differences in chainring BCD.

[mig]

apart from the ability to fit smaller rings are there any engineering based reasons why there are several different BCD sizes?

eg why is the common track based BCD 144? does it have a certain quality about the size that is specifically applicable to the usage?

or has each size just been adopted through history for branding etc reasons in the same way as, say, standard gauge railways, lengths of nails, screws, measurements of timber and so forth.

[thatsnotmyname]

It pretty much is the move towards smaller rings that has driven the development of smaller BCDs. I think Shimano were the originators of the 130 standard and 144 is still in use on the track, partly as a way of keeping flex down, and partly because 'that's the way it's always been' ...

[Brucey]

you can make almost any BCD and any number of bolts work, but it just makes it easier/better for some jobs if you have it 'about xxx'. As well as limitations on chainring size here are factors such as being able to slip a chainring over a pedal, or being able to swap all the chainrings without disturbing the RH crank which may or may not be important.

The rest of it -which is in truth most of it- is fashion, marketing and historical accident. 144 BCD x5 wasn't really much of a thing before campag started to use it on Nuovo Record chainrings. It displaced the previous Campag 150mm BCD x5 standard almost immediately and has been the de facto track standard for the last 50 years. It was also the de facto road standard for at least 15 years, until folk worked out that they wanted 39T chainrings (for some reason).

At one time nearly every manyfacturer made their own road chainset in 144mm BCD. The odd one out was arguably shimano; they copied the 'old' 150mm BCD in Dura Ace track but they used 130mm BCD on the road. Eventually they became the leaders and everyone copied them rather than campagnolo.

One standard I'm a bit fuzzy on is 110mm BCD. That was a thing before MTBs were a big thing, and it became very popular as a result of MTBs. I never expected it to become a road bike standard, because in ~50T size the chainrings always seemed to be rather flexy. I'm not sure who did it first though.

Small BCDs are hardly a new idea; look at the old three-bolt chainsets (eg the Herse one which has recently been resurrected). The stronglight 86mm BCD was popular for years, and you can't get much smaller than the old TA cyclotouriste bolt circle...?

cheers

[The utility cyclist]

Choosing different BCD to your competitors I reckon is in part down to tying in people to buy chainrings only from you or your designated suppliers.

Brucey, the mid 70s Dura Ace 7000 pitch 10 c/sets were actually 122mm, the 7500 was the only 151mm cranks Shimano used, slightly later produced than the pitch 10 and by 1986 the 7600 series were 144mm, the 7710 followed suit.

[scottg]

Small BCDs are hardly a new idea; look at the old three-bolt chainsets (eg the Herse one which has recently been resurrected). The stronglight 86mm BCD was popular for years, and you can't get much smaller than the old TA cyclotouriste bolt circle...?

cheers

The Stronglight/TA can be thought of as direct mount chain rings, circa 1937.
One ring bolts to the crank arm, the other ring bolts to the ring.
Just like a modern Easton or White Industries crankset.

TA did make adapteurs for other bcds that worked with the Pro Vis 5.

Spent Sunday drilling a Simplex Tourist chain ring to use modern chain ring nuts,
use a 9.9mm drill bit. 46/28, bolts hold the the 28 to the 46.

[Marcus Aurelius]

I’d have thought that having a larger BCD would also mean there’s less chance of a failure due to fatigue creep of one hole to an adjacent hole as well, as long as the holes were the same size. But it’s mostly practical considerations ( like ability to change rings over an in situ crank. as mentioned above ) and aesthetics / fashion / marketing guff.

[mig]

i always wonder why those numbers though.

so would a rider be able to flex a chainring of 86bcd more than say a 150 bcd? or t'other way around? or is flex more perceived than real? more to do with cutouts? or number of bolts? BB setup?

sub topic. looking to buy a 144 ring and 'gebhardt' keep coming up. yay or nay? hard or cheese?

[thatsnotmyname]

so would a rider be able to flex a chainring of 86bcd more than say a 150 bcd? or t'other way around? or is flex more perceived than real? more to do with cutouts? or number of bolts? BB setup?

It's not so much about the 'absolute' BCD, it's more the BCD in relation to the chainring size/tooth count.

sub topic. looking to buy a 144 ring and 'gebhardt' keep coming up. yay or nay? hard or cheese?

For what application?

[mig]

for winter fixed / 1/8th chain / good chain line.

[thatsnotmyname]

for winter fixed / 1/8th chain / good chain line.

Nothing particularly special about Gebhartd, but nothing wrong with them either. I'd go with whatever you can get for the least amount of spend. Miche, Stronglight, TA, whatever. Assuming you don't want to pay Dura Ace or Sugino Zen prices

[Brucey]

for winter fixed / 1/8th chain / good chain line.

Personally I'd favour a 144 setup not least because I have an old biscuit tin full of chainrings.

But otherwise I'd probably choose 130 BCD, simply because it is 'good enough' and the size is almost ubiquitous now.

BTW a larger BCD doesn't guarantee better stiffness, but it does make it easier to achieve.

However in your case there is another element to consider; I strongly suspect you are very intolerant of chain slack in your setups and that means that your chainsets need to be spot on, not eccentric in any way. You can get lucky at any price point but most cheap chainsets just ain't round enough for minimal chain slack when running fixed. No point in shelling out on a posh chain and sprocket if you have an eccentric chainset.

cheers

[thatsnotmyname]

Brucey - he's already got 144.

Agree about the chain slack/chain ring roundness - especially in a road application, where there is more likelihood of chain drop. But if the chain line is good, then this might be less of an issue.

[Marcus Aurelius]

for winter fixed / 1/8th chain / good chain line.

However in your case there is another element to consider; I strongly suspect you are very intolerant of chain slack in your setups and that means that your chainsets need to be spot on, not eccentric in any way. You can get lucky at any price point but most cheap chainsets just ain't round enough for minimal chain slack when running fixed. No point in shelling out on a posh chain and sprocket if you have an eccentric chainset.

cheers

Quite.

[Brucey]

the eccentricity concern applies just as well if you are only buying a chainring for a fixed gear setup.

Most derailleur setups with notionally round chainrings actually have slight eccentricity to them; those chainsets/chainrings would never pass muster for a 'just so' fixed gear transmission.

cheers

[Marcus Aurelius]

i always wonder why those numbers though.

It’s almost certainly tradition, given we are talking cycling here, and a lot of things that are done with cycling really don’t hold up to reasoned scrutiny. You’ll probably find that the original bikes used in ( now ) famous races, used chainrings machined by someone using a machine that was supposed to used for making tank parts, in the army ( or something similar) because that’s what was available en masse, because of the end of some military conflict somewhere. That’s the sort of thing that you find a lot of in cycling.