The short answer
You want roughly one inch to one and a half inches of up-and-down play in the chain, measured at the midpoint of the bottom run between the two sprockets, with the bike off the stand and nobody sitting on it.
If you can barely move the chain, it is too tight, and that is the dangerous direction. A chain with no slack destroys sprockets, wrecks bearings, and can snap.
How to check it
- Bike on level ground, engine off, in neutral.
- Find the middle of the bottom stretch of chain, halfway between the front and rear sprockets.
- Push the chain up with a finger, then pull it down. The total travel between those two points is your slack.
- Roll the wheel a few inches and check again. Then again. Check it at several points around the wheel, because chains wear unevenly and you must set the tension at the tightest spot you find.
A chain that measures perfectly at one spot and is drum-tight a quarter turn later is worn out and needs replacing, not adjusting.
Why too tight is the dangerous mistake
When the suspension compresses, the distance between the two sprockets actually increases. The chain has to be slack enough to absorb that. Set it tight with the bike sitting still and the first time your kid lands off a bump, the chain is pulled bar-tight.
What that does:
- Loads the countershaft bearing sideways until it fails, which is an engine-case job
- Wears sprocket teeth into hooks in a fraction of the normal time
- Stretches the chain fast
- Can snap the chain, and a snapped chain either wraps the sprocket and locks the rear wheel or comes through the case
Too loose is less dramatic. A loose chain slaps the swingarm, wears the slider, and can jump off the sprocket. Annoying and worth fixing, but not the same order of problem.
How to adjust it
- Loosen the rear axle nut. Not all the way off, just enough that the axle can slide in the swingarm slots.
- Find the adjuster bolts. One on each side of the swingarm at the back, each with a lock nut, pushing on a block the axle sits against.
- Turn both adjusters the same amount. This is the part people get wrong. Clockwise pulls the wheel back and tightens the chain. Whatever you turn on one side, turn exactly the same on the other, or the rear wheel ends up crooked in the frame and the bike will not track straight.
- Use the alignment marks. Most swingarms have matching notches stamped on both sides. The same mark should line up with the axle on the left and the right.
- Re-check slack at several points around the wheel, aiming for one to one and a half inches at the tightest spot.
- Tighten the axle nut firmly, then tighten the adjuster lock nuts.
- Spin the wheel and watch the chain run onto the sprocket. It should feed on straight, not pulled to one side.
Lubrication
A clean, lubed chain lasts several times longer than a dry one, and on a bike ridden in dirt this is the maintenance that pays back most.
- After every few rides, or any ride in mud, sand or water.
- Brush the worst of the dirt off first with a stiff brush. Lubing over caked grit turns the chain into a grinding paste.
- Apply chain lube to the inside of the chain run, where it wraps the sprocket. Centrifugal force carries it outward through the links. Lubing the outside just makes a mess that flings onto the wheel.
- Rotate the wheel a full turn or two while applying so you cover the whole chain.
- Let it sit ten minutes before riding so it can creep in and the carrier can flash off.
- Wipe the excess. Excess lube is a dirt magnet.
Do it on a warm chain right after a ride if you can. It penetrates better. Chain care sits inside the wider 50cc maintenance routine for parents, alongside cleaning the air filter, and it matters most before you put the bike away for winter.
When the chain is done
Replace it when you see any of these:
- Tension varies wildly at different points around the wheel
- The axle is at the very back of the adjustment slots and it is still loose
- You can pull the chain away from the rear sprocket enough to see daylight under a tooth
- Links are kinked, seized, or will not flex sideways freely
- Sprocket teeth are hooked or pointed instead of symmetrical
Replace the chain and both sprockets together when you can. A new chain on worn sprockets wears out fast, and worn sprockets on a new chain do the same in reverse.
Questions
How often should I check chain tension?
Before every ride, as part of the same walk-around where you check the brakes and the bolts. It takes fifteen seconds. New chains stretch quickly in their first few hours, so check a new bike often, and especially while you are breaking a new engine in.
Can I use WD-40 as chain lube?
No. It is a water displacer and a light solvent, not a lubricant. It will actually strip what lube is there. Use proper chain lube, it's a few dollars.
The chain came off while riding. What now?
Stop immediately. Check the chain and both sprockets for damage, check the rear wheel alignment, then reset the tension properly. A chain that came off was either far too loose or the wheel was out of line, and it will do it again until you fix that cause.
Should I check it with my kid sitting on the bike?
Set it with nobody on the bike, using the one to one and a half inch figure. Then have them sit on it and confirm the chain still has visible slack. If it goes tight under their weight, loosen it.
Setup and maintenance walkthroughs at manual.pluggedby50cc.com, or text 623 320 8984.
Plugged by 50CC is an independent retailer. We do not manufacture, assemble, modify or service the bikes we sell. Every bike ships sealed in its factory crate exactly as it reaches us, about 90% assembled from the factory, and final assembly and setup are completed by the buyer. Off-road use only. Riders under 18 should always be supervised by an adult.