I was just about to spend 30 bucks online for a splined single speed cog when I saw the Dimension ones for sale. They're only 4 bucks. Has anybody ever used these? Is there anything wrong with them? I noticed that more expensive cogs have a very wide base.

https://www.jensonusa.com/Singlespeed...or-Singlespeed

I think you risk damaging your freehub body, because they are narrow. They will likely dig into your freehub. I guess there okay for testing out gear ratios, but not for the long run.
BTW, I have never actually bought one, but that is what I have read on MTBR.

surly makes wide ones.

Of course you can get replacement freehub bodies for $18 so replacing that with next cog still cheaper than $30. Freehub destruction a concern if you've got Ultegra or better or XT or better freehub body though. Don't wanna waste expensive parts. The fancier ones are lighter Al units so easier to chew up. If you're on a cheepo freehub body to start with then no worries.

OP, I was on my way to get Dimension for a project but a closer bike shop had Shimano for $5 so went with one of them instead.

Those cogs are primarily for BMX racing...they are the same width as Shimano splined BMX cogs. They are designed to work with a shimano type freehub body, so you shouldn't really worry about it.

I have have been using a mixture of Dimension, Shimano, and Surly cogs for years on my SSMTB with a tiny 32T chainring, and I ride up hills with it that require all the strength I can muster. The Dimensions work just as well as the Shimanos and Surly's. I mostly use Dimension 20T, 18T, and 17T cogs. I prefer the narrower cogs over the surly cogs because it gives me more freedom to tweak the chainline with thin mm spacers.

My steel freehub body still looks like new. It might dent an aluminum freehub depending on your strength and the size of your chainring. However 10 speed cassette cogs, which are commonly used with aluminum freehub bodies, are narrower still.

Edit: On second thought, I don't think any SS-specific freehubs come with a aluminum freehub body, so you probalby have nothing to worry about. And if you happen to be using a road or MTB hub with an aluminum freehub body, Novatec makes an aluminum freehub body with a single steel spline....the single steel spline adds a few grams but is sufficient to prevent notching. You could replace your freehub with this and be done with it.

Thanks guys. Learned something new!

My bike is a Redline Monocog and has Redline hubs. The splines are threaded and seem to have a nut on either end which screws down to sandwich the cog in place. I'm not sure what the freehub is made of. Either way, it seems like a cheap dimension cog will save me a bunch of money.