Trail and steering

Michael Press said:

One of my bicycles has 73 mm trail, 622x25 tires. What can
anyone say about its steering, handling, and stability?
What about estimates of its other dimensions?

How did you measure the trail? Are you talking about the distance
between the contact patch and the extention of the steerer axis, or are
you talking about the offset of the bend in the forks?

Not enough information. You need to state the rake angle as well.
Also, tread style influences apparent trail. Changing from nobbies to
slicks has the effect of making the turning more skittish.

Remember that what the trail is all about is putting in the appropriate
or desired response to lean. Too much trail, and the front wheel
castors and will not turn without having to push on the outside of the
handlebars. Too little trail (or lead!) and the bike will be unstable
at all times.

You can test the relativity of this by simply leaning the bike over
gently while at a standstill, and watching how the wheel "flops" to the
leaned side. A bike with too much trail (too much curve of the fork)
will actually flop the wrong way.

In article <[email hidden]>,

Road Man said:

Assessing it is actually a complex question. In another thread, Tim
McNamara framed the compexity very well, IMHO:

"Part of the problem in this discussion is the subjectivity involved.
What's "more responsive?" More responsive to what? To leaning the
bike? To turning the bars? To road inputs? How stable is too
stable,
and not stable enough? For what purposes?"

Can you turn inside another experienced rider? We all
know when a bicycle turns quicker. One bike I rode a
long time. Got a new bike. I tried riding it like the
older. Riding down the street I'd think, "Here comes
this corner. Let's get ready to turn, ..." and the
bicycle was turning long before I expected. Had to
compensate to keep from turning up the curb and down
the sidewalk.

Quoted message said:

My first reaction is that 73 mm is a lot of trail. Road bikes seem to
have (some spot-chacking based on reading websites) values ranging
from the high 40s to the low 60s. Some think 60 mm (many Trek frames
are 59 mm) is the "sweet spot," but what does that mean for me as a
rider? I'm not a Lance.

The bike is a quick and sharp turner. Not simple to ride no hands.

Quoted message said:

Quoting Tim again:

""Normal" trail seems to be between 50 and 60 mm. Yet many bikes were
made with 73-74 degree head tubes and 50-60 mm fork offset, resulting
in
very low trail figures. And yet those bikes were prized for the
ability
to ride right down the fog line for hours at night, even though
conventional wisdom suggests that they should have been very
unstable."

I pasted in the technical details from the reply to Fred.
Trail decreases with increasing fork offset, which can be
unexpected.

Assume 340 mm wheel radius for a tubular tire.

head tube angle fork offset effective trail ground trail
74 50 43.5 45.5
73 60 39.5 41.0

Quoted message said:

So what aspects of handling do you value?

I ride only a few hours at a time.
I like a quick and sharp turning bicycle.

Quoted message said:

Michael, if you have time for this, please do a deep dive into the
scientific literature on single-track vehicle dynamics and provide us
the comprehensive answer. I think Tim has identified the value
criteria fairly completely.

I have spent a bit time, but have yet to find a comprehensive
technical essay on bicycle dynamics.

Quoted message said:

IMHO, we don't even have a consistent and objective language to talk
about steering/handling/stability, much less any commonly held views.

I think the value of trail is that it unifies two incomplete metrics:
head tube angle and fork offset, and it relates directly to a
restoring force on the fork as it is turned and the bike is leaned.
Not magic, just engineering. The problem is to interpret it into
subjective terms.

Trail can be calculated from wheel radius, head tube
angle, and fork offset. These three latter quantities
are what really tell us about steering qualities. Call
this quantity ground trail. The perpendicular distance
from the contact patch to the steering axis is called
effective trail. Effective trail is a physically
meaningful quantity as it is the actual length of the
lever arm on which the side force acts when there is a
slip angle. Slip angle occurs when the steering is
turned and the contact patch is displaced away from the
line of travel

R: wheel radius
beta: head tube angle
B: fork offset
N: ground trail
E: effective trail

E = R * cos beta - B = sin beta (R * cos beta - B).

As you can see both ground trail and effective trail
_decrease_ as fork offset increases.

--
Michael Press

"Michael Press" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Can you turn inside another experienced rider? We all
know when a bicycle turns quicker. One bike I rode a
long time. Got a new bike. I tried riding it like the
older. Riding down the street I'd think, "Here comes
this corner. Let's get ready to turn, ..." and the
bicycle was turning long before I expected. Had to
compensate to keep from turning up the curb and down
the sidewalk.

Quoted message said:

The bike is a quick and sharp turner. Not simple to ride no hands.

Ok, this observation is clear and easy to understand. I had the trail
increased on my Trek 610 because I thought it would add straight line
stability, and it did make the bike turn quicker when intended.
Compared to my Woodrup it turns sharp and sometimes surprises me. It
now has about 55 mm of trail. It became much easier to ride no-hands,
because (I think) at the same time I had the frame aligned and
replaced the headset, so I eliminated biases.

Quoted message said:

Quoted message said:

Quoted message said:

I pasted in the technical details from the reply to Fred.
Trail decreases with increasing fork offset, which can be
unexpected.

Assume 340 mm wheel radius for a tubular tire.

head tube angle fork offset effective trail ground
trail
74 50 43.5 45.5
73 60 39.5 41.0

Quoted message said:

So what aspects of handling do you value?

I ride only a few hours at a time.
I like a quick and sharp turning bicycle.

I don't want more quickness, but more stability. I want to be able to
blow my nose and do other fidgety things on a tiring ride without
stopping or drifting into traffic. I don't have too much experience
with twisty descents, so I'm not sure how descent stability fits into
the perspective, either subjectively or objectively.

Quoted message said:

Quoted message said:

Michael, if you have time for this, please do a deep dive into the
scientific literature on single-track vehicle dynamics and provide
us
the comprehensive answer. I think Tim has identified the value
criteria fairly completely.

I have spent a bit time, but have yet to find a comprehensive
technical essay on bicycle dynamics.

My shallow dive revealed the same thing, David Wilson noted the same
thing in Bicycling Science 4th. Ed, "Unfortunately, the mathematics
purporting to describe bicycle motion and self-stability are difficult
and have not been validated expermentally, so design guidance remains
highly empirical. The most significant design detail is a geometric
quantity called " mechanical trail." (chapter 8) His mechanical
trail seems to be the same as your "effective trail."

Quoted message said:

Trail can be calculated from wheel radius, head tube
angle, and fork offset. These three latter quantities
are what really tell us about steering qualities. Call
this quantity ground trail. The perpendicular distance
from the contact patch to the steering axis is called
effective trail. Effective trail is a physically
meaningful quantity as it is the actual length of the
lever arm on which the side force acts when there is a
slip angle. Slip angle occurs when the steering is
turned and the contact patch is displaced away from the
line of travel

R: wheel radius
beta: head tube angle
B: fork offset
N: ground trail
E: effective trail

E = R * cos beta - B = sin beta (R * cos beta - B).

As you can see both ground trail and effective trail
_decrease_ as fork offset increases.

These are exactly the relationships I was referring to. I don't know
why they are not the whole story, except for there isn't a
comprehensive engineering theory that says they are, and that there
are numerous other factors, such as CG position and possibly bike
length that seem to my engineer's perspective to be significant.

As an engineer, I find it hard to accept that trail is an adequate
metric for handling when a strong theory that says so isn't
well-known.

I guess you're at the same point in the quandary as the rest of us.

With so much trail, I fear your bike might have some handling
characteristics you won't like, but I can't tell you what they are. I
did read your other post where you said straight descents were fine,
(and where the rest of the geometry seems unremarkable to me) but what
about road disturbance resistance, body english glitches, and loaded
descents with curves and bumps? Don't you ever turn, find bad roads,
or fidget when you descend?

Ken Freeman