SRAM A2 Automatix; introduction to the internals

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fastpedaller
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Re: SRAM A2 Automatix; introduction to the internals

Postby fastpedaller » 30 Mar 2020, 11:00am

Thanks, that makes more sense than the explanation on the SA site, which uses a linear explanation, hence my inclusion of Pi.

mig
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Joined: 19 Oct 2011, 9:39pm

Re: SRAM A2 Automatix; introduction to the internals

Postby mig » 30 Mar 2020, 4:11pm

i'd have a lash at one of these i think but not if the shift point is daft. that would drive me bonkers. happy to put the miles in on fixed (with ratios to which i'm used) but always looking for alternatives.
why would the manufacturer choose to build them for small wheeled bikes? far more 700c or MTB type wheels around no?

Brucey
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Joined: 4 Jan 2012, 6:25pm

Re: SRAM A2 Automatix; introduction to the internals

Postby Brucey » 30 Mar 2020, 4:57pm

mig wrote:i'd have a lash at one of these i think but not if the shift point is daft. that would drive me bonkers. happy to put the miles in on fixed (with ratios to which i'm used) but always looking for alternatives.
why would the manufacturer choose to build them for small wheeled bikes? far more 700c or MTB type wheels around no?


the big sales are probably in hubs for new bikes rather than aftermarket parts. Buyers of new bikes with the hub included may not appreciate the niceties/subtleties of the gearing, so in terms of making money they may have made the right choice; a 'gear down' hub won't give a viable setup on a small wheeler, so it is a worse compromise than using a 'gear-up' hub on a large wheeler. Both Sachs/SRAM and SA have made 'gear down' 2s hubs in the past, but chose not to make those versions in more recent times.

Arguably the shift point is always a bit daft; suppose that the shift point is at 14mph road speed (which IIRC it is likely to be) and you don't want to pedal faster than 100rpm, then you need to use a 47" bottom gear which leaves you with a ~65" top gear. Even then when the shift goes into top the cadence will drop from ~100rpm down to ~73rpm.

on the same hub if you set the gearing 59"-81" then you will never pedal faster than 79rpm in bottom gear before it will shift. Unless you are lucky and the gearing you prefer happens to give shift points that coincide with your preferred cadence too, then you can expect to have to make compromises or find some way of adjusting the shift point.

It is a likely consequence of widely spaced gears that there may be 'forbidden speeds' , such that you are between gears at times. This is more likely to be annoying when the going is hard than when it is easy. Still, it can be better to have the choice than not.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

mig
Posts: 2257
Joined: 19 Oct 2011, 9:39pm

Re: SRAM A2 Automatix; introduction to the internals

Postby mig » 30 Mar 2020, 5:59pm

Brucey wrote:
mig wrote:i'd have a lash at one of these i think but not if the shift point is daft. that would drive me bonkers. happy to put the miles in on fixed (with ratios to which i'm used) but always looking for alternatives.
why would the manufacturer choose to build them for small wheeled bikes? far more 700c or MTB type wheels around no?


the big sales are probably in hubs for new bikes rather than aftermarket parts. Buyers of new bikes with the hub included may not appreciate the niceties/subtleties of the gearing, so in terms of making money they may have made the right choice; a 'gear down' hub won't give a viable setup on a small wheeler, so it is a worse compromise than using a 'gear-up' hub on a large wheeler. Both Sachs/SRAM and SA have made 'gear down' 2s hubs in the past, but chose not to make those versions in more recent times.

Arguably the shift point is always a bit daft; suppose that the shift point is at 14mph road speed (which IIRC it is likely to be) and you don't want to pedal faster than 100rpm, then you need to use a 47" bottom gear which leaves you with a ~65" top gear. Even then when the shift goes into top the cadence will drop from ~100rpm down to ~73rpm.

on the same hub if you set the gearing 59"-81" then you will never pedal faster than 79rpm in bottom gear before it will shift. Unless you are lucky and the gearing you prefer happens to give shift points that coincide with your preferred cadence too, then you can expect to have to make compromises or find some way of adjusting the shift point.

It is a likely consequence of widely spaced gears that there may be 'forbidden speeds' , such that you are between gears at times. This is more likely to be annoying when the going is hard than when it is easy. Still, it can be better to have the choice than not.

cheers


yes i see now. i have the feeling that , using one of these hubs, i'd be making my way down the road cursing the thing for changing both ways at an unwanted time.

fastpedaller
Posts: 2575
Joined: 10 Jul 2014, 1:12pm
Location: Norfolk

Re: SRAM A2 Automatix; introduction to the internals

Postby fastpedaller » 30 Mar 2020, 10:16pm

[quote="mig"

yes i see now. i have the feeling that , using one of these hubs, i'd be making my way down the road cursing the thing for changing both ways at an unwanted time.[/quote]
I've come to the same conclusion. I rather like the idea of the Sturmey Archer S2 with the 'kickback' facility so that I can choose when I change gear, but a 38 % increase just wouldn't be suitable. I could suffer a 20% increase, so use a 60" and 72", but i guess that would present a problem with either a small axle diameter or a large hub diameter to achieve?

Brucey
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Joined: 4 Jan 2012, 6:25pm

Re: SRAM A2 Automatix; introduction to the internals

Postby Brucey » 30 Mar 2020, 11:13pm

Achieving small increments in gear ratio is difficult in simple planetary gear trains inside bicycle hubs; practically speaking If you are going to avoid having stepped planet pinions, the sun pinion size can't be made smaller than the axle, and this places a limit on the gear increment. You can make the ring gear larger, but only up to a point. So within the 'conventional' SA architecture a 60T ring gear is standard, and I think a ~15T sun pinion is the smallest that can be used; even this may not be advisable since the axle is weakened by the presence of the gear teeth, which add stress concentrations. So this way (i.e. with a simple planetary gear train and a smallish ring gear), the ratio increase is always likely to be 1:1.25 or more. Even this might be useful though; you could get 60", 75" gears this way.

If you go to a stepped sun pinion you can get about half as much increase, e.g. in the AM hub, but this does cost a small amount in efficiency terms (there is more net drag in the planet pinion bushings when a stepped planet pinion is used). If you want smaller gear increments (inside a smallish shell) then it has to be compound gear trains; in the simplest terms you can take the difference between two widely spaced gears and have that as a smaller increment between different ratios. This is (more or less) how the old close-ratio Sturmey hubs work; it is also a trick that is used in modern hubs, e.g. the middle three gears in a Nexus Seven hub. Clever stuff right enough, but it does nothing for efficiency.

FWIW when riding the SRAM automatix there is some (partially controllable) hysteresis between gears; the upshift from 1 to 2 will happen even under full load above the critical speed; if you go over a bump just below this speed it can also trip the shift. If you don't want the high gear then all you need to do is slow your pedalling rate momentarily and it will drop back into gear 1 (and stay there, at least until you go over another bump....) provided you are still below the critical speed. By contrast the shift from gear 2 to gear 1 can be delayed well below the critical speed, provided you pedal in nice circles and maintain torque at all times. However unless you have set the gearing so that the upshift cadence is incredibly high, then delaying the downshift merely allows you to use a gear that is probably too high, at a cadence that is too low. For example if the upshift cadence is set at 100rpm, delaying the downshift allows you to use a cadence below ~72rpm; you may not find this very useful.

In truth what you find with any limited selection of gears is that you can't maintain constant effort and apply that effort over just a narrow range of cadence; whether you have manual or automatic control over the shift point makes a difference for sure but with just two gears on offer you are always going to have compromises, they are just different ones, that's all; you end up riding around the characteristics of the gears either way.

It'll be interesting to see how the SA A2 hub compares with the SRAM Automatix.

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

Sid Aluminium
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Re: SRAM A2 Automatix; introduction to the internals

Postby Sid Aluminium » 31 Mar 2020, 4:29am

mig wrote:why would the manufacturer choose to build them for small-wheeled bikes? far more 700c or MTB type wheels around no?


Just guessing, but in Asia - where 61% of the world's population of humans lives, and where the preponderance of the world's bicycles are manufactured - there's a sizeable market for machines that stow compactly for short-distance urban transportation use. I've seen some 16" and 14" wheel folding machines with three-speed derailleur drivetrains. This results in relatively light, efficient and very low geared cycles. Just capturing a portion of this OEM market would result in product success for the Sturmey S2 and A2.

Screen Shot 2020-03-30 at 10.26.04 PM.png
Screen Shot 2020-03-30 at 10.26.04 PM.png (193.68 KiB) Viewed 175 times


Like I said, just guessing here.

fastpedaller
Posts: 2575
Joined: 10 Jul 2014, 1:12pm
Location: Norfolk

Re: SRAM A2 Automatix; introduction to the internals

Postby fastpedaller » 1 Apr 2020, 4:33pm

That looks a nice little hub - I wonder if it's available as a separate item and with 36H drilling for a larger wheel?