Oooh! Great minds…

A bike actually using it:

http://kentsbike.blogspot.com/2008/11/building-retro-direct-drive-bicycle.html

and there's a load of vids:
http://www.google.co.uk/search?q=retro+direct+bike&hl=en&prmd=v&source=univ&tbs=vid:1&tbo=u&ei=cRB9TMr9MIqQjAf99O3TDg&sa=X&oi=video_result_group&ct=title&resnum=5&ved=0CDMQqwQwBA

By the time you've added up the weight of the centrifugal clutch and the two chainrings, chains, sprockets and tensioner, you might as well run 1×9

a) no and b) weight isn't the point.

It'd be like singlespeeding except not spinning out on downhills and roads. And it'd be automatic.

I think it's an ace idea.

Brompton now have a wide range 3-speed hub that could be rather neat. Doubt it's MTB compatible though…

Brompton now have a wide range 3-speed hub that could be rather neat. Doubt it's MTB compatible though…

nope only 120oln… it is just a SA hub with slightly diff ratios

though it combined with their 2 speed dérailleur could be neat but would require work..

still think the disc shimano inter nexus is the best bet

thepodge – Member
…I also imagine (though I could well be wrong) that the SA hub was cutting edge when it was released but has not been updated with the times.

Not much to update in a light hub that's good for 50,000 miles, but it has been done.

is there no way interal hubs could be made lighter ?

is there no way interal hubs could be made lighter ?

possibly, but essentially they are almost solid lumps of metal.

but are they *that* bad?

compare a rear hub + cassette + dérailleur weight

A 3 speed S-A weighs slightly over 1,000 grams. When you compare this to a derailleur hub and the weight of the cassette and derailleur, it's not a great difference.

Especially when you consider the S-A hub will still be going strong long after you have worn through several sets of derailleur hubs and cassettes.

"It'd be like singlespeeding…"
No, it'd be like twospeeding with a more complicated alternative to a two speed derailleur. 😉
I like the idea, but I don't see how it could be made to work.
All gear systems need the power to be reduced while the gear shift takes place.
Easy enough on a manually controlled derailleur or hub gear for the rider to back of a bit while they make the shift.
Not so easy on an automatic where the rider may not be expecting a shift.

If you shift from 7th to 8th slowly on a Rohloff while pedalling lightly, it will momentarily engage 14th.
This is intentional. The gears are effectively in two ranges, 1st to 7th and 8th to 14th. Going from top gear in the low range to bottom gear in the high range, it's safer to have the hub engage high range first so that the rider hits a sudden resistance to pedalling in 14th rather than engage the low gear first and suddenly hit almost no resistance at all in 1st.

A centrifugal gearbox would need to do something similar, especially as the rider may not be aware that it is about to shift gear. Catching the rider unawares with a neutral between gears, however brief, wouldn't be acceptable.
It would also need to be designed so that it shifts down at a slower speed than it shifts up.
Supposing you wanted it to change gear at 15km/h.
If the rider is riding at about 15km/h, it will be constantly shifting up and down.
Somehow, you would need to make it shift up at 16km/h and down at 14km/h.

StumpyBlurRider – Member
is there no way interal hubs could be made lighter ?

I reckon it would be pretty cool to weight weenie a S-A hub.

It's on my list of future projects bodges.

There's a number of hefty bits of steel in there. Things like the lhs end and the bit that carries the planetary gears. I am contemplating a bit of drillium, but if you mix too much drillium with steel you create a new alloy called disintegratium.

If I had any competency on a lathe I'd try turning up alloy replacements for some of the heavier bits. Titanium would be better for niche points though 🙂

If I had any competency on a lathe I'd try turning up alloy replacements for some of the heavier bits. Titanium would be better for niche points though

some of SA alloy shell hubs are heavier than the steel shell versions

The steel 3 speed hub is 100g heavier than the alloy one and it's about the same for the 5 speed.

All gear systems need the power to be reduced while the gear shift takes place.

Not really. This would be like the VW DSG gearbox, where you've (effectively) got two gears each with its own clutch, and as one clutch comes out the other goes in.

Good point about downshifting at a different speed to upshifting though. That could be arranged I am sure, but it'd make the innards a lot more complicated than simply spinning weights.