I'm looking for a 10 tooth cog for a Shimano hub. And before every goes all "Sheldon Brown" on me with gear inch calculators and questions about speed at X rpm etc, let me explain. This is for a friend who loves to push big gears, and there is a local TT that has a long downhill, that normally has a tailwind. It's a short TT and the average speed is around 32-33 mph for winners. He has a 58 tooth front chainring that he can put on, but that makes all of his gears taller, and he really only wants this one gear for a specific section of the TT. So, anyone have a lead on such a thing? I know it exists, but where to find one?

Are you sure it exists? I believe that smaller than 11 tooth simply is too small for to fit over a Shimano freehub body. You'd have to get into BMX hubs or have a Capreo freehub (that is incompatible with standard freehub parts) to get 10 tooth.
And just to go "Sheldon Brown" on you, try this.
https://www.sheldonbrown.com/harris/k7.html#capreo

There's no 10t cog for normal Shimano hubs. It wouldn't fit. You can go to a 9t cog on the Capreo hub- it's made for bikes with small (20") wheels. The freehub body is smaller than normal hubs.

Saw the Capreo, but the hub body is a smaller diameter. I think SPR, or SRP made a 10 tooth cog awhile back.

Not to get all gear-inchy on you, but is the diff between a 58-11 and 58-10 even enough to feel? I wouldn't know--I can't turn either one.

yes, as the cogs get smaller the more pronounced the difference. As cogs get bigger, its harder to tell the difference.

maybe run 52 or 53 or something like that as a small ring and the 58 as the big ring. it might take some work as you would have to flip the small ring around and remove any pin for the crankarm. just a thought...

or just use the 58. a 53x12 and a 58x13 are pretty much the same gear inches and a 53x11 and a 58x12 are similar as well

About 5 mph at 120 RPM. I could turn one... if the back wheel was lifted off the ground.

I know you said not to go Sheldon on you, but 53/11 at 120rpm is 46.1 mph I'm thinking that if you're over 46mph on the downhill portion it may be more efficient to really tuck in and recover anyway, given that the power it takes to go from 46 mph to 47 mph is a hell of a lot.

And I'm no TT expert, but my understanding of why some time trialers run bigger chainrings is not mostly to get a gear bigger than 53/11, but to make the other ratios, shifts, and chain angles work better. Going to a 10, as oppossed to putting on a bigger chain ring will not help with those issues.

Finally, the smaller the cog, the less efficient the drivetrain, so you'd have some frictional losses going to the 10.

Where did 120 rpm come from? Is that what you guys spin during TTs?

I doubt many people TT at 120 rpm's. However anybody should be able to spin effectively at 120 rpm, So for a short dowhill effort it would not be unreasonable to spin at 120 rpms.

But even take 100rpms and you're at 39 mph.

I'm betting on most courses for the vast majority of people that's plenty enogh gear, and if its not, you're still better off going to a bigger chanring.

Sorry, I just put 120 RPM in there because it provided a constant, and the higher the RPM, the greater the mph difference. As Waterrockets said, though, it's questionable as to whether there is any gain in pedaling vs. just tucking more aero after about 40 mph.

what if the downhill is at the end?

still, tuck and run over 40mph usually. You're better off getting aero.

If you're really hell bent on a 10 tooth cog, take a larger cog to a gunsmith and have them machine it for you. I know a few guys that used to do this to get an 11 tooth before they became common. But still, not 100% sure there's enough material to actually achieve it and/or have it strong enough to work.

Here is a pic of an SRP



Link

And another one

Oneup Components 10 Tooth Cluster > Components > Drivetrain > Expander Cogs | Jenson USA

I understand the naysayers, but I say GO FOR IT! Totally savage.

Edit: Oh shoot; I didn't realize this was a zombie thread!

Zombie thread apocalypse is alive and well.

Hatchets and sawed-off shotguns seem to be the tools of choice for the zombie apocalypse, but what are the tools of choice for the zombie thread apocalypse?

A torque wrench, of course.

Aside from thread necromancy, that requires a custom Oneup hub, so it wouldn't even work for the OP....

Was it just me, or did anyone else notice that everyone was missing the point? He wanted the 10 because he did NOT want to use the big 58t chain ring that was the alternative - perhaps because it would necessitate undesirable shifting to the small ring or something like that.

So, a gunsmith can take an 11-tooth or 12-tooth cog and turn it into a (useable) 10-tooth cog? That would be quite a trick.