Another indexed steering thread

"ZeeExSixAre" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

I've been making observations lately between two of my bikes: my BMX bike
and my road bike.

Whenever I rode my road bike (100psi), I would look down and see the wild
fore-and-aft deflections of the fork dropouts in relation to me over bumpy
pavement. The deflections were probably up to a 1/2 inch. I naturally
attributed this to the fork's lighter steel construction and relatively thin
blades.

After I got my BMX bike, I was riding it home (60psi) when I looked down and
saw the fork doing the exact same thing! This is on a bike that has legs
that are about 4 inches shorter, has fork blades that are 1 1/4" in diameter
and a wall thickness of 6mm!

The fork fore-aft motion can't possibly be attributed to much else other
than the headset, unless the fork is bending that much at the crown.

In relation to the indexed steering thread, does this mean that the bearings
are being pushed into the races enough to cause the deflection, and since it
does not hit the yielding stress of the materials, the bearings bounce back
with no deformation?

Did you allow for the fact that your eyeballs are not rigid?
--
Mark South: World Citizen, Net Denizen

ZeeExSixAre said:

I've been making observations lately between two of my bikes: my BMX bike
and my road bike.

Whenever I rode my road bike (100psi), I would look down and see the wild
fore-and-aft deflections of the fork dropouts in relation to me over bumpy
pavement. The deflections were probably up to a 1/2 inch. I naturally
attributed this to the fork's lighter steel construction and relatively thin
blades.

After I got my BMX bike, I was riding it home (60psi) when I looked down and
saw the fork doing the exact same thing! This is on a bike that has legs
that are about 4 inches shorter, has fork blades that are 1 1/4" in diameter
and a wall thickness of 6mm!

The fork fore-aft motion can't possibly be attributed to much else other
than the headset, unless the fork is bending that much at the crown.

In relation to the indexed steering thread, does this mean that the bearings
are being pushed into the races enough to cause the deflection, and since it
does not hit the yielding stress of the materials, the bearings bounce back
with no deformation?

almost all the flex you describe /is/ the frame [and fork], not the head
set.

all "diamond" frames flex in plane about the head tube, even sturdy ones
like bmx. even tho it's called a diamond frame, the tip of the diamond
is usually truncated by the head tube, so you don't get perfect
triangulation. if you built a straight parallelogram from erector set,
you'd have free play about the axes. if you built a triangle, it would
be as rigid as the materials could stand. if you built a truncated
diamond like a frame - a hybrid between the parallelogram & the
triangle, you'd get minimal play about the "seat tube", and exaggerated
play about the "head tube".

the head tube exaggeration can be minimized by making the tubes and
their joins at this point as rigid as possible by using big thick tube
ends, and in the case of giant mtb frames, even flare the end of the
tube to make the contact with the head tube even larger then normal, but
the tendency is still there.

regarding the bearings, yes, they deform elastically under load, but
their deformation is measured in microns, not halves of inches.

Quoted message said:

almost all the flex you describe /is/ the frame [and fork], not the
head set.

Oh.

Quoted message said:

all "diamond" frames flex in plane about the head tube, even sturdy
ones like bmx. even tho it's called a diamond frame, the tip of the
diamond is usually truncated by the head tube, so you don't get
perfect triangulation. if you built a straight parallelogram from
erector set, you'd have free play about the axes. if you built a
triangle, it would be as rigid as the materials could stand. if you
built a truncated diamond like a frame - a hybrid between the
parallelogram & the triangle, you'd get minimal play about the "seat
tube", and exaggerated play about the "head tube".

Yes... this is likely, as the frame size for this BMX bike is about 10.5"
and the top tube is 20.5". The top and down tubes are about 2 inches in
outer diameter.

Quoted message said:

the head tube exaggeration can be minimized by making the tubes and
their joins at this point as rigid as possible by using big thick tube
ends, and in the case of giant mtb frames, even flare the end of the
tube to make the contact with the head tube even larger then normal,
but the tendency is still there.

My Giant XTC has this Fluidform applied to the downtube and top tube...
looks cool but dunno if it makes as big of a diference as they claim.

Quoted message said:

regarding the bearings, yes, they deform elastically under load, but
their deformation is measured in microns, not halves of inches.

Okay, so if this is true, then that means the frame, around the headtube, is
deflecting a degree or two, enough to cause the 1/2" deflections at the
dropout, and fork flex is minimal in comparison?

--
Phil, Squid-in-Training

ZeeExSixAre wrote:
<snip>

Quoted message said:

Okay, so if this is true, then that means the frame, around the headtube, is
deflecting a degree or two, enough to cause the 1/2" deflections at the
dropout, and fork flex is minimal in comparison?

no, forks flex also. the fork crown area tends to be quite rigid, but
the blades have flex as does the steerer tube.