Discussion
Can anyone tell me how the power gets split between the front and rear wheels on a 4 wheel drive sports car. I remember seeing somewhere that on a skyline the rear wheels drive most of the time but power is fed forward when needed. How can the power slit not be equal between front and rear wheels?
PS sorry not really an engine thing but its not suspension or body either
PS sorry not really an engine thing but its not suspension or body either
Depends on the system, most modern systems use a complex series of electronically controlled clutches to regulate the power to the appropriate wheel.
Although watching 5th gear on monday, apparently the new fiat panda 4x4 uses a mechanical friction plate that expands when the front wheels accelerate too quickly and then transfers some of the power to the rear wheels.
Although watching 5th gear on monday, apparently the new fiat panda 4x4 uses a mechanical friction plate that expands when the front wheels accelerate too quickly and then transfers some of the power to the rear wheels.
My previous car (Skoda Octavia 4x4) had an electronically controlled system (VW 4-motion) that senses wheelspin at the front wheels and couples the driveshaft to the rear wheels. You could feel it when it engaged. My current car (Audi A6 quattro) has permanent 4wd, I think it has Torsen diffs that limit wheelspin.
Some cars have a fixed split, with the power being divided front rear in set amounts.
Some cars have a fixed split, with the power being divided front rear in set amounts.
There are also "viscous couplers" they use a fluid,heat and a series of metal plates with holes in them. The heat caused by wheel slippage causes the power to be redirected to wheels that have traction.
[url]http://auto.howstuffworks.com/differential9.htm[/url]
Viscous couplers are the most common IIRC.
[url]http://auto.howstuffworks.com/differential9.htm[/url]
Viscous couplers are the most common IIRC.
[quote=annodomini2]Depends on the system, most modern systems use a complex series of electronically controlled clutches to regulate the power to the appropriate wheel.
[quote]
perhaps in some exotic machinery, not in the majority of road cars.
Most simply use a Viscous centre diff and rear diff, with an open front diff.
Cheap and nasty, but it works fairly well. Some lucky ones may get a plated diff, or a torsen to replace the VC units.
Some do have electronic control to a degree, but very few.
[quote]
perhaps in some exotic machinery, not in the majority of road cars.
Most simply use a Viscous centre diff and rear diff, with an open front diff.
Cheap and nasty, but it works fairly well. Some lucky ones may get a plated diff, or a torsen to replace the VC units.
Some do have electronic control to a degree, but very few.
On a decent car with permanent AWD you need a centre differential. That means that rear and front wheels get power all the time no matter what. Examples are Subaru WRX, Range Rover, Sierra 4x4, Lancia Integrale, Mitsu Evo 8, Jaguar X-Type, Audi A4, A6 & A8 quattro. In many cases they also incorporate a slip limiting device such as a torsen, plate or viscous coupling diff, but these don't drive, just limit between the two outputs of the diff.
Some newer cars have partial AWD that has primary wheels driven with secondary driven by means of a clutch set or coupling. There are two sets - RWD with front actuation optional e.g. BMW X3, and FWD with rear actuation e.g Volvo AWD, Audi A3 and TT quattros. Here the clutch is 'intelligently controlled' i.e. commanded when front wheel speed is in excess of the rears or vice versa by a certain amount. The issue is that the clutch has a certain capacity, e.g. 500 Nm which means that the clutch can never send more than that to that axle. The clutch is wet, and the fluid is temperature controlled to protect the diff, so when it gets to a high temp it goes to open mode, i.e. back to 2WD.
The crudest system is a partial AWD with a viscous coupling, so it doesn't need a centre differential, just a power take off, the coupling to the secondary wheels then sends some torque back in a 'crisis'. Basically it's fit only for cheap 4WD vehicles like Fiat Pandas where handing is not a must, merely the ability to get from A to B in all weathers.
So which is better?
Basically the partial system is reactionary, it waits until the problem happens and then channels power to the other wheels, and is not able to get a lot of power to the other wheels in first gear on a slippery surface when you want to get going because the clutch has a limited capacity. Electronics mask the effect and flatter the untalented but as most have never driven at the limit they will never know. It's a cheap system that's easy to Engineer but it's not that good.
The best is full time AWD for a road car. In addition the bteer systems will have an epicyclic diff to split torque to give superb handling to the limit by optimising the torque distribution to the tyres. Better still there will be some slip limiting, preferably on all three drive axles, usually viscous, torsen or plate diff for cost reasons.
The ultimate system would have active traction and yaw by having plate (ie. clutch) diffs that are electrohydraulically controlled that can limit slip considerably in off the line situations, then open the diffs for tight corners eliminating understeer, then clamp them under power, then use locking motions to channel torque to non-slipping wheels under power to maximise handling and cornering under power. It would work similarly to the way an actve ABS system does on upmarket cars, except rather than slow the car down to resolve the 'problem' it can do it under full power. Very useful on a rally car.
Some newer cars have partial AWD that has primary wheels driven with secondary driven by means of a clutch set or coupling. There are two sets - RWD with front actuation optional e.g. BMW X3, and FWD with rear actuation e.g Volvo AWD, Audi A3 and TT quattros. Here the clutch is 'intelligently controlled' i.e. commanded when front wheel speed is in excess of the rears or vice versa by a certain amount. The issue is that the clutch has a certain capacity, e.g. 500 Nm which means that the clutch can never send more than that to that axle. The clutch is wet, and the fluid is temperature controlled to protect the diff, so when it gets to a high temp it goes to open mode, i.e. back to 2WD.
The crudest system is a partial AWD with a viscous coupling, so it doesn't need a centre differential, just a power take off, the coupling to the secondary wheels then sends some torque back in a 'crisis'. Basically it's fit only for cheap 4WD vehicles like Fiat Pandas where handing is not a must, merely the ability to get from A to B in all weathers.
So which is better?
Basically the partial system is reactionary, it waits until the problem happens and then channels power to the other wheels, and is not able to get a lot of power to the other wheels in first gear on a slippery surface when you want to get going because the clutch has a limited capacity. Electronics mask the effect and flatter the untalented but as most have never driven at the limit they will never know. It's a cheap system that's easy to Engineer but it's not that good.
The best is full time AWD for a road car. In addition the bteer systems will have an epicyclic diff to split torque to give superb handling to the limit by optimising the torque distribution to the tyres. Better still there will be some slip limiting, preferably on all three drive axles, usually viscous, torsen or plate diff for cost reasons.
The ultimate system would have active traction and yaw by having plate (ie. clutch) diffs that are electrohydraulically controlled that can limit slip considerably in off the line situations, then open the diffs for tight corners eliminating understeer, then clamp them under power, then use locking motions to channel torque to non-slipping wheels under power to maximise handling and cornering under power. It would work similarly to the way an actve ABS system does on upmarket cars, except rather than slow the car down to resolve the 'problem' it can do it under full power. Very useful on a rally car.
Does anyone know of a JCB like vehicle with dynamic power distribution, among the wheels?
I know they're not exactly "performance cars", but since they already have the hydraulic infrastructure, it'd be pretty easy to add a computer to do the sums and deliver the power appropriately.
I know they're not exactly "performance cars", but since they already have the hydraulic infrastructure, it'd be pretty easy to add a computer to do the sums and deliver the power appropriately.
GavinPearson said:
On a decent car with permanent AWD you need a centre differential. That means that rear and front wheels get power all the time no matter what. Examples are Subaru WRX, Range Rover, Sierra 4x4, Lancia Integrale, Mitsu Evo 8, Jaguar X-Type, Audi A4, A6 & A8 quattro.
You sure about the Sierra 4x4? I think they had a cheap and nasty viscous coupling system; I remember thinking Ford were cheapskates for avoiding a proper centre diff when they first came out.
Most types use a VC centre diff.
I think I read that some of the VW 4-motion had open diffs all round, relying on the intelligent braking system to solve any traction loss.
If one wheel lost traction and began to spin, brakes were applied, causing drive to be retained at the other wheels.
I think I read that some of the VW 4-motion had open diffs all round, relying on the intelligent braking system to solve any traction loss.
If one wheel lost traction and began to spin, brakes were applied, causing drive to be retained at the other wheels.
stevieturbo said:I think some of the Uber Porsche use that system.
Most types use a VC centre diff.
I think I read that some of the VW 4-motion had open diffs all round, relying on the intelligent braking system to solve any traction loss.
If one wheel lost traction and began to spin, brakes were applied, causing drive to be retained at the other wheels.
Pigeon said:
On a decent car with permanent AWD you need a centre differential. That means that rear and front wheels get power all the time no matter what. Examples are Subaru WRX, Range Rover, Sierra 4x4, Lancia Integrale, Mitsu Evo 8, Jaguar X-Type, Audi A4, A6 & A8 quattro.
You sure about the Sierra 4x4? I think they had a cheap and nasty viscous coupling system; I remember thinking Ford were cheapskates for avoiding a proper centre diff when they first came out.
No, it's got a torque splitting epicyclic centre diff, that has a viscous coupling slip limiting function.
IMHO it's infinitely better than anything Quaife makes.
Yep, it's a pretty good design. A 1/3:2/3 Front:Rear split that gradually tightens up as one of the axles slips.
I'm sure that, for racing, a configurable pre-loaded one is better, and for mud plugging a fully lockable diff is better, but for "making progress" around wet UK roads or towing a horsebox/caravan/trailer/boat across grassy muddy fields then it's a good design.
I'm sure that, for racing, a configurable pre-loaded one is better, and for mud plugging a fully lockable diff is better, but for "making progress" around wet UK roads or towing a horsebox/caravan/trailer/boat across grassy muddy fields then it's a good design.
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