A lot has been said about the Lazer ZX standard center diff. The manual shows the setup, but it doesn’t really explain how it works and is very vague with the ‘types’. I will do my best to do so here.
Let’s split the diff into the two drive directions.
Rear drive
The rear drive hub (LA-6) is a one-way system that spins freely on the layshaft when the layshaft is ‘accelerating’. But let’s not focus on the layshaft, as the rear drive hub doesn’t get its drive from the layshaft, rather it gets its drive from the pinion-spur-thrust balls-inner thrust plate construction.
Front drive
The front drive hub (LA-7) is connected to the layshaft through a one-way system that is locked when the layshaft is ‘accelerating’. The layshaft gets its drive from the LA-5 drive plate, which slots onto the layshaft. And this again gets its drive from the spur through the outer thrust plate that is sitting on the thrust balls. The front drive hub spins freely on the layshaft when the layshaft is ‘breaking’, so the front wheels don’t offer any braking.
Tuning the diff
The standard setup acts according to the ‘type 1’ description in the manual, with around 60% of the power going to the front.
The diff itself can be tuned through the locknut on the spur side. The more you lock it, the more the front and rear are connected together.
On the other side is the limiter nut. Tighten it for more drive to the front, loosen it for less drive to the front.
As standard (setup according to the manual), the car is very front wheel drive biased. Tightening the locknut on the spur means you will naturally get more rear wheel drive because you’re limiting the diff function. It’s a hard system to get right and it’s recommended to have a play with it to feel the car and get the hang of the interaction between the two nuts.
Using the asbestos OT-88 protection plate
This setup is the ‘type 2’ description in the manual.
Coming from the mid, this optional part (2 asbestos discs) can be installed and will create a kind of slipper type clutch. The balls are removed from the spur. This system offers better lock / grip than the thrust balls and it gives the benefits of a slipper-like setup.
It’s asbestos. There’s been a recall for a brand of RC cars in Australia due to the presence of asbestos in the slipper pads. These pads are probably not legal to use as a slipper pad at this time due to the release of fibres when undergoing friction. At the lease, exercise great caution when handling these parts and the dust they may create.
The LAW-4 reverse one-way rear drive hub
This replaces the LA-6 one-way drive hub and inverts the action of the one-way bearing. So, LAW-4 is locked to the layshaft, and will always receives the full drive, be it through that layshaft (taking power from the spur through the outer drive plate) or through the spur directly by the inside connection (spur friction to LA-6 with the thrust plates). It gives a 50/50 drive.
The reversed one-way also means that when braking, the layshaft ‘disconnects’ from the rear drive hub and the braking force comes from the spur-to-hub connection. In practise, this means a bit less breaking is coming from the rear.
Tips
It is crucial to use the correct ball diff grease on the balls and thrust plates as per the manual. Typically, you will need to clean the spur setup regularly as any other grease will contaminate it and cause the balls to slip (and overheat / wear quickly).
While it was a popular action to lock the diff completely because it eliminated getting used to the tuning, this also puts a lot of pressure on the entire setup. Typically, this wore out the LA-9 thrust bearing races and I have seen more than one Lazer ZX in which the front-side plastic bearing carrier (part of the rear gearbox, on which you mount the motor brace) being distorted, requiring further tension,… until it all fails.
Conclusion / racing
All in all the Lazer ZX diff offers a lot of possibilities. Kyosho engineers must have thought really hard on how to turn the typical spur-layshaft setup into a center ball diff. However, it’s definitely hard to get precisely right and while it could offer a very controllable car on loose dirt surface races, the system was complex and not that easy to understand. Many racers completely locked the diff by completely tightening the spur locknut, getting as close to full 50/50 front to rear drive as possible, and then tuning the adjustment nut on the other side of the layshaft to get more or less drive to the front. The LAW-4 offered a nice solution while keeping some of the tunability of the car. But in fact it was the LAW-8 parts bag, upgrading the spur and drive to a real slipper setup, that simplified the setup and made it easier to finetune. That setup also featured on the Lazer ZX-R car.
