Spin resulting from off-center impacts with irons

[nkurz]

"Gear effect" is a surprisingly large effect for drivers and other clubs with a center-of-gravity that is significantly behind the club face.  But with irons, where the center-of-gravity is much closer to the face, the effect is much less.  Dave Tutelman, in his excellent article on gear effect, goes as far as saying (in the caption for an explanatory image) that irons have "No gear effect": 

 

 

This would leave open the possibility that there is some other effect that produces spin for non-center impacts with irons, but in the text he says that for "there isn't any special effect on spin" and "You don't have this effect with an iron."

 

Is this true?  Is there really no difference in spin from off-center shots with irons, whether they are high, low, left, or right?  And given that we know that the off-center shots have lower Smash and hence lower ball speed, does this mean that the spin stays the same as a full speed shot or is it proportionally less in accordance with the difference in Smash?

 

I can find a few pieces of evidence that the effect is present but just less with irons.  This article for example cites 2009 Trackman research (PDF bottom of page 3) showing a significant gear effect for a 6 iron.  Here's the chart:

 

 

So which is it?  Is there a gear effect for irons or not?  Or maybe there is a small gear effect, but it's countered by some other larger effect on spin?  Is there better info somewhere regarding the effect of off-center impact on spin for irons?  

 

 

[Chunkitgood]

This is a good question.  Whenever high moi clubs are discussed, it’s usually in terms of getting better distance from off center hits, but rarely (as far as I can see) in terms of getting straighter shots.

 

Some advertisements seem to imply you get straighter shots with high moi clubs, but I’ve never heard an outright claim to that effect.

 

There could be a difference in direction due to deflection of the club face even without gear effect, it seems to me.

[nkurz]

Hmm, not a lot of discussion so far.  Was a there a better place on the site to post this?  

 

For another opinion, here's the copy for Wishon's new wedge series: 

 

Quote

 

The uniquely innovative 309HCG wedges draw upon the proven science of vertical gear effect to achieve an increase in backspin.  Experienced clubmakers are aware that impact below the CG (Centre of Gravity) with a driver increases backspin due to the principle of the vertical gear effect.  

Wishon Golf has employed this lesser known principle of clubhead dynamics to create an all new family of wedges that deliver enhanced backspin by moving mass higher on the head to position the CG above the typical point of impact on the face.  Impact below the CG causes the head to rotate slightly forward which causes the ball to roll up the face to offer a slight increase in backspin over the normal reaction of the ball to the friction of the areas between scorelines.

 

 https://wishongolf.com/309hcg-wedges/

 

Clearly they believe that irons (or at least wedges) can have a gear effect.  It would be nice to know how large an effect they are seeing. 

 

[TKS]

I've neen playing around with this in an unscientific way.  Wishon's current EQ1-NX wedges are hollow in the sole area and lighter than std for SL applications. I have built one to "std" length and started to add lead tape to the back of the topline. If it stops snowing or I can get some time on a monitor, I'll report back.

 

[aggiegolfer21]

On 12/25/2022 at 7:55 PM, Chunkitgood said:

This is a good question.  Whenever high moi clubs are discussed, it’s usually in terms of getting better distance from off center hits, but rarely (as far as I can see) in terms of getting straighter shots.

 

Some advertisements seem to imply you get straighter shots with high moi clubs, but I’ve never heard an outright claim to that effect.

 

There could be a difference in direction due to deflection of the club face even without gear effect, it seems to me.

 

High MOI clubs do travel straighter.  This is due to the fact that there is less twisting of the clubface on off center strikes thanks to the perimeter weighting.

[nkurz]

15 hours ago, aggiegolfer21 said:

 

High MOI clubs do travel straighter.  This is due to the fact that there is less twisting of the clubface on off center strikes thanks to the perimeter weighting.

 

It's almost certainly true that high MOI clubs twist less from off center impacts, but it's less certain how large this effect is compared to other possible effects.  The impact time is so short (.0005 seconds) that there just isn't much time for the club to turn while it's still able to influence the ball flight.   How many degrees do you think a low MOI iron turns during impact?  How much less would a high MOI iron turn?  

 

I searched, and I couldn't find an easy precalculated answer.  My quick attempts at estimating (based on known gear effect side spin for a driver and some probably undefensible assumptions) suggest something less than 1 degree of opening for a strike 1/2" to the toe, thus quite a bit less than 1 degree difference between different iron designs.  So real, but less than the typical accuracy of an iron shot.  Anyone have a better answer to this question?  

 

My quick attempt at a search did turn up a recent very promising paper that might answer the topic of this thread.   I'll try to write up a summary tomorrow, but if anyone wants to read it before then, it's here: The influence of 5-iron clubhead mass distribution on clubhead presentation and initial ball launch conditions: Part I: Golf robot tests.  The ultra quick summary is that the found a significant gear effect for cavity back irons, but not for blades.

 

[aggiegolfer21]

30 minutes ago, nkurz said:

 

It's almost certainly true that high MOI clubs twist less from off center impacts, but it's less certain how large this effect is compared to other possible effects.  The impact time is so short (.0005 seconds) that there just isn't much time for the club to turn while it's still able to influence the ball flight.   How many degrees do you think a low MOI iron turns during impact?  How much less would a high MOI iron turn?  

 

I searched, and I couldn't find an easy precalculated answer.  My quick attempts at estimating (based on known gear effect side spin for a driver and some probably undefensible assumptions) suggest something less than 1 degree of opening for a strike 1/2" to the toe, thus quite a bit less than 1 degree difference between different iron designs.  So real, but less than the typical accuracy of an iron shot.  Anyone have a better answer to this question?  

 

My quick attempt at a search did turn up a recent very promising paper that might answer the topic of this thread.   I'll try to write up a summary tomorrow, but if anyone wants to read it before then, it's here: The influence of 5-iron clubhead mass distribution on clubhead presentation and initial ball launch conditions: Part I: Golf robot tests.  The ultra quick summary is that the found a significant gear effect for cavity back irons, but not for blades.

 

I would say you're probably right about this.  There is a difference but it's pretty small.  Another contributing factor to high MOI irons going straighter is the reduction in spin.  This probably plays a greater role in producing straight shots than the reduction in gear affect.

[Socrates]

Having breezed through the tests and graphs and cut to the conclusion, it appears that the cavity back has more gear effect because of it's deeper CG location.  This results in higher spin and a lesser rate of face angle deflection.  The important bit of information from what I can see regarding ball speed, is the face angle.  "Thus, the horizontal impact location appears less important than the face angle and vertical location for maintaining ball speed."

 

I guess from what they tested for, if you can find a cavity back that doesn't have a deep CG location, you might be better off if you are a 'better' player.  You get better ball speed off a mis-hit while not getting too much 'gear effect.'  Which is what you see in a lot of the better player irons.  

Should be interesting when they incorporate human players into the test.

[nkurz]

On 12/31/2022 at 11:35 AM, Socrates said:

I guess from what they tested for, if you can find a cavity back that doesn't have a deep CG location, you might be better off if you are a 'better' player.  You get better ball speed off a mis-hit while not getting too much 'gear effect.'  Which is what you see in a lot of the better player irons.  

Should be interesting when they incorporate human players into the test.

 

I'm not ready yet to summarize their robot testing paper, but Part 2 where they use human testers is here: https://journals.sagepub.com/doi/10.1177/1754337120953309.  It's a bit of an anti-climax.  They found that  although the cavity backs gave a higher launch angle (as expected) but they weren't able to show a much other advantage:  "However, results in Part I relating to the forgiveness of the cavity-back club did not manifest more consistent shot outcomes when used by golfers."

 

Separately, I'm not sure that a high MOI cavity back with less gear effect would necessarily be better. What I think you want is the "right amount" of gear effect so that it cancels out the opening or closing of the face caused by hitting it off-center.  I think this is going to be swing speed dependent, and I'm not even sure where current clubs fall on the scale of under- or over-correcting.  

[nkurz]

OK, let's try to summarize the paper: The influence of 5-iron clubhead mass distribution on clubhead presentation and initial ball launch conditions: Part I: Golf robot tests; Wallace, Corke, Jones, Bettler, and Otto; 2020.    From what I can tell, the paper is based on Corke's 2015 Doctoral Thesis (which I haven't managed to read yet).  

 

They used two different 5 irons on a robot swing tester.  One was a "blade" (actually just a very small cavity) and the other a "cavity back" (not clear how extreme).  The blade had a center-of-gravity (CG) 10 mm behind the face, and the cavity had a CG 12.6 mm behind.   The cavity back also has a moment of inertia (MOI) that is 30% greater.  They showed pretty clearly that cavity back 5-irons have a clear gear effect for off-center hits.  

 

The main "money shot" is in Figure 8. It shows the spin resulting from shots that are horizontally centered on the CG but at different heights on the club face:

  

 

It's easier to understand if you look only at the Red dots, which represent a square face angle.   It shows that for the Blade (chart on left), the spin is highest when the vertical impact is at the CG (rightmost set of dots at 0), and very slightly lower as the impact is moved down the club face (-5, -10, -15).   The lowest spin is when the ball is lowest on the club (positive offset is up the club face).   This is what you would expect if the dominant effect was the club "wrapping around" the ball due to the impact below the CG.

 

But for the Cavity (chart on right), we see the opposite.  The spin is lowest at the CG (rightmost set of dots at 0), and then gets slightly higher as we move down the club face (highest spin at -15 mm of vertical offset).   This is the same as you see for drivers, which are known to have a large gear effect, and thus they conclude (correctly) that this is evidence that cavity irons experience gear effect.   Presumably the club still tilts downward (although less due to the increased MOI), but the ratchet effect of the club face moving down is enough to overcome this.

 

Realistically, this also means that Blades also have a gear effect, just that it has less effect on spin than the larger effect of reduced loft due to hitting the ball lower on the club (the face tilts forward at impact).  Apparently, the extra 2.6 mm of rearward  CG plus 30% higher MOI about the horizontal axis is the difference between the tilt of the club dominating the spin effect and the gear effect dominating.   One thing I found lacking from the paper was an attempt to split the tilt effect from the gear effect so as to compare them.

 

The results for horizontal impact are similar, but harder to interpret because of the way they present the data.   Instead of decomposing the spin into backspin and sidespin, they use "Spin Axis" presumably because that's what their Trackman gave them.   What would be really nice to know (future research) would be whether the resulting shot ends up under or overcorrected.  We know that for a driver, it would be overcorrected if not for "bulge", but they don't try to answer this.  

 

Anyway, from what I can tell, it's a pretty good paper, and conclusively shows that irons do exhibit a gear effect.  Further, it shows that at least for backspin with a cavity back 5-iron, the gear effect is large enough to be dominant.  Just a like a driver, you should expect higher spin for impacts low on the club, and lower spin for impacts high on the face.   Tutelman's page on Gear Effect should be updated.  

[joostin]

@nkurz Nice summary and finds with the research papers.  I'm slowly working up some diagrams that support this (similar to Tutelman's but with more detail), and should be able to post sometime soon.  I'll probably post in the WRX Tech subforum because there might be more interest there with technical nerdy equipment related things (that not too many care all that much about if it doesn't help their game :), but will link here.

 

Basically since it's known that the clubhead rotates about it's CG on an off-center hit, as long as the CG is anywhere behind the face, there is a gear effect.  It might be too small to notice in real life especially if the CG is very close to the face, but the farther back it moves, the more the effect is amplified and noticeable.

[nkurz]

3 hours ago, joostin said:

@nkurz Nice summary and finds with the research papers.  I'm slowly working up some diagrams that support this (similar to Tutelman's but with more detail), and should be able to post sometime soon.  I'll probably post in the WRX Tech subforum because there might be more interest there with technical nerdy equipment related things (that not too many care all that much about if it doesn't help their game :), but will link here.

 

Thanks, and I look forward to your post.  Since it sounds like you are recreating Tutelman's derivations, I'll mention that I'm confused by some of his choices.  I'm not sure he's wrong (read: he's probably right) but some of his choices aren't terribly well defended.  

 

As an example, consider his diagram on Figure 1-3 on https://www.tutelman.com/golf/ballflight/gearEffect1.php: 

It's not obvious (to me at least) that we want to use wC instead of wR.   He implies it's because we only want the "sideways" component, which is the same as wC.  Maybe, but I'm not immediately convinced.  Once the ball is "splatted" on the face, I'd think wR (which for an iron might be quite a bit larger than wC) might be more relevant, since you can apply spin at any tangent.  Or maybe it depends on the compressibility of the ball?

 

Maybe I don't have the right mental model of the impact.  I also have trouble following his "impulse" argument just below that.  I think maybe he's working from a frame of reference where the clubhead velocity is always 0 (where all post impact momentum of the club is angular) in which case Vb needs to subtract the post impact clubhead velocity?  Or I'm just lost.  In any case, an even clearer explanation that works for both irons and woods would be welcome.   

[joostin]

On 1/11/2023 at 9:21 PM, nkurz said:

 

Thanks, and I look forward to your post.  Since it sounds like you are recreating Tutelman's derivations, I'll mention that I'm confused by some of his choices.  I'm not sure he's wrong (read: he's probably right) but some of his choices aren't terribly well defended.  

 

As an example, consider his diagram on Figure 1-3 on https://www.tutelman.com/golf/ballflight/gearEffect1.php: 

/cdn-cgi/mirage/712d2f5be28e06d6ec41d6bbc5d363376ac1b20519421febfc060ec433120614/1280/cdn-cgi/mirage/712d2f5be28e06d6ec41d6bbc5d363376ac1b20519421febfc060ec433120614/1280/https://www.tutelman.com/golf/ballflight/gearEffect/velocity.jpg

It's not obvious (to me at least) that we want to use wC instead of wR.   He implies it's because we only want the "sideways" component, which is the same as wC.  Maybe, but I'm not immediately convinced.  Once the ball is "splatted" on the face, I'd think wR (which for an iron might be quite a bit larger than wC) might be more relevant, since you can apply spin at any tangent.  Or maybe it depends on the compressibility of the ball?

 

Maybe I don't have the right mental model of the impact.  I also have trouble following his "impulse" argument just below that.  I think maybe he's working from a frame of reference where the clubhead velocity is always 0 (where all post impact momentum of the club is angular) in which case Vb needs to subtract the post impact clubhead velocity?  Or I'm just lost.  In any case, an even clearer explanation that works for both irons and woods would be welcome.   

So the gear effect is imparted by friction.  Friction force is along the surface (parallel or tangent to the surface at impact).  The ωR angular velocity points in the direction the point of impact is rotating, but the ωC is its component along the surface that imparts a friction force, and the reason it's needed.

 

As far as the collision, yes the clubhead will slow down as the ball speeds up, but to gear effect his calculations only need to derive from how the clubhead deflects based on the clubhead properties and the the impact conditions, which can be derived from the ball mass and it's exit speed.

 

He uses impulse to get to angular momentum (L) equations:

 

For the clubhead:

L=Iω (moment of inertia of clubhead... in this case the horizontal MOI... times angular velocity of the head)

 

Relating to the ball:

L=x*m*Vb (if you look up angular momentum it's similarly seen as L=rmv)

 

From there you can solve for angular velocity of the clubhead and then the gear effect spin on the ball.  One thing is I think there could be a factor for slip, or a reduction in efficiency of the transfer of angular velocity (say water or a reduction in friction) to the ball spin.

[nkurz]

9 hours ago, joostin said:

So the gear effect is imparted by friction.  Friction force is along the surface (parallel or tangent to the surface at impact).  The ωR angular velocity points in the direction the point of impact is rotating, but the ωC is its component along the surface that imparts a friction force, and the reason it's needed.

 

Yes, this is what I take to be Tutelman's explanation, and it's right in the case of an "instantaneous" collision with no deformation, but I'm less sure it completely correct for an actual driver-ball impact. I don't think the simplified version of the ball rolling along the face is actually what generates spin.  Instead, I think it's more a matter of the point of highest compression moving to a spot where it is no longer in line with the center of the ball, and then rebounding in a way that produces spin.  

 

Try to picture the effect of the club turning with the ball "splatted" on the face like this:

 

 

It's not just simple gear-on-gear rotation.  My intuition is that while the perpendicular motion of the center of the face is a good starting point (from decomposing the motion at the off-center impact site into components) it might not  give the exact answer and we might need to consider the tangential movement somewhere between the center of face and center of impact.  I'm not sure of this, but I think the explanation for why this is not the case needs to go beyond Tutelman's "obviously".  

 

9 hours ago, joostin said:

He uses impulse to get to angular momentum (L) equations: ...

 

Good explanation, but I fear I'm still confused.  It seems counterintuitive that  your equation to calculate the final spin of the golf ball is ignoring the angular momentum in the spinning golf ball, and looking only at its linear motion.  My intuition (old, rusty, never that good to start with) is that might be a good first approximation but isn't strictly true, and might give something close to the right answer, but I have no sense of how close it actually is.  Am I wrong, and the integration across the entire ball actually does cancel out perfectly?  For that matter, what single frame of reference are these calculations using?  

[joostin]

15 hours ago, nkurz said:

what single frame of reference are these calculations using?  

He's definitely using a "lab" frame of reference - top view at impact with ground at 0 speed - because he uses ball speed Vb as actual measured speed (150mph in the calcs) vs the ground, not vs a moving frame following the moving clubhead (in which the relative ball speed would be around 50mph).

 

15 hours ago, nkurz said:

Yes, this is what I take to be Tutelman's explanation, and it's right in the case of an "instantaneous" collision with no deformation, but I'm less sure it completely correct for an actual driver-ball impact. I don't think the simplified version of the ball rolling along the face is actually what generates spin.  Instead, I think it's more a matter of the point of highest compression moving to a spot where it is no longer in line with the center of the ball, and then rebounding in a way that produces spin.  

You're right in that it is a simplified analysis:  Elastic collision, pure gear effect, not looking at compression.  The best analysis actually would be a computer FEA similation using accurate models of the club and ball, and specifying impact conditions.

 

I think though the hand calculations will still apply somewhat close to reality.  Reason being, if you watch slow mo vids like the one from the pic you showed, the center of the ball doesn't seem to move until after max compression, but definitely as it's decompressing.  So the point of initial impact probably isn't too far off the center of max compression (with relatively square contact).  The net force exerted on the clubhead (and ball) would be the same whether the ball is rigid or it squashes into a pancake because that's just mass of the ball times it's acceleration.  The impulse exerted on the club and on the ball would still remain equal to each other, though the time portion of the impulse equation would go up the more it compresses.  The collision is still creating the same torque on the clubhead - same net force, same moment arm - that is causing the clubhead rotation and tangential friction force, whether a rigid ball or squashed.  The ball will still see it, ending up with the spinning rebound that you mention.

 

I agree it doesn't account for everything.  True conservation of energy and momentum between club and ball is not the case with losses due to vibrations, non-elastic deformation, heat, whatever inelastic things happen.  The resulting spin numbers will actually be less than calculated because it won't be true gear transmission efficiency.

 

16 hours ago, nkurz said:

seems counterintuitive that  your equation to calculate the final spin of the golf ball is ignoring the angular momentum in the spinning golf ball, and looking only at its linear motion.

You mean Tutelman's 😉 (trust me I don't think I'd be able to derive what he did without taking college classes again).  As far as the ball's angular momentum, that's a result of the collision.  Of course he's assuming a true gear effect, so whatever tangential velocity the clubhead sees at the impact point, ωC, is the same tangential velocity the ball has in his calcs.  Where I see something that could play a role that I didn't see factored in is the MOI of the ball, that's resisting going from 0 spin to having spin, and the coefficients of friction of the clubface and ball.

 

16 hours ago, nkurz said:

might give something close to the right answer, but I have no sense of how close it actually is.

I haven't come across anyone that has verified anything tbh.  It would be cool to see a controlled robotic gear effect study in reality vs his calcs, or a simulation like I mentioned before.  Probably the closest anyone can measure is from launch monitor data for zeroed out impact conditions (0 face to path, 0 face angle, 0 AOA), measuring how far off the strike was vs a measured CG, then deriving gear effect spin based on the clubhead at its dynamic loft, ball speed, spin, spin axis... 

 

16 hours ago, nkurz said:

 Am I wrong, and the integration across the entire ball actually does cancel out perfectly?

I hope so because I don't have fond memories of calculus classes lol.  But yeah they cancel out because the impulse integrals equate to ball momentum and clubhead momentum, and the impulse is physically the same for both equations, so we're left with momentum equations.

 

16 hours ago, nkurz said:

My intuition (old, rusty, never that good to start with)

Same

 

Tbh I wasn't expecting to dive deep into his gear effect calculations, and definitely not recreate them or derive some other way.  I was only working on creating prettier diagrams because I thought the one he has for an iron can use a better look.  I certainly don't have the drive, skill, or knowledge of Mr. Tutelman when it comes to analyzing and deriving the math and physics (nor the patience to look through so much at a time), but can only try to interpret it.  I'm learning as I go too, so the discussion helps.

[joostin]

Linking the new thread I put in the Tech subforum to your original thread here.