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Gearing

Old 07-17-19, 09:53 AM
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tomkcook
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Gearing

I have a tadpole recumbent trike made by ZZ Merck (a Chinese import to the UK) with 36V 250W electrics.

I generally like it, but I'm finding the gearing on it rather low-range. I spend almost all my time in top gear and spin out on anything that's not uphill. Admittedly my legs aren't made for spinning around really fast & I prefer a slow, hard push.

It currently has a 3-ring set of sprockets on the front and a seven-ring freewheel sprocket set on the back. The front has 42, 34 and 21 (I think? not sure about the 21) tooth sprockets.

As far as I can tell, my options for increasing the gearing are:
  • Put bigger sprockets on the front. This would mean replacing the whole set because the ones that are there are not made to be mixed-and-matched. My local bike shop has found a set that goes up to 52 teeth and we can go bigger than that, but it starts to get pricey (I'm being quoted upwards of £60 per sprocket for 60-odd teeth) and eventually there's a limit to how big a wheel you can put on. 52 would give me just under 25% extra gear ratio - good but not as much as I'd like.
  • Put smaller sprockets on the back. I'm not sure how feasible this is with what's there, I haven't tried taking it apart. I suspect there's not a whole lot of room to move here.
  • Something else. There is an idler half-way along which sits on a bolt that's screwed into a nut that's welded to the frame. I'm considering replacing the idler with a stub axle with two sprockets on it, locked together, in about a 2:3 ratio. I'd then split the chain into two loops. This would give me 50% extra, but I'm worried about the forces on the stub axle & the nut weld.
So three questions:
  • What sort of gearing setups do people use on their recumbents?
  • What do people think of the idea of replacing the idler?
  • Is there some other option that I've missed?
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Old 07-17-19, 11:28 AM
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You can use a gear calculator to find out your exact gear range on your trike - https://www.sheldonbrown.com/gear-calc.html or Mike Sherman's Bicycle Gear Calculator. It's a quick way to compare the results for changing the sprockets or changing the wheel size.


The manufacturer chose to use a mountain bike set for the chainrings on your trike. There is no cheap way to increase the gear range on your trike. You could have the rear wheel rebuilt with a 3 speed hub but that's not cheap either. You also can't just switch out the largest chainring for a bigger one without possibly having to replace the front derailleur. Each FD is designed to operate with a specific range (# teeth in large chainring - # teeth in small chainring) and if you exceed this number it will not work properly.


I have a trike equipped with a 700C rear wheel, 11-36 tooth cassette and 52 for the largest chainring and a gear range of 21.9 to 124.2 gear inches, pretty much what you find on most road bikes. If you want sticker shock look up the EU price for a 65 tooth Schlumpf drive chainring like the one that comes on my Greenspeed GTO.


I wouldn't mess with the idler. Who knows what kind of stresses that would place on the frame? If you want a better idler, look at Terracycle idlers. They are expensive but very good quality.
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Old 07-17-19, 11:46 AM
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What he says is true but there’s another option. You have a mountain bike crank, but you can find a very similar hybrid crank for not very much money. It will have 48, 36, 26 front rings, or thereabouts. That will take you up about two shifts. You will need to move up the front derailleur and might need to add some chain links.

if you have a 20 inch rear tire a road triple is best, my Terra Trike came with 53-42-30
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Old 07-17-19, 02:23 PM
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To do it right you'll have to replace your current mountain crankset with a road crankset, which these days seems to be 50/39/30; and also replace your front derailleur with a road derailleur. The larger chainring on the road set will have a larger circumference than the mountain set, and your current FD is made to match the smaller circumference of the mountain set. Will the current derailleur work? yes, but not well.

Not being familiar with that trike, what size is the drive wheel, and is there a mid-drive?
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Old 07-25-19, 11:17 PM
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Stub axle would be subject to considerable force, not likely to be durable unless over-engineered.
(My 2 wheel bent has a stub axle mid-drive and took a year or so to work out the kinks.)
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Old 07-26-19, 05:15 PM
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Also, won't changing the gearing affect the motor too?

My thought is that if you need a motor, you probably need the low gearing.
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Old 07-30-19, 09:46 AM
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When triples are unit construction like the o.p.'s they are almost always in fixed steps of 10 teeth. I've never known any exceptions. Some careful tooth counting is in order I believe. I don't think a mid-drive is in use because I don't know of any that work with a triple crank but of course I don't know everything. 250W doesn't sound like a lot of power but it is like having an Olympic caliber cyclists on the trike with you. Without the power speeds would drop then a rider would understand why those particular gears were chosen. IF the rear wheel is 20" then, yes, those are lowish gears. A road triple wouldn't be a terrible idea but some idea of the o.p.'s cadence and/or expected cadence (in revolutions per minute) would be instructive. It's one thing to enjoy a slow hard push and very high road speed due to e-assist but battery life will not be high. A much higher cadence may take some practice but allows better performance from the human side of the hybrid power scheme.
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Old 08-29-19, 11:03 AM
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Thanks for all the feedback. I'm still figuring out what to do here. Thinking seriously about a fairly major rebuild, involving a larger rear wheel and a few other adjustments. To respond to some points:

Originally Posted by BlazingPedals View Post
Not being familiar with that trike, what size is the drive wheel, and is there a mid-drive?
It's a 20 inch drive wheel. I am seriously considering asking an engineering shop to cut the fork off and weld on a 700c fork, replacing the rear wheel completely.

Originally Posted by sch View Post
Stub axle would be subject to considerable force, not likely to be durable unless over-engineered.
(My 2 wheel bent has a stub axle mid-drive and took a year or so to work out the kinks.)
I'm perfectly prepared to over-engineer this, but I'm also trying to be at least moderately practical.

Originally Posted by BlazingPedals View Post
Also, won't changing the gearing affect the motor too?

My thought is that if you need a motor, you probably need the low gearing.
Well, it depends. The bike is used for commuting and my commute has a ~200ft climb over about 3/4 mile. If the electrics are not working then yes, this goes pretty slowly in places, though I'm still not nearly down to the bottom of the gearing. The lowest gear ratios on this thing are that ridiculous. In practice, once I've figured out the basics of not letting the battery go flat, I never do it without the electrics. The motor is not subject to the gear ratios and so its performance is not affected by changing them.

Originally Posted by Leisesturm View Post
Without the power speeds would drop then a rider would understand why those particular gears were chosen. IF the rear wheel is 20" then, yes, those are lowish gears. A road triple wouldn't be a terrible idea but some idea of the o.p.'s cadence and/or expected cadence (in revolutions per minute) would be instructive. It's one thing to enjoy a slow hard push and very high road speed due to e-assist but battery life will not be high. A much higher cadence may take some practice but allows better performance from the human side of the hybrid power scheme.
I'll measure my preferred cadence next time I'm riding but my guess would be somewhere around 30-40RPM. The rear wheel is 20" and yes, this is a significant part of the problem. Yes, the battery life will be affected but then as I only use it for a 9 mile commute, battery life is not a big problem at present.
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Old 08-29-19, 01:16 PM
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By the way someone mentioned above replacing the rear hub with one with an integrated 3-speed gearbox. I'm having trouble finding any IGH with an electric motor - does such a thing exist? Or would this mean moving to a mid drive?
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Old 08-29-19, 01:27 PM
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Originally Posted by tomkcook View Post
I'll measure my preferred cadence next time I'm riding but my guess would be somewhere around 30-40RPM. The rear wheel is 20" and yes, this is a significant part of the problem. Yes, the battery life will be affected but then as I only use it for a 9 mile commute, battery life is not a big problem at present.
I'm thinking you can (need to) do much better than 30 - 40 RPM! Like double that. Even at 60 y.o. I get to 100 RPM with relative ease but on a 9 mi. commute in rolling country with few stops I'd probably try to stay in the high 80's with bursts up to 90 - 100. That's on a bike without e-assist so YMMV but 30 RPM seems pretty low. IF the rear wheel size is a problem you need a new trike. No engineering shop is going to take on a re-design of your trike. The SRAM Dual Drive hubs are the go to resource for people needing to get 700C type gear inch range from 20" rear wheels. They are no longer produced but can be found on Ebay. Sturmey-Archer makes an equivalent which for some reason does not appear to get as much love as the SRAM unit did. With a hybrid IGH/cogset rear hub on your trike AND a front triple crankset you would have a very, very impressive gear range. Somewhere in there would be your perfect gear for every situation. Minimal engineering, pretty moderate cost too. It's what I recommend.
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Old 08-29-19, 03:19 PM
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Originally Posted by Leisesturm View Post
I'm thinking you can (need to) do much better than 30 - 40 RPM! Like double that. Even at 60 y.o. I get to 100 RPM with relative ease but on a 9 mi. commute in rolling country with few stops I'd probably try to stay in the high 80's with bursts up to 90 - 100. That's on a bike without e-assist so YMMV but 30 RPM seems pretty low.
That's a pretty wild guess - I'll check next time I'm riding.

The SRAM Dual Drive hubs are the go to resource for people needing to get 700C type gear inch range from 20" rear wheels. They are no longer produced but can be found on Ebay. Sturmey-Archer makes an equivalent which for some reason does not appear to get as much love as the SRAM unit did. With a hybrid IGH/cogset rear hub on your trike AND a front triple crankset you would have a very, very impressive gear range. Somewhere in there would be your perfect gear for every situation. Minimal engineering, pretty moderate cost too. It's what I recommend.
Sorry if I'm being horribly dense, but using one of them would preclude using a rear hub motor for the eassist, right? That's not totally out of the question I guess - a mid drive could be fitted to the front of the tadpole. Or am I missing something and there are eassist motors that can be fitted around an IGH?
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Old 08-29-19, 04:25 PM
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Originally Posted by tomkcook View Post
Sorry if I'm being horribly dense, but using one of them would preclude using a rear hub motor for the eassist, right? That's not totally out of the question I guess - a mid drive could be fitted to the front of the tadpole. Or am I missing something and there are eassist motors that can be fitted around an IGH?
Nope, you're not being dense. You are correct. IGH rear hub means no hub motor. Yes, a mid-drive could work but AFAIK none work with a triple. I'm not sure any work with a double! I think the road triple (52/39/30) and the rear hubmotor with however many cogs it has is about as good as it gets.
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Old 08-30-19, 07:26 AM
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Originally Posted by tomkcook View Post
Originally Posted by Leisesturm View Post
I'm thinking you can (need to) do much better than 30 - 40 RPM! Like double that. Even at 60 y.o. I get to 100 RPM with relative ease but on a 9 mi. commute in rolling country with few stops I'd probably try to stay in the high 80's with bursts up to 90 - 100.
That's a pretty wild guess - I'll check next time I'm riding.
Yep, a very wild guess. On my ride in this morning I think the numbers were more like:
  • Comfortable - 70-80 RPM
  • Doable but uncomfortable - 80-90 RPM
  • I can spin my legs this fast but it's not adding - 100 RPM
This is again an estimate while riding but I have a reasonable musician's sense of time - these are somewhere in the right ballpark.
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Old 08-30-19, 10:33 AM
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Originally Posted by tomkcook View Post
Yep, a very wild guess. On my ride in this morning I think the numbers were more like:
  • Comfortable - 70-80 RPM
  • Doable but uncomfortable - 80-90 RPM
  • I can spin my legs this fast but it's not adding - 100 RPM
This is again an estimate while riding but I have a reasonable musician's sense of time - these are somewhere in the right ballpark.
Since I have no experience with e-assist I am unable to offer much more advice. I am curious though as to how much you use your electrics. Is this a pedelec system or is it hand throttle? I wouldn't be surprised if you were spinning out any but the top gear with the motor running. You probably have more stringent laws governing e-assist than those in the U.S. and the maker of an e-trike bound for the U.K. cannot gear it so high that with motor power the rider can easily exceed 20mph. The assist has to cut out at that point and riders would quickly complain that they aren't getting any of the assist they paid for. I'm thinking you need to spend some more time with your trike trying to work with its idiosyncrasies until you can see more clearly what you need. I also would suggest you do some very careful tooth counting of all the front chainrings and all seven rear cogs and tell us what they are.
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Old 08-30-19, 02:54 PM
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There's a lot of difference between a 42 and a 52 big front sprocket. That's what I would do.
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Old 08-31-19, 12:28 AM
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Originally Posted by Retro Grouch View Post
There's a lot of difference between a 42 and a 52 big front sprocket. That's what I would do.
Weeel ... about that ... one of the reasons I've suggested the o.p. carefully count the teeth ... all the teeth, is because for me, the matter is not settled as to whether it actually is a 42T big ring. My money says not. I could be wrong. Tell me where to Paypal my money in that event. I have a 20" rear wheel folding bike with a 53 x 11-32 8sp cassette. I spend a lot of time in top gear. My 700C bike with the 50/34 x 11-27 I spend most of my time on the 34 and only use the big ring for downhill bombs. There is simply no way a triple with a 42T big ring is possible using a 20" back wheel. The middle ring would be barely useful and the granny would be completely and utterly unusable. But please do not think for a minute o.p. that this is criticism. We are all but flesh.
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Old 08-31-19, 09:58 AM
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Originally Posted by Leisesturm View Post
Weeel ... about that ... one of the reasons I've suggested the o.p. carefully count the teeth ... all the teeth, is because for me, the matter is not settled as to whether it actually is a 42T big ring. My money says not. I could be wrong. Tell me where to Paypal my money in that event. I have a 20" rear wheel folding bike with a 53 x 11-32 8sp cassette. I spend a lot of time in top gear. My 700C bike with the 50/34 x 11-27 I spend most of my time on the 34 and only use the big ring for downhill bombs. There is simply no way a triple with a 42T big ring is possible using a 20" back wheel. The middle ring would be barely useful and the granny would be completely and utterly unusable. But please do not think for a minute o.p. that this is criticism. We are all but flesh.
You have a point but you aren't the person who asked the question. My answer was intended to be a relatively simple and do-able solution to the OP's question not as a do-all for every possible recumbent configuration.
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Old 09-04-19, 06:15 PM
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What I'm hearing is that you're ready to charge off and modify your trike without learning how to ride it first. You want bigger gears because a.) your cadence is half what it should be, and b.) you've got a motor doing all the work. My advice is don't change anything until you fix your cadence and then put 500 miles on it, preferably without the motor. The motor is a crutch. Some people need crutches, but I haven't heard the words that you do.
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Old 09-14-19, 08:13 AM
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Removing one tooth from your smallest rear cog will do as much as adding four teeth to your front one, so looking at your cassette is the obvious first line of investigation.

I've done something different. I didn't like the huge chain, with or without tubes, so I got rid and use two chains. They join together at a relay pulley, but this isn't a 1:1 ratio pulley - it's geared up.

The front chain turns a 12t cog, which pulls the rear chain with an 18t cog. This allowed me to keep my crank nice and small, and still have long gearing..

It does of course lengthen all gears, so your low gears are longer. As I have e-power as well, plus an mtb cassette, this is no issue for me.
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Old 09-14-19, 08:40 AM
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Originally Posted by MikeyMK View Post
Removing one tooth from your smallest rear cog will do as much as adding four teeth to your front one, so looking at your cassette is the obvious first line of investigation.
I disagree strongly. How many trikes have you seen that don't already have an 11 tooth small cassette cog? Different chainring sizes are much easier to source.
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Old 09-14-19, 10:07 AM
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Originally Posted by Retro Grouch View Post
I disagree strongly. How many trikes have you seen that don't already have an 11 tooth small cassette cog? Different chainring sizes are much easier to source.
Mine doesn't (13t)

They look kinda goofy with 60t on the front, the cost rises further with more chain and suitable derailleurs, too.

Technology has moved down to 8t now but admittedly they're not as smooth and require a hub movement assembly conversion.

I'll never run one long chain again, either.
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