yet more on trail

Archive-URL: http://search.bikelist.org/getmsg.asp?Filename=internet-bob.10601.0802.eml
Date: Tue, 10 Jan 2006 00:59:40 -0700
From: Mitch Harris
Subject: [BOB] Trail talk

Like Jan said in a previous post, trail is only one design feature that
affects bike handling, and works in concert with the whole bike design.
Since we've been talking a lot about trail, I thought I'd offer this
description of what trail is about and how it affects handling. This is my
take on it that I offer for others to add to and dispute. Most of you
probably know more about this than I do, but here it is. Hopefully this
is standard enough that it will help the trail discussion generally. This
comes from mostly 80s sources like John Forester, corroborated by my own
experience.

The gist of the trail issue is that lower trail makes the bike a little less
responsive to the rider's leaning or other body movement and more responsive
to steering with arms/handlebar. Higher trail makes the bike more responsive
to rider lean and subtle body movements and small weight shifts, and less
responsive to arm handlebar movements. This is more true the faster you go.

So a higher trail road bike, like 2.1" or 2.2" of trail, will feel a little
less stable at slow (parking lot) speeds, even twitchy as the bike squirrels
around a bit in response to your body leans and weight shifts at slow
speeds. You get accustomed to this enough that you'd have to ride the two
kinds of bikes one after the other to really notice.) But when you get that
2.2" of trail up to speed (say, 15mph) the bike feels very stable
because the rider unconsiously steers by subtle weight shifts without having
to steer with the bars much. The design challenge is getting this kind of
road race stability at speed while still allowing the bike to be responsive
to tossing into corners by arm steering (and with both high or low trail you
want to avoid wheel flop by getting the right balance). This kind of long
trail has become the road bike standard and people have found it comfortable
for most kinds of distance riding, especially unloaded or lightly loaded
riding (or at least rear loaded riding). I understand Jan's posts on low
trail to be exploring how French low trail is also great for most kinds of
distance riding, and better for some, especially perhaps with cushy 650b
tires, and especially with wieght in a handlebar bag.

In the late 70s and early 80s racing bikes in America started to favor low
trail, 1.8" or 1.9" because most American racing was criterium racing on
short flat tight multi-corner courses where quick arm steering seemed like
an advantage. Achieved with very steep head angles, it was known as crit
geometry and is described as responsive by those who like it, and twitchy by
those who don't. By contrast non-criterium road races tend to favor the high
speed stability of a balanced 2.1"/2.2" trail classic road racing geometry.

In the meantime, designers of touring bikes built for riding with a load
knew to design for lower trail because once you have a heavy load on your
bike you don't want the bike to be responding to every little subtle weight
shift. Also you want a heavily loaded bike to be stable at low speeds. So
you wanted the bike to ignore more subtle weight shifts at low speed and
obey arm steering more. This applies even more to bikes that are going to
carry any significant weight on the front/handlebar because heavy bar bag
weight with long trail will make the steering over-react to subtle weight
shifts. This is unpleasant in my experience with my 2.2" trail long
wheelbase 73/73 degree race bike when I tested it with bar bag and 10 or 15
pounds. That bike tours very well with 25 pounds in a Carradice on the back.

Jan would probably say this is the kind of bike that needs to carry weight
in the back and not the front (as I discovered). He's also pointed out that
the softer wider tires of low trail 650b bikes affects trail. Do a search
for his idea on how tires contribute to trail (he covers this in VBQ more
fully I understand). I wonder how that idea about tires and trail meshes
with the mature designed rigid mtb of the late 80s early 90s that had very
long trail figures more in the 2.8" range plus wide knobby tires. Grant's
MB-1 had the steeper 72 degree head angle (and somewhat less trail than most
mtbs) which he promised in the catalog wouldn't be twitchy because of the
fat tires reducing twitchiness. I've found that to be true with my MB-1.
Interesting how many different takes on this there have been on trail. Tony
Oliver in his very interesting book on bike design has still more
and different ideas on trail, especially for tandems. All of the track bikes
I've had have longish trail in the 2.4" range (even with a steep head angle
the fork offset is minor enough that trail is still high) and are very
stable at speed. This surprised me at first since you think of track racing
as being about maneuvering and not about stability. Over the years of racing
it came to make sense to me because on a good banked track you don't arm
steer much (except for certain maneuvers), and on a very steep banking you
can feel like you are on a continuous straightaway as the track leans you
right over on your side on the turns.

Happy trails,
Mitch

***

Archive-URL: http://search.bikelist.org/getmsg.asp?Filename=internet-bob.10606.2302.eml
Date: Thu, 29 Jun 2006 14:53:46 -0500
From: "Kogswell Cycles"
Subject: Re: [BOB] Now: trail, was ew bike from Riv -- the A. Homer Hilsen

On 6/29/06, Philcycles@aol.com wrote:

> High trail is kinda imprecise. There's a sort of normal trail for non touring
> bikes the runs in the 58cm area-that's what I use for non touring bikes.
> Here's where low trail comes in: loading the front end. French touring bikes
> were built with long rake/low trail because French touring bikes were loaded
> with front panniers and bar bags and very little weight on the back. In this
> configuration the low trail fork works superbly. Americans and British don't
> load that way, we tend to load the back of the bike. Low trail in this
> circumstance always feels kinda floppy, especially at low speed when
climbing.

Exactly right.

The French were front-loaders and
English and Americans were rear
loaders.

We figured that since we were using
French tires, why use an English
geometry?

The answer is that everyone
has her own way of loading a bike
so why not offer a choice of forks
to accomodate them all rather
than forcing everyone to adapt to
our view of the world?

And don't forget tire pressure.
Lower pressure tires need lower
trail front geometries.

Matthew

***

Archive-URL: http://search.bikelist.org/getmsg.asp?Filename=internet-bob.10601.1022.eml
From: Mitch Harris
Date: Jan 11, 2006 1:04 AM
Subject: Re: [BOB] Trail talk
To: Murray Love

This all sounds right to me too, Murray, according to what I know. After
reading your description, it occurs to me that I should have said that low
trail bikes require more arm movement for steering instead of saying that
they are more responsive to arm movement. You're right that they are less
responsive to arm movement which is why they require a bit more of it. I was
thinking of it this way: you steer a high trail bike with lean, and a low
trail bike with arms. Another way to put it, after reading your post, is
that you steer a higher trail bike by leaning and turing your arms, while
you steer a low trail bike by leaning a bit harder and steering with your
arms a bit more emphatically. In practice the difference is subtle and you
can steer either bike with arms or lean, of course, but that is the key
difference.

The caster you write about is another way to talk about the high speed
stability that is usually what people are after when they build a higher
trail bike. The last sentence of your wheel flop paragraph is another way to
talk about how you tend to steer a higer trail bike with wieght shifts.
Interesting to think of that as wheel flop since I usually only hear that
term when it means "too much wheel flop," where the wheel turns or kneels
more than you want it to. But it makes sense. The body movement and weight
shift/lean stuff is pretty subtle and anybody who can ride a bike already
does it all the time. It's just that higher trail makes a bike more
responsive to those movements. That's why a higher trail bike rides
no-handed a little more easily than a lower trail bike, because you're
steering with body movement/lean only. And because low trail bikes respond
a little less to lean, they do require subtly more arm steering to turn. I
have race-oriented bikes with both low and higher trail and I like both
kinds of steering geometry a lot. Both descend canyons well at speed which
is where I rely on steering geometry. The only adjustments one needs to make
going from one to the other is that the low trail bike feels a little less
able to ride no-handed at low speed right at first (adjusting to exaggerate
lean a bit takes a couple of seconds), and they feel different climbing out
of the saddle too.

Eddy M.'s bike bike business was getting big in the 80s right when the US
market moved toward higher trail bikes (away from Crit geometry), and I
always thought that might have been part of the reason he kept his front en
d
geometry a trade secret (he wouldn't publish head angle or fork rake at
all) because he didn't want people to overlook his lower trail bikes if they
saw how his steering figures differed from 80s Pinerellos, etc. He knew
they'd ride great.

One thing about low trail bikes, I think (bike designers please comment), is
that it seems like a bigger chanllenge to design well for low trail. With a
lower trail design you are flirting closer to a no-trail bike, flirting with
the lower limit of what is ridable (at least no-handed), while you try to
find that sweet spot that has a low trail advantage while still having
enough trail to have some caster. That spot would be somewhere between
maybe 38mm and 46mm of trail. By contrast, the advantages of high trail are
available across a wider range. Jan's says higher trail begins about
52mm. Higher trail bikes then would range from 52mm to 75mm if you include
mtbs and track bikes, etc. (Obligatory mention that trail is just one
variable that affects the handling, and not that all high trail bikes would
be nice to ride accross that range.) Think of how many times you've seen
someone out on the street riding a bike with a crash-bent-back fork that no
w
gives huge trail figures (like 100mm) and they seem fine with it. Bend that
fork forward the same amount and you'd make the bike unridable.

--Mitch

***

Archive-URL: http://search.bikelist.org/getmsg.asp?Filename=rbw.10608.0082.eml
Date: Thu, 3 Aug 2006 15:13:19 -0700 (PDT)
From: alex wetmore
Subject: Re: [RBW] question about Atlantis handling with front load, etc.

On Thu, 3 Aug 2006, dave(AT)wrightwoodlodges.com wrote:
> I see a number of Atlantis builds with a small Nitto front rack with
> a Bert houd bag mounted on top, or to the bars and the rack. These
> are mounted low on the sides of the wheel, obviously, and so deviate
> from the typical touring load distribution where a lowrider rack
> might be used to counterbalance a heavy load in the rear. These
> builds with the small rack / boxish front bag are more like a
> randonneur set-up.

I had a Quickbeam, not the Atlantis. It is also a high trail bike
though, and I do use it with a handlebar bag. I've also used
handlebar bags with my 57cm Heron Touring, which has a geometry that
is almost identical to the 58cm Atlantis. I also own a Kogswell P/R
and use it with the same handlebar bag (each bike has a Nitto small
front rack and I move the bag between them).

High trail bikes work well with handlebar bags. The high stability
given to you by the high trail makes the bike very stable even with a
heavy front load. The Quickbeam does require more muscle than the P/R
when turning, and I can't ride it no hands while turning, but these
are more minor issues.

> Is it important to have rear weight as well, or does your Atlantis
> handle fine with only a front load.

I've been riding the Quickbeam with only a front handlebar bag.

I prefer the handling of the low trail bikes, but they are more picky
about how the load is distributed on the bike. My P/R requires that
the majority of the load be carried up front if there is a moderate to
high load on the bike. The Quickbeam doesn't care and handles well no
matter where the load is located. I expect that the Atlantis would be
similar.

> Additionally, I'm interested in how large 45 mm+ tires affect the
> handling of an Atlantis, as well. I have read that one of the
> reasons randonneurs are oft en designed with low trail is to
> complement the widish 36-38 mm tires a 650 B- bike generally has.

Wide tires help a low trail bike by adding some of the stability that
the low trail gives up. A low trail bike with narrow tires can feel
twitchy and like it wants to turn too easily. The wider tires
resist these minor turns and make the bike feel more stable.

A high trail bike like the Atlantis already resists the turns due to
having a high trail. The extra resistance of a wider tire isn't
necessary (but you still may prefer the bike's handling with one size
of tire or the other).

Try a handlebar bag on your Atlantis, I think you'll be happy with
the results.

alex

***

Archive-URL: http://search.bikelist.org/getmsg.asp?Filename=internet-bob.10601.0898.eml
Date: Tue, 10 Jan 2006 22:28:36 -0800
From: Murray Love
Subject: Re: [BOB] Trail talk

Thanks, Mitch. My understanding is a little different--though not by
much--and I'll lay it out below. I'll leave aside issues of weight
shifts and body english since I'm not sure I understand them all that
well just yet.

My take on the trail issue is as follows:

When you increase geometric trail, you also increase two factors that
counteract each other to some extent: Caster and wheel flop.

CASTER refers to the tendency of the front tire's contact patch to
want to fall in line behind the steering axis, and resist deviations
from that plane. Any bike with positive trail will exhibit
self-correction in some proportion to its speed: the faster you go,
the stronger the self-correcting (or "righting") moment. High-trail
geometry increases caster, but even high-trail bikes have little
caster at low speeds--the bike can easily be steered quite
aggressively with little resistance.

WHEEL FLOP refers to the tendency of the front end of the bike to
"kneel" under steering input. When you turn the handlebars, the front
of the bike pivots about an axis passing through the contact patch and
parallel to the steering axis, and the axle height drops. My
understanding is that wheel flop is what we use to initiate lean via
countersteer--the greater the trail, the higher the wheel flop, and
the easier it is to initiate a lean by very small movements of the
handlebars.

So. If I have this right (and I make no guarantees), we can say the follow
ing:

HIGH-TRAIL BIKES are juggling the effects of both higher caster and
greater wheel flop. At speed, caster makes the bike want to continue
in whatever straight line it's in, but relatively minor steering
inputs can cause the bike to begin a lean, and to veer as a result.
At lower speeds, caster is diminished, while wheel flop is unchanged
(and steering angles are higher), leading to lower stability. The
upshot is that high-trail bikes require LESS steering input to
initiate a lean at speed, despite the fact that they have greater
straight-line stability.

LOW-TRAIL BIKES have inherently lower caster and therefore less
straight-line stability, but most bikes with positive geometric trail
will have enough caster at speed to inspire confidence. They also
have less wheel flop, so are not as prone to begin a lean if the
handlebars are nudged. One implication of this is that you need to
turn the handlebars MORE to initiate a lean on a low-trail bike (can
anyone confirm this?).

Last month, Dan Goldenberg wrote on KOG:

"I recall that Merckx designed his frames (and presumably the ones he
rode also in
competition) with low trail, because he liked how a low trail frame handled
on the cobbles and poor roads."

This makes sense when you consider that pave is one of those rare
on-road situations where most of the steering input is coming from the
road surface, as the wheel tries to find its way around and over the
cobbles. If it's correct that a high-trail bike is more sensitive to
steering-axis inputs, it would follow that the bike would be falling
from lean to lean in this situation. A low-trail bike, which is less
sensitive to steering inputs, would be less affected.

Thoughts?

Murray

***

Here's what Tom Kellogg has to say about trail on the Spectrum Cycles
web site:

TRAIL & ITS EFFECTS
First the definitions;
Fork rake; The distance that the front axle leads (usually) the
imaginary line drawn through the center of the head tube (or steering axis).
Head angle; Is the angle described between the ground and the steering axis.
Trail; The distance between the front wheel contact patch with the road
and the imaginary point where the steering axis meets the road.


As a general rule when dealing with 700-C wheels, a rake of about 5.6mm
will give a frame set "neutral" handling. My use of the term "neutral"
here refers to two things. First, neutral handling means that a frame
set will respond to steering input in the same manner no matter what
speed the bicycle is traveling. Second, while cornering, a neutral
handling bike will have neither a tendency to climb out of a turn nor
have a tendency to dive into the turn, it will simply hold the line that
the rider sets up unless further rider input is applied.

Decreasing trail below the neutral range has a couple of effects as you
might expect. The first thing a rider will notice about a low trial bike
is that it appears to resist attitude changes (or lean angles). It
requires more physical effort to get the bike to lean into a corner and
more effort to get it to straighten up. The second thing that you will
notice is that while cornering at higher speeds, the bike will have a
tendency to climb out of the turn on its own. Finally, you will find
that the way the bike responds to rider input is effected by the speed
of the bike. As you might have guessed by now, at lower speeds, a low
trail bike will have a tendency to want to go straight and do so pretty
much on its own. What you will find at higher speeds (like over 30mph)
is that a low trail bike will become quite vague in the front end. The
front wheel will feel as though it is wandering a bit and the contact
patch feel will go away.

Increasing trial above the neutral range will cause opposite effects for
the most part. At lower speeds, handling response will be light and
consequently, attitude changes will be much easier. During cornering,
the bike will have a tendency to drop into a tighter arc than the rider
might have intended. Finally, speed's effect on handling is reversed.
While low speeds give a light feel during handling maneuvers, high speed
sets up a very solid front end feel.

Although high trail frame sets give safer (more inherently stable)
handling than low trail frame sets do, high trail frame sets are still
inconsistent in the way they respond to rider input. Interpreting from
the basics above you can see why we usually aim for neutral trail. It
does not require the rider to consciously hold a bike down during hard
cornering, nor does it require different rider input depending on
changing speeds.

For some frame designers though, it is not always that simple. For
example, look at the way Eddy Merckx designs most of his frames. He
usually uses less trail than the "ideal" as he did much of his racing on
the pave and likes the way a low trail frame tracks under really
horrible conditions. Granted, they do not act as consistently under a
variety of speeds on good roads, but they really work on northern
Europe's country tracks.

1 comment:

Harry said...

You misplaced a decimal point when you quoted Tom Kellogg. Neutral trail, claims he, is 56 mm. Not 5.6mm. Most of your readers will note the error.