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  1. #231
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    Quote Originally Posted by NickRally View Post
    then maybe the centre diff on the Fiesta is not of the usual epicyclic gear type with clutch pack to vary the degree of locking, but maybe it uses two clutch packs each controlling the flow of torque to the front and rear axles. This would permit infinite degree of variability in the proportion of torque send to the front and rear axles. I am not sure how reliable such device on a current WRC car will be, but to the best of my knowledge it is used on the current Focus RS to distribute the torque between the two rear wheels in place of the usual epicyclic type rear differential.
    I don't think that is any chance to have just 2 clutch packs and not epicyclic gear type center diff.Imho such application will not be reliable and capable at wrc level,its almost not reliable at street cars(focus rs)

  2. #232
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    Quote Originally Posted by Kris82 View Post
    The way I understand it the active center differential can be either open or locked and the state is hydraulically controlled with help of electronics. ?
    no they are not just ''opened'' or ''locked''
    they can be opened.locked,and semilocked at countless theoretically positions depenting if you are at a corner,at brakes,if you are with steering at straight or some degres left/right ,the gas pedal position etc.

  3. Likes: A FONDO (21st April 2017)
  4. #233
    Senior Member SubaruNorway's Avatar
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    Doesn't seem like the Focus RS is a car you would use on an ice track all day, some diff problems in this video from Team O'Neil.
    https://www.youtube.com/watch?v=F33BQEXBvok
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  5. #234
    Senior Member Fast Eddie WRC's Avatar
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    Ostberg was saying that although he has previous experience driving active-diff cars, this has not really helped him with the 2017 Fiesta as it a brand new car and there are so many variables to adjust.

    He confirmed his car can be 'faster' with the centre diff locked (with better traction), but this also makes the Fiesta more difficult to drive especially setting it up for corners. His team are keeping his car set-up as 'simple' as possible as its impossible to adjust too many settings and know which is actually improving the car's performance.
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  6. #235
    Senior Member itix's Avatar
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    The center differential very likely has a clutch that operates with hydraulic pressure that locks the differential. That means that when there is no hydraulic pressure, the power is sent to where there is the least grip. As the car accelerate, the front lift and the power spins away on the front wheels. That means only the rear wheels have grip which is probably why he had to drive it like a rear wheel drive car.

    That's my take on the dynamics anyway.

  7. #236
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    Quote Originally Posted by dimviii View Post
    no they are not just ''opened'' or ''locked''
    they can be opened.locked,and semilocked at countless theoretically positions depending if you are at a corner,at brakes,if you are with steering at straight or some degres left/right ,the gas pedal position etc.
    I know there is open and locked and everything in between. I hadn't said it clearly, or you overanalyzed what I had said.

    Quote Originally Posted by itix View Post
    The center differential very likely has a clutch that operates with hydraulic pressure that locks the differential. That means that when there is no hydraulic pressure, the power is sent to where there is the least grip. As the car accelerate, the front lift and the power spins away on the front wheels. That means only the rear wheels have grip which is probably why he had to drive it like a rear wheel drive car.

    That's my take on the dynamics anyway.
    That's what I was leaning towards as well. I was just confused by what Ogier and one of the commentators before him (if I remember correctly) had said during the coverage.

  8. #237
    Armchair General Mirek's Avatar
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    Quote Originally Posted by Fast Eddie WRC View Post
    Ostberg was saying that although he has previous experience driving active-diff cars, this has not really helped him with the 2017 Fiesta as it a brand new car and there are so many variables to adjust.

    He confirmed his car can be 'faster' with the centre diff locked (with better traction), but this also makes the Fiesta more difficult to drive especially setting it up for corners. His team are keeping his car set-up as 'simple' as possible as its impossible to adjust too many settings and know which is actually improving the car's performance.
    Locked center diff = natural understeering, i.e. slower car everywhere except full acceleration on straight and a car which needs to be driven more sideways to turn.

    Quote Originally Posted by itix View Post
    The center differential very likely has a clutch that operates with hydraulic pressure that locks the differential. That means that when there is no hydraulic pressure, the power is sent to where there is the least grip. As the car accelerate, the front lift and the power spins away on the front wheels. That means only the rear wheels have grip which is probably why he had to drive it like a rear wheel drive car.

    That's my take on the dynamics anyway.
    No expert here but that makes little sense to me. With opened differential You can't have more torque on one side than on the other. The side with less grip determins the torque going to the other, i.e. the lesser grip on one side the lesser torque the whole system brings but the split ratio is not changed. In extremum when You lift one complete axle it spins but the other stands still as there is no torque on the other side except mechanical friction.

    Correct me if I'm wrong.
    Last edited by Mirek; 20th April 2017 at 22:52.
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  9. #238
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    You are spot on Mirek. I think that is what itix meant but you described it better. The thing is that in that situation the front would spin out hence making it feel more like a front wheel drive car. I guess in the end the car feel would depend on how the front and rear diffs were set up.
    Last edited by Kris82; 21st April 2017 at 00:29.

  10. #239
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    I like Focus RS system, because it can do centre and rear diff work only with two clutches (very good price performance solution). But it is actually a FWD car meant to be pushed just here and there for a little while. Still it cant do more than locked diff...

    More options has EVO X (if I am not wrong) rear diff, that can accelerete one wheel over the speed of locked diff. But I see no need for such system in a WRC, because it is just a compensation for poor drive skills (it make turn car faster than driver can perform by his own pedal/steering work).

    In WRC I think there is no more than a diff with clutch, that can be opened locked or everything in between. It means that it performs at equal for both axles (open) or equal speed (if no gearing is involved) for both axles (locked) or in between.

    Regarding Ogires RWD feel... I think it can happen if he had a front diff problem, so that front diff remains open. It means that front axle had just as much torque transfer as the "less grip wheel" allows i.e. less than usual. It means that most of the torque went trough rear wheels.

  11. #240
    Senior Member NickRally's Avatar
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    You wouldn’t be able to send more torque to the rear wheels (relative to the front) with an open centre diff as explained by Mirek, the only difference opened or locked front diff will make is to provide “the least common denominator” for the whole transmission system, i.e. the path of least resistance or in other words the max torque that can be transmitted through the system.
    Looking at it from another angle (which might be what you meant), the combination of open centre diff, open front diff and some state of locking on the rear diff is likely to make the car more stable, i.e. give degree of understeer in general. Whether such comination will make the driver think or feel he has something akin to a rear wheel drive car, don’t really know, might be possible.

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