Built up a play/jump bike so i can go out with my nephew. Toyed with a few ratios and ended on 36×18 as the 36×16 was too slack even with a tensioner. Thought i had the magic ratio for no tensioner (frame has normal dropouts) but then noticed the wheel was not properly in the dropout. Got it to go in with a little bit of force and tightened it all up but the chain is now super tight, as in could not be any tighter.

So… can i run it like this on the assumption it'll stretch a bit and then be perfect or will it explode and cause me to lose my testicles?

The bike won't be taken proper offroading or even go up any hills, it'll just be thrown around off steps, jumps etc if that has any bearing on your advice. Thanks.

you'll be overstressing the hub and freewheel bearings if the chain is as tight as you describe

get a half link or a tensioner with more adjustment in it … or get a worn chain off another bike

I think it's not true that a tight chain causes bearing failure. How tight does it get when you stomp on the pedals?

Don't put the spindle all the way into the slot, but do the QR up really tight until it stretches.

yes it does, the stress has nowhere to go with a tight chain but turns into forward motion when on the pedals

rotate the wheel (or back pedal) on a very tight chain and you can feel the grinding, plus there is almost always a tight spot on the chainring (they're not completely round)

another way to ease the tension is to file off one edge of the axel. I think Charlie the bikemonger has some advice on this. I tried it once; worked (after a fashion) but really doesn't give you much.

The chain is effectively 'solid' – ie it doesn't change tension when you pedal – it's already at maximum tension.

My other thought was to run it with the 16t cog for a bit to stretch the chain then swap back to the 18t after a little while?

Will anything actually break though? – the bike is an amalgamation of cheap bits so i'm not too bothered about risking breaking bits and it'll only get used once in a blue moon. I just don't want to face plant after one of my poxy jumps when the chain explodes. Freehub bearings i can live with as long as they don't fail straight away?

Edit – i can feel a definite grinding noise and feeling when i pedal the bike in the work stand.

I don't think the chain will go (but ofc it may…it's an old one?), more likely the freehub (esp if it's a converted wheel). if you're not cranking on the pedals much then you're unlikely to come too unstuck but sudden loss of drive while stomping is probably the biggest cause of SS accidents (after getting your hairshirt and/or beard caught in the chainring ofc 😉 )

that you can feel / hear the grinding tells you that it's too tight. If it survives until stretched in then you'll probably be OK

if it's all cheap spares, I'd try filing off the leading edge of the axel where it passes through the dropouts. Doesn't give you much but maybe enough (and it's fettling, which is always good)

Sure I read somewhere that you should be able to move the chain about a 1/2 inch up or down. Can't remember why, but that's how I always ran my singlespeed with no problems at all.

Chain is a brand new 3/32 KNC SS specific, rear hub is a Deore one on a 2nd hand wheel bought off here. A 17t cog would be the simplest answer i guess but i'm tight and i can get a 16t or 18t (Gusset, Shimano etc) for around £3 each but the only place i can find a 17t is On One and they're £16. The chain had just enough play in it to be perfect before i noticed it wasn't in the dropout properly.

I shouldn't be stomping on the pedals much as it won't be ridden any distance, just very local.

take a 17 from an old cassette?

Just take a file to the front of the dropout on both sides and you should solve the issue – ok it'll get a bit baggy as time goes on but it shouldn't ever fall off. (have done this on an aluminium frame before to get a magicish gear)

I did a magic ratio like that. The chain was like a bowstring.

The spider on my cranks broke off after about 5 rides.

The geared riders I was with were awed my immense leg strength.

I didn't enlighten them 🙂

No, honestly, try only partially inserting the axle into the dropouts. It works.