Fork Rake and Stuff
 

There's an interesting thread which started with measuring rake to talking about steering/handling/trail. Andy


https://www.bikeforums.net/bicycle-m...l#post20180356

FWIW, VAR used to make a tool to directly measure trail on a fully-assembled bike. Probably wouldn't work on those new-fangled straight-blade forks, though:

http://www.sheldonbrown.com/var/images/var0005.jpg

How does the tool know the angle and depth of the fork blade? Looks like a great tool for getting it wrong.

Now we can NEVER take our bikes to any shops that are not equipped with these two tools!

I have a 171 tool and have used it only a few times to measure seat tube angle. Works well as long as you level the drop outs.


I would lone to have a 170. Not too hard to make, I know. To answer Kontack- The tool knows the angle because it sits on center with the steerer and upper blade, which back then were in line with each other. But the head tube angle need not be know as the trail is read directly from a scale. Once trail is known a head angle can be calculated, if needed.


I think it's cool to see these old tools. Reminds one that what we talk about here is the same as what was talked about 50 years ago. Andy.

Yeah, I was thinking about blade-to-steerer angle, like on a Colnago fork. But there must be forks that had some curve high enough up to cause the measure to be off even if they didn't look like straight blade forks, like the Henry James 3° crown.

My heavy touring bike's fork was built with a bi plate crown, offset there parallel to the steering axis, and blades raked less..

opposite end of the (gram) scale from something for a Colnago..

I must be dense, I don't understand how the rake tool works at all.

Eric- the tool established a line parallel to the steerer axis, which also intersects the road along a scale zeroes om the tire contact patch. Andy

ok, I didn't really see that the pointer is at the ground level. There are plenty of forks that wouldn't work on, even back then, I think. Needs 2 attachments at the steerer and an adjustable bar to go across the blades.

I don't disagree that this tool is now very dated in current application. I still think it's cool. Some year I'll get around to making one, maybe :) Andy

My issue with both 170 and 171 is that they both make assumptions: 170 that the fork axis is along the steerer axis. (I have heard it said that Peugeots of the day often ere built with forks that "trailed" a touch, ie where the fork axis was steeper than the steerer axis.) The 171 assumes the top tube is horizontal. On traditional bikes, usually, yes. But on a hastily jigged bike. maybe not.

I would prefer tools that give correct measurements without assuming good workmanship.

Ben

I wonder what part of the 171 tool interpolates the two measures into an angle. Or do you have to do the math yourself?

I wonder if it came with a table. I am pretty sure you could get the angle out of it with another part. Although I might be missing something there too.

I think you all are missing what these two tools were all about. In the era these were made in this was pretty leading edge stuff. The knowledge base that we have these days is so much greater then what was commonly known back then. These tools needed no other instrument then the supplied ruler and the fully built bike. What a quick way to check bikes that one only has for a few minutes.


With the directly measured dimensions these tools make easy to get one can design a frame, no angles are needed to be known, just rise over run. I think this thread has once again shown that we are so caught up in the way we currently look at frame parameters that we are blind to other ways to do the same thing, methods that don't depend on computers, design programs or calculators.


Sure these tools have their limitations and assumptions but that can be said for any measuring method. Andy

So the "Seat tube angle checking jig" doesn't measure an "angle" at all? Was there a point in cycling history that seat cluster set back was a parameter builders or buyers looked at?

I think if you were replicating someone's bike, checking the seat angle that way would probably be a good idea. I think the trail measuring device would be pretty interesting to use, updated to match current bikes a bit better.

Andy, I think people probably solved for the angle. You could get Nervex lugs in every half degree size.

Var 171 measures set back from the vertical line extending up from the BB's center to the seat tube's center. This is the short side of a right triangle. The vertical is the long side and the seat tube is the hypotenuse. From these you could derive the angles but this set back, for a certain "frame size", could be directly transferred to a drawing (and therefore establishing the seat tube angle). Here's a shot of my 171 and it's horizontal scale. Andy

I think we all understand how it works, just not what you do with that 14cm result it produces, especially when the tool doesn't give a vertical distance to go with it. What process does that number assist?

In theory, you could build a frame just by knowing all the center to center lengths of the tubes without referencing angles at all.

A ruler/tape measure will give you the other sides dimensions. Not every tool lives in a world independent of others. The 148mm isn't a number without relationships.


Since most all main frames are not true triangles but 4 sided, with unequal angles/sides, it's configuration can shift. The four sides can maintain their dimensions but the angles between adjacent sides can range/vary. Establish any one of the 4 angles and with set side lengths the remaining angles are also set.


I've tried to explain how this tool's data can be used. It seems that I can't find the words to do this. Sorry. Andy

Yes, you need at least one angle to copy a frame, not just tube lengths. And tube lengths are somewhat difficult to measure with the accuracy required, some redundancy is nice. In a world with only horizontal top tubes, it's a pretty useful tool. I have a fitstik, which is a pain to use. That hook system would be nicer than sloppy velcro. I want to make an x-y tool eventually.

My dim memory of the Var 171 tool is that it was simply meant to be used to ensure that the relationship between the forward/back position of the nose of a rider's saddle and the center of the bottom bracket would be reproducible for setting up the rider's next bike (or the same bike after disassembly).

Just looked it up, and there's an illustration on page 134 of the 1972 edition of the C.O.N.I. "Cycling" book that shows how to measure that dimension for fitting a bike to a rider.

On the other hand, the illustration for the Var 171 tool does include a note indicating that the tool is to be used for checking the seat tube angle ("very important for frame building").

Looks like more of a tool for bike fitting than for frame building as mentioned above.

I find drywall square and measuring tape works pretty well.

drywall square is too prosaic. Although I do have a metric drywall square in my wishlist on amazon.

Have to think about where my CONI book is, I'd like to see the picture of it being used for fitting. Would have been nice if they had put the ruler on a separate slider, it would be really great for positioning saddles if it had that.