Brompton BWR hub and related 5 speed efficiency question

Alan H Bradley · Post by **Alan H Bradley** » 24 Oct 2008, 1:24pm

I noticed Bromptons new 2 speed BWR hub gear/2speed derailleur combo to give an efficient 302% range: ery impressive.

I have the SA5 speed which is noticably inefficient in 1st (and thus top, although less noticably): in this mode its wide range epicyclic is in use, which has a slightly smaller step/up/down ratio (1.5 up) than the BWR:

so where is the inefficiency coming from: is it the compound planet pinions (and thus smaller outer ring)?

Could the five speed be made as efficient in outer ratios as inner 3, simply by using two completely separate sets of planets, which could then share the same outer ring gear?

Would a 6 speed hub made of a 3 speed epicyclic followed by 2 speed epicyclic of direct drive or step up/down (eg like brompton hybrid combo) be more or less efficient than the above suggestion or standard 5 speeds?

MLJ · Post by **MLJ** » 24 Oct 2008, 4:39pm

There is a good explanation in the latest A to B magazine which looks in depth at the BWR 6-speed. As a user of the Sram 6-speed system on the Brompton, after first having a 5-speed, I can say that the gearing is far superior. However, you cannot upgrade the 5-speed Brompton as it does not have the necessary braze-ons to the rear triangle. It is less costly to sell and buy new!

Alan H Bradley · Post by **Alan H Bradley** » 24 Oct 2008, 5:25pm

I read the AtoB. I don't think it explained the large difference. I was hoping CJ would explain.

rogerzilla · Post by **rogerzilla** » 24 Oct 2008, 8:02pm

It is apparently because the tooth profile doesn't match properly in certain gear combinations.

abradleyInf · Post by **abradleyInf** » 27 Oct 2008, 5:19pm

Any further expanations/answers?

rogerzilla · Post by **rogerzilla** » 27 Oct 2008, 10:05pm

From someone who knows more than me about these things:

SA 5sp hubs have 2 epicyclic drivetrains but they only use one at a time, never both together. One is a wide range 3sp, the other is an extra-wide range 3sp, losing a gear because both have direct drive. Reason for extra drag in 1st and 5th is because SA saved money by not changing the tooth profiles to match the changed gear wheel diameters.

Sadly, "saving money" was a bit of an SA hallmark, which is why we got the AW with its neutral position; it was cheaper than fitting a third set of pawls as in the original 3-speed hubs (and now revived by SunRace for the current AW and S-RF3). Taiwanese ownership has done SA a lot of good, as there's actually an R&D budget and some proper product testing going on.

CJ · Post by CJ » 28 Oct 2008, 9:57am

rogerzilla wrote:From someone who knows more than me about these things:

SA 5sp hubs have 2 epicyclic drivetrains but they only use one at a time, never both together. One is a wide range 3sp, the other is an extra-wide range 3sp, losing a gear because both have direct drive. Reason for extra drag in 1st and 5th is because SA saved money by not changing the tooth profiles to match the changed gear wheel diameters.

Well: if that's the full quote I'm not so sure.

Certainly SA had to resort to ingenious cost-saving exercises in order to keep manufacturing such an item as long as it did (in a country whose banks would rather stoke the property market than invest in something worthwhile!). But it does not cost any more to cut (or mould) a geometrically correct involute tooth profile than one that isn't, so I think somebody has misinterpreted something. These cost savings may nethertheless have resulted in all gears having a less perfect tooth profile, which defect is likely to become more apparent, the more work those teeth are called upon to do.

It is a fundamental characteristic of gearboxes, that all other things equal, the greater the difference in speed between input and output shafts, the greater the frictional losses become. That, rather than anything else, is the reason gears 1 and 5 of the SA 5-speed were less efficient than gears 2 and 4. For the same reason: gears 1 and 7 of my Sram 7-speed are less efficient than 2 and 6, which are less efficient than 3 and 5. These are all the same basic type of gearbox: a compound epicyclic, with two or three suns and stepped planet gears of which the smallest drives the internally toothed ring gear. Depending upon which sun is locked to the axle, you get a bigger or smaller speed ratio between planet cage and ring gear.

In these simple epicyclic gearboxes the planet gears run on plain bearings, and as the torque transmitted via the planet cage increases with increasing gear ratio, planet bearing friction accounts for much of the variation in efficiency referred to above.

Years ago, back in the glory days of British manufacturing, SA brought out a de-luxe hub gear with ball-bearing planets. It was more efficient, but it was just a medium ratio 3-speed and not enough people felt enough benefit to pay the higher price than other models with plain-bearing planets, so it didn't remain on the market for long. A shame, because if ball-bearing planets had become the norm, there would be much less variation in efficiency across the wider gearing ranges provided by later 5 and more-speed hubs.

The extreme ratios of the Rohloff hub would be totally impracticable with plain bearing planets, so it necessarily has either ball or needle bearings throughout - and you know what that costs!

I have to admit that I don't know what sorts of bearings the BWR has, or any of the more recent models of internal gear. Unfortunately those techy cut-away views, that I used to find so fascinating, do not have a place in modern marketing methods.

rogerzilla · Post by **rogerzilla** » 28 Oct 2008, 1:19pm

The SA 5-speed is said to be unusually bad. I've not ridden one myself, but supposedly the pedal force it requires is just as high in 1st as in 2nd, although you're travelling slower. A modern Shimano 8-speed doesn't have drag I can feel in any gear, although obviously there's some there.

The SA 5-speed is a hack of the 4-speed. However, the 4-speed uses its second planetary geartrain only to provide a lower bottom gear, and maybe it's optimised for that.

The AW and the AM 3-speeds are the best of the classic SA hubs - they're both simple, reliable and reasonably efficient. I use an AW to get to work most days, and they can be picked up for 99p from eBay - spend another £7-8 to replace the parts that wear (clutch/planet pins/pawl springs/bearings) and the hub is as good as new. It's probably a good deterrent to bike thieves as well

EDIT: I believe the Shimano 8-speed has roller bearings for the planets. The current SA 3-speeds still use plain planet pins.

abradleyInf · Post by **abradleyInf** » 25 Nov 2008, 1:38pm

The SA supposed to be the worst of five speeds: in 1st you pedal faster for same effort as in 2nd: therefore I hoped they would /could fix it and wondered what the cause was.

Sunrace SA are apparently redesigning it to give 250% range, so perhaps it will get more efficient.

pioneer · Post by **pioneer** » 26 Nov 2008, 9:29am

Oh b----r!

I'm in the middle of getting a new S/A 5 speed (XRF-5). I hope the quality is better than their previous 5 speed hubs.

In reality, I don't think there's much difference now between the S/A 5' and Sram 5'. Though the S/A is a lot cheaper.

nurgles · Post by **nurgles** » 27 Nov 2008, 4:49pm

simple question on this subject

Will I be able to fit the new hub gear to my SL2x without any problem except rebuilding the rear wheel ?

I know it may be considered silly due to the weight issue but I really need a few low gears

MLJ · Post by **MLJ** » 29 Nov 2008, 3:49pm

The normal Brompton rear triangle has an attachment and pully for the gear cable: does the SL2X have the necessary braze-ons? It is not quite clear from the images.

Alan H Bradley · Post by **Alan H Bradley** » 30 Nov 2008, 3:43pm

CJ could always ask Brompton for some diagrams, as they designed it.

MLJ · Post by **MLJ** » 1 Dec 2008, 9:51am

Look at the rear triangle on your bike - there should be a braze-on part way up the RH rod towards the brake for attachment of the pully for the cable.