differences in chainring BCD.

For discussions about bikes and equipment.
thatsnotmyname
Posts: 397
Joined: 23 Jan 2020, 10:23am

Re: differences in chainring BCD.

Postby thatsnotmyname » 21 Feb 2020, 11:03am

Brucey wrote: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


Eccentric chainrings - that is to say chainrings which are not quite round, as opposed to actual 'oval' rings - don't really matter on a derailleur setup, as the rear mech cage will compensate for variations in tension.

For track use, you can run a chain with much more slack than you might want to on the road, so 'imperfect' chain rings are far less of an issue.

scottg
Posts: 717
Joined: 10 Jan 2008, 8:44pm
Location: Highland Heights Kentucky,, USA

Re: differences in chainring BCD.

Postby scottg » 21 Feb 2020, 1:24pm

Marcus Aurelius wrote: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.


The French constructors had their cranks and brakes made by companies whose primary
customers were aviation related. Aluminum forging and machining, common on aircraft.
Thin wall steel tube, more aircraft bits, aircraft designers are the original weight weenies.
+++++++++++++++++++++++++++++++++++++
Why not the best, buy Cyclo-Benelux.

mig
Posts: 2165
Joined: 19 Oct 2011, 9:39pm

Re: differences in chainring BCD.

Postby mig » 21 Feb 2020, 3:40pm

yes i can't say that i like transmissions where the chain runs slack due to poor concentricity of the chainring. i do quite a few miles on that steed so like it to be at least 'okay.' it has a stronglight 144 set up on it and i'll probably change chainring, sprocket and chain when the clocks change and it gets put to the back of the shed again. i always found that 86bcd stronglight rings were quite poor in terms of roundness and had an eggring made many years ago. now that was good quality!

Brucey
Posts: 37471
Joined: 4 Jan 2012, 6:25pm

Re: differences in chainring BCD.

Postby Brucey » 21 Feb 2020, 7:34pm

thatsnotmyname wrote:
For track use, you can run a chain with much more slack than you might want to on the road, so 'imperfect' chain rings are far less of an issue.


you can do, but many riders absolutely hate it, and it is an obvious safety issue. You rarely if ever see much slack on chains these days whenever big track events are on. Needless to say if the chain is at all slack and the frameset is a floppy plastic one, it might be possible for it to come off in use; not good, not good at all...

FWIW a common reason why chainrings are not concentric is that cheap chainrings are manufactured by being pressed/blanked out. You will be able to see this in the finish on the cut edges. Mid quality chainring profiles are part blanked, part machined. Posh/good chainrings are machined all over and the process needs to go badly wrong before the chainring ends up eccentric.

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

thatsnotmyname
Posts: 397
Joined: 23 Jan 2020, 10:23am

Re: differences in chainring BCD.

Postby thatsnotmyname » 21 Feb 2020, 8:14pm

Brucey wrote:you can do, but many riders absolutely hate it, and it is an obvious safety issue. You rarely if ever see much slack on chains these days whenever big track events are on. Needless to say if the chain is at all slack and the frameset is a floppy plastic one, it might be possible for it to come off in use; not good, not good at all...


Not sure I agree, sorry. A lot of riders run track chains with as much slack as they can get away with, as it will minimise friction losses. As long as you can't lift the chain off the side with your fingers, or as long as the chain doesn't fall off when you spin the pedals and hold the bike up sideways, then it's unlikely to unship during use. The term 'slack' is relative though. At the very least, it needs to be slack enough so that it doesn't bind anywhere during the crank's rotation.

drossall
Posts: 4700
Joined: 5 Jan 2007, 10:01pm
Location: North Hertfordshire

Re: differences in chainring BCD.

Postby drossall » 21 Feb 2020, 8:24pm

Most modern chainsets seem to have a racing heritage. Anything that will only take a ring of 40T or larger. And I'd have thought the lack of standardisation was lack of incentive. Most manufacturers have been catering for a racing and derivative market for decades, at least where doubles are concerned. Everyone wants their bikes to look like the pros, so they buy styles that limit ring size, or else buy triples.

Brucey wrote: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...?


This. The TA and lookalike took tiny rings. But it lost out in the market, around the late 70s when the touring market had really shrunk and racing lookalikes were all that were on offer, except from really specialist shops.

Cyril Haearn
Posts: 12389
Joined: 30 Nov 2013, 11:26am
Location: Between the woods and the water

Re: differences in chainring BCD.

Postby Cyril Haearn » 21 Feb 2020, 8:26pm

Stronglight 49D with the bolts right by the crank is optimal, still got a 49D ring in use on my Gillott
Takes rings as big or as small as one chooses, simples :wink:
Nice one Cyrille, nice one son..
Cycling-of course, but it is far better on 49" fixed
We love safety cameras, we love life "1330"

Brucey
Posts: 37471
Joined: 4 Jan 2012, 6:25pm

Re: differences in chainring BCD.

Postby Brucey » 21 Feb 2020, 8:54pm

thatsnotmyname wrote:
Brucey wrote:you can do, but many riders absolutely hate it, and it is an obvious safety issue. You rarely if ever see much slack on chains these days whenever big track events are on. Needless to say if the chain is at all slack and the frameset is a floppy plastic one, it might be possible for it to come off in use; not good, not good at all...


Not sure I agree, sorry. A lot of riders run track chains with as much slack as they can get away with, as it will minimise friction losses. As long as you can't lift the chain off the side with your fingers, or as long as the chain doesn't fall off when you spin the pedals and hold the bike up sideways, then it's unlikely to unship during use. The term 'slack' is relative though. At the very least, it needs to be slack enough so that it doesn't bind anywhere during the crank's rotation.


Whatever chain slack you think you have, it will (transiently) get far far worse if you put a strong rider on and he starts mashing away full bore, even on a bike where you think everything is quite stiff, it ain't stiff enough to prevent this from happening. Unless you have been riding with your eyes closed it won't have escaped your attention that a chain which is almost taut at a standstill will be able to flap about like crazy when the rider is 'going for it'. Checking for excessive slack with no load is "necessary but not sufficient".

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

Cyril Haearn
Posts: 12389
Joined: 30 Nov 2013, 11:26am
Location: Between the woods and the water

Re: differences in chainring BCD.

Postby Cyril Haearn » 21 Feb 2020, 8:58pm

What happens, does the chain stretch, then get shorter when not loaded, does the frame flex?
Nice one Cyrille, nice one son..
Cycling-of course, but it is far better on 49" fixed
We love safety cameras, we love life "1330"

Brucey
Posts: 37471
Joined: 4 Jan 2012, 6:25pm

Re: differences in chainring BCD.

Postby Brucey » 21 Feb 2020, 9:03pm

everything flexes.

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

thatsnotmyname
Posts: 397
Joined: 23 Jan 2020, 10:23am

Re: differences in chainring BCD.

Postby thatsnotmyname » 21 Feb 2020, 10:01pm

Brucey wrote:Whatever chain slack you think you have, it will (transiently) get far far worse if you put a strong rider on and he starts mashing away full bore, even on a bike where you think everything is quite stiff, it ain't stiff enough to prevent this from happening. Unless you have been riding with your eyes closed it won't have escaped your attention that a chain which is almost taut at a standstill will be able to flap about like crazy when the rider is 'going for it'. Checking for excessive slack with no load is "necessary but not sufficient".

cheers


Brucey - sorry, I don't understand any of that. I'm simply saying that some level of slack in a track chain is desirable, to minimise friction loss and mitigate 'non-round' components. Nobody runs a tight chain on a velodrome, indoor or out. I've read the above a few times and it's not clear to me what you are trying to say.

Brucey
Posts: 37471
Joined: 4 Jan 2012, 6:25pm

Re: differences in chainring BCD.

Postby Brucey » 21 Feb 2020, 10:40pm

what I'm trying to say is that if you run some eccentric chainring and pass the test as you suggest, the chain might still unship when you are going for it, because everything flexes.

If you have the chain slightly too tight (which BTW I am not advocating), it often only makes a difference when you are not really trying; the rest of the time the bottom run goes slack because everything flexes.

A simple demonstration of this is to have the front brake on (or on a bike with no brakes, up against a wall) and to stand on the pedals, so as to pedal forwards. On most bikes you will see the bottom chain run visibly slacken under the applied load.

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

thatsnotmyname
Posts: 397
Joined: 23 Jan 2020, 10:23am

Re: differences in chainring BCD.

Postby thatsnotmyname » 21 Feb 2020, 10:46pm

Brucey wrote:what I'm trying to say is that if you run some eccentric chainring and pass the test as you suggest, the chain might still unship when you are going for it, because everything flexes.

If you have the chain slightly too tight (which BTW I am not advocating), it often only makes a difference when you are not really trying; the rest of the time the bottom run goes slack because everything flexes.

A simple demonstration of this is to have the front brake on (or on a bike with no brakes, up against a wall) and to stand on the pedals, so as to pedal forwards. On most bikes you will see the bottom chain run visibly slacken under the applied load.

cheers


Still not sure I'm with you tbh. I know there is no hard and fast rule, but how would you assess and set chain slack on a track bike? Given that the 'conventional' way is to spin the wheel and check the chain is not pulling tight anywhere in the crank rotation.

mig
Posts: 2165
Joined: 19 Oct 2011, 9:39pm

Re: differences in chainring BCD.

Postby mig » 21 Feb 2020, 10:57pm

ahhh yeah....i remember now.....open the sleeve of an 86bcd stronglight chainring and there's a little shower of tiny needle type bits coming out of the packet. some are still stuck to the teeth edges.
i did run some stupido road rings on a TT bike once that were TA. 58T or something like that. they were really well finished though. i'm sure they said 'vento' on. think i'll give their 144 track rings a bash if they're nice and round.

re. slackness. so if you're going for it on your track bike and the slop in the chain happens to coincide with flex in the frame then you could unship the chain. that's how i see it any road up. and i agree. my track bike is now used for little trundles on warm summer evenings. it feels lovely and secure under reasonable loads but i can still feel it flex when i welly it down a slight gradient on a quiet road. 631 frame, dura ace track chainset, open pro 32H clincher rims. i love riding that bike though. so quiet, so pure.

Brucey
Posts: 37471
Joined: 4 Jan 2012, 6:25pm

Re: differences in chainring BCD.

Postby Brucey » 21 Feb 2020, 11:18pm

thatsnotmyname wrote:Still not sure I'm with you tbh. I know there is no hard and fast rule, but how would you assess and set chain slack on a track bike? Given that the 'conventional' way is to spin the wheel and check the chain is not pulling tight anywhere in the crank rotation.


the worry is if the slack gets to be enough that the chain might unship. So if the chainring is eccentric you should be just as interested in the maximum amount of slack. It'll vary with the bike and rider but for sake of argument lets say that the flex under load gives you another 1/2" of vertical slack. The danger also varies with the chain and sprocket combination. IME the very worst combination is a slightly worn 3/32" setup where you have a 3/32" chain with no camber on the inner sideplates, you have a poor chainline and insufficient chamfers on the chainring teeth. If the vertical slack, mid-run, gets to be 1" or so (unloaded) then it might be 1-1/2" under load and that might be enough to see it coming off.

With an unknown setup you can vary the wheel position and chain slack to see what you can get away with, unloaded.

If the chainring is not eccentric then you can set the slack up more or less as you wish; IME up to 1/2" (unloaded) is unlikely to cause trouble.

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~