When creating the triangle for the head angle, you'd
want to use a small right-angle triangle and tape the
edge of the long leg of the right angle to the edge of
a ruler. You'd then line up this long edge with the
top tube centerline and when the legs of the right
angle intersect the head-tube centerline, you then
measure these legs. With the given 90 degree angle of
the triangle and the two "legs" or sides, you'd then
solve the triangle. From both triangles created from
the seat tube and head tube, you'd solve for the
smaller acute angles and then subract from 90 degrees
to get seat and head tube angles. I've never done this
before; I wish I had a bike with known specs to
compare my calculations to.
Joe Starck,
masidon, wi
> --- Chuck Schmidt <chuckschmidt@earthlink.net>
> wrote:
>
> > Measuring:
> > For fork rake I have a plexiglas gage called a
> "Gabe
> > Meter" made in
> > Germany in the early 1980s that has a v-block
> (with
> > magnets) on the back
> > that attaches at the top of the fork leg that has
> a
> > scale you read at
> > the pointed end of the QR.
> >
> > For head angle and seat angle I use a 4' long
> metal
> > straight edge along
> > top tube and a draftsman's 14" adjustable triangle
> > that has a degree
> > scale on it to measure the angle.
> >
> > I think this is much more accurate than one of
> those
> > angle finders that
> > relies on leveling the top tube first (fluid
> filled,
> > with floating pointer?).
> >
> > I also use the 4' metal straight edge and a
> vernier
> > caliper to measure
> > the BB drop.
> >
> > Anyone have a better method to measure fork rake,
> > head and seat angle?
>
> Lemme ease into this question. At Masi, we had some
> tools to make this task easy. We had two, machined,
> surrogate axles, one for the fork, one for the rear,
> that were attached to two heavy bases; these two
> fixtures were about 10" tall of identical height.
> The
> frame and fork(with dummy headset) were clamped into
> these fixtures placed on a granite surface plate to
> ready the frame for measurement. We then used a
> shop-built height guage to check that the top tube
> was
> level, and then measured the seat and head angles
> with
> a machinist's adjustable bevel protractor(Mitutoya).
> A
> digital protractor, of course, is the most accurate,
> and it's fun to use, as long as you don't drop it on
> concrete. The BB drop was measured by subtracting BB
> height from axle height.
>
> To measure specs on a complete bicycle, at home, I
> figure you, Chuck, have a flat table of sort. You'd
> check the flatness with a straight edge or meter
> stick. And then you'd have to keep the bicycle
> upright, by having another hold it, or by some other
> method, even boxes of bicycle parts book-ending each
> side of the wheels, or maybe have the bike hang from
> hooks in the ceiling with two lengths of string,
> whatever. Or maybe you just lean the bike against a
> wall. Your measuring methods seem accurate to me,
> but
> I'd have to see.
>
> But OK, lets say you've got a table or floor that's
> flat and level with the world. Irregardless of top
> tube, your angles are whatever they are, relative to
> the two wheels on a flat surface. You can then
> measure
> your angles with a right-angle triangle, a plum-bob
> and some trig-o-nometry. For the seat angle, measure
> from the center of the BB to the intersection of the
> top and seat tubes. That's one side of the triangle.
> Then, drop a plum bob from the top tube to the
> center
> of the BB. The distance from the string, along the
> top
> tube, to the intersection of the top and seat tubes
> is
> the second side. So, you've got two sides and one
> angle(the 90 degree one). And I'm pretty sure that's
> all you need to then calculate the rest with that
> sine
> and cosine stuff. I can't calculate it right now. I
> forgot how to do it. If you Chuck, or nobody else
> has
> the formulas, I'll look it up later. For the head
> angle, it gets interesting. Line the edge of the
> long
> side the right angle of your right-angle triangle
> along the center of the top tube, with the
> right-angle
> facing forward and short side aimed at the ground.
> Slide the triangle forward or back until the edge of
> the short side intersects the center of the head
> tube.
> You've now formed a triangle that possesses your
> head
> angle. You can measure three sides and you already
> have the right angle. Time to calculate with trig'
> formulas.
>
> I measure BB drop by subtracting BB height from
> wheel
> radius. (You have to use that square-with-the-world
> table or floor or whatever.)
>
> I'm not sure about your fork measuring methods. Fork
> offset is measured relative to the steerer, not the
> blade. Some blades cant forward or backward, and
> then
> there's the differing radii along the blade, so if
> your tool allows it Chuck, I'd see about settin'
> that
> vee-block on the steerer and then measuring out,
> taking care that the "measuring out" is square and
> all
> with the steerer. Here's an accurate way to measure
> an
> individual fork; this is the way I used to do it:
> With
> a vise or fixture secured to a table to hold the
> steerer, you clamp the fork such that the virtual
> axle
> is vertical to the ground. You can use a pencil or
> something as a surrogate axle. Mark this axle center
> on a piece of tape. Then flip the fork 180 degrees
> and
> mark the axle center again. Measure the distance
> between marks and divide by two.
>
> As to those protractors with the floating needles.
> The
> better ones are more heavily weighted and have less
> friction than the cheap ones. I dont think these are
> the fluid-filled ones, I'm thinking of. If you
> rotate
> it and the needle doesn't "stick," then it should be
> fairly accurate, assuming it's indexed properly, but
> that can be checked on a flat surface.
>
> As for my Trig' methods above, I wish I could send a
> drawing, or demonstrate it; It'd be clear to anyone
> in
> five minutes. I hope my written explanation is
> understandable.
>
> Joe "fun with trig'" Starck,
> masidon, wi
>
>
>
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