In article <[email hidden]>,
Quoted message said:Tim McNamara said:In article <[email hidden]>,
Quoted message said:Hi All,
I am making some forward progress on my build-my-own-frame
project. I am now working on the design. Here are my
thoughts/desires that have dictated the design so far:
Fixed gear commuter/fun bike 175mm cranks manouverable, yet
stable at speed, tracks straight, easy to ride no hands comfort
comes from tire and saddle choice I don't know if I have long
femurs or what, but I like the saddle WAY back.
Here are the specs:
61cm c-t seat-tube 58cm c-c top-tube 75deg head tube 73.5deg
seat-tube 28mm rake (gives trail of about 6cm depending upon tire
size) 5cm BB drop 405mm (minimum, but dropuouts are horizontal so
some range here) chainstays clearance for 28mm tires
So what seems strange given my "brief"?
The overly steep head tube and long trail combined with the short
chainstays. The wheelbase will be short and you may have toe clip
overlap despite the large frame. This bike will probably be overly
sensitive to leaning and difficult to ride no-handed at low speeds.
In turns it will have an inclination to cut towards the apex and
it will be difficult to change lines to avoid obstacles on the
road. Basically you've gone with track geometry and fattish tires.
I've never had a bike that didn't have toe overlap, so that doesn't
bother me.
Wow, we are not all that different in height, I suspect, and I have
rarely had bikes with TCO. I've got some on my 61 cm Ritchey, but never
had any on my Raleighs or my Bianchis from my racing days. However, the
Ritchey handles far better than any of my other race bikes. That guy
knows geometry (and how to fillet braze. Amazingly beautiful fillets).
Quoted message said:Overly sensitive to leaning does not sound good. Some
bikes I have tried are such that a quick glance back over the
shoulder and the bike wiggles all over the place. Others (like my
Pinarello road bike) track like a freight train. That's what I want.
A bike that can ride on the white stripe as long as I want. But I
want to be able to avoid obstacles (potholes and fallen riders) if
need be. This bike will never see north of 50kmh, and if so it will
be on a straight road. Mostly the speed will be 30-40kmh.
Track bike steering geometry is designed to go in a straight line on a
very smooth surface. Track racers never really corner, the banking of
the track takes care of most of the radius of the turn. I used to race
on the Blaine velodrome for a couple of years (until I crashed badly and
separated my shoulder, at which point I realized that at 40 years old
bike racing could cost me a whole lot of money if it laid me up for a
few weeks). After I stopped racing I rode my old track bike on the road
for a few years, it was kind of fun but the ride was very harsh due to
the short wheelbase. The track bike was not as agile as a good road
bike BUT let's face it, for the vast majority of riding situations it's
not a big deal.
The old randonneurs designed bikes that could run down the fog line all
day and all night and do it again the next day, even when the rider was
utterly exhausted, and yet nimble enough for fast mountain descents.
The steering geometry doesn't seem to make any sense to modern thinking,
though, such as 73 degree head angle with 60 mm rake resulting in trail
of 40-45 mm or lower. We would expect that to be terribly unstable, in
large part due to Jones's URB experiments that suggested stability is
dependent on trail. There are clearly more factors than that.
One possible explanation is that randonneurs use wider tires at lower
pressure. Jim Papadopalous has proposed the idea of "pneumatic trail"
suggesting that the wider softer tire increases stability. I have not
ridden and ultra-low trail bike myself, so I can't comment on the ride
quality. But I have bikes with trail in the 50-55 mm range and they
ride well. I think that a longer wheelbase is necessary with low-trail
geometries. Also, I have read that low-trail geometries are very
sensitive to alignment issues.
Quoted message said:Quoted message said:I'd drop the head angle back to 73 or 73.5, with a fork rake of
45-50 mm to give a trail of 50-55 mm with chainstays of 42 cm or
even longer. The steering will feel lighter and the longer
wheelbase will give it stability. It will be agile and stable at
the same time. You'll be able to change lines in the middle of
turns easier and it will tend to turn at a constant radius rather
than dropping towards the apex.
This bike will not be doing any cornering. Any cornering is done so
slow it makes no difference how the bike feels. Basically straight
line cruising, and obstacle avoidance.
Obstacle avoidance is changing lines. So you want nimbleness. And yet
you also want straight line stability to zip down the edge of the road.
Hence my suggestion for a fairly low trail front end and a longer
wheelbase to go with the fairly fat tires you're planning on using. I
think you'd get the combination of features you're looking for.
IIRC the old French "bike boom" bikes of the 70s used this sort of
geometry- large fork offset, low trail, longish chainstays. Maybe you
can find an old PX-10 to take for a spin on your regular route and see
what you think about how it handles.
Quoted message said:Quoted message said:I'd also drop the BB if you're not planning on riding this on a
velodrome. Actually, I'd recommend dropping the BB 2 cm and using
165 cranks, since it's a fixed gear. You'll need to be able to
spin at 120 rpm or more and that's harder to do with long cranks,
even at your height.
It probably won't ever see a velodrome, but I don't want to preclude
any visits. I have 3 track cranks now (170, 165, 172.5) that I'd like
to experiment with, so it should be able to accomodate them. What
will lowering the BB accomplish?
The higher the BB, the higher your center of gravity. The higher your
center of gravity, the harder it is to lean your bike into and out of
turns. Try balancing a 1 foot long stick on your palm and then a 3 foot
long stick to get the idea. The lower BB makes the bike more
maneuverable. IMHO anyway. Probably some smart person knows how to
build bikes with high BBs that are very maneuverable and just knows
something I don't.
Quoted message said:Quoted message said:I built my own frame along the lines I described, but with an even
lower BB (8 cm drop) because it's a single speed rather than a
fixed. It rides and handles very nicely.
http://www.frostybobs.org/bikebuild.html
I've been there more than once! Thanks for sharing the info.
Sure! It was a fascinating experience and I really enjoy riding a bike
that I built.
