I do argue that even on single speeds chain tension is not constant around the whole chain. Common recommendation is that when one pulls the chain up and down in the middle between cog and chain wheel, there should be about an inch of play. This means that when pedaling the bottom of the chain really is slack.

I do ride single speeds and fixed gears and I have a visible amount of slack on the chain. It is not a problem. Overtightened chain makes the chain stretch fast and a stretched chain eats the gears. The only place where tight chain is needed is where there must be an instant reaction from backpedaling (that would be fixed gear trick riding and fixed gear trial).

I argue that fixed gear commuting is not the kind of riding where instant reaction from backpedaling is an absolutely mandatory feature.

A few comments, though:

On geared bikes, derailleurs do the chain tensioning. If you ever jam on the pedals hard and suddenly, your derailleur might flop a bit, but the chain as it passes from the derailleur around the cassette should be properly tensioned. And never on a geared freehweel do you have forces coming from the top of the chainweel backward to the cassette (as opposed to fixed-gear). The chain is only ever taught on the top (from cassette to chainwheel) when pedaling, and never taught on the bottom (chainweel to derailleur).

On a single speed, your chain tension is constant through the whole chain. Because the chain is a determined length, Any tension or stretch is communicated through the whole chain, without the alleviation of tension that a derailleur provides.

This is why professional BMX bicycles and track racing bicycles come equipped from the factory with chain tensioners or “chain tugs.” Chain tension is significantly higher on a single-speed.

And finally, 80s 10-speed frames tend to ghost-shift when climbing steep hills, due to frame flex. This problem solved by single-speed!

Usually the reason for slipping rear wheel is that the drop outs are not parallel. If drop outs are not parallel, then the track nuts do not get full grip from the drop out and will slip.

So check that the drop outs are parallel with each other and parallel with the rear wheel.

-M