Re: Making a Unicycle
Greg and Alexandra wrote:
> I have been considering making a MUNI for about 6 months and have some
> questions and ideas to bounce off the experts since I personally lack the
> exposure and experiences many of the readers of this news group have.
> I have 3 major areas of concern; axles, bearing holders, and the frame.
interesting that you do not consider the seat a concern aswell, When
manufacturing my own uni’s the seat has often been difficult to manufacture
> Axles: After reading the news group posts and web pages, it seems the axles
> are a weak point for many munis today. I think strong axles could be made from
> high strength steels or titanium. Does anyone know what specific material
> suppliers use to make commonly available axles? Are most axles a standard
> size? (length and diameter) How does the axle interface with the hub? (if they
> are typically welded together, then dissimilar materials will propose a
> problem) I thought I would have a machinist make 5-10 axles from a high
> strength material and sell them for cost to spread the expense over a few
> parts. Any recommendations on a material or an alloy would be welcome. On the
> other hand, I understand the Semcycle axles are very strong. Maybe there is no
> need to go through the hassle of making axles…
The long and short of it is that the hardest steels available have been used and
have failed as unicycle axles. That is not to say that a hub machined from a
decent billotted steel with purticular attention payed to the machining around
the 5/8" square taper, i.e. no sharp edges and generious radi where ever
possible, then hardened…, will not last the life of the unicycle. Go for
it! What would be better, would be to look at the top end BMX hubs as they
experience the same if not higher forces than unicycles. They use up to 1"
diameter round bar (or bigger and hollow) with a broached or rolled spline. They
then use a different design for the crank, a broached tube with one or two pinch
bolt to the rear. In general terms this is the direction I think you should be
looking in. There is only a few problems I can see at the outset:
a: You need to manufacture the cranks as well as the BMX ones are
definately too long.
b: The standard design used on BMX’s have the pinch bolts at the opposite end to
the pedals. This would be bad for crank idling. I think the pinch bolts could
be on the same side as the pedals (but I have not layed it yet).
c: The bearing need to be bigger, you can get 42mm od bearing with a 25mm id,
they are not common, but the load area higher than those on the
40mm/17mm ones used on the better unicycles - so you cannot retro fit this type
of hub to a standard unicycle.
d: One advantage that is worth mentioning is that the cranks could be fitted at
different widths depending on the riders preference. I am seriously looking
at manufacturing some like this, I have the machinist, one of the best welder
in this region and even a company who can do the broaching. I just need the
time to get it sorted!
> Bearing holders: I found myself in an redundant design loop when I considered
> the frame and bearing holders and determined I need more information. I have
> exposure with two different types of bearing holders. My first unicycle was a
> Zefer and it has a holder made from an aluminum casting. The bearing is
> pressed into the casting. The casting is attached to the frame by sliding it
> inside the tube of the fork and is secured by two bolts, as shown below.
> | O |
> | | <== This part slips into the fork
> | O | and the holes are tapped for the bolts.
> | |
[i]> / [/i]
[i]> / ___ [/i]
> | / \ | ___/ | The casting is shaped some what like this
> \ / and has the bearing pressed inside. ______ /
> I think I read someones description of a Pashley Munis bearing holder and it
> sounded similar to the one shown above.
> My other unicycles are Semcycle XLs. Their holder is a two piece hat shaped
> clamp. One half is welded to the frame and the other half captures the bearing
> and is fastened to the other half with small bolts, as shown below.
> | | <== bottom of frame
> ||_ / \ T/ _T_ | | T \ / T Two clamps with
> bearing _______/ held inside.
> Of the two designs, I like this one the best. I would like to know how Miyata,
> the other Semcycle, Schwinn, etc. make their bearing holders.
I agree with your conclusions, I would add that there is a difference between
the clamp designs available. The pressed steel ones are of a conciderably lower
quality than the ones that are machined (look at DM’s Unihock for an example of
good design). If the bearings are not held correctly they tend to fail. Another
point worth noting is that if bolts are used to secure the bearing holder in,
then they should be from front to back not side to side as they can catch your
ankle when riding.
> Frame: I have considered buying a mountain bike tire for my Semcycle XL and
> there was not enough space between the forks. I read that this is an issue
> with other standard unis available today. So we should consider making a
> custom frame for a muni.
> I am sure many people have looked at mountain bike forks and thought it would
> be a good start for a unicycle frame. Mountain bike frames use a variety of
> high strength materials. The have relatively high production volumes which
> keep prices reasonable. They also are the prefect size for a mountain bikes
> tire. [thats obvious I suppose ] [this section is focused on frames,
> however I am leaning towards a 26 inch wheel for a muni with approximately 6
> inch cranks] When one considers a mountain bike fork, you must consider
> suspension. I have no clue if a suspension is a desirable feature on a muni,
> however it would be neat. Cannondale has a fork with a suspension in the head
> set instead of the blades of the fork. (see pictures of them at
> www.cannondale.com or more directly at www.headshok.com/html/products.html)
> Check out the DD60, MC60 and the
> OK, the down side to a bike fork: The part of the fork where the bike axle
> attaches would have to be removed and replace with a bearing holder. The
> spacing between the fork would have to match that of the axle and hub, and
> the cranks would have to have plenty of clearance. Then the seat post must
> attach to the fork and with the fancy Cannondale fork, this would be tricky
> (although not impossible)
If you are interested in suspention then don’t use the Cannondale one as it has
an adjuster on the top. Look at the one on the Action-Tec bikes, it is a lot
better and it is the one that Cannondale copied. It uses an oil damped, duel
spring suspention into a single shaft, with CrMo forks. It is manufactured in
Silverardo in South California by Russ who is a very skilled designer/machinist
and I am sure would talk to you if you would like to go down this avenue.
Warning though, you need to be tall to ride one, I cannot as I only have an
inseam of 30".
> Well, buying a bicycle fork and modifying it may be a waste of time and money.
> Especially if the suspension is a undesirable feature. So I began thinking
> about making a frame from scratch. I have had lots of ideas but my lack of
> exposure to bearing holders is a major stumbling block. Is a light weight
> frame an issue with these unicycles? (As a very proud owner of a Tom Miller
> giraffe, weight is NO concern!) I think one could make some nice muni frames
> in aluminum with tubes and extrusion. (I know an extrusion die costs in the
> neighborhood of $1000, but I am a dreamer)
Weight (and ridgidilty) is a big issue in my books as it effects the properties
of the riding, the lighter the uni the easier it is to use. I have a 7lb
mountain uni and it does perform rather nicely. I am on a mission at the moment
to manufacture a 6’ giraffe that weighs only 11 lbs, my problem at the moment is
getting a 150mm aluminium crank set with a single small chain ring (38 tooth
would be great). Anyone have any ideas? One thing I have learnt from the early
CF uni’s is that the crown width of Mountain bikes is too big and catches on
your knees (it doesn’t bother me luckly). I reduced the size on the Mk4’s by
10mm and I still get comments that they are too big. I am about to layout for an
investmant casting tool for the crowns for the Mk5 CF uni which will be smaller
again by 10mm and bonded instead of clamped. My dream at the moment is to
manufacture some CF uni’s with ovel tubing, but I don’t know anyone with an ovel
mandrel. oh well…
North East England