Spin decay for ST?

[BWJones]

SkyTrak_Seth , I jus thaw some posts from senior members of this forum discussing the effect of spin decay on ball flight. I recently noticed differences in draw and fade extent going from my ST compared to real life. I think the ST is an awesome product for the price. This is one of the few areas that I see could use improvements. Is it possible for your team to look at this slight difference in spin axis, and potential spin decay as some have mentioned here?

[Zevon]

Thanks for bringing it up. Certainly open to exploring if it has potential to increase our flight model accuracy. I will address with our team next week.

This idea seems like an epiphany to me.. what was your team's reaction?

[SkyTrak_Seth]

I think we're onboard, but we have other club and ball physics stuff on the front burner right now. We are pretty happy with the ball flight model as it stands, so I'm not in a huge hurry to muck it up I will definitely get this slated for testing early next year.

[Js1010]

Seems trackman says a ball loses 4% of its spin every second its in the air.

http://tutelman.com/golf/ballflight/spinDecay.php

[GungHoGolf]

SkyTrak_Seth - is factoring in spin decay still on your team’s road map? It’s still the most significant problem with SkyTrak’s flight algo in my opinion. A high baby draw or fade curves initially then falls relatively straight down in real life and on Trackman shot tracers, while on SkyTrak it just keeps curving at a constant rate until it hits the ground.

ProTee United can you confirm that TGC 2019 is now factoring in spin decay?

[GungHoGolf]

SkyTrak_Seth - just wanted to make sure you revisited this thread. I firmly believe that the SkyTrak flight model exaggerates the effect of off-axis spin, and it could be that factoring in spin decay would solve the issue. It could also be a factor in the shorter distances seen with long clubs, as the artificially-high spin towards the end of the ball flight continues to slow it down, and then also shorten its roll-out.

[trumb1mj]

I think this goes along with spin decay but I wish the ball responded more realistically when it hit the green. As it stands, the ball doesn't skid forward before catching like it does IRL.

[SkyTrak_Seth]

Thanks for bringing this back up. While we are re-working the environment, we are also having to make changes to the ball flight model to port it to pure 3D. I will make sure that spin decay is part of that body of work.

[AndyF]

I'll copy this in here as it was something I wrote to Seth a while ago but it might help things...

I've been reading the thread regarding spin decay for the Skytrak flight model and, if you're interested, I have a formula that I've used in my own flight model that seems to give results that are consistent with a number of other models that are out there.

Now unfortunately I can't remember where I got the original equations from however they are as follows:

Spin reduction (dw/dt) = -0.00004 * (v²/R²) * S

where the spin reduction is given in rad/s = (rpm * 2 * PI) / 60

v = overall ball velocity in metres per second
R = ball radius in metres
S = (ball radius (in metres) * spin (in rad/s)) / velocity (in m/s)

I use this to calculate a spin reduction for each specific time point in my spreadsheet (which is set to 0.01 seconds) and apply this reduction to the overall ball spin rate before then splitting it into it's appropriate x, y and z vectors

As an example, if I've got initial launch conditions of a regulation golf ball (with a radius of 0.02134 metres) an initial velocity of 150mph (66.67m/s) and an initial spin rate of 2500rpm then first I calculate my spin rate in rads / second

Spin rate = (2500 * 2 * PI) / 60 = 261.80 rads/s

I then calculate the S factor

S = (0.02134 * 261.8) / 66.67 = 0.0838

And then I use these numbers to determine the spin decay over my time factor

dw/dt = -0.000004 * (66.67² / 0.02134²) * 0.0838 = -3.272 rads/s

So over my time period of 0.01 seconds I know that the spin reduces by 0.03272 rad/s and then I recalculate again for my next time period based on a new spin rate of 261.77 rad/s

I don't know if this will help or not but hopefully you can get something useful out of it.

Many thanks for your continual improvement of a really great product and long may it continue.

Best regards,

Andy

[Morini]

On a somewhat related note, it would be nice if we could have realistic ball behavior in terms of both fairway and green roll out. Else why bother modelling it? Ideally with user input, so we can dial in iron and pitch shots to the roll out we see on our real courses. All that is needed are factors for ground firmness, slope, and green speed.

This paper goes over the math involved: https://www.researchgate.net/publica...d_Roll_on_Turf