How to make a car skid
It might not win races or set record lap times, but holding huge, glorious powerslides and drifts is one of the most fun things we can do with our cars. To do this we need to make sure the power to the wheels is being delivered reliably and predictably, which means we need a limited-slip differential (LSD for short).
If you've watched THE FINALE of our BIG-BLOCK MIDGET BUILD then you'd have seen the lads fit a Cusco limited-slip centre to the tiny truck. This job is very different from other differentials they've worked on (see how the GR Yaris & Corolla diffs work HERE) so it was a voyage of exploration for them... but the results were worth it!
A differential works by using a pinion gear (pictured below), which is attached to the tailshaft and turns the big ring gear you may have seen in images of diffs that have been pulled apart.
The ring gear is the big gear next to Marty's finger that he's jamming in the back of the diff looking for cashews. The pinion spins with the tailshaft and turns this gear, which is bolted to the diff centre. Inside that centre are spider gears all meshing together to turn the driving force 90-degrees out to the axles, which are bolted to the wheels.
The skiddy success of this episode all comes down to the Midget now being able to transfer what little power it has to the wheel with the most traction, as this is the function of a limited-slip differential. So, while doing big skids is the goal it is actually our old enemy traction which lets us do it - remember it is a LIMITED-slip diff, not a NO-slip diff!
In an open (or "single-peg") diff the power escapes out the wheel with the least amount of traction as it is the easiest to spin. The wheel with all the traction is more difficult to turn and the differential has no way of pushing the engine's power to that wheel, so the one-tyre fire continues until glory is reached (as seen below in the amazing photo I found on Reddit).
Here is the open diff centre with the ring gear unbolted and knocked loose. You can also see the spider gears which transfer drive out to the axles - these are what are welded together when someone talks about a "welded diff", which is just a crude, cheap way of locking both wheels together so they spin at the same time.
This can be fun on a track-only car but is a pain in anything else as you need the inside wheel to spin at a slower speed when turning a corner, so the car will pivot around the bend easily - welded or locked diffs will offen chirp the inside tyre when making tight turns and the force exerted on them can often lead to the weld or an axle failing. Just buy a proper LSD, guys.
With most LSDs the inside wheel won't be fully locked together with the outside wheel, meaning you'll still get a nice, smooth, non-chirping, tight-turning car. It will only lock the inside wheel up when accelerating or putting load on the system.
Below is a photo of the ring gear being fitted to the new, Cusco clutch-type LSD centre. An LSD (limited-slip diff) can come in a few different operating methods, and the common ones you'll hear include Torsen (or torque-sensing), viscous, and plated or clutch-type. There are multiple reasons manufacturers will use different types of LSDs in different cars, but it is widely agreed that a plated LSD is best for a motorsport or high-performance appllication.
In the below image you can see the clutch packs, which look like round discs. Just like a clutch in a manual transmission these plates all grip together to transfer the engine's drive.
As one wheel loses traction the clutch plates will lock up, sending equal drive to both wheels and giving more traction to the wheel with grip, moving the car forwards... or sideways! The clutch plates lock more tightly than other LSD types which can rely on fluid, or springs or sometimes a really aggressive pigeon... no, wait.
Some motorsport-oriented cars, like special edition Spec C STi Subarus, actually have plated LSDs fitted from the factory.
The down side to plated LSDs is these plates can wear with aggressive use, requiring them to be shimmed tighter to maintain the proper locking operation. Basically, they can be pulled apart and have spacers or extra plates installed to make the diff tighter, or they can also have plates removed to make the diff softer and act a bit more like an open diff.
The photo above shows an Eaton TrueTrac, which is an aftermarket helical gear limited-slip centre. This is the modern replacement to the clutch-plate LSD as the diff operates as a full open centre when there is no load (no acceleration), but gradually locks up under load, eventually locking up solid.
We used one of these centres in the diff build for my Pontiac. If you want a look at how to build a live-axle rear-end from scratch you can CLICK HERE to see Matt from Geelong Diffs put together a 1000hp-rated Ford 9inch.