[python] Re: Position of Steering Pivot Axis
- From: "25hz" <25hz@xxxxxxxxxx>
- To: <python@xxxxxxxxxxxxx>
- Date: Mon, 9 Jan 2006 22:06:39 -0500
> > "Does the pivot axis have to go through the middle between both
> > hip joints in order to minimize pedalling/steering interferences (PSI)?"
> >
> > To gather the infos I have set up a new site:
> > http://www.python-lowracer.de/modifications.html
> > Some pictures are already there.
> >
> > Any ideas?
>
> while riding my improved TT clone (as you can see on the modification
page) there is no PSI.
> In the original TT the leverarm A is smaller than the leverarm B, there
PSI is strong. So as long
> as A = B or better A > B, no PSI will arise. I measured from bb
horizontely to the pivot.
It's hard to see in the picture, but "B" ssems to be the BB height from the
ground, and "A" seems to be a horizontal line drawn from the BB to the
intersection point of the pivot axis. This being correct, your assumption
that A>B or A=B will reduce the steering/pedalling issues is likely only
applicable to the TT design (and there's no name on them to be able to tell
which is the "improved" one?) because Jurgen's PX5 definitely has A>B and he
says there's problems.
For me, I'm not exactly sure what is meant by the PSI. On trikes and other
2 wheeled bents a commonly mentioned issue is "pedal steer". Pedal steer is
when the force of pedalling induces steering input and the bent ends up
going down the road from side to side in the same motion a fish uses to
swim. Is that what you all mean by "pedalling/steering interferences"? If
it is, the most common source of the pedal steer is the way you pedal.
Other less common factors (unless on poorly designed or homebuilt bents) are
major weight distribution issues or alignment problems. Predominantly the
pedal steer problems arise because of drastically uneven forces throughout
the pedal stroke without any counterbalancing force on the other pedal.
Especially on bents, an uneven pedalling technique not only causes the
uneven torque on the front end of the bent, but also makes for uneven weight
control/shifting of the rider who is pretty much locked into the seat. Any
weakness/flex in the frame will amplify this. This is no news flash to most
python riders, but I think varying amoutns of pedal steer are going to be
inherent in any design where the weight, momentum and inertia of your legs
are being thrown out front and higher than the conatc patch of the front
wheel, the pivot point, or both.
By pedalling circles, with practice, you can keep remarkably even pedalling
power on both pedals for just about all (if not all) of the entire pedalling
motion - pushing and pulling. I was a terrible one-legged grinder when I
first started riding bents. With concentration and practice, and listening
to the noise the tires make on the road, you can tell when you are pedalling
smoothly or when you are pedalling poorly and you get a "pulsing" noise from
the tires. With clipless pedals and practice, you can produce a constant
volume and pitch of noise coming up off your tires. On the python, I get
pedal steer when I have been riding hard and my legs are nearly fried, and I
can no longer smoothly either pedal in circles or make fine motor control
adjustments for steering input. The other time is when I am in too high of
a gear and pushing too hard. When I get the rpm right, I can maintain a
nice high cadence, maintain a nice smooth pedal circle, and put out the
least amount of power/effort for the speed I want to cruise at. I've ridden
4 different pythons now, and each one of them, for me, seemed to have their
own "frequency". By that I mean a cadence and torque level that produced
nice smooth power, steady steering and good speed. Until I found the "sweet
spot" for each python, I had some pedal steer but as I approached the sweet
spot, pedal steer reduced and when I went past it (by either too high a
cadence, too much torque or too high/low a gear) the pedal steer slowly
started to return. Every time someone gets on a bent for the first time,
they always say they're "twitchy" and they go down the road looking like a
salmon. If however, the person is any kind of a bike rider and takes a
little bit of effort to try to spin smoothly, the twitchiness and pedal
steer disappear, and they come back with the standard "recumbent grin". I
really enjoy the science of pedalling now because on top of all the other
power/speed/efficiency bonuses of a smooth stroke, my painful little knees
absolutely LOVE it when I am pedalling properly and reward me with a
reduction in pain :)
I don't think there are major weight distribution issues with the python's
design, or anyone's designs that have built one. Unless the builders have
really screwed up their builds and built a seriously misaligned python
(which I doubt), the only other source would be pedalling. The thing is
though, with the 4 pythons that I've ridden, their designs, geometry and
angles are all different and all give them a different characteristic for
pedal steer. Once you can ride your own python reasonably well, I believe
you can ride just about anyone's. For the python, because of it's design, I
don't think it will ever excel at high power riding like 1/4 mile/200m
sprints from standing starts, because the low rpms and high torque to get
hard acceleration at the start makes steering and efficient power
transmission very difficult. Once you get the python going however, it's a
very different story. I look forward to the next 10 lap race at our local
velodrome this summer as I can hold a lot higher cruise speed now that I can
balance much better than the last time I raced it. :)
============================================================
This is the Python Mailinglist
//www.freelists.org/list/python
Listmaster: Jürgen Mages jmages@xxxxxx
To unsubscribe send an empty mail to
python-request@xxxxxxxxxxxxx
with 'unsubscribe' in the subject field.
============================================================
Other related posts:
