I reported a problem with excessive and irregular chain slackness due to eccentric sprockets on my Semcycle 6’ giraffe. I isolated the problem to the crank sprocket which has 2 more teeth than the wheel sprocket. The problem coincided with each revolution of the cranks, not the wheel.
Sem Abrahams was kind enough to send me a replacement crank sprocket with the disclaimer that all of the sprockets for giraffes are slightly eccentric because they are welded with marginal precision. He was right. It was interesting that the new crank behaved EXACTLY the same way as the old one did, though.
I tried to indicate the bottom bracket axle the best I could by clamping a wire to the frame, bending it over to the axle, and rotating the axle and eyeballing the wire tip. The axle, square taper, and 8mm threaded hole all seemed to run true.
I decided to reinstall the cranks offset 90 degrees from the original installation. Although this did not solve the problem, it reduced it considerably. The chain tension remains much more uniform through a complete crank revolution now.
Sorry for bringing this old thread to life again but couldn’t find another better one for this topic.
Recently I bought the 2007 CrMo Nimbus 5’ giraffe (single chain version). I really like it but there is the problem Harper already addressed: the chain is at some pedal position very tight and in other pretty slack. Obviously this thing is noticeable while riding and especially while idling
After an inspection I found the sprocket (on the wheel) is the root of all evil. While turning the wheel it can be seen that it is slightly oval.
They solved the problem with sprocket slipping but there is still this very unpleasant aspect.
First I think of trying to rotate the wheel, sprocket and chain-wheel in relation to each other so that hopefully I will get a better position…
…but if this will not work I have to change the sprocket! Now where to get a bolt mountable one?
The sprocket is made out of solid steel but has one oval hole (8-10mm) in the position where a BMX chainwheel would be bolted on the crank arm (on 3 piece set). This leads me to thinking of it being adapted right from a BMX
chainwheel - 6 bolt holes for mounting and a bigger center hole to fit the hub would be then all post-processing task. So, if this is the case, I tend to believe they rather did a poor drilling job (not perfectly centered) than the sprocket not being made round in the first place. What do you people think here?
Eventually I, myself, may try to adapt one BMX chainwheel…still wonder why is it so difficult for a factory to do things properly!?!
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Additionally, I will upgrade my giraffe to a cartridge BB (with sealed bearings) as the original 3 piece BB (loose balls and cups) is not satisfying me much!
For those interested, the BB shall of the Nimbus giraffe frame (maybe other brands also) complies with ISO standard (68mm, 1.37 in x 24 TPI). The original axle length is 116mm, also common in bike world!
I wait for your comments, especially on the sprocket issue.
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…
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. 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…
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 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.
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.
First, sorry for late feedback - in weekends I try to ride as more and stay on the computer as less as possible
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
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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…
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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…
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?
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!
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.
what i did when i built my last 26inch giraffe unicycle is used a mtb front wheel with disc hub standard 6 bolt pattern and i took the disc rotor off and held it onto a sprocket and drilled the holes and bolted it back to the hub as if it were the disc rotor.
