Thorn Cycles Forum

Technical => Transmission => Topic started by: julio on December 20, 2017, 10:17:40 PM

Title: What is the ideal ratio transmission
Post by: julio on December 20, 2017, 10:17:40 PM
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 ?
Title: Re: What is the ideal ratio transmission
Post by: Danneaux on December 20, 2017, 10:49:38 PM
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.

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.
Title: Re: What is the ideal ratio transmission
Post by: mickeg on December 21, 2017, 02:46:21 AM
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.
Title: Re: What is the ideal ratio transmission
Post by: John Saxby on December 21, 2017, 03:16:16 AM
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 (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.) 
Title: Re: What is the ideal ratio transmission
Post by: Andre Jute on December 21, 2017, 07:09:03 PM
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.
Title: Re: What is the ideal ratio transmission
Post by: julio on December 22, 2017, 03:53:19 PM
Thanks all !

So as i've in stock a 16t sprocket and a 17t both are almost new, it would be interesting after reading to you, to buy a 36t ring.
For information about me, i weigh 70 kg, i'm not a rider like Amstrong or Dan  ;) i'm better at running, and my Nomad will be moderately loaded (about 16 kg in back, 8-10 kg in front) total with water : 30 kg

After reading to you 36x16 (2.25) or 36-17 (2.12) would be fine with a Nomad loaded

As well, may be i can start with my actual transmission 40x17 (2.35) and bring with me my 36t ring..
Title: Re: What is the ideal ratio transmission
Post by: RobertL on December 22, 2017, 06:48:15 PM
If you start with the direct gear, what gain inches are people looking for, 65-70?
Title: Re: What is the ideal ratio transmission
Post by: Danneaux on December 22, 2017, 07:16:37 PM
Quote
If you start with the direct gear, what gain inches are people looking for, 65-70?
With a 36x17 combo and 26x2.0 tires, my direct-drive Gear 11 is 55 gear-inches. I most often use it or my 62 gear-inches Gear 12 for level cruising.

This is very close to my favored 58 and 62 gear-inch cruising gears on my randonneur bikes with half-step derailleur drivetrains.

At my preferred high cadence of 120+rpm, I spin out of the 80 gear-inch top gear at about 28mph/45kmh. Any faster than that and I am happy to coast...so far, the fully loaded Nomad has proved stable coasting just past the 62mph/100kmh mark with no problem. The limit at the top end ensures I will have an adequate number of gears below 20 gear-inches for wincing expedition loads up steep hills.

Best,

Dan.
Title: Re: What is the ideal ratio transmission
Post by: mickeg on December 23, 2017, 07:24:54 PM
...
As well, may be i can start with my actual transmission 40x17 (2.35) and bring with me my 36t ring..

When I tour with my 36, I noted that around home I use a 44.  For touring I put the 44 in the outer position on a double crank and the 36 on the inner position.  I have never changed from one ring to the other on a tour, but if I wanted to it would have been quick.

If you start with the 40 and switch to the 36 later, make sure your chain tool is capable of  taking out one link when you make that switch.  I use two quick links when I use my 44, then take out three links and one quick link when I switch to the 36. 

But it would be harder to use two quick links on your system because you would have one link with inner plates in between the quick links, those inner plate links are not held together by anything when loose.

If you look at my crank in teh photo you can see that my chain is on a smaller chainring behind the bigger one.
Title: Re: What is the ideal ratio transmission
Post by: julio on December 24, 2017, 02:49:45 PM
Well done Mickeg  :)  but as i don't want to change my lovely stainless ring bolts, i will prefer eventually stock my 36t ring inside my Ortlieb pannier..
Title: Re: What is the ideal ratio transmission
Post by: PH on December 25, 2017, 12:53:11 AM
Now we have splined sprockets, which take moments to change, I wonder if these might be a better option for those wanting to take alternatives?
 I have no advice in the search for a perfect ratio.  I seem to be unusual in that I don't mind getting off and walking, once the speed has dropped below 4mph and I'm huffing and puffing, I'm more comfortable pushing and can do so at a lower heart rate.  It's marginally slower, but when touring in a group it's often me at the top of the hill waiting for those who've ridden up to catch their breath before we set off again.
I have 11th set at around 70" which gives a range of around 20 - 100
Title: Re: What is the ideal ratio transmission
Post by: Danneaux on December 25, 2017, 01:49:04 AM
Quote
Now we have splined sprockets, which take moments to change, I wonder if these might be a better option for those wanting to take alternatives?
  :)! This sounds a brilliant idea (if Chainglider-free) compared to swapping chainrings, as the changeover is not only quicker, the space required is less, should one wish to carry a spare sprocket (and the length of chain required to wrap it).

Well done.

Best,

Dan.
Title: Re: What is the ideal ratio transmission
Post by: mickeg on December 25, 2017, 09:08:35 PM
.... I seem to be unusual in that I don't mind getting off and walking, once the speed has dropped below 4mph and I'm huffing and puffing, I'm more comfortable pushing and can do so at a lower heart rate. ....

Agree.  I think my power output for short hills is maybe 180 watts, taller hills maybe 150 watts.  More and my heart rate gets to a rate that I am sure my physician would find objectionable.  Plus using a few different muscles when walking can be a nice change.

Now we have splined sprockets, which take moments to change, I wonder if these might be a better option for those wanting to take alternatives?...

Agree for the reasons cited by Dan. But, that would only be if you find you find that the range options you want are available.  It is quite easy to buy a five arm 110mm BCD chainring (the crank I use is a road compact) in 36, 38, 40, 42, 44, etc., thus a wider choice of options with chainrings.  If you are lucky, you might not have to add or subtract chain links.
Title: Re: What is the ideal ratio transmission
Post by: pavel on January 05, 2018, 09:41:19 PM
I find that too low a gear and I can't balance the bike while pedaling as fast as I tend to be able to.  I had a 41 front and 17 rear so according to this calculator ( http://www.sheldonbrown.com/gear-calc.html ) I had a 17.5 to 91.8 range of gear inches.  I found the top almost never used as I tend to coast down hills and the lowest on the edge of too low.  But I did use first gear often enough that 17-20 gear inches is about right for me.  I consider the lowest ratio the more important one to get right, as do most people, I believe.  I tend to do better mashing versus the smarter spinning.  Just built that way. I also like to never go fast.  At about 35 miles per hour I put the brakes on so I can enjoy the scenery. So this ratio works for me whether I'm lightly loaded or at max weight, both.

The only change I'll make is to go to a larger 19 tooth rear next time and whatever gives me similar ratios.
Title: Re: What is the ideal ratio transmission
Post by: julio on January 09, 2018, 12:42:05 AM
So for you Pavel, a transmission 41X17 is ideal even bike loaded ..

But what about climb a pass ? ..

You seem in very good physical condition, is not it ?
Title: Re: What is the ideal ratio transmission
Post by: pavel on January 09, 2018, 01:23:30 AM
So for you Pavel, a transmission 41X17 is ideal even bike loaded ..

But what about climb a pass ? ..

You seem in very good physical condition, is not it ?

That was the ratio that I used when I cycled from North Carolina to Texas in 2012 with my daughter.  I was carrying between 45 and 60 pounds on that trip, depending on food and water.  Virginia has some very impressive climbs, the most elevation gained of any state on the Trans-America tour, and I was in terrible shape and for weeks on end the temps were over 38-42 degrees with the horrid humidity that the South Eastern US has in summer.

That ratio was a guess on my part but it turned out perfect.  Not because my legs and body were up to the task, but rather by virtue of the fact that I can't seem to go in a straight line well enough under about 3 miles per hour, so a lower ratio turned out to be undesirable for me for that reason.  Both Bianca and I agreed that being that being macho and never pushing the bike up the hill was not something that worked for us. 

So in summation, I'd have to spin too fast with a lower ratio, (and I don't like to go over about 110 rpm) and I don't like to weave like a drunk, so we'd get off our bikes and push quite frequently on the worst of the climbs.  I think that getting off and pushing increased our daily miles, not that we cared about mileage for mileage sake at all, due to the change up the muscles got.  We normally did between 70 and 130 km each day and boy oh boy some days were tough.

I should mention that I was in very bad shape.  I was suffering from  stage four follicular lymphoma  and had about 8 kilos worth of a tumor  on the right side of my belly which often meant no sleep for days.  I had to stop to vomit a lot of the time. Strange .... that it was very  best 54 days of my life.

http://way-word-way.com/page/6/