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question on adding a second chainring

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question on adding a second chainring

Old 04-11-22, 10:25 AM
  #1  
totalnewbie
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question on adding a second chainring

the bike is HASA F-SL which has only one chainring at 47T and one single cog at 9T at freewheel. I have seen pictures of someone adding a 39T to the big ring to bring down the gear ratio (see picture) the original 47T becomes unused (since it is attached to the crank) and just shield the smaller 39T chainring. I know that the chainring is a BCD130 5-holes. If I want to do something similar, is it a simple operation (I have no experience in bike modifications before) by removing the crankset and bolting the small ring to the back of the big ring? Or does it involve replacing/adding a whole bunch of other components? I have attached a picture of the spec if that helps.

If such operation is relatively simple, instead of a typical 39T circular ring, can I use a 39T Rotor Oval ring (and pair it with the soon-to-be decorative big circular ring)? Please note that there won't be any derailleur involved.

Or, can i even get rid of the big ring (but keep the crank) and just have ther oval ring replacing the big circular ring?


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Old 04-11-22, 11:09 AM
  #2  
GamblerGORD53
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WHY did you buy a FLAWED bike in the first place?? Start over and get one that you WANT. Some have a SA 3 speed in the rear.
Those things are meant for FLAT land.
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Old 04-11-22, 01:03 PM
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Can you take a photo from an angle that shoes what you've got? I see two different chainring setups in your photos and cannot find a Saga 47t on the web (that's related remotely to bicycles anyway). What you want to do sounds straightforward, but without seeing what you've got I cannot help.

Ignore the previous poster. I've been modifying cranksets to do what I want them to do for half a century. All part of the fun!

Edit: I just looked further and I think I see what you've got. Top photo. the 47 hiding behind the outer ring. Bottom with a round 39 in place of the 47. You are wondering if you can do this with an oval 39. Probably yes but the oval ring will probably have an effect on chain slack. I drew up an arbitrary oval and measured a 1/10" of difference in chain circumference as the cranks are changed. (5mm, so half on top, half on the bottom - basically 2.5mm difference in "perfect" hub placement for ideal slack. Your actual chainring, over quality of the drivetrain and your tolerance of less than perfect slack will decide if this is OK.)

All this is assuming you do not use a chain tensioner which is an easy solution as long as you have no ideas of going fix gear.

Now, going to the outer position will look better but if the bike is set up right , you will be moving the chainring 4mm or so out of line with the rear cog. OK but not ideal. More likely for the chain to be thrown off, especially at the slacker portions of the cycle. (You could go to the outer position and get a shorter bottom bracket axle to bring the chairing back to in-line but that is expense (including tooling or bike shop time) and another go-round of "does this work? Can I get it?")

Quote me (with more/better photos if possible) if you have more questions. I am not the expert on parts that are out there for modern bikes.

Last edited by 79pmooney; 04-11-22 at 01:24 PM.
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Old 04-11-22, 07:26 PM
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Originally Posted by 79pmooney View Post
You are wondering if you can do this with an oval 39. Probably yes but the oval ring will probably have an effect on chain slack.
thanks for your input. I have not thought about the slack but I think you might be right, w/o a tensioner at the rear it might not be ideal. If so, then I will likely stick with a circular 39T and stick it behind the big ring to avoid the offset you mentioned. or better yet, I will see if I could remove the big ring and replace it with th smaller one (while using the same crank) so all the offset placement remains the same.

I don't have the bike yet... just considering buying it. The reason I am considering this is that I like the lightweightness of the bike (6.9kg) for my commuting (just doing the last mile), which likely will be mostly flat. however I would prefer a lower gear-inch ratio thus the thought of replacing/adding a smaller ring.
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Old 04-11-22, 07:59 PM
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The simplest thing is to replace the chainring,1-for-1. Adding a second ring to the crank will require, at minimum, new bolts, and spacers, if the crank will support 2x at all.
You may be able to find a ring with a chain guard so you don’t have all those teeth on the outside (chain guards on city bikes are to keep the chain from getting on stuff, rather than the other way around)

Finally, this looks like a 16-18” wheel bike; small-wheel bikes often run what appear to be oversized chainrings; the roll-out of the wheel (the distance it travels per revolution) is much smaller than a full-size bike, so they employ those big-ratio drivetrains to provide acceptable ground speed at normal “relaxed” riding cadence
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