Four-wheel steer turns both axles instead of just the front. It looks impressive in videos. It opens up steering tricks that are otherwise impossible. It also adds real complexity, real electrical risk, and real cost. Here's what 4WS actually does, what it takes to install, and whether it's worth doing on your build.
What 4WS is
A second servo at the rear axle that steers the rear wheels. The front keeps its stock servo, drag link, and knuckles. You add a rear servo, drag link, and steering-capable rear knuckles. Then you decide how the two servos talk to each other.
The three signal modes
- Mirror (rear opposite of front). Tightest turning radius. The truck pivots around its center. Best for tight gates and switchbacks.
- Same direction (crab walk). Both axles steer the same way. The truck moves sideways. Useful for fine positioning on technical lines.
- Front only (rear locked center). Stock behavior. Useful for fast sections where rear steer would feel twitchy.
- Independent (advanced). Front on stick 1, rear on stick 2. Lets you mix between modes mid-line. Requires a 4-channel radio.
The mode you can use depends on how you wire the second servo: Y-harness off the front signal (mirror or same-direction with a swap), separate channel (independent), or a dedicated 4WS controller (preset modes).
What you need to install 4WS
- Rear steering knuckles. Aftermarket required. INJORA, RCAWD, Mofo RC all sell rear-steer knuckles for SCX24. Around $20 to $30 per pair.
- Second servo. Sized to match your front. AGFRC A20CLS is the safe choice.
- Rear servo mount. Specific to the axle you're running.
- Rear drag link. Often comes with the knuckle kit.
- Wiring. Either a Y-harness (cheap, $5) or a 4-channel-capable receiver and ESC setup.
- Optionally, a 4WS controller. INJORA makes one. Lets you switch modes from a button.
The electrical risk
This is where 4WS gets dangerous and where most failed builds happen. Two big issues:
- BEC current overload. Two servos draw twice the current. Stock 2-in-1 BEC is 1A continuous. Two HV servos can pull 3A combined under load. The receiver browns out, the ESC resets, and the truck stops mid-line. You need either an aftermarket ESC with a beefier BEC (Furitek Lizard Pro, Hobbywing) or a separate BEC for the second servo.
- Wrong polarity on the Y-harness. Plug a Y-harness backward and you can backfeed voltage into the receiver. Smokes the board.
If you're going to do 4WS, plan on aftermarket electronics. Trying to bolt 4WS onto the stock 2-in-1 is asking for failure.
The mechanical complexity
- Rear axle gets less rigid. Steerable rear knuckles add joints that the stock rigid setup didn't have. The truck can feel less planted under throttle.
- Driveshaft geometry. The rear axle wasn't designed to steer. CVDs or U-joints are mandatory; a stock rear shaft will pop out the first time you steer.
- Suspension binding. At full droop with full rear steer, some setups bind. Check on the bench before you commit to a build.
- Tire wear. Rear tires now scrub like front tires. They wear faster.
Is it worth it?
The honest answer:
- For comp builders running tight technical courses: sometimes. Mirror mode helps on tight gates. Crab walk helps on precision positioning. Most comp builders try it, then either commit or remove it.
- For trail drivers: rarely. The added complexity outweighs the benefit on most natural terrain. The truck is more capable but also more fragile.
- For scale builds: mostly as a party trick. Looks cool, doesn't change practical capability much.
My recommendation: don't add 4WS as one of your first five upgrades. Build the truck to a solid Stage 5 or 6 first. Then if you're still curious, build a second truck specifically for 4WS. Bolting it onto a build that's already doing well usually creates more problems than it solves.
If you do install it
- Buy the right rear knuckles for your specific axle (stock, MEUS, LGRP, etc.).
- Plan on an aftermarket ESC with a 2A or higher BEC.
- Mount the rear servo with the same care as the front. Threadlock everything.
- Set rear endpoints conservatively. Less travel than the front works fine and avoids binding.
- Test on a bench before you take it to a trail. Sweep every steering combination at full droop and full compression.
Background on servos is in the servo explainer. The install procedure for the front servo is in the servo install guide, and the rear is the same procedure mirrored.