Fork Offset and Handling?

pinnah said:

My current road bike has a 62cm (tall) frame with a 73.5 degree head tube angle and a fork offset of 4.3 cm. I can not ride it comfortably with no hands. the calculated trail for my current bike is 5.4 cm

Quoted message said:

my commuter/errands bike and another sport touring bike

that I used to have both have/had 73 degree head tube angles and 5.5
cm of offset. I could ride those bikes no hands while sleeping. The
old bikes have a calculated trail of 4.5 cm.

Quoted message said:

I have found an aftermarket fork that is nearly identical to my

current fork in all respects with the only substantial difference
being that it has an offset of 5.0 cm. This fork would give me 4.7 cm
of trail

Here's a link to a good discussion of trail:
http://www.phred.org/~josh/bike/trail.html

In general, more trail results in a more stable ride (stronger
centering force). Less trail normally results in a less stable ride
(quicker handling, twitchy steering). But other factors come into play,
and I think that's what happening in your case.

Even though your touring bike has less trail, you say it feels more
stable than your road bike. That may be because of the stouter wheels,
wider tires, longer wheelbase, etc.

Changing the fork on your road bike to one with a lot more offset may
actually have the effect of making the bike less stable(assuming
everything else stays the same).

Your road bike might even have an alignment problem that makes "no
hands" riding difficult.

The specs on your current bike (73.3 degree HA, 4.3cm offset) sounds
quite normal. I wouldn't mess with it.

Art Harris

-----BEGIN PGP SIGNED MESSAGE-----

In article <[email hidden]>,

pinnah said:

If I replace my current fork with one that has more offset, will it
make the handling of the bike more likely to track straight (increased
centering)?

Some details....

My current road bike has a 62cm (tall) frame with a 73.5 degree head
tube angle and a fork offset of 4.3 cm. I can not ride it comfortably
with no hands. It is possible, but it takes a lot of concentration.
For those who understand trail (I guess I don't) the calculated trail
for my current bike is 5.4 cm according to this calculator. Note, I'm
using a measured 67cm for the diameter of my 700x25 wheels.
http://www.kreuzotter.de/english/elenk.htm

In contrast, my commuter/errands bike and another sport touring bike
that I used to have both have/had 73 degree head tube angles and 5.5
cm of offset. I could ride those bikes no hands while sleeping. The
old bikes have a calculated trail of 4.5 cm.

_ Look at the back chainstay length between the bikes and overall
wheelbase. In theory more trail should mean a more stable bike,
but it only means a more self-centering bike. I'm very convinced
that wheelbase and stem length matter nearly as much if not more
to "perceived" stability than trail does.

Quoted message said:

I have found an aftermarket fork that is nearly identical to my
current fork in all respects with the only substantial difference
being that it has an offset of 5.0 cm. This fork would give me 4.7 cm
of trail according to the calculator but again, I have no good
understanding if that will achieve my goal.

Just to reiterate, I want would like to eek up more straight line
stability out of my bike. I'm willing to give up some agility in
cornering to get the bike more willing to ride straight and true with
less input from me. Will the new fork help me with this or will it
just cause other problems?

_ The standard answer is that less trail will make the bike less
"stable". But bikes are not shopping carts, and trail stability is not
really what you are looking for. You want a slower reacting
bike. In my experience that means a long wheelbase bike and
getting that extra wheelbase from just the fork doesn't seem
like a very good solution. Could be worth a shot if it's not
too much $$$. However, my ideas about bike handling are not
the accepted theory. Many people will chime in soon that
a cm of wheelbase does not matter.

_ Also if you are using a stem that is either longer or shorter
than you're used to than can have an affect on the steering as
well. The other thing is that I've found every new bike has
a no-hands learning curve. Even if you can do miles at time
no hands on your old bike, no hands on the new bike feels
weird and unstable. With practice this feeling goes away.

_ Booker C. Bense

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pinnah said:

If I replace my current fork with one that has more offset, will it
make the handling of the bike more likely to track straight (increased
centering)?

Some details....

My current road bike has a 62cm (tall) frame with a 73.5 degree head
tube angle and a fork offset of 4.3 cm. I can not ride it comfortably
with no hands. It is possible, but it takes a lot of concentration.
For those who understand trail (I guess I don't) the calculated trail
for my current bike is 5.4 cm according to this calculator. Note, I'm
using a measured 67cm for the diameter of my 700x25 wheels.
http://www.kreuzotter.de/english/elenk.htm

In contrast, my commuter/errands bike and another sport touring bike
that I used to have both have/had 73 degree head tube angles and 5.5
cm of offset. I could ride those bikes no hands while sleeping. The
old bikes have a calculated trail of 4.5 cm.

I have found an aftermarket fork that is nearly identical to my
current fork in all respects with the only substantial difference
being that it has an offset of 5.0 cm. This fork would give me 4.7 cm
of trail according to the calculator but again, I have no good
understanding if that will achieve my goal.

If your calculation is correct, the aftermarket fork would give you less
centering force. I suspect the reason the old bikes rides easier may be
because they have heavier front wheels. To ride without hand, you rely
on the gyroscopic effect of the front wheel to give you the steering
control. The heavier the wheel (more angular momentum), the easier the
control. You may want to try the experiment using a front wheel from one
of the old bikes.

GH

G. Huang said:

To ride without hand, you rely
on the gyroscopic effect of the front wheel to give you the steering
control.

Do you? I thought it was the castor of the front wheel which kept it
centred, and you steered with your thighs using the saddle nose.

Art Harris said:

Here's a link to a good discussion of trail:
http://www.phred.org/~josh/bike/trail.html

Yes, I had found that link before but remain confused (a statement
about me, not the web page!).

Quoted message said:

In general, more trail results in a more stable ride (stronger
centering force). Less trail normally results in a less stable ride
(quicker handling, twitchy steering). But other factors come into play,
and I think that's what happening in your case.

So, it would seem that I need to reduce rake to increase the bike's
self-centering tendancy then?

Quoted message said:

The specs on your current bike (73.3 degree HA, 4.3cm offset) sounds
quite normal. I wouldn't mess with it.

I goofed up on this. I double checked the specs. The 56cm version of
the frame had a 73.5 HA and 4.3 cm rake, but the 62cm version has a HA
of 74.0 degree. They used the same rake throughout the size run
(almost). I note that today (some 10 years) it is common for HA to
increase and rake to decrease as frame size goes up. I can see now
with the calculator that this should help ensure that the different
sizes end up with similar trails.

Quoted message said:

Even though your touring bike has less trail, you say it feels more
stable than your road bike. That may be because of the stouter wheels,
wider tires, longer wheelbase, etc.

I may need to double check this too. There are differences in the
tires, which may account for some discrepancy.

Here is what I don't get though....

Why are touring and randoneur type frame associated with shallow HAs?

Putting the question another way.... Would 2 frames with the same
trail but different HA and offsets handle differently with respect to
self-centering?

For example, my Trek 1220 has HA of 74 deg and offset of 4.3cm while
the same year's touring 520 has a HA 72.5 and offset of 5.2 cm. Both
have exactly the same trail of 5.1 cm

Thanks for the help...

-- Dave
==============================================
"It is impossible, or not easy, to do noble acts
without the proper equipment."
Aristotle, <<Politics>>, 1323a-b, trans Jowett
==============================================

pinnah said:

Art Harris said:

In general, more trail results in a more stable ride (stronger
centering force). Less trail normally results in a less stable ride
(quicker handling, twitchy steering). But other factors come into play,
and I think that's what happening in your case.

So, it would seem that I need to reduce rake to increase the bike's
self-centering tendancy then?

Correct.

Quoted message said:

Quoted message said:

The specs on your current bike (73.5 degree HA, 4.3cm offset) sounds
quite normal. I wouldn't mess with it.

I goofed up on this. I double checked the specs. The 56cm version of
the frame had a 73.5 HA and 4.3 cm rake, but the 62cm version has a HA
of 74.0 degree.

1/2 degree isn't enough difference to be significant.

Quoted message said:

Quoted message said:

Even though your touring bike has less trail, you say it feels more
stable than your road bike. That may be because of the stouter wheels,
wider tires, longer wheelbase, etc.

Why are touring and randoneur type frame associated with shallow HAs?

For one thing, a shallow head angle (and the corresponding large offset)
produces a longer wheelbase which gives a more comfortable ride for tourists
and randoneurs.

Quoted message said:

Putting the question another way.... Would 2 frames with the same
trail but different HA and offsets handle differently with respect to
self-centering?

As far as self centering, they would be the same. But as I mentioned before,
other factors influence stability.

Another interesting thing is that while bikes with lots of trail are stable
at normal and high speeds, they can be difficult to ride at very slow speeds
due to wheel flop.

Art Harris

Zog The Undeniable said:

G. Huang said:

To ride without hand, you rely on the gyroscopic effect of the front
wheel to give you the steering control.

Do you? I thought it was the castor of the front wheel which kept it
centred, and you steered with your thighs using the saddle nose.

I don't think you can apply turning force to the bike to turn the wheel
through the caster of the wheel while sitting on the bike. You can do
that if you are walking the bike. What happens as you "steer" the nose,
I believe, is that you tilt the bike along the longitudinal axis. This
creates the gyroscopic force which turns the wheel. I remember someone
sited an experiment in which a counter-rotating wheel was installed on a
fork to counter the gyro force and the result was that the bike could
not be ridden without hand. It would be another interesting experiment
to eliminate the trail and substitute with a weak artificial centering
force (ie. from a spring mechanism) and see if the bike can still be
ridden no-hands.
GH

pinnah said:

My current road bike has a 62cm (tall) frame with a 73.5 degree head
tube angle and a fork offset of 4.3 cm. I can not ride it comfortably
with no hands. It is possible, but it takes a lot of concentration.
For those who understand trail (I guess I don't) the calculated trail
for my current bike is 5.4 cm according to this calculator. Note, I'm
using a measured 67cm for the diameter of my 700x25 wheels.
http://www.kreuzotter.de/english/elenk.htm

In contrast, my commuter/errands bike and another sport touring bike
that I used to have both have/had 73 degree head tube angles and 5.5
cm of offset. I could ride those bikes no hands while sleeping. The
old bikes have a calculated trail of 4.5 cm.

My track bike, with very steep angles for the day (74-degrees) and a
short-rake fork, was always trivial for me to ride no hands, while my road
bike was nearly impossible to ride no hands. Turned out the road fork was
out of alignment. Check that before you ascribe all your problems to bad
geometry.

--

David L. Johnson

__o | "A foolish consistency is the hobgoblin of little minds, adored
_`\(,_ | by little statesmen and philosophers and divines." --Ralph Waldo
(_)/ (_) | Emerson

In article <[email hidden]>,
gang@removethis_bell-labs.com says...

Quoted message said:

If your calculation is correct, the aftermarket fork would give you less
centering force. I suspect the reason the old bikes rides easier may be
because they have heavier front wheels. To ride without hand, you rely
on the gyroscopic effect of the front wheel to give you the steering
control. The heavier the wheel (more angular momentum), the easier the
control.

I have my doubts about that -- an experimental fork that duplicated
the steering geometry of my standard fork with a 700C front wheel,
but used a 6" scooter wheel, was still stable riding no-hands. A 6"
scooter wheel has much less gyroscopic stability than a 700C wheel.

--
[email hidden] is Joshua Putnam
<http://www.phred.org/~josh/>
Updated Bicycle Touring Books List:
<http://www.phred.org/~josh/bike/tourbooks.html>

Hi --

Quoted message said:

Why are touring and randoneur type frame associated with shallow HAs?

I believe a big part of the answer is simple fender clearance. I
recently retired a Trek 520 which had only marginal clearance from my
winter boot tip to fender, even with that shallow head angle. These
bike types are more likely to have fenders than a road bike is, and the
tires are larger too. So to make room for tire + fender, the hub is
pushed away from the BB with the HA, then the rake is adjusted to
balance the trail.

My view of trail is that small trail optimizes stability at high
speeds, while large trail optimizes low speed. So you need to decide
what situation you really care about the most. When I bought the 520, I
really liked the handling, including stability for no-hands riding.
Then as I gradually built up the ability to ride faster, I hit a shimmy
issue and then became dissatisfied with its performance in ruts. The
worst situation was descending in car-rutted slush, where the peaks of
the ruts would aggressively steer the bike back into the rut. (I
should say I've never used front paniers, so if the 520 is optimized
for that, maybe I bought the wrong bike.)
My new bad-weather bike is very different, a modified rigid MTB, but
it has less trail than the 520. It is dramatically better at slush
ruts, and also cracks in the pavement closely aligned with the bike's
centerline. The shallow trail simply maintains its line rather than
reacting to the pavement. So I now view the 520 trail as excessive, at
least for the kind of riding I do now.

-- Gary.