Head tube angles & trail thread from Tandem@Hobbes



From Glenn Erickson.....
Traditional tandem geometry is considered to be "laid back", a smaller "trail"
than the more modern single road bikes. Trail is the distance between the
point where the steering axis intersects the ground and where the tire hits
the ground, less trail is more laid back, more trail gives quicker steering.
When a bicycle is leaned to initiate a turn, the tire / ground point moves out
of the center line of the bicycle as preasure is applied sideways to the tire
patch (please forgive my poor wording here). The longer the trail, (consider
trail as a lever arm working on the steering axis), the greater the force
turning the front wheel, that means "quicker". More trail = quicker, less
trail =slower = laid back. Tandems traditionally have been laid back because
as a stoker moves around on the mike there is the resulting steering input.
Too much trail(quicker) would cause the tandem to take many unwanted little
turns, therefore laid back could be better. However, if the stoker is
experienced and has learned to not move around, causing all those little
turns, then the tandem can be built with more trail (quicker), resulting in
bike that responds quicker to driver input and ultimately a bike that becomes
more stable. It actually requires less driver input, slight leaning movements
can acurately steer the bike.

In the case mentioned with the flattened front tire, my guess is that the
amount of trail had nothing to do with the stability of the bike in that
cituation. In a panic situation the captain puts all focus on steering and
controlling the bike, overruling the natural tendencies of the steering. That
they stayed upright is due to his very good bike handling skills not the
steering geometry of the bike. If the bike had been on its own (bear with me)
the steering geometry would have been overruled by the interaction of the flat
tire and the rim with the road which would more than likely have thrown
everything to the ground.

Every builder has their own philosophy about steering geometry and often it
can depend on the level of expertice the captain and stoker have. I tend to
build my tandems with quicker steering, but most of my tandems are second or
third tandems and the drivers generally are very experienced. One should not
say this or that geometry is "right", just that they are different.

Glenn Erickson
Erickson Cycles



From Todd Schusterman at daVinci....
Our tandems, and most others I believe, are built with a little less trail
than most singles. The difference is usually more in the fork offset than
the head tube angle. If you build a tandem with as little fork offset as a
single you get a bike that turns like a truck, very heavy, due to the extra
weight. You will get a similar feel with a slack head tube angle. Too much
offset and you get a bike that is characterized as "twitchy", a steeper
head tube will also give you a twitchy feel. How you combine head tube
angle and fork offset to arrive at a given trail will make the difference
of how the bike feels when you lean into a turn, and slow speed handling. I
believe a tandem should handle light and stable without too much of either.
A tandem with less fork offset (more trail) may feel more stable for the
first couple miles, but just a little less trail is easier to handle and
more maneuverable after you have spent a few hours with the bike, no matter
how "squid like" your stoker is. Some may disagree with me, but I
consistently hear that our tandems feel very light and stable. OK, let the
controversy begin.

Todd Shusterman
da Vinci Designs
Denver, Colorado


 
From Andy Dyson at Bilenky.....
Hi all,=20

Sooner or later we had to get into the fray on this one, though there=
probably isn't much of a fray, at least in terms of advancing our thinking=
and knowledge on this subject as a species. A lot of it was spelled out a=
long time ago - my copy of the reprint of "Bicycles and Tricycles, an=
Elementary Treatise on their Design and Construction" by Archibald Sharp,=
published over a century ago in 1896 has more to say on the subject than=
anyone wants to read here in a posting. It is also a mind-blowing treasure=
trove and recommended bed time reading - but that's a whole 'nother story. =
The disc wheels and suspension bikes keep me smilin' as much as " Simpsons"=
repeats (US: re-runs). And while I'm stringing along this chain of=
consciousness there was a tandem spotted in Springfield this week.

Anyway, Jon said:

>I have read that Bilenkys and Santanas, to name two, handle in a laid
back, stable (yawn, yawn) manner. =20

I think this is praise, though we'd like to point out that we can and do=
make frames with differing steering geometry depending on what we perceive=
the riders' preferences and needs to be. I'd be surprised if any custom=
builders didn't do this. This amounts to only very small changes, though.

Louise & Philip (who have a cool web site) said:
> Lighter, less stiff frames would not have been as easy to control.

Good point, echoed by Dwan S. The steering geometry isn't worth much if the=
fork moves about and the frame twists.

What has been established in this discussion is that trail depends on fork=
rake (offset) and head angle. It also depends on wheel size - a lot. =
That's why our 26" wheel bikes have different front ends to the 700c. =
Here's the Steve Bilenky take: assuming we're talking about bikes with the=
same wheel size it follows that you could have front ends with the same=
trail but different head angles, as long as they had different rakes, too. =
Would all these bikes handle the same? The handling we are most=
interested in optimising is when the bike is moving pretty fast. If you=
have more trail the bike takes more force to move off of a straight line,=
so at high speed the bike is more stable. Less trail means that it will=
come off of a straight line more easily. But.. once the bike is leaned,=
steeper head angles have a quicker steering response, part of the reason=
track riders like 'em steep. Slacker head angles require more movement of=
the handlebars once the bike has leaned, which may seem slower to some=
people. So we're looking for a balance between trail and head angle for=
our sweetest handling tandem, not just a particular trail value.

>>Glenn Erickson wrote "It should also be noted that most of the terms that=
=20
>>Dwan and Todd used were subjective, neither one of them stated what trail=
=20
>>they use on their tandems."

Well, anyone who has purchased a tandem from Bilenky in the last decade=
either did or could have had a print-out of the geometry, so they aint a=
secret. As I said above, they do also vary. It is safe to say that most=
are the same in the front end, though we never assume that a team want a=
standard geometry when we get an order:

Rake 2.25" 5.7cm
Head angle 73.5 deg.
Trail 1.65" 4.2cm

This is for road bikes with a 700c wheel with an outside tyre diameter of=
27". A tandem like this handles great at all speeds - that's my best shot=
at a subjective term! =20

Dwan Shepard mentioned "traditional tandem geometry" If this survey pans=
out, I guess we're going to find out if there is such a thing, and who's=
bikes are it! For my part, I ride as many tandems as I can and have yet to=
ride one that I like more than my personal bike (Bilenky), which has a 74=
deg head and 2 1/8" rake, 700c wheel. My favorite 26" road bike to come=
out of this shop was an S&S tandem called "Priscilla" which has been ridden=
around the world. It happened to fit me perfectly as a captain, and when I=
get around to ordering a personal S&S tandem it will be a duplicate of=
Priscilla unless I ride something sweeter in the mean time. Priscilla has=
a 72 degree head angle, 2" rake, which gives a 1.96" trail with a 25"=
actual diameter tyre. I tested this bike with Ritchey Cross tyres that=
have a smaller diameter, so that would mean a little less trail. I have=
yet to establish who Priscilla was named after.

In 1896 Archibald Sharp wrote of tandem design " A great variety of frames=
are in use at the present, the processes of natural selection not having=
gone on for such a long time as in the case with frames for single=
machines" Thankfully, I think this is still true today. We have tandems=
with two or three wheels, recumbent, semi recumbent, upright, dozens of=
frame tube configurations most of which work pretty well and several of=
which are excellent, functional folding and take-apart designs and an=
increasing number of frame materials, geometries and suspension systems. =
As far as I can tell the only things that have become extinct are=
components that don't work and tubing that's not stiff enough. Diversity=
is increasing because the number of niches has grown. If we had this=
diversity 100 years ago, and we have all the variety I see today, then the=
tradition of tandem design is one of constant innovation.

Best to all,

Andy Dyson.
***** Bilenky Cycle Works - Artistry in Steel *****
Home of the world's finest fillet-brazed bicyles,
solo and tandem, touring, recumbent and racing=20
http://www.bilenky.com
artistry@bilenky.com



 
Dwan Shepard at Co-Motion Weighs In
Hiya,

Okay, Dick Powell, we'll take a bite on this thread! We have gotten a lot
of mileage out of the phrase "handles like a single" over the years. But how
many singles handle alike? Not many. In the single-bike world you'll also
find enormously different philosophies applied to bikes which you might
expect to be relatively generic. For that reason, it would be inaccurate to
say that our tandem geometry is taken directly from single bikes.

Glenn Erickson says quicker steering is a simple matter of increasing
trail. We believe this creates a false impression that more trail = good,
less trail = bad handling. It just ain't that simple. Good handling is a
delicate balance of well applied materials, good fit and appropriate
geometry for the job at hand. I recently read an article by Lennard Zinn
(Velo News) equating short trail with quick handling and long trail with
stability! Another piece, written by Seven's Rob Vandermark (7 Cycles'
newsletter) makes a more accurate case. Rob equates long trail with
high-speed stability and short trail with low-speed stability. That's a
theory I can agree with! It makes sense then, that in order to create a
bike that handles well at all or most speeds is a matter of finding a happy
medium between the extremes.

A bike with lots of trail is more susceptible to lean input as a means of
steering control. As Glenn suggests, too much of this tends to cause an
overabundance of necessary steering corrections. Remember, a major factor in
bicycle steering is the person who is responsible for controlling the bike.
A captain accustomed to a long-trail road bike might not be able to handle a
"laid back" tandem at all, and the reverse is also true. The trouble with
too much trail is that a turn is easy to initiate with a little lean, but
can be more difficult to get back upright, especially with a heavy couple on
a flimsy tandem. If you've ever tried to steer a leaned-over bike away from
a hazard in its path, you know what I mean.

The so-called "laid back", or short-trail tandems remain more upright when
steered, but are more resistant to changing direction. This what causes the
skate effect when attempting high speed maneuvers. In extreme examples, you
can actually hear the front tire scrubbing noisily against the pavement
while attempting a turn. Purposely leaning the bike improves its ability to
get through high speed turns, but leaning the bike won't initiate
directional change as easily as with a longer-trail bike, making such
maneuvers awkward. The "laid-back" geometry remains popular in part because
it instills a sense of stability at a time when most tandemists are taking
their first baby steps: the initial test ride, which is usually too short
and straight to reveal enough information!

Naturally, we have our own opinions about how much trail is appropriate
for a nice, precise-handling Co-Motion tandem. We have consistently received
praise for the balanced handling character of our bikes, and it's no
accident. We design our tandems for what we call neutral handling, a fine
balance between the extremes. A neutral handling bike responds easily,
without demanding constant steering corrections. At the same time, a neutral
handling tandem will not resist your input, resulting in a natural,
intuitive feel at high or low speeds. For people accustomed to traditional
tandem geometry, this approach feels remarkably different, but a quick
no-hands test and a few sharp turns put most people at ease right away. It's
quite an eye-opener to learn that all-'round stability and responsive
handling are not opposing factors at all.

I'd like to mention three other factors directly related a tandem's
handling character beyond head angle and fork rake. First, the position of
the driver: The steering axis should line up reasonably well with the
shoulders, while the rider's upper body weight is easily carried without
strain. Placing the driver too far ahead of or behind the steering axis
dramatically changes the movement required to get the body into position for
a turn. Good positioning demands proportional frame/stem sizing, and helps
make bike steering feel more intuitive. Second, the fork must be rigid
enough to inspire confidence during a hard lean, while retaining its ability
to absorb impacts as passive suspension. Massive fork blades look
impressive, but they can detract from this extremely vital aspect of the
fork's job. A fork with little lateral strength and zero linear or vertical
compliance is the worst possible combination, making you feel perfectly
confident until you attempt a difficult maneuver such as a winding, high
speed mountain descent. Third is the overall character of the frame, which
should be rigid enough laterally to enhance good teamwork while retaining
enough vertical compliance to allow both riders enough comfort to remain
seated and keep both tires in contact with the road.

I agree with Glenn and Todd that Jon and Joan primarily have their
hard-earned skills to thank for their success at staying upright. They
should also be thankful that they didn't have a brake setup that wouldn't
have allowed separate rear brake function! Certainly, riding a good quality
tandem was a factor here, too. A good combination of the right skills
applied to the individual characteristics of the team and the tandem they
ride makes for safe handling.

Whose geometry is best? Only you can decide, since there are advantages and
drawbacks to every philosophy applied to the current crop of tandems. If you
are comparing tandems, try some different handling techniques with each one
until you find a tandem that handles in a way that best serves your skills.

Ride together!

Dwan Shepard
Co-Motion Cycles Inc.
222 Polk St.
Eugene, OR 97402
http://www.co-motion.com
tel. 541-342-4583
fax 541-342-2210

 
Glenn Erickson expounds on early comments and those of others....
In reference to both Dwan's and Todd's postings, ti is important to remember
that I said that there is no right or wrong geometry, only different. As
builders we try to best fit our customers, different customers different
geometries. It should also be noted that most of the terms that Dwan and Todd
used were subjective, neither one of them stated what trail they use on their
tandems. We can all say different subjective things leading everyone to think
that we use different trails and that one is "best". Actually I think
Comotion and I use very similar trails, maybe this is because we build for
similar customers, or maybe it is just coincidence, but most important is that
the bikes both handle well as our customers have told us. ZIf our customers
had told us the tandems handled like "merde" we would probably change the
handling.

Bike handling is very complicated to understand, what words do we use, no
reference books to tell us the right awnser, different for different people
and so on.
It is also far more simple than we have lead you to believe, every one of you
can get on a proper fitting tandem from any of the builders and ride it just
fine. One might feel a little different than the other but no big deal, we
are all in the same ball park. The major handling criteria is the tandem
team.

I like the high speed stability of more trail, plus the quick responce once a
turn is initiated. Here goes, with experienced tandem teams I like to use a
trail of 58 mm or about 2 1/3 inches. This is considerable more than anything
mentioned so far. I like this steering feel and my customers seem to love it.
Is it better than other tandems, I dont't know, but it certainly is different.
This is similar to many single bikes, and I have to say my own tandem(s)
handle like my single bikes and this seems to work well for me.

I was afraid that if we start listing trails that it would soon become an
argument over which trail is best, hopefully this will not happen.

Glenn Erickson


And the last word; Dwan at Co-Motion sums it all up....
Dear folks,

Glenn Erickson said:
"in reference to both Dwan's and Todd's postings, ti is important to
remember that I said that there is no right or wrong geometry, only
different."

I agree with Glenn. I also should have noted that every builder has his or
her own way of trying to achieve the kind of handling balance which they
feel is most appropriate. There's no right or wrong, at least most of the
time, "just different" as Glenn says.

Dwan Shepard
Co-Motion Cycles Inc.
222 Polk St.
Eugene, OR 97402
http://www.co-motion.com
tel. 541-342-4583
fax 541-342-2210