27-tooth chainring?
 

The hills in Vermont have grown steeper in the 25 years or so that I have been frequenting Bike Forums--something to do with the road crews not maintaining the grades properly, I think--so I have lately been reshuffling some of my gearing. One of my solutions is a very nice half-step and granny setup that gives me an acceptable high, nice even steps, and a maximally low low while still staying within the 34-tooth total capacity of a Suntour Cyclone RD and a reasonable 16-tooth drop from the middle ring to the granny.

The only problem is that it requires both a 27-tooth freewheel cog and a 27-tooth, 5-bolt 74 BCD inner chainring. I already have the necessary Suntour 27-tooth cog, but a desultory internet search hasn't turned up any 27-tooth chainrings. Did anyone ever make such a thing? If not I will sadly lower my expectations and sadly return to the gear calculator.

In my 40+ years of working in and around the bicycle industry I have never seen a 27-tooth chainring. It might exist but it would be a very very rare bird indeed.
IMO, exceeding the published capacity of your derailleur by one tooth is unlikely to cause issues. I’ve often pushed Shimano derailleurs well beyond their “limits” with no ill effects aside from a slightly slack chain in the small-small combination.

Here’s TA chainrings for 74mm bolt circle:
TA chainrings on Peter White Cycles

You can make one?

There is the rare 27T Suntour A sprocket and the Avocet freewheel sprocket to chain ring adapter. If you can find both. :innocent:

Jon, I have several 28t 74bcd rings, so maybe chop off a tooth, and call it good?

That sounds like a Cyclone GT mk2 to me - I use 2 of those.
Recently I was playing with the gearing on my new build and got a Cyclone GT mk1 for the larger chain wrap - 36 (but max cog 32 instead of 34).

It can just handle 37 wrap: 32+50 works fine but a bit slack in 13+32.

back 13-15-17-20-24-32 SunTour New Winner Ultra 6, front 50/32 (18 gap)
https://www.gear-calculator.com/?GR=...N=MPH&DV=speed

https://cimg5.ibsrv.net/gimg/bikefor...46d0b6c99a.jpg

Yeah, I wish! I'm not a machinist myself--I just play one on TV.

[gives self Homer Simpson-style fore head slap] I didn't think of that! It's way better than my idea of getting someone to tig-weld an additional tooth onto one of my 26s!

I have had a couple 27t TA Cyclotouriste (6-bolt). Not that it helps your situation, just for the record, TA made them.

I used one to do this Campy GS (steel) triple conversion. Obviously I had to lop off 3 of the original 6 bolt hole locations.

https://cimg0.ibsrv.net/gimg/bikefor...4d241b3232.jpg

I wonder if Willow ever made a 27t in 74 mm 5-bolt? I know they made a 29t, I used one for this Campy Record Triple, with the "factory" 100 mm BCD, the smallest ring you can put on a 100 mm circle:

https://cimg1.ibsrv.net/gimg/bikefor...257e7735d5.jpg

Did Avocet make one too or are you thinking of the Mountain Tamer? I'd love to see the Avocet if anyone has a pic.

https://cimg9.ibsrv.net/gimg/bikefor...7c2c1d12d7.jpg

They also made one to take two freewheel sprockets to turn your crank into a quad. Not very popular since most F.der.s can't shift 4 chainrings. Somebody (maybe as a joke) set one up as a quint:

https://cimg4.ibsrv.net/gimg/bikefor...c8d66a820c.jpg

Could work well as the captain's crank on a front-drive tandem, with a custom or modified F.der., like Routens and so many other Frenchies did back in the '40s-'50s. Didn't Jan Heine make one recently?

Okay, I guess I have my answer. But I did go back to the gearing chart and found something with a 26-tooth low that I hope will do just about as well. I knew that 27-tooth cog and the 47-tooth chainring I'd been hoarding would come in handy if I lived long enough.

https://cimg0.ibsrv.net/gimg/bikefor...a2ad776d21.png

Heine made a fake pic of a 5 for an April fool joke IIRC.

This looks good with a evenly stepped spaces on your two outer chainrings and using only a 5 speed freewheel. This is why I like triples and half step gearing. About a 21" low gear and a 93" high.

"One by? One By? We don't need no Stinkin' one by"

Ahhhhh--- my memory was slipping. I dug mine out and it is an Avid and not an Avocet. Also, it uses a 56BCD chainring instead of Suntour sprockets. :rolleyes:
https://cimg9.ibsrv.net/gimg/bikefor...7c8d3e9b01.jpg
https://cimg3.ibsrv.net/gimg/bikefor...e606c1b7d9.jpg
It comes with a 20T 56 BCD. A quick but incomplete search on eBay didn't show a 27T, 56 BCD chainring. Infact, I didn't find any 56 BCD rings.

TA made 27T rings for the "Cyclotouriste" crank; my wife has one on her bike. I don't know if they're still made. TA also used to make custom rings in any tooth count; mostly very large, often used for those land speed record attempts. I don't know if they still offer that service.

Thanks for the info.

Given current CAM capabilities it would be straightforward for someone to make one from flat plate.

I recall that there was a program available from the IHPVA that would print a pattern for a chainring of any random tooth count. It was up to the user to print this out, glue it to a sheet of aluminum, then drill and file until done. That’s how they did it in the ‘80’s.

Wow. That's a lot of work. It sounds like the kind of thing someone would do to make a key in a prison-escape movie.