Product Description
As every sports car with front wheel drive, Abarth 500 (or Abarth 595) may suffer low traction, due to the high torque unloading to tyres.
This situation usually happens in corner exit, where internal wheel is partially unloaded by current weight because of centrifugal load drive; so, one wheel is able to unload to the ground an amount of torque far smaller than the other one.
Note that a free differential is characterised by the fact that it always divides the torque in two identical shares; so the maximum torque which can be unloaded to the ground is conditioned by the grip of the less loaded wheel.
Abarth 500-595 is normally equipped with an electronic device called TTC, which SIMULATES a self-locking differential. When a drive wheel loses grip, ECU gives an electric pulse to brake caliper of the slipping wheel; the negative torque added to the wheel allows to mislead the normal (“open”) differential, leading an identical torque amount to higher grip wheel.
TTC is a considerable and safe system, that permits a good control of the vehicle, but – we insist – it’s not a self-locking differential and it doesn’t guarantee the performances which only a well-made self-locking differential can give.
If you use to drive your Abarth 500 on track days or you are annoyed by unpleasant slipping of internal wheel at every wild corner exit, a real self-locking differential is your solution.
Prometeo self-locking differential with helical gears (also called “Limited Slip Differential”), tested on cars with more than 30 kgm (~300 Nm) engine torque, allows to share in an asymmetric way the driving torque coming from the engine. This means that the wheel with more grip will get an extra torque respect to the other (more unloaded) wheel. Locking rate of Prometeo differential in driving is about 35%. We prefered not to go over this rate, because Abarth 500 or Abarth 595 is, and still remains, a road car.
Helical gears differential works in a very progressive way and does not generate unpleasant effect in standard drive. Locking rate in engine-brake mode is limited to 25%, so not to affect the braking and the cornering.
It’s very difficult to explain in words the effect achieved, but the sensation experienced when you open the throttle after the apex of a curve and don’t feel the front is going off on a tangent but is leading to the internal, is something unknown for a front drive standard differential.
Furthermore, Prometeo differential working is compatible with TTC.
How does a self-locking differential work?
First of all, consider the open (=standard) differential in the front drive.
Differential is a device able to share the torque coming from the gearbox in two exactly identical halves, irrespective of wheel speed difference on the same axle.
So, as you can see in the image, said 100 (arbitrary units) the torque coming to the differential, every wheel will receive a torque of 50.
There is another important factor to be taken into account: the grip of the tyres to the ground. Torque can be unloaded to the ground until grip limit of the wheel; this limit is proportional to the load rate acting on the wheel and to the grip factor, which is strictly dependent from soil condition, as well as tyres, obviuously.
So, if one of the wheels of front drive goes over a zone of less grip (i.e. slippery asphalt), or it is low-loaded in driving a fast curve (internal wheel), the torque that this wheel can unload to the ground is reduced because it reaches its slipping limit. Due to the standard differential feature said before (mechanism sharing in equal parts the coming torque), the same (low) torque value will go to the other wheel, although it would have a good grip to the ground.
As you can see in the previous picture, said 30 the slipping limit of front left wheel, right wheel cannot do nothing better than unload 30 to the ground. So whole traction of front driveline reduces to 60, significantly less than 100 of previous example.
If we install Prometeo self-locking differential removing the standard one, traction is improved. Let’s see how and how much.
First of all, in condition of good and uniform grip, self-locking differential behaves as an open differential, as shown in the following picture.
If a wheel loses grip, respect to the case of an open differential, an extra torque of about 35% goes to the other wheel.
So, whole front driveline torque unloaded to the ground is higher than the torque unloaded by an open differential.
Following picture is very explanatory of the way of working.
The rate of 35% is the best compromise found during plenty of tests made in different conditions. Don’t forget that an excessive locking rate would make the car too understeering in normal drive, with consequent troubles to the driver. On the other hand, a too small locking rate would have reduced self-locking differential advantages, making it substantially unuseful.
