the twin tube design used by rick and myself is stiffer under front-back and torque loading than a single tube because of the very deep “effective diameter” of the tubes in that direction.
In my opinion these are the directions that most need stiffness.
Except when you add a brake. Then you need a frame that doesn’t bow outwards when the brake is applied. I don’t think the twin tube designs are going to be as stiff as the KH in that case.
I don’t think using enough braking force to bend the tubes significantly is likely to happen on a uni, especially seing as the difference in theoretical stiffness is small.
Yeah, I was thinking the same. But there is a way to make the hunter-style frames a lot stiffer.
When the brakes are applied they push out on the brake bosses, which tends to bow the stays out. Since the bosses on both styles of unicycle are near the crown (or second bridging element), then this mode of deflection probably isn’t significant. It will be twice as large with the smaller tubes, since they have only about half the bending stiffness of the larger tubes, but still it’s small and not terribly important since the braking mechanism has more than enough range to compensate.
The small tubes are, however, much less resistant to torsional flexion around the axis of the tube. Torsional stiffness scales with something like the 4th power of the tube diameter, which means it’s about 1/16th as stiff as the KH stays. This outward rotation not only allows the brake mechanisms to move apart; it mis-aligns the braking surfaces which can lead to dramatically reduced braking effect.
Fortunately the fix is simple and should lead to a frame that is much more resistant to torsional deflection than the KH frame. Weld (or braise) a short piece of tubing between the two legs. It will act like a wrench and keep the brake boss from rotating under load:
New
tube
|
o | o
| V /
|-------/= <--- Brake boss
| /
| /
| /
| /
| /
| /
| /
| /
O
Cheers,
Tim
On edit: The newsgroup software removed all the spaces in my wonderful ASCII graphics, but the idea is still there.
Re: KH or Hunter (stiffness and flex)
“jagur” <jagur.w1jvb@timelimit.unicyclist.com> wrote in message
news:jagur.w1jvb@timelimit.unicyclist.com…
>
> cyberbellum wrote:
> > *Just out of curiosity, why is this important? *
> cause im shoppin for a new 24" muni frame and i want one with the least
> amount of frame flex as possible.this is most important because i will
> be useing a brake and i dont want my rim flexing into the brake
> pads…make sence?
>
I think your question about the frame is addressing the wrong point of
flexure. I believe the wheel flex is more the trouble spot in brake rub.
The test with flexing a single tube vs. a two tubes vs. an oval tube is
interesting but that’s not so meaningful as the design of the fork. Think
about it, the top of the fork is fixed at the top via the crown. The bottom
is fixed via the axle. You’ve got a basic parallelogram or nearly so. When
you torque on the frame you are basically racking the parallelogram. You
can show this to yourself using a couple of drinking straws held in your
hands as if they were uni forks. To reduce the racking consider the crown
design first.
I may be all wrong but after my little drinking straw experiment I went out
to the garage, laid my uni across a stack of boards. Once set under the
axle, one set under the seat post. Then I stood on one side fork near the
crown trying to get the frame to flex enough to move the wheel. Wouldn’t go
but about a mm. But with finger pressure only I was able to get the wheel
to flex that much. Your mileage may very based on wheel stiffness, etc.
-Cubby
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This is certainly what I do - I believe rick does the same. it also helps to distribute the actual brakeing force on the frame across both tubes.
Do you have pictures of your frame? I thought you used to have info about your frame at http://www.pedal.me.uk/
heres one off the unicycle.uk website
i still dont understand what the welded piece of tubing thing is that cubby01 is mentioning and i also got lost with the drinking straw referance…
Indeed - the unicycle bit of the website is currently out of action - it will however be up again soon.
Currently the frames are only available through Roger at unicycle.uk.com unless you have a particular custom request.
Cubby - your wheel shouldn’t be moving much laterally if at all. there certainly shouldnt be much flex in the wheel - check the bearings.
That said - I don’t know what force you’re putting it under. Wheel flex can be a problem on cokers fitted with brakes.
thread update
i built up my KH frame tonite and can confirm now that a KH with the single oval legs is stiffer and less flexy from side to side than the thinner double tube legs of the Hunter.
Just using the stock Coker frame, which is uni-tubular of soft steel, I found that, for me, brake rubbing on idling and handle-intensive uphill riding was primarily due to wheel flex. It was only with a wide hub and very high tension on the Airfoil rim that the rubbing went away.
Is it a KH Pro-frame or a Taiwanese one?
Tim, you can retain your wonderful ASCII spaces by putting it in code brackets (use the # button).
Fortunately the fix is simple and should lead to a frame
that is much more resistant to torsional deflection than
the KH frame. Weld (or braise) a short piece of tubing
between the two legs. It will act like a wrench and keep
the brake boss from rotating under load:
>
> New
> tube
> | o | o
> | V /
> |-------/= <--- Brake boss
> | /
> | /
> | /
> | /
> | /
> | /
> | /
> | /
> O
>
Cheers,
Tim
I found out about the code brackets before posting a thread about Unicycle ASCII art.
Cool. Thanks!