# Maths question - Golf swing speed **URL:** \ **Category:** Factual Questions\ **Created:** [July 12, 2017, 5:09pm UTC](https://boards.straightdope.com/t/maths-question-golf-swing-speed/791106 "2017-07-12T17:09:39Z")\ **Posts on this page:** 16\ **Page:** 1
**Author:** ![jimdigritz](https://avatars.discourse-cdn.com/v4/letter/j/91b2a8/32.png) [@jimdigritz](https://boards.straightdope.com/u/jimdigritz)\ **Post date:** [July 12, 2017, 5:09pm UTC](https://boards.straightdope.com/t/maths-question-golf-swing-speed/791106/1 "2017-07-12T17:09:39Z")
So I’ve just bought a Mobitee and PIQ golf swing analyzer for my 11 yr old son to help develop his swing. FYI it’s this: [https://piq.com/golf?gclid=Cj0KCQjwkZfLBRCzARIsAH3wMKqknvBQNLsntx\_u2fgrP2KhERLmPg4Lt-g0IF-mtABVXSY-cKodVJ0aAiqtEALw\_wcB](https://piq.com/golf?gclid=Cj0KCQjwkZfLBRCzARIsAH3wMKqknvBQNLsntx_u2fgrP2KhERLmPg4Lt-g0IF-mtABVXSY-cKodVJ0aAiqtEALw_wcB) It uses an accelerometer sensor at the grip end of the club to calculate club head speed. He’s loving it however we’ve noticed that the reported swing speed looks very high. He was radar tested a few months ago and his swing speed was around 78mph. He might have improved, but… The Mobitee PIQ sensor consistently shows his swing around 105-115mph! That speed would put him up with the PGA Pro Tour players!!! He is using a 38 in. finished club length Driver. I believe a standard Men’s driver is 45 in. Can anyone help me figure out how I can calculate the actual club head speed given that the club is 7 inches shorter than a standard club? 😕 By the way I’m prepared to discover that this is maths that a 6 year old could figure out!!! Thanks! ---
**Author:** ![HoneyBadgerDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/honeybadgerdc/32/1033_2.png) [@HoneyBadgerDC](https://boards.straightdope.com/u/HoneyBadgerDC)\ **Post date:** [July 12, 2017, 5:23pm UTC](https://boards.straightdope.com/t/maths-question-golf-swing-speed/791106/2 "2017-07-12T17:23:56Z")
How does it know when ball contact would be made? Possibly the swing is faster past the sweet spot. I would guess about 84% of what a standard driver would read. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [July 12, 2017, 5:36pm UTC](https://boards.straightdope.com/t/maths-question-golf-swing-speed/791106/3 "2017-07-12T17:36:07Z")
Presumably the “sensor” is a gyro + accelerometer. Accelerometer tells you how fast the grip is moving, and the gyro tells you the angular speed. Add the linear velocity of the grip + rotational velocity of the grip _x_ length of the club, and you get the velocity of the head. But without knowing the separate measurements, it’s impossible to correct for the length of the club. It’s not a fixed percentage error. The best you can do is assume (approximate) that the velocity of the head is much greater than the velocity of the grip, in which case you can ignore the velocity of the grip and just apply a 38/45 correction factor. Are you sure the software doesn’t allow you to input the length of the club?? ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [July 12, 2017, 5:39pm UTC](https://boards.straightdope.com/t/maths-question-golf-swing-speed/791106/4 "2017-07-12T17:39:22Z")
> [@HoneyBadgerDC](#): > > How does it know when ball contact would be made? Possibly the swing is faster past the sweet spot. I would guess about 84% of what a standard driver would read. Wouldn’t the club head speed be at its maximum as it hits the ball? It must be accelerating all the way up to the collision, and then lose a fair amount of kinetic energy in the collision. ---
**Author:** ![HoneyBadgerDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/honeybadgerdc/32/1033_2.png) [@HoneyBadgerDC](https://boards.straightdope.com/u/HoneyBadgerDC)\ **Post date:** [July 12, 2017, 5:57pm UTC](https://boards.straightdope.com/t/maths-question-golf-swing-speed/791106/5 "2017-07-12T17:57:09Z")
> [@scr4](#): > > Wouldn’t the club head speed be at its maximum as it hits the ball? It must be accelerating all the way up to the collision, and then lose a fair amount of kinetic energy in the collision. I believe when they are testing club head speed there is no collision. ---
**Author:** ![jimdigritz](https://avatars.discourse-cdn.com/v4/letter/j/91b2a8/32.png) [@jimdigritz](https://boards.straightdope.com/u/jimdigritz)\ **Post date:** [July 12, 2017, 6:01pm UTC](https://boards.straightdope.com/t/maths-question-golf-swing-speed/791106/6 "2017-07-12T18:01:42Z")
> [@HoneyBadgerDC](#): > > I believe when they are testing club head speed there is no collision. Thanks for the responses so far - the PIQ/Mobitee sensors have no way of knowing impact speed or smash factor - hence they can only give actual distance measurements if you walk to the ball (via GPS) Some other systems (like Zepp) DO allow you to edit the club length to allow for ladies/juniors clubs, however at the moment the Mobitee system doesn’t - hence why I was trying to see if there was a way to manually calculate based on the club length and reported speed (which assumes 45inch length) - if Zepp does it using almost identical hardware - surely it must just be a calculation… Cheers Jim ---
**Author:** ![D18](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@D18](https://boards.straightdope.com/u/D18)\ **Post date:** [July 12, 2017, 6:27pm UTC](https://boards.straightdope.com/t/maths-question-golf-swing-speed/791106/7 "2017-07-12T18:27:24Z")
Is there any chance the readout of your machine is set in kilometers per hour instead of miles per hour? 78 mph is 126 or so kph, so the difference you are citing is in the ballpark of the conversion from imperial to SI. ---
**Author:** ![jimdigritz](https://avatars.discourse-cdn.com/v4/letter/j/91b2a8/32.png) [@jimdigritz](https://boards.straightdope.com/u/jimdigritz)\ **Post date:** [July 12, 2017, 6:55pm UTC](https://boards.straightdope.com/t/maths-question-golf-swing-speed/791106/8 "2017-07-12T18:55:35Z")
> [@D18](#): > > Is there any chance the readout of your machine is set in kilometers per hour instead of miles per hour? 78 mph is 126 or so kph, so the difference you are citing is in the ballpark of the conversion from imperial to SI. Great idea, but no, the original readout was KPH, I changed to MPH and got the 100mph readings… ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [July 12, 2017, 7:45pm UTC](https://boards.straightdope.com/t/maths-question-golf-swing-speed/791106/9 "2017-07-12T19:45:48Z")
> [@jimdigritz](#): > > … if Zepp does it using almost identical hardware - surely it must just be a calculation… But as I said earlier, just because it’s a calculation doesn’t mean it can be reversed. In this case you have _Vh = Vg + L x Wg_, where _Vh_ is the velocity of the head, _Vg_ is the velocity of the grip, _L_ is the length of the club and _Wg_ is the angular velocity of the club (measured at the grip). To recalculate for a different _L_, you have to know _Vg_ and _Wg_ separately. You can’t backtrack from just _Vh_. The closest approximation you can make is to assume _Vh \>\> Vg_, in which case _Vh = L x Wg_. So in this case you can make the correction (_Vh2 = Vh1 \* L2 / L1_). In other words, multiply the readout by 38/46. ---
**Author:** ![jimdigritz](https://avatars.discourse-cdn.com/v4/letter/j/91b2a8/32.png) [@jimdigritz](https://boards.straightdope.com/u/jimdigritz)\ **Post date:** [July 12, 2017, 7:59pm UTC](https://boards.straightdope.com/t/maths-question-golf-swing-speed/791106/10 "2017-07-12T19:59:42Z")
> [@scr4](#): > > But as I said earlier, just because it’s a calculation doesn’t mean it can be reversed. In this case you have _Vh = Vg + L x Wg_, where _Vh_ is the velocity of the head, _Vg_ is the velocity of the grip, _L_ is the length of the club and _Wg_ is the angular velocity of the club (measured at the grip). To recalculate for a different _L_, you have to know _Vg_ and _Wg_ separately. You can’t backtrack from just _Vh_. > > The closest approximation you can make is to assume _Vh \>\> Vg_, in which case _Vh = L x Wg_. So in this case you can make the correction (_Vh2 = Vh1 \* L2 / L1_). In other words, multiply the readout by 38/46. Thanks, I think my relatively innumerate brain can understand that, so 100mph could be approximated to 88mph… Sounds about right, thank you for the explanation scr4, my 11 year old will be gutted to hear that he is in fact NOT swinging as fast as Rory McIlroy 🆒, however he is (probably) swinging faster than 3 months ago! ---
**Author:** ![smithsb](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/smithsb/32/12107_2.png) [@smithsb](https://boards.straightdope.com/u/smithsb)\ **Post date:** [July 12, 2017, 8:08pm UTC](https://boards.straightdope.com/t/maths-question-golf-swing-speed/791106/11 "2017-07-12T20:08:14Z")
I think you will have to do manual as suggested above. From the FAQ at the website: “Club speed is measured in miles per hour how fast your club head is traveling at the point it impacts golf ball. Value is calculated from hand speed measure with **standard club height** (base on which club you are using).” Bolding mine. You could ask them the question. [http://www.mobitee.com/en/mobitee-piq-features](http://www.mobitee.com/en/mobitee-piq-features) ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [July 12, 2017, 8:29pm UTC](https://boards.straightdope.com/t/maths-question-golf-swing-speed/791106/12 "2017-07-12T20:29:50Z")
**scr4** , why do you think the sensor needs to take a separate measurement of angular velocity? The required data are the speed at the sensor, along with two fixed distances - from the center of rotation to the sensor, and from the center of rotation to the club head. The speed of the club head is just the speed of the sensor scaled up by the ratio of these two distances. Although this assumes that you don’t add much velocity to the clubhead by turning your wrist. Maybe that’s not a reasonable assumption? Ignoring wrist action, the adjustment you would need to make is as follows, with all distances taken from the center of rotation which I assume is somewhere mid-chest (I’m not a golfer): (a) for a full-length club held by a full-sized human, estimate the distance to the end of the club, and divide this by the estimated distance to the sensor position near the grip. (b) for your son holding his club, do the same thing. Adjust the speed by the ratio (b)/(a) The values (a) are the lengths that the device is assuming to make its calculation. Of course, if you can find those assumed numbers directly in the manual, of course you don’t need to measure them. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [July 12, 2017, 8:47pm UTC](https://boards.straightdope.com/t/maths-question-golf-swing-speed/791106/13 "2017-07-12T20:47:11Z")
> [@Riemann](#): > > **scr4** , why do you think the sensor needs to take a separate measurement of angular velocity? > > The required data are the speed at the sensor, along with two fixed distances - from the center of rotation to the sensor, and from the center of rotation to the club head. The speed of the club head is just the speed of the sensor scaled up by the ratio of these two distances. That would be true if the entire club was rotating around a fixed point, but I think it’s more complex than that. See [this photo](http://www.golfdigest.com/gallery/todd-anderson-fix-your-swing-fundamentals) for example. There is quite a bit of wrist motion involved. Approximating this motion as a rotation around a fixed point wouldn’t be very accurate. Besides, the head moves _much_ faster than the grip, so it wouldn’t be very accurate to measure the linear speed of the grip and extrapolate that to the head speed. ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [July 12, 2017, 8:50pm UTC](https://boards.straightdope.com/t/maths-question-golf-swing-speed/791106/14 "2017-07-12T20:50:46Z")
> [@scr4](#): > > That would be true if the entire club was rotating around a fixed point, but I think it’s more complex than that. See [this photo](http://www.golfdigest.com/gallery/todd-anderson-fix-your-swing-fundamentals) for example. There is quite a bit of wrist motion involved. Approximating this motion as a rotation around a fixed point wouldn’t be very accurate. Besides, the head moves _much_ faster than the grip, so it wouldn’t be very accurate to measure the linear speed of the grip and extrapolate that to the head speed. Ah, yup. Clearly not a reasonable assumption to ignore the wrist action. ---
**Author:** ![HoneyBadgerDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/honeybadgerdc/32/1033_2.png) [@HoneyBadgerDC](https://boards.straightdope.com/u/HoneyBadgerDC)\ **Post date:** [July 13, 2017, 12:21am UTC](https://boards.straightdope.com/t/maths-question-golf-swing-speed/791106/15 "2017-07-13T00:21:54Z")
Another thing that would throw off the meaningfulness would be that it simply takes more strength to accelerate an object at the end of a longer pole. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [July 13, 2017, 4:16am UTC](https://boards.straightdope.com/t/maths-question-golf-swing-speed/791106/16 "2017-07-13T04:16:56Z")
You could make measurements using an iPhone in slow-mo video mode (240 frames per second). Put a calibrated grid in the plane of the swing so that you can measure how fast the club is moving by the change in position in each frame.