Andrew Prince]

LR diffs are generally smaller and the half-shafts much smaller.

There are lots of variations on diffs but the basic concept is the same - to split power from one source into two and alter the rotational speed. Different kinds of LSDs, auto-lockers, gear types/cuts and so on.
I'm sure the internet can enlighten you!

 

[Crispin]

We are the internet

I thought there was a type which was a worm onto a gear (compact) but having had a look around, that is not a diff per say just normal splitting the power.

Thanks

 

[Chris]

I was off road a couple of weeks ago when a LR broke the rear diff. Half an out later it was out. It wasn't the teeth, it was the pins in the planet gears. "Typical" they described it as. Someone produced a spare, we stuck it in an 5 mins later that was broken. I don't know how much they are but they certainly saw them as consumables. The rear one looked less capable than an 80 front that's for sure. Very similar design, but much less well engineered.

Chris

 

[pugwash]

I was off road a couple of weeks ago when a LR broke the rear diff. Half an out later it was out. It wasn't the teeth, it was the pins in the planet gears. "Typical" they described it as. Someone produced a spare, we stuck it in an 5 mins later that was broken. I don't know how much they are but they certainly saw them as consumables. The rear one looked less capable than an 80 front that's for sure. Very similar design, but much less well engineered.

Chris

Having been an LR boy for 15 years before buying the cruiser it always raises a wry smile when i see people disucssing the "weak" front toyota diff and how you shouldn't do recoveries on them. From my LR perspective the front 8" yota diff is as strong as a FC101 salisbury diff- they aren't inherently weak, just start to get movement in them over time, and could do with a rebuild- you can drive like a complete tosser off road and it will be thousands of miles before they break!

the LR diffs do come in at least 4 different varieties though- the standard rover 2 pin diff- this is like cheese- the 2 pins break frequenly and i've seen numerous diffs where the planet gears have been forced out of the diff case!

There is a 4 pin rover diff that is a bit stronger, but still let down by the righ and pinion and the diff cage.

There is also a "salisbury" diff which is pretty chunky and basically a Dana 60- these are strong, but only usually found on the back of defender 110s, they were used on the front of large forward control 101s (well a beefed up version was- just about the only "strong" rover axles)

None of the rover diffs are of a hypoid design, which to my mind at least, makes them inherently weaker (although there is debate about this)

 

[Crispin]

None of the rover diffs are of a hypoid design, which to my mind at least, makes them inherently weaker (although there is debate about this)

Guessing this was my question then - if they are not hypoid design, what are they and what is the other design?

Standing in neutral ground, why would LR choose that (and stick with it) if it is apparently not a better design? What made Mr-T stick with his decision?

 

[pugwash]

LR diffs are spiral bevel

the main difference is that LR diffs work as you imagine most gears work- they connect at the midpoint of each gear- basically the rotation of the pinion pulls the gear around- there's little slip between the pinoin and ring.

in a hypoid the poinion acts as a worm gear. As you might expect this is a much stronger design BUT it creates way way more heat- you need a specific lubricant that can withstand the huge load on the surfaces as the pinion slips down the ring gear and pulls the gear round at the same time.

The other difference is that you need a reverse rut R&P on a hypoid diff, which whilst you "should" have a reverse cut R&P on a spiral bevel diff, its not essential- hence why all Rover diffs (and by rover i mean the ones made by rover themselves- ie not salisbury) can be swapped front and back- a huge benefit out in the bush if you do a back diff- you just stick the front in!

 

[Chas]

May be of interest
http://www.youtube.com/watch?v=gIGvhvOh ... re=related