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I got a SH Commencal 24" MTB for my son. He loves it - except on climbing steep slopes where he runs out of steam.

It comes with a 7-speed 14-28T freewheel (Shimano MF TZ21) + Acera derailleur, and a single 36 tooth chain ring.

I added a photo in case it helps.

What would you suggest for getting the lowest gear ratio -- lower ? The cheaper, the better.

I have basic knowledge of bike mechanics (but I'd use this as an opportunity to learn) and basic tools (but then again, there's eBay).
If it's over my head, I'll ask a bike shop to do it for me, but it will add to the price and be less fun.

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  • Nice work - do bear these limitations in mind when the boy outgrows this bike, and you're looking for lower-low gearing some years in the future.
    – Criggie ♦
    Sep 22, 2020 at 0:28
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    Just two non-technical suggestions: Make sure his saddle is high enough (as a rough estimate, when he puts his heel on the pedal at the 6 o'clock position the leg should be fully extended). Second: It took me a long time to realize that it’s actually possible to pedal at only 60 or 70rpm.
    – Michael
    Sep 22, 2020 at 5:47
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    Depending on how strong the slopes are, there may not be a satisfactory solution with the given setup: The cassette only has a 2x gear spread, and even if you upgrade to a 12-36 cassette, you only get a 3x gear spread. And if you ajdust a 3x spread bike to climb 10% slopes, you won't be happy on the flats or marginal descends. Real mountains call for larger gear spreads like 5x or more. Imho, 3x is barely enough for stop-and-go in the city, 2x is plain too narrow for any use.
    – cmaster - reinstate monica
    Sep 22, 2020 at 9:57

3 Answers 3

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The specific model of rear derailleur you have is an Acera RD-M360 SGS, produced about 2009-2011, for 7 or 8 speed drivetrains. Shimano very helpfully provides archive specifications on the product info page.

Looking in the 2009 spec PDF we can see that the max size of the largest sprocket the derailleur will handle is 34. I believe 12-34 freewheels do exist, but you might have to hunt around to find one. That would be the cheapest way to lower gear ratios. (I'm guessing the crank on this bike does not have replaceable chainrings.)

Removing and installing freewheels is very straightforward, but requires a special freewheel removal tool to get the old freewheel off, and there are many different types of tools for different freewheels. See this Park Tool Repair Help video for how to select the correct tool. Other than that you only a large adjustable wrench in addition to basic tools.

A larger range freewheel may require the chain to be a few links longer, which unfortunately means sizing and installing a new one. I installed an 8 speed rear derailleur on my son's single chainring bike recently and used the chain sizing directions for 1x drivetrains in this Shimano manual. This is working fine so far.

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  • Thanks for the info! I'm now reading through the docs. I added a photo of the chainring, not sure what to look for. How can you tell the exact version of the derailleur? Lots of experience? :)
    – Cristian Diaconescu
    Sep 21, 2020 at 14:56
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    As I suspected the crank has a stamped steel chainring/spider permanently pressed onto the crank axle - these are very common on kids bikes.
    – Argenti Apparatus
    Sep 21, 2020 at 15:06
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    Re identifying the derailleur: The Wikipedia Shimano page has tables listing current and past groupset models for both MTB and road bikes. I just googled for images going back along the Acera models (M390, M360 etc.) until I found a match.
    – Argenti Apparatus
    Sep 21, 2020 at 15:11
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    @CristianDiaconescu I forgot to mention sizing the chain - answer now edited to address that.
    – Argenti Apparatus
    Sep 21, 2020 at 16:29
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    @mikes, I have not found a hub which needs the axle removed to remove the freewheel. I don't think this is true, you just need an appropriate tool.
    – abdnChap
    Sep 22, 2020 at 8:49
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While not suggesting its the best option, another option from a new cassette is a 30 tooth chainring. which will give close to same ratios as a 14-34 cassette.

Square taper cranks can often be picked up free or very cheap at recycle centers or local bike coops. A 30th chainring will cost about the same as a new cassette, and you can shorten the chain (saving cost of new chain).

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Shimano makes the TZ510 7-speed freewheel which is 14-34 teeth. It generally retails for under $20. You'll need to get a "freewheel removal tool" which is a splined tool on one side while the other side has a 1 inch hex nut. From Park Tool, the model FR 1.3, are less than $10 and a well equipped bike shop generally has these in stock. Be aware that a very similar looking tool is for cassette removal and doesn't work well on freewheels. The freewheel removal tool is longer overall than a cassette tool.

The increase in tooth count of the TZ510 will lower your gear ratios quite a bit. When I first experienced a rear end tooth count above 32 teeth, I thought for sure I could peddle right up a wall. Hills still require a good amount of rider input no matter how low a ratio one has, however they are a tremendous way to gain strength and endurance, especially so for a growing youth.

Back on the maintenance side of this, note that removing a freewheel oftentimes takes a very large amount of torque as the freewheel tends to tighten itself as the bike is ridden. I use a 1" box wrench that is about 18" long. A crescent wrench ( a brand of adjustable wrench) could be used as well, but my point is, the longer the better because it's gonna be tight.

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    I like to clamp the tool in a fixed bench vice and then apply force to the wheel.
    – Pzy
    Sep 21, 2020 at 22:50
  • @Pzy Absolutely. Prolly the best way, given a good vise and some room.
    – Jeff
    Sep 22, 2020 at 2:50
  • But don't be 'that guy' and use the vise technique to do it up.....
    – mattnz
    Sep 22, 2020 at 3:57

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