markizschnitzel

I need a small folder for commuting. I see used Dahon Curve i3 in my local yellowpages, but not local enough to go and try it out.
So, what kind of speed can I expect with this thing?
Chainring does not look especially big.
I'd like to be able to go as much as 25km/h or 15.5 mph, with reasonable cadence (<80?). I can't really get into the rebuilding the bicycle to make it suit my needs exactly, at least not yet, so need the stock bike to be fast enough.

Duragrouch

First, do you mean a Dahon Curve, or Dahon Curl? They use much different frames, but similar size wheels. The Curl folds very similar to a Brompton, with a swinging rear triangle and front frame fold. The Curve is a simple bi-fold, the frame folds in half.

They may both use the same gearing, so let's assume so for now. The Curl i3 uses an Internal Gear Hub (IGH), and for most of those in 3-speed, gear 1 is a reduction, gear 2 is direct drive, and gear 3 is an overdrive, which is the main area of concern for you, for top speed. You need to know what model IGH is used on the i3, either to select on online gear calculators, or to calculate yourself.

The current Curl is a model D9, using 9 derailleur gears (no overdrive) but a quick search online indicates the i3 uses a Shimano Nexus 3 hub. I'm going to assume the same size tires as the D9, so 16"/349x37, and the same 46T crank and 16T cog as used on the i4. So plugging this all into gear calc for mph at 60 rpm cadence, that yields 11.2 mph. Not very fast. You'd need to spin 80 rpm cadence to get 15 mph.

The wheels are small, so have inherent top speed limitations, and top gear, IIRC, is only about 33% above direct drive, not a lot. To go faster on reasonable cadence, you need a wider range IGH with a higher overdrive, and/or bigger wheels. A smaller rear cog at the hub may not be possible, selection is limited. A bigger chainring at the crank is possible, but not by much, 52 or 53 tooth, before getting into special parts like a 60T.

EDIT: You could add a Schlumpf drive at the crankset to get wider gearing, this is a two speed gearing inside the bottom bracket shell, with a button sticking out each side, that you shift by hitting sideways with your heel. I've seen them used on Bromptons (same size tires I think) to get a wider gear range.

markizschnitzel

Thanks.

Actually, 80 RPM@25km/h is not all that bad, considering my current bike is a 14" ebike that does this at ~85+ RPM. It means with a slightly bigger chainring it could be decent.

Can you recommend me a calculator that is simple enough for a lay person?
I want to check another bike, Pony Classic 3.
It has 20" wheels, Sturmey Archer RS-RF3 3 speed IGH and 44T chainring. I don't know what is the rear cog for certain, but I think maybe 18.

tcs

Hmm. Perhaps a Dahon Curve D3?

That's got a 16T rear sprocket. It's easy and cheap to replace this with a 15T, 14T or even a 13T.

Plugging in a 46T chainwheel, 13T sprocket and ISO305mm wheels to Sheldon Brown's gearing calculator gives 17mph @ 80rpm.

markizschnitzel

Thanks.
Is the sprocket size dependent on the IGH?
If a bike has Sturmey Archer RS-RF3 IGH and 18T rear cog, can I just replace it with something smaller?
As a simple commuter, bike rebuilding is foreign to me, so I need to figure some stuff out before comiting to buy.

Schwinnsta

No.

tcs

An RS-RF3? Hmph!

Sturmey's specifications say you can mount 13T to 24T sprockets on this hub.

https://www.sturmey-archer.com/files...0IGH%20C30.pdf

Visual inspection of my RS-RK3 hubs would seem to confirm this.

The hub gear sprockets that fit your RS-RF3 are the old standard 'three-tab' 35mm ones manufactured in the last 70 years by Sturmey-Archer, Fichtel&Sachs (SRAM), Shimano and others. They all interchange!



If you have a 1/8 chain, either a 3/32 or 1/8 sprocket will do. If you have a 3/32 chain, you'll need a 3/32 sprocket.

You'd need to shorten the chain by a link or two or three.

Duragrouch

I typically use Sheldon Brown Gear Calc, just search for that. The pulldown menu options are not too difficult, I use results in gear-inches but you can also select meters development and also speed per cadence.

Gear-calculator.com has more features, shows visual representation of gear steps, and you can just move the gear icons to change things quickly.

Duragrouch

Always smart to look at things on paper before buying, that is the essence of engineering, predicting results before you buy or build. Online gear calculators have made this a lot easier for the average person. It's a bit harder to know what options you have available in both gearing, and drivetrain system options.

Just a small note in general: I use a folding bike with 20"/406 wheels, because that is the smallest wheels I can use and still get a high enough top gear (for me, 85 gear inches, not a racer high, but it's enough), without an internal hub gear or Schlumpf drive, using only derailleur gearing, with "typical" 11 tooth small cog, for easy of maintenance and low cost. If you go with smaller wheels, you'll need to either accept a slower top gear, or need an internal hub gear for an overdrive, or Schumpf drive for same, or, a more special freehub body and cassette with a 9 tooth high cog (I had run numbers for that assuming a Curl D9 on 16"/349 wheels).

Duragrouch

Sorry but this thread title is reminding me of a tuba joke.

What's the range of a tuba?
Twenty yards if you've got a good arm!

50PlusCycling

It’s geared well enough for your needs. Unless you live in a hilly or mountainous area, the gearing is sufficient. A bike’s speed has more to do with the rider than the bike itself.

Fentuz

I doubt it, you would need a very big chainring, very low rolling resistance tyres etc.
i get these values with my lightened helios.

While the gearing calculations are a good start, they don t tell the truth as they do not consider rolling resistance or aero. Even indoor on rollers they are not quite right.

Ron Damon

Hmmm... 🧐