6 speed 14-47 tooth sprockets

[PAB855]

Currently I have a 6 speed 14-28 tooth block on an electric trike. The trike can only pull away using the motor from rest if the pedals are turned, even by a very small amount. However, there is a problem. If the trike is negotiating a barrier of the zig zag type on a cycle path on an uphill section and comes to a stop, MBH hasn't got enough strength in her legs in first gear to move and requires a shove from me to get going.
I have looked at the possibility of using a Shimano megarange which has a 34t bottom gear. That is better and drops the gearing from 37in to 30in ( The wheels are 26in and it's a 40t chain wheel). However, from a previously owned trike, that's still too high a gear for her.
The trike frame has bags of room for the largest sprocket and the nearest piece of frame tubing is approx 4in from the drive shaft centre.
By my calculations, that should provide enough room for something like a 47t sprocket if it's OD was about 7.5in.
Is it feasible to have a 6 speed block 14-47 ? Do they exist?

What practical difficulties have I overlooked?

Hope you can advise me Thanks.

PAB855

[mattsccm]

Doubt it. Freewheels are an older design from the days when we didn't use big sprockets. Even modern MTB cassettes only go to 42 and thats an 11 speed cassette.

[pickerd]

It might be easier to reduce the chainring size?

I'm a trendy consumer. Just look at my D5803 using hovercraft full of eels.

[gaz]

Is it feasible to have a 6 speed block 14-47 ? Do they exist?

What practical difficulties have I overlooked?

As others have said freewheels are somewhat dated technology, even BITD I do not recall seeing sprockets larger than 34T.

Having ridden a 14-34T five speed I'd also add that IMO the jumps between sprockets are too large for comfort.

The other practical difficulty may be the rear derailleur. There is no guarantee that a mech that can handle a 28T sprocket will handle anything larger.

[cycleruk]

It might be easier to reduce the chainring size?

It's a composting toilet -> my D5803 using hovercraft full of eels.

Or possibly fit a double chainwheel?
40/30T or lower.

[PAB855]

Your information on MTBs is useful to know, thanks.

As for changing the chainring, that would result in far too low a top speed and even although it's an electric assist trike, pedalling at a cadence to match the road speed still is part of the enjoyment and the top gear of about 74in is fine. A small chain wheel to give a very low bottom gear would also give a top gear nowhere near high enough.

And the addition of a second chainring if anyone is thinking of that is not an option. That has too many practical difficulties in suitable components and there are already enough' buttons and levers without adding more..

Back to the drawing board!

[PAB855]

Ok, I could be persuaded. What's the smallest size chainring that could be added to the existing 40t? I will worry about the operating mechanism later!

[pickerd]

Sjs do a 13-32 freewheel. The 13t would enable you to reduce your chainring size to maybe 36t nearly maintaining your high gear, but reducing your low gear quite a bit. A 36t ring will depend on your bcd though.

I'm a trendy consumer. Just look at my D5803 using hovercraft full of eels.

[Brucey]

you don't say how the electric motor drives the trike; could that be changed to provide more torque?

On any other machine I'd say 'get a cassette hub' and then you could use a standard wide range cassette to get the gearing you want, in combination with a smaller chainwheel. However without knowing how the trike is built it is difficult to make any suggestion.

Re the cassette option; 14-47 is only a ratio change of x3.35 whereas 11-40 is a range of x3.6 and you can buy a mech to run a 40T sprocket. A 32T chainring will give the same top gear.

Also I note that your required gear range is just a guess; it might need to be more, it might be OK with less.

cheers

[rualexander]

.....Even modern MTB cassettes only go to 42 and thats an 11 speed cassette.

No, 46t is common and there are now 11-50 tooth cassettes for 11 speed MTB.

[PAB855]

More food for thought, thanks everyone.

The frustrating thing is that MBH has a mobility scooter which would negotiate such cycle path OBSTACLES with ease. We have also had the chance to try four other makes of electric trikes and not one pulls away from rest on small inclines the way the scooter does.

I will go and have another think.

[531colin]

From a position of complete ignorance, it sounds to me like the trike is designed to provide electric ASSIST.....ie the motor only cuts in when the thing is moving under pedal power, but the scooter is entirely electrically DRIVEN, ie from standstill.
A cycling friend who is ill has 2 "electric bikes". One is a "conversion" which he can ride like a motor bike, ie battery power from standstill, whether he is pedalling or not. The other is a newer machine bought as an "electric bike" which provides electric "assist" only when he is pedalling.....I think this may be the current traffic regulations.....electric assist is a bicycle, electric all the way might be a motorcycle.
Its obviously absurd that the same person can use on the public road a scooter which is in effect a motor bike, but the electric bike has to be "assist only", and I would be looking for an electronic solution......if necessary, on the quiet.

[borisface]

Will using a 47 tooth sprocket mean that the chain scrapes on the ground?

[thirdcrank]

Will using a 47 tooth sprocket mean that the chain scrapes on the ground?

It shouldn't. For a broad comparison, think how much clearance there is under a front chainwheel of that size; a typical BB is a tad lower than a rear axle. You might have to be a bit innovative about how to wind up all the spare chain. Talking of being innovative, it ought to be possible to lash up a 47T rear by bolting a suitable 47T front onto the largest sprocket of a standard freewheel.

[PAB855]

You are absolutely correct Colin. The current trike is a retrofitted motor of 2014 vintage so the power can be fed into motor from a wee lever just like the mobility scooter, but there's not enough power for anything other than a level surface. The whole point of the electrics is to solve this standing start problem as well as to assist while pedalling. The control on the new trike (2017) has four levels of power so it is stepped rather than continuously variable to satisfy the new regulations. It also has an override button for use without pedalling which gives a road speed of 6kph once it is pressed. That's great, but will only start on a level surface!

All this discussion is very useful.

Thanks