Re: [CR]Now: 1960 Cinelli SC measurements Was: 1960s Masi measurements

(Example: Framebuilders:Brian Baylis)

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Date: Tue, 1 Feb 2005 13:49:37 -0800 (PST)
From: "Joe Starck" <josephbstarck@yahoo.com>
Subject: Re: [CR]Now: 1960 Cinelli SC measurements Was: 1960s Masi measurements
To: classicrendezvous@bikelist.org
In-Reply-To: <41F96A3A.454898D4@earthlink.net>
cc: chuckschmidt@earthlink.net

--- 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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