Pneumatic trail
Ted Bennett or Lisa Kaskan
is? Jobst?
Is it significant for bicycle tires?
Would it be of greater significance for a tire smaller than a 700 in
diameter but larger in section?
TIA,
Ted Bennett
Jobst Brandt
> Could some tire-knowledgable person explain to me what pneumatic trail
> is?
It has also been called dynamic trail and is the shift of center of
pressure caused by hysteretic losses in the tire. Statically the tire
has no losses and its trail is geometrically determinable by castor
angle, offset and rolling radius. When rolling, energy absorption
appears as though the tire were rolling uphill, changing the center of
pressure of the tire against the road. It reduces trail.
On a bicycle, this shift is so small as to have no effect, although
rolling resistance can be measured. The change in trail is insignificant.
Jobst Brandt <jbr...@hpl.hp.com>
Doug Milliken
To add to what Jobst has said, but in an _automotive_ context:
SAE J670, Vehicle Dynamics Terminology <www.sae.org> defines Aligning
Torque (Aligning Moment, Mz) as torque about the Z axis (vertical axis).
Mx is another name for pneumatic trail.
For _car_ tires, Mz has a very nonlinear characteristic, typically plotted
against slip angle. At small slip angles, Mz tends to steer the tire in a
direction to reduce the slip angle (thus the term pneumatic trail). At
medium slip angles the Mz reaches a peak. At high slip angles, near or
above the friction limit where the tire is sliding, Mz is near zero, or may
even reverse sign so that it acts as reverse trail. We have a more
complete explanation in Chapter 2.2 of our book, "Race Car Vehicle
Dynamics", < http://www.sae.org/products/books/R-146.htm>, which includes
information on the effect that Mz has on car steering.
I'm in agreement with Jobst that pneumatic trail (by any name) is
not significant with high pressure (road) bicycle tires, the tire
footprint is so small.
-- Doug
Douglas Milliken <bd...@freenet.buffalo.edu>
Milliken Research Associates Inc.
Ted Bennett or Lisa Kaskan
Ted