Re: Eccentric sprockets on giraffes

Just to let you know,

Yesterday I removed the sprocket from the hub to check it more
attentively and found, like I expected, its radius to vary with no less
than 2mm!!! Additionally it seems to me its teeth do form a round
circle and the deformation comes right from the poorly centered inner
holes.
And yeah, I am almost convinced now that the sprocket is adapted from a
BMX chainwheel -> 6 bolt holes added and the one in the center
increased.

As a note: the sprocket flanges (there is one on each side) of the
Nimbus hub is identically spec'ed to a standard bicycle disc brake
flange! This is nice as you can even fit a disc rotor on this unicycle
hub... 🙂

Another detail: As I told before, I upgraded the BB (Bottom Bracket) to
a Shimano UN26 (68/117) as it was a very close match to the original 3
piece BB. I haven't had time to test it yet but will do in the weekend.
Ahh, I think I will reinstall it with a little Blue Loctite 243 (Medium
Strength) before...
Here you can see it:
http://tinyurl.com/2fl2s6

This being said, it's still sad I didn't figure out how to eliminate
(reduce) the chain slack variation due that poor cog :o

Flaviu

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Sheldon Brown describes a technique for centering the chain ring on a
singlespeed or fixed gear bike. Singlespeed bikes have the same
problem with chain rings and sprockets not always being exactly round
or bolt holes not exactly in the right place.

The technique is described on his 'Fixed Gear page'
(http://sheldonbrown.com/fixed.html). Scroll down to Drive Train >>
Chain Tension. Use that technique to center the giraffe sprocket on
the hub as best as it is able to be. A big factor is the sequence that
you tighten the bolts. It may take several tries before you end up
with acceptable results.

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john_childs said:

Sheldon Brown describes a technique for centering the chain ring on a
singlespeed or fixed gear bike. Singlespeed bikes have the same
problem with chain rings and sprockets not always being exactly round
or bolt holes not exactly in the right place.

The technique is described on his 'Fixed Gear page'
(http://sheldonbrown.com/fixed.html). Scroll down to Drive Train >>
Chain Tension. Use that technique to center the giraffe sprocket on
the hub as best as it is able to be. A big factor is the sequence that
you tighten the bolts. It may take several tries before you end up
with acceptable results.

Thank you very much John!
I use to red on Sheldon Brown's site quite a lot; it's a very good
source of information! However in my case the sprocket at the wheel is
clearly the problem while the chainring is much more acceptable.

My belief is that both cogs (from wheel and from crank) have bicycle
origins - the chainring (and crank arms) is from children cheap bikes
and the sprocket is from low-level BMX bikes. Both of them are
manufactured in much bigger batches than Nimbus giraffes and happen to
be (in my case at least) of satisfactory quality (round enough to say
so) BUT the post procession applied to the sprocket for mounting it to
the hub is done in a rather amateurish manner.
-------------------------
Some comments about the hub:
The new Nimbus hub is very nice. I like the aluminium shell they made
(were they or someone else?) although the flange separation is quit
small (~45mm). Additionally I would upgrade its axle and the bearings
maybe...

SO, it is king of a flip-flop hub with 110mm axle dropout (like bicycle
track hubs). It has one flange for sprockets on each side (designed
symmetrically). By the way, I checked and found the flanges perfectly
fit a bicycle disc rotor 🙂
Each flange has a circular lip about 32mm in diameter designed to hold
the sprocket as an additional safety feature against radial movement!
-------------------------
Now, coming back to the sprocket:
They misplaced the 32mm hole (and obviously all 6 bolt holes too) with
about 1-2 mm from the center. So, I just can't apply the instructions
from SB's site as the tolerances are too tight to compensate the
error.
The only way to repair the thing and still keep the same sprocket would
be to enlarge the center hole (to let's say 33-34mm...or only where
necessary) and do other 6 bolt holes, intercalated between the original
ones). I believe I can do them better but then all the sprocket strain
(torsional and also radial) will go to the bolts as the center hole
will be bigger than the hub lip...I not sure if this is dangerous or
not!??

I still wait your comments and will keep you informed if any
interested...

Thanks,
Flaviu

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You might be better off starting from scratch with a new and high
quality BMX chainwheel and getting the necessary holes machined by a
competent machinist. If you're concerned about the size of the center
hole being too big you could get an adapter machined similar to a 'BMX
chainwheel adapter' (http://www.danscomp.com/457004.php?cat=PARTS) to
get the hole down to the correct size. But I don't know if the center
hole fitting snugly on the hub is even important. I'd think the six
bolts would be good enough.

A local Seattle area rider upgraded a Torker TX giraffe with high
quality BMX and bicycle components. Used a bicycle disc brake hub and
had the six holes machined in a BMX chainwheel to use as the hub
sprocket. This was before the UDC hub was available. He also replaced
the bottom bracket with a higher quality one. Replaced the cranks with
higher quality ones. And used a quality BMX or singlespeed chainwheel.
All the upgrades made a big difference. The unicycle rode very very
smoothly. There were no tight spots in the chain rotation. Everything
was nice and round. Sounds like the UDC giraffe could use some similar
upgrades.

BMX and singlespeed specific bicycle parts are generally more round
than regular bike parts. Even better are the track bike fixed gear
parts.

A bit of a pain though having to get things custom machined. And a bit
expensive too since machinists don't work cheap. There might be a
small market for someone to machine up a small batch of quality hub
sprockets for the UDC giraffes and sell them aftermarket.

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Well, you're about as far from Bucharest as you can be and still be in
Romania. Otherwise, I would have you call my friend, Dr. Cristina
Bourdeanu at Horia Hulubei National Institute of Physics and Nuclear
Engineering and tell her to just get it done for you. She owes me big
time.

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Thank you John and Harper!

First, sorry for late feedback - in weekends I try to ride as more and
stay on the computer as less as possible :p

harper said:

Well, you're about as far from Bucharest as you can be and still be in
Romania. Otherwise, I would have you call my friend, Dr. Cristina
Bourdeanu at Horia Hulubei National Institute of Physics and Nuclear
Engineering and tell her to just get it done for you. She owes me big
time.

Indeed I'm pretty far from Bucharest (~550km) but still I used to go
there 5+ times a year to see some of my favorite metal bands
(Metallica, Megadeth, Kreator, King Diamond, Obituary, Therion,
Nightwish...). This year I was more lazy though 😄
..........................................
OK, now back to the sprocket:
Last time I said I may try to do other holes in the same cog but I
worried this would be a difficult task to do with hand tools only (as I
don't have any special machines) and, even more, enlarging of the
middle hole will compromise it's purpose of holding the sprocket
radially...
However, as the chain tension was so odd, this Saturday I decided I
don't have much to loose so I give my idea a try...

So, here are the steps I did:
- bond the sprocket with super-glue to a wood board (flat and bigger
than the cog...)
- determine the real center of the cog in relation with its teeth
(using a digital caliper)
- on the wood board, in the center point, fit a metal pivot (the pivot
was from the accessories of the math-compass I was using...)
- measure the diameter of the imaginary circle containing the centers
of the former bolt-holes (again with the caliper)
- draw a circle with the above diameter, in the real center this time
(using a good math-compass and a thin-tip marker)
- on this circle, determine the centers of the new 6 bolt-holes,
even-spaced and somehow intercalated between the original bolt-holes
(using a center-punch)
- drill the new holes (I used 5.5mm drill bit and a hand-held drilling
machine - don't have a vertical one)

At this point I had 12 holes disposed in a cicle, close to each other
but still far enough to withstand the forces...

Now with a round hand file I adjusted the big hole in the middle to be
able to fit the sprocket on the hub using the new holes I made.
Note: as the cog's big hole main purpose was already compromised, I
filed it until I evened-out the space towards the hub lip (now ~1.5mm
where it was a snug fit before...) - so I centered this big hole too,
not necessary but at least aesthetic 🙂

Finally, I installed everything back (with the 6 bolts still not tight)
and only after following the good hints from SB's site I tightened the
bolts hard (as they do the whole job now) and went out for a test
ride...

Right from the first pedal strokes I noticed great improvement 🙂 much
smoother, less chain lag...better feeling
I rode the thing some km's on various roads (inclined, bumpy, flat...)
to see how it's gonna hold and found it has no problem. I still verify
the 6 bolts pretty often but they don't seem to loosen and the cog is
steady in the center. I can say now the achievement paid the effort 😄

I really hope it will last some time too...
.................................................................
I would like one additional thing to upgrade maybe on my giraffe - to
convert somehow the hub to sealed bearings and 12 mm axle
ends...similar to BMX hubs...
Note: I think bearings with 30mm OD might be fitted inside the hub
after removal of the original steel cups. Not sure though???

And yeah, for this maybe I'll need a machinist to make the custom
axle...

Any suggestions?

Thanks everybody,
Flaviu

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This is an interesting problem and one that we have not seen before.
The plates are produced with a 0.25 tollerance on the holes from what I
remember so the maximium that it should be off is this and it should be
easy to centre. Is it something you could send me a picture of to show
the problem or is it too small to show?

Roger

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rogeratunicycledotcom said:

This is an interesting problem and one that we have not seen before.
The plates are produced with a 0.25 tollerance on the holes from what I
remember so the maximium that it should be off is this and it should be
easy to centre. Is it something you could send me a picture of to show
the problem or is it too small to show?

Roger

Roger,

Thank you for posting on this thread!
The big hole in the middle (with all 6 satellite holes) is about 1mm
off-center. So the difference between the lowest tooth (closest to the
real center) and the highest one is about 2 mm. However the sprocket
itself is round within a tolerance smaller than 0.2 mm. I may try to
get a good picture of the cog although I'm not sure if it will reveal
the problem to you. Additionally, as I wrote above, I did other 6 holes
(now there are 12...) and enlarged the one in the middle thus the
picture may be a little confusing.

Finally, Roland Wende (Municycle.com) sent me a free replacement for
this sprocket which I measured to be, like you said, within 0.2 mm.
This means 10 times better than before - namely very good!
By the way, Roland is a GREAT guy!

All the best,
Flaviu

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Glad that is sorted. I had a feeling it was a faulty product. I will
make sure that this info is fed back to the manufacturer so that this
does not happen again.

Regard

Roger

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