I have a Basis Osprey folding bike. It has a rear wheel hub and small wheels as per most folding bikes. It was a gift so please don't suggest I bought the wrong bike. When going along in full power on the flat or downhill the highest gear ratio is too low and I have to pedal like fury to keep the motor going. Does anyone know if the rear sprocket can be changed to provide a higher gear or maybe add a second smaller front sprocket? Please don't get too technical with your answers. Thank you.
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4Why do you need to keep the motor going? Maybe it is meant to only give a little support when needed, for example when going uphill. Downhill it works just like a non-electric bike.– quaragueCommented Mar 11 at 11:38
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1Does your bike have a crank sensor, which has to be activated by pedalling in order to make the motor assist you? Second, does your bike have a "maximum assist speed" at which it cuts out? Many ebikes are limited to assist up to a set speed, (perhaps 25 km/h) where they stop assisting. This number is frequently legislated, so depends on country of sale. Based on e-bikesdirect.co.uk/brands/basis/… I presume you're in the UK? Is this your bike ?– Criggie ♦Commented Mar 11 at 12:12
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2If I did the math right and the bike has a 44t front chainring (as shown on the product site) and (assuming a Shimano 6-speed cassette, 14-28t), you'll top out at ~25 kph and 85 rpm with these wheels. A 11t cassette bumps this to ~32 kph, for the same speed, you'd need a 56t chainring, which is rather unrealistic. I only found MF-TZ510-6 as cassette(14-28 only), so it could be that we're pretty limited in gearing with this bike.– DoNuTCommented Mar 11 at 12:39
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1@ChrisH And what is "spinning fast"? Assuming the bike cuts out at 25 kph like it should, the standard gearing makes you pedal about 85 rpm at these speeds. Maybe I'm a bit biased as a road cyclist but that doesn't sound like crazy high cadence. I'm aware that people on e-bikes often tend to pedal slower because the motor does the torque. In that case, every tooth helps, every tooth increment (46/48/50) buys you approximately 1 kph theoretical top speed or ~3 rpm lower cadence, so I'd say at least a 48t chainring would make a notable difference.– DoNuTCommented Mar 12 at 13:11
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1@DoNuT spinning fast is whatever the OP means by fast. I'm thinking of the system behaviour, and whether it changes with speed, on the basis of checking that everything is working consistently before changing anything. It's the wording "to keep the motor going" that got my attention. Folders often seem to be under-geared so that is quite likely to be the issue, but best to check for faults before considering upgrades.– Chris HCommented Mar 12 at 13:54
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2 Answers
Unfortunately I think your options are a bit limited.
As Criggie says, your options are to change to a larger front gear or smaller rear gears. From what I have been able to find the front gear is on a standard square taper axle (this one) and this means it is easily changed. However if you make the gear much larger its front edge will hit the bracket where the bike folds, or its rear edge will hit the bicycle frame. I've shown where the gear will hit the frame or bracket here:
Or a slightly different view here:
It's hard to tell how much you could increase the size without actually looking at the bike, but I would have guessed not much more than 10% bigger. A 10% increase might be useful as it's a bigger change than it sounds. It would in effect shift all the gears up by one. If you want to try this I described my experiences of doing this to my bike here.
The other option is to change the gears at the back. From what I've found I think the Osprey uses a type of gearset called a freewheel and the smallest gear you can get on a freewheel has 14 teeth. I cannot tell from the pictures but I would guess your bike already has a 14 tooth smallest gear at the back and that means you cannot reduce it further.
The only way round this would be to modify the rear wheel to use a different type of gearset called a cassette, but even if this is possible it would be very complicated and expensive and it would be more cost effective to sell the bike and get a new one.
If you decide you want to try changing the front gear we can help, but you need to measure up the bicycle to find out how much space is available for increasing the radius of the front gear.
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1Looking at the quick release on the hinge, I reckon the chainstay would be the issue. You've got about 50% more pixels to the bend in the chainstay than to the hinge pin, starting from the chainring guard ring (in the upper photo). Such a guard is probably quite useful on a folding bike of course. 4 hole chainrings are very common, so that would be a good place to start - if the ring is bolted to the crank and can be replaced without interfering with the cadence sensor. However I'd look at a wheel build as well, because 14-28 is a very small range (2:1).– Chris HCommented Mar 12 at 14:04
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Excellent point on the freewheel with 14 teeth high, I had that on my 20" folder and ended up re-building the rim onto a different hub that takes cassettes, to get that 11 tooth. I also swapped a ~42 tooth chainring for a triple with a 53 and that worked fine, but there's no way to fit a front derailleur so it lives in the big ring and I stop and shift to smaller rings by hand if needed. Retention hasn't been a problem.– Criggie ♦Commented Mar 12 at 20:49
Might seem obvious, but you are changing gear, and the chain is moving to a different cog, right? At the rear a smaller cog (lower number of total teeth), combined with the larger-tooth-count front chainring will give a higher ratio.
Most cassettes are limited to 11 tooth as the smallest gear - there are some that offer a 10 tooth and a very few that go to 9 tooth overall.
The easiest way to increase the gearing is to swap the front chainring for a larger one with more teeth. This will require a longer chain too.
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Thank you yes I'm gonna look into changing the front sprocket. Commented Mar 12 at 13:29