Shahmatt

I've just checked the gear inches on my folding bicycle according to Sheldon Brown's Bicycle Gear Calculator here:

https://sheldonbrown.com/gears/

I came up with the following numbers:

Gear 1: 42
Gear 2: 57.3
Gear 3: 78.1

I use a Shimano Nexus 3 Speed, with a 46T chainring and 15T sprocket on the hub. The tyre size is 20" - 406. The crank length input is not necessary for gear inches I think.

Have I got this right? I'm not sure but the lowest gear seems still a bit high at 42. There aren't many hills where I live so I don't struggle too much. But I've been considering going lower on a bike I want to build and I'm using my existing bike as guidance. What's a good nominal amount of gear inches to keep to? How much should I try to lower this by?

fietsbob

the gear ratio of your 2nd gear is the cog and chainring combination [(Ft)'/,(Rt)x (W) ... wheel diameter ]
1st is 0.73 of that, 3rd is 1.36 x the 2nd gear..

lower the external gear , to lower all 3,... less teeth on the chainring or more on the hub cog ..

GamblerGORD53

I've been riding a heavy bike with a SA RD5w. My GIs were 43.5 to 111 last year and 45.5 to 116 this year. I chug up 10% hills no problem. I went from 165 to 180 cranks. Yes it doesn't change GIs but it IS 9.1% easier/faster going up hills and in any situation I shifted 1 or 2 mph sooner. Not everyone can do this. I use mid 90s GI with the wind on flat land. Your current 3sp is set up pretty good for city riding.

As for your new build, you need to provide more info about what load you want to carry and how many gears, IGH or not.
With an Alfine8 you can set it up nicely to about 31 to 100. I very seldom used my 30/30 gear, 27GI. Most tour bike guys crow about derailler GIs in low 20s or less, but I just don't get it.

noglider 

Shahmatt, you are correct that crank length doesn't figure into gear inches. I get different results with your numbers, though. 46/15*20 = 61.3. That makes your bottom gear 46 inches and your top gear 82 inches.

As for what gear you need, only you can answer that. You'll feel a difference if you make it 15% lower or more.

fietsbob, most 3-speed hubs are .75/1.0/1.33. Not a big difference, but I thought I should toss that in there.

Dan Burkhart 

You did not say what the tire size was other than 20 inch. I threw the numbers into this calculator using 20 x 1.35 and came up with this. Looks like your numbers are pretty close, maybe a shade low.

fietsbob

I looked it up, on referenced SB gear chart.

Seems the nexus 3 is more like Sachs in that , than Sturmey..


John Allen & others keeps updating Sheldon's site with data about stuff made since SB's passing.

JohnDThompson 

The Sturmey-Archer AW is probably the most popular 3-speed hub and has the 0.75/1.0/1.33 ratios you mention, but the Shimano Nexus is 0.73/1.0/1.36 as fietsbob stated:

https://www.shimano.com/publish/conte...03%20Speed.pdf

Other hubs had different ratios, e.g. the Sturmey-Archer SW was 0.72/1.0/1.38, the ASC was 0.75/0.9/1.0, the S3X is 0.625/0.75/1.0, etc.

Shahmatt

Thanks all for the replies.

OK so a GI of around about 42 or 46 is then probably about right. I couldn't believe that this bike was geared so high after reading through random google posts where claims of GIs circa 20 are mentioned.

I'm trying to build a folding recumbent with a Cruzbike conversion kit. I have no experience with building bikes so I guess I'll have to learn as I go along. The purpose is for commuting to work. I'm looking at a comfortable range of around 10 miles. The load will be around 4lbs max = backpack, tools, books and a change of clothes. I weigh around 175lbs.

A folding frame is likely to weigh heavier, as well as an IGH. But recumbents are aerodynamically efficient machines, and the cruzbike drivetrain is stiff on account of the short front triangle, so I'm hoping that this will help, at least theoretically.

Anyway, I'm considering the Nuvinci N360 with 52T on the chainring and maybe a 13T (if this is available) on the rear.

This will yield me a GI of 37.4 to 134.5, which seems to me to be a comfortable range.

fietsbob

Nuvinci N360 is set up like a 3 speed .. direct drive of hub-shell, by the cog , in the middle .

overdrive above , and reduction gear below, are 180% ... just step less ..