Bmach

I have a Cannondale Topstone 1, it has Shimano GRX with 46/30 up front and 11/34 in the back. I like to go to an 11/42 in the back, can I just change the real derailleur and cassette or is there more to it?

Thanks
Brian

csport

Most likely you will need a longer chain. Check the "big/big +1 " sizing method.

Badger6

Oh boy…

Yes, in theory that’s all you need to do, in practice it won’t be straightforward, and you’ll need to use a RD that is going to be beyond its capacity. There is plenty of anecdotal evidence that’s it can be made to work, but in practice, when folks are re bing honest, to get decent shifting on the big cogs (low), they sacrifice shifting performance on the small cogs (high). The good news is that Shimano RDs are well know to be spec’d conservatively in terms of their capacity, so it is definitely achievable if you are willing to have a bike that won’t shift as crisply or accurately as designed at the top of the range. So let’s run through it…

1- the cassette is easy swap. 11-34 cassettes require a 1.85mm spacer just like the MTB cassettes, such as 11-42, so this will already be present and you won’t need to source it. JUST DON’T LOSE IT! The bike will never shift properly without it.

2- RD. Pay very close attention to capacity. The 1x specific GRX RD actually has less capacity to wrap a chain than the 2x. But, the 2x RD also is spec’d to a max large cog of 34t. There are plenty of folks that have made it work with an 11-36 cassette (made by SRAM, and using an HG interface), which means 2t larger on the large cog BUT only 1t in excess of spec’d capacity (on a 46-30 crankset). Moving to an 11-42 so you can have gearing lower than most MTBs is where you need to be very careful. That is 8t larger than the specified large cog, and no matter how you get to making the pulley at the top of the cage clear the large cog, it will be be a long ways off from the little cogs at the high end and shifting will be imprecise on the last 3 or 4 gears. The most reliable way to get the RD compatible with a large cassette is to use a Wolftooth Roadlink. Just bear in mind that it doesn’t solve your capacity issue…and it was designed for use on “mullet” builds, no spec 1x systems.

3- You will need a longer chain, and you will not be able to use the 2 largest cogs with the large chainring, and 2-3 smallest cogs with the small chainring.

Lots of words, bottom line, it can be done. It cannot be done with a result that provides shifting performance as crisp or precise as the system was designed.

Bmach

so what is needed to get better climbing gears?

Badger6

Either better legs (I jest), or a wide range 1x (MTB RD on a massive cassette being driven by a small chainring 28-32), or….the manufacturers build out truly wide range 2x systems. The last part has not been done from what I can tell because of the engineering challenge of making a cage long enough to wrap up all the chain from top to bottom of the cassette that is also not hanging so low in the low gears that it risks being damaged by striking rocks/roots.

Di2 solves some of this problem, in that GRX (and road shifters) can shift the Di2 MTB RDs. On traditional cables, there is something called a Tanpan also made by Wolftooth that is designed to match the cable pull between a road (and now GRX) shifter and a MTB RD. I’ve never used or installed one, so I don’t know how well they work, but they’ve been in existence for a while.

I am not saying it cannot be done…I am saying it can be done as long as you understand the trade offs. Are you searching for lower gears because you’ve discovered you need them, or is it “insurance” in case you need them?

Bmach

Will doing the D2R2 this past weekend I discovered I needed them.

Thanks for the replys

dsbrantjr

(cough) triple (cough)

DiabloScott

If 30x34 isn't low enough, you just might not be a climber. Honestly, it probably won't help, even if you could make it work.

DaveSSS 

I don't always assume that everyone is extremely fit and under 30. A 30/34 is a relatively low gear. Most triples don't have a chain ring any smaller. I'm lightweight and very fit for my 68 years. I use sram axs with grx 46/30 or 48/31 chain rings and up to a 36T sprocket for 10% grades. I've gained some strength and now use a 10-33 cassette, but probably not in the winter or spring when I'm not in top shape.

Bmach

Ouch! Just did a 100k mostly gravel ride with 6600ft of climbing and was thinking bigger gears might help. I guess I will just curl up and give on these type of rides then.

HTupolev

What derailleur were you thinking of changing to? The RD-RX810 that's on your bike already is (along with the RD-RX400) just about the highest-capacity rear derailleur that Shimano makes for their 11-speed road cable pull. The RD-RX812 is spec'd to tolerate a 42-tooth cog (and it can readily do more), but it has less chain wrap, and its geometry isn't really designed to work well in multi-chainring drivetrains.
I do have multiple friends who are using the RD-RX810 with 16-tooth front differences and an 11-40 or 11-42 cassette, and getting it to work reasonably well. The derailleur has no trouble firmly wrapping several more links than rated (its cage is VERY long), so only a little bit of small-small cross-chain range is lost from slack. Shifting on their setups isn't as crisp as a narrow-range racing drivetrain, but it's reasonable. You may end up needing a hanger extender like a Wolf Tooth RoadLink.

S-Ride makes a derailleur called the RD-M520C that is nominally more in-spec for the setup you're intending to put together, although I have no direct experience as to how well it really works on such a setup.
You could also use a pull adapter like the 11-speed Wolf Tooth Tanpan to make your shifters compatible with an 11-speed MTB derailleur. Something like the XTR-series RD-M9000-SGS would nominally be pretty close to in-spec... the MTB SGS Shadow derailleurs are a bit of a compromise geometry when used in multi-ring drivetrains, but it does a good enough job that Shimano spec'd it for wide-range triples.

You'll also want to replace the chain. If our current chain was sized for your current gearing setup, it'll be too short for the new setup, and getting into the larger cogs while in the large chainring could have bad consequences.

A great solution, but unfortunately there aren't a lot of options for 11-speed drivetrains. There's always the "throw a friction shifter at some random crankset" option, but with 11-speed chains, that can involve quite a bit of kludging around to get adequate function.

My gravel bike has a 3x8, using a 48-38-24 crank paired to an 11-13-15-18-21-24-28-32 cassette, and it's a great do-everything setup for me: the 113" top gear is nearly always enough for spirited pavement, the 19" low gear is usually adequate for the mountainous gravel climbs, and the two larger chainrings 1.5-step the cassette to give me tight ratios when I want them. The gearing feels at home in a paceline cruising along paved flats at 25mph, and also feels at home crawling up loose double-track at 5mph.

There are very few contexts in cycling where someone bottomed out on their gearing wouldn't benefit from having lower gears. And 30-34 isn't all that low in the context of mountainous gravel.

DiabloScott

Didn't mean that in a pejorative sense... how to get lower gears is one of the most common questions in here, usually by people who aren't familiar with the issues, and use non-standard terms like "rear cassette". So I gave you advice assuming you were in that group.
I also assume you mean "smaller" gears... bigger cog.
And certainly gravel is different from road riding.
But when you get that low, your cadence has to be pretty high just to get any forward motion, and not fall over.

HTupolev

Even grinding away at 50rpm, a 30-42 gear on 40mm 700c tires is nearly 3mph. That's an awkward speed to ride at on flat ground when you're not getting any back-force from the pedals to balance your legs against, but on the steep ascents where you'd normally use these gears, it's nowhere close to where most riders start to have issues balancing a diamond-frame bicycle.

Low speeds can make it more difficult to stay rolling through loose or rough or technical spots. However, that's a tangential issue that doesn't really de-incentivize low gears, since snap and acceleration is useful when approaching the tough spots. There's a reason that mountain bikes often come with even lower gears than we're discussing.