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Gear slipping

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    Gear slipping

    Hey kids,
    New to the forum, so some background first. Bought a new bike back in April and added a bafang bbsd kit. Everything worked perfectly , but I found myself using top gear, 7th, almost exclusively.

    So I decided to swap to a larger chainring to make better use of the gears. Went from a 42t to a 52t. It worked great, but the chain was too short to get into first gear. Installed a longer chain, and here's the problem.

    In 6th and 7th gear, the chain slips. Much worse in 6th than 7th. There were no problems with the original chainring and there was no slippage using the new chainring with the original chain.

    Google searches point to worn gears, but like I said, they worked fine before the chain swap. Bike only has 1000km on it.

    So Any ideas.


    Is this just a 7 speed? How many miles did you put on it before the change?

    This must be fairly small wheels like 20's? Or you are really into speed?


      Yes, 7 speed , 26" wheels. Just did the change last week, About 600 miles before and about 30 since.


        Just based on the little we know so far I'm gonna guess that it could just be wear. 7 speeds don't come in high quality anymore and people can wear out high quality stuff in 1000 miles so 600 seems possible. A 7 speed small is likely at least a 14 which doesn't wear as bad as a 11 on a typical cassette but either way you have fewer teeth engaged to the chain so each tooth has to carry more force.

        The fact that a 42 wasn't enough for you kinda implies you like to go fast which also means a lot of stress on the driveline. Most likely the chain that came with that 7 speed wasn't the highest quality either so my guess is the chain and sprockets on the freewheel wore together which was still working for the moment because they wore together. The new chain now doesn't match the worn teeth so especially when you stress them they slip. New freewheel should get you back to normal. Note they take a special tool to remove so be sure to order that too if you are doing it yourself.

        You should probably buy a chain stretch gauge and monitor your chain wear so you can change it out before it causes extra wear to the freewheel. Maybe also up your chain lube and cleaning frequency to try and get a little more life out of stuff? Although 600 really isn't that bad especially for what is likely low end stuff under a lot of stress. There are others here running middle of the road quality stuff and only getting 1000 miles out of a chain. It can really vary depending on conditions and how hard you are pushing things.

        Just to put things into prospective a normal person can output 250w in short bursts and an athlete can maintain that. IF you are running a BBSHD hard you are running at least 4x more force through everything than they ever expected. Being a low end bike they likely didn't even plan on a constant 250w. I don't think they really expect cheap bikes to get ridden hard or often and you (and many of us) are doing both.


        • Retrorockit
          Retrorockit commented
          Editing a comment
          Just remember to factor in that top gear on a freewheel will usually be 13-14t and not 11t like a cassette.So I could see 42t not being enough on a 26" bike. But I run 50x11 so what do I know?
          DNP is a freewheel brand that has some fans for Ebikes. They make them in 11t.They seem to be supporting that market specifically.
          They make a 7speed 11-30t range.

        Thanks for the help, Eldo.
        It's not that I want to go fast all the time. I wasn't using the lower gears so a larger chainring would give me more range.
        One more question, how can you visually tell whether you have a freewheel or cassette without removing the wheel?


          Seven speed is likely a freewheel and you can tell them apart but it's hard to explain - do some looking for diagrams and you'll probably be able to figure it out... diagrams are likely better than photos

          I would look to Sunrace for a replacement, they are usually best bang for buck and they have both cassettes and freewheels in seven speed and you may be able to find one with steel cogs which last much, much longer... also don't lug in the high gears - keep the pedals/ring spinning as fast as comfortable... the tension on the chain is proportional to the difference in pedal cadence - e.g. pedaling at 70rpm will have half the tension as 35rpm for the same power delivery

          If you do have a cassette it's possible to change just the cogs but a seven speed is so inexpensive (like $20 range) you might as well just replace the whole thing



            Thanks AZ, does a pic help? Click image for larger version

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              That's a freewheel. Those would be the part numbers that start with MFM in that Sunrace link.

              Looks like a fairly low end bike so if you are going to push it hard and put the miles on its gonna need lots of extra care and likely upgrades or keep an eye out for a better bike to convert. Now that you know you like it its a little easier to spend some money assuming you have some to spend. Cheap bikes just are not built to get the miles especially hard miles. My cheap bike eats up a lot more maintenance time and money than my good one which I ride a lot harder.


                Thanks guys


                  Sunrace freewheel and tool arrived today. Easy swap and problem solved, no more gear slipping.


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                    We appreciate the update

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