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Saturday, October 13, 2007
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
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
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
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
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.