Advice sought re upgrading chainring fix folding bike

Good evening,

I have a Fiat branded (think its a Dahon) folding single speed bike. Absolutely tiny thing, smaller than the Dahon Uno. The previous owners upgraded the chainring to 44tooth and think it is 12 at the back. I find that on the flat I tend to top out quite quickly. I have seen larger chain rings on Dahon Uno ss bike and wondered what advice you have about potentially increasing the size of the chainring? Any advice grateful received.

If you want to do this yourself, you’ll need to find or measure the ‘bcd’ aka ‘pcd’ measurement of your crank in order to buy the right replacement ring, a new chain and a chain ‘breaker’ tool to remove unwanted links. Take a look at somewhere like Spa Cycles for TA or Stronglight rings, good quality and loads of options.

What else do you need to know? Get a bigger chain ring, try it. Maybe a 48t? Spinny is good for your knees, don’t go mad.

You should check whether a larger chainring and the resultant chain angle will clear the chain stay and seat stay.

Thanks @Skülly @LWaB. Appreciate your advice. With regards size, note your point about being kind to knees. I tried using a online tool to work out how much faster it will be with different size rings. Do you think 50+ is silly, dangerous or problematic for the forces being put through folding frame etc?

I tried using a online tool to work out how much faster it will be with different size rings. Do you think 50+ is silly, dangerous or problematic for the forces being put through folding frame etc?

You need to take into account the front chainring, rear sprocket and wheel size, and then it may be helpful to work with gear ratios expressed as the equivalent diameter of the wheel or its roll-out. These are commonly measured in inches diameter or metres development (how far the bike moves forward in metres for one revolution of the crankset).

First you need to specify your actual wheel diameter (and check the number of teeth on the rear sprocket, is it really 12t?), I have seen Fiat bikes online that take either 20" or 14" tyres. If it is this one described on YouTube, then it will take a large chainring, but you would need to watch the chain clearance with the rear stay:
https://www.youtube.com/watch?v=A5k6pMyTsC4

I haven't found an online calculator for 14" wheels, so here I'll use the imperial formula for gear ratios expressed as diameter in inches, which is:
(chainring_teeth/sprocket_teeth) * wheelsize

to get the ratio in metres development multiply the above value by 0.0798.

Assuming it is the smaller 14" tyre, then with 44t chainring and 12t sprocket your current gear is 44/12 * 14 = 51" (i.e., 4.10m development), which is rather low.

For comparison, here are some common values often used for single-gear set ups:

• for some hills, a gear of 59" (i.e., 4.71m development),
  
• for all round use about 65" (i.e., 5.19m development) and
  
• for a relatively flat area up to 70" (i.e., 5.59m development).
  
  

Changing your chainring would result in these values:
50t chainring 50/12 * 14 = 58" (i.e., 4.65m development),
52t chainring 52/12 * 14 = 61" (i.e., 4.84m development),
56t chainring 56/12 * 14 = 65" (i.e, 5.21m development),
60t chainring 60/12 * 14 = 70" (i.e., 5.59m development).

Bearing in mind the compromises of a folding bike (frame fit and flex, their tyres), with a 14" tyre and 12t rear sprocket I would maybe look at the 52t chainring size, which is readily available for many types of crankset.

As this is LFGSS, there will be plenty of views on favourite single-speed ratios. If your wheel size is larger than I guessed, then just multiply the values above by (your_wheelsize/14).