Over the weekend I tried out a modified version of a trapezoidal seatpost
shock. The shock I used was a quadrapivot design called a Moxey post. I
believe the design is similar to the newer model of Thudbuster (Thudbuster
sued Moxey a while back for patent infringements).
Although trapezoidal shocks should be way better than telescoping shocks
(because they don’t require a long seatpost and the travel distance of the
shock is larger), they don’t work well on a unicycle because the component
of force is wrong- a rearward force is required to activate the shock
mechanism and sitting on a unicycle seat gives a vertical downwards force.
However, they can easily be modified to correct this. The following
description references this picture: Click on “seatpost” at <http://www.canecreek.com/index_t.html>. In this picture you can see that
the post attaches to the shock by two pins. By removing the pins, flipping
the shock around by 180 degrees, and re-attaching the pins, the shockpost
is now “kinked” by about 20 degrees. When you put this back on your
unicycle, the shock now works really well and you get about 3.5 inches of
travel. Your frame no longer sits vertically but this doesn’t matter as
your center of gravity is still directly over the axle. Note that I did
this on the Moxey post, not this one, so I’m not 100% sure that this will
also work on the Thudbuster.
Riding results were somewhat mixed, though. Definately it is very
comfortable and you can sit down while riding over rough terrain. However,
my uni seemed to be slightly less responsive to quick turns. Climbing
hills while sitting down was about the same but it seemed slightly harder
to climb when standing up and pulling on the seat.
If anyone out there has a Thudbuster brand seatpost it would be great if
you could try this alteration and report back on the results.
Cheers,
Kris Holm.
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Re: — Chris Reeder <reed8990@uidaho.edu> wrote: > >By removing the pins, flipping the shock around by 180 degrees, and > >re-attaching the pins, the shockpost is now “kinked” by about 20 > >degrees. > >…So what holds the pins in place, anyway?
The pins are composed of two pieces- a hollow tube and an allen machine
screw. Two pins are pressed into holes through both the shock and post
(holding the two together), and the allen screw then threads into one end
of the tube to hold it in place. The bottom part of the quadrapivot
rotates on these tubes.
> > > Climbing hills while sitting down was about the same but it seemed > > slightly harder to climb when standing up and pulling on the seat. > > Was this even with the preload set fairly hight? Any ideas where the > flex might be coming from, or is just a geometry thing?
It wasn’t a flex thing at all- there is no flex when standing up climbing
and the seat flexed in a comfortable way when sitting down and climbing.
Sitting down and climbing didn’t change with the shockpost, only when
standing up.
On steep climbs, where you’re standing up and pulling up on the seat,
there’s that awkward point where the cranks are vertical and you have to
briefly depend on momentum. For some reason it seemed that this “dead”
spot was harder to pull through. I’m wondering if this has something to do
with the kinked (open V) shape of the post and frame (the V was oriented
forwards).
BTW has anyone tried this on the Thudbuster brand? John Foss you have a
quadrapivot style thudbuster post don’t you- maybe tonight could you take
a look at it?
-Kris.
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> BTW has anyone tried this on the Thudbuster brand? John Foss you have a > quadrapivot style thudbuster post don’t you- maybe tonight could you > take a look at it?
Sorry, I have a Thudbuster Uni-pivot. It rides great, even on a vertical
post (though not as nice as on an angled post like a Telford). The
Uni-pivots are no longer made, though I just saw a really inexpensive one
in the Nashbar catalog (Nashbar brand). This might be great for someone to
try out without spending a lot of money.
Sadly my Uni-pivot is on my carbon MUni, which doesn’t get much action.
Since the frame only fits tires up to around 2.1" I almost always choose
to take my DM with 2.6" tire instead.