A differential is a small box of gears between a car's two driven wheels. It lets the two wheels turn at different speeds when the car goes round a corner, while the engine still pushes both of them. Every car on the road has at least one. It is also one of the most satisfying mechanisms you can build, because once it clicks, they start seeing corners differently.
Why the wheels need different speeds
Ask a building partner to walk in a circle holding your hand, with you on the inside. Who has to walk further? The person on the outside. Car wheels have the same problem: in a turn, the outside wheel travels a longer path than the inside wheel, so it has to spin faster.
If both wheels were fixed to one solid axle, one of them would have to skid in every corner. The car would scrub its tyres, hop and fight the steering. The differential fixes this by letting the two wheels share the push but not the speed.
Try it below: drive the car round the corner and compare the two back wheels, then switch to a solid axle, tighten the turn, and put the inner wheel on ice. The prediction cards ask what happens before you drive.
Interactive: Differential cornering simulator. Drive a car around a corner and compare inner and outer wheel speeds, then try a slippery wheel with the differential open and locked.
How it works, in one sentence
The engine turns the differential's case; inside the case, small gears called spider gears push on a gear for each wheel; when one wheel is slowed down, the spider gears roll round and let the other wheel speed up by the same amount.
That's a lot for one sentence. It makes sense in about ten seconds once you hold one.
Build one and watch it
Differential Demo Cart is a push-along cart built to show a differential with nothing hidden. It takes about 22 minutes and is level 2 of 5, with the gearing visible throughout the assembly.
Try this with the finished cart:
- Push it in a straight line. Both back wheels turn at the same speed, and the little "speedo" knob on top spins along.
- Lift the back, hold one back wheel still with your finger and turn the other one. It turns freely, and the differential's gears roll round inside to let it.
- Put it on the floor and push it round a tight circle. Watch the outside wheel spin faster than the inside one, while the speedo shows the average of the two.
Step 2 is the moment it clicks. The differential page has the same test in plain language, plus questions to explore next.
The catch, and the fix
Testing also reveals a limitation: if one wheel is in the air or on something slippery, the differential sends all the turning to that wheel, and the car doesn't move. Real off-road vehicles solve this with a differential lock, which joins the two wheels together again when you need grip more than you need to turn.
The Diff-Lock Mud Truck has one you can switch on and off, and the differential lock page explains when drivers use it. It is a level 4 motorised build, a useful follow-up once the basic mechanism is familiar.
Where differentials show up
- Cars and trucks: at least one differential per driven axle.
- Four-wheel drive: often a third one in the middle, between the front and back axles.
- Racing: the Pull-back Diff Racer adds a differential and steering to a pull-back car, so it can corner without skidding.
- History: the South-Pointing Chariot is an ancient Chinese machine that uses a differential to keep a figure pointing the same way however the cart turns. It is the hardest build in the set (level 5) and a challenging project for experienced builders.
Questions to ask
- "Which wheel goes further in a turn?"
- "What happens if one wheel is stuck in mud?"
- "Why don't bikes need a differential?" (one driven wheel)
- "Why don't the front wheels of a push cart need one?" (they are not driven; each spins freely)
Four quick questions to check the idea before you build.
Interactive: Check yourself. A few quick questions on the ideas in this guide, each with an explanation.
- A car turns left. Which back wheel has to travel further?
- With an open differential, the inner wheel turns at 0.8× and the outer at 1.2× the case speed. How fast does the differential case turn?
- One wheel is on ice and the differential is open. What happens?
- Why do drivers switch the diff lock off again on a dry road?
In the classroom
Differentials are a useful lesson for builders ready to study more complex mechanisms. The Blueprint Lab Engineers track has an Advanced mission on steering and differentials, with a fair test, a rubric and printable plans. The car builds collection has every car in order of difficulty, so a class can split across levels.
