Regular readers (hello to both of you) will be aware I’m going to ride my BSA Prima ‘Major Tom’ on the Eroica Britannia this year. The ride is 55 miles, and it’s advertised as being quite a challenge. By which I mean ‘hilly’. You know it’s going to be hilly when the rules specifically state “change of the gear ratios is allowed due to the difficulty of the ride.” This is making me think a little. My corner of Staffordshire isn’t really that hilly, and they don’t call the place where Eroica’s taking place the Peak District for nothing!
Both my rear derailleurs (a SunTour Cyclone MkII currently fitted, and the original SunTour Vx as shown above) are limited in the size of rear sprocket they will accept to a maximum of 26 (which is what I have), but has some capacity left over to accept a smaller chain ring than the 42 I’m running. The Vx has 24 tooth capacity, the Cyclone has 26. Currently the capacity used is 22, from 52/14 to 42/26, but to change the chain ring, one first has to know what size it is…
This is new to me, and I didn’t at first understand what BCD meant – Bolt Centre Diameter. As someone versed in engineering drawing I’d been looking for PCD – Pitched Circle Diameter. Same thing. Anyway, if you know geometry and trigonometry pretty well (and I’m one of those who actually does use trigonometry, Pythagoras etc fairly regularly) all you have to do is measure the distance between two holes, divide it by the sine of the angle between the bolts (72 degrees on a five arm crank, since 360 divided by five is 72) and you have your PCD. Sorry, BCD.
Well I measured it, it was almost exactly 8cm. This gave an answer of 136mm, which seemed pretty close to the established 135mm size. Just one problem. Sheldon said no.
Now if the subject is old bicycles and the late Sheldon Brown says it, it almost certainly is. He does not list 135mm as being a Sakae Ringyo size, and if it was, he would. It’s a Campagnolo size. So I trawled through some old Sakae catalogues I found, and just to muddy the waters some of their cranks are advertised as ‘interchangeable with Campagnolo chainrings’, but it wasn’t my particular model. These models are 144 mm BCD. So you think, well, look in the catalogue, find your model, find the BCD, Bob’s your uncle. Not so fast.
It’s not too hard to find a catalogue with my model in. It’s a Sakae Custom, specifically CTC-5DRG2, manufactured around 1982. But the catalogue didn’t have the BCD in, it just said ‘interchangeable with Sakae AE-5RG’ which means nothing to me. Maybe I was a millimetre or two out with my ruler, maybe it’s 130? I should measure again and check. Only problem is the bike’s 65 miles away at the moment!
Well I posted my conundrum on Facebook, and I got quite a lot of low estimates around the 110mm mark. Then someone finally said it was probably a kooky Sakae proprietary size that was long dead. This was disappointing in terms of readily available spares, but at least it was progress. I looked up Sheldon again and he listed 118mm as a likely candidate.
And what of my 8cm measurement? Well it turns out I measured the right number but remembered it wrong! If you divide 70 by sin36 you get 119. Sure enough, I finally did find a catalogue listing the BCD for my exact model, which uses a size Sakae calls Apex, as 118. What an idiot…
Now that the BCD is established, it’s on to the tracking down of some chain rings that fit. Thankfully after a brief look around I found a mail order place in the Netherlands with a few new-old-stock Sakae chain rings for sale. They’re not dirt cheap but not prohibitively expensive either, so I bought a pair – 53 and 40 – as with postage it made more sense to by two for £40 than one for £30.
I picked the 40 as it’s the smallest they had, and the 53 because I know I don’t need a smaller big ring than the 52 – I’ve been spinning out occasionally downhill recently. My cadence has increased a bit lately, up to around 80RPM, whereas before when I was riding just the three-speed it was probably around 65-70. Also I wish I had more of a difference between changing between rear sprockets and between chain rings, so I get a bigger effect when I encounter a sudden incline.
Here’s what my speeds are for each gear at 80RPM currently: (as I mentioned in a post previously, my ten-speed basically offers six gears in one of three ways, the transition from red to green is when changing from big ring to small ring)
And here’s what it would look like with a 53-40 combination:
Now this time the step between big ring and small ring is about double the step between rear sprockets, which feels like what I want without actually being able to try it out. The rings should reach me in a week or two, then I’ll try out the 40 on some of the hillier streets round about, and we’ll see if the reality matches my rather clinical calculations.
As for BCD vs PCD, I’m sure it’s just that someone somewhere misheard the P as a B, figured B must stand for bolt, so it’s just become an established thing.