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I will try to explain it again, what I think.
When accelerating from a corner you have: open front diff (defective locking system), locked centre diff (works OK), locked rear diff. (works OK).
In that situation Front and rear axle spin at same speed (locked c. diff) - rear axle works OK, - front inside wheel spins (because it is unweighted) outside rear wheel transfers same amount of torque (open f. diff) as inside one (very little or zero, if wheel is lifted)... effectively you have RWD.
I see, thanks for clarifying J_n_z – yes, if the problem was on the front diff only, then this will be possible.
Question is, does rally Corsica has so many sharp corners to cause front insight wheel to spin?
I think that it can happen in all corners not only in tight ones, it can happen also when cutting corner over grass, or it is half dirty... It also can become difficult to save oversterings.
Definitely WRC car need some locking in f. diff.
guys you must be a bit confused,front diff at wrc cars have no hydraulics at all.They are plated diffs.
Focus rs 4wd system is a simple and cheap way to have 4wdrive. There is no comparison to active center diff from Mitsubishi evos various editions,which is a proper active diff.
no its not like that.Accelerating from corners you don't have an opened front diff,neither locked rear diff.Quote:
Originally Posted by J_n_z
The point was IF it was defective, it could act as an open diff. All speculation, as we really don't know at the moment.Quote:
Originally Posted by dimviii
according to Msport they had problems with hydraulics.Quote:
Originally Posted by Ctesibios
Front diff has no hydraylics at all,thats why I said that some are confused.
Anybody know the list of all devices that use hydraulics in the current, 2017, WRC cars?
Thanks dimviii, just from curiosity point of view - do we know this for certain (about the mechanical front diff on the current WRC cars) and if yes, one more question – how do we know this :)?Quote:
Originally Posted by dimviii
they cant use something else,they have to use mechanical diffs,as they are not allowed active diffs, at front/rear.Quote:
Originally Posted by NickRally
Hm, I went through the tech rules and couldn’t see a restriction in terms of the operation (i.e. mechanical/hydraulic) of the front and rear diffs.Quote:
Originally Posted by dimviii
@ dimviii: I know that f.& r. diff are not active and almost certainly have a mechanical locking system...
By (locked diff.) I ment somehow functional mechanical LSD that probably perform quite a good lock at full throttle.
I my thinking is based on Ogiers RWD statement... but I don't know all details that was said, so it may be that I am completely wrong.
PS. As far as I can understand Focus RS system can simulate centre an rear active diff. quite good, but not for very long time of real pushing.
Gearbox, center differential and handbrake (different circuit I think).Quote:
Originally Posted by Ctesibios
Frankly I haven't chacked the rules myself but afaik all the official announcements before the introduction of 2017 cars were only about center differential.Quote:
Originally Posted by NickRally
Mirek, gearbox hydraulics are for shifting only? I am guessing for the center diff it's for the"actuators" however they are made according to each manufacturer. And for the handbrake, is it for disengaging the rear wheels from the drivetrain or some kind of power assist?
dimvii is right these new cars have mechanical front and rear limited slip differentials (same as the 2011-2016 spec). Mechanical meaning that their state changes mechanically while accelerating, breaking and coasting, although preload is what determines their behavior while coasting. The only differential controlled by a computer via a hydraulic system is the center differential, hence its name "active".
I don't think there is a rear wheel disconnect on these new models. While a lack of a center diff warranted a use of a clutch pack disconnecting the rear axle to allow handbrake use, the cars with an active center diff. set the differential to an open position while the handbrake is used allowing for both front and rear axles to move independently.Quote:
Originally Posted by Ctesibios
Agreed with all the statements about the active centre and passive front and rear diffs, just wondering how FIA controls this, but as it isn't in the main rules (or maybe I have missed it), it must be through one of the numerous Technical Directives that get distributed among the competitors, but are not publicly available.
4. Citroen C3
Time for the last remaining team. I have to say before the season started, I, as many other people, thought this is the car to beat and dominate the championship. After four rallies, we can (luckily) say that this is not quite the case, with all four cars capable of winning and keeping us entertained. At the same time, I am still very impressed with the attention to detail in the C3. After four rallies, if I didn’t know any better, I would have said they have the most mature design overall without necessarily excelling in any one particular area.
Below are few images with notes.
http://www.geocities.ws/rallytech/ae.../C3_aero_1.jpg
http://www.geocities.ws/rallytech/ae.../C3_aero_2.jpg
http://www.geocities.ws/rallytech/ae.../C3_aero_3.jpg
http://www.geocities.ws/rallytech/ae.../C3_aero_4.jpg
http://www.geocities.ws/rallytech/ae.../C3_aero_5.jpg
http://www.geocities.ws/rallytech/ae.../C3_aero_6.jpg
Nick, what is the effect of the air flow hitting the rotating wheel? I mean with a difusor ahead of the wheel the air coming on the wheel is slower and differently spread than without it. I don't know anything about this phenomena. Is it even being considered in the design of these small difusors?
That's a very good question Mirek, I will sit down and write something next week (busy weekend).
Very well explained article about Active Central Diffs.
https://www.full-race.com/store/driv...roller-1.html/
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.
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.
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.
Thanks for the additional info Mirek, these reasons make sense.
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.
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.Quote:
Originally Posted by NickRally
Basics of diffusers.
http://www.racecar-engineering.com/t...-aerodynamics/
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.Quote:
Originally Posted by OldF
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.
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.Quote:
Originally Posted by OldF
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.
Even looking at the era I like the best, the Group A days between say 88 to 94 its seems that everybody just made all the diff stiffer than hell..Front , center, rear, crazy stiff.. Now add wide tires and tight narrow tarmac roads and you have a car horrible to try and do tight corners...Quote:
Originally Posted by OldF
There there was toque split on those cars...who know but I DO know that many in the Sierra/Sapphire/Escort Cossies did go to 50/50 and to quote one multiple BTRDA Gold Star winner, Pete Doughty, "It handled like a pig, like the old Quattros but 50/50 was the fastest so that is what you run"
Jerónimo Aliaga*@jerowrc
Diferencias aerodinamicas ente @TOYOTA_GR y @HMSGOfficial en la previa del #WRC #RallyArgentina2017
https://pbs.twimg.com/media/C-WJyrmXcAAFyZx.jpg
https://pbs.twimg.com/media/C-WJyr_XoAAU4LB.jpg
https://pbs.twimg.com/media/C-WJyrqXUAgPczV.jpg
https://pbs.twimg.com/media/C-WJyrpWAAAhaZS.jpg
Well I can see the Toyota "pods" are wider at the bottom, but can't see any change to Hyundai at first sight.
Watching the redbull.tv rally Argentina day 1 recap where, as usual, they start by talking about the previous rally, Corsica in this case, and at 3:23-3:33min from the start of the program Ogier talks about his problems, though I don't know what day of the rally that applies for (I am yet to watch the wrc+ daily highlights from Corsica) - anyway, when talking about his problems he says "diff was locked" and "no hand brake". The significance of this is that if the issue was lost hydraulic pressure, then it means the diff is normally locked when no hydraulic pressure is applied to it. This is possible though I haven't got experience with such arrangement. I am more familiar with the usual normally non-locked diffs that need hydraulic pressure to lock them.
In my view though, it is more likely that they use the normally unlocked diff and the problem would have been in the control electronics, that must have necessitated to manually force the system into a locked state or the control mechanism reverts by default to locked diff state when issues are encountered. The locked diff would then of course explain the lack of hand brake.
The issue above would be the exact opposite of having rear wheel drive only car, but there might have been different problems each day, though the handling of the car from the first Sunday stage on TV appeared to match a car with locked centre diff and no hand brake (overshooting hairpins).
And now Ostberg - no centre diff and no hand brake
Hey NickRally, I enjoy reading your analysis on aero, it's always good to hear an outside opinion :) Keep it coming!
I think every team did well to hide the extra girth required for the new side impact regulation. Only when you see one from this angle that you realise they are basically pushed out to the regulation width at the waist.
Attachment 1325
Very interesting to read through and try to follow.
Watching the WRC show with a couple of rear diffusers being lost in water splashes, the footage showed it as a significant loss of rear traction with spins in tight corners. Is the down force at slower speeds really likely to be that affected or is that just the footage that was available?
Sorry for the poor grammar, hopefully it makes sense.
Thanks Mrpengski, I absolutely love the appearance of these new cars.
Hi fecksfx – I doubt the contribution of the rear diffuser to rear/overall grip at low speeds is significant enough to be the cause of the spins you mention. Let’s make an estimate (probably way too optimistic) for the total downforce in gravel trim as 200kg at 150kph and let’s say the contribution of the diffuser to this number is 30 percent (again probably way too optimistic considering the gravel trim ride heights) - this would represent 60kg at 150kph. Now if we reduce the speed down to say 50kph, then the contribution of the rear diffuser will be [(50/150)^2]*60 which is 6.67kg – figure low enough to be lost in the overall noise of the “experiment”. If you add to this that the cars will be very much sideways in such corners (i.e. significant yaw angle), then the contribution of all aero devices will drop even further, though some teams appear to have paid more attention to downforce creation in yaw than other teams. And to add even further point here, the Fiesta diffuser appears to be the most benign of the lot, which means if lost will have less of an effect on downforce. All three other teams have more aggressive diffusers and I suspect they will feel it a bit more if they lose it, but again not at these low speeds. Nevertheless the Fiesta diffuser attachments appear to be a bit fragile with all these instances of broken ones – I am sure the M-Sport guys will be discussing how to improve the attachments in their faults meetings.
I suspect the most significant part that a missing diffuser plays, is in the head of the driver, which in itself is of course a problem.
Quote:
Originally Posted by Mrpengski
This is actually the most important change of the 17 rally cars, and also the most road car relevant. It should actually be used in road cars as soon as possible, because as it is for a rally car, side impact is the weakest link also for road cars.
I don't think that such large safety side zones are realistically possible for road cars. It would add a lot of cost and reduce practical use (or internal space). For example imagine that You add 100 mm on door thickness. Now You park at a mall. Your car is wider, i.e. You have less space around and moreover You need to open the door 100 mm more to be able to get outside. My car is not that big (Octavia III) but very often I can barely get out of it when parking in the city. Another problematic thing is that especially for women the wider and heavier door would be likely difficult to manipulate (especially when You don't park on a leveled ground but on the slope), maybe they would require electric motors, i.e. more cost and more potential issues after crashes.