"Chris Neary" <
[email protected] > wrote in message
news:
[email protected]...
> >If we look at concrete examples, say a current Cannondale touring frame,
> >and a similar size racing frame (T2000 vs. R1000, size J vs. 63cm) we
see
> >the following:
> >
> >HT: racer = 73.5 deg, tourer = 73
> >rake: racer = 4.5cm, tourer = 5.3
> >ST: racer = 72 deg, tourer = 72.5
> >TT: racer = 60cm, tourer = 59.7
> >
> >WB: racer = 101.2, tourer = 108
> >CS: racer = 40.8, tourer = 45.7
> >
> >Now, your contention is that there is significant difference in trail,
is
> >0.7cm (0.28") significant?
>
> The useable range of trail is narrow, John Schubert lists a recommended
> range for a touring bike of 5 to 5.75cm
> (http://www.adventurecycling.org/features/buyersguide.cfm),
The sum total of what he has to say on the subject:
"Head tube angle
In racing circles, head tube angle is sometimes made ridiculously steep (75
degrees or more). On your bike, it will probably be 72 or 73 degrees, less
if you have a small frame. There is usually little to shop for in head tube
angle. "
"Fork rake
Rake is joined at the hip with head tube angle. Together, these two
dimensions determine trail, which determines the bike's steering stability.
"
"Trail
You want about 2 to 2-1/4 inches of trail. "
He also has the following gems, which I think are very wrong:
" It has been a long-held belief that touring bikes should have long
chainstays, and indeed, if you're carrying extremely heavy loads, you'll be
better served by the longest-possible chainstays. But my opinion is that
this dimension is overrated. "
" A lower bottom bracket makes a bike more stable. "
"Wheelbase is the sum of other dimensions, and is not an important
dimension in and of itself "
All that's getting off track, but I don't find him particularly
informative, or accurate.
> while John
> Forester complied values ranging from 5cm (Holdsworth Touring bike)
> to 7cm (Paramount track bike)
> http://www.johnforester.com/Articles/BicycleEng/dahon.htm (I've thrown
out
> the more extreme examples at either end of his listing), so yes, a 0.7cm
> change in trail should be significant.
To quote JF:
" increasing the trail increases the stability and decreasing the trail
decreases the stability. Therefore, when a design shows an improper amount
of stability, we know that the first corrective measure is to change the
amount of trail. The easiest way to do this is to change the fork rake,
although the designer may choose to change the head angle or, conceivably,
the wheel size. "
" Because steering angles and wheel sizes are pretty generally rather
similar, different bicycles have similar trail distances. Trails run
between 1-3/8" and 2-3/4". In general, racing bicycles have long trails
while utility bicycles have short trails. The difference is that racing
bicycles require great stability and are ridden by well-informed and
skilled cyclists, "
And in another section:
"Corbett, a professor of mathematics and bicycle frame builder, built an
experimental bicycle whose front fork could be adjusted to provide trail
from -7/8" to +4-5/16".
"He found that the steering characteristics fit into the pattern described
above, i.e., with trail in the low forties [approx. 1-5/8"] the bike felt
nervous, with a trail of 55 mm [1-3/16"] it had the sort of hands-off
stability which seems desirable yet still turns easily, and with a trail of
74mm [2-15/16"] it was very heavy feeling." (Chris Kvale, A Fresh Look At
Steering Geometry, Cycling U.S.A., Feb 1981)"
> The only personal experience I can offer is comparing a Santana Sovereign
> (4.5cm trail) to a Co-Motion Speedster (5.7cm) (both tandems). The bikes
> handled very differently, with the Santana requiring large inputs to
steer
> while the Co-Mo required much smaller inputs, yet remained stable. I felt
> like I was driving an oil tanker on the Santana, while the Co-Mo behaved
> like a (long) single.
Your experience doesn't agree with JF's fundamentals. The long trail bike
should be more stable, requiring larger forces to steer.
> > I think not, and would also point out that's a fork dimension, not
frame, FWIW.
>
> No, trail is a *FRAME* dimension, since it is determined by both the fork
> rake and head tube angle. You cannot look up a fork spec alone and know
what
> the trail will be.
>
> On second thought, it is actually a dimension applicable only to a
complete
> bicycle, since the tire size will also effect the value. Still not a fork
> dimension, FWIW.
That was my point, echo'ed by the JF quote, that trail is the important
dimension and that it can be changed with fork selection.
> >The same observation applies to the HT angle, is 0.5 deg really
significant? I don't think so.
>
> I'm oversimplifying, but it's the equivalent of 0.2cm change in trail in
the
> example we're discussing, FWIW.
> REF: http://www.kreuzotter.de/english/elenk.htm
Plugging the Cannondale numbers into the calculator, I get 2.6cm trail for
the "race" bike, and 2.0cm for the tour bike, using the nominal wheel
radius. That makes the race bike more stable, as JF predicted. Changing to
much larger tires might increase the touring trail to perhaps 2.3cm,
according to the calculator.
> >The only significant difference between the 2 frame geometries is the
> >chainstay & wheelbase length. Does that make a bike more "stable"? It
does
> >make it harder to wheelie.
>
> Sheldon sez it promotes stability, he also believes touring bikes are
more
> stable due to their relaxed front end geometry. Maybe you wish to take up
> this discussion with him?
> REF: http://sheldonbrown.com/gloss_ta-o.html#touringbike
I have had several discussions with Sheldon on the subject since he is my
LBS, and did frame selection and setup on several of my bikes. He doesn't
expand on his claim of stability in his glossary, and I don't want to put
words in his mouth. He may be speaking to weight distribution, not to
steering response, though.
> >How this could conceivably affect the ability to "keep up" on a downhill
> >road continues to baffle me.
>
> There's a lot we don't know about the original poster's problem keeping
up
> on downhills, that's why I made a point of saying *Maybe*.
>
> If we are talking about descents on straight roads, you are correct that
> steering geometry will have *NO* effect on the ability to keep up. The
> important factors will be: weight, aerodynamics, and losses from items
such
> as the large, low pressure, tires the poster mentioned.
>
> OTOH, if we throw a few curves into this downhill, then rider skill and
bike
> geometry rise in importance. I've made my point on bike geometry. We have
no
> idea what skills the original poster possesses or how they compare to his
> fellow riders.
I think your point on geometry is mistaken, as demonstrated by your own
references. It still remains to be proven how stability in steering can
affect the ability to "keep up" (your original claim) on downhill courses.
You imply that it has to do with cornering speed. I have no argument with
the part rider skill plays in this, but that is not related to frame
geometry. Jobst Brandt has often described the importance of rider skill in
fast, curving, descents.
It is commonly claimed that touring bikes handle like trucks, while racing
bikes handle like sports cars. This is just so much misinformation, the
kind of BS that's slung around to make artificial distinctions between
bicycle models. Changing stem lengths also affects steering response, yet
doesn't seem to change sports cars into trucks. Ridiculously short
chainstays are strictly a fashion statement that negatively affects weight
distribution, tire clearance and chain angle, yet are required to make a
bike look "racey". Bikes in modern form have been around for about a
century, all this has been long worked out. Form follows function, at least
until the marketing department gets involved.
