Firstly observe that Shimano currently sell three types of chainset designed to fit a standard threaded bottom bracket shell: Hollowtech 2, which consists of a left crank arm and right crank arm/chain ring mount/axle, Octalink (aka Hollowtech 1), and square taper. Octalink was an attempted improvement on square taper, except it wasn't, so it's slowly dying. Square taper doesn't fit Shimano's marketing goals so if you want one then you're likely best looking elsewhere.
To this end there are three 'levels' of HT2 bottom bracket.
These are essentially: XTR/Dura-Ace, Ultegra/105/XT/SLX, and everything else. The most noticeable difference is the size of the external cups, 39mm, 41mm and 44mm respectively, each requiring a different tool. The smaller cupped BBs use smaller bearings, and there's ~10g between each one in weight. The three levels are completely interchangeable for any given HT2 crankset, but axle lengths vary between MTB and road, hence roughly six HT2 bottom brackets.
It might be the middle level has durability improvements over the bottom, but I have not seen evidence for that.
In terms of crank arms there are two major construction differences. Solid crank arms, which can be recognised from their sculpted inner arms:
And the confusingly named 'Hollowtech'
Essentially Shimano only markets a chainset as 'Hollowtech 2' if it uses both the Hollowtech 2 bottom bracket AND has hollow crank arms.
You can find Shimano chainsets with Octalink BBs and hollow crank arms as well, though these are I think EOL models now.
This is all just marketing in reality - the bottom-line is that the hollow crank arms shave pretty much 100 grams off the weight of a chainset, and you will pay a premium for this. Do they work better? I'd say it's safe to say that they do not.
Note that all Shimano crank arms are made of aluminium, but they do not specify the specific alloy. XTR & Dura-Ace crank arms are a tiny (perhaps 10g) amount lighter than lower levels as a result.
Currently hollow crank arms are a feature on 105, Ultegra and Dura-Ace, and SLX, XT and XTR. However there are some Octalink/hollow-crank-armed Deore cranks around.
If you have solid crank arms, then they will either be painted or anodized. Anodized should be more durable in its finish. There is not quite consistency with this, so check before buying - the cheapest MT200 chainset has painted arms, MT210 (which costs the same at retail) is anodized, but the supposedly higher level Alivio M4050 is painted. Current Sora & Claris chainsets are also painted.
All the HT2 cranksets have 24mm steel axles, which are likely identical across the range.
Besides paying for less weight in the form of hollow crank arms, you also get aluminium bolts rather than steel ones on the Ultegra/Dura-Ace and XT/XTR level. This saves the princely sum of 7g on a double.
Apart from hollow crank arms the other main form of weight saving comes in chain ring materials.
We can note the approximate weight of steel chainrings:
* 50t 220g
* 44t 170g
* 40t/39t 100g-120g
* 32t/34t 80g-90g
* 22t/24t 30g-40g
The cheapest Shimano chainsets are riveted and will weigh even more than this. These are found on the MT200/MT210 chainsets.
Half or more can be shaved off these weights using different construction techniques.
The first of these was a laminate, typically between aluminium and plastic, but also between steel and plastic. All but the smallest ring on a Shimano chainset has ramps. These ramps are designed to aid shifting from smaller to larger chain ring. Thus a 30t small ring for a road triple (which does no 'lifting') is different from a 30t middle ring on a MTB triple. With the laminate, instead of being metal, the ramps are plastic (or 'carbon fiber' if you must), so that the back of the chainring is plastic, and the front metal.
The chainset above has 3 steel chainrings, while the one below has alloy/plastic ('carbon fibre') laminate outer ring with alloy inner ring. This is quite visible, and your money is paying for these lighter materials.
All Shimano chainsets are ramped so your money is not buying better shifting, merely lighter materials.
Shimano boast about 'hollowglide' chainrings which are supposedly stiffer blah blah blah. This is obviously marketing nonsense, and is just a buzzword they came up with to reflect previous weight-saving techniques which are likely less relevant now they are using laminates.
A high quality steel should be more durable than aluminium, but the most important thing when selling bicycle components is weight, and durability is secondary. Thus for example, a double XT 11-speed chainset uses alloy/plastic laminate for the outer ring (34/36/38), and alloy for the inner ring (24/26/28). SLX 11-speed however uses alloy/plastic for the outer and steel for the inner. This makes the XT chainset around 30g lighter (another 7g coming from the alloy bolts), a rather trivial amount, but important for the marketing rundowns. The single XT 11-speed chainset is using steel + plastic for its sole ring.
However, the 12-speed chainsets use a new 'direct mount' system, which was copied from SRAM. These now uses a construction where the teeth are steel, the body plastic, and the arms alloy on the single chainsets.
The double uses all-alloy for the outer, and steel for the inner, on both XT & SLX:
There is a somewhat inverse relationship between the size of the chainring and the amount of wear it experiences, to the extent that Shimano is wary of alloy on small chainrings. Alloy seems to be 'good enough' on road, in that Tiagra, 105, Ultegra, Dura-Ace doubles are all-alloy, as are 105 triples.
Steel is inherently harder, and it seems that for single chainrings and for small rings in a MTB double it is objectively better.
As for whether your (now largely dead) 30t ring in a road triple should be alloy or steel, that seems slightly moot, since there aren't too many choices, and road double 34t rings are largely alloy as well. However Claris manages a steel/plastic 50t, and a steel 34t, and Sora alloy/plastic 50t, with steel 34t.
It may be that alloy is a good material for a 50t ring, and steel a better choice for 34t, hence Sora might be a better chainset than Tiagra which is all-alloy.
In terms of what this all means in terms of weights, Shimano's MTB triple (40/30/22) HT2 line-up consists of:
(aside to note that Shimano considers their chainsets to be 7 & 8-speed, 9-speed, 10-speed, 11-speed, 12-speed. I.e. 7 & 8 are the same, but the others are different. 7/8 is not provided for in HT2)
9-speed:
* MT200 (loosely 'Altus'), which is steel, riveted , with painted crank arms. This is likely ~1100g.
* MT210, which is the same with anodized crank arms.
* MT300 (loosely 'Acera'), with removable steel chain rings and anodized crank arms. Weight should be below 1100g
The above three are all very good value, since they are typically supplied with a HT2 bb. Here in Indonesia (they are made in Malaysia) they all cost around £25. The chainrings are the -AX type, i.e. 40t-AX, 30t-AX, 22t-AX.
* M4050 (loosely 'Alivio'), with the identical steel chain rings as on MT300, but with the chainring arms fitted to the crank made of alloy instead of steel. This weighs 1036 grams. Pricing here is ~£40. Budget users will probably prefer the cheaper model.
10-speed:
* MT500: This has painted crank arms and an alloy outer ring. It should be under 1000g.
* Deore M6000. This uses a steel/plastic middle ring. Weight ~900g.
* SLX M7000. This gets the hollow crank arms. 800g.
The 40/30/22 chainrings are all the -AN pattern hence interchangeable, but with weight differences where different materials are used.
There is only around £20 between these models, so the SLX is the best buy.
11-speed:
* XT M8000: There is an alloy 22t ring. 774g.
* XTR M9020: Uses titanium and other materials. 656g.
These chainrings are different patterns - BA for XT and AR for XTR. XTR is obviously ludicrously expensive.
Doubles:
9-speed
MT210-2: riveted steel rings
10-speed:
MT500 Aluminium/plastic and steel chainrings. Painted crank arm.
Deore M6000 Same but anodized arm.
11-speed:
MT600. As MT500 ~855g
SLX M7000 Hollow crank arms. 761g
XT M8000 Now alloy small ring. 718g
XTR M9020. Part titanium chain rings. 649g.
12-speed:
SLX M7100. Direct mount rings. Alloy + steel. 673g
XT M8100 . Direct mount rings. Alloy + steel 650g
XTR M9100. Direct mount, all alloy rings. 597g
The current touring triples are:
9-speed
* Alivio T4060, which has steel spider arms, unlike the M4050 or the discontinued M4060 (which is the same as T4060, but with narrower q-factor). It weighs 1100g in 44t variant, more in 48t. This is presumably an older design. Chainrings are the same, steel.
10-speed
* T551 Alloy/alloy/steel rings. 982 grams
* Deore T6010 Alloy/steel/steel rings, hollow arms
* Deore XT T8000 All alloy rings, hollow arms. 868 grams.
These three chainsets use the same chainring patterns, the AL type.
It's worth noting at this point that Shimano has apparently discontinued its Deore LX brand, and that T6010 might be considered to be LX level.
The new GRX 'gravel' groupset is essentially a road type, with either subcompact double or single, in both 10-speed and 11-speed variants.
It worth noting that the 10-speed and 11-speed doubles use the 100% same alloy inner chainring, while the outer ring has a different part number.
https://si.shimano.com/pdfs/ev/EV-FC-RX600-4579.pdf
Confusingly, both the outer rings have the same code 46T-NF. The chainrings are marked differently - 11s 46-30T NF, and 10s 46-30T NF
Whether there is any difference at all between the two chain rings beyond the printed code is something I would suggest to verify in hand.
Available visual evidence suggests that they are identical, and hence 10-speed and 11-speed chainsets are fungible
Anyway, if you pay the extra pounds for FC-RX810 you get hollow arms and alloy chainring bolts. Saving you 100g - 810g vs 710g.
Road chainsets go from:
* Claris R2000 '8-speed' steel chainrings, painted arms 1082 grams
* Sora R3000 '9-speed' alloy, steel & steel (triple) or alloy & steel chainrings , painted arms. 960g as a double, more as a triple
* RS400 '10-speed' painted arms, all-alloy rings ~800g
* Tiagra 4700 '10-speed' alloy/plastic & alloy, or alloy/plastic, alloy & steel chainrings, anodised arms. ~800g in double, ~900g in triple
* RS510 '11-speed' alloy/plastic & alloy, anodized arms ~800g
* 105 R7000 '11-speed' alloy/plastic & alloy, hollow arms (713g)
* Ultegra R8000 '11-speed' alloy/plastic & alloy, hollow arms, alloy bolts (674g)
* Dura-Ace R9100 '11-speed' alloy/plastic & alloy, hollow arms, alloy bolts (614g)
It's worth observing that you get reasonably (for certain values of 'reasonable') substantial weight savings up to 105 level, whereas beyond that it's largely burning money for no possible gain.
Also that RS510, R7000, R8000 and R9100 use the same chainring pattern ('MS' for a 50/34), but there are weight differences between each version.
The 5800/6800/9000 '11-speed' chainrings are cut differently ('MA') so don't work between generations.
Also worth noting is that Shimano used to have 5 arms on their chainrings, up to the latest Dura-Ace, Ultegra and 105 10-speed generations, but 11-speed is 4-arm only. Subsequent groupsets in 8/9/10 speed have a choice of 4-arm or 5-arm chainrings. The 5-arm variants are noted with -CG suffix.
So you pays more your money (to buy less) and you takes your choice. It seems that more expensive chainsets may be worse for some values of 'worse' than cheaper ones. In particular it is not obvious that an 11-speed XT double with alloy small ring is 'better' for the 40g saved than SLX with a steel small ring. And it costs more money. The extra money for XTR over XT is a lot, and is presumably LESS durable.
Aside from that inner chainrings are completely 'speedless', i.e. there is no difference between 9-speed, 10-speed and 11-speed chainsets, when it comes to inner rings. See also here https://si.shimano.com/pdfs/ev/EV-FC-M4050-B2-4288.pdf
9-speed and 10-speed outer rings appear to differ. While 10-speed and 11-speed chainsets appear not to differ. The differences can come in the ramp patterns on outer chain rings reflecting different chain widths.
It follows that if you have a single chainring, then they are speed-agnostic. Shimano's direct mount '12-speed' single chainsets are quite a bit lighter than their 11-speed options at the same money.
It seems that the very cheapest are the best value, since they should work as well as the most expensive, but if you want to save weight then there is some merit in doing so up to around the SLX/105 mark.
