Suspension Seat Posts

Hi All,

Hope everyone’s enjoying the same freakish weather conditions I am at the
moment - sunshine that is :wink:

I’m in the process of bankrupting myself buying a Wilder and Profile
dream machine and about the only area left to throw some money at is the
seat post. I’m wondering if those with experience of suspension seat
posts can help…

I like the rigidity of my current solid post and carbon fiber seat base.
I have no doubt that suspension posts are great for soaking up bumps and
lessening impact from drops but I’m wondering about them flexing when
pulling up on the seat while hopping / jumping etc. Does this actually
happen or is there something in the design that stops this? Can they
take the punishment of drops or are they only for soaking up tree roots
and the like?

As for specific models… I’d ask anyone for their own recommendation. The
Thudbuster listed on Unicycle Source
(http://www.unicyclesource.com/Cane_Creek206456.html) looks pretty good
and can be shortened - I’m 5’ 11", the uni is a 24x3 and the post is some
370mm long so it shouldn’t need shortening that much, if at all.

All words of wisdom gratefully appreciated, Neil

There are definate pros and cons to different suspension posts and whether
you need them at all.

My experience with a modified Rock Shox post is that it is nice for
cross-country riding where you are actually sitting down. For technical
downhill they reduce fatigue because you can sit down a little more, but
most of the time it works just as well to drop your seat lower.

All telescoping posts also become wobbly over time, although you can buy a
fairly inexpensive rebuild kit for the Rock Shox post that fixes this.
Modifying the telescopic posts is also a lot of work.

In theory, trapezoidal shockposts like the thudbuster might be good (and
much stronger)- with one big caveat. They are designed to flex down and
back, not straight down. This means that they work OK on a Teleford cycle
(which has an inclined seatpost) but the geometry is not that well-suited
for a regular unicycle.

I think it would be awesome if someone out there wanted to experiment with
modifying the geometry of a thudbuster post to be better suited to unis.

-Kris.

— Neil Dunlop <neild@roachmill.demon.co.uk> wrote:
> Hi All,
>
> Hope everyone’s enjoying the same freakish weather conditions I am at
> the moment - sunshine that is
>
> I’m in the process of bankrupting myself buying a Wilder and Profile
> dream machine and about the only area left to throw some money at is the
> seat post. I’m wondering if those with experience of suspension seat
> posts can help…
>
> I like the rigidity of my current solid post and carbon fiber seat base.
> I have no doubt that suspension posts are great for soaking up bumps and
> lessening impact from drops but I’m wondering about them flexing when
> pulling up on the seat while hopping / jumping etc. Does this actually
> happen or is there something in the design that stops this? Can they
> take the punishment of drops or are they only for soaking up tree roots
> and the like?
>
> As for specific models… I’d ask anyone for their own recommendation.
> The Thudbuster listed on Unicycle Source
> (http://www.unicyclesource.com/Cane_Creek206456.html) looks pretty good
> and can be shortened - I’m 5’ 11", the uni is a 24x3 and the post is
> some 370mm long so it shouldn’t need shortening that much, if at all.
>
> All words of wisdom gratefully appreciated, Neil
>
>


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> Kris Holm wrote: I think it would be awesome if someone out there wanted
> to experiment with modifying the geometry of a thudbuster post to be
> better suited to unis.

I have considered doing this myself. My plan was to buy a Thudbuster post
without the seat post glued in yet. Then the aluminum post that it comes
with could be removed and replaced with a piece of CrMo tubing that has a
kink near the end right before it goes into the suspension unit. Then it
could be used with a standard uni frame.

Another option to buying one unglued from the factory would be to just cut
off the post right before it hits the suspension unit, and use CrMo tubing
whose O.D. is the same as the I.D. of the original aluminum tube. Or we
could leave an inch of aluminum tube sticking out, cut a slit in it, and
experiment with different kink angles with a quick release clamp before we
finalize the job with epoxy.

How does this sound to you guys?

Chris

— Chris Reeder <reed8990@uidaho.edu> wrote:
> > Kris Holm wrote: I think it would be awesome if someone out there
> > wanted to experiment with modifying the
> geometry
> > of a thudbuster post to be better suited to unis.
>
— Chris Reeder <reed8990@uidaho.edu> wrote:
> I have considered doing this… myself. My plan was to buy a Thudbuster
> post without the seat post glued in yet. Then the aluminum post that it
> comes with could be removed and replaced with a piece of CrMo tubing
> that has a kink near the end right before it goes into the suspension
> unit. Then it could be used with a standard uni frame.

OK, but I wonder about the following: If there is a “kink” in the
suspension, ie the post goes out and forwards (or backwards) before
angling back to a location directly above the seat, does this affect how
the unicycle behaves? Ie. would this affect what it feels like to:

  1. twist while riding
  2. push the unicycle directly sideways (no twisting) while hopping
    seat-out-in-front.

This is a question for all you people out there who know about statics and
moments of inertia.

-Kris.


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> OK, but I wonder about the following: If there is a “kink” in the
> suspension, ie the post goes out and forwards (or backwards) before
> angling back to a location directly above the seat, does this affect how
> the unicycle behaves?

Yes, the center of mass of the frame will move forward a couple inches.
I spent about a half an hour crunching numbers just to see what kind of
effect we’d get. I assumed a Gazz 24x3 off road combination. 2 lb
frame, 3 lb seat, 3 lbs of cranks and axle, 4.5 inches of tread on the
ground (150 lb rider with about 18 psi), coefficient of friction of .8
on dry pavement.

>Ie. would this affect what it feels like to:
> 1) twist while riding

By my quick calculations, the unicycle’s moment of inertia for twist
turning would increase about 4 percent.

> 2) push the unicycle directly sideways (no twisting) while hopping
> seat-out-in-front.

It will try to make the unicycle twist just a bit. If you push 20 lbf
sideways on your seat (this is about what it feels like when I push
really hard), I calculated that you get about 3 Nm (27 inlb) of torque
that tries to twist the unicycle. It’s not going to move very far, though,
since it takes about 20 Nm (180 inlb) to twist my Gazz on dry pavement.
Obviously the tire can flex a bit without actually moving the tread, but
it would be on the order of 0.5 degrees. (I can twist about three degrees
without the tread actually moving, so assuming a linear relationship
between torque and amount of twist…)

In short, it’s not likely that we’d ever notice the difference. But I know
my unicycle could use some 3 inches of travel-- that I know I’ll notice!

Now for another matter. Would it be better to run out the front (like a
Telford) or run out the back before kinking in to the shock? Front might
be better as it would put your footrests ahead of the seat (like on a
bike) but back might be better because it would give you more tire area to
use for wheel walking. Of course we can try both just by turning the seat
around at the rail mounts.

Chris

> OK, but I wonder about the following: If there is a “kink” in the
> suspension, ie the post goes out and forwards (or backwards) before
> angling back to a location directly above the seat, does this affect how
> the unicycle behaves? Ie. would this affect what it feels like to:
> 1) twist while riding
> 2) push the unicycle directly sideways (no twisting) while hopping
> seat-out-in-front.
>
> This is a question for all you people out there who know about statics
> and moments of inertia.

I never took a physics class, but I’ve ridden a lot of unicycles. Adding a
kink to the seat post would put the mass of the unicycle in a non-straight
line, but as far as jumping to the sides or even twisting, I don’t think
you will notice much difference.

It would be similar to riding a Telford, which has the frame mass
distributed in more of a front-to-back arrangement than on a typical
unicycle. It still rides fine.

I don’t think you would be affected by this more than being able to feel
the difference. My question is do you really think it’s necessary? Since
you are (hopefully) not using the suspension to land on when doing big
drops, I don’t think it matters if it compresses to the rear. Since you
weigh more than the unicycle, it just means the frame will pivot forward
as the post compresses. I think your butt and your hand will stay in about
the same place.

My 2 cents,

John Foss, the Uni-Cyclone jfoss@unicycling.com www.unicycling.com

“Someone who thinks logically is a nice contrast to the real world.”