Here's a question for you peeps while I'm bored on nightshift!!!!
Does anybody know how the torque split works on the mt75 4x4 box asfitted to Escos and saph 4x4?
I know it has something to do with epicyclic gears but how is the front/rear 36/64% bias worked out, and how would you change it to a 50/50% torque split as has been done onsome featured cars?

 

Its 66 rear and 34 front, and I am not sure exactly how it works, but know that there is a viscous coupling in there somewhere

50/50 is done by changing the VC for a solid one.

 

No it's not

The drive is taken into the epicyclic through the planet carrier.
The planet gears have 13 teeth
The rear output anulus has 58 teeth
The front output sun has 32 teeth

So you have a rear output ratio of 13/58, and a front output ratio of 13/32, or 4.46:1 versus 2.46:1

Wont bore you with the maths, but you end up with 0.64reccurring to the rear, and 0.35reccurring to the front.
The viscous coupling connects between the sun and the annulus to PROMOTE equal drive between the two.

 

Bernie

 

 

Sorry, should have said.... You can increase the promotion of equal drive by increasing the poise of the VC (make it stickier).

There were some 50:50 planet sets available many years ago (allegedly) but I have never seen one

 

if you just remove the front prop, does it make the car just drive on 66% of its power??

 

No.

 

if u remove the front prop will the VC try to take up the difference until it locks and then drive only the rear wheels ?

 

The VC isn't designed to lock. There are a series of fine plates immersed in a viscous silicon fluid.
VCs are rated dynamically, for example; 50NM/SEC. That is; You will need to apply 50NM to gain a slippage rate of 1 revolution per second, or 60 rpm.
50NM/sec is a typical first upgrade to an MT75 centre VC.
For a torque to occur, you need an equal and opposing load.
By removing the front prop, the load is removed from the centre differential sun gear. The planets, faced with a load at the annulus, versus an easy life at the sun, will choose to simply spin the latter, revolving inside the stationary annulus. Yes, there is the VC providing some encouragement to drive the two at the same rate, but unless the VC is rated to pretty much match the available torque at the planets against the load of the vehicle, you will simply "fry" the VC.
Remember, if you have, say, an engine capable of supplying 300NM, then in 1st gear, with a reduction of 3.3:1 (from memory on an MT75), you will have an available torque of 990NM

 

Bernie - first rate explanation there mate!!

 

Well this is turning out to be quite interesting



Red=Annulus, Green=planet carrier, Yellow=sun, Blue=planet



So, which bits (sun, planet, annulus) are conected to?

Output from gearbox?
Front prop?
Rear prop?

 

So does that mean that the annulus driving the rear prop, and the sun gear driving the front prop, are rotating at different speeds and the VC tightens up to stop the slip between them to give an equal rpm beween the two outputs providing there is an equal amount of grip at all four tyres when driving in a straight line?????????!!!

Hope that makes sense!!!!!!!!!!!!!!!!

 

this has been done before dudes but may have been lost somewhere on here m but , i might add , did not have bernies top quality 1st class explanation of the centre vc !!!

 

no - when all wheels have equal grip and rotate together, the sun and annulus rotate at the same SPEED, and in this condition the VC does nothing, as both sides of it are rotating at the same speed too.
The gear ratios between the sun/planets and planets/anulus create the TORQUE split, not a SPEED split - however the epicyclic gear assembly does allow differential speed to occur between sun and anulus (ie front and rear prop, when there is loss of grip at one end), this is when the VC starts working as it now has both halves moving with respect to each other which effectively causes 'drag' if you like, between the front and rear outputs, which is the bit Bernie refers to as 'promoting' equal drive.

Remember, the centre diff action (the epicyclic gear assembly) is TORQUE sensitive, whereas the VC is SPEED sensitive. Apart from the viscous 'drag' effect when the front and rear shafts turn at different speeds during front/rear slippage conditions, the VC plays no part in the torque split front-to-rear.

 

50/50's do exist.i have one!

 

Some clever peeps here tonight!

My Saph has very little drive on the front wheels (according to the guys at the rolling road).
I'm told thats very common.
Whats happening there?

 

when it shows up on the RR like that, usually means the centre Viscous Coupling is goosed. On the road you probably won't notice it whilst there is a reasonable amount of grip.
Get it on grass and see if the rear wheels spin
Alternatively, jack up one front wheel and try and turn it... should be very difficult to turn by hand if the centre VC is ok..

 

Good point Rich,

I'ts not unusual at a rolling road shootout for at least ONE ot the Saph 4x4 or Es Cossies to have i'ts power run aborted aas soon the RR opperater notices that only one pair of rollers is being driven

Thats a £ 450 - 500 job too (new/recon centre VC + labour)

 

There's an interesting side issue here, regarding how 4wd rolling roads actually work. I'm assuming the roller pairs are independent, as the applied torque will be different front and rear (although speed must be the same)..., and hence why they can tell that the centre VC is knackered..
But how are the rollers loaded? does the operator have to load them up progressively in relation to each other, whilst maintaining the same speed on each?

Surely they can't rely on the centre VC to equalise the speeds on the roller pairs?? But if that's the case, then what does it matter if the VC is faulty?...

 

Dont know, but Dave at Interpro wont continue a power run if he notices that one or the pointers is not moving (twin pointers on a single dial).

 

interesting post this... up for the day crew!

 

Rich, I had wondered the same about the RR, anybody know?

TF Rallyesport, is that the 50/50 gearset bernie talks of?

 

Bernie -
if you get a few minutes, I for one would be happy to be bored by the maths I'm sure others here would too..

 

Yes mate, I'm just starting to get my teeth ( ) into this.

Rich, http://en.wikipedia.org/wiki/Planetary_gear is quite good I think

 

Ok.
Rear toothcount is 13/58
Front toothcount is 13/32
To work out the relationship we must first convert to fractions of the same total, so:
Multiply out the fractions to get 416/1856 and 754/1856 (58x32, 13x32, 13x58)
Now we have a relationship.
Add 416 and 754 together which forms our whole: 1170.
Now divide 416 by 1170 = 0.3555555555555555555555555etc
Divide 754 by 1170 = 0.64444444444444444444444etc

To achieve a 50:50 split you would need double planets, which are effectively two sets of planets in the same carrier.
One set will mesh with the annulus, with the other meshing with the sun.
Toothcounts are monkeyed with to achieve identical ratios
For example: 20/40 is the same ratio as 15/30 (2:1)

 

We do an uprated centre VC for £175.00+VAT exchange

This is an interesting anomoly.
I have had people bring supposedly knackered centre VCs in for rebuild/uprate because of this very issue. You take the thing apart and find nothing wrong

Has me thinking that maybe some r/roads bring the load up the same on both sets of rollers, and with the already discussed torque bias in the transfer, you will simply stop the front wheels, maybe.........

 

According to the guys at Power Engineering, the front and rear rollers are independant. With my car on the rollers, the rear wheels are visibly turning much faster than the front.
Apparently its fine on the road where everything turns at roughly the same speed.
The difference between the axle speeds on the RR would fook it up completely.

Gearboxman - Interested in the £175 +v VC. Do you fit?

Anyone know how many labour hours I am looking at for fitting??

 

2-3 hours, we charge £40.00 per hour. HTH

 

I've been having a chat today with a 4wd rolling road operator, and there are some points of interest.

When doing a power run you will dial in fixed load conditions, which supposedly replicate vehicle weight, "steepness of hill" you'd like to climb, etc.
There is no facility to input torque split at the centre diff.
If a high enough load is dialed in, then theoretically you could quite possibly beat the VC.
When doing a speed hold, apparently you dial in a speed that you don't want to exceed, and the roller retarders do just enough to keep to that speed. This, apparently, is not so hard on the centre diff.

The trouble with a rolling road, as opposed to the real road, is that in the real world your 65% rear bias is helping to push the car up the road, so taking a lot of the load off the front. I don't know that 4wd rolling roads can replicate this.....

I would be interested to hear any input from RR operators.
Incidentally, the guy I spoke with has sent me several VCs over time, and the ones he sent were knackered through old age. As I said earlier, I see many that have been condemmed by a rolling road, only to find nothing wrong with them.
Interesting topic this

 

gearboxman, very intresting

i must be honest i've never thought about it,,, but the more i do think about it,, the more i dont understand how the rr workd in 4wd mode

 

What about RR with only one roller for each wheel? Cannot remember the name of the RR, but its using rolls that are 2 meters in diameter and that are around 2000 kg each.

Will this kind of RR be closer to replicating real onroad condition?

Great tech stuff Gearboxman

 

nor_cossie, Inertia dyno

and no it wouldnt replicate real onroad conditions

 

you would really need to replicate a road, a large wide long belt , like a huge running machine to allow th car to drive as it should do through each wheel i suppose??

 

not replicate the hole road thing with drag and surface differences etc.

But replicating the force on the vc.

 

yeh, replicate to get an as true to real life as practicle anyway.

 

Expect a phonecall once I get my credit card back down to a useable level!!

See my sig!!!

 

So where can I get a 50/50 torque split centre diff from?
Do they have to be custom made to order?

 

I don't think you'll find a new one now, they were a Group A option on the Sapph. At the time, I seem to remember them being IRO £2000.00

Why do you feel the need for one?

 

Given the above post about the ratios, would it not have sufficed to say

2.46/(2.46+4.46)=35.5 %
and
4.46/(2.46+4.46)=64.4%

Just to make life easier for us simple mathematicians