Technical > Transmission

What is the ideal ratio transmission

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jul:
Hi all,

Actually, on my Nomad my ratio is 40 x 17.

If i'm planning a big tour (plains, mountains, deserts), which will be the ideal ratio with my rohloff ?

Many are in 38 x 16 .. but is there a big difference to compare with my ratio ?

Danneaux:
Hi Julien!

First, I think it is important to define what "ratio" means in terms of an internally-geared hub. "Ratio" is chainring teeth/sprocket teeth and Rohloff specifies a minimum ratio, as shown here:

--- Quote ---To prevent overstraining the hub, a minimum sprocket ratio of 1.9 must be used. With the available sprockets these minimum possible sprocket ratios are: ~32/17, ~30/16, ~28/15 and ~26/13.
--- End quote ---

You can achieve the same or close gearing (in gear-inches or meters of development) via different chainring/sprocket tooth-counts so long as the ratio remains the same.

With a 38x16 combination, you have a ratio of 2.375.

For riding steep hills fully loaded, I prefer a 36 x 17 combination, for a ratio of 2.1. With 26x2.0 tires, this gives me gear-inches of:
15, 17, 20, 22, 25, 29, 33, 37, 42, 48, 55, 62, 71, 80.

I certainly use and value my gears below 20 gear-inches and use them. With my fast, light cadence, I have no problem climbing at about 2.5mph/4kmh, but everyone is different; it might be too slow for you.

If you wish to try lower gearing, it is not too expensive or difficult to fit a smaller chainring to pair with your 17t stprocket, then shorten the chain and retension it using the eccentric bottom bracket.

All the best,

Dan.

mickeg:
Around home where I am not riding a heavily loaded bike and my steepest hills are a 7 percent grade, I ride my Nomad with gearing that is quite different than I use for touring.  But touring, I want the lowest gear that I will find to be practical.

The net result is that I use a 44T chainring around home, sprocket is 16T.  I set the gearing to be in a range that I found to pretty much meet my needs around home with a dérailleur bike.  With these gears I never find that I wished I had higher or lower gears, I have the range that I want covered.  Ratio = 2.75.

For touring, I felt that the lowest speed where I could maintain vertical and directional stability without excessive steering was about 3.5 miles per hour (5.6 km/hour).  And I wanted a minimum cadence of 72, slower and my pedaling is not as smooth.  With a bit of calculations based on a 26 inch wheel diameter with a Marathon Extreme of 57mm width, I figured out that I should use a 36T chainring with my 16T sprocket to give me that gearing.  My highest gear is too low for shallow downhills where I usually would pedal down the hill with a low cadence and low torque, this gearing is too low for that.  But if I have to lose something, I would rather lose my high gears than lose my low gears, so that is the gearing I use.  Ratio = 2.25. 

When it comes to the Rohloff criteria for minimum ratio, I am about 80 kg in weight.  I do not recall what their rules are, but they have different minimum ratio for body weight over 100 kg.

You asked about different gearing and you listed a change in both sprocket and in chainrings.  I would suggest you only change chainrings and not the rear sprocket if you wanted to run different gearing for a tour like I do than you would for normal use.  It is easier to change one instead of both.  When I change chainrings, if I switch from the 36 to 44 I add a second quick link and a few more chain links, or remove those extras when I switch to the 36.

Regarding your question, would you notice a difference between 40/17 (ratio = 2.353) and 38/16 (ratio 2.35), those gears are about 1 percent apart.  Upshifting a Rohloff by one gear is about 13 percent.  You would probably never notice the difference between those two setups when you consider how small the change is compared to the change of one gear shift.

Big tour, mountains, plains, deserts, etc., I think you might want to lower your gearing a bit but you should not fall below the Rohloff minimum ratio for your body weight.  Since you have  a Nomad, I assume you are talking about a heavy load.  I have a slightly lower ratio (2.25) with my tour gearing than you have (2.353), but not by that much.  Your first gear is between my first and second gears, but your first gear is closer to my first than to my second gear.

We both have a higher gear that Dan.  But he has a higher cadence than me so I think it would make sense that he have a lower gear than me.

In the photo, the hill that I had just climbed, I got off the bike and pushed the bike up the hill.  It was too steep for my gearing.  But, I would rather get off and push than have a heart attack, we have to know our physical limitations.  A lower gear would not have helped me there because I do not have the power to maintain speed while climbing a hill that steep with a heavy load.

Good luck with your trip planning.

John Saxby:
Julien, "it depends"... On things like fitness, how much weight you're carrying on your bike, what cadence you like, etc.

Like Dan, I use a 36 x 17 ratio -- I have a Thorn Raven. When I first bought the bike, I used a 38 x 17, but I have found the 36 x 17 much more to my liking. The difference between the two is only about 5%, but it feels as if it's much more. The 36 x 17 gives me a lower 1st gear, and that helps on the steep and/or long hills, but it also gives me a lower 11th gear, and I've been surprised and pleased to learn that I spend much more time in 11th gear than I used to.

The 36 x 17 has made my Raven an easier and hence more enjoyable bike to ride.

Hope that's helpful, and good luck.

John

PS:  Changing the chain ring from a 38 to a 36 was fairly quick and easy. Changing the ring took maybe 15 minutes. The more fiddly bit was getting the length of the chain right, as I was replacing a partly worn longer chain with a new shorter one.

PS #2:  There is a long thread on this topic, in the section "Rohloff Internal Hub Gears", here: http://thorncyclesforum.co.uk/index.php?topic=4412.0  (It may have more than you ever wanted to know about the subject!) (But I found it useful, when I was considering lowering my gearing by fitting a smaller chain ring.) 

Andre Jute:
For a touring/commuting/utility bike I tend to think the ideal ratio matches your most comfortable long-distance cadence in your bike's most efficient gear on the flat to the lowest gear/speed at which you can keep your balance on a really steep hill.

That's a lot of variables to accommodate, so usually we deal with it as rules of thumb enlightened by experience, as in the advice you already received above.

But, given only two inputs (your preferred cadence and the circumference of your preferred tyre), you can make a table of gear inches and speeds in various available gear ratios (chainwheel and sprocket teeth, plus, in the case of a hub gearbox, internal gearing) and choose among them to match your lowest speed requirement according to your riding requirements, and then seeing if the top end you need is still there, or if you need to look for different components (hub gearbox, chainring, sprocket or cluster). If you want to see how it is done, there are some worked tables, set up to compare various chainring/sprocket possibilities for Rohloff and Shimano internal gearboxes in the spreadsheet Excel, at http://coolmainpress.com/BICYCLINGHebieChainglider.html
-- which allowed me to see quite clearly at which angle of ascent (in fact when my wife wanted to live up the steepest hill in town) it became worth my while to switch from Shimano Nexus internal gearboxes to the much more expensive Rohloff gearbox.

Or you might just take the word of the wise men for it; it's generally the same answer, without the math and the migraine.

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