Aardwolf

Hiya Folks,
I've got a cheapy spoke tension meter (ZTTO TC-1) which apparently has the same conversion table as the ParK Tool TM-1.
Previously I've tried the Park Tool app (https://www.parktool.com/wta) and that gives TM-1 conversion values to 2 decimal places when you look at the tension results.

So putting the Park Tool app values for 2mm round steel spokes into a statistics package (R https://www.r-project.org/) I determined that the conversion curve is an exponential.
So for 2mm round steel spokes

kgf = 16.126 + 3.8987 exp (0.13127 * reading)

With an R squared value of 0.99994, ie it's an exact match,
Obviously the coefficients will be different for different spokes etc.
But if you're doing your own spoke tension diagrams in a spreadsheet it's quite useful

Aardwolf

I'm now using it for spoke tension charts in a spreadsheet, but thought I'd confirm it works.
One advantage is you can estimate the TM-1 reading to about nearest 1/4 and using the formula converts all values.

Seems fine:

Now I have to think about calibrating my tension meter.

IPassGas

It is relatively simple and not too expensive to build your own calibration setup with something like in the pic. The first time I used it I found my park gauge was off by 2 divisions. Whenever I check wheel tension I also check calibration.

Aardwolf

Cheers, I did see something similar in another thread.
I also saw a lot of comments saying tension meters can be off by quite a bit.

I'm building a wheel for the first time so I'm trying to avoid unnecessary errors.

I've got a travel scales, but it only goes up to 50kg.
It should be enough to check my tension meter though, or I could buy another one for £16.

headasunder

buy another one 50kg wont get you in the ballpark

Aardwolf

Yep, definitely needs calibrating.
I took some readings for 20 - 50kgf and fitted an exponential curve and suddenly the new spokes have gone from 80 to 110 kgf.
Think I'll buy another digital scales

Aardwolf

Next day delivery is a fine thing
Now I've got a scales that does 150kg:



So the readings are still on the high side versus TM-1, but now I know the curve coefficients that doesn't really matter.

It did occur to me that the meter goes from 0 to 45 and I'm only really interested in the 70kgf to 120kgf bit, which is 18.15 to 23.19 on the meter.
Think I'm going to try backing off the spring on the meter adjustment screw - should result in a flatter curve and 70-120 kgf being spread over a wider range.

The Park Tool TM-1 has 70-120kgf as 20.09 to 24.95 for these spokes, but their conversion table shows Aluminium Round 3.3mm spokes use the 33-40 readings.

Edit: Seems turning the adjuster screw on the tension meter just moves the curve up and down, it doesn't change the slope.

MeganCook

Accurate spoke tension is crucial for building strong and durable wheels. Tension meters use Hooke's Law to measure the amount of deflection or stretch in a spoke when a known force is applied. Spoke tension can vary depending on position in the wheel and different math concepts, such as radial and lateral runout, dish, and spoke length calculations, also come into play when building and truing a wheel. Understanding these concepts is important for building better performing wheels. Moreover, https://plainmath.net/algebra-ii/925...onds-year-1000 can be useful for you if you need some help with equations or other math stuff. I had a recent exercise that made me be confuse. The questions was which operation to perform to find the number of milliseconds in a year, and I was so satisfied when on the service of math homework help, I found the answer. So, keep experimenting and making adjustments as needed, and hopefully you'll be able to get the results you're looking for.

Road Fan

I think you get an A or at least B+ for engineering methodology, but what about validation? How does your curve compare to say, plotting points collected by looking at a bunch of finished wheels using a TM-1 to actually collect the data? Or more generally, how does your cheapie tension meter's curve compare to the same spokes and wheel checked with the Park tool? I would expect it to be close, since I bet it's pretty easy to reverse-engineer the Park tool, or any other which may be thought to be the state of the art.

Can any mechanical engineering analysis (leverages, spring constants, free-body diagrams, or four-bar linkage models) be used to derive the curve for your meter and for a Park and perform a comparison of the modelled tension meters?

Aardwolf

If you do enough test values (green points in my graph) then the exact theory of why the curve should be exponential or not doesn't really matter - the curve fits the data so you can use the curve to extrapolate intermediate values as long as you've tested over your range of interest.

I just tried it because I've used the software for tuning accordions.
Nope I'm not joking - the frequency of notes goes up exponentialy (doubles per octave) so calculating what frequency A6 is with 15 cents of musette (at A4) can get quite complicated

I don't have a Park TM-1 but they do produce an app (mentioned in first post) which I very much doubt is using a lookup table and exactly fits an exponential curve so I'm fairly sure Park have worked out it's an exponential curve.

However in the big scheme of things it doesn't really matter, I use the meter for 2 things:

1. Check I'm not exceeding what I think is the max desired tension - requires some calibration
2. Check all spokes are at roughly the same tension, within my allowed margin - doesn't require calibration.

So it's only the max tension that requires absolute meter calibration, and I'm fairly sure people are going to allow some margin for error.

I set my Mavic Monthlery Route to a max of around 90kgf because that's what I believe Mavic said 40+ years back, also when I got the wheels they were at about that.
I've done over 1000 miles on them since, no issues at all.

FBinNY 

The key issue with spoke tension is not absolute precision where there's plenty of latitude, but rather, the tension uniformity spoke to spoke.

So, what matters is the repeatability and consistency of multiple readings. Therefore that's what you want to test for by checking tension on the same spoke a few times.