Differing front/rear torque split - how?

Differing front/rear torque split - how?

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xRIEx

Original Poster:

8,180 posts

147 months

Wednesday 25th March 2015
quotequote all
I've been trying to get my head round various online articles on the matter, but I can't understand how a torque split of e.g. 45:55 F:R is achieved, or the components used to do it.

I know some systems use planetary gear sets, but whatever the system it seems to come out with the same result: differing torque means different rotational speeds (of prop shafts). How would that work? Surely if the front and rear diffs had different final drive ratios to bring the rotational speeds at the wheels back to the same figure (assuming same diameter wheels all round), that would also equalise the torque at the wheels?

Does every system have some amount of slip somewhere to allow for this?

anonymous-user

53 months

Wednesday 25th March 2015
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What you have identified is that all these so called "torque biasing" systems are largely irrelevant in terms of controlling a vehicles yaw dynamics (ie, the amount of tyre slip at each end of the car)

In order to control yaw, you don't control drive tyre TORQUE, you control tyre SLIP!


And to do that, INDEPENDENTLY of the road surface, you need to be able to vary the rotational speeds of the front and rear wheels separately. To do that, you need a mechanical device that can change the front to rear drive ratio, in effect, if it adds 1rpm to the rear prop, it removes 1rpm from the front prop etc. Luckily, such a device exists and every car already has one, it's called a differential drive!

The difference of course between a normal open cross axle diff, and one that can actively overspeed, is you need someway of slowing down one of the outputs, and that is generally through a set of clutch plates that "brake" one side of the outputs to ground (the casing). Generally, when used as an active centre differential, a bias-able epicyclic geartrain is used, where changing the speed of one of the rotating components (sun/planet/annulus etc) varys the effective drive ratio split without changing the overall transmission ratio.

(it's easier to think about normal cross axle diffs here, as people are more familiar with those. if you turn the crown wheel at 1rpm, both halfshafts turn at the same 1rpm, but grab one halfshaft at force it to stop, and the other halfshaft increases in speed to 2rpm in response)



So, all these "haldex" based systems that can move torque around are largely irrelevant for vehicle dynamics, because the tyre slip is still fixed by the road surface passing underneath!

anonymous-user

53 months

Wednesday 25th March 2015
quotequote all
xRIEx said:
Does every system have some amount of slip somewhere to allow for this?
SHORT ANSWER: YES


;-)

xRIEx

Original Poster:

8,180 posts

147 months

Wednesday 25th March 2015
quotequote all
Cheers, I think I understood most of that! thumbup

Just to make sure I understand 'overspeed', your example of on open diff with one braked halfshaft, crown wheel at 1rpm and unbraked halfshaft at 2rpm is a demonstration of overspeeding? Am I right in thinking that on a normal open diff with no braking facility, if a wheel slips and 'overspeeds', it's not actively overspeeding because the wheel has no traction and the torque is providing no drive?

anonymous-user

53 months

Wednesday 25th March 2015
quotequote all
xRIEx said:
Cheers, I think I understood most of that! thumbup

Just to make sure I understand 'overspeed', your example of on open diff with one braked halfshaft, crown wheel at 1rpm and unbraked halfshaft at 2rpm is a demonstration of overspeeding? Am I right in thinking that on a normal open diff with no braking facility, if a wheel slips and 'overspeeds', it's not actively overspeeding because the wheel has no traction and the torque is providing no drive?
Correct, in an OPEN cross axle diff, the overspeed is a result of tyre slip on the lightly loaded inside wheel, and because there is nothing to react the "excess" torque against, the outer tyres drive torque has to fall to the value set by the inner tyre. Hence, you use a limited slip diff to increase traction.
However, in an actively driven diff, clutch packs inside the unit react the excess torque (into the casing/mountings), effectively acting as brakes, so that the maximum torque that is transmitted is maintained

GavinPearson

5,715 posts

250 months

Thursday 26th March 2015
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xRIEx said:
I've been trying to get my head round various online articles on the matter, but I can't understand how a torque split of e.g. 45:55 F:R is achieved, or the components used to do it.

I know some systems use planetary gear sets, but whatever the system it seems to come out with the same result: differing torque means different rotational speeds (of prop shafts). How would that work? Surely if the front and rear diffs had different final drive ratios to bring the rotational speeds at the wheels back to the same figure (assuming same diameter wheels all round), that would also equalise the torque at the wheels?

Does every system have some amount of slip somewhere to allow for this?
A free body diagram of the epicyclic differential would show you how they send the torque to the wheels.

anonymous-user

53 months

Thursday 26th March 2015
quotequote all
GavinPearson said:
A free body diagram of the epicyclic differential would show you how they send the torque to the wheels. send most people to sleep
EFA ;-)




Mroad

829 posts

214 months

Saturday 28th March 2015
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The Integrale centre diff set-up shows it quite well (purely mechanical diff with front/rear viscous coupling):



Red is input from gearbox.
Yellow is front axle.
Blue is to the rear axle.

(this is an early Delta HF4WD diff with 56% front and 44% rear, later versions were rear bias)

The viscous coupling between front and rear is in the bottom right (not coloured). The coupling can be seen better here:




Edited by Mroad on Saturday 28th March 09:29

garagewidow

1,502 posts

169 months

Monday 30th March 2015
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interesting,

so that's like,for example a sierra diff with a viscous coupling working in reverse?

the gears for the shaft to the rear are not 1/1 ratio so must be reduced again to have parity with the front axle ratio?

if a front wheel slips the extra torque ratio is sent to the rear?