How to find out chain wrap capacity?
 

[EDIT: How to find out rear derailer's MAX chain wrap capacity?]
I want to convert my Raleigh Olympian from 52/40 teeth chainrings to 50/34 teeth chainrings. Poking through this forum taught me that I need to somehow make sure that my rear derailleur will be able to handle the extra 4 teeth chain wrap capacity ((50-34) - (52-40)=4). How do I know if my rear derailleur will be able to handle this difference? Velobase.com has my rear derailleur but the entry for max chain wrap has been left blank. Can you tell me if my conversion will be successful?

Components:
Front deraulier: "SUNTOUR 7" written on it.
Rear derailleur: "SUNTOUR SEVEN" written on it. The exact model is SunTour Seven 1b.
Current chainrings: 52/40
Future intended chainrings: 50/34
Current cogset: 6 cogs 30/14 teeth (yes they are 6 and not 7)
Current chain wrap capacity = 52-40+30-14 = 28
Future intended chain wrap capacity = 50-34+30-14 = 32

2 related but less important questions:

1. Where can I find a 50/34 crankset in vintage-looking grey/silver color? Did vintage bikes even have those? Amazon and Ebay are only showing black colors.

2. When they designed my Raleigh Olympian back in the day (~1984), who was it targeted towards? Did they think that people wouldn't do some serious climbing on it? The reason I want a smaller small chainring is that climbs become grueling sometimes. In my backyard where there is a road called "Old La Honda", the average grade is 7.5% with max grade = 18% inside switchbacks. Those 18% regions have me out of the saddle and if I am touring with large panniers, I can only climb those regions if I am weaving.

You can calculate your chain wrap capacity by subtracting the number of teeth on your largest front chainring from the number of teeth on your smallest front chainring, doing the same with the gears on your rear cassette, then adding the two values. So necessary chain wrap capacity on a bike with a 44-tooth big ring, 22-tooth small ring, and 11-34-tooth cassette, would work out like this:

(44-22) + (34-11) = 45

Not my words but it came from this website chain wrap capacity ? CanooterValve

Thanks for the response! I actually got the terminology wrong. I don't want to calculate chain wrap capacity. I know it for my gears. I only want to know if my rear derailer can handle an additional 4 chain wrap capacity.

Oh gotcha... Look on velobase and if you can find your derailleur, it should have specs.
http://velobase.com/

VeloBase.com - Component: SunTour Seven GT (version 1A)

says max chain wrap = 36T

Thanks again for the responses. Velobase.com looks like an amazing resource! I found my rear derailleur on the website. It is actually Suntour seven 1b. But the entry in front of "Max chain wrap" is blank.

The GT series have the long cages, with that, probably not a problem- with a standard cage-
I don’t think so.

Another approach - try what you've go. Change the chainring. Shift into small-small. Does that work? I'm guessing the derailleur cage will be roughly straight back and unable to get full tenison of the chain. The chain may be hitting itself. But neither of these is a deal breaker if it doesn't bother the matter between your ears. It is a gear that you shouldn't be using very much. Working but meeting nobody's idea of kosher is OK.

If it doesn't, there may be other easy options. First, if you cannot make that derailleur work, it will probably be because it is a medium cage, not a long cage. SunTour cages could be swapped. I'm sure there are tutorials on line and if not, we can help you. I used to tour and race on the same Cyclone, just different cages. Cyclone - a lot lighter and sexier than the 7 and not interchangeable but no different in concept. You may be able to go to a coop and find a 7 with a longer cage. (It can be crashed and have a broken upper body. I used to rebuild my Cyclones after crashes and wear by buying crashed derailleurs from bike shop wrecked parts bins for $5 with the half I needed intact. Kept a Cyclone going for about 20 years and an absurd amount of miles doing that.

Ben

Exactly; as a sadist who insists on brutalizing all my RDs to their extreme limits, I feel trial and error is the surest way to calculate your chain wrap. It's especially fun on bikes with really long dropouts. It really expands the possibilities, if you're not afraid to mess with chain lengths.

"OK, let's see what this little sucker can handle..."

Look for 110 mm BCD cranks, often listed as 110/74 (74mm being the granny ring bolt circle), but you don’t have to put on a granny ring. There are plenty of those in silver. Excellent high quality C&V cranks include Sugino AT (actually the original 110/74), many Shimano models, Specialized “flag”, and lots more. Sugino still makes one as their XD600 model, although that one looks a bit less C&V.

"Old La Honda Rd." sounds like a segment of my first century-long (double metric, actually) back in the early 1970s, with my cousin, who lived in Los Altos at the time. I see what you mean about wanting some lower gears. :)

Any long cage SunTour rear derailleur should have no trouble with your proposed gearing scheme.

That was going to be my answer: "Check velobase or disraeli gears." :D