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My girlfriend has an oldish Cannondale CAAD8 bike with Tiagra 9 speed triple. The rear derailleur is the RD-4500 GS with a 12-26 cassette. The front is a 50-39-30. She'd like to get a lower bottom gear as we plan on touring up some intimidating hills this summer... The issue is that the RD-4500 doesn't support larger than a 27 tooth sprocket at the back.

Would we be able to switch the RD-4500 for the Sora RD-R3000 GS? My hope is that it's 9 speed, so should be compatible. It has a much wider size (supporting up to a 34T at the back, total capacity of 43), whilst still supporting front chainring difference of 20.

I would then throw on a 11-34 cassette or something like that, changing the chain at the same time.

Is she likely to run into any issues? Is current-Sora worse or better than 15-year-old Tiagra in terms of smoothness etc. (though of course she'll lose out on some smoothness by widening the cassette)?

Is there anything else I should be checking - or any other good ways to achieve a similar range of gears? (She doesn't care too much about the fastest gears, rarely finds herself in them so if you have any options that involve sacrificing a bit of room at that end please go for it.) We're thinking about this option because at least she won't have to upgrade to 10 speed and change all the shifters, etc.

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    MTB rear derailleurs are a good option for getting larger max cog size. You can also swap the smallchainring for a 28 or 26. I currently have a bike with 3x9 Tiagra with 26-39-50 up front and 11-28 in the rear with a 9sp Deore XT rear derailleur.Another bike has 3x8 Sora with the same chainrings and an 11-32 cassette with an STX rear derailleur – Andrew Feb 25 at 0:00
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    If Shimano's documents say you can go up to a 27-tooth maximum sprocket, you almost certainly can use a 28-tooth sprocket without any issues and there's a real good chance you can use a 30-tooth sprocket. Shimano's documentation is notoriously extremely conservative - if the documentation says it works, it WILL work. Which means you can normally go a decent distance beyond what the documentation says works and still get acceptable performance. At 90 RPM, a 28-28 combination only gets you 7 mph at 90 RPM - much lower than that and you'll have to spin 100+ rpm just to stay upright – Andrew Henle Feb 25 at 0:27
  • If you can find a 30-34 tooth 9 speed cassette on your local auction website then buy it, Even if worn it will show you whether things might fit together. – Criggie Feb 25 at 2:45
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    You're effectively converting to what I've got on the tourer (3x9 Sora throughout, but I've got 11-32 at the moment) and RD-R3000-GS isn't hugely expensive.. As @Andrew says, MTB parts work too. I've borrowed the RD-M-3000 from my hardtail and it barely needed tweaking to get decent performance – Chris H Feb 25 at 6:36
  • I’d go for a 9 speed Deore ”shadow” rear derailleur especially if you are touring with low hanging panniers. – Michael Feb 25 at 9:39
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Others are mostly suggesting alternatives, but I think you deserve the answer to the option you asked about.

Yes, there should not be any problem with doing that change. It should work perfectly fine, you will just need a longer chain. I have this Sora with 11-32 on my road bike and I do not think the steps are unbearably large. My main bike even uses the same range on an 8-speed Claris and it works well.

I think you can go for your plan. I would not go for a rebuild of more of the drivetrain.

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  • I put my info in a comment precisely because I wasn't actually answering the question. Kudos to you for giving a direct answer! – Andrew Feb 25 at 14:31
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You have several options depending what is important to her, I am presuming cost is a factor.

11-34 cassette with new derailleur and chain (given cassettes and chains are consumable) is cost effective, but it does mean the jump between gear changes increases. If she does not need the higher gears then there are ways to bodge a cassette of say 14-34 that would close the gap (at the risk ramps would not line up making shifting less smooth)

Decreasing the chain rings from 30, as commonly suggested, means to to get the same ratio as 30/34 would require a 23/26 (reality 22 or 24) chainring, which almost certainly won't work with the existing derailleur and frame, and if it did, would also require new cranks, i.e. not cost effective.

A hybrid solution of a smaller chainring and larger, but not as large as 34 cassette, adds the cost of a chain ring but reduces the jump between gears, but allows you to tert some things before spending money that may not be needed This could be done in steps-try say 11-30 cassette with existing derailleur (see comments - pushing it, but should work acceptably), if that works, but low gear is still to high, smaller chainring. If shifting is unacceptable, upgrade derailleur.

A final option would be go to a 10 or 11 Speed rear. If you are already upgrading the derailleur, cassette and chain, it adds just the cost of a shifter (and cable inner). 10 and 11 speed will play acceptably with a 9speed front end and the wide range cassette means front shifting is reduced anyway. Obviously the decision to do this needs to be done before purchase.

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50-39-30 chainrings with the existing GS variant mech

  • Total Capacity: 37 teeth or less
  • Largest sprocket 27T
  • Smallest sprocket 11T
  • Chainwheel tooth difference: 22 teeth or less.

Big/Big of 50:27 and little/little gear of 30:11 is a total chain difference of (50+27)-(30+11) or 36 teeth.

Nominally you're at Shimano's posted limit for that mech

However I suspect you would get a 30 tooth to work well enough, but you'll need a longer chain. The B tension screw may need tweaking to make it work, which will decrease the engagement in small cogs a little.


Your other option is to leverage that 8/9/10 speed all use the same cassette depth, and simply go for a complete replacement of the rear end.

  • Tiagra 4700 10 speed with a medium cage derailleur goes to a 34 tooth and can probably run 36 fine.
  • 105 R7000 11 speed might work too - this depends on whether the existing rear wheel hub can take the overhang of the cassette. This has a clutch.
  • GRX RX400 might be suitable too - that can go to 36 tooth on a long cage derailleur, again with the clutch

However this will cost significantly more because you need a right hand shifter, rear mech, cassette and chain.

Depending on the rider, you may have problems with the two shifters not matching visually, which leads to a slippery slope of replacements.

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