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  1. #261
    Senior Member jparker's Avatar
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    Very well explained article about Active Central Diffs.
    https://www.full-race.com/store/driv...roller-1.html/

  2. #262
    Senior Member NickRally's Avatar
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    Thanks jparker - interesting reference in the link above about Nissan’s and Subaru’s differential systems.

    Also while browsing the net for WRC transmission information, I stumbled across this not very new link, which I am sure it must have already been posted around here, but I will do it for reference, containing interesting info about the diff settings of the previous generation active diff cars:

    http://wrcbehindthestages.blogspot.c...ravaganza.html

    Also pondering few points about differentials below:

    1. WRC cars up to and including 2016 – no centre diff, i.e. spool – the rules read:

    Front/central/rear differentials
    Only the housings and mechanical limited slip differentials
    homologated in the WRC extension may be used (without any
    modification).
    "Mechanical limited slip differential" means any system which
    works purely mechanically, i.e. without the help of a hydraulic or
    electric system.
    A viscous clutch is not considered to be a mechanical system.
    Any differential with electronic management is prohibited. The
    number and the type of the plates are free.

    The significance is that the rules specifically mention mechanically locking central differentials as being allowed, but the common knowledge suggests they were not utilised, i.e. the competitors decided it is better to have a spool (locked front and rear transmission paths) rather than mechanically locking central differential.

    2. Torque split of a spool often given as 50:50 – to state the obvious, this is of course seldom correct. It might apply for near perfect conditions where all the gearing is the same, tyre dynamic dia’s are identical and none of the tyres are slipping, otherwise greater proportion of the torque will flow towards the axle/wheel with greater grip/resistance.

    3. I was thinking how would I approach the challenge if I was given the task of designing an independent clutch packs type central diff, what would it take? And then I thought the chances for this working on a current rally car are probably greater than implementing it on a road car. A current rally car has got to cover, let’s say, 350 competitive km per event, which is equal or less than F1 race + Qualy + FP3, which in turn is what the transmission there has got to cover in a race weekend – now, before anybody objects saying the transmission stresses in F1 are lower, I am pretty confident it will be the opposite due to the higher engine power coupled with the enormous downforce levels - the stresses on a F1 differential would be higher than WRC car central differential (which also does not see all the gearing of the complete WRC car transmission). With that in mind, in my opinion, to design a reliable independent clutch packs only central diff, will take proper sizing and material selection for the discs as well as sufficient cooling and top that up with plenty of hours on the transmission dyno and the test course. It is maybe unlikely that current WRC teams’ budgets stretch that much, but it is good to dream.

    4. Continuing from point 3 above, I suspect the biggest hurdle in making an independent clutch packs central differential work is the actual control strategy – in what manner do you actually use such device. There is a lot of experience in making hydraulically operated epicyclic diff work, but hardly any when it comes to making the clutch packs one working. I suspect the number of options going from the normal hydraulic epicyclic diff to the clutch pack one will increase exponentially. So although the clutch packs will give you way more opportunities, they will be easier to become lost in what you are doing, which means the epicyclic diff will be much easier to work with.

  3. #263
    Senior Member NickRally's Avatar
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    Continuing my “rant” about differentials, I had a chat with one of my colleagues at work (he is a very common sense person) and after some discussion, we thought the most likely reason for Ogier’s complaints that he ended up with a rear wheel drive car after the hydraulic issues is that the Fiesta had an asymmetric central epicyclic diff, which when in its free state sends more torque to the rear than the front (which of course is not uncommon). As the locking system fails, it will give the driver the feeling of reduced front traction/stability and, talking from experience, very often that would be enough for a driver to proclaim “no front traction at all” or “rear wheel drive only”, although in my field of activity that would sound more like “terrible, no front grip whatsoever” or “horrific, no rear grip at all”, even though the reduction in front or rear grip is actually a very small amount.

  4. #264
    Senior Member Mirek's Avatar
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    Nick, the pre-2017 WRC rules are basically nothing more than slightly modified S2000 rules. Those were introduced around 2005. The very first S2000 actually had clutchpack-type center differential (Toyota Corolla (RSA), VW Polo (RSA) and Fiat Punto). Peugeot in 2007 was the first one to bring transmission without center diff at all. I believe these were the main reason why not to use it:
    - for naturally aspirated S2000 engine the power lost in the center diff was not negligible
    - such simple clutchpack brought no significant advantage in handling performance over the variant without center diff at all
    - it was an additional weight; in that time the weight limit for S2000 cars was 1100 kg and I know from Barum scrutineering that the limit was totally not achievable with the given rules; i.e. the less weight the better

    All other following cars had no center diff at all even though for example Škoda used both variants in testing for comparison. Later even Fiat changed transmission in the Punto for one without center diff.

    Honestly I thought that starting with the new WRC rules (2011) the center diff was explicitly banned.
    Stupid is as stupid does. Forrest Gump

  5. #265
    Senior Member NickRally's Avatar
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    Thanks for the additional info Mirek, these reasons make sense.

  6. #266
    Senior Member OldF's Avatar
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    In the past a centre diff with asymmetric torque split was in use. The example is from “few” years back in the 1986 Portugal rally which had both tarmac and gravel stages.
    The Lancias had a front/rear torque split 25/75 on gravel and 30/70 on tarmac.
    The torque split for the Audis was 40/60 all the time.
    Malcom Wilson had with the MG Metro a torque split of 35/65 on tarmac and 45/55 on gravel. The other Metro drivers had 25/75 on tarmac and 35/65 on gravel.
    The Fords had a torque of split 37/63 all the time.

    The active front and rear diffs was banned from the beginning of 2006. If I recall correctly there was also talk about banning the active central diff but at least the drivers wanted to preserve it because it enabled the use of the handbrake. https://en.wikipedia.org/wiki/World_Rally_Car

    The rule changes from season to season can be followed on http://juwra.com/seasons.html
    The ban of the front and rear diffs is not on Juwra’s 2006 rule changes http://juwra.com/rules_2006.html
    Jonkka must have missed it for some reason.

    All the manufacturers attending the 1995 wrc season had an active centre diff by the end of the year.
    “Don’t eat the yellow snow” Frank Zappa

  7. #267
    Senior Member OldF's Avatar
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    Quote Originally Posted by NickRally View Post
    I was thinking how would I approach the challenge if I was given the task of designing an independent clutch packs type central diff, what would it take?
    If you mean an independent clutch pack for both front and rear at least the other one have to be normally locked and released by the hydraulic system.
    “Don’t eat the yellow snow” Frank Zappa

  8. #268
    Senior Member OldF's Avatar
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    “Don’t eat the yellow snow” Frank Zappa

  9. #269
    Senior Member Mirek's Avatar
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    Quote Originally Posted by OldF View Post
    The Fords had a torque of split 37/63 all the time.
    I believe the RS200 could have been run as RWD only and such configuration was used on asphalt stages. 4WD of that time were rather clumsy on asphalt especially due to very bad turning ratio (caused by angular limitation of driveshaft joints I believe) combined with no rear axle releasing mechanism. I may be wrong though, I don't remember it very well.

    By the way C4 WRC also did not have even torque split. Asphalt center diff was 40/60 if I recall. That was often mentioned as a reason why the car was tricky for privateers and gentleman drivers in the later years and why many used gravel center differential even on asphalt.

    This article states that Escort gr.A (prior active diff) could have 30/70 torque settings as well: http://tech-racingcars.wikidot.com/f...-v-rs-cosworth

    There are some interesting links inside but they open in Greek language. I don't know if it's possible to switch them in English somehow.

    Quote Originally Posted by OldF View Post
    The active front and rear diffs was banned from the beginning of 2006. If I recall correctly there was also talk about banning the active central diff but at least the drivers wanted to preserve it because it enabled the use of the handbrake.
    Development of S2000 cars started around 2004-2005 and those were equiped by the simple hydraulic clutch to release the rear axle with handbrake therefore the solution of the issue was probably already known. I believe the real reason was that people were sane enough not to design all new gearbox, center diff and rear axle releasing mechanism in the name of cost saving.
    Stupid is as stupid does. Forrest Gump

  10. #270
    Senior Member NickRally's Avatar
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    OK, let’s attempt to answer Mirek’s question on the tunnels/Venturis ahead of the wheels. In open wheel racing, traditionally a wing or a tunnel ahead of a wheel does two main things – first of course it retains its main function of creating downforce and second, by placing it ahead of the wheel, in effect it partially masks the wheel reducing its drag or more appropriately the drag of the overall wing/tunnel and wheel system is less when compared to the wing/tunnel and wheel on their own. This gives slightly better lift to drag ratio.
    It is important to mention that wing on its own in free air, both upstream and downstream, will create slightly more lift/downforce for the same angle of incidence than when placed ahead of a wheel, but the combination of a wing ahead of a wheel appears to create a very good overall lift to drag number, i.e. more efficient system overall.
    Now, switching to closed wheel cars, there are other dimensions to consider – the air from the tunnel (I will switch to talking about tunnels only as this is the predominant option on a closed wheel car) must be allowed to go somewhere, otherwise imagine if you place a flat plate at the back of the tunnel and block it, it will simply stop working, and as such the wheel arch provides a nice open-ish space for this. There might be better areas to expel the tunnel air, but the rules rarely allow much freedom for tunnel exits in close wheel racing. Expelling the tunnel air in the wheel arch, in turn, can benefit massively from providing good air evacuation from the wheel arch itself usually using louvers on the outside of the arch and of course through the large natural opening on the side of the wheel.
    The best example of front tunnels are the current WEC sports prototypes, which are allowed to exit the air from the large front diffusers behind the front wheels in a very well faired and channelled manner.
    Going back to the current generation of WRC cars, from the ones that so far appear to have tunnels at the front, Toyota and Citroen, the Toyota’s appear just inside of the wheels, but I need to revisit the images to see where they expel the air, while the Citroen’s are possibly just inside the front wheels but expelling the air in the wheel wells with nice louvers on top of the arches.
    One think that I am not sure is how a front tunnel works in combination with a front splitter vertical skirt, which is present on all current generation WRC cars.
    Last edited by NickRally; 30th April 2017 at 01:18.

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