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I have a 20" folding bike with a shimano Nexus Inter-3 (most probably this one), and I find that even on flat terrain (not to mention downhill) the 3rd gear is simply too slow: I often find myself cranking very fast exerting a very low torque, and 2nd gear is good for most slopes (<15%). I mainly use the bike for commuting, on mostly flat terrain.

My current top speed seems to be around 20-25 km/h, I would like to be able to do 40 km/h. That means an increase by a factor of two.

I am searching for a way to up the gear ratio, my options seem to be:

  1. Increase the size of the front chainring (45 currently):
    • This would make folding harder.
    • I would also need to change the chain protector and possibly the pedals.
  2. Decrease the size of the rear cog (18 currently):
    • This would increase the strain on the individual cogs and increase the chance of slipping (that already happened) and chain fall-off.
  3. Change the gear hub to one with a higher upper ratio:
    • May need to change the shifter and corresponding wiring
    • May not fit in the current frame
    • The current ratio is 1.360, I would like at least 1.8 or even 2.0
    • I do not really need gears below 1.0, and certainly not below 0.73 (my current 1st gear). Unfortunately most gear hubs concentrate on the lower gear ratios, but I am yet to find a comprehensive table of available gear hubs (wikipedia's, and Sheldon Brown's have different flaws). Shimano's 4-speed hub seems promising, but is discontinued.

Which solution (or combination of them) do you recommend? If it is the third, what specific gear hub(s) do you recommend?

  • 1
    side note: 15% is a lot it is very steep, and very rarely encountered. – njzk2 Sep 30 '16 at 13:41
  • @njzk2 it depends where you live. Round here there are plenty of such hills you can't avoid. – Chris H Sep 30 '16 at 15:28
  • @ChrisH when touring on the french alps road, the steepest section I encountered were 12-13% (avg value over a km), and that was hard (and rare). – njzk2 Sep 30 '16 at 18:49
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    Have you ever ridden at 40km/h on your 20 inch folder? Down a hill even. 40 km/h is quite fast, and I wonder how well a 20 inch folder would handle at those speeds. – Kibbee Sep 30 '16 at 20:48
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    @njzk2 averaging over a km isn't really appropriate though. On my old commute the less steep option was 25% for about 200m. But averaging over a km it would have been less than 10%. Such hills aren't uncommon round here. – Chris H Oct 1 '16 at 8:04
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Decreasing the rear cog is by far the easiest of your options. On road bikes, chain rings of 34-39 teeth and rear cogs of 11 are common, so you would be within range. The force on the chain is higher for smaller chain rings, not larger ones. Slipping probably comes from not having the chain tight enough. You may be able to move the rear axle aft to tighten it.

  • Can you go into more detail about how you would fit a 12T cog off a derailleur onto a 3 speed IGH, please? Otherwise your answer makes no sense. – Nuі Oct 2 '16 at 0:22
  • I know the single speed people change gears to suit their needs. I took the fact that you cited option 2 to say you could change the gear, but I don't know the hub specifically. The Shimano page you linked to does not show a gear on the hub, so presumably you buy that separately and might be able to buy a different gear. 12T may be too small because the diameter may be smaller than the hub, but even 14 or 15 would be a big improvement. You might need to shorten the chain. – Ross Millikan Oct 2 '16 at 2:15
  • At last, I decided to go with this option, and although the mechanic could only fit a 15-cog gear, it is about 20% faster, and feels much better. I could hit about 35 km/h as max speed and steering is no longer wobbly at 30 km/h and I can maintain that speed for much longer. It was also real cheap, about $15 with chain change included. It is a compromise, as a higher gear would be even better, but even this small change made the bike much more useable. – P.Péter Mar 8 '17 at 14:44
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I've got a folder and had similar problems with top end speeds.

I can't speak for the IGH specifically, but increasing the front chain ring size and/or decreasing the rear cog will help with the ratios.

The freewheel I had was 14-28, and that's all that was available in 6 speed. So I rebuilt the rear wheel onto an 8 speed freehub axle and have fitted an 11-32 cassette and 8 speed shifter.

If you can fit a larger front chain ring then all your gear ratios will move proportionally.

Finally, you can get a very minor improvement in top speed by increasing the rolling diameter of your tyres. Downside this will probably increase their weight too, which won't help.

Lastly, consider that a folder is not a racing bike and remember you're still going faster than walking.

  • I should add that even on 45/14 I managed to hit 40 km/h on this bike. You have to pedal with a cadence around 135 RPM, which means employing your ankles and calf muscles more and your thighs/butt muscles less. Search for "ankling" or "twiddling" – Criggie Sep 30 '16 at 21:23
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There is a fourth option, which you have omitted:

  1. Pedal faster.

By my calculations, at 25 kph you are pedaling at a cadence of 76 to 77 pedal strokes per minute. That's low.

By far the cheapest option is to learn to pedal faster. Aim for at least 90, preferably 100 (or more) strokes per minute when you're flat out. Depending on whose opinion they follow, most road cyclists (ie people who ride road bikes fairly seriously) aim to cruise at 90 or 100.

The big downside of what you're currently doing (it's called mashing) is that it can cause knee damage in the long term.

The downsides to aiming for a higher cadence are

  • It takes a while to get train to do it, and to feel comfortable. To many people, it just doesn't seem like they're working hard enough.
  • You may find that you have trouble keeping your feet on the pedals. In this case you can do two things: learn to pedal smoothly, and / or get clipless pedals.

By my calculations, if you pedaled at a cadence of 100 then you'd be doing 33 kph.

But there's a different problem that you are not addressing: how much power does it take to do what you suggest?

To do 33 kph takes about twice the power of 25 kph, because the amount of power required rises as the cube of the speed. To go at 40 kph takes more than three times the power that 25 kph does.

So, in the end, my advice is learn to pedal faster. It costs nothing, and while you're doing it you will build up the muscular and aerobic capacity to get to those higher speeds. At some stage, an upgrade will be required, but you'll still have the money to do it.

Happy cycling.

  • I will have to do measurements to verify your numbers (measure speed with GPS and measure cadence with a clock). However, without clock I seem to like 60-70 per minute and am comfortable up to about 90. Over that, I really have problem keeping my feet on the pedals, and still feel that I am crawling. So this solution was not mentioned, as I already tried it. And failed. :( – P.Péter Oct 3 '16 at 6:58
  • @P.Péter The main assumption I made was in the circumference of your wheels. Building up cadence does take practice, and sometimes clipless pedals. It's certainly your choice which solution you choose. I think the important thing is that you and future readers have the full range of advice to choose from. What's right for one may not be right for another. – andy256 Oct 3 '16 at 12:36

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