Author Topic: What's your Rohloff combo (chainring, cog)?  (Read 115593 times)

Dunroving

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #375 on: September 24, 2021, 12:48:25 PM »
Thought I would come back and update my situation, and say thanks to everyone who provided input to my Rohloff neophyte problem. Special shout out to Mickeg, who replied first and also pointed me towards the gearing modeller, which was invaluable, along with the Sheldon Brown calculator to confirm my workings out. As a measurement specialist, I value the opportunity to use at least two tools to measure something in order to check the reliability of my output.

Brief recap: I was trying to decide on gearing for my new Rohloff-equipped Smithy Mule. My goal was to match the bottom end of my current go-to bike, a 1997 Litespeed Obed. It has a low of 99.3 gear inches (3.1 mph at 60 rpm) and a high of 17.3 gear inches (29.5 mph at 100 rpm). I'm also switching from 175 mm cranks to 165 mm, to save my already-wrecked knees.

Update #1 (desired speeds and cadences):

On a 5-day C2C tour in the UK, I paid special attention to my speeds and cadences. Although I didn't climb any "long and steep" climbs, there were some "long and middling" climbs (such as Hartside Pass at 15.1 km, with 540 m of climbing and sections at 12%), and several shorter, steeper climbs (with some sections at 20%). I didn't need to stop or walk on any of these climbs or sections, although I did need to use my lowest gear frequently. This confirms that my new bike needs to go down to my Litespeed's lowest gearing, or even slightly lower.

Update #2 (what I learned from using the various modellers):

(a) After much work with the Gates Sanity Checker, it eventually dawned on me that this was oranges and apples - the numbers on a Gates chainring have no relation to teeth on a regular chainring! I had forgotten that I originally was thinking of a Gates belt, which is why Chris at Smithy had pointed me towards the Gates modeller. So I scrapped my Excel page of extensive data from the Gates modeller. Doh.

(b) From using Sheldon Brown's calculator and the gear-calulator.com site, I realised that comparisons (between the two, and to my Litespeed calculations) was confounded by the lack of standardisation between the two. For example, Sheldon has a 27.5 x 2" tyre combination and a 27.5 x 2.5" combination, but nothing in between. I intend to use a 42-44mm slick on the roads and a 2.3" knobbly off-road; neither modeller had these. Not a big deal, as you can do calculations either side and guesstimate what's in between.

Update #3 (where I have arrived at):

First grateful discovery is that the two modellers generally agreed, within rounding (Sheldon calculates to 1 d.p, gear-calculator to 2 d.p.). Phew.

I ran calculations for the following parameters: (a) 38mm and 50mm (Sheldon)/51mm (gear-calculator) tyres, as these were closest to my intended 42mm and 2.3" tyre plans; (b) 165 mm cranks; (c) low cadence of 60 rpm and high cadence of 100 rpm. My 5-day trip confirmed this is about what I use when going very slowly up a steep hill and very fast on a downhill, respectively; (d) 38T chainring, as the new frame will fit up to 42T and this gives wiggle-room for upward adjustments if needed, and is a fairly large-ish chainring.

Combining the results for the two tyre sizes, a 38x18 combination gave me 2.7mph (60 rpm) to 25.1 mph (100 rpm) and 15.3 to 84.4 gear inches. 8x17 combination gave me 2.9mph (60 rpm) to 26.6 mph (100 rpm) and 16.2 to 89.4 gear inches. 38x16 combination gave me 3.1mph (60 rpm) to 28.3 mph (100 rpm) and 17.2 to 95.0 gear inches.

I have focused on the bottom end to make a decision, as this is the more important end for me. I discarded the 38x16 combination, as it is either the same low end (38mm tyre) or higher lower end (50mm tyre) as my Litespeed.

I'm swithering as to whether to go with 38x17 or 38x18. I guesstimate that the 38x17 combo will give me the same low end as my Litespeed if I use similarly sized tyres (2.2"), and a lower low end if I use the smaller tyre size. The 38x18 combo will give me a lower low end than the Litsepeed for all tyre sizes I would use - but limits the top end to about 24 mph to 25 mph. On a level, smooth road with a following wind and day-trip luggage, I reckon I'd rarely hit 25 mph, and would certainly not sustain that speed for a long period. Going downhill, meh, if I have to coast, fair enough. On my 5-day trip, I saw very few people power down (i.e., pedal down) long hills, and my days of trying to see how fast I can get are gone. Over 6 recent days of cycling, all with downhills, on a relatively unloaded bike, my max daily speeds were 22.0 mph, 25.0 mph, 29.4 mph, 31.1 mph, 31.9 mph, 32.0 mph and 32.0 mph, respectively.

Sorry, lots of information! If anybody has an opinion on the choice between 38x17 or 38x18 above, I'd ppreciate it, but I think I can hardly go wrong with either.


Matt2matt2002

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #376 on: September 24, 2021, 01:40:11 PM »
38*17 in my Raven Tour.
1.75 Schwab tires.

Goodness' moi. I'm in awe of all the technical calculations here. Helmets off to you all.

The Raven is my only bike due to a very small shed (6'*4') and a wife ( petite ).
I've used it to tour, loaded & also as my runaround from home.

Sorry I can't contribute more to this discussion but as always, enjoy the technical calculations and maths.

Best

Matt
Never drink and drive. You may hit a bump  and spill your drink

Dunroving

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #377 on: September 24, 2021, 01:46:22 PM »
38*17 in my Raven Tour.
1.75 Schwab tires.

Goodness' moi. I'm in awe of all the technical calculations here. Helmets off to you all.

The Raven is my only bike due to a very small shed (6'*4') and a wife ( petite ).
I've used it to tour, loaded & also as my runaround from home.

Sorry I can't contribute more to this discussion but as always, enjoy the technical calculations and maths.

Best

Matt

Hi, Matt, out of interest can I ask what size wheels you have? 38 x 17 is one of the two combinations I am choosing between for my 27.5"-wheel new-build.

I hear what you are saying about the calculations, but the online calculators are excellent for making quick work of it. As I am spending a fair bit on the new build, and in the past have had to spend additional cash on the bikes I have bought, to change the gearing, I wanted to get it as right as possible from the get-go. I'm a bit of a scientist, too, so the data collection and analysis is enjoyable to me. I can understand why it doesn't float the boat for other people.
« Last Edit: September 24, 2021, 02:04:50 PM by Dunroving »

Aleman

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #378 on: September 24, 2021, 02:16:47 PM »
Can I suggest that you go for two prime numbers for your gearing. According to the Thorn "Bible" two primes or an non prime with only two factors, should result in less chain and cog wear as it takes a while for the same tooth to engage the same link ... thats why I went for 47-17.

mickeg

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #379 on: September 24, 2021, 03:24:41 PM »
...
I'm swithering as to whether to go with 38x17 or 38x18. ...

One gear change on a Rohloff is about 13 percent, upshift one gear and you go about 13 percent faster for the same cadence.

The difference between an 18 and 17 is that the 18 would give you about 6 percent more speed in a given gear than the 17 at the same cadence.  Thus, roughly a half gear.

Sprockets are not that expensive, maybe you should buy both 17 and 18.  You might find that for some things you prefer one over the other, they are not that hard to change.  (This assumes you could adjust the chain without having to add or subtract a link)

Sometimes it makes sense to do a bit of field testing your equipment instead of arduous analysis.

Dunroving

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #380 on: September 24, 2021, 04:59:56 PM »
...
I'm swithering as to whether to go with 38x17 or 38x18. ...

One gear change on a Rohloff is about 13 percent, upshift one gear and you go about 13 percent faster for the same cadence.

The difference between an 18 and 17 is that the 18 would give you about 6 percent more speed in a given gear than the 17 at the same cadence.  Thus, roughly a half gear.

Sprockets are not that expensive, maybe you should buy both 17 and 18.  You might find that for some things you prefer one over the other, they are not that hard to change.  (This assumes you could adjust the chain without having to add or subtract a link)

Sometimes it makes sense to do a bit of field testing your equipment instead of arduous analysis.

Your comment is spot-on, as usual. I was thinking through things this afternoon while plodding up a big climb on Zwift, and decided to go with the easier gear (18T cog), as my first few rides with the new bike will be distinctly off-road, with lots of climbs. As you say, if it ends up being too low, I can switch a 17T onto it later.

Matt2matt2002

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #381 on: September 24, 2021, 05:15:05 PM »
38*17 in my Raven Tour.
1.75 Schwab tires.

Goodness' moi. I'm in awe of all the technical calculations here. Helmets off to you all.

The Raven is my only bike due to a very small shed (6'*4') and a wife ( petite ).
I've used it to tour, loaded & also as my runaround from home.

Sorry I can't contribute more to this discussion but as always, enjoy the technical calculations and maths.

Best

Matt

Hi, Matt, out of interest can I ask what size wheels you have? 38 x 17 is one of the two combinations I am choosing between for my 27.5"-wheel new-build.

I hear what you are saying about the calculations, but the online calculators are excellent for making quick work of it. As I am spending a fair bit on the new build, and in the past have had to spend additional cash on the bikes I have bought, to change the gearing, I wanted to get it as right as possible from the get-go. I'm a bit of a scientist, too, so the data collection and analysis is enjoyable to me. I can understand why it doesn't float the boat for other people.

26" CSS Wheels.
Suit me fine. I've never experimented with other combinations.
At nearly 69 with probably one long tour left in me, I think I'll leave things as they are rather than juggle around with what ain't broken and works well for me.
To be honest after riding it this way for 9+ years it may be a question of me fitting the bike rather than the bike fitting me.

Good luck with your set up.

Best

Matt
Never drink and drive. You may hit a bump  and spill your drink

geocycle

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #382 on: September 24, 2021, 06:20:41 PM »
38*17 in my Raven Tour.
1.75 Schwab tires.

Goodness' moi. I'm in awe of all the technical calculations here. Helmets off to you all.

The Raven is my only bike due to a very small shed (6'*4') and a wife ( petite ).
I've used it to tour, loaded & also as my runaround from home.

Sorry I can't contribute more to this discussion but as always, enjoy the technical calculations and maths.

Best

Matt

Iím a bit concerned about your petite wife in the shed, hope she is ok and doesnít mind sharing with the bike.

I run 40 x 17 on my RST with 26*1.6 tyres. I find this great for loaded touring and ok when it is in service as the commuter come utility bike. Iíve certainly never wanted a lower bottom ratio and am fine with freewheeling down hills. Thatís what cycling is about isnít it?
 

Andre Jute

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #383 on: September 24, 2021, 06:42:56 PM »
The Raven is my only bike due to a very small shed (6'*4') and a wife ( petite ).
Iím a bit concerned about your petite wife in the shed, hope she is ok and doesnít mind sharing with the bike.
Heh-heh!

PH

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #384 on: September 24, 2021, 08:19:27 PM »
I'm swithering as to whether to go with 38x17 or 38x18. I guesstimate that the 38x17
Can't make your mind up? 39 x 18 will be roughly in the middle  ;D
Really, there's no way of knowing.  The transferring of gearing from one bike to another, or between derailleur and Rohloff, isn't an exact science, the numbers are just a part of it. OK, it'll give you a basic idea, but IMO not enough to be decisive.  There aren't going to be many hills you could ride with one and not the other, I'd be more concerned about what it did the the ratios in my most used gears (10.11,12 in my case) that's where I'll probably spend 70% of my time, I wouldn't want to compromise them for the one or two percent of the time I spend in bottom gear.
I'd agree with mickeg's point that sprockets are not that expensive, with that in mind I'd suggest starting with a cheap chanring and chain as well.

mickeg

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #385 on: September 24, 2021, 10:12:47 PM »
...
Iím a bit concerned about your petite wife in the shed, hope she is ok and doesnít mind sharing with the bike.
...

I was working on a line, but you were much more eloquent than I was capable of, so I will remain silent.

JohnR

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #386 on: September 24, 2021, 10:54:26 PM »
(a) After much work with the Gates Sanity Checker, it eventually dawned on me that this was oranges and apples - the numbers on a Gates chainring have no relation to teeth on a regular chainring! I had forgotten that I originally was thinking of a Gates belt, which is why Chris at Smithy had pointed me towards the Gates modeller. So I scrapped my Excel page of extensive data from the Gates modeller. Doh.
That the belt has different sized teeth to a chain is irrelevant. It's the ratio of the teeth on the chainring equivalent and the sprocket which matter.

(b) From using Sheldon Brown's calculator and the gear-calculator.com site, I realised that comparisons (between the two, and to my Litespeed calculations) was confounded by the lack of standardisation between the two. For example, Sheldon has a 27.5 x 2" tyre combination and a 27.5 x 2.5" combination, but nothing in between. I intend to use a 42-44mm slick on the roads and a 2.3" knobbly off-road; neither modeller had these. Not a big deal, as you can do calculations either side and guesstimate what's in between.
I attach (if the system permits it) my own gear inch calculation spreadsheet. A good reference for tyre sizes is https://www.cyclinguk.org/cyclists-library/components/wheels-tyres/tyre-sizes but actual sizes often differ slightly from the nominal numbers. Rim width will also change the tyre circumference.

Matt2matt2002

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #387 on: September 25, 2021, 03:10:06 PM »
...
Iím a bit concerned about your petite wife in the shed, hope she is ok and doesnít mind sharing with the bike.
...

I was working on a line, but you were much more eloquent than I was capable of, so I will remain silent.
In actual fact it's my Raven that doesn't mind sharing with my wife......
Never drink and drive. You may hit a bump  and spill your drink