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We've been in the market for a mountain bike for my wife, and we've been looking at a BMC Twostroke AL 5 that's on clearance for a very good price at our LBS. We're almost ready to pull the trigger, but in test riding, my wife noticed that if she's pedaling hard and then stops and coasts, she'll get a lot of slack in the chain, and then the chain will snap back to being tight, creating a noticeable backwards jerk against the pedals. At first, I kind of doubted it was much of anything, but I took for a quick spin and noticed the exact same thing.

I managed to get a video of it here:

Notice how much the chain slacks up when I stop pedaling in the video (there's at least 3 good examples). I was wondering if we were making a mountain out of a molehill, but when we got home, I took my MTB and my hybrid (which has the same drivetrain as this BMC) out for a spin, and it's definitely not happening on either of those bikes. As somebody who rides several times a week, I'm very confident that this is unusual at least for the bikes I've ever ridden.

Despite this, the tech at the LBS insisted everything was normal. We even had him take it out for a test ride himself, but he did not think anything was out of the ordinary.

I guess my question is: is this a normal amount of play in the chain? Is this something that can be easily adjusted? I would think it's probably an easy fix, but it's concerning to me that the LBS disagrees that anything is wrong. I don't want to blow up the deal over it, but it's quite annoying to have the chain jerking back on the pedals like that while riding.

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  • Did you size the chain or is this as shipped?
    – Johnny Fiver
    Commented Oct 4 at 1:19
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    This is new from the store, so it's either the correct chain per the manufacturer's specs, or the LBS accidentally put the wrong chain on somehow... I have not personally checked the sizing, though.
    – John Chrysostom
    Commented Oct 4 at 1:49
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    Is the clutch on (assuming clutch derailleur)
    – Johnny Fiver
    Commented Oct 4 at 1:59
  • Thanks... it looks like a Shimano RD-M4120, which does not have a clutch, but another good thing to confirm.
    – John Chrysostom
    Commented Oct 4 at 2:03
  • Do you have a pic of the chain and RD on the largest sprocket?
    – Johnny Fiver
    Commented Oct 4 at 2:07

2 Answers 2

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The official term of this is "chain throw", it's caused by the inertia of a heavy cassette, which would be the case of an all-steel 11-46 cassette. This can cause derailments on the front, if this happens on not so smooth surfaces.

When you are rolling fast while pedaling and stop pedaling, the cassette will continue to rotate because of its inertia and lower the tension on the upper part. That part is normal. What will stop the rotation of the cassette is the internal friction of the cage pivot (even unclutched derailleurs have some friction, but for heavy cassettes it may not be enough to stop the cassette rotation instantly). If the chain in too long, the slack needs to be taken by the rotation of the cassette before the pivot frictions takes over, so that can be an worsening factor as well.

The angle of the video doesn't allow to see if the derailleur cage moves, but if it's the case, a clutched derailleur would solve the issue. But anyway, if the bike is intended to be used on rough terrain (even if not technical), a clutched derailleur on a 1x is a requirement for me. But if the bike is a really good deal, taking the bike and changing the derailleur can make it "only" a good deal.

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  • Thanks for this! The derailleur specs apparently max out at a 42 tooth cassette and this has 46, so I wonder if that's on to something... could be it doesn't have enough friction to overcome the extra inertia from that larger cog? I will definitely look into the chain length, and consider the advantages of a clutched derailleur as well...
    – John Chrysostom
    Commented Oct 4 at 10:12
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    @JohnChrysostom BMC made a sad cost saving move here: IIRC Shimano intends the M4120 as a 2x derailleur only (the front derailleur cage being a good enough mitigation against chain throw) and limits it to 42T because it's the biggest cassette for 2x. They also proposes the M5120 (which is identical, except for the clutch) for 2x (with the same limits as the M4120 in 2x, but that works with a 1x as well, and the max cog side is 46T in this case). The price difference between the M4120 and M5120 is 2-3€ when bought online, so strange that BMC spec'd this RD.
    – Rеnаud
    Commented Oct 4 at 10:44
  • Thanks. I'm going to tell the LBS today that this is a dealbreaker for us unless they can solve it. Sounds like shortening the chain or swapping to the M5120 would be possible solutions. I don't know that they're going to want to put a $40 derailleur on to make a $700 sale, but we'll see! If they can't get it sorted, there will be other bikes.
    – John Chrysostom
    Commented Oct 4 at 12:02
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    So, went back to the LBS, and they still insisted that everything was totally normal, and were uninterested in troubleshooting or (understandably) switching derailleurs. They disagreed that the M4120 was intended for 2x drive trains with 42T max, even though my searching seems to confirm that? I feel a bit gaslit at this point. Would have appreciated them at least saying "yeah, we know it's annoying and this doesn't happen with other drive trains, but it's not a safety or functionality issue so you might just have to treat it as a downside to this model and decide accordingly."
    – John Chrysostom
    Commented Oct 4 at 15:49
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    @JohnChrysostom For me, no clutch on a 1x is a functional issue, even if not ridden hard. I forgot a few times to enable the clutch on my wife's bike (a trekking e-bike, ridden on gravel at worst, with a RD-M6000 — similar specs than the M5120), the reminder was a derailment on flat gravel (or rough asphalt). FWIW, I did some test on my fun bike (RD-M5120, 11/42 10-speed), no chain throw with the clutch on, more chain throw with the clutch off.
    – Rеnаud
    Commented Oct 4 at 18:35
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Edited : As the bike is single chainring and my first answer presumed a double).

The gears are on the smallest cog, so there is not much tension on the chain. It is most likely be the chain is a little too long and derailleur is not providing any tension, or it could be the freehub (The bit that allows coasting) is binding a little.

I looked up the specs for the bike and rear derailleur. The bike is speced with an 11- 46thooth cassette, the derailleur has a specified maximum of 42 teeth. This limit may be because it is also marketed for double chainring setups more than any technical detail, and Shimano are known to have conservative specs. As long as shifting to the large cog works well I would not worry too much on this front.

At 41 tooth, the deraileur has more than enough capacity (required is 46-11=35). Given the capacity of the derailleur, it should be possible set the chain length to cope with the lowest gear (largest cog) and have good tension on the smallest cog.

Its a small job for the bike shop to shorten the chain if it is the problem. Get the shop to check it out, as is, while not bad, I would not be happy to accept the bike. If it is not the chain, it is the freehub. If it is the hub, it might just been sitting too long and will come right after a few kilometers of riding, might benefit from a strip down and relube, or might need a new freehub - practically a new wheel often works out cheaper.

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  • This is a 1x11 drivetrain, so can't be running small/small or large/large... Counting the other answer and a friend that I've asked about this, you're the third person to suggest that chain length may be the issue. I'll definitely have the LBS take a look at that. My wife and I were just talking, and we've decided we're not going to take the bike in its current state... even if there's nothing "wrong" it's very annoying to ride.
    – John Chrysostom
    Commented Oct 4 at 1:51
  • Thanks for the detail in the update. I don't know if the shop will be open to shortening the chain (assuming the right chain was installed in the first place), but I'm going to ask them to evaluate it. One of the other answers mentioned that cassette inertia causes this, and if it's a 46 tooth cassette with a derailleur that's only spec'd for 42 tooth, I wonder if the derailleur just doesn't provide quite enough tension to overcome the inertia of the heavier cassette?
    – John Chrysostom
    Commented Oct 4 at 10:10
  • With a bit more chain tension you can reduce, or even eliminate, the slop from inertia. A derailleur with a clutch would also help and a potential upgrade. However, a brand-new bike should be better than it is.
    – mattnz
    Commented Oct 4 at 21:00
  • Agreed... this was my first experience with BMC and, sadly, it has soured me on the brand. I have multiple Trek bikes in the same price range, with similar 1x Deore drivetrains, and none of them have had this kind of quirk when brand new. It's worth getting right.
    – John Chrysostom
    Commented Oct 7 at 16:44

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