Chain drop issue

**Dantheman** · 09-02-2016, 11:53 AM

I could use a little help diagnosing the cause of chain drops that are happening on my son's bike. The chain is occasionally dropping off the chainring during shifts. It's a 1x10 GX drivetrain with a 32t SS non-NW chainring. It's happening while going uphill so it's not the chain being bounced off. The first time it happened he was shifted all the way up the cassette so I thought it was a chainline issue at first (it's a 20" bike with 330 mm chainstays so the chainline is pretty severe), but it's happened several times now while in the middle cogs when the chain was dead straight. If he way up the cassette it drops to the inside, but otherwise drops to the outside. When they've happened the chain tension was probably higher than ideal, but he didn't appear to be mashing on the pedals particularly hard and at 45 lbs he can't put that much pressure on them anyway. I've never seen similar drops before, it's really puzzling.

The obvious solution is to put on a NW ring, but I want to be sure I'm not missing anything before I spend the money. It's all brand new stuff, initial cable stretch has been taken out, chain length looks good, and it shifts perfect in the stand.

**mntlion** · 09-02-2016, 12:24 PM

NW and a derailer with a clutch or higher tension.

Kids 20" wheels are inflated hard, for a 45p kid, so the bike still bounces a lot, even on ups, Also any suspesion is not really set up for someone that small. And 20" wheels are deflected more then a bigger wheel.

All this makes is easy for a chain derailment.

Also adding a F derailer/chain guide can help too.

**grskier** · 09-02-2016, 12:47 PM

Throw a NW and/or chainguide?

It's not designed for that geometry is probably what it boils down to.. like you said, it's exaggerating the chainline.

**toast2266** · 09-02-2016, 01:48 PM

^^ what those guys said. There's a reason that lots of single ring kids bikes come with those "chainguide" plates on either side of the chainring.

**Dantheman** · 09-02-2016, 01:59 PM

It's a clutched derailleur (GX 2.1), tires are inflated to a reasonable pressure for a human that size (15 psi), and the fork is a custom kids fork that actually works. He uses the full 80 mm of travel on bigger hits, though it is a hardtail. It's happened in some pretty smooth spots at low speeds. So far it has only happened when shifting, and never while descending rough stuff at speed. The chainline is severe when shifted all the way up the cassette, but most of drops have happened in the middle of the cassette when the chain is straight.

Like I said, buying a NW will probably solve it but I want to make sure I'm not missing something obvious first. Thanks.

**Telemahn** · 09-02-2016, 04:35 PM

Bent tooth? A small twist can cause this intermittently. it will only happen when that tooth is in the right position when the shift occurs.

**Flea** · 09-02-2016, 04:55 PM

How's the drivetrain wear? Something sounds fishy. The chain drop shouldn't be happening from normal use.

**Flea** · 09-02-2016, 06:20 PM

Edit: somehow missed the "all brand new parts" duh.

**JerseyDave** · 09-03-2016, 07:44 AM

My kid is on a spawn with suspension. The Bike is crazy expensive but my folks got it for him so WTH. This same thing is happening. I looked at BB spindle length, the shop measured it at 110mm. They said they could get 103mm. The chain line is so bad, even from the 5th cog up. Seems silly to replace a new BB with a new BB but the crashes resulting from him trying to pedal after the chain is dropped are some of the worst he's taken.

**gravitylover** · 09-03-2016, 08:46 AM

It's not silly if it's necessary.

**farmguy** · 09-03-2016, 01:42 PM

A loose bottom bracket? With just a little side to side play that is the same as a shift or dropped chain.

**stuckathuntermtn** · 09-03-2016, 06:05 PM

^^^square taper crank?

**Dantheman** · 09-03-2016, 09:48 PM

Originally Posted by DaveVt

My kid is on a spawn with suspension. The Bike is crazy expensive but my folks got it for him so WTH. This same thing is happening. I looked at BB spindle length, the shop measured it at 110mm. They said they could get 103mm. The chain line is so bad, even from the 5th cog up. Seems silly to replace a new BB with a new BB but the crashes resulting from him trying to pedal after the chain is dropped are some of the worst he's taken.

Interesting, which Spawn? I assume that is without a NW chainring?

Originally Posted by farmguy

A loose bottom bracket? With just a little side to side play that is the same as a shift or dropped chain.

BB is rock solid. It's an external BB with 2-piece cranks. I'm not sure what the shell width is.