Hi,

I am part of a team of students attempting to design and make a human powered boat hoping for a bit of advice on derailleurs.

I am wondering if it is easy/possible to buy or create a hub with 3 different sized sprockets which uses a rear style derailleur (able to tension the chain) but will not freewheel?

Also, could anyone possibly explain to me why freewheel mechanisms never seem to be included on the hub which is being directly driven by the cranks?

If anyone is interested here is a little backround info on the project:

We’re planning on having two recumbent riders pedaling crankshafts which eventually power a propeller.

I’m currently trying to work out the best way to connect the two riders together so they can deliver maximum power to a drive shaft which is located between the riders.

One option would be to use a standard tandem but with the angle of the drive chain changed. However, we ideally would like independent freewheeling which this cannot achieve. I am considering connecting the two cranks with separate chains to a shaft between the two riders where freewheels could be attached.

Any help/advice on this would be hugely appreciated!

Many thanks,

Emily

Emily-

Fixed (no freewheel) with derailleurs doesn’t work because rear mechs can’t handle tension in the lower chain run. You need an internal hub gear or a fixed gear. Probably the latter.

The very cheapest of current newly manufactured derailleur systems come in five speed screw on blocks. But for the reasons above that is irrelevant.

If the freewheel mechanism was in the cranks then when freewheeling momentum energy would be lost turning the redundant drivetrain. Plus tension in the lower chain run is a no-no as above.

I really think you will not manage to fit gears to a fixed system, and derailleurs won’t work.

I’d guess a good crank rpm is going to be about 90 – a basis for selecting your gearing and propeller pitch.

it will work fine with freewheeling jackshaft sprockets but if the mech jams for some reason it may go very wrong.

surely in a boat, there will only be one most efficient speed to spin the prop. just gear it to that.

If you really wanted a fixed hub with 3 gears, you could get the Sturmey Archer S3X hub gear. Though it is rather expensive (over £200).

Also, could anyone possibly explain to me why freewheel mechanisms never seem to be included on the hub which is being directly driven by the cranks?

Do you mean “why is the freewheel on the rear hub, and not the crank?”
Some trials cranks do this – that’s the route I’d go down…

DrP

Drive side…

DrP

have a look at the davinci tandems independent peddling set up – might be something useful there

ttp://www.davincitandems.com/dv2.html

Ooh – clever!
So presumably they’re 16t freewheels, and each crank has a 32t single chainring… (hence the 2:1)

DrP

Huge numbers of gears and huge range as well complex tho

gonna wear out in no time too.

Gave someone something to do in the evenings for a while at least! 😛

have you never seen scrap heap challenge?

chain drive always fails

LOLs 😛

Hi,

Thank you all for your replies, I really appreciate your thoughts.

I have been considering a modified version of the Davinci system but it seems the luxury of independent freewheeling does not warrant all the added weight and complexity.

A hub gear may well suit us nicely but seem out of our price range unfortunately.

Emily

hpsb – Member
…A hub gear may well suit us nicely but seem out of our price range unfortunately.

If no-one closer comes up with anything for you, I have a 7 speed Sturmey-Archer hubgear you can have free. It’s brand-new but an old model. I don’t have a shifter for it.

The deal is you put pics of your boat up on here when it’s built. 🙂

Good luck.

Well if you used the cranks I linked to, you could have independent pedalling quite simply, no?
Just need 2 pairs of the cranks, and 2 freewheels, linked to a standard drive gear….

DrP

It might be good to look these guys up http://www.torpedalo.com look under drivetrain or design

surely in a boat, there will only be one most efficient speed to spin the prop. just gear it to that.

This. Before you go re-inventing the wheel as it were I’d have a good look at the engineering solutions used for current prop driven HPVs (or is checking up on previous designs not allowed for your project? 😯 ) For example http://www.recumbents.com/wisil/hpb/compact_drive.htm

Or are we all missing the point, and the derailleur is only for chain tension rather than to change gear? In which case given a custom built setup you’re far better off adjusting the distance between the two ends of the chain and getting rid of the tensioner.

just place two simple freewheels on the prop shaft.If you use a belt drive, machine the freewheel teeth off and and thread the pulley on that