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I recently bought a used Trek Checkpoint ALR4, with which I'm very satisfied. The gearing is ok so far for going on plane ground an light uphills, but when going on long and steeper hills it gets really hard to push which I don't want to sustain on longer rides.

It has a 50 and a 34 tooth chainring in the front and I was looking for a way to replace the 34 tooth chainring with a smaller one. But as far as I can see, I would have to change the whole groupset with both chainrings ant the cranks.

My question is if there is a way to do this without replacing the whole crankset or at least keeping the front derailleur. I couldn't find any evidence online that this is possible without replacing everything.

EDIT:

Rear Cassette: 10x 11-34.

I've got spare a 11-42 rear cassette from my mtb, would this one possibly fit without chainging the derailleur?

Update with the solution I went for:

I changed the rear cassette from 11-34 to 11-36 (CS-HG 81-10) which worked straight out of the box without any adjustments.

For the crankset, I went for the Shimano GRX 2x10 46/30 crankset which was easy to install. I had to shorten the chain by 2 links. It took me quiet a while to adjust the front derailleur and at the point where it shifted correctly, there was some friction when shifting into the higher gears on the bigger front chainring (The chainline of the GRX is 3.4mm wider than the line of the Tiagra crankset). With no possibilities for adjustemnts left, I ended up bending the metal of the derailleur a bit. Im pretty sure every professional would have told me to buy a new derailleur, but now it shifts even better than before all those modifications and the disfigurement that I did to the derailleur is not noticeable if you don't know it.

I'm pretty happy with the gearing of my new setup. Costs for the complete conversion approximately 120€ and 2 hours of my time.

Thanks for the help!

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  • What cassette do you have on the back. It might be easier to replace the cassette than to go with a smaller front chainring, as a 16 tooth gap is already quite large. Reducing the small front chainring with a small one might have some effects on shifting if you don't reduce the larger chainring as well.
    – Kibbee
    Jul 8, 2019 at 13:39
  • @Kibbee edited my question. But wouldn't changing the rear Cassette also lead to some negative side effects as well? And how much bigger chainring can i mount before I need a new Derailleur?
    – Gistiv
    Jul 8, 2019 at 13:44
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    Not a direct answer to the question, but a nice (and recent) article about bodging lower gears with a shimano setup: road.cc/content/feature/…
    – Andy P
    Jul 8, 2019 at 13:48
  • Shimano specs are pretty conservative. The RD-4700-GS Tiagra rear derailleur has a 34t maximum supported sprocket size. That means a 36t cassette would almost certainly work just fine. A 42t cassette might work, but it's really pushing things - the derailleur might not be able to take up that much chain, for example, meaning your chain would either go slack on the small sprocket and cause problems, or be too short to make it over the large sprocket and cause even bigger problems.
    – Andrew Henle
    Jul 8, 2019 at 13:55
  • (cont) Going to a 36t cassette is almost a 6% difference - that's probably enough to be noticeable.
    – Andrew Henle
    Jul 8, 2019 at 13:57

1 Answer 1

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The Trek Checkpoint ALR4 has a Tiagra 4720 series groupset. The FC-4700 crank as a bolt-center-diameter of 110mm which will not support a small ring below 34 teeth, so yes you are looking at a whole new crank to get smaller rings.

The RD-4700 derailleur will not work directly with an 11-42 cassette, although 11-36 will probably work OK.

You should be able to keep the front derailleur - 48-32 or 46-30 sub-compact cranks have the same tooth count difference between the rings. You would just need to drop the derailleur on the seat tube a little. From the pictures on the page you linked to the derailleur appears to be a band-clamp type so there should no problem moving it (some braze-on style brackets prevent the derailleur from being dropped to accommodate smaller rings).

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