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The Retrospec Judd folding bicycle has rather high gearing, 48T front, 18T back, around 53 inches gearing. Is it possible to replace the front gear with a 40T, say? That would give the bike about 44 inch gearing. For a fixie, not bad. Maybe my legs (knees) can handle that.

This answer, How can I replace my front cog..., has a comment that reads: "If its like mine [folding bike], the chainring will be a cheap stamped steel 38 to 42 tooth, and it will be welded to the right side crankarm."

So, my concern is whether the chainring can even be replaced, if anyone knows in advance. Alternatively, what should I look for to verify that it is a bolt on?

Inline image from manufacturer's web site linked, to prevent link rot.

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  • Aside - this bike is specced with only a single rear coaster brake. Be cautious that the brake functions as expected after any changes.
    – Criggie ♦
    Commented Jun 29 at 22:44

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Alternatively, what should I look for to verify that it is a bolt on?

Easy. Look for bolts.

Bolts can be differentiated by rivets in that they have a Torx / Allen head that allows turning them with wrenches.

I have never seen a rivet with a Torx / Allen head, or a bolt that doesn't have the possibility to turn it with a wrench.

So you should attempt to see such a bike in real life and look at the chainring.

Note that being riveted is not your only concern. Theoretically it might be possible that it's not a standard bolt circle. Standards are usually 110mm/5-bolt, 130mm/5-bolt, and 104mm/4-bolt. The standard chainrings have equally spaced bolts. Non-standard chainrings include some stupid designs by Shimano that don't allow rotating the chainring to distribute its wear evenly.

The ideal bolt circle would be a 4-bolt one, since that allows perfectly distributing wear. But even 5-bolt ones are common and easy to find chainrings for.

Bring a caliper with you and look for the spacing between two bolts. Check that the spacing is equal and measure how big it is. Count the number of bolts. Then use some math to turn it into the bolt circle, to check if it's one of the common ones (110mm/5-bolt, 130mm/5-bolt, 104mm/4-bolt).

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  • This provides the info needed. Thanks!
    – casualcoder
    Commented Jun 29 at 22:11
  • 5-bolts allow even better distribution of wear, provided you are ok re-bolting them five times
    – fraxinus
    Commented Jun 30 at 14:11
  • Well the wear occurs symmetrically on top and bottom, with no wear left and right, so for the number of directions (top, bottom, left, right) 4 bolts are ideal and adequate. 5 bolts have the problem the bolt count is not divisible by 2, but the wear occurs symmetrically on top and bottom.
    – juhist
    Commented Jul 1 at 15:03
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It has the look of non-replaceable, and that's in line with the price point. Internet wisdom gives non-replaceable ring cranks an unfair rap sometimes; when a bike is trying to come in at $300US, it's what you would want and expect.

48x18 on 20x1.75 (406-44) gives 49.8 gear inches. "High" is a subjective term but there's basically no world where that's a high gear. You say "for a fixie" so presumably you're talking about converting this to a fixed gear. At 49.8 gear inches, your legs are already hitting around 100rpm at 15mph, a fairly gentle downhill speed. Most people wouldn't be able to manage any more than that, so making the gearing lower yet is suspect.

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  • You were right, for sure, about the non-replaceable aspect of this chain ring. It is part of a one-piece, flat stamped steel spider and ring. Your fixie riders might make short work of this assembly.
    – casualcoder
    Commented Jul 1 at 1:59
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From the pictures in your link it looks to me like a square taper crank, which is usually the case for lower and mid range bicycles. If so you can easily change the whole crack. You will need to buy a crank puller tool, but they are not very expensive.

Have a look at the question I asked: Can I (easily) change this chainring? That wasn't on a folding bike but the answers will apply to your bicycle as well. It turned out to be pretty straightforward and cheap.

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  • Not necessarily easily. Each crank model requires its specific bottom bracket length. There is no standardization, and actually there are incompatible ISO and JIS tapers. With Hollowtech II for example, it might be easier since the bottom bracket is standard, working for all Hollowtech II cranks (maybe with "road" / "MTB" distinction).
    – juhist
    Commented Jun 30 at 16:26
  • But you don't need to change the bottom bracket. Just change the crank arms.
    – John Rennie
    Commented Jun 30 at 19:29
  • I did not measure the length of the crank, but folding bicycles have unusual crank arm lengths. I doubt an MTB crank would work - it would be too long. Though it would be an idea if one would fit. REI has a Tern folding bike that is much better outfitted than this thing, but at way over twice the cost.
    – casualcoder
    Commented Jul 1 at 2:06
  • @JohnRennie You don't need to change the bottom bracket IF the bottom bracket length is correct and IF the taper is correct (ISO vs JIS). Considering that there are a huge number of different lengths, and two incompatible tapers, chances are it isn't correct.
    – juhist
    Commented Jul 2 at 16:34

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