fietsbob

OK take the fork out

Kontact 

And then what? Use magic to levitate it in the exact same position it was when it had a front wheel?

fietsbob

choosing to appear clueless or just confrontational ?

Kontact 

Duder, your completely impractical suggestions are clueless. And instead of continuing to demonstrate what an absurd idea measuring trail directly is in real life, you have switched over to attacking me.

As you have before.



It is okay to be wrong about something. You don't have to keep digging a bigger hole.

Road Fan

All depends how accurate you need to be. All measurements are estimates. All results of calculations that involve measured data as inputs are also estimates. To know something exactly is an engineer’s or scientist’s will o the wisp (Webster’s definition: a delusive goal), which is never possible to achieve and might not be beneficial.

But how good is good enough? What dimension has the most control over your desired result (sensitivity functions with a Pareto chart)? What is the best technique for maximizing that measurement with tools on hand? Are the assumptions you may have to make reasonable?

It’s too complex a problem for anyone to just walk in and summarily dismiss another person’s method for a measurement.

1. I like to remove the fork, clean it, and prop it up on a flat surface (kitchen counter checked to be level ot at least flat) with a bare hub clamped in the drops.

2. Put a level on the steer tube and raise it so the steer tube is as level as you can see, with your level.

3. Block up the steer tube so it stays in that position.

4. Now your steering axis is as close to parallel to the ground plane as you can get it.

5. Now with a decent grade digital, dial, or vernier caliper you can measure the height of the hub axis, top of the steer tube, and bottom of the steer tube above the tabletop.

6. With basic arithmetic you can calculate the height difference between the hub axis and the steer tube center axis, to whatever number of significant figures you like. This is the fork offset. You should not have to do this frequently.

This is how I like to attack measuring trail for my bikes. I think taking off the fork is a good idea.

Kontact 

How accurate does it need to be? Accurate enough to at least distinguish a 40mm from a 45 or 50mm fork, otherwise, why bother measuring it?


And the technique matters, because if you are going to invest the time in trying to do it, what's the point if your method is so inaccurate that you could have just held a framing square up next to the fork and read the apparent rake right off the bottom. Sometimes we mistake really accurate components (lasers, complex calculations, dial calipers) with accurate measurements, but it doesn't work like that. If your measuring system is crude, it doesn't matter how straight a laser is.


If "whatever" is the level of accuracy you need, just use a framing square or T-square. If your eyeball is good at picking out the center of the stem top and lower headset race, you'll be as accurate with this method as anything else that doesn't use perpendicular triangulation.

If you want accuracy but don't want to remove the fork, measure to something perpendicular to the axis of the fork, like the BB. Much, much more accurate than anything else.

And if you want a super accurate rake number, pull the fork and use the better tested methods in this thread:
https://www.bikeforums.net/bicycle-m...sure-fork.html

Road Fan

I want a few millimeters of accuracy, but I also want repeatability.

We seem to be in agreement on a few high-level points.

You say there are "super-accurate" methods that are well-tested, in the thread you cited. But a skim (admittedly a sketchy reading) through it does not show me any testing, and a few of the suggested techniques do not say it's tested. Have you tested any? What reference did you use to check if the results are accurate?

And why do you jump to the conclusion or at least suggestion that what I suggest is not tested? How do you know?

I can confirm the data given in some of the classic bike catalogs, Trek and Rivendell. I'm happy with that accuracy.

Kontact 

I don't see where I said you didn't test your suggestion.

What I said was that your suggestion is unlikely to be accurate because it relies on small degrees of angle measurement accuracy with a level. Additionally, it relies on getting fine angles measured repeatedly, and every time you have to re-measure a small angle (like with a level), you add a new possibility of error - which is like tolerance stacking. It isn't that you can't get the right answer from your method, it is just that there are so many opportunities to get the wrong answer and no controls to prevent that from happening - you are essentially eyeballing your test. If you had a surface that is useful for putting a level against it, why not simply measure directly from that surface without adding the step of leveling? Would the steerer and your measuring surface not remain parallel in any axis once you locked them together?


If you look in the other thread I tested FBinNY's string method and my pot and glass table method. My results for my method was within a quarter mm, so I think it could be regarded as solid. It also doesn't suffer from some of the problems I mentioned that cause tolerance stack.

Road Fan

Bubble levels and digital levels have very clear indications of zero, so I see no concern in determining when it says zero. There can be a concern whether the level is contacting the steer tube clearly, but that is largely a matter of whether the contacting surfaces are clean, straight and free of debris. These two types of levels are much easier to read clearly than, say carpenters' or roofers' angle finders. I use scales and a caliper but in my jobs and education over the years I have been trained in use of those tools. I'm not overly worried about errors. If you've been trained in school in how to use a slide rule (late '60s high school, for me!), you are halfway there.

Plus, for each measurement I wait a little while for the measurement to stabilize. My digital levels can take some time to settle. There's clearly an integrator in the algorithm.

You speak so much of untested, error-prone methods, and I wasn't sure which methods you saw as such. Because you spoke of this in the post following my process description, I assumed you were responding to what I said. I see you were mainly making general comments.

A method that gives you a quarter millimeter is solid if it is consistent physically and in the reading of the result. Part of this is the user and the technique. We're not in the business of providing lessons in how to measure (at least I'm not), so I don't think the skill points in what I'm doing is a problem for me. I have something that works for me with the tools and location that I have. If someone else can get results he/she is happy with in another way, that's fine for me, and kudos to them!