Show HN: Racketmeter – Measure Badminton String Tension Using Sound Frequency
12 comments
·May 18, 2025nwatson
The Gates company make the Gates Carbon Belt to replace bike chains with a carbon fiber belt (encased in some polymer). My Priority Bicycle 600x has this belt drive, along with matching sprockets and a Pinion C1.12 "transmission", where all gearing/shifting takes place.
To check proper tension, Gates supplies an app [0] that also has you pluck the belt to measure the frequency. For my bike the appropriate tension is at 60Hz. Tension is adjusted by moving the rear axle backward or forward.
[0] https://play.google.com/store/apps/details?id=com.gates.carb...
EDIT: the advantages are ... no fragile derailleur; no chain lube; long belt lifespan; low maintenance
bloak
It sounds like you've done plenty of experiments and don't need any theory to make this work, but have you looked at the physics/mathematics of the vibrations? Does the frequency you detect correspond to https://en.wikipedia.org/wiki/Vibration_of_a_circular_membra..., for example? I'm guessing that the interlocking strings are best modelled as a membrane rather than individual strings (https://en.wikipedia.org/wiki/String_vibration) but the racquet head isn't circular, so it's a bit beyond my level of maths to do that properly.
I suppose that to do the maths we'd need to know the spacing of the strings and the mass per unit length of the strings. (And the dimensions of the racquet head, of course, but that must be easy to look up.)
Also, would this work just as well for tennis and squash racquets?
EDIT: Perhaps https://en.wikipedia.org/wiki/Dimensional_analysis would give you the formula, and the constant you could get by experiment, and then the same formula might be applicable to any similarly shaped racquet.
bloak
Ah, the string spacing cancels out, doesn't it? I'd expect the frequency to be proportional to sqrt(t/d)/a, where t is the string tension in Newtons, d is the linear density of the strings in kg/m, and a is the diameter of the racquet head (assuming they're all the same shape and measured in the same way, which probably isn't true, unfortunately).
zhacker
yeah. Spot on. For the app, I don't take into account the density, but racket head is taken into account. Prominent shapes are oval and isometric, so I have separate co-efficients for those, which were learned using regression. One challenge that I haven't figured out yet is when 2 different types of strings are used. It seems, some advanced players use different strings at different tensions for the horizontal and vertical strings. Like 32x28lbs or other similar configurations.
HyprMusic
Very useful. I just tried it with my spare racquet, and it seems about right. I have always wondered if my main racquet has lost tension, so it'll be really useful for that.
For making money, I'd suggest reaching out to professional re-stringers and asking if they want to advertise (once you have some nice analytics to brag about). Maybe even localise them based on geolocation data so you can have more re-stringers without cluttering it up. It's a value-add to your users so everyone should be happy.
You could probably even reach out to some YouTubers (I personally like BadmintonInsight). Since it's free, I would imagine some of them would do a video on it just to help their viewers.
zhacker
Finding stringers nearby is really cool idea. Yeah, BadmintonInsight is a really good channel. I'll try reaching out.
BarryGuff
Proves that there's literally an app to do anything. I'm amazed. Good job!
zhacker
thank you :)
fock
While I don't play Badminton (and so can't test with a racket on hand) this seems very cool! I also thought about something similar for judging bikewheel spoke tension - I guess I have to research this a bit more now.
As for monetization: I personally don't have problems with static ads served from your domain. Find some celebrity or brand and ask them if they want to have you serve their banner.
zhacker
if spokes come in standard guages, it should work for bikewheels as well. There may be some differences due to the bike rim material, but in case of badminton, I found out that frame material could be ignored safely.
Racketmeter lets badminton players measure string tension using the sound frequency produced when tapping the racket strings. It's 100% free, works in your browser on mobile and desktop, and requires no sign-up or installation.
I built it to solve a personal problem. I started playing badminton regularly in 2016 and quickly learned that players often ask stringers to string rackets at specific tensions (like 22 or 26 lbs). But after a few stringing jobs, I began to feel like the tension was inconsistent. Other players told me they just tap the strings and go by ear where "sharper sound meant higher tension."
One day while tuning my guitar, I could see exact sound frequencies on my tuner app. That’s when it clicked. It should be possible to build a tuner for badminton strings as well!
I searched online and found some tension-frequency data shared by professional stringers, but it wasn’t clean or comprehensive. So I visited 5 or 6 local stringers, gave them a frequency measuring app, and asked them to record racket head size, string thickness, tension, and sound frequency for each job. Some asked for a small payment, but most helped for free. Within a week, I had over 200 solid data points.
I trained a simple regression model using that data and validated it with newly strung rackets. It turned out to be surprisingly accurate. I shared it with friends and fellow players, and it started to spread in badminton forums.
There was another app that launched a few months later with big celebrity endorsements, but it was less accurate, harder to use, and required in-app purchases. Mine wasn't built to compete, but it ended up being more useful.
I originally released it as a mobile app, but constant changes in Google Play policies kept taking it down. So I rebuilt it as a simple browser-based tool.
Would love feedback, suggestions for improvements, or ideas on how to sustain it without cluttering it with ads or paywalls.
Let me know what you think.