[python] Re: The old qustion of PSI
- From: George Durbridge <gdurbrid@xxxxxxxxxxxxxx>
- To: python@xxxxxxxxxxxxx
- Date: Fri, 18 Jun 2010 13:35:32 +1000
Howard,
Friction dampers are usually deprecated, because they give too much
static friction for a given amount of friction in motion. Hydraulic
dampers, though usually less compact, give a damping force which
increases with the speed of the motion to be damped. This would be
particularly applicable to a steering damper, which would tend to start
moving with a jerk (to overcome stiction), making it hard to control
precisely. Modern motorcycle steering dampers seem to be generally
hydraulic, probably for this reason.
Esko Meriluoto used a hydraulic motorcycle steering damper on his
centre-steered Hipparion trike, and was emphatic that all trikes need
steering dampers. That was an overstatement, but it may be true for
centre-steered trikes.
In my experience, you can get perfectly adequate traction on a slick or
narrow tyre on tarmac: when necessary, I have lifted the rear wheel on
an MR Swift under brakes on Tioga Comp Pools and 28mm Marathon Slicks,
which implies braking acceleration of about 1g, and corresponding
adhesion. The same tyres spun miserably on loose gravel or mud.
Knobbies would have gripped on those surfaces, but would have been very
poor tyres elsewhere. As well as high rolling resistance, they actually
have less grip than slicks on tarmac, even wet tarmac.
So I think you need to decide just where your present tyres are letting
you down, and whether you need more tread for loose and soft surfaces.
If you do need more tread, there are three ways of getting it. First,
knobbies from K-Mart: don't do it. Second, shallow-tread high pressure
tyres, such as Big Apples or Hookworms: some possibles here. Third,
tyres with a fairly smooth crown on the tread, and knobs along the
sides, which engage only when you sink in. I'm not sure if anyone is
still making these, which were actually a very nasty compromise for
bikes, but might work OK on trikes.
I doubt that wide tyres will be more stable directionally: I have seen
no evidence of it myself. There is a move to wider tyres for comfort
and grip, however. Schwalbe are making wider tyres and lowering maximum
pressures from about 100psi to more like 70psi, without losing anything
in rolling resistance or puncture resistance. Look at some of their
recent Marathons, particularly the Supreme, and the Big Apples.
You can also get several very solid high-pressure tyres with shallow
treads from Maxxis, such as the Hookworm. I have tried these tyres, and
dislike them intensely. Heavy, high rolling resistance and a very rough
ride. They do seem to be indestructible, however, and are fairly cheap:
they might do nicely on a wheelbarrow.
George
On Thu, 2010-06-17 at 21:46 -0500, Howard Stevens wrote:
> Hi Pythonites. Like some others I have some problems with PSI with my
> Python-like trike. The steering pivot is 100mm in front of the line
> of my hips and I cannot get it closer than this with my design. I
> have tried bungy cords which make it quite rideable but when peddling
> up hills the PSI becomes more noticeable, despite paying attention to
> pedalling style and gearing. My idea is like the old motorbike
> dampers which reduced movements by increasing the friction of the
> joint movements. (see the attachment) By having a hollow pivot pin
> with a cam lever and skewer with a nut on the lower end, and adjusting
> the nut at the lower end of the pivot screw you could adjust the
> amount of friction and hence movement needed, but avoiding any locking
> which could be dangerous. By releasing the cam lever at the top, all
> the friction would cease and so you could easily turn. The friction
> would be between the bottom aluminium plate and the carbon fibre base
> of the front wheel housing. The old motorbike dampers had a couple of
> bronze plates as well, which I understand made good friction plates.
> Another thing I mentioned was the idea of putting a wide tyre on the
> front wheel, thus giving increased traction for pulling on hills and
> for braking. The extra rubber to road contact would also make the
> wheel less likely to wobble, according to some experimenters.
> So any comments ? Happy cycling Howard Stevens
>
>
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