Another indexed steering thread

[Zeeexsixare]

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?

--
Phil, Squid-in-Training

 

[Mark South]

"ZeeExSixAre" <[email protected]> wrote in message
news:F%[email protected]...
> 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

 

[Jim Beam]

ZeeExSixAre wrote:

> 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.

 

[Zeeexsixare]

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

Oh.

> 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.

> 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.

> 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

 

[Jim Beam]

ZeeExSixAre wrote: <snip>

> 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.