If you watch his videos, his urethane is "performing" more aggressively than his reactive. . .

BUT if you look at the start of his rack-attack video, you can see that his urethane is MUCH more sanded than his rack-attack . . .

so, all things being equal - but they're not! LOL.

Would be much interesting to see them both at 500, same board, same pattern.

Yes, in the videos, the Rack Attack is at stock 500/4000 with a low flare layout.
The Karma is at 500 grit with a high flare layout.
On these lanes, that day, the Karma was way stronger. Too strong, and rolled out and had no power.
The Rack went way long and had good power.

The point of the videos was not to compare the performance of those two balls per say, but to point out the difference in ball reaction characteristics.

Urethane is early and reactive is late.

Now at Parkway bowl I have the idential high flare layout on my Hammer Dark Legend Solid and my Crow urethane and both balls are at 1000 grit.

On the fresh the Dark Legend will have difficulty reading the lane where the Crow will read it well. After one game with the Crow I can go to the Dark Legend and it will work fine.

Or, I can simply stay with my Crow urethane and slowly move right as the set goes on to maintain the reaction as the urethane ball gets more oil soaked and loses reaction.

Basically with urethane I do the reverse from reactive.
With reactive, as the ball breaks the lanes down we tend to move in with our feet and target.

But with urethane I move right with feet and target. This is because the ball gets more slippery from the oil despite wiping it off. As I move right eventually I hit an equilibrium point and I no longer need to move and can just stay put for the rest of the set.

So on the fresh I will throw my Dark Legend right up second arrow and it will not even move, but if I throw my dry Crow up second arrow it might go Brooklyn! The ball is fully dry and the first throw is always far too strong!

But as the Crow get wetter and wetter it loses that strong reaction it had when it was dry and now I have to move right into the dry to keep the reaction going.

One slight point on the videos.
The target path that both balls, the Rack and the Karma followed were very close to each other.
I have attached screen caps to illustrate.

Rack going up 7.5 board and Karma going up 7, maybe 7.25 board.

Problem is that you have two different surfaces on the two balls - and two different layouts.

Thus, we're not really comparing urethane to reactive, we're (this time) comparing 500 vs. 500/4000

I'm not sure which delivery I was comparing - the one I looked at seemed to be at least a board outside - and not just at one POINT, but rather at delivery and then continuing.

Have you ever come across a site that describes Jesper Svensson's layout? He seems to have urethane figured out. I've never seen him throw reactive yet.

I understand.

Again, I was just using these two videos as extreme examples to illustrate the difference in how a reactive and a urethane read the lane.

Urethane hooks early and rolls long and reactive hooks late and rolls short and sudden.

By increasing the grit on urethane the skid phase can be lengthened but it will always hook early and roll longer than reactive.

You can see that here in this video on the same lanes after I took my Karma up to 2000 grit.
Now the ball gets more length but still rolls pretty early.
Towards the end of the video you will see the oil guy start to oil the lane.
Now look at how long the Karma goes on the fresh after he oiled it.
It is more like a reactive ball, being very long and late, but without the sudden back end reaction that reactive provides.

https://www.youtube.com/watch?v=T40TnHpvzbQ

I agree. Axis rotation with urethane is always problematic unless you can generate really high RPM's. Forty years ago, with oil everywhere, even those with 250 RPM's would still have some rotation left at the pins.

Today, 250 RPM's at 38 feet, will straighten out and deflect. A person needs almost twice as many RPM's to maintain axis rotation down lane. It's why you only see 2-handers and Tackett doing it.

If the pattern will allow, urethane pointed up right off the corner isn't too bad, but any fluctuation in speed leaves a corner or chops through the face leaving ugly splits.

As I age and speed/RPM's drops, the writing's on the wall. I can only hope for a tech breakthrough that allows a ball to wind-up without any speed :-)

This is from Joe Slowinski -

Coverstock Basics

Reactive Resin bowling balls skid more in oil and have a large amount of hook potential when making contact with the friction on the backends of the lane. So, the ball tends to skid and snap. The cover is also tacky. In the 1990s, manufacturers added resin to urethane material leading to a more porous, tacky and higher friction bowling ball that also hydroplaned in the oil. Since these bowling balls skid in oil, they store more energy and lead to higher scores due to the reaction when the ball hooks abruptly and hits the pins.