Chainring compatibility

[Tomred]

Hi,
My bike came with an FSA gossamer 50/34 crankset. Its 5 arm and uses bcd screws. I'd assumed I have to use the same manufacturer to replace the rings - I changed the outer a while back - but looking around there are lots of chainring out there from many different manufacturers that look like they will fit.
Are the number of arms and BCDs the only factors I need to worry about as far a compatibility is concerned?
There are a range of prices out there. Are there other things I should consider?
Thanks in advance.

[Brucey]

bcd means 'bolt circle diameter'. Probably you are have 110mm BCD five-bolt chainrings; this is the most common sort with modern 34-50 compact doubles.

You need to worry about whether

a) you have 8,9,10, 11s chainrings or whatever. (You are best off with a close match)
b) if the chainrings you are buying are meant to work with one another
c) the chainrings are meant to work as part of a double or a triple or a singlespeed etc
d) that the chainrings are designed for a spider arm of exactly the same thickness as your chainset (they vary)
e) the timing of the chainring is correct for your cranks.

On the last point most chainsets have the crankarm set between two bolt holes, so all five chainring bolts are visible. However some have the crank in the way of one bolt, so that you can only see four of the bolts when looking at the RHS of the chainset. The latter sort of big chainring has the chainpeg fitted 180 degrees away from the former type and/or a special fitting for the hidden bolt, so you need to buy the correct one.

There are aftermarket chainrings that will fit and work other chainsets but many of them are not perfect because they are slightly different in some subtle way. Very much the safest option is to buy the correct chainrings that are specified for your chainset.

cheers

[The utility cyclist]

You do not have to worry about whether the chainrings are 8/9/10/11, they WILL all work, I've two FSA cranks, Stronglight, Shimano, TA rings, they all fit, the Stronglight is a 9 speed, the Shimano was from an 8 speed era MTB chainset, In fact I've fitted an old Sakae compact chainring from the late 1980s (six seed era) that fitted fine and worked with a 10 speed chain. Recently I've had an 11 speed set up on my old Carlton, the 1970s chainrings worked perfectly with an 11 speed chain and 80s front derailleur plus 70s rear derailleur.

The chainrings don't have to 'work' with one another, this is another misleading bit of information, this is manufacturer nonsense so they can keep you coming back to them for their products.
Also the timing is not important, plenty of rings have no ramps on them and shift perfectly fine.

[Brucey]

If you want your bike to work as per new, you need to worry about all the things I have mentioned. This is why I mentioned them....

For example I have seen some idiot try to run a 9s chain on a brand new 8s chainset (contrary to Mr Shimano's explicit instructions) and it nearly scrapped the frame.

You can cobble together all kinds of arrangements that look as if they work, but they are liable to not work perfectly and/or only work at all by accident e.g. because the parts are used (worn) already. For example you can usually run 9s chain on worn 8s chainrings, provided the chainrings are worn enough and not too dirty. Otherwise you usually end up in (big) trouble.

You only need look a few posts away to find that someone is not able to run 11s chain on a 9s chainset after all....

In this case my advice most certainly remains to buy the correct parts if you can.

cheers

[Tomred]

I certainly want good shifting.
My current crankset is a 10 speed compact. I've added a snap of the rear of one of the cranks.

I was a bit overwhelmed with the choice. I notice there's quite a lot of difference in where the bolts holes are positioned. Perhaps that what Brucey means by spider.

I shall have to search more throughly.
Thanks.

[Brucey]

if you want to understand how modern chainrings work there is good article here

https://janheine.wordpress.com/2018/08/20/rene-herse-11-speed-chainrings-in-new-sizes/

In which an ancient chainset layout is brought kicking and screaming into the 21st century, where old-style chainrings without shifting aids are replaced with matched sets of pinned chainrings giving modern shifting performance on 10/11s drivetrains.

This photo show much of what makes the biggest difference.



the chain is shown mid upshift onto the big ring. You can see that the shiftpin on the big ring has snagged the chain so that the shiftpin lies in the middle of one chain side plate, and is fed thence to the teeth on the big ring, quite cleanly. For this to be reliably achieved, the pins on the big ring need to be exactly timed w.r.t. the teeth on the small chainring. If the rings are not a matched pair, the chain might snag near one of the chain rivets (so it won't be lifted cleanly) and then even if it lifts, it won't be fed cleanly onto the big ring; it will 'sit on top' for a fraction of a second before dropping into the teeth. That's if you are lucky; otherwise it might just drop off the ring altogether.


So modern big rings are usually marked with the chain type eg "10/11s" ; (they are close enough in width that an 11s chainring will also work as a 10s in most cases, but not always vice versa) and perhaps "50/34T" (on a 50T ring). This indicates that this 50T chainring is specifically intended to work with a matching 34T chainring from the same manufacturer. That chainring will have a specific timing of the teeth with the bolt holes; for example IIRC on most FSA 34T five-bolt chainrings the bolt hole 'ahead' of the crank is lined up exactly with one tooth on the chainring. A shimano 34T chainring might have a different alignment, such that it might be possible to make it work with an FSA big ring or not, depending on how the inner ring is installed.

With triple chainrings not only do you need to keep track of the alignment of (say) a 30-something- T ring with the big ring, but also how the inner chainring is timed to it as well. The middle ring of a triple also has ramps and pins on it.

Of course for decades chainrings and sprockets didn't have ramps and pins etc. But the shifting was a bit (or a lot) crap too, and (IME) the front shifting just gets worse as you run ever narrower chains and try and use plain, (or worse yet mismatched) chainrings. If you are used to modern niceness you will certainly find it a bit of shock to be dragged back into the dark ages, and that is if it works at all, of which there is no real guarantee.

cheers

[ElCani]

For example I have seen some idiot try to run a 9s chain on a brand new 8s chainset (contrary to Mr Shimano's explicit instructions) and it nearly scrapped the frame

I’m interested in this, I know of 9 speed bikes sold with eight speed chainsets (Shimano). I worked on one with terrible front shifting, the chain was ending up between the rings and all sorts. Investigating the whole set-up led me to discover the mismatch in parts, but the bike came from the factory like that.

How was the frame nearly destroyed?

[Brucey]

For example I have seen some idiot try to run a 9s chain on a brand new 8s chainset (contrary to Mr Shimano's explicit instructions) and it nearly scrapped the frame

How was the frame nearly destroyed?

in the simplest terms it was chainsuck; the 9s chain got jammed on the most heavily manipulated teeth and was carried into the chainstay. Oddly enough it seemed OK on the workstand but manifested the fault when the bike was ridden.

cheers

[Valbrona]

I'm guessing they're FSA Pro chainrings - widely available, decent price. Just as well to stick with the same. Or maybe FSA Super?

Where it says on your chainring 'N10' or 'N11' or whatever, the 'N' denotes Shimano and the number the speed. It might in fact say 'N10/11', meaning the chainting is suited to both 10 and 11 speed Shimano set-ups.

[Tomred]

Ah ok. My googling for that part didn't show much.

It looks like I have the N10/11.

[Tomred]

This looks like the right one
https://www.wiggle.co.uk/fsa-super-compact-road-n1011-chainring/
It's out of stock at the moment.

[Tomred]

Touching on my original query, as finding an actual FSA N10/11 50t is proving a little tricky, could I install this Shimano one
https://www.merlincycles.com/shimano-105-5700-chainrings-65549.html
It looks like it will fit and is 110 BCD, 50t and 5 arms that look like they'll align.

[Brucey]

yes, superficially the chainring looks like it will fit but

1) the lateral offset/dish in the ring may be different and/or
2) the phase of the teeth may be different.

On the latter point the clean-ness of the shift onto the big ring (as per the photo upthread) may well be adversely affected if the phasing of the inner ring doesn't match the phasing of the outer ring exactly. If the alignment is wrong you can rotate the inner ring around to any one of five positions and (for a 34T ring on a five bolt pattern) you can move the tooth phase 1/5th of a tooth per bolt hole. However you may always be left with an error of ~1/5th of a tooth which may not sound like much but it is enough to mess everything up.

On the former point most 10/11s chainrings are slightly dished on the left side, and the small ring that matches it is often made with a slight joggle rightwards. Both features are to allow such chainrings to fit to a RH crank arm ('spider') with an arm thickness which is the same as cranks for < 9s use. However not all cranks are made the same in this respect. If the chainrings are wrong in this respect then you can get various problems both subtle and obvious, the obvious including such as the chain sitting on top of the small chainring instead of meshing with it, or the chain hitting the edge of the big ring when using the small-small gear combinations.

I have known folk just buy a new chainset when their chainrings wear out; it often isn't that much more money and you get to buy something that you definitely can buy new chainrings for. Others seem content with aftermarket chainrings which don't give exactly the same shifting performance, but at least you can buy them. Weirdly it seems it is easier to buy FSA chainrings that are meant to replace shimano ones than it is to buy chainrings that are designed to fit their own chainsets.

FWIW I think the FSA and SRAM chainsets are often almost exactly the same thing, so you might find a SRAM branded chainring is actually a good fit on an FSA chainset.

cheers

[Tomred]

just buy a new chainset when their chainrings wear out

Is a chainset just the rings? If so I could buy a pair of rings and avoid any potential conflicts.

[Valbrona]

https://www.tweekscycles.com/uk/fsa-pro ... ng-900771/