Sturmey SC-RF3 ?

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Tinnishill
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Sturmey SC-RF3 ?

Postby Tinnishill » 26 Jun 2016, 5:49pm

The idea is to build a 24” wheel for a Tern Node with a Sturmey SC-RF3 hub using an 11/32 8 speed cassette and 30t chainwheel. Sheldon Brown’s gear calculator says that will produce a gear range between 16.9 and 87.3 inches. Can anybody spot any obvious pitfalls ?
Cheers.

Actually, I think it's called "CS-RF3"
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Brucey
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Re: Sturmey SC-RF3 ?

Postby Brucey » 26 Jun 2016, 11:00pm

I think it is a reasonable idea; it either contains all the best bits of both derailleur and IGH systems... or the worst of them.

BTW unless you need a super-low gear, might I suggest that you fit a larger chainring, eg a 46T one will give you gears from 26" to 133" , and 26" is low enough for many people. The high gears are pointless (more or less) but with this setup you can spend most time riding in the middle gear of the IGH, which is the most efficient. By contrast, with your proposed setup there is no gear over 61" in the middle IGH ratio (and that with a stupid tiny sprocket and a crummy chainline...), so you will likely spend most time riding in the high IGH ratio, which is probably less efficient overall.

cheers
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Geoff.D
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Re: Sturmey SC-RF3 ?

Postby Geoff.D » 27 Jun 2016, 11:10am

Brucey wrote:I think it is a reasonable idea; it either contains all the best bits of both derailleur and IGH systems... or the worst of them.

BTW unless you need a super-low gear, might I suggest that you fit a larger chainring, eg a 46T one will give you gears from 26" to 133" , and 26" is low enough for many people. The high gears are pointless (more or less) but with this setup you can spend most time riding in the middle gear of the IGH, which is the most efficient. By contrast, with your proposed setup there is no gear over 61" in the middle IGH ratio (and that with a stupid tiny sprocket and a crummy chainline...), so you will likely spend most time riding in the high IGH ratio, which is probably less efficient overall.

cheers


+1
The point about riding mostly in the middle gear of the IGH is quite right.

You don't say why you need such a low gear. If it's for getting up steep hills (20% or more), you might find the limiting factor is going so slow that you're down to stalling speed, or that you're lifting the front wheel at that steepness and effort.

However, there's another way of looking at this. I ride a trike as well as a bike. It's normal range is about 16" - 108". The 16" gear gets me ( a reasonably fit 67 yr. old) up 25% gradients when lightly laden. But, I wanted to ensure that I wouldn't fail on such hills (Yorkshire Dales, etc) when laden with camping gear. I wanted this for two reasons - it's a bugger trying to push a trike up a hill and I had (until surgery this last year) arthritis in hip and ankle. So I put a CS-RF3 hub on, IN ADDITION TO the front derailleur. This had been suggested by XAP Bob who is active on the recumbent forum.

The effect is that I use the direct drive of the IGH 98% of the time, but I have the option of dropping to the low range (to an 11 inch gear) should I need to. Of course I have no problem with a stalling speed, or lifting a front wheel. But, I never have to get off and push.

I notice that the Tern Node has a front derailleur. You could keep the existing gears, ride in direct drive most of the time, but have both a low range and an overdrive available if you need.

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[XAP]Bob
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Re: Sturmey SC-RF3 ?

Postby [XAP]Bob » 27 Jun 2016, 1:57pm

Yes - the hub is great way to extend range. It requires it's own shifter, so if you have the dedangler capacity anyway it's an interesting choice.

I see it has two main uses:
- Increase range of a single chainring bike - one without capacity for multiple chainrings for whatever reason
- Increase the range of a multiple chainring bike - to make extreme range possible

I don't see a huge advantage in using it as a front dedangler replacement. The one key advantage is that you can change the 'chainring' at a standstill.
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Tinnishill
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Re: Sturmey SC-RF3 ?

Postby Tinnishill » 18 Oct 2016, 11:54am

As the 24" wheeled folder style of bike isn't very common I thought that I should report back on this.

I bought the 24" wheel Tern Node for train assisted touring and it proved to be OK at this; it folds compactly enough to bag and carry on as luggage without pre booking, or to go on a rail replacement bus, while riding pretty much like a full size bike. The Airnimal is a similar format, though more "take apart" rather than folding. I bought a D8 Node version on the grounds that was all I could find at the time. It came with an 8 speed 12-32 cassette and biggish single chainwheel.

After a while I hankered after a wider range of gears. The first attempt was to fit a double chainwheel and front derailleur. The frame downtube has a diameter of 42mm, so it took while to find a big enough derailleur clamp. I then started to find subsequent problems. The stock rear derailleur didn't have enough capacity to cover a double chainwheel; I had to fit a new gear hanger to allow a long cage derailleur. The Shimano cyclocross spec' CX70 front derailleur (optimised for a 30/46 double) worked really well, but the very short chainstays of the Node combined with the fat down tube pushing the derailleur to the right of the bike resulted in uncomfortably sharp angles on the chain. I could fit a double chainwheel but not a triple and the smallest chainring I could find was a 32t. Changing the cassette to a 7 speed with a shim helped the chain angles a bit, but I found myself mostly just using the 32t of the 32/42 double I had fitted. Then I remembered about the Sturmey and SRAM hybrid gear hubs. Off came the front derailleur.

The Sturmey CS-RF3 was cheaper than the SRAM dual drive and easier to find. Obtaining decent 507 rims in 36 hole took a while; Airnimal sell some but at the time they were universally out of stock. I hung out for 36 hole as a result of a history of broken spokes at 32h or 28h; these are the struggles of a fat cycle camper. I eventually bought a pair from Zweiradnetz.de of Berlin; they don't do an English language website but it's fairly simple to use online translation and it switches back to English when you reach Paypal. A local wheelbuilder thrashed the rim and hub together for me and on it went.

What gear lever to use for the epicyclic part of the hub was a bit of a puzzler. The Sturmey website says "Compatible with standard MTB-ATB left hand shifter for front triple chainwheel". I presumed that "standard" meant indexed rather than the friction levers I usually use and had a rummage through some boxes. I came up with an old Shimano indexed front changer trigger lever. The control cable connected up like any SA three speed but lo and behold, low gear is in position 3 with high gear in position 1; it worked fine once I had got my head around it. After a while I obtained a modern grey plastic Sturmey 3 speed lever and fitted it back to front on the left hand handlebar; it worked absolutely OK. Although I preferred the feel of the Shimano lever I have left the Sturmey one in place because the numbers are in the right order.

The end result is a hybrid gear set up with the CS-RF3 and a 12-32 cassette on the back, driven from a 32 tooth chainwheel. Bearing in mind Brucey's point about direct drive being Sturmey gear 2, this gives a "normal use" range of 24 to 64 inches with an over drive in epicyclic high gear of 73 and 85 and an epicyclic low choice of 22 and 18 inches. Overall range for touring purposes, then, of 18 to 85 inches. In real life use I find myself using mostly the middle ratios with occasional recourse to the very high or very low.

I have now done about 250 km of mixed riding on the hub now and it's all been fine. These hybrid gear hubs seem particularly suited to the 24 inch wheel format. It makes me wonder why this isn't a standard option on smaller wheeled bikes; business inertia I suppose.
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horizon
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Re: Sturmey SC-RF3 ?

Postby horizon » 24 Jun 2018, 2:51pm

Geoff.D wrote:
You don't say why you need such a low gear. If it's for getting up steep hills (20% or more), you might find the limiting factor is going so slow that you're down to stalling speed, or that you're lifting the front wheel at that steepness and effort.



I'm just picking up on this old post as it's been linked from here:

viewtopic.php?f=5&t=122974&p=1246759#p1246759

The Tern Node 16 has a very generous range of gears (27" - 108"). That's easily enough for city use unladen. However the Node does come with rear and front pannier eyes so it is intended for some serious work. Those gears will take luggage or they will take hills. Cornish hills and my kind of luggage put together require something a bit lower. It's a shame they didn't offer it in two gearing versions - but then this isn't a Spa Cycles bike! :D
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