# Upgrade drivetrain to allow faster top speed

I have a bicycle with a Shimano Altus 2x9 drivetrain. It has 36 and 22 tooth chainrings and a 11-34 tooth cassette.

I am hoping to upgrade this to extend its high range (i.e. 36/11) without sacrificing the low range (22/34), and keeping it 2x. Is this possible at all? I understand that product recommendations are not allowed here, but I am not asking "what is the best". I am asking if there exists a solution to my problem at all (perhaps I am trying to get too much range from a 2x?), and how I might go about finding it and researching compatibility.

I am not experienced with bicycle technology, and I became quite lost when trying to understand what the compatibility constraints are, and what can be mixed with what. For example, I found out that chainrings cannot always be swapped freely, and that not every small / large chainring combination is possible due to pin placement. It is also unclear what are the limits on largest rear cog / large front chainring combinations, what the relevance of the chainline numbers are, etc.

To make this question more concrete, here's an example of trying to find a solution:

Ignoring compatibility, I can do the following quick calculation: I am looking for at least a 10% improvement. At the rear, I seem to be limited to a 11 tooth cog, which means that I need a larger large chainring, with at least 40 teeth to get the 10%. I see that Shimano has 11-42 tooth cassettes for 2x10 systems. In order not to lose the low range, I would need 42/34*22 = 27 tooth chainring. Let's say 28, as that would bring minimal loss.

After some googling, it seems that 40/28 cranksets exists for 2x10 systems in the form of FC-M785, although it seems to be an old model with low availability and high price. It is unclear if it is possible to just change the chainrings on my current Altus crankset (chainrings seem more available). Recently, I learned about the concept of "capacity". For the combination I propose, it seems I would need a capacity of 40-28 + 42-11 = 43. However, the 2x10 rear derailleurs on Shimano's website (RD-M5120-SGS and RD-M4120-SGS) seem to have a capacity of only 41 teeth.

At the moment I am "running out of gears" and want to pedal faster than the 36/11 combo permits. On a good day I am pedaling continuously in top gear, and I would like to be able to go faster.

• 22-34 is a very low/easy gear. Are you sure that you need such an easy gear? How often do you use that gear in your typical ride? You didn't mention your tire size, but at on a 700 (29 inch) by 35 wide tire, you'd be going at a walking pace at 80 RPM. Typically you wouldn't need such an easy gear unless you are carrying heavy loads going up big hills. Jun 21 at 20:37
• @Kibbee Yes, it is walking pace, but I do use it occasionally in the woods, to get up very steep sections of uneven paths. I could just walk up instead, but I would prefer not to lose a gear that I do in fact find myself using from time to time. Jun 21 at 20:55
• This is an every day use, do-it-all bike for me. I use it for everything from daily commutes through weekend trips to accessing parts of the woods which are much too far to walk, sometimes on rough trails. Jun 21 at 20:58
• If the bike has Altus components, its unlikely to be worth the money it will cost to do this upgrade. One problem that increases expense quickly is parts compatibility is easy to get wrong. You are probably better to sell the bike and buy a replacement more suited to the riding you are doing. Jun 21 at 22:23
• Holding 36/11 on a 29er at 100rpm is 45km/h. If you can regularly hold that speed on the flat, then you DESERVE a whole new better bike, ideally one that somebody else has paid for. Jun 22 at 6:36

Your math and the reasoning are correct.

The main issue in your problem is to find a compatible crankset. The points to consider: the bottom bracket width, the kind of bottom bracket that you have and the clearance between the frame and the chainrings.

If your bottom bracket is 73mm, you are limited to MTB components (or trekking if you fancy the idea of 3 chainrings), 68mm, you can install road cranksets and MTB ones with spacers (but I would be surprised that you can).

For the bottom bracket kind, that would depend on your bike: bottom brackets in the Altus range are not Hollowtech II in Shimano's parlance, so you might have to change the bottom bracket for increased compatibility.

And last but not least: clearance between chainring and the frame: you simply have to measure and see if there is enough space to fit bigger chainrings. That is often a limiting factor, especially with mountain bikes. The only thing to do about it is changing the frame (or the bike).

For the market research: assuming you want to stay as close as possible of the Atlus range, the easiest is to go Microshift Advent. They have a 11/42 9-speed cassette, with a matching clutch derailleur (long cage, that has a capacity of 47T). For the crankset, there is a 46/30 in the Acera range (FC-MT210-2, Hollowtech II). You will also probably have to change the front derailleur (Acera/Alivio or Microshift). Except for the 30 teeth instead of 28 for the small chain ring, it will match your reasoning. You will also need to change the chain, the one you have with your current setup will probably be 46-36+36-34 = 12 links too short. Unless your chain is brand new, given it's a consumable part, better to change it entirely rather than add the missing links.

11-teeth sprocket is indeed a lower bound, unless you change the rear hub to have a SRAM XD or a Shimano Microspline, that would allow a 10-teeth small sprocket. But then you will enter the philosophical question whether it's "worth" putting high-end components on an entry-level bike. I don't want to seem pedantic: given components are proportionally more expensive if you buy them separately and the margins are increasing with price, it might be a better option to sell the bike as it is and buy a new one with the money of the sale and what you wanted to spend in upgrades.

The reason why product recommendations are not allowed is because the goal of the site is to make a "knowledge database", and product recommendations tend to be short-lived.

• One other reason why the 11-teeth sprocket is the lower bound, besides requiring special hardware to go any lower, is that the efficiency suffers greatly. The smaller your sprockets, the more effect any friction in the drivetrain has, and the higher the stress on the components. The SRAM method of having a tiny chainwheel in front requiring an ever tiny-er one in the back to go any speed at all, is, frankly, stupid. If you ask me, even 11 tooth is a bit on the small and inefficient side, and I'd prefer having larger chainrings both front and back. Jun 22 at 5:53
• @HorrorVacui It’s the smallest cog on a MTB - who cares about 5% efficiency or whatever at 40+km/h? You get a lot more bang for your buck the smaller the cogs go. Larger chainrings make frames worse because of the extra clearance needed. You also lose ground clearance, which is already at a premium. Jun 22 at 6:30
• The bottom bracket appears to be Hollowtech II. I don't know which series it comes from. Jun 22 at 9:44
• @HorrorVacui As pointed out by MaplePanda, I don't think efficiency is the biggest concern on a MTB for the smallest sprocket. But anyway, the market has decided, now that Shimano has decided to join the 10-teeth bandwagon, they also scrap all chainrings bigger than 36T from the XT/SLX/Deore ranges, if you want a bigger chainring, you can only take XTR or Alivio and below. Sourcing part is an issue now, so you can either stick to parts that are very difficult to get if not impossible, or make a small compromise on efficiency and have a wider choice for your parts. Quick choice for me. Jun 22 at 10:18
• MaplePanda, Renaud, Well, practical considerations notwithstanding, I was talking about the technical merits (or rather the lack thereof) of smaller cogwheels. Being primarily a roadie (and engineering nerd), I do care about the efficiency top end. That Shimano are switching as well is not a proof of the technology, just a proof of demand. 10-tooth cassettes are solving the problem they have caused themselves to begin with, all at a price of new, incompatible (and insanely expensive) components, honi soit qui mal y pense. Anyway, I HAVE ran out of gears on my MTB (and that's a 40/11 29er). Jun 23 at 12:03

It may be that your requirements would suit a wide-ranging triple chainset rather than a double. A common MTB triple might be 22/34/44 tooth which would suit you nicely, and 44:11 would give the top-end ratios you want.

Buying new is likely to be costly, so a cheap donor bike bought from the local recycling depot could be the best source of parts.

You will require

• a 3x left shifter
• a front shifter mechanism that expects cable in the direction your frame routes (ie top/bottom pull as appropriate)
• a triple crankset with chainrings that aren't too worn, with
• cranks of a length that suit you.

Note a lot of cheaper MTBs would have rivetted chainrings, which are essentially unchangeable. So don't get a worn one.

Also note that your frame could have areas where the larger chainring could touch the frame, and that a triple is wider which may throw the chainline off somewhat. This means that big-big is worse on a triple than it was on your current double chainring.

Additionally, the front mech has to push the chain a long way out to the big chainring, which can be a challenge for some frames - the tolerances sometimes just don't work. By sourcing used parts you can test out these ideas without a large financial risk, and you can always revert to the old parts.

• Agreed that the requirements are more suitable for a triple. However you don't necessarily need a new front derailleur, there should be no difference between a double and a triple here The difference in distance of travel is covered by adjusting the limit screws. Jun 22 at 5:46
• @HorrorVacui the derailleurs sold as doubles and triples have different cages. It is possible to limit a triple derailleur to work with double at least with non-indexed shifter, but I'm not convinced that it works the other way.
– ojs
Jun 22 at 6:05
• @HorrorVacui maybe a front mech for a double would work on a triple chainset, I know I have used one like that. But the FD for a triple tends to have more "range" to get across three chain rings, and they often have bigger plates on the left (inner) side. I know I couldn't get great performance shifting a quad chainring set, even using a triple FD mech.
– Criggie
Jun 22 at 10:20
• The front shifter can eventually be reused. I have a 2x9 on my bike (Acera), at it looked to be a 3x shifter with a different "indicator" (where what would be the small chainring is blanked). It can shift to a third imaginary small chain ring, but I have to adjust the stops on the derailleur to make sure that it doesn't happen. Jun 22 at 10:41
• @Renaud I have an Acera front shifter just like that on this bike, but I didn't know it can actually be made to shift to the third, blocked out setting. Jun 22 at 20:05