5 replies to this topic

[CADBLACK]

Guys,

A long time ago, in a place far away, I worked on a Brabham BT-29 (in the US a "Formula B") as a (very young) mechanic.

For the life of me, I can't remember how differential ramp-angles worked; i.e. really what the term "ramp angle" really meant. I do remember changing the numbers of clutches in the diff...as I recollect, the greater the number of clutches, the more "gentle" the "wheel-connection" was (my words) on both acceleration or breaking. But what is the term "ramp-angle," exactly? I know it had something to do with linking the wheel rotation with non-equal amounts of slip on both acceleration or deceleration, but can't for the life of me remember which number really meant. Was it a percentage of slip number? Which number came first...the acceleration side of the equation, or the deceleration side, etc.

The transmission we were using was a Hewland FT-200.

Thanks to anyone who can explain!

CC

[Dave Wright]

I'm searching for info on Limited Slip Diffs myself. Came across this on the net - may be of use

Limited Slip Differentials

In an effort to solve the problem of the open differential gear train designers have come up with a number of solutions over the decades. One of these uses several clutch plates coupling the axles to the bevel gears. It was made famous by General Motors during the muscle car era as the "Posi-Trac,' but the proper name for this type is the 'Salisbury Differential.' In the early Salisbury units like the Posi-Trac, the amount of wheel spin allowed is controlled by the preload on the clutches. The theory was great, but in practice the preload was difficult to get right and changed as the plates wore. Many refinements to the Salisbury differential have been added by other manufacturers over the years, including one by Xtrac, the Hewland Powerflow and a new diff called the Variloc introduced by Taylor Race Engineering. Although there are differences among them, all use ramps splined to the inside of the housing which load the clutch discs through the end gears when power is applied. The loading of the discs is therefore controlled by the ramp angle employed.

Since we want differential action entering a turn and need to reduce wheel spin when the throttle is applied, the ramp angles are different between the coast and power sides. The coast ramps are usually 80, 83 or 90 degrees to load the clutch plates little, if at all, allowing speed differentiation under braking. The drive ramps can be anywhere between 30 and 60 degrees depending on how much lock-up is needed out of the corners. Additional adjustments can be made by the type and stacking order of clutch plates used and by preload on the plates. These units can be set up anywhere between an open differential to something approaching fully locked, both entering and exiting turns. Although they are very effective when properly set up, some power is lost as heat when the plates are slipping.

[Wolf]

CADBLACK- the 'variables' with limited slip diffs are ramp and coast angle, and number of clutches. Ramp angle regulates distribution of power on application of throttle (the greater the angle, more like open diff behaviour, the smaller the angle the more behavour like 'solid axle, e.g. go-kart like behaviour). Same applied to coast angle, except it determines power-off (and braking) behaviour. Clutches, as You corretctly remember, determined how abruptly or gently diff behaviour 'kicked in' (more clutches make the limited slip effect kick in slowly and gently)...

IIRC, I have seen the info that '67 cars used settings like 45/80/1 (ramp/cost/clutches), but in GPL the settings I find most usable would be completely opposite e.g. 80/30/5...

[MrSlow]

A good (IMO) read about differentials is Richard Nunninis site Grand Prix Legends Driving and Setup Foolishness

The differential section:
http://website.lineo...fferential.html

[tagfat]

You might also find this usefull:

http://forum.racesim...&threadid=58721

[felttip]

There are other diffs besides the clutch-plate limited slip diff. There's also the Torsen and viscous diffs. The Mazda MX-5 used to have a Torsen diff as an option.