MUR Blog - Differentials…What’s The Difference? (Pt. 2)
This post is part two of a two-part series. If you haven’t read part one, click here to take you to it.
Previously, we discussed the pros and cons of both the spool and open differential. We needed a differential that didn’t exhibit the heavy understeer that was one of the characteristics of a spool. The open differential solved this but had disadvantages of its own: the traction difference problem. Thankfully, there is a happy medium between a spool and open differential: the limited slip differential (LSD)! The LSD was designed to tackle the traction difference problem by limiting how much a wheel could slip, before becoming more spool-like.
Clutch Plate (Salisbury) Limited Slip Differential
The Drexler LSD that we use is a clutch plate type differential. At its core, it adapts the open differential mechanism with the addition of ramp angles and clutch plates.
Exploded view of Drexler 2010 FSAE LSD (Image courtesy of Drexler)
Upon acceleration, the spider axle is wedged into the ramps of the pressure rings, with a component of the resultant force pushing them outwards and compressing the clutch plates. The increasing pressure and therefore larger friction on the clutch plates increasingly binds the driven wheels as more acceleration is applied making it harder and harder for them to spin at different rates. Thus, the shallower the ramp angle, the more aggressive the locking factor.
Ramp angles determine how much pressure is applied to the clutch plate to increasingly bind the wheels. The torque is transferred from the chain and sprocket to the differential housing. As the housing accelerates relative to the spider axle, the spider axle is wedged in between the ramps of the pressure rings; forcing them outwards.
Drexler built-in ramp angle options (Image courtesy of Drexler)
The ramp angles dictate the torque split between the rear wheels. For example, a 30-degree ramp angle will lock up more than a 45-degree ramp angle, whilst a 90-degree ramp angle will not compress the clutch plates at all. Various combinations can be selected on the Drexler by positioning the spider axles with the corresponding set of ramp angles.
Note: Lock in a clutch plate differential is progressive in the sense that the friction from the clutch plates increases such that it becomes harder and harder for the wheels to rotate independently as acceleration increases, not necessarily that it is completely spool-like.
The Drexler FSAE differential is a 1.5-way differential. For a 1.5-way differential, the ramp angle on the acceleration side is different to that on the braking (or coast side); as opposed to a 1-way differential which only locks in acceleration, or a 2-way which locks equally under acceleration or braking.
1.5-way differential: As the differential accelerates in the driven direction, the spider axle is forced into the acceleration ramp angles (40°). When braking it is forced into the opposite coast side (50°). (Image courtesy of Drexler)
It is desirable to have a shallower ramp angle (smaller angle, more spool-like) on the acceleration side for higher powered vehicles, limiting slip to a greater degree and sending power to the wheel with greater traction. However, the car then does experience more understeer. Lower powered cars can get away with a steeper (larger angle, more open) ramp angle setting as the torque produced is not sufficient to overcome the tire traction limit.
On the coast side, greater locking induces better braking stability but again comes at the cost of understeer during braking.
With shallower ramp angles, we must also be careful of snap oversteer. Snap oversteer occurs when the car is cornering and sudden application of throttle spools the differential up. This sudden increase of torque to the inside wheel whilst the car is turning can kick the tail of the car outwards potentially spinning out the car.
Oversteer: Rear end kicks out
As such, finding the sweet spot for acceleration and coast ramp angles is key. It is a complex combination of a whole myriad of factors, but ultimately, the driver must feel comfortable in predicting the behaviour of the car. In that way, consistent and therefore faster overall lap times can be achieved.
MUR 2015 and 2016 both ran with 30/45 ramp angles, but have not experimented much with the other combinations. This year, we hope to compare the 30/45 setting and 40/50 settings and quantify whether a more ‘open’ setting will increase drivability to the point where it can improve lap times.
Our drivers this year will be less experienced in driving FSAE cars than previous years so the theory is that making the differential more ‘open’ will aid in maneuverability at speed and hence be easier to drive. A more ‘open’ differential is also aimed at counteracting the negative effects on maneuverability due to a 20mm increase in track-width this year and the 30mm increase in wheel-base compared to the 2015 and 2016 cars.
One of the appeals that the clutch plate differential offers is that the locking characteristics can be tuned in many different ways be it by varying the number of active clutch plates, the preload provided by the disc spring, or the ramp angles as discussed. Any combination of the variables can make the differential behave like a spool at one extreme, to an open differential on the other extreme, to anything in between. This is a very useful feature for a differential used in racing applications. Especially one that needs to work across different set-ups as it gets passed on from year to year.
Preload is another parameter that can be tuned. It can be thought of as a baseline degree of locking. It is achieved by using a disc spring to apply a minimum pressure to the clutch plates, and so the wheels are always coupled via friction to some extent.
The torque difference in the wheels must overcome this baseline resistance before being able to spin at different rates. This is called the breakaway torque and is quantified by measuring the torque required to turn one wheel whilst the other is held stationary. Dynamically, tuning for the ideal preload aids in maintaining stability of the car during the transition between throttle and braking.
Over time, this preload will lessen as the clutch plates wear down and become thinner, so if consistent characteristics of the clutch plate differential is desired, it must be regularly maintained by adding shims (spacers) to maintain the spacing, changing the spring stiffness or renewing worn clutch plates.
The friction discs are splined to the side bevel gears whilst the clutch plates are splined to the differential housing. When pressure is applied to the clutch pack, the friction increasingly ‘binds’ the axles and the housing to rotate as one.
The order between clutch plate and friction discs can be varied to alter the number of active clutch interfaces. The more interfaces, the more friction provided to couple the wheels. Typically, the clutch plates and friction discs alternate: 0101, but 0011 is also possible, reducing the active clutch plates and thus lessening the resistance, making it even more ‘open’. Given our tight schedule this year, it is unlikely we will test this parameter in a controlled manner, but the hope is that this can be a task for MUR 2018.
The differential setting selected will very much influence our suspension set-up. As opposed to the stiffer suspension set-up for a spool that encourages the inside wheel to lift off the ground during corners to reduce understeer, it is more beneficial for an open differential or LSD to be softer in roll. (Roll is the tendency for a car to lean outwards during a corner.) Our 2017 suspension design allows us to tune the activation through spring rates, dampers, anti-roll bar settings, and a whole lot of other suspension magic. It is intended that a less aggressive anti-roll bar setting at the rear wheels relative to the front will ensure the rear tires closely follow the contours of the road maintaining traction, giving us the best chance at pushing the car as hard and fast as possible.
So, was the transition to a Drexler worth it? We think so!
The Drexler LSD affords us ample tuning capabilities to adjust with our evolving design platforms and dynamic needs. It is reliable and is sure to be with us for more years to come.
Hopefully this blog has given you a little insight into our set-up for the Drexler differential and a bit of background to why we’re using an LSD.
Deconstructed Drexler LSD
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About the Author:
Lead Brakes & Drivetrain Engineer, 2017
Junior Brakes & Drivetrain Engineer, 2016
Junior Brakes & Drivetrain Engineer, 2015