Make it fit: mismatched chains, powermeter spiders, aftermarket bottom brackets

27 October 2023
There are official compatibility charts — and then there is what in fact works well.

Sometimes, what is officially not compatible can work in combination even better than standard. Rarely, what is officially compatible can work not that well. Anyway, here is how to fit together something that they don’t want you to.
MINIMAP | Chains — ChainringsCranksetsBottom BracketsAlgorithm

1. Chains/cassettes

All bicycle chains have the same spacing between the pins — also called the pitch — 12.7 mm (1/2 inch).

The internal width of chains is also easy:
• single speed — 3.175 mm (1/8 inch),
• 5, 6, 7, 8 speeds — 2.38 mm,
• 9, 10, 11, 12 speeds — 2.18 mm,
• 13 speeds — 1.75 mm*.

(Note that a chain has in fact two inner widths, which are alternating. The above parameter is for the narrower one — the wider one is less important as it doesn’t restrict the width of teeth that must fit in. With one exception, to be mentioned later.)
Now, let’s forget about anything less than 9-speed and remember that modern chains have same internal width, except for Campagnolo Ekar (* The only other 13-speed groupset in 2023, Rotor, uses the 12-speed dimension standard — and hydraulic shifting!)

The overall width of the cassette has always been the same (-ish), so with each extra gear added, the sprockets had to squeeze closer together — and thus, the chains kept getting narrower externally to fit. I will not bother you with many digits, just remember that chains for different numbers of speeds have different external widths.

(For my fellow geeks who are into the numbers, note that some hundredths of mm are within manufacturing tolerances, and when I say 0.05 mm — that could in fact be 0.03 or 0.07. Doesn’t affect real-world compatibility, so let’s not overcomplicate.)

General rule: you should not try and fit a wider chain (so, for less gears) to a cassette with narrower spacing (so, for more gears). But you can do the opposite — an externally narrower chain can fit in-between wider-spaced sprockets just fine! In fact, oftentimes it even improves shifting performance, as many people report. Despite the idea that every manufacturer makes intricately shaped links so that they would work especially well with same manufacturer’s cassettes. In real life it doesn’t feel like that has any significance at all. And that shape is clearly not always functional.
That said, there is one more parameter that does affect chain compatibility — the roller diameter. It was supposed to be easy — there is ISO standard that prescribes 7.9 mm (5/16 inch) — but then for some reason Shimano has been making it 7.65 mm. And it seems every aftermarket chain manufacturer followed suit. But Campagnolo and Sram happened to have their own ideas.

So now we have the following situation:
• most of the chains on the market have circa 7.65 mm rollers;
• non-Ekar Campagnolo and non-AXS (non-flattop) Sram have 7.7 mm;
• Ekar has 7.75 mm;
• flattop chains for Sram AXS groupsets have 7.9 mm.

In the real world that means the following.

A) Non-Ekar Campagnolo and non-AXS (non-flattop) Sram chains are compatible with any cassettes and chainrings, except for — who would’ve thought — Ekar and AXS. However, if you install a non-Ekar/non-AXS chain to a non-Campagnolo/Sram drivetrain, it will slightly deform the teeth, and then you will have to keep using Campagnolo or Sram chains — or otherwise a new chain may skip.

B) If, vice versa, you use a Shimano, KMC, YBN, whatever other chain on a non-Ekar Campagnolo or a non-AXS Sram drivetrain, it will behave like a slightly worn one. You’d want to replace it sooner not to cause accelerated wear to the cassette and chainrings.
C) Also, most of the wear-checkers are calibrated for the majority of the 7.65 mm chains. Thus, for non-Ekar Campagnolo and non-AXS Sram chains they will be showing less wear percentage than in fact there is. (Even worse on Ekar and AXS chains, to the extent that regular percentage wear-checkers are not usable and you need either one operating in mm — of course with understanding of what millimeters are relevant for your chain — or a special roller-independent one.)
D) Ekar and AXS (flattop) chains are not compatible with other drivetrains. The rollers are just too big and will restrict teeth from fitting in. However, despite the 0.15 mm difference in roller diameter — which is more than with the rest of the groupsets — flattop chains somehow work on Ekar cassettes. Don’t ask me how, I have no idea. They just do.

(The most attentive of you could notice that this contradicts the above rule of “don’t put 12-speed chain to a 13-speed cassette”, but flattop chains are narrower than the rest of the 12-speed, and are actually closer to Ekar’s outer width.)

E) The exception mentioned in the beginning is Shimano 12-speed chains. This requires a brief explanation. As already noted, the chain inner width alternates between narrow and wide. Sram have invented the so-called narrow-wide single chainrings (and bottom pulley wheels in the rear derailleur) where the teeth also alternate, hence, matching the chain spacing. This makes for a better chain retention — the chain is less likely to jump off the chainring (or the pulley) when it is tossed around.
Shimano could not use this technology because of patents, and so they invented their own — where the inner chain plates extend outwards, effectively getting rid of the “wide” internal spacings and making all of them “narrow”. While the Sram narrow-wide technology could only be used where chain doesn’t have to shift and always stays on the same cog, the Shimano “narrow-narrow” technology makes the chain fit as snug everywhere, including cassette sprockets and double chainrings. This is good for performance — but makes Shimano 12-speed chains not compatible with narrow-wide chainrings and derailleur pulley wheels.
MINIMAP | Chains — Chainrings — CranksetsBottom BracketsAlgorithm

2. Chainrings

All double chainrings have the same circa 5 mm spacing between them. So, external chain width doesn’t really matter and it’s all about the internal. (Well, there is a rare scenario where the chain is narrower than the double chainring spacing — in which case it can fall in-between the chainrings.)

Remember I said above that the inner width of all 9–12 speeds chains was the same? Well, that’s not entirely true. See, in theory it should be 2.18 mm (11/128 inch), but in practice e.g. Shimano 11-speed is 2.32 mm while Sram AXS/flattop is 2.21 mm. (By now you must’ve gotten used to the idea that the written standards are not exactly cared for in the cycling industry.)

Luckily, it doesn’t affect the chain compatibility — as there are more serious factors in play anyway — but it matters with chainrings. See, Sram states clearly that AXS chainrings are backwards-compatible with non-flattop chains— which makes sense, they really should be.

But in fact, the teeth milled for AXS/flattop inner chain spacing are a bit small, both in width and length, for the majority of chains out there. And while they are technically compatible, the chain retention is promising to be poor with such combination. (Also, those teeth are sooo ugly :)
In this post I’m trying to strike the balance between not being like Sram and just saying “it is compatible” while there at least should’ve been a huge asterisk next to that claim — but also not delving deep into performance peculiarities of various component combinations. At that, here’s a separate article just on chain retention:

I will leave it at the following conclusion: in theory, all 9–12 speed chainrings are compatible with any multispeed chain — except for Ekar. And maybe AXS. And Shimano 12-speed if the chainring is narrow-wide. In practice, the performance may vary a lot (and it’s not just about brand-matching — read the linked article for details). Now you at least know why your chain keeps falling off even if your have same-groupset chainrings.
MINIMAP | ChainsChainrings — Cranksets — Bottom BracketsAlgorithm

3. Cranksets

Crankset consists of the left and right crankarms, joined by the spindle (axle, shaft). Where the right crankarm meets the spindle, chainrings are connected, either through a spider, or directly. For now, let’s assume the presence of the spider. Also, I’m only talking road and gravel here — as covering also mtb standards would bloat the post — but all the principles apply to the latter, only particular implementations may differ.
3.1. Spiders, direct-mount & BCD interfaces

The interface between the spindle and spider depends on the manufacturer and, sometimes, particular model. Shimano and Campagnolo use “spiders” integrated into the right crankarm. This solution limits your choice of chainrings — only those intended for said manufacturers’ cranks will fit — and doesn’t allow for using powermeter spiders (I’ll get to that).
A better solution is the direct-mount interface (to which both spiders and direct-mount chainrings can connect).
Sram mostly uses its 8-bolt direct-mount interface (other manufacturers have various implementations of their own). However, some models also have integrated spiders. E.g. the current generation of Rival has two different types of cranks: for double chainrings with an integrated spider, and for a single chainring with a direct-mount interface. That’s while higher-tier Force have just one direct-mount version which is used for either chainring config. So, they could do this — which is just better for users, and I hate the fact that manufacturers keep making things more complicated (presumably, because the more non-compatible standards, the more stuff they will end up selling).
If you’re lucky enough to have the direct-mount interface, you can choose a fitting spider with any BCD-standard to connect a multitude of various chainrings (you could even mix-match different small and large chainrings at one spider with two BCDs). BCD stands for “bold circle diameter” so e.g. “110 BCD” means that the bolts are placed on a circle with a diameter of 110 mm. That said, BCD interface can be 4-bolt or 5-bolt — or asymmetrical, which is another manufacturers’ shot at making things proprietary instead of universal/standardised — Shimano, Sram and Rotor are doing their own asymmetrical layouts which are not intercompatible.
The Chinese manufacturers (Magene, Xcadey, Sigeyi) have to be more competitive, and so not only they make nice powermeter spiders, but also some of them can fit both 4-bolt and 5-bolt chainrings with the same BCD — and even asymmetric ones — that’s one user-oriented approach!
(A powermeter spider is the cheapest way to get double-sided power — the price is comparable with a powermeter left crankarm from Stages or 4iiii, which are good options, but only measure power from one leg, which may produce less accurate numbers.)

While a separate spider with BCD interface is good, at some point there appeared chainrings that were too small to accommodate it (first in mtb, but now also in gravel and even road — which refers to my post on easier gearing that is linked at the bottom of this webpage). Hence, direct-mount chainrings were born. Basically, that’s just a chainring (or two) integrated with the spider — with the BCD removed from the scheme. Luckily, the manufacturers still haven’t got an idea to integrate chainrings right into the crankarm. (Although Sram is already integrating powermeters into direct-mount chainrings, hehe.)
3.2. Spindle diameter and length

Shimano, Sram, and Campagnolo had originally used 24–25 mm spindles made of steel. Apparently, someone thought they were too heavy or something, or maybe there was just not enough standards in the world :) Anyhow, they came up with aluminium spindles — but since 25 mm was not enough for those to be sufficiently stiff, they’ve been made 30 mm in diameter.
This caused an issue. The external diameter of classical threaded bottom brackets did not leave enough space for bearings to fit comfortably around the 30 mm spindles — while there exist such bb, they are too compacted to work optimally, and so there are all sorts of problems with them. Thus, on one hand, more roomy bottom bracket standards have been created (more on that later), and on the other, Sram have made its DUB aluminium spindle standard that has the diameter of 28.99 mm. Yeah, just 0.01 mm shy of 29. Because Sram, that’s why. Anyhow, just 1 mm smaller spindle solved the bearing fitment problem while still being stiff enough.
So, currently we have the following major spindle diameter standards:
• Sram GXP — 22/24 mm steel (Sram wouldn’t be Sram if it made just a spindle);
• Shimano Hollowtech II, Rotor 3D, etc. — 24 mm steel (some Chinese use aluminium, alas);
• Campagnolo — 25 mm steel (or, in top models, titanium);
• Sram DUB — 29 mm aluminium;
• Cannondale Hollowgram, Rotor 3DF/3D+, etc. — 30 mm aluminium.

Then there is spindle length but for that one we should talk bottom brackets first.
MINIMAP | ChainsChainringsCranksets — Bottom Brackets — Algorithm

4. Bottom brackets

To fit a crankset into a frame, a bottom bracket is required, in which there are bearings that allow the spindle to rotate. Being an interface between the frame and crankset, bb has two sets of measurements:
• frame-related — frame bb width and frame shell diameter (with threads or plain);
• spindle-related — overall bb width (affecting spindle length) and spindle diameter.

4.1. Frame bb standards

Originally, bottom brackets had been threaded — the classical “English” is still up and running under the name BSA, while “Italian”, “French”, “Spanish” & “Swiss” variations are things of the past. The BSA has two variations: road, with frame bb width of 68 mm, and mtb with 73 mm. Both are used in gravel bikes.
As explained in the previous section, the BSA bottom bracket has one downside — it is too small in the external (frame shell) diameter to fit 30 mm spindles inside it without the mentioned issues. Thus, Chris King and co have made another threaded standard — T47. Basically, it is the same as BSA but with a larger thread (frame shell) diameter. It also comes in the frame bb widths of 68 mm (road) and 73 mm (mtb) — as well as newer 77 & 86 mm variations. (There is now also T45 Colnago standard — just because the more standards the better.)
The threaded bb used to be all that was needed for steel or aluminium frames — just cut the threads in the bb-tube (shell) of the frame and screw in the bb — easy. However, when carbon has become the prevalent material for sporty frames the scheme got more complicated: make openings (holes) in the frame, then press-in/glue-in metal rings with threads — only to be able to screw-in a metal bottom bracket in which bearings are pressed-in. (Currently they can make threaded carbon, but that’s a very new thing.)

Clearly, if instead of all this you could just press-in the bearings right into the frame itself, that would make the scheme so much simpler, save weight and, potentially, costs. So, the cycling industry believed in its ability to meet the tolerances required for that to work — and the rest is history: infamous press-fit bottom brackets were born. (As we know today, the manufacturers were too confident in their ability to fit perfectly round bearings into supposedly round frame holes, so now the threaded bb are in trend again.)
There are 10+ press-fit bottom bracket standards, but we only need to understand now that at the frame shell side they are different combinations of:
• one of four existing external diameters (37, 41, 42, 46 mm — the latter is best), and
• a frame width out of the eight possible options (61, 68, 73, 79, 86.5, 90, 92, 95 mm).

There is also the PF30a/BB30a — the Cannondale asymmetrical bottom brackets. By default these “bb” consist of just two cartridge bearings (same as the symmetrical PF30/BB30) which are pressed-in from the opposite sides of the frame, with nothing in-between, so the frame width and asymmetry does not affect the bb itself, but only the choice of crankset/spindle (if not to try and fit an aftermarket bb which is a rare scenario — only for those who are really sick of C’dale signature bb creaking).
4.2. Spindle bb standards

Now that we understand how to fit a bb to our frame, it’s time to get back to the spindle size. In section 3.2 above I have listed the existing diameters (and prevalent standards that use each of them), but then there is also the spindle length. And that’s a minefield.

Spindle length can be measured in a different manner depending on the crankset. If the spindle is bonded with one of the crankarms, it is usually measured from the inside of that crankarm to the end. Which is not at all a precise measurement, because we don’t know how much of that spindle end can maximally (or should minimally) go into the other crank. If the spindle is a separate unit, same unknown is added to its overall length on the other end.

Thus, the only way to try and understand what spindle length will fit what bb — apart from guesstimating or trial-and-error — is to check the crankset manufacturer’s bb compatibility chart. However, there is a couple of potential issues with that.
Above is an example of such a compatibility chart. We can see from it that for an “mtb” two different spindle lengths are recommended: L131 for BSA 68 mm — or L136 for BSA 73 mm.

That said, a bottom bracket can have either internal bearings, that go inside the frame, or external ones, that are placed just outside of the frame — and, hence, increase the overall bb width (and, thus, the spindle length).
As can be seen from the picture above, external bearings can add some 2 cm to the spindle length.

So, when the manufacturer recommends spindle L131 for T47 73 mm — do they mean internal bearings or external bearings? I mean, it will simply not fit in the latter. Or why “mtb boost” has the same 148 mm spacing as “mtb wide” but the spindles recommended for them, with same bb, are 5 mm different? I could continue, but you get the picture. In the bottom of the chart you can see the list of “known” compatible and not compatible bb — which says it all.

Another issue is that that was a good compatibility chart. Many manufacturers don’t have even that.

Where a crankset manufacturer doesn’t give you a clear indication of spindle length for your frame/bb, you could ballpark by q-factor — parameter covered in detail in my post on bikefit & geometry linked in the bottom of this webpage — and/or chainline. In any case, you should consider these parameters when matching a crankset to a frame.
MINIMAP | ChainsChainringsCranksetsBottom Brackets — Algorithm

4.3. Crankset-to-frame fitting algorithm

1) Research the frame:

1.1) find out frame shell bb standard;
1.2) search what cranksets the frame has been equipped with as standard, and look up their —
— largest chainring combination (smaller ones should fit, but the larger may not),
— q-factors (you can most likely go with the narrowest — or wider),
— chainline range (the chainline can deviate within several mm);
1.3) for an existing bike, eyeball how close the cranks are to chainstays. Since you already know the q-factor, that will give you idea of whether you can go narrower. This also works for chainring clearance.

2) Choose the crankset:

2.1) I advise the minimal q-factor that would leave some clearance between the cranks and chainstays — even if you biomechanically need a wider stance, as you can always make it wider (I refer again to my bikefit & geometry post down below);

2.2) try to stay close to the original chainline (which mainly depends on whether you have 130, 135, 142 or 148 mm rear hub) — otherwise you could get issues such as accelerated chain/cassette wear, noisy drivetrain, and/or poor shifting quality;

2.3) provided same chainline, you are safe going same size or, better yet, smaller chainrings (I refer again to my easier gearing post down below) — but if you want to go bigger, consider clearance between the chainrings and chainstays.

3) Search for a bb that fits both your frame and your chosen crankset. Some cranksets have interchangeable spindles, and you can choose one that fits best. You could try and find a spindle length & bb compatibility chart for your cranks. There is a chance that there is no bb for a certain frame/crankset combination — well, you just can’t fit a square peg in a round hole.
5. Brake rotors

Somewhat odd here, but as a bonus I’d like to quickly share a couple of things regarding fitting non-standard brake discs.

First, while the 6-bolt standard is straightforward, centrelock may require different tools (regular freehub wrench — don’t confuse with a bb wrench — or a dedicated “external” centrelock tool) — or even non-standard lockrings (e.g. you cannot use your regular ones with Fulcrum hubs).
Second, brake rotors differ in thickness — the standard are 1.8 mm but there exist the 2 and even 2.3 mm thick. Thicker rotors weight more but — provided they fit in the calipers — will serve you longer and are harder to bend accidentally (the easiest to bend are Shimano Ice Tech, the braking tracks of which are made not of full steel, but a steel-aluminium-steel sandwich).
Third, and this is not a documented feature — take a look at how wide your brake pad is. Then take a look at the width of the rotor braking surface. Chances are the rotor braking track is narrower than the pad. I try to choose rotors that make use of all the pad width — not the most obvious, but a thing affecting braking performance.