SA AT5 internals into FG/AG hub?

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steve browne
Posts: 111
Joined: 8 Oct 2009, 12:30pm

SA AT5 internals into FG/AG hub?

Postby steve browne » 5 Jul 2019, 11:32am

Has anyone done this modification or knows if it is possible? The reason I am interested is I have a couple of old rod brake roadsters which would be much more fun to use in Devon with lower gears. The rims have 40 holes which makes simply switching in an AT hub much less convenient.

Brucey
Posts: 41477
Joined: 4 Jan 2012, 6:25pm

Re: SA AT5 internals into FG/AG hub?

Postby Brucey » 5 Jul 2019, 11:46am

AG hub is completely different to FG hub. Only AG internals (or very similar 3s ones) will fit an AG shell. Only FG internals will fit an FG hubshell.

Anyway the whole premise is fundamentally flawed as should be evident from a glance at the gear ratios of these hubs; AT5 has the same internal gear ratios (1-4) as FG anyway; the 'extra gear' is an extra high gear. It isn't going to make the slightest difference to going up hills, only going down them.

AT5 axle assembly may work with an FG internal, but (as well as axle length differences) there are subtle differences in the clearance available where the cup that locks the secondary sun pinion is mounted. This would have to be checked. It is not difficult to convert an FG to 5s anyway; you simply fit a 5s primary sun pinion to an FG axle and then to arrange a LH pushrod to control the sun shift. AW toggle parts can be used on the RH side of the hub.

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

steve browne
Posts: 111
Joined: 8 Oct 2009, 12:30pm

Re: SA AT5 internals into FG/AG hub?

Postby steve browne » 5 Jul 2019, 12:39pm

Thanks Brucey. It's the "This would have to be checked" bit that I was hoping to get answered, hence wondering if anyone else has gone there first.

The lower gears come from using larger sprockets. Installing a wider range hub and keeping top gear roughly the same will give the lower bottom gear I am after. The limiting factor here will be the full chain case - I know 22t sprockets will fit, not sure about larger ones.

The three speed roadster is going to need the most work. I think I shall have to swap in an FG hub in place of the AG and take it from there...

Brucey
Posts: 41477
Joined: 4 Jan 2012, 6:25pm

Re: SA AT5 internals into FG/AG hub?

Postby Brucey » 5 Jul 2019, 4:39pm

you can get 23T and 24T sprockets but like you say it isn't clear that they will fit inside the standard steel chaincase.

Another option (which preserves the look of the thing) is to weld a smaller chainwheel to a standard crankset. I think you should be able to use a 40T chainring inside the chaincase.

The wider range of the FG/5s hubs only lowers the low gear ratio a relatively small amount; IIRC in an AW the low gear is 0.75 and in the FG it is 0.6667, i.e. it is about a 10% difference.

Other things to think of;

- building a different wheel and using a SRAM or shimano 3s gear; these have slightly wider ratios than the SA AW.
- as above but using a more complex gear eg 7s or 8s.
- using a Brompton BWR gear.

The other thing is that every bike comes with a 24" gear, i.e. two feet.....

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

steve browne
Posts: 111
Joined: 8 Oct 2009, 12:30pm

Re: SA AT5 internals into FG/AG hub?

Postby steve browne » 19 Jul 2019, 5:00pm

Things I found out this week!

A 1986 set of AT5 internals does fit into a 1950 Dyno Four hub. I kept the quadrant changer for the right hand toggle chain and added a handlebar trigger for the left hand toggle chain. Gives gears of 32.6, 38.6, 49.0, 62.0, 73.4

A 23 tooth cog just fits into the 1939 Raleigh chain case and a 24 tooth cog is too big.

Brucey
Posts: 41477
Joined: 4 Jan 2012, 6:25pm

Re: SA AT5 internals into FG/AG hub?

Postby Brucey » 19 Jul 2019, 5:03pm

steve browne wrote:Things I found out this week!

A 1986 set of AT5 internals does fit into a 1950 Dyno Four hub. .


not unless you retain the FG planet cage they don't.

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

steve browne
Posts: 111
Joined: 8 Oct 2009, 12:30pm

Re: SA AT5 internals into FG/AG hub?

Postby steve browne » 19 Jul 2019, 5:31pm

No! They went straight in. The only original bit from the FG hub that was retained was the shell (and the dynamo). Turned out much easier to do than I feared!

Brucey
Posts: 41477
Joined: 4 Jan 2012, 6:25pm

Re: SA AT5 internals into FG/AG hub?

Postby Brucey » 19 Jul 2019, 7:42pm

steve browne wrote:No! They went straight in. The only original bit from the FG hub that was retained was the shell (and the dynamo). Turned out much easier to do than I feared!


thats not been my experience. The FG uses plunger pawls for the low gear drive and the five speeds don't.

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

steve browne
Posts: 111
Joined: 8 Oct 2009, 12:30pm

Re: SA AT5 internals into FG/AG hub?

Postby steve browne » 20 Jul 2019, 8:03pm

Screen Shot 2019-07-20 at 20.01.19.png

Brucey
Posts: 41477
Joined: 4 Jan 2012, 6:25pm

Re: SA AT5 internals into FG/AG hub?

Postby Brucey » 20 Jul 2019, 8:07pm

ah OK, the AT5 uses plunger pawls too, that explains it!

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