On Tuesday, August 4, 2020 at 11:06:48 AM UTC-7, sms wrote:
On 8/4/2020 9:12 AM, AMuzi wrote:

snip

https://www.ebay.com/itm/202930103988



We agree.
That does exactly the same job in the same way as a used Tourney
derailleur with the stop screw turned in.
If you're putting that device on a bicycle with double or triple front
crank then it makes some sense, but for single front crank why would you
not just shorten the chain?

That description is quite amusing: "This single speed converter is very
suitable for the bicycle that suits for mountainous terrain" and
"Suitable for people who like cycling."
If you have vertical drop-outs you invariably have a slack chain that can throw the chain off of the chain ring. So whether you have one, two or three rings you're stuck using a chain tensioner of some sort. Andrew probably has an entire junkbox full of useful old derailleurs that would take care of the problem but since I've been cleaning everything out I'm stuck purchasing new.