Do I really need a 'chain tensioner' thingy or chain 'guide' thingy or can I actually use one naked chainring on the front and a 'spaced', roughly in line, cassette ring on the back with an old 8 speed chain ?
Just a LITTLE worried about it slipping on hill climbs as my knees strain under the pressure…

Yes you'll need the gubbins, otherwise your chain will fall off, and you'll chin yourself on the stem, or smash your bollox on the toptube, OK? Oh, and you'll need a bit better than "roughly in line" other wise the chain will fall off, and you'll…

Depends if you get luck or not. I get by with a chain tensioner, a couple of spacers and an old cassette cog.

it'd probably work but not having chain at a good tension will risk it jumping off lots, which would be a royal pain in the ass. and normal cassette ring/chainring wouldn't last that long either.

You might be lucky and have a magic ratio meaning that you don't need the gubbins immediately but as it all wears then you will.

And obviously you'll also need the proper SS gubbins – beard, roadie cap, hipflask, etc 😉

Unless you have horizontal/sliding dropouts or an EBB, or are lucky and get the magic ratio, then you'll need some way of tensioning the chain to stop it jumping off when you really give it some welly… so a tensioner is a good idea.

you could actually use an old rear mech as a chain tensioner. This will also help with any alignment issues.

If you get the right ratios you may have a tight enough chain without a tensioner. This is known as the magic ratio. Ask on here what you need for your type of bike and someone might have done the hard work for you.

Suppose it could work if it so happens that you can fit the chain at the perfect length so it has the right tension, although I'd think this is pretty unlikely. So I'd say that yes, you do need a tensioner.

It isn't the ratio that's magic, it's a fortuitous combination of ring, cog and chainstay sizes that will allow a tensioner-less setup.

Magic ratios are not so magic once you've ridden them a bit (what seemed tight at the start of a one hour race ended up coming off) even more so if you are using a ramped ring/cog. So yes, a non-ramped ring/cog and a tensioner are pretty much essential if you are on non-SS frame and doing anything other than pottling about town.

Using a ramped cassette cog which has been designed for the chain to easily slip off is probably a bad idea. Especially if you're using a fixed tensioner, magic ratio, ebb, anything which allows the chain to stretch without automatically adding tension.

shortcut – Member
you could actually use an old rear mech as a chain tensioner

Better than tensioners that I have used.

A ghost chainring might also do the trick.

No need for spacers on the freehub though, the sprocket will self align …

Sam… I was just thinking that after I posted, but then I thought about my Solitude, it's got sliding dropouts and I only have to change the tension on it every now and again, and certainly not after just an hours riding, so it shouldn't make that much of a difference…

But of course, when it has stretched(I know, it wears really) that's when the magic ratio loses out, as you need to fit a new chain as you can't take the slack out.

Cheers everybody, that's really helped. I'm sure I've got an old rear mech kicking around. Shame though, adding the extra bits – just like a 'geared' bike really – adds the bit of extra weight I was hoping to lose in this project !
So….why was I making this bike singlespeed again ?? Lighter ? Cheaper ? Less dangly bits hanging down ? mmmm…probably not then !
More pain for my knees ? That's the one !
Oh shoot !

So long as you get a straight chainline you can get off with some slack for most situations, but a TTTI* event will soon persuade you that it's better to play safe and have the "gubbins"

*Testicle Top Tube Interface

You might be lucky and have a magic ratio meaning

Although most vertical dropouts allow enough variation in wheel insertion to find good tension pretty easily. As the chain etc wears you just drop the wheel a little further in. Disc brakes soon let you know if you've got the wheel in straight and it'll let you know if it's not in far enough by falling out. Doing this I quite happily SS'd* a Soul, E8, Inbred (geared one), Armadillo, Merlin, Pastey, 456, Rock Lobster and Prince Albert. You really should use a non-ramped cog and chainwheel tho.

*Having said that I've been back on the gears for a couple of years now.