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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 and the Gates Carbon Drive Bicycle Calculator 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

1 Answer 1

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With the existing 50T chainring and 19T cog your gearing ranges from 19.8 gear inches to 104.2 gear inches (assuming 622 wheels)

So you're essentially loosing about a gear at the top end and gaining a gear at the bottom when going from 19->22 and from 22->24.

Cog Teeth
Count
Low/
easy
G E A R I N C H E S High/
hard
19 19.8 22.5 25.6 29.1 330 37.5 42.6 48.5 55.0 62.6 71.1 80.6 91.8 104.2
22 17.1 19.4 22.1 25.1 28.5 32.4 36.8 41.9 47.5 54.1 61.4 69.6 79.3 90.0
23 16.4 18.5 21.1 24.0 27.2 31.0 35.2 40.0 45.4 51.7 58.7 66.6 75.8 86.1
24 15.7 17.8 20.3 23.0 26.1 29.7 33.8 38.4 43.5 49.6 56.3 63.8 72.7 82.5

The full table was calculated at https://www.sheldonbrown.com/gear-calc.html

For chains, cogs are not a complex item, and don't cost a great deal. I'm not sure how much belt drive cogs cost but they are definitely going to be more expensive.

All belt drive bikes have to have some way to tension the belt - I suggest you get a feel for the current belt tension, then loosen yours and see how much slack you gain.

Going from 19 to 24 is adding 5 more "links" but they're on both sides so effectively the belt is getting 2.5 links longer on both sides. Visually see how much you can move the eccentric.

If your bike had old-style trackends it would have 30+mm of adjust overall, but its all down to how much your Eccentric offers.


Your other option is to replace the belt and cog together, and then store the current pair for future use if your riding patterns change.

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    Thanks this is a great help. If I use the calculator I get about two inches higher in every result field but I do get the overall picture. I'd like to gain 2 low gears and lose 2 high gears so a 24t would be the right choice, but i don't know if my belt will take that. I do have an eccentric bottom bracket, but have no idea how much slack there is as I'm very inexperienced in that field. Belt rear sprockets and front chainrings cost about 100€/piece here so quite expensive and that's why I only want to replace the rear sprocket. Any tips on how to find out how much slack my bottom bracket has?
    – Joseph
    Apr 30, 2022 at 13:23
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    @Joseph: With the few eccentric bottom brackets I’ve seen you can see its current orientation and how much slack you could gain. Usually their whole adjustment range is just a few millimeters, so I doubt it’s going to be possible to compensate for the new sprocket.
    – Michael
    Apr 30, 2022 at 18:01
  • @Joseph your other option is to get a shop to do it. Sure it will cost more but you know it will work straight off without a gamble. Then note down your combination for the future.
    – Criggie
    Apr 30, 2022 at 22:55
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    Thanks a lot, you guys have been a great help. Since I was thinking of buying a spare belt anyway for my next tour, I might was well immediately go for the largest rear sprocket (24t) and buy a new belt for that configuration. That should solve my problem and keep costs balanced. For the actual replacement I'll go to the shop anyways as it's far beyond my beginner's expertise :-) Thanks again! Tailwinds, Joseph.
    – Joseph
    May 1, 2022 at 8:20

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