hi, Nexus seven hub changing the rear sprocket from 21t to 16t

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James9000
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hi, Nexus seven hub changing the rear sprocket from 21t to 16t

Postby James9000 » 11 Oct 2018, 9:38pm

hello. I recently bought a basic ebike that is front wheel drive. The gears are all set up for a normal bike and are a bit too low for an electric bike. Two questions I'm going to swap the rear cog which is 21 tooth to a 16 tooth which is the smallest one they do, will that make too extreme a difference? Also does one need a special new bicycle chain when changing the rear cog. Thank you

Brucey
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Re: hi, Nexus seven hub changing the rear sprocket from 21t to 16t

Postby Brucey » 11 Oct 2018, 11:24pm

42/21 compared with 42/16 gearing (for 35-622 tyres) here;

http://ritzelrechner.de/?GR=SNI7&KB=42&RZ=21&UF=2185&TF=90&SL=2.6&UN=MPH&DV=gearInches&GR2=SNI7&KB2=42&RZ2=16&UF2=2185

Speeds are assuming you pedal at 90 rpm.

if the chainring size is correct, even if you pedal at a glacial 60rpm you will still do 16mph (which is the legally permitted speed for an e-bike) with the gearing you have. If you are going faster than this you have presumably derestricted the machine, in which case in the eyes of the law, you don't have an e-bike, you have an electrically powered motorcycle.

Said "motorcycle" will be untaxed, un-MOT-ed, uninsured, you won't be wearing the right helmet, and you won't have the correct licence either. In other words you may get away with it for a while but if you get caught and/or have an accident you can expect to get the book thrown at you.

cheers
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the snail
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Re: hi, Nexus seven hub changing the rear sprocket from 21t to 16t

Postby the snail » 12 Oct 2018, 9:59am

Brucey wrote: ...If you are going faster than this you have presumably derestricted the machine...

Or you're pedalling without assistance - there's no speed limit for e-bikes afaik?

James9000
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Re: hi, Nexus seven hub changing the rear sprocket from 21t to 16t

Postby James9000 » 12 Oct 2018, 11:09am

Thank you very much for your reply Brucey, I'm very encouraged because what you wrote sounds promising. I am slightly disabled in my right leg and a glacal pedal speed is exactly what I'm after. I'm not a speed merchant either, I have four power settings and even on one and two, I lose resistance so I'm peddling for about five seconds and then coasting for about 20 seconds and repeating that.

That ready reckoner calculator looks amazing, I'll be honest right now I'm completely baffled, just because I'm unfamiliar with the term but I will figure it out. By the way what is the "chainring"? Definitely not a de-restricted bike, and I take your point about the legal issues, I would dearly love a throttle control bike, it would be an absolute dream but there is no way I want the legal jeopardy.

Just to confirm, you think the 16 tooth would check out. It's 60 rounds per minute, that's not weirdly slow is it? What sort of impact would it make on starting the bike from stationary? The cogs themselves are quite cheap but it's about £45 to get the new one fitted, which is why am trying to get the best estimate of which cog to put in. Many thanks

Brucey
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Re: hi, Nexus seven hub changing the rear sprocket from 21t to 16t

Postby Brucey » 12 Oct 2018, 12:04pm

'chainring' in the calculator is the size of the front chainwheel, in 'teeth'. Bike with hub gears quite often have a 38T chainring, in which case the gears are about 10% lower than first indicated. Obviously the wheel/tyre size affects the gearing too. In the UK gears are measured most often in 'inches'; this corresponds with the diameter of a wheel that is driven directly by the cranks, i.e. like on a pennyfarthing.

It isn't that difficult to fit a new sprocket; if you are handy you can probably do it yourself. The cost you have been quoted is at the upper end of what I'd expect to be charged for this job.

You don't have a very large range of gears so if you raise the whole lot too much, starting from rest may be a struggle (for you and/or the motor), likewise steep hills, and of course should you find yourself without a working battery or something riding the bike will be very difficult.

Most seasoned riders pedal at about 80-90prm. If you pedal more slowly (eg at 60rpm) then you have to push harder on the pedals to get the same power. At 60rpm you need to push half as hard again as at 90rpm. This is fine for the first few miles but seems like very hard work after a while. Inexperienced cyclists feel that it is somehow 'wrong' because they don't feel much resistance when they are pedalling at about 90rpm but that is the whole idea; the gears etc are to be used to make pedalling easier.

Note that with a 16T sprocket fitted, if you want to pedal at 90rpm you won't have a gear low enough anytime you are going less than ~12mph. 12mph is about what you would manage on the flat without a battery and without breaking sweat. However for going up hills it is 'very fast'. For normal bikes those gears would be 'way too high' and on an e-bike the thing is going to be pretty useless on hills unless the motor is working flat out.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

James9000
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Re: hi, Nexus seven hub changing the rear sprocket from 21t to 16t

Postby James9000 » 12 Oct 2018, 1:47pm

Hi Brucey, thank you for what you wrote, I am mindful that I don't want to drive you mad, or exhaust you because I'm such a beginner, I need to do my own research, what you have said has been enormously helpful.

If you can bear it just a couple of quick questions. What are the numbers by the black triangles on the calculator? Are they the revolutions per minute of the pedals? So am I right in thinking that in the lowest gear the pedal rpm would be 45.4 rpm at 12 miles an hour and obviously much less at slower speeds. By the way I checked and the front cog is aAlloy forged 170mm, 42t.

From that calculator you linked, can you tell what my estimated pedal rpm is with my existing setup of 42t front and 21t, shim Inter7 in the highest gear, travelling at 16miles an hour? (If it shows that information) Because it is a bit too advanced for me to understand. Many thanks

goatwarden
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Re: hi, Nexus seven hub changing the rear sprocket from 21t to 16t

Postby goatwarden » 12 Oct 2018, 4:52pm

James9000 wrote:What are the numbers by the black triangles on the calculator?


Those numbers are the "size" of the gear in inches; this means the effective diameter wheel a penny farthing would need to give the same progress forward for each revolution of the pedals.
The example set up by Brucey is for a 42 tooth chainwheel, which is what you have. The upper box demonstrates your gear range currently (with a 21 tooth sprocket) and the lower box shows the range for a 16 tooth sprocket.

To establish what speed you will achieve for a particular cadence (r.p.m. at which you rotate the pedals) you need to play with the "cadence" slider in the lower left hand corner. By default it is set at 90 r.p.m. (general consensus of a comfortable cadence) but the slider will allow you to show the effect down to 60 or up to 120 r.p.m. Once you have set your desired cadence value, you can read off the speed that would achieve in each gear from the "mph" scale at the lower edge of the lower graphic box. Thus currently for your current gearing, pedalling at 90 r.p.m. you would achieve approximately 9.2 mph in gear 1 and approx. 22.6 in gear 7.

The Gear calculator is a really useful tool and very simple to use and understand if you play with it for a while.

James9000
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Re: hi, Nexus seven hub changing the rear sprocket from 21t to 16t

Postby James9000 » 12 Oct 2018, 7:05pm

goatwarden wrote:<SNIP>

Thank you very much, your explanation was very helpful, finally the penny has dropped, I really see how useful that gear calculator is now. I'm going to have to do some further experiments because I simply don't believe what gear calculator says. It could be that I'm totally out of condition as a cyclist but I tested the maximum speed I could get to which was about 16 mph and I felt my legs were going like bees wings but according to the gear calculator, in high gear, travelling 16 mph on the flat my cadence should only be 60 revolutions per minute, which Bruciey said was glacial .

Maybe for me 60 cadence feels really fast, I'm very curious to do some experiments and count the revolutions versus speed, I will let you know the result. Thanks again to you and Brucey, am enormously grateful to understand how gears work.

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andrew_s
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Re: hi, Nexus seven hub changing the rear sprocket from 21t to 16t

Postby andrew_s » 12 Oct 2018, 9:25pm

Pedalling quickly is something you have to get used to.

It's not all that uncommon to get a post from a new (young, usually) cyclist asking for advice on higher gears because they are spinning out their top gear (i.e. they can't pedal any faster), and then, when you ask, finding out that the gear concerned is about 120", which I'd call good for 50+ mph (given a suitable downhill).

brynpoeth
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Re: hi, Nexus seven hub changing the rear sprocket from 21t to 16t

Postby brynpoeth » 12 Oct 2018, 9:39pm

Best not to change the sprocket yet, wait a while, read more, ride more until you are sure about changing it or not. Might just not be necessary

You could spend some time searching & browsing these fora, lots more relevant info posted already
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asinus
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Re: hi, Nexus seven hub changing the rear sprocket from 21t to 16t

Postby asinus » 14 Oct 2018, 6:55pm

Changing to a 16T sprocket will take the direct drive ratio to 2.6, above the manufacturer's recommendation, but it doesn't matter really if it's too high: it's too low a ratio that makes it more likely that the teeth on the sun pinions in the gear hub will strip.
I guess the question of the bottom gear depends on where you are and what hills you have to go up. With a 16T sprocket your bottom gear would be 40-45" depending on wheel size, which would be too high for many places. However, on the elderly pedelec I use as a load-lugger (with a less powerful motor than newer ones and a not-very-sporty riding position), I find that I use a 41" gear on an incline that I go up on a 29" gear on my unassisted hybrid, so that the higher bottom gear might not affect you. You would be pushing hard above 25 km/h once the electrics cut out, though, in your new top (106" if you have 26" wheels, more if bigger).
Hope this helps.
Edmund

James9000
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Re: hi, Nexus seven hub changing the rear sprocket from 21t to 16t

Postby James9000 » 15 Oct 2018, 3:04pm

asinus wrote:<SNIP>

I really appreciate yours and other information containing replies, I'm trying to learn as much as possible. My situation is weird, I've not ridden a bike in about 20 years because I've become increasingly disabled. I have Ehlers-Danlos syndrome which is a connect if tissue deficit which is hereditary. My hamstring tendons are in a bad way as I repeatedly injure them over the course of time. I couldn't ride a bike because the lean forward position put too much strain on the hamstring, however I discovered Amsterdam and then sit up and beg bikes and found I could manage the position and pedal.

I'm 6 foot two but on a medium frame Raleigh so the handlebars are quite near and is quite comfy. When my legs not bad I can cycle short distances, purely for recreation. Long story short, the bike is basically instead of a mobility scooter and I can't store one of them. Using my electric bike can be painful if my leg is already sore. Having a fast cadence does irritate my leg. What I want to do is breeze along the flat my legs turning barely any energy going into the pedals, I feel this way I will be able to travel further.

The issue is I don't want it too hard so it's difficult to start the bike when it is stationary if I swap the rear cog to a 16t. What I would really like to know is, whether anyone can tell me what gear on my current Nexus seven 21t cog would simulate the lowest gear with a 16t cog, so I could see if I was able to start the bike easily. For example would gear number five be the same as gear number one on a 16t cog? Thanks

PS thanks Bruciey found a cheaper place to swap the cog, they charge £25

Brucey
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Re: hi, Nexus seven hub changing the rear sprocket from 21t to 16t

Postby Brucey » 15 Oct 2018, 5:20pm

gear 3 with a 21T sprocket is almost identical to gear 1 with a 16T sprocket.

cheers
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James9000
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Re: hi, Nexus seven hub changing the rear sprocket from 21t to 16t

Postby James9000 » 30 Oct 2018, 12:29pm

Brucey wrote:gear 3 with a 21T sprocket is almost identical to gear 1 with a 16T sprocket.

cheers

thank you for this Info, I see how it matches up with the gear chart. Just wanted to give a little, a sort of Figured out what the problem was, it was actually my riding position that was to forward which was putting strain on my hamstring, so every time I rotated the pedal it would irritate my hamstring. So rather than changing gears my new strategy is to change the handlebars so I'm even more up Right. When I hold the bars with my fingertips I can actually turn the pedals comfortably. Many thanks