Frame geometry - effects, impressions, rules of thumb?

[Frank Krygowski]

So, as I've stated many times, I don't have much of a connoisseur
mentality. In particular, I don't seem to perceive the minute
differences in riding characteristics that magazine road testers claim
to perceive. In fact, I'm pretty sure most road testers would fail any
double-blind test of similar bike frames.

But at the same time, I'm sure that frame dimensions or geometry do
affect riding properties. I've ridden old Schwinn "ten speeds" that
were as stable (for no-hands riding) as railroad cars. And many years
ago, I rode a brand new low-end Japanese road bike (maybe Fuji?) that a
friend bought for his wife. It was so twitchy she couldn't ride it.

I'm sure that longer wheelbase means slower response, other things being
equal. And of course, on touring bikes, longer chainstays give heel
clearance for rear panniers. Cross bikes and crit bikes tend to have
higher bottom brackets for obstacle or pedal-to-ground clearance.

But what does the group know about the effect of things like front end
"trail"? Some people seem to like more, some less. How about top tube
length? If a longer top tube is used (maybe to avoid wheel-to-toe
overlap) and a shorter stem is used to compensate, how does that affect
handling? If trail is held constant, what does a lower steering angle
achieve? How about a lower bottom bracket on a touring bike? Does that
do anything beyond lowering standover height? Some have claimed low BB
adds stability - yet an "ordinary" or tall bike is far easier to balance
than a recumbent.

Care to discuss?

--
- Frank Krygowski

[John B.]

Well, a longer wheel base, as you say, usually makes for a more stable
bike, increasing Trail seems to slow down handling and bottom bracket
height also effects handling by effecting the CG.

There is a document - "A FRESH LOOK AT STEERING GEOMETRY
by Chris Kvale" - which goes into quite a bit of detail.

One of the problems is that everything is interrelated. A lower BB
height increases stability but may be a determent for track and cross
country. A longer top tube and a shorter head tube, to some extent,
effect the strength and flexibility of the frame.

Trail, while the easiest to change has some unexpected effects. In the
study I mention the writer describes a Cinelli track frame with 68mm
trail. He described it as:

"extraordinarily stable no-hands, but was very heavy feeling in the
corners, seeming to require actual physical steering rather than mere
leaning. It is important to note that this bike handles perfectly in
its event - steady track time trialing, the very long trail making it
easy to stay right on the pole line without wandering".

Another point. the head tube length is largely determined by the front
wheel size, seat tube length, top tube length and slope. A short seat
tube with a level top tube, for a rider with long arms, with a 300C
wheel, would result in a shorter, then perhaps ideal, head tube.

I believe that the rider also enters into the equation as an
experienced rider may be quite happy with a faster handling bike than
a "newbe" and generally, a stage race bike is more stable then a
criterion bike as the "frisky" crit bike is tiring to ride for long
periods and I would guess that the TT or Iron Man bike would be
similar.

I wonder if there has been a comparison of the bikes used in the RAAM.
I know that they use aero bars but I also read that they adjust them
for comfort rather than minimum wind resistance.
--
cheers,

John B.

[Tosspot]

This is the best explanation I found;

http://www.cyclingabout.com/understanding-bicycle-frame-geometry/

Adding longer stems to short top tubes works, ime, for 5cm or so
extensions, after that it feels very odd in the steering department.

[avag...@gmail.com]

you would 'get over it'


as previously noted, muh Raleigh sports tourer...a fine balanced ride...

was to nervous for sand skim berms alternating with loose asphalt good asphalt corse sand....

I wuz working too hard on 1 3/8th's 3 rib kendas

a donor gave me a same model mixte double tube bike

Measuring n eyeballing the woman's fork seemed to say the trail was greater.

by an 1/8th"

the difference calmed the balanced frame down immediately n off I rode into the sunset happy as a clam