WRC now without the central differential. What changes?

[747heavy]

from an recent article in Racetech Magazine #121



As far as I understand it, the drivetrain/gearbox is based on S2000, just the engine is a 1.6 I4 turbo now. There is no center diff, and the front and rear LSD´s can only have mechanical locking (ramps/friction plates).
Visco couplings are not permitted.

So, I would like to think, that the Xtrac value of 50/50% still holds some weight,
put I can´t say 100% for sure.

There will be a convnetional sequential/ or H-pattern g/box - no pedal shift at the steering wheel.

And the handbrake cut off for the rear axle, is via an hydraulical activated clutch (like in the mid 90`s), very simple concept/system.

In the article it says that power of the new engines is ~50hp less then the current 2l turbos and torque is said to be at ~50% of the current engines.
The engine must have direct fuel injection
max. body width of the cars is 1820 mm.

[Arunas]

747, Thank you. Still sounds strange - transmission with any degree of freedom between front/rear. Handling must be awkward.

[Formula None]

As far as I understand it, the drivetrain/gearbox is based on S2000, just the engine is a 1.6 I4 turbo now. There is no center diff, and the front and rear LSD´s can only have mechanical locking (ramps/friction plates).
Visco couplings are not permitted.

So, I would like to think, that the Xtrac value of 50/50% still holds some weight,
put I can´t say 100% for sure.

So, its either an issue of semantics, wherein no center diff would mean around 50:50 torque distribution most of the time (before traction is lost at any other wheel) or there's a non-adjustable spec center diff of some sort. Am I thinking correctly?

In the article it says that power of the new engines is ~50hp less then the current 2l turbos and torque is said to be at ~50% of the current engines.
The engine must have direct fuel injection
max. body width of the cars is 1820 mm.

... ...

[747heavy]

This type of all wheel drive does not have a center differential - when all wheel drive is engaged, the front and rear driveshafts are mechanically connected and rotate at the same speed.

When a vehicle is turning, the front wheels travel a greater distance than the rear wheels.



Because this all wheel drive system does not have a center differential, the front wheels cannot go faster than the rear wheels.
This type of all wheel drive cannot be used on pavement.
Turning on pavement (even on a wet pavement) with all wheel drive engaged causes transmission windup and increases the chances of the transmission breakdown.
When all wheel drive is engaged, the vehicle heavily understeers and this can lead to an accident.

The all wheel drive mode shoud only be used on surfaces with low traction (mud, snow, ice, sand), for short periods, and at low speeds. In these conditions the transmission windup is eliminated by slipping of the wheels.

If they use a "solid/permanent" drive between front and rear axle, this could get interesting in tarmac rallies.

I don´t know what they use, but a possible solution would be a HOC (hang on clutch)
on one axle, which is basicly a "free wheel" mechanism (like you have on your push bike) allowing the front wheels to travel faster, then the rear wheels, but in this moment you would have 100% rear wheel drive.

When the rear axle looses traction (rpm in the drive line increases) and the front wheels get connected again.

This would make for an instant change between 100% rear and 50/50%, and the rear axle would need to loose traction for the front to engage.
This makes the car basicly a rear wheel drive car, unless the rear axle looses traction.
This would explain the comments of M.Wilson that the cars are a lot more sideways, because you basicly drive a RWD car when you start turning the steering wheel.

The other option would be a "clutch pack" where you turn any temporary difference in rpm into clutch slip. But there was no mentioning of such a system in the article.

[747heavy]

for the ones interested in the possible "clutch pack" connection between front and rear axle.

A similar system was used in the Toyota Celica ST165 Rally car in the early 90´s, it was using an Xtrac developed 4WD system which was quite uniqe at the time, making use of this pinciple.

Hydraulic clutch couplings

Again, not really a differential, but another type of device used in AWD cars to engage the rear differential. With these types of coupling, the front and rear differentials drive hydraulic pumps - normally filled with oil. Any difference in the speed of the two pumps causes a pressure imbalance in the system that activates a clutch pack in-line to the rear differential to engage it. So again, when the front wheels spin faster than the rear (meaning slip), the clutch pack is engaged and the rear differential comes into play. These types of coupling typically also have braking and thermal overrides so that if the gearbox oil in the rear differential becomes too hot, or the car is braking, the clutch pack can be overridden and disengaged (without this, ABS-equipped vehicles would not be able to sense all four wheels correctly under braking).

with such a system, the "cut off" for the handbrake is very easy to implement, in the Celica it was just a brake light switch on the handbrake lever, telling the system to disengage the rear axle/opening the clutch pack.

on the free wheel system, as used in early Land Rover(1948-1952) 4WD systems

The freewheel controlled drive from the gearbox to the front axle, which disengaged on the overrun.
This allowed the vehicle to have a permanent 4 wheel drive system by avoiding 'wind-up' forces in the transmission.
This system worked, but produced unpredictable handling, especially in slippery conditions

Sounds like they diged this one out again.
It´s as cheap and simple as it get´s for a 4WD system, and maybe adds to the show.

[Tim.Wright]

The no centre diff seems crazy but possible. V8 Supercars in Australia have no rear diff. I've heard its not uncommon for the rear axle to permanently twist up 45deg after use. Though in these cars they often have nearly 100% weight transfer mid corner so this helps take the load off the locked inside wheel.

For a rally car I imagine it will be quite a harsh environment for the drivetrain.

Tim

[747heavy]

Here the FIA sporting regs for the 2011 WRC.
No news on the tech. regs or gearbox/center diff homologation yet.

http://argent.fia.com/web/fia-public.ns ... 202011.pdf

64.7 LIMITATION OF TRANSMISSION & SPARE PARTS
64.7.1 For a Manufacturer or a WRC Team, a list of linked rallies intended to be run with the
same transmission and spare parts must be notified to the FIA prior to the first rally in which they will
take part as:
- 4 links of 2 rallies per car and 1 link of 3 rallies per car for a Manufacturer or a WRC team which
has nominated all Championship rallies.
- 4 links per car for a WRC Team which has nominated 10-12 rallies.
- 3 links per car for a WRC Team which has nominated 7, 8 or 9 rallies.
64.7.2 For rallies which are not linked, Articles 64.9.2 & 63.3 will apply.
64.8 TRANSMISSIONS
For Manufacturers and WRC Teams 2 gearbox and differential assemblies per car will be sealed on
the first rally of each link. Only these units must be used on the linked rallies. If 2 gearbox and
differential assemblies have been used on the first rally of a link, only once per season, 2 different
gearbox and differential assemblies may be sealed without incurring a penalty.