JusticeZero

Giant Simple 3. I'm a clyde and I use this bike for utility, so I know I load it up a lot. Shortly after I got it, it threw the chain. Went back and had it dealt with; I don't have room for a stand or anything and it seemed like a warrantee thing. Other issues came up (broken rear spokes). But the chain kept throwing. They took the bike in, drilled a new hole to take the rack completely off of the axle, and put some sort of restrainer to keep the rear axle from moving, tightened it all up, gave it back. I ride it a couple days and the chain comes off during moderate acceleration on flat ground.
When it throws, it comes off of both sprockets. Don't know if that's relevant.
Someone was looking at it and remarked that he thought that the chain was tight when I was sitting on the seat, but when he checked again in the exact same spot after I stood it was loose. I can't verify that.
Any thoughts what might be going on? This is getting old; i'm getting tired of having my chain come off in intersections and having to coast out without brakes.

dabac

Re: coming off at both sprockets
- A chain is a fairly floppy thing, with not much to guide it once it's off. Assuming it comes off at the back first, a moment extra of pedalling may well shift it off at the front too. If it came off at the front first, I'd expect the rear to stay on. With the wheel coasting there shouldn't be any incentive fo the chain to dislodge.
re: coasting to a stop
-That's why I'd never ride a bike with only one brake in traffic. Have a front brake fitted. Not only will it be a nice safety feature, as long as traction is good it'll also enable you to brake harder.

The Simple 3 that came up on my net search had forward facing dropouts. Now for rearwards facing dropouts, fitting a tug nut/chain tensioner etc is a simple thing. But I've never seen a stock item meant to do that job for a forward facing dropout. Also, "my" S3 didn't have the rack bolted to the axle. maybe yours are older?
There are a few possible options with more aggressively toothed axle locknuts and things like that. But the Anti-rotation washers supplied by Shimano tend to do a fairly good job of that from the start.
Strong/heavy riders are sometimes able to cause the chainring to flex/fold, but there's not enough detail on the web to speculate if that's a realistic option in your case or not.
Chainrings/sprockets aren't noted for their centricity, so tight/loose spots as the chain goes round are common. But if the chain is really changing tension when the drivetrain is stationary and you're getting on/off the bike, then that'd suggest a frame failure. If the chain goes tight with you in the saddle (and no load on the pedals), check the chainstays real close, particularly the welds by the bottom bracket.

oldroads

Also, be sure the front chainring is not bent.

fietsbob

Might be a stretched chain, + /or the axle not being tight enough ,
Anti rotation grip washers should, if not, add some star washers
on the other side, to bite into the metal of the dropout.

For my IGH, I prefer full bushing chains
over using bushingless derailleur type, chains.

I replaced the original equipment Rohloff (bushing-less-type) chain on same name hub,
with a full bushing Whipperman [3/32''] .. it's been fine.. a couple years, now.

MichaelW

If the chain is tight, then again, its lose, check how round your chainwheel is. If you want to switch, get one with extended teeth and no ramps, for singlespeed use.
I've never had a problem with de-chaining even with a very slack chain.

JusticeZero

How much difference should there be in chain tension when I load the seat up? I didn't see any frame damage, but I might have missed something.
And how can I check how round the chainwheels are at home with minimal tools?

fietsbob

you mean when you sit on the bike?

A Bit esoteric, sounds like you have to visit a bike shop,
where people can see what you are talking about

dabac

None, squat, zero. If the chain changes tension with you on/off the bike(w/o touching the pedals) that means the distance between bottom bracket and rear hub is changing. And that shouldn't be happening on a rigid frame.

(strictly speaking, a frame will bend a little under load. But the amount should be too tiny to be visible as chain tension differences for the naked eye.)

It's fairly easy. Clamp/pinch something to the frame so that it just touches the tip of a tooth, then rotate crank/sprocket while looking for how your point of reference meet up against the passing teeth.

Sixty Fiver

If the chain tension is correct and the axle nuts are torqued the wheel should not shift... a lot of people do not torque axle nuts properly and as a bigger rider you may want better lock washers.

If a chain ring was bent or it, or the cog was severely out of round this might cause the chain to climb off one or the other.

If you get on the bike and the chain tension changes without you moving the crank, you have a frame problem which could be something like undue lateral flex at the bottom bracket which would manifest under high pedaling loads.

I am just a little runt but used to lay down enough power to bend lesser chain rings on standing climbs due to lateral flex in the frame... the flex always happens at the highest load point of the cranks and always affects one area of the chain ring so over time the chain ring can bend.