Question to RDC owners

FranzS

Rookie
Is it possible to clamp the racquet with the stringbed facing upwards (parallel to the ground instead of perpendicular) for the swingweight measurement?
In case it is, would you mind trying and sharing the different measurements (parallel vs. perpendicular)?
I'm working on a way to determine twistweight from the difference of those two measurements (pure math speculation for now, but who knows...).
Thanks to everyone who's willing to try and share.

bfroxen

Rookie
That is a valid way to measure twistweight, but you need fairly high precision (0.1 kg•cm^2 or so) for the two measurements to get a meaningful twistweight result.

FranzS

Rookie
That is a valid way to measure twistweight, but you need fairly high precision (0.1 kg•cm^2 or so) for the two measurements to get a meaningful twistweight result.
Could you please explain the details? I haven't done the math yet but I suppose I should get a greater difference b/n the two measurements.

bfroxen

Rookie
Twistweight is on the order of 14 kg•cm^2. If you're measuring the swingweight and "90° swingweight" to integer precision in kg•cm^2, you could get results of, for example, 325 and 340, when the actual values are 325.49 and 339.51. You'd calculate 340 - 325 = 15 for the twistweight when the actual value is 14.0 kg•cm^2. That's a potential error of ±7% (before accounting for other errors in the measurement), which is quite large. It might tell you something, but it's not precise enough do anything.

Other swingweight machines may have better precision, or you may be able to devise a method to get better precision from an RDC (assuming Babolat RDC) by adding known weights and averaging several measurements.

The method is valid, though. If you know the moment of inertia in two perpendicular axes, the moment of inertial of an axis perpendicular to the other two axes is the difference between the known values.

My Microgel Radical MP's (re-issues) measure a difference of 12.7 to 13.0 kg•cm^2 using this method and a swingweight machine I'm developing.

• joohan

FranzS

Rookie
I'm dumb, I forgot the perpendicular axis theorem. I was going to go through some calculus! Thanks for saving a few hours of mine! And yes, of course you're right about the resolution of 1 kg*cm^2 being too large.

bfroxen

Rookie
Ha. I get into a lot of math with my tennis interests, but I don't think I've used much calculus (maybe some basic stuff).