Is Exit Speed off Tee, just physics? Is Zepp Bat Speed better?

Groundout, What I'm seeing at home definitely coorelates to your data on PG.  We are not talking a huge difference in my sons case as far as exit velocity.  I would conservatively say 8mph to 12mph due to size.  This again is based on data out there suggesting that range due to the trampoline effect on top of bat speed.  The more we discuss it, the more confident I am about it.  At this point, I think you can look at it like this.  There is one bat swinging around an Axis (your center of gravity).  The bat size will depend on the player size and bat size together in some relationship.  I know many won't believe it and assume my zep reading is wrong, but it is what it is.  I also confirmed the bat speed with an old Doppler device that I used to measure my club head speed, but also has a bat setting.

2020, I'm not taking away from the bigger man here.  I am big enough and Junior will be too in a couple years.  My conclusion in all this, is bat speed should be weighted more than many are currently think.  Bat Speed buys you time at the plate and we all know what that means to a hitter.  Exit Velo will come.  Bat speed is a different story.  I can tell you my bench is 2x that of juniors yet his bat speed is the same as mine.

Groundout, Before I call it a night,  I am that 200lb player, actually 210lb now, with the 90Mph exit, and I am un-impressed as well .  About 10 years ago between coaching, I was hitting a few out over the softball fences though... Ha

Linedrive_07 posted:

Adding to my thoughts on the center of gravity, there is a mass rotating around that fulcrum/axis or whatever you want to call it.  As far as the ball is concerned, it would have no idea where the bat stopped and where your body began.  It would just see the total force transferred at a very small location on the bat.  Truman, does this change the idea of how you would look at it in relation to physics?  

If you recall, I mentioned harmonics.   Every bat has its own harmonics and will change depending just what's attached to the bat and where.  Also, how its harmonics reacts to impact will change depending on much mass hits the bat and where it hits on the bat.  The sweet spot is the place on the bat where there is the least displacement resulting in more energy being transferred from the bat to the ball.  Note too that the speed of the bat at impact with the ball is much slower at the nob than at the other end of the bat. Right?  Yet, even though the speed of the bat is faster above the sweet spot than the speed at the middle of the sweet spot, the exit speed off the end of the bat like that will not produce as much exist speed because the resulting harmonic displacement is so great (eating up the energy) and why that energy hurts the hands when a ball is hit there.

Now, when hands are attached to the bat (which of course includes arms), the harmonics of the bat is changed according the the harmonics of the the hands and arms . . . but not significantly (IMHO).  Big strong hands and arms will have very different harmonics than small weak ones and  so, will also affect the harmonics of the bat and how much energy might be transferred to the ball.  The rest of the hitter's body only functions as a platform to generate angular acceleration, which is added to the angular acceleration of the hands as they release through the ball.  And as I'm sure you'd agree, the timing for combining bother those angular accelerations in affecting bat speed.

But, we're talking about a constant bat speed with different exit velocities.  And the best thing that explains that to me is the difference in harmonics.  Your example of dropping a bat at 75mph as it hits a ball will affect the the balls exit velocity negatively compare to having the bat attached to the car or being held by a person at impact.  If the bat is attached to the car firmly and hits the ball, there would be no difference if it were attached the same way to an Abrams Tank going at the same speed.  The harmonics of the bat and the energy that is displaced would not be any different and to F would be constant along with the exit speed.

 

Take it a step further, let go of the bat right before contact.  The total mass is reduced.  Is it possible to hit a homerun?  Yes, depends on bat speed and size of bat.  Now let's say magically half of the bat broke free right before contact.  Would that reduce the exit velocity even more?  Logically to me yes. 

 Yes . . . but, for a different reason.  You're saying it the mass that's gone, I'm say there's a change in harmonics where what's left of the bat has less energy displacement and can't and can't be transferred to the ball.

 

 If you are following my train of thought, this should continue to add all the way back to the center of the axis the bat is rotating around. 

 The radius of that axis will have a harmonic sweet spot just like the bat itself has and is influenced by they the position of the hands and arms making up that radius in the axis.  

 

 I'm am not one to just assume something is the way it is just because someone in the past performed their own experiment and said it was so.  It has to add up to me period.  Truman, I would love to hear your thoughts.

 If made a machine consisting of a rotating disk that had jointed arm and hand like attachment to hold a bat and had that moved just as a human does, I'd bet that machine (without a body below the disk) and programmed to a fixed bat speed the exit speed wouldn't change not matter what that disk was attached to (other than needing to be built to a minimum of withstanding the rotational forces the disk generates to keep from falling apart).  There is such a machine fin golf and it's mass has nothing to do with the ball's exit speed . . . only the club head speed.

Well, anyway . . . I hope you find these thought of some interest.  Someday, not too far in the future, we may have a robot that can quantify and answer your question definitely.  

there is a Video of todd frazier hitting a home run with the bat flying out of his Hands (bottom Hand is off the bat and top Hand just losing the grip). that supports the theory that the Hands can't affect the bat during contact.

https://www.youtube.com/watch?v=fczUzv0kcNw

Truman, thanks for the thoughts.  For me, the harmonics are not as intuitive, but  I would bet it is all the same in the end.  The reason I like to use Mass is because in the end a Force vector is transferred to the ball which also can be described as MA.  I like the spinning disk thought; I will contend the heavier the disk assuming it is rigid in attaching to the bat, it would yield a higher exit velocity.  I think you can take it a step further though, and just spin a single object with a contact area for hitting a ball.  There should in that case be a Total mass outside the radius that would add to yield an exit velocity.  Scaling that object up using same proportions which increases mass would produce a higher exit velocity (IMO).

Dom, I'm not saying it's not possible to hit a HR with no hands.  I'm saying it's all additive, and with hands it would have gone even further. At what percent, I don't know.  I just have real world observations right now, which I can only estimate at 10-15mph (This is my bet Truman), based on what I'm seeing with my son and I.  I suspect that there is percentage, of total mass as it compares to the baseball.

Truman, your spinning object must have a total weight of 120lbs for one experiment, and then 200lb for the 2nd.  Mass will not affect the exit velocity until hitting a certain weight, and I'm betting its around 125lb (Some percentage relative to the ball), and maxes  out at some weight (200lb sounds good to me) where the gain is negligible, probably to do with your harmonics and the ball which I'm not fully able to grasp .  Anyway, that's my story and I'm sticking to it as it fully explains what groundout and I are seeing.

Dom, I would love to know the bat speed of the homerun.  Bat speed on average translates to roughly 3.333ft per 1mph.  350ft shot would require a bat speed of say 105mph.  Reducing by say 12mph in my scenario or letting go with hands should reduce the ft by roughly 40ft to a 310ft shot.  Remember though you are also talking about a live pitch and we know the pitcher adds to the exit velocity but not as much as once thought.  There again the higher the pitch velocity the smaller the gains.  Truman, just as we see this additive exit velocity due to the pitchers velocity, I will pretty much guarantee at this point, you will find the same thing due to the hitters mass on the other side of the pitch

Dominik85 posted:

there is a Video of todd frazier hitting a home run with the bat flying out of his Hands (bottom Hand is off the bat and top Hand just losing the grip). that supports the theory that the Hands can't affect the bat during contact.

 

https://www.youtube.com/watch?v=fczUzv0kcNw

Hmmmm???  I don't know why one would conclude this supports "Hands can't affect the bat during contact" . . .???  

If one looks closely, we see this fast ball hitting the bat on the bat's sweet spot and to me it's interesting to see the slow motion harmonic response of the bat (see how it vibrates upon contact and as it flys away).  If the hands were gripping the bat more firmly, that harmonic response would be different and therefore less energy would be lost in that harmonic response if the hands stayed firmly on the bat resulting in the ball going further.

well it was a bomb so not that much energy could have been lost. the bat did vibrate but not sure it is the lack of support could also be a slight off center hit due to the bat altering path due to the losing of the grip. 

the HR "only" was around 370-3800 feet (mlb average is 400) so he might have lost some distance but not sure. 

SultanofSwat posted:

 Prophetic.

Its not a rabbit hole.  Dr. Nathan is just incorrect.  Dont always assume someone has got it right just because they have PhD and came to a conclusion.  You have to think outside the box.  Many theories in the past including your "world is flat" have been proved wrong

But I have pictures on the flat earth.

Dominik85 posted:

well it was a bomb so not that much energy could have been lost. the bat did vibrate but not sure it is the lack of support could also be a slight off center hit due to the bat altering path due to the losing of the grip. 

I've watched quite a few MLBer slow motion hits like that (either during a live game broadcast and on YouTube) and I've always seen that harmonic vibration when the ball is hit and the bat has always shown a vibration like that regardless of where the ball strikes the bat.  When the ball is struck on the sweet spot, there is way less vibration (the visible harmonic evidence).  So, even when the batter is gripping the bat rather tight as they make contact, it can't be stopped because of the nature of the bat and its harmonics.  "The lack of support" as you say does indeed change that harmonic just as gripping the bat harder does.  Yes, it was a bomb alright and it would have even a bigger bomb if less energy was displaced/lost at contact. 

The ball is gone from the bat long before the first vibration reaches the hands.  It doesn't matter if hands are on the bat or not, nor how big the batter is....only how fast the bat was swung.

Go44dad posted:

The ball is gone from the bat long before the first vibration reaches the hands.  It doesn't matter if hands are on the bat or not, nor how big the batter is....only how fast the bat was swung.

The harmonic is generated as soon as the ball makes contact with the bat and it's amplitude will vary depending on various factors (e.g. bat size, bat shape, material of the bat, where on the bat the ball is struck, dampening affect of anything attached to the bat and where such attachment might be, etc.).  It all affects how the energy is transferred to the ball for its maximum exit speed for any given bat speed.

It's just physics.  

I agree with you that it doesn't matter how big the batter is except for the strength/power it takes for acceleration to be generated to get as much bat speed as possible to the point of contact.

I have long since forgot what I was arguing about.  I may have not ever known in the first place.  My only certainty is that I was right.  If I could just remember where I put my car keys....

Haha, not right at all.  I think you guys want to explain the phenomena away by saying my Zepp is wrong.  Sorry guys, my Doppler says the same thing as  well.  Funny how we try to explain things away when they are inconsistent with pre-conceived ideas.

What I really like about Science is that no matter how right you think you are, the conclusions/assertions are ALWAYS questioned; new data arrives and conclusions are refined and/or changed accordingly.

Go44dad posted:

I have long since forgot what I was arguing about.  I may have not ever known in the first place.  My only certainty is that I was right.  If I could just remember where I put my car keys....

. . . just wait till you get to be my age.  

Since my previous postings were are based on my limited knowledge and understanding of physics, and since I'm retired with little demand on my time, I did a little web searching and found few interesting things:

http://ffden-2.phys.uaf.edu/21...jon_drobnis/bmb.html

“Every time a ball is hit it gets deformed from all the force that is applied to it. The ball actually compresses to about an inch. In this process a lot of energy is lost. It is lost into the form of heat, friction, sound and vibrations. These vibrations are also known to have broken bats. When the ball collides in the areas that cause more vibrations, the batter will know! These vibrations cause a stinging sensation, however if the batter hits the ball right in the sweet spot, located about 5 inches in from the bat, then less or no vibrations are caused and more energy goes into the ball causing it go a further distance.  “

http://www.acs.psu.edu/drussell/bats/sting-damp.html

http://www.acs.psu.edu/drussell/bats.html

http://baseball.physics.illino...chicago-tribune.html

“As it turns out, the complex calculations needed to understand the sweet spot resemble those used to analyze the vibration of airplane wings or bridges, Nathan said. Such motion is important in baseball because every vibration in the bat takes away from the ball's speed as it leaves the plate.”

http://www.acs.psu.edu/drussell/bats/bend-sweet.html

http://hyperphysics.phy-astr.g...du/hbase/soushm.html

 I found another publication that mentioned the dampening of the "vibration" that the hands do on the bat and the effect the the energy transfer that's added due to the batter continuing to add acceleration at the point of contact.  Their conclusion was that the addition of this factor wasn't warranted given that the amount of energy attributed to it was so small because of the short time the bat is in contact with the ball.  The "dampening effect" of the hands being on the bat does have some significance in reducing the amount of energy lost upon contact.   

I didn't same the location of that article and could not find it again.  

Still can not find anything in the realm of baseball physics that supports the idea the the mass of the batter contributes in any way to more exit speed given a fixed bat speed.

Truman, cool information, thanks for sharing.  I feel like my son and my swing are just the perfect storm to see an effect that is just not understood yet.  To see this effect, you must have high bat speeds with a smaller mass attached to it.  We do know that on average, a batter can get let's say 10-20% additional exit velocity ontop of their bat speed.  In my case that is a 76mph + 14mph = 90mph.  That is the trampoline effect like in your ball getting squashed.  This is just a fact, however; I would theorize that at a given bat speed, there has to be enough mass to get the additional 14mph at contact.  I think as the bat speed goes up, that mass must be even greater.  For example my son's bat speed is 76mph, but he only gets 73'mph exit.  Groundouts kid is in the 100s for bat speed but high 80s for exit.  For those that want to bring up launch angle, sweet spot, etc., please just go back through the thread as I have already addressed that.  

Truman, I think you want to address this by saying my strength comes into play at contact.  I'm not going to say 100% that is not what's going on, but I really don't think so.  I believe once I or my son have gotten the bat up to max bat speed (76mph in our case),  that the force to do that must be the same and become static as it approaches the ball (force on the bat)  I think at that moment prior to contact the force is identical.  At contact is the issue.  I think because of my mass, I can contunue the force through contact, but my son's mass can not.

Here is a factor I completely missed:

http://ffden-2.phys.uaf.edu/21...0template/Slide4.htm

In trying to think of all the things that keep the bat from transferring all its energy to the ball, I forgot about Newton’s 3^rd Law and how exactly it applies to the hitting of a baseball.  According to it, unless the ball is contacted at just the right angle, the stored energy in the bat will not all be transferred to the ball.  So, this (at least in part) can explain why two people with the same bat speed can have different exit speeds. 

http://www.efastball.com/hitti...-speed-by-age-group/

My Bat Speed is the same as my teammate, but his Exit Speed is higher/lower

Three factors could create a different exit speed with a given bat speed.

• Different bats with different sweet spots and/or performance
• Hitting the sweet spot... or not
• Max bat speed is achieved at a point in the swing other than contact with the ball

The last bullet point is a good point too and one that one needs to keep in mind regarding this question.

If a mass (let’s call it vM) has no velocity, then it has a force of zero and no energy which can be transferred to another mass.   No matter how massive vM is, it will always have a zero force vector when at rest.

Torque and angular acceleration of vM can transfer energy into another mass and again, it doesn’t matter how much mass vM has, the torque and angular acceleration is what’s generating the force/energy. . . mass is not part of the equation.

In the case of a batter, the same rules of physics apply.  If bat speed is the same and measured at the exactly the same point of contact and the same vector, exist speed will be the same.  If exit speed is not the same, then there’s a number of variables and combinations of them than can explain that difference (e.g. loss of energy due to the harmonics of the bat because of where on the bat it was struck; the plane of the bat in relation to the plane of the ball exiting the bat, etc).

 When a batter has forward movement when planting his front foot, one might think the mass of the hitter would then come into play.  But it doesn’t because all that forward movement stops before contact with the ball and that movement is then transferred into angular acceleration and the energy is transferred to the bat.

 The bigger the bat (more massive), the more energy it can store and transfer to a ball (note too that a ball has limits as to how much energy it can absorb and release to get its maximum velocity).  And in addition, the more acceleration that can be generated, more mass is added (E=MC²) and more energy is added than can be transferred to a ball.   Of course, there are limits to the size of a bat a human can accelerate sufficiently.  At some point mass of the bat becomes more than a human can accelerate well. So, there is a optimum size.  From what I’ve been reading today, the optimum bat size (mass/weight) is between 32oz to 42 oz.  Modern MLB players prefer the lower end of this range to have a good balance between bat control and power.

Linedrive_07 posted:

 I think at that moment prior to contact the force is identical.  At contact is the issue.  I think because of my mass, I can contunue the force through contact, but my son's mass can not.

That's kind of what I was considering and wasn't sure how much of it difference it makes.  But rather than thinking in terms of "mass" of your body, I was thinking in terms of brute strength to overcome or off set the energy transfer from the ball to the bat that occurs.  According to that one article I read (that I can find now), that factor is so small they they pretty much ignore it in there calculations.

Like I said, I am positive Bat Angle, Sweet Spot, etc. Is absolutely not the culprit here. See again, what I explained to Atlantic:

Another thought.  Imagine this scenario, I swing a bat off normal ground, and then once I get it up to my 76mph bat speed, I am instantaneously moved to an ice skating rink.  I am willing to say 100%, I would not be hitting the ball at my max exit velocity of 90mph.  I believe it would drastically be lower.  Does that mean I am weaker?  No.  It means I do not weigh enough or just call it friction to generate the same exit velocity (Transfer of Force) to the ball.  That is about the best logical comparison that I can make to explain what is going on.  Could my son actually be slipping a little at contact, moving back as I would on my exaggerated ice example?  Possibly.   To me that is all the same (weight/mass/friction).

Thinking about your comment regarding forward movement as a hitter not impacting the swing, I don't agree with that either.  I used to box a little and can say I through a much harder punch stepping into it that standing flat footed. With hitting, if I just stand there or not shift my weight from back to front, I don't get nearly the same exit velocity.  

Last comment and I'm calling it a night.  You bring up a larger bat again, which I mentioned before.  We all agree a bat that is 2x as heavy, swung at the same bat speed as a normal say 30oz bat, then that would produce a bigger exit velocity.  The point I keep bringing up that has to be addressed is this:  What constitutes weight as far as the ball is concerned in the bat?  Is is a quarter sized area on the bat that constitutes weight that can be transferred as a force into the ball?  Are we saying only a specific part of the bat, where that section only matters and the rest could be PVC tubing?  This has to be answered, because as you know, I'm going to say again, the ball has no clue what a bat it is.  It does not know where the body begins and the bat ends.  Your arm may as well be an extension of the handle and so on.  This is just one of those things where we have to look at it differently than it has been in the past.  I think if you guys were to experience and have the same observation that I have, you would realize it is in fact real, so it must be addressed.  I don't know what else to say because we are not going to explain it away.

Linedrive_07 posted:

Another thought.  Imagine this scenario, I swing a bat off normal ground, and then once I get it up to my 76mph bat speed, I am instantaneously moved to an ice skating rink. I . believe it would drastically I am willing to say 100%, I would not be hitting the ball at my max exit velocity of 90mph be lower.  Does that mean I am weaker?  No.  It means I do not weigh enough or just call it friction to generate the same exit velocity (Transfer of Force) to the ball.  That is about the best logical comparison that I can make to explain what is going on.  Could my son actually be slipping a little at contact, moving back as I would on my exaggerated ice example?  Possibly.   To me that is all the same (weight/mass/friction).

When you've reached your 76 mph bat speed the bat has stored "x" amount of Kinetic Energy due to angular acceleration.  If angular acceleration stops right there, no additional Kinetic Energy will be stored in the bat, even if you are in a zero gravity environment or not.   When the bat hits the ball with that "x" amount of Kinetic Energy, you're always going get the the same transfer of energy (assuming there no change in the factors that bleed energy from being transferred to the ball).  In your scenario, there's no other force acting to transfer energy to the bat.  The "weight/mass/friction" is not producing energy than can be transferred to the bat.  But, what you seem to be referring to has to do with Newton law for every action there and equal and opposite action.  That is, if the transportation to ice, in your scenario, reduces that amount of energy transfer to the ball, it would have to be as a result of the ball's energy being transferred to the bat, into the arms and into the body causing a vectored offset of force at impact for a loss in some exit velocity.  That amount of force lost here is so small, it can't account for anything close of the difference you're seeing.   

Linedrive_07 posted:

Last comment and I'm calling it a night.  You bring up a larger bat again, which I mentioned before.  We all agree a bat that is 2x as heavy, swung at the same bat speed as a normal say 30oz bat, then that would produce a bigger exit velocity.  The point I keep bringing up that has to be addressed is this:  What constitutes weight as far as the ball is concerned in the bat?  Is is a quarter sized area on the bat that constitutes weight that can be transferred as a force into the ball?  Are we saying only a specific part of the bat, where that section only matters and the rest could be PVC tubing?  

Certainly, referring to the actual weight of the bat is an over simplification when talking about stored and released energy into the ball.  The actual weight of the bat has more to do with acceleration, which adds Mass (not weight) to the bat.  COP (Center-Of-Percussion) is affected by a bats weight and it's shape.  MLB players like heavy end weighted bats at that's how to maximize the bats total weight to maximizing energy transfer into the ball . . . even when COP isn't achieved on a hit.  If you take a particular shaped bat and keep that shape and dimensions, but you change its weight, you're going to get different exit speeds accordingly (and to do that to an extreme to really see dramatic results; you'd have to use different materials).

 

This has to be answered, because as you know, I'm going to say again, the ball has no clue what a bat it is. 

 Well, true. . .. but then, in a sense it does.  

 

 It does not know where the body begins and the bat ends.  Your arm may as well be an extension of the handle and so on. 

 The hands and the arms do not change the COP of the bat.   If you build a bat that is as long as the sum of the length of the arms, hands and the bat, there would be a different COP than the hand held bat.  This doesn't happen when holding onto the bat because the hands are not really part of the bat and the hands are only connect to the point keeping the bat from flying away as angular acceleration is performed then acting as a harmonic damper at impact with the ball.

 

This is just one of those things where we have to look at it differently than it has been in the past.  I think if you guys were to experience and have the same observation that I have, you would realize it is in fact real, so it must be addressed.  I don't know what else to say because we are not going to explain it away.

 The only way to know is to have precise measurements of each variable that we've been discussing and look at the math. And of course, to do it in a scientific manner rather than anecdotal.  

 BTW, thank you for this thread and your input.  You got me looking at this a lot closer than I ever have.  The last baseball research I did has been some time ago when I was trying to find out how I could help my son with his pitch velocity. 

Linedrive_07 posted:

Thinking about your comment regarding forward movement as a hitter not impacting the swing, I don't agree with that either.  I used to box a little and can say I through a much harder punch stepping into it that standing flat footed. With hitting, if I just stand there or not shift my weight from back to front, I don't get nearly the same exit velocity.  

that is an indirect effect, the forward momentum is transferred into rotational energy when the front leg braces. 

Let's approach this from a different way.  First, you have to accept my previous statements.

1.  Both my Son and I have the same max bat speed at contact: 75mph

2.  Sweet Spot, Bat angle, Launch angle, moving up, back etc in the stance, are not causing the effect as I have already addressed those ideas.

3.  Two separate devices have confirmed bat speed:  Dopplar device and Zepp that use different technology.

4.  Exit velocity confirmed by Three different devices:  Stalker at local cages, cheap Bushnell, Hittrax at team practice (Although I didn't get to try this so he is the only one with 3 devices confirming exit)

5.  I'm 6' 210lbs and Junior is 5'7 120lbs. His mom is also 5'7 for those wondering.

6.  My exit velocity Maxes out at 90Mph. My average is around 85mph and low on shanks 79mph.   Juniors at 73mph max.  Average 70mph.  Low around 64mph, but he rarely misses, while mine is much more volatile in terms of hitting sweet spot.

5.  Strength: Yes, I'm considerably stronger.  We don't max, but I can rep 225lbs 16x.  He reps 115lbs 15x.  Using the online max calculator (accurate within some percent, puts me close to a 350lb bench.  Juniors close to 170lb.  I don't have all the rest of the numbers for other lifts, but I am stronger in general across the board.  He may actually have me in core though because he is extremely strong there as he works it very hard and I half a$$ that .  Actually he may get me in pull-up number where I'm around 15 and he is like 17 (1st continuous set) but I'm still stronger there due to pulling more weight.  He is very strong for his weight because he works hard due to being a late bloomer physically and has to make up for that and does a great job at it (probably most ripped kid at his age in our area (not bragging, just is what it is as I am trying to get the word strength across pertaining to his size). Starting SS for HS and summer teams (Athlete).  The reason I bring this up is because, I'm still slightly open to all this being due to overall strength difference but not very for reasons already discussed.

Ok, I would love to hear someone provide a logical reason why this is.  Obviously, I am not going to accept the word magic.  If you are able just to accept what I said above is true, you quickly understand while I feel it is Mass, but I'm open to any theory even though most of you are not open to mine.  Lastly if you are not will to accept what I stated above, then I no longer want to discuss it any further.  That would just say you do not believe me and you are being close minded, so I'm wasting my time.

Lastly you have to understand where I'm coming from and why I'm confident in my assertions above.  Junior is a rising sophomore and we are trying to get him ready for his showcases coming up in Feb.  You all know he is approaching a big year recruiting wise.  A few months ago when I discovered his exit velocity compared to his bat speed, I was like you guys because I know darn well the exit is supposed to be considerably higher than his bat speed.  We did EVERYTHING mechanically with regard to stance, etc to fix it, and NOTHING mattered.  After banging my head against a wall there for a couple months, You can see how I arrived at mass.  Ok now it's your turn, and no using the word magic .

I think the answer is :  Joachim's Razor.  

Based on your previous athleticism you likely get more extension of the bat through the ball and are able to transfer your coiled energy to your left side more aggressively creating greater bat ball collision than your son, thus greater exit velocity.

Also, your son could have hit a line drive and be on base and you hit a hard ground ball and are out at first.

I spoke with a top D1 coach the other day and he said his coaches are laughing about the PBR hype on exit velocity.  You can have a beast of a human being have a higher exit velocity but his swing looks like crap, works inefficiently, is inconsistent and for the life of him can't hit a 90mph FB.

Gov,  I appreciate the thoughts.  I also agree 100% with the last statement knowing that while I can still get 90mph exit on a Tee, I would be horrible with my current mechanics hitting a 90mph fastball.  Im sure my exit back in the day was much higher along with my bat speed.  

Here is the thing and lets say the extension is true for a second.  My issue is, my son's bat speed does not correlate at all with his predicted exit velocity (at least as what should be expected).  While it should be higher than his bat speed (probably 15mph higher), its actually lower, which does not add up at all.  We could say ok, he just is not athletic, and mechanics are more in line with rec ball.  I can tell you though, that is the farthest from the truth.  Not because he is my son, but he just in fact is very athletic with pretty good mechanics (100x better than mine for sure),  great hand speed and strength.  I should of added my hand speed on the zepp is averaging 30mph at best and his at 34mph.  He has hit 82mph bat speed with 36mph hand speed, but that is too much of a down plane and not getting on plane compared to when he is 76/34mph.

Given how you described your son, it sounds to me like he's not getting off his backside to release his harnessed energy through the ball.

A lot of kids were taught squish the bug with the back foot.  Not that he was taught that, if a hitter is doing that he's limited with how much energy he can get transferred because the hips are not in a position to fire. The back knee needs to travel a bit with the stride foot to unweight the back foot to allow the hips to fire, transferring the energy through the ball.  

Went through this with my 2018.  Had no idea what was going on... He was too rotational, not getting off the backside and by default not really getting squared up contact and creating extension through the ball.  He's corrected it, results are more piercing line drives, back spin with carry.  His exit velo shot up from 84 to 89 quickly.  Once he gets it all in sync he'll get higher exit velo, but then he'll also be getting a lot stronger along with an even finer tuned swing.

Linedrive. Why not post a video? 

Gov,  I hear you and that is the same thoughts I would pass along to someone as well.  I am definitely not a fan of squashing the bug.  I have head coached up until last year, but with him hitting HS, it was time to let all of that go.  I just work with him on the side outside of his team practices now.  He definitely gets transfer into his front stiff leg at contact.  Im sure you have seen the harper swings where his back foot is not even on the ground at contact.  Trust me, we have tried to recreate that with his back foot off the ground to ensure all energy is transferred, so am just telling that is not it either.  I would love for someone to have an example of a 120lb kid that can get more than say 75mph exit on a bbcore bat with a bat speed of 75 for example.  If it were a man, that would be even better because we could assume with some man strength he would be stronger and that would put it to bed.  If anyone has an example out there, please share.

I will consider posting a video.  I just dont want to get hung up in a perfect sweet spot, mechanics discussion when I know with out a doubt, that is not it.  Here is zepp, that I shared on a different thread on a bat speed discussion.  Yes too much down angle and I know its not what you are looking for, but for what its worth, to see some of the athleticism.  Believe me, if I thought it was a mechanical issue, Im not too proud and would be hitting the community up about it:

Had an earlier discussion regarding MOI and why you want a low MOI bat..... Advil and video would be helpful to solve your question.

Normally not one answer, but a combination of a few things.  But I'd lay odds your son is not getting off his backside like you think he is...

video & 3 Advil required

Against my better judgement, I will upload a video we took today.  I am not going to say his mechanics are perfect, but I will say they are in no way bad enough to justify not getting an exit velo that is higher than his bat speed.  That is the standard and even I can do that.  I will pose an easier question though.  Anyone got an example out there with someone 120lb or less getting exit velocity from a bbcore that is higher than their bat speed?  The only caveat is his bbcore is balanced, so not end-loaded.