dallara F302, Manual

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15 POWER FLOW DIFFERENTIAL
This differential is designed with versatility as its major asset. Many parameters will lead
you to the required setting. A car with good grip and low power may require a completely
different arrangement than that required for a high power/low grip car.
Working principles: Ten friction plates within the diff, six connected to the side gears, four to
the diff casing, control the amount of ‘differential’ action available. The amount of limited slip
depends only on the frictional resistance between these ten plates.
Four factors contribute in defining this frictional resistance:
1. The bevel gears thrust apart as soon as the car moves. This is a feature of bevel gears and is not
adjustable. The contribution of this on friction is minimal.
2. The ramp angle on the side gear ring influences the amount of the driving force on the diff that gets
directed sideways and onto the plates. E.g., on the power/drive side ramp, 60 degrees transmits less
force sideways than a 30 degree ramp. Likewise, on the off-power side ramp, an 80 degrees angle will
transmit little force while 45 degrees locks much more. 60°/80° is normally fitted as standard;
3. The pre-load with which they are assembled to start. In each diff there is a pre-load spacer that looks like
one of the B plates, but thicker. Depending on diff model, it is either the first or the last component
assembled into the diff casing. Its thickness dictates to what degree the plates are pre-loaded / forced
against each other. The pre-load is set and checked on each diff by holding one side gear locked, via a
dummy output shaft held in a vice, and by turning the other with a torque wrench. If the measured
resistance is deemed too high, the spacer is ground down until the desired figure is achieved. The figure
should be checked periodically as it tends to reduce as the diff runs, meanwhile a new A, slightly thicker
spacer will allow re-setting;
4. The final and easiest adjustment is the re-arrangement of the contact order of the friction discs. The
arrangement 1, with a disc succession A, B, A, B, A, has the maximum number of working friction faces.
It gives the maximum resisting torque. The arrangement 3 has the minimum of working friction faces
and gives the minimum resisting torque.
Standard Hewland available ramp angles are: 30/60; 45/45; 45/80; 60/80; 80/80
Differential settings have an important influence on the cars’ balance, especially on corner turn-in and exit.
•
The torque on the differential in drive (acceleration) is much bigger than the torque on the
differential given by the engine brake (deceleration). Typical in line acceleration gets to about 1g,
off-power/braking by the engine only gets typically up to 0.3g.
•
The disc configuration (2, 4 or 6 faces) has the same effect on drive and off-power, the ramps are
the only tool to differentiate the friction force or ‘lock’ between drive and brake.
•
The discs wear off, just as a clutch, and should get checked regularly. This also means that the pre-
load is ‘wearing’ down, especially when using the 2 friction discs configuration.
•
Pre-load is kind of a ‘constant lock’ and the effect is felt in slow and fast corners in entry, mid-corner
and exit. The ramps and disc configurations have more effect in slow and less in fast corners, and
affect corner entry and exit, less so mid-corner.
•
Pre-load blocks the differential (both wheels turn at the same speed) until the difference in torque is
bigger than the pre-load. Once passed the pre-load, the remaining lock is achieved by the ramps
and disc configuration only.
•
Most circuits require little lock to prevent the inner wheel from spinning coming out of the corners,
depending though on tyres, track, driving style and weather conditions. Excessive lock might result
in power understeer.
• Some amount of lock in off-power helps to stabilize the rear end, excessive lock might cause turn-in
understeer.
This table shows the % of lock from minimum to maximum lock.
Lock%= (slower wheel torque – faster wheel torque)/ total torque
LOCK% 2.5 5.0 9.5 12.0 18.0 24.0 25.0 33.5 42.0 44.0 55.0 68.5
RAMP 80 80 80 60 60 45 60 45 30 45 30 30
DISCS 2 4 6 2 4 2 6 4 2 6 4 6