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Gearing

Old 07-17-19, 09:53 AM
  #1  
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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