Thorn Cycles Forum
Technical => Transmission => Topic started by: Joseph_ on April 30, 2022, 10:16:30 am
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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 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 :D
Cheers, Joseph
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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.
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Thanks, that's a good gear ratio you've got there :D
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..
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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.
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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.
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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.
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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 :D
Cheers, Joseph.
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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 :D
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.
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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 :-)
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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 :D
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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.
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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.
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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.
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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 :D
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.
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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.
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Thanks again JohnR, really helpful all this.
Indeed that's why I was confused, thanks for the clarification.
I've no idea wether my rear sprocket's splined or threaded, but in attachment's I made a quick pic, maybe you can make it out?
But I'm reaching out to my local Koga dealer on Tuesday, he''ll know.
I guess 2nd hand is always an option though for this upgrade I'd prefer to buy new because I wouldn't know quality from problematic parts, and I'm pretty sure I'm going to love the lower gears so will be using them for a long time to come.
Thanks mickeg, I thought changing teeth in the front chainring (beltring? ;D) would have more effect since it's bigger but it seems it's the other way around.
For this I'm also going to reach out to my Koga dealer as he'll have to perform the change anyway, so I guess he'll be able to tell me my options.
Part costs here are : rear cog 95€ front chainring 90€ belt 90€, but as you said very long life so well worth it (imo).
The only way to be certain about which combination is ideal for me is trying out the different combinations beforehand, which is pretty impossible or too expensive.
So I'm just going to use calculators and aim for two extra lower gears and two less upper gears, which a 24t rear sprocket would provide (I'm told).
As for possible combinations, the Gates Carbon Drive Bicycle Calculator https://www.gatescarbondrive.com/Apps/DriveCalculator/index.html is pretty good at that, but the chainstay length remains a gamble for me as I don't know how much slack my EBB has.
Cheers, Joseph.
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I can't figure out the fixing detail in your photo. I therefore attach a photo of a standard (chain) Rohloff splined sprocket with the lockring clearly visible. A Gates sprocket has different teeth but the mounting on the carrier is (I think) the same.
I also attach (if the forum software) will let me a simple spreadsheet where you can change the chainring and sprocket teeth and quickly see how the gear inches change. For example, changing the chainring from 50T to 39T (which seems to be one of the standard Gates sizes https://www.bike24.com/p2181007.html ) moves the gearing by two Rohloff gear steps (change the numbers in the yellow boxes).
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I think you have the splined sprocket carrier, but it has a threaded fitting on it to hold the sprocket on it instead of a spring circlip.
https://www.rohloff.de/en/service/handbook/speedhub/workshop/splined-carrier-lockring-conversion
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Thanks again JohnR!
I can clearly see the locking ring, good to know.
And that spreadsheet is really handy too, easy to see know how many gears I'd win/lose, awesome.
Yes mickeg, I also think I've got splined (because Alee Denham who has the same bike as me also has a 50-22t config and I'm sure he went as low as he could go in the rear cog) I'll know for sure as soon as my Koga dealer gets back to me.
I don't reckon there exists a splined-to-threaded 'adapter' or anything of the sorts? (just wild guessing here)
I got a lot of solid information here from you guys, really appreciate it.
Now all I have to do is think about all the possible combinations and make a decision, which will be very personal one so that one's on me ;D
Cheers, Joseph.
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Now all I have to do is think about all the possible combinations and make a decision, which will be very personal one so that one's on me ;D
One unfinished business is whether the desired gearing combination will work within the variation provided by the EBB. Using the calculator at https://www.gatescarbondrive.com/Apps/DriveCalculator/index.html shows that a 39 : 19 combination (which drops the gearing by two Rohloff gear steps) needs a 115 tooth belt and a chainstay length of 471.70 mm. I would expect that to be well within the adjustment range of the EBB.
I would also note that while belts are not inherently messy in the same way as chains, they do like to be kept clean. I used a bike with a CDX belt one winter and, thinking it was maintenance free, let it get dirty. The bike developed worrying creaking noises which disappeared when I gave the belt a good scrub.
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Ccorrectomundo.
In the meantime I've got more info on the front sprocket; apparently there's only 1 smaller size, a 46T (CDX CENTERTRACK) https://www.gatescarbondrive.com/products/front-sprockets?productline=cdc%20centertrack (https://www.gatescarbondrive.com/products/front-sprockets?productline=cdc%20centertrack)
Which means my two best choices are :
- if threaded sprocket : 24T rear sprocket + new compatible belt.
- if splined sprocket : 22T rear sprocket + new front sprocket 46T (belt should fit I think because the new chainstay length would be 471 mm vs 469,5mm now).
So far no noise problems on my current belt (about 8000 km), but I've only ridden in about 15-25 degrees weather mostly.
I did read that in very dry/dusty environmonts it can get very noisy too, and that a Silicone Belt Lubricant can fix that.
Other than that, a plain old toothbrush and water ;D
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In the meantime I've got more info on the front sprocket; apparently there's only 1 smaller size, a 46T (CDX CENTERTRACK) https://www.gatescarbondrive.com/products/front-sprockets?productline=cdc%20centertrack (https://www.gatescarbondrive.com/products/front-sprockets?productline=cdc%20centertrack)
That table is for the CDC range. Your photo of the rear sprocket suggests it is CDX in which case the relevant product range is at https://www.gatescarbondrive.com/products/front-sprockets?product_line=CDX . One potential obstacle is that the 39T front sprocket is only available as 4 bolt 104mm BCD so I hope your 50T sprocket is not 5 bolt 130mm BCD. The 46T sprocket is available for both BCD options so your option to change both sprockets but keep the belt is feasible although the gearing ratio is nearer 1.5 Rohloff gear steps than the 2 steps of the 39:20 option.
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Dear God, you saved my life here JohnR!
I was about to send the order email to my Koga dealer when I saw your reply come in, great timing!
These last few days I've been so busy researching all this that I simply failed to see this difference, which is of utmost importance, phew!
And I should've read your posts more carefully as you've suggested the 39:19 option two times already, so sorry my bad :-[.
I indeed have a CDX front sprocket, a CDX:EXP to be more exact and part# CT1150MBA-5.8 to be very exact.
Apparently the surface area on the EXP is 25% larger but the 39T should be an option : https://www.cyclingabout.com/new-gates-cdxexp-expedition-carbon-belt-drivetrain/ (https://www.cyclingabout.com/new-gates-cdxexp-expedition-carbon-belt-drivetrain/)
So first thing tomorrow morning I'm calling my Koga dealer to know exactly which front and rear sprockets are possible, after which I can finally make a decision, hopefully 39:19!
Thanks again you lifesaver! ;D
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So first thing tomorrow morning I'm calling my Koga dealer to know exactly which front and rear sprockets are possible, after which I can finally make a decision, ...
Given the high costs, you might consider asking their best Rohloff expert when will he or she be in and available so you can go talk with them, and bring your bike along so that you can make sure it gets decided right.
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Apparently the surface area on the EXP is 25% larger but the 39T should be an option : https://www.cyclingabout.com/new-gates-cdxexp-expedition-carbon-belt-drivetrain/ (https://www.cyclingabout.com/new-gates-cdxexp-expedition-carbon-belt-drivetrain/)
So first thing tomorrow morning I'm calling my Koga dealer to know exactly which front and rear sprockets are possible, after which I can finally make a decision, hopefully 39:19!
39T CDX:EXP is listed by Gates https://www.gatescarbondrive.com/products/front-sprockets/39t-4-bolt-cdx-exp . The good news is that the 50T part is also 4 bolt 104 BCD https://www.gatescarbondrive.com/products/front-sprockets/50t-4-bolt-cdx-exp .
Given the high costs
Ideally one only gets a belt drive system after knowing the preferred gearing for the intended use. I haggled a discount on my belt drive bike because I knew I would need to change the gearing.
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Evening, just a quick update.
Went to my Koga dealer today and he confirmed I do indeed have a splined rear sprocket.
As I understand it threaded is older technology :
"Previously, belt cogs screwed directly onto Rohloff hubs, but now there is a much more user-friendly ‘splined carrier’ (part #8540L) with three splined belt cogs available (19t, 20t, 22t). This carrier makes cog changes much easier. There was a known issue for the 1st-generation splined carriers, but this was fixed mid-2018 (and warranty parts sent out), so won’t be an issue going forward."
Tomorow morning I have to call our national Rohloff dealer and I'll be able to get prices, availability and delivery times (which could be long I'm told).
I guess there's only two viable options now :
46:22 > change both sprockets and keep the 120t belt
39:19 > only change the front sprocket and change the belt to 115t
Costs are about identical and the gear ratios are also very closely knit together : http://ritzelrechner.de/?GR=RLSH&KB=46&RZ=22&UF=2281&TF=60&SL=2.6&UN=KMH&DV=speed&GR2=RLSH&KB2=39&RZ2=19&UF2=2281 (http://ritzelrechner.de/?GR=RLSH&KB=46&RZ=22&UF=2281&TF=60&SL=2.6&UN=KMH&DV=speed&GR2=RLSH&KB2=39&RZ2=19&UF2=2281)
The 39:19 has as a bonus that it resets the belt longevity, but the 46:22 distributes the drive train force over 10 more teeth (68 vs 58) and someone told me this is a good thing?
Please advice, it's the final choice ;D
Cheers again, Joseph.
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The final decision may depend on parts availability and price. You will struggle to notice the difference in gearing. There are those who believe that an odd-numbered total of teeth spreads out the wear on chains but I doubt it is significant with a belt. You may want to investigate getting the parts from a USA distributor if not available in Europe - some advertise on UK ebay so they may also advertise on European ebay platforms and it's also worthwhile checking Amazon.
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These ratios that you have picked of about 46/22 or 39/19, that is 2.05 to 2.09. That is pretty low. I use 2.25 for my lower gears. Your gears would be about one half of a gear lower than mine when I use my lower touring gearing. You would in effect be gaining almost two entire lower gears and never having the use of your existing 13th and 14th gear that you now have.
I am assuming that your body weight is under 100 kg. Rohloff has a requirement that the ratio be higher for riders over 100 kg.
Bigger chainring will mean you put less tension on the belt when you pedal. I would expect that is a good thing, with chains it is but since belts are supposed to have some pre-tension then maybe it is less important to a belt user.
If you are really convinced that you need gearing this low, if I was you, I would change both chainring and sprocket to the larger ones (46 and 22), buy a new belt that is the same length as my old belt, my old belt would become my spare belt that I carry on tours.
And I would keep my older chainring and sprocket, there might be a reason to have them later.
If the cost of tools to change the parts did not cost much more than the cost to have someone do that at a shop, I would learn how to do it and buy the tools. And I would figure out how to set belt tension, although you probably should only need to do that once.
Good luck.
The final decision may depend on parts availability and price. You will struggle to notice the difference in gearing. There are those who believe that an odd-numbered total of teeth spreads out the wear on chains but I doubt it is significant with a belt. You may want to investigate getting the parts from a USA distributor if not available in Europe - some advertise on UK ebay so they may also advertise on European ebay platforms and it's also worthwhile checking Amazon.
I am in USA, most of my Rohloff related purchases were not from a USA source but instead from a German retailer. I bought my hub and 250ml oil bottles from Bike24.
The odd number of teeth would be a chain thing, as unless someone makes an effort to install a half link in a chain, when you have an even number of teeth, half your teeth will always have a link with outer plates and half always have inner plates. The outer plate links elongate, the inner plate links don't. Belts, all teeth are the same.
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These ratios that you have picked of about 46/22 or 39/19, that is 2.05 to 2.09. That is pretty low. I use 2.25 for my lower gears. Your gears would be about one half of a gear lower than mine when I use my lower touring gearing. You would in effect be gaining almost two entire lower gears and never having the use of your existing 13th and 14th gear that you now have.
I agree that the result will be low gearing but is consistent with the objective noted earlier.
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.
In a separate comment 30% slopes were mentioned.
Given the cost of the Gates components it's almost worthwhile fitting a normal chain-based drivetrain to use while determining the optimum gearing before spending money on the final belt solution.
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In a separate comment 30% slopes were mentioned.
Given the cost of the Gates components it's almost worthwhile fitting a normal chain-based drivetrain to use while determining the optimum gearing before spending money on the final belt solution.
When I get up close to 10 percent on a loaded bike, my heart rate monitor tells me a number that I know means I better get off and walk. (I am 68 years young.) So, I do not think of slopes that steep.
When I built up my Nomad Mk II, I knew that I wanted two ranges of gears:
For touring I wanted the lowest I could go in first gear at a reasonably smooth cadence (I picked 72 for that), but still at a speed that was not so slow that I fall over (I picked 3.5 mph for that). It took some calculations to work that one out. But I am an engineer so it was not THAT complicated. There were a few hills where I let my cadence fall to low or mid 60s, but with good tarmac and lack of side winds, I could maintain balance at speeds a bit lower than 3.5 mph.
And for my higher range of gearing for all other things, I wanted it pretty close to my Thorn Sherpa derailleur touring bike. My Sherpa has a lowest gear of 19.2 gear inches and highest of 106.8 gear inches with 40mm wide tires, for my Nomad Mk II gearing for around town I chose 19.8 to 104.0 gear inches which almost perfectly matches that.
But if you are new to touring and your new custom bike is your first one for touring, that might be the ideal way to figure out what you want, start with chain and figure it out. If your crankset can be used for both chain and belt, your redundant costs should not be too high.
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I keep forgetting that the OP does not have a Thorn, he has a 700c bike, where my gearing is on a 26 inch bike, so his proposed gearing is not as low as it would be on my bike.