Re: Sturmy Archer 8 speed

Andrew Muzi writes:

>> This has happened to me, slipping in 3rd even with the indicator
>> chain removed!

>> I had things adjusted properly so the no-drive zone was avoided,
>> and then found the hub slipping in third. I took the chain out to
>> make sure it stayed in that gear, and sure enough it would spin as
>> it should, slip once, then re-engage. This would happen at random,
>> and always under more pressure. Not very confidence inspiring when
>> trying to clear an intersection in a hurry...

>> The hub stays in 2nd without issue - no slipping into the no-drive
>> zone, but then into 3rd and it's a guessing game. I'm told this is
>> due to wear on the inside of the hub's shell, so it's not just a
>> matter of replacing the guts, but the whole hub, which means a
>> wheel rebuild. Arghh...

>> It's almost enough to make me rebuild it with a old Shimano
>> 3-speed! (Too cheap to buy a new internal hub, and I like
>> recycling whenever possible.) But everything I've read says those
>> hubs can be problematic. Too bad, because they don't have the
>> no-drive zone, and the shifter is easier to use (this is my winter
>> bike, and of course I wear thick gloves). So I guess I'll install
>> another AW and hope for the best. Or try the Shimano. Hmm...

>> Ideally I would go single-speed or fixed for winter use, but being
>> car-free I need to tow a trailer every now and then, and need a few
>> extra gears.

> Although Jobst had a less than positive experience I've never had
> that happen in a literal lifetime of commuting on Sturmey hubs. Not
> once.

> I am still unclear about the syndrome.

> Let's rule out any worn bits such as the clutch and pins ( pedaling
> while shifting can chip the edges). And Jobst agrees the
> easily-kinked control wire is not at fault here.

> Jobst says a generally sloppy fit and axle deflection can allow the
> driver and clutch to move - where? Not much room inside!

It's not movement from sloppy fit but rather to driving elements
facing each other with a bow in the shaft rotate with their elements
going in and out of engagement. Since there is a net slope, going
into engagement hardly occurs under heavy torque. The two elements
separate as do sliding gears in gearboxes with a bent shaft.

You can join the many SAE engineers that were convinced it couldn't
occur but they overlooked the dynamics of it. When the "clutch" moves
away from the top gear position it engages the heels of the direct
drive pawls, leaving the low gear pawls to over-ride, so there is
clicking in second and high but not in low where the straight drive
pawls are lifted and the low gear pawls over-run. There must be a
space between high and second because the drive cannot be in two gears
at once.

> A Shimano is smaller and the parts fit more closely so I suppose
> that's an alternate. Jim Adney's opinion of Shimano hubs is that if
> they saw any care or lubrication they might have worked. Since
> 'better' bikes all had Sturmey in the day, Shimano hubs on XMart
> bikes were poorly adjusted and seldom lubricated. My own opinion is
> not as generous.

I haven't taken one apart but I am sure they addressed the problem by
means other than crowding.

> [I haven't talked with Jim about this in a good long while. He may
> have further thoughts]

The mode of failure is that under full torque the "clutch" disengages
from the ends of the planet pins. The five speed uses the same method
except that it drives slots in the planet housing instead of the ends
of the four pins. It does not jump out and I have not investigated
the pressure angle but I could be sloped to not disengage.

> You were told, "this is due to wear on the inside of the hub's
> shell". Since the high and direct gears are driven through the same
> pawl/ratchet set so that can't be true.

The hub make no contact with any working parts except the end caps
that are screwed into it and in some pressed in on a straight knurl.
That the housing wears it bogus. That the end caps wear is likewise
bogus.

> I can accept that there is something wrong in there (no more
> credible observer than Mr Brandt) but I still don't get it and this
> is the third iteration in as many years.

First, the flexing of the axle AND the loose fit in the planet housing
generate a skew between "clutch" and the pins that it drives. That is
a disengaging mechanism. When the clutch pops off the end of the
pins, it is free to rotate forward before any other drive mechanism
engages. At this time the standing rider pitches over the bars as his
downward foot is as food as off the pedal, freely falling.

Jobst Brandt
.

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