SRAM A2 Automatix; introduction to the internals

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fastpedaller
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Re: SRAM A2 Automatix; introduction to the internals

Post by fastpedaller »

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

Post by mig »

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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Re: SRAM A2 Automatix; introduction to the internals

Post by Brucey »

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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mig
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Joined: 19 Oct 2011, 9:39pm

Re: SRAM A2 Automatix; introduction to the internals

Post by mig »

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
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Re: SRAM A2 Automatix; introduction to the internals

Post by fastpedaller »

[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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Re: SRAM A2 Automatix; introduction to the internals

Post by Brucey »

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 planet 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
Last edited by Brucey on 9 Nov 2023, 3:41pm, edited 1 time in total.
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Sid Aluminium
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Re: SRAM A2 Automatix; introduction to the internals

Post by Sid Aluminium »

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 (179.71 KiB) Viewed 5538 times


Like I said, just guessing here.
fastpedaller
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Location: Norfolk

Re: SRAM A2 Automatix; introduction to the internals

Post by fastpedaller »

That looks a nice little hub - I wonder if it's available as a separate item and with 36H drilling for a larger wheel?
fugglesby
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Joined: 18 Nov 2021, 5:51am

Re: SRAM A2 Automatix; introduction to the internals

Post by fugglesby »

It's such an interesting hub- shame it's discontinued. The SA alternative seems to be a bit of a non entity also these days.
Some parts of this design would have a lot of benefits in a fast, urban fixed gear setting, but would require a redesign. If the springs were set correctly you could have a relatively light and nimble gear for acceleration then a chunkier gear for when you're spun out on the flats or to control yourself better downhill. Then as soon as braking force is applied to slow the hub you'd end up in your original lighter and spinnier gear to make skidding kinder on the knees.

Issues:
The 1.37x jump is pretty extreme. While it could be great for people looking to set the bike up with a granny gear for bailing out on hills it's a lot to lump on top of a more sensible cruising gear. The previously described minimum of 1.25x within standard planetary gear architecture would make a lot more sense. For greater durability the hub shell could potentially be upsized to allow for a bigger and brawnier sun gear/ axle. Chub hubs were popular for some time in the fixed gear scene so there's precedent at least.

The spring adjust tuning method is less than ideal with the amount of different gearings individuals run. A redesign could have a lighter weight with a threaded hole in it. A series of thin steel slices could be bolted down to each to allow for repeatable, indexed adjustments of shift point.

The biggest issue is getting it to run fixed in the first place though. My understanding is that this hub runs two separate sets of pawls, each performing their own freewheel function. Smooth shifting between low and high is achieved through the slower pawls freewheeling as the faster pawls engage which wouldn't be possible fixed- there would need to be a neutral or dead spot in between. From my understanding all fixed internal gear hubs use a lateral sliding mechanism which requires a fair bit of movement and force. In comparison this sram hub uses a small amount of rotational movement to change the angle of some pawls so they'll engage. Even if the inertia counterweight could be adapted to produce lateral movement the action could lack the power to engage the kind of mechanisms seen in fixed internal hubs like the S3X. Furthermore the action back down to a lower gear could be quite sluggish.

Something fun to think about though, and thanks to this old thread for providing a great resource on how to understand the internal mechanisms better.
Last edited by fugglesby on 19 Nov 2021, 6:07am, edited 1 time in total.
ratherbeintobago
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Re: SRAM A2 Automatix; introduction to the internals

Post by ratherbeintobago »

So is the SA A2 basically the same idea as the SRAM Automatix?
rogerzilla
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Joined: 9 Jun 2008, 8:06pm

Re: SRAM A2 Automatix; introduction to the internals

Post by rogerzilla »

Yes. Hubjub have them.

Along with other modern SA stuff, I would expect it to look good but not to work very well.
alexbrown981
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Joined: 21 Dec 2021, 7:51pm

Re: SRAM A2 Automatix; introduction to the internals

Post by alexbrown981 »

Hi all, I have been running one of these on my city bike for 5 or so years and have really liked it. I have it shifting at about 15.5 mph.

However the LHS (usually brake side) bearing case has just shattered leaving just the 3/16th balls and a load of casing. Does anyone know what size it is or if it is the same as the RHS one?
Thanks
Alex
Indi
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Joined: 8 Nov 2023, 3:51pm

Re: SRAM A2 Automatix; introduction to the internals

Post by Indi »

Have been running SRAM Automatix for a while and broke a bit- the very thin wire circlip.
SRAM are being very helpful ;)and say you can’t buy just the circlip, here’s the part number for the entire internals, also that part’s not made anymore so good luck getting hold of it :)

Anyway has anyone got a solution- made the clip themselves or found a source from another industry etc. ?
Any suggestions appreciated.
How it happened
I was riding up hill and heard a noise that sounded expensive, so I slowed thinking to stop before I stopped (maybe a pedal stroke or 2) the entire wheel seized (and came out of the drop outs one side). It took a lot of force to extract the wheel as it jammed solid in one dropout and out of the other even with all the bits loosened (hammer). Very happy it didn’t happen going fast as it STOPPED would have been a big mess going down hill. Only issue seems to be the circlip snapped and I assume got caught in something.
Brucey
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Re: SRAM A2 Automatix; introduction to the internals

Post by Brucey »

Indi wrote: 9 Nov 2023, 12:49am Have been running SRAM Automatix for a while and broke a bit- the very thin wire circlip.
I'm not sure what bit you mean, did take any pics? IIRC there was nothing inside the hub that was made of wire that I could not remake fairly easily if needs be.
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Brucey
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Re: SRAM A2 Automatix; introduction to the internals

Post by Brucey »

alexbrown981 wrote: 21 Dec 2021, 7:55pm Hi all, I have been running one of these on my city bike for 5 or so years and have really liked it........
However the LHS (usually brake..........
if the clip that holds all the balls has broken you are in schtuck.
IIRC German firm 'das zweirad' were offering a shimano part that is just 0.5mm different in size; this may be a suitable alternative [not tried it
myself]..It may well be that you need to use a metal clip on the LHS if you have a coaster brake; the heat may kill it otherwise. I think [again not certain here] the LH cone is the same as was fitted to the final version of the 'torpedo' 3s gear.
I thought a similar SA part could be made to work,[in place of of the NLA SRAM part] despite the fact that it is obviously the wrong size. The SA part is made of acetal plastic; I suspect it can be modified to work inside the RHS of a SRAM hub. My mods would include changing the shape of the bit between the balls and then cutting it to make a 'c' shape [rather than an 'o'] and thus altering the length/dia. to suit.
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