Saturday, October 13, 2007

Zero Trial


Use a fork from a large wheel bike.





                                     



This fifty degree steering will eliminate 'swing steer' effect. Wheel flop is a stability problem when climbing hill very slow




This is another way to have .5" trail; rotate the fork up to about 79 degrees. It does change direction fast, but works fine for me.







STEERING GEOMETRY.


Most people can't even begin to understand this, because they do not know about tiller effect. 7" to 10"of tiller for counter balance stabilizes a 20" wheel even at 50mph.


A 50 degree seer axis (like on choppers), designed to create a long trail for stability at high speed, will create way too much wheel flop for slow speed maneuverability. (Wheel flop pulls the line of travel to one side in a turn.)



At very low hill climbing speeds zero trail is very maneuverable because the front wheel will turn the bike with less input from the hands.



The long wheelbase eliminates the twitch that a short wheelbase produces by shifting your weight back and forth across the line of travel.



All the factory bikes that I know of use the same mass-produced forks made for short bikes. This dictates that the steer angle be around 60degrees and the trail about 2-2.5".



Note that this picture is of a long trail!




Mark Stonich (this man is a genius):





For a certain angular deflection of the steer tube the contact patch of the bike with the steeper head angle will be at a sharper angle WRT the bikes frame. However this has little to with the ability to turn sharply. The rider will be able to make the tightest turn on a bike that has geometry and ergonomics that give the rider the most confidence at low speeds. All other things being equal I'd bet on the low trail bike.

We regularly went over 50MPH, one handed, on our loaded tandem, w/1/2" trail. Steady as a motorcycle.

Gyroscopic isn't a major factor IMHO. But a wider front contact patch has significantly more self-aligning torque.

Weight distribution is more important than wheelbase.

If you can't get self-centering from your steering ergonomics, you may find that low trail leaves a vague feeling to the steering that's worse than resisting the wheel flop from too much trail. The only SWB I've ever ridden that handled OK had an 80 degree head angle.

Weight distribution and distance from the riders shoulders to the steering axis are what determines how well a 'bent can be made to handle. I have MWBs with wheelbases of 42" and 45" that are so stable that on steep, fast downhills I ride with one hand in my lap to reduce wind drag. They have 2/3:1/3 weight distribution, 6 and 9" of tiller with optimized hand grip orientation.

MWB = BB between 2" ahead of the front axle to 6" behind the axle. Easy to get good ergonomics and weight distribution. Once you get the BB even with or slightly behind the axle heel strike is not a problem.

I was locked into some of the geometry on this one so it could use a little more tiller and I rarely get a bit of heel strike. 42" WB

On the 2nd MWB I moved the front wheel 2" forward and used a slightly steeper head angle to eliminate heel strike and optimize tiller. I moved the rear wheel one inch back to maintain weight distribution. 45" WB

This one, built for a customer, has a shallower head angle and a 54" wheelbase.








All of my LWBs and the tandem have had a 60 degree angle and the trike has 50. All the singles have standard, medium priced 1" headsets.
 The tandem is also 1" and now has a Stronglite roller headset that is no longer made. I had a Campi Record headset in it for several years and it held up fine till I needed it for an older Paramount.

Always use a long head tube (think leverage) and remember that a really good headset feels smooth when it's too tight. Back off the adjustment in tiny increments till you can just feel a tiny bit of play. Then go back just a wee bit. Also, always use loose balls without a cage as you can get more balls sharing the load.

If you use a quill type stem adjust the headset with the stem in place and tight. Doing so actually shortens the steer tube a tiny amount. Years ago, Campi warned you to do this because so many of their headsets failed prematurely due to over-tightening.





For a trike I wanted a bit more trail. .75". But with the extremely shallow head angle I needed 5.75" of rake; too much to bend so I used a tubular crown.




Mark Stonich; BikeSmith Design & Fabrication, Minneapolis, Minnesota http://bikesmithdesign.com/Design/12Steps.html http://mnhpva.org/


















Even motor cycles are using the zero trail concept: But with a springy to add trail to the steering when hitting a bump.