What rear sprocket do I need to help me get up hills and mountains easier?

[Joseph_]

Hi I'm new here 
I'm the proud owner of a Koga Worldtraveller Signature 2.0 with Rohloff and Gates belt. I have a 50t front chainring and a 19t rear cog with a 120t beltdrive plus Schwalbe Almotion 28" 50mm tires with 2293mm wheel circumference.

On my last trip I noticed that I lacked low gears when climbing and now after a lot of research I am still wondering which rear sprocket I should fit to improve this? Many say the lower the better, but I also have to take into account that I don't have to replace my belt too because that would make it twice as expensive. So basically I'd like to put the lowest rear sprocket possible while keeping my current belt, and using the tools Ritzelrechner www.ritzelrechner.de and the Gates Carbon Drive Bicycle Calculator https://www.gatescarbondrive.com/Apps/DriveCalculator/index.html I get the following possibilities (FYI my chainstay length is 460mm, my crank is 170mm) :

A 22t sprocket is best compatible with a 459mm chainstay so can I keep my current belt with that? (at 60 cadence = 18,2>95,8 Gear Inches - 1,45>7,65m development - 3,25>17,1 mph / 5,23>27,5 km/h Speed)
A 23t sprocket is most compatible with a chainstay of 456mm so a little too short. (at 60 cadence = 17,4>91,6 Gear Inches - 1,39>7,31m development - 3,11>16,4 mph / 5,01>26,3 km/h Speed)
A 24t sprocket is most compatible with a chainstay of 465mm so a little too long. (at 60 cadence = 16,7 Gear Inches / 1,33-7,01m development / 2,98>15,7 mph / 4,8>-25,2 km/h Speed)
Now someone told me that if you have an eccentric bottom bracket (EBB) like my bike has, those few mm in 23+24t can be adjusted for somehow? And is a 24t sprocket a good choice or is this much too low? FYI, I am 47 years old but have a very good fitness and mostly ride on flat terrain (+- 60%), but often I do slopes and/or mountains (+- 30%) and occasionally I go off road (+- 10%).

If you guys want to give me a gamble with this information (because I know it's still a gamble), I would be eternally grateful 

Cheers, Joseph

[in4]

FTR I ride 45 x 19 on my Nomad MK2; as per Thorn Mega Brochure. Combination works well for me. Perhaps a look at the brochure might be useful for you.

[Joseph_]

Thanks, that's  a good gear ratio you've got there 
I already read the brochure which is a great resource but I wasn't able to get the information I needed from it as my question is very specific to the Gates Belt Drive I guess..

[mickeg]

For heavy touring or for when I set up my Nomad Mk II with suspension front fork for mountain biking, I use a ratio of 2.25 for chainring to rear sprocket (36/16), chain drive.  This is a heavy bike, a bit over 20 kg, designed to carry a load.  With 57mm tires (26 inch wheels), I have lowest gear of 16.2 gear inches with this gearing.

But for riding around near home where I rarely have more weight on the bike than a pannier of groceries, I run a ratio of 2.75 (44/16).  That is 19.8 gear inches for unladen riding.

I have a chain drive because (1) that was the only option with that model but also (2) because I knew before I bought it that I would want different ratios for different purposes.  Thus, I can swap chainrings and with a second quick link I can add or subtract the 4 chain links to do that.  I am clueless on belt drive and choose to stay that way.  I considered buying a different model but chose not to because it was belt drive.

When touring on that bike there are times that I wish I had higher gears when riding down a shallow slope hill and I am spinning out due to lack of higher gears.  But for touring I value the lower gears more.  When riding around near home with a light load, those low gears are unnecessary so I gear it higher to obtain a couple gears that are only used on downhills.

I am clueless on belt drives and choose to stay that way.

I have three touring bikes, the other two are derailleur drive.  For lighter loads I usually use the derailleur bikes, in part because they have a wider gear range.

[JohnR]

For what it's worth I've got an almost new 46T 104BCD CDX chainring and an almost 115T belt which came off a bike where I found the gearing too low and did a few miles while alternative parts were in the post. I've also got a 118T belt which has done about 3k miles before I dismantled the bike to use the Rohloff hub on a new build. One of those combinations might work if the chainring will fit - you would need to check the max and min chainstay lengths provided by the EBB.

The parts are gathering dust in my garage so I'm open to an offer.

[CycleTourer]

For heavy touring or for when I set up my Nomad Mk II with suspension front fork for mountain biking, I use a ratio of 2.25 for chainring to rear sprocket (36/16), chain drive.

I ride heavily loaded and have a 38/16 setup. I appreciate the ability to get up tough hills slowly but effortlessly, but I regret the inability to pedal at all on downhills. I can't imagine going even lower to 36.

[Joseph_]

Thanks for all the info, it's a great help.
And if anyone could provide any more information on my belt options, that would be great too.
Also thanks for the offer JohnR, but I live in Belgium 
Cheers, Joseph.

[JohnR]

And if anyone could provide any more information on my belt options, that would be great too.
Also thanks for the offer JohnR, but I live in Belgium 

I would suggest that you measure the effective chainstay length (crank centre to rear axle centre) with the eccentric bottom bracket set to both the minimum and maximum distances so you are certain of the range that the calculated chainstay length must fall into for the various chainring / sprocket / belt combinations. You mention that the chainstay length is 460mm which is the same as the specs at https://www.koga.com/en/bikes/trekking/worldtraveller?frame=H but the data at https://geometrygeeks.bike/bike/koga-signature-worldtraveller-s-2-0-2021/ says 470mm (which is closer to the calculated chainstay length for your combination of components). Checking the range provided by the EBB is essential. If you don't know how to adjact the EBB then now is the time to learn.

As you are located in Belgium then it will be much easier to buy whatever parts you need.

PS: If the spocket is splined (much easier to remove / replace) then there are three sizes: 19T, 20T and 22T. If the sprocket is screw-on (much more difficult to remove) then there are 19T, 20T, 22T and 24T options.

[Joseph_]

Thanks again John,

I should've mentioned that mechanically-wise I'm really a beginner but very eager to learn more.
And you're correct about the discrepancy in chainstay length between the standard Worldtraveller and the Signature version, it's something I too only found out today and am still wondering about.
I think the Signature value is correct (it's actually 469,5mm), I thought the Worldtraveller frames were identical, apparently they're not.
I never looked at my EBB before so I guess now's a good a time as any though for the actual replacement of parts I'm going to have to go to my local dealer anyway :-)

[Joseph_]

Just quickly to confirm that the chainstay length is indeed 469,5 mm : https://www.koga.com/en/koga-signature/technical-information.htm
Thanks JohnR for that correction!

Additionally, may I ask where you found the info on the sprocket size options?
That's also something I've been confused about since browsing Gates documents I sometimes saw 22t as the maximum rear sprocket and sometimes 24t.
I must've missed something because of my inexperience 

[mickeg]

For heavy touring or for when I set up my Nomad Mk II with suspension front fork for mountain biking, I use a ratio of 2.25 for chainring to rear sprocket (36/16), chain drive.

I ride heavily loaded and have a 38/16 setup. I appreciate the ability to get up tough hills slowly but effortlessly, but I regret the inability to pedal at all on downhills. I can't imagine going even lower to 36.

I checked the gearing, your gears are all almost exactly one half of a gear higher than mine if you have the same tire size.

When I built up my bike I knew that for carrying a heavy load up a hill I would want the lowest gear that I could get that would allow me to go the lowest speed that I have good directional and vertical stability without oversteering, at the lowest cadence that I find to be a smooth pedaling cadence.  And my calculations indicated I would want the 36/16 gearing to get a speed of 3.5 mph with a cadence of 72 with Schwalbe Marathon Extreme 559X57 tires.

I would like higher gears, but not at the cost of giving up my lowest gear.  There are shallow long downhills where I spin out, an inconvenience, but they are more rare than the steep uphills where I need that gear.

There are some that have lower gears than I have, but they would have me going too slow up the steepest hills to maintain stability without oversteering.
 
I think the Pinion people were smart to design their gearbox to have a wider range of gears, but they had years to analyze the shortcomings of the Rohloff before they designed their system.

[Matt2matt2002]

38*17 on my Raven. Happy with that set up for running around town and loaded touring.

As with many things in life, a compromise to a degree.

[Joseph_]

Hi mickeg, my tyre size is 28" 50mm so 50-622? I agree on your gear logic, I almost never use the top 3 gears (12-13-14). Ideally I'd like to lose 2 top gears and win 2 low gears and I'm told a 24t will achieve that.
But in the meantime I found out a 24t may not fit so I may have to replace the front chainring and the belt. It means double the cost but I'd have much more options for gearing.
Could someone explain to me if there's any other benefits or disadvantages for replacing the front chainring instead of the rear sprocket? Thanks.

[JohnR]

Just quickly to confirm that the chainstay length is indeed 469,5 mm : https://www.koga.com/en/koga-signature/technical-information.htm

Additionally, may I ask where you found the info on the sprocket size options?
That's also something I've been confused about since browsing Gates documents I sometimes saw 22t as the maximum rear sprocket and sometimes 24t.
I must've missed something because of my inexperience 

All the Gates sprockets are listed at https://www.gatescarbondrive.com/products/rear-sprockets . Your confusion is probably caused by there being two types of fitting onto the Rohloff hub: Screw-on and splined. The latter is easier to change. You can easily check which is on your bike as the splined sprocket is held in place by a metal lock-ring which has a gap in it (show in the photo at https://www.sjscycles.co.uk/sprockets/rohloff-splined-sprocket-carrier-8540/ ). If the bike has the screw-on carrier then a 24T sprocket is available.

Is it worth searching your local ebay or similar for slightly used Gates chainrings and belts? There must be many people who found their belt drive bike came with the wrong gearing and have had to replace some of the parts which didn't get much use.

[mickeg]

Hi mickeg, my tyre size is 28" 50mm so 50-622? I agree on your gear logic, I almost never use the top 3 gears (12-13-14). Ideally I'd like to lose 2 top gears and win 2 low gears and I'm told a 24t will achieve that.
But in the meantime I found out a 24t may not fit so I may have to replace the front chainring and the belt. It means double the cost but I'd have much more options for gearing.
Could someone explain to me if there's any other benefits or disadvantages for replacing the front chainring instead of the rear sprocket? Thanks.

So, you have a 50T front and 19T in back.  Based on that, a one tooth change in back will have roughly two and a half times as much change in gearing as a one tooth change in front.

If you dropped 2 teeth up front and added 2 teeth in back, that would give you a ratio of 2.29 which is not quite as low as mine but is pretty close.  I do not know if those components are made, just talking numbers here.  I assume (could be wrong) if the sum of your chainring and sprocket teeth after the change is the same as before, that the same belt could be used, but you still would need to adjust belt tension. 

If you start making these changes yourself, it would be a good idea to invest in whatever you need to measure belt tension instead of having a shop adjust your belt.  This guy runs looser belt tension than suggested, and he has a lot of experience with belts.  Note he cites a smartphone app for this.
https://www.cyclingabout.com/koga-worldtraveller-touring-bike-after-30000km-use/

I have no idea what belts, chainrings (is it beltrings?) or sprockets cost for belt drive.  But it is my understanding that they are quite expensive, but have a very long life.  Before you make any changes, you might want to be really certain that those are the changes that will make you happy before you start incurring a lot of additional costs.  That is one reason I wanted chain drive, I can change the gearing for different purposes.

Keep all the old parts, if you sell this bike some day, the new owner might want the different gear range.

One last thought, since Koga builds these and they know your chainstay lengths, they should be able to tell you what sprocket and chainring sizes are possible with with different belt sizes.