[python] Re: max downhill speed --lost mail, second try

  • From: Ray Schümacher <mtb@xxxxxxx>
  • To: python@xxxxxxxxxxxxx
  • Date: Fri, 06 May 2005 08:36:39 -0700

Hi Dirk, Odd mail behavior for you too?

At 02:16 AM 5/6/2005, you wrote:
>Well it lies around 55 - 60kmh. After that it gets really scary.

My fully loaded commuter feels a bit sensitive and worrisome at 60kmh, and 
requires full attention on the hill home from work. As Mark alluded, it might 
be faster than you want to crash! I've crashed off-road at good speed and just 
scraped the gravel out of the skin, but the few road falls, all <30khm, caused 
real pain and injury. I think it's the friction of the surface; on dirt/grass 
you loose energy slowly, sliding. I've fallen on ice, skating as fast as I can 
- I would never want to fall on rollerblades on the street. <shivers>

>At least I get the feeling that when something happens I
>won't be able to correct it. And the consequence will have an explosive
>effect.

My mode of falling (still somewhat regular on the python) follows an attempt to 
correct or over-correct, which results in a sudden capsize with the wheel 
rapidly turning to the side simultaneously. This is typically done around 
15kmh. Everything is going fine, then a combination of bad road and inattention 
leads to a sudden feeling of unease and an even more sudden capsize.

>It is like something in the steering is *acting* to fast for me to react
>to, and I doubt very much that braking in the rear would help at all
>(except bringing the speed down).

Will you try some sort of damping or friction with the pivot next? Hand and the 
other authors assert that the rotational moment about the steering axis due to 
trail is proportional to the square of the speed, whereas all other 
contributing factors are linear or constant. Since no one needs much steer rate 
at speed anyway, the only solution might be to really dampen it, directly. 
It may also be that your smaller wheels offset the smaller trail somewhat 
(smaller turn radius and gyroscopic effect).

After some more practice riding, I am really considering shortening the 
wheelbase 15cm, which I think will improve low speed response and 
maneuverability - the biggest requirement around town. Because of the angle in 
my rear frame, shortening the rear will also add a few degrees to the pivot 
angle, maybe to ~68.  Hopefully I can ride the boardwalk soon, it is our usual 
fun spin. Though a bit narrow for my current python skill 
http://efgh.com/bike/bdwk1.jpg it is often distractingly scenic 
http://www.acclaimimages.com/_gallery/_SM/0001-0409-1813-3208_SM.jpg

Cheers,
Ray


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