I'm glad to see we have gotten this right, and in fact what we are saying here goes way beyond what most people understand. We don't care why the bounce is higher or lower, we only care that it is. Anything that makes the bounce higher is more like clay. Anything that makes the bounce much lower is more like grass. HC remains in between but with quite a bit of variation. I don't understand all the factors that make indoor tennis on HC slower, but I have read that often the surface is laid over something that is less firm. Is that true? Can someone confirm that?And I think its actually wrong. The concrete base just means its a bit higher bouncing. Grit level effects speed. My statement that the higher bounce in and of itself would slow the courts down is largely incorrect. Friction level (or grit) determines this. I'm still wrestling with the whole thing. At this point we know that concrete base for asphalt is a significant change.
Regardless, I think we all know that generally indoor courts bounce lower.
That's one factor, and it's huge.
The other factor is what causes friction, what the ATP apparently labels as COF or coefficient of friction. I'm in danger of embarrassing myself if I go any further, but as I understand it anything that adds friction slows down forward motion. The only balls not affected spin at the same rate they are moving forward, and that is very VERY rare. It demands something like a 5000 rmp spin (39 mph) with the ball slowing down to no more than that at the moment it bounces. A bit of thought tells us that most shots are going to "brake", with moderate topspin slowing down and flat balls slowed a lot. Most of all slice is hugely reduced at the moment of the bounce.
So it becomes obvious that making the courts very smooth will reduce this friction to a minimum, and for sure HC surfaces could play as fast as grass or faster if you make them very smooth and reduced the bounce. But I don't think we are ever going to see such a low bounce in events like the USO or even the AO. The bounce there is reasonably high.
And the bottom line is that anything that causes the balls to "put on the brakes" changes the angle more at which they bounce, so the bounce will be more vertical.
In addition - and I finally learned this last week - the less spin, the more the speed of the ball contributes to the height of the shot. The reason is that less spin means the ball is spinning slower than the forward motion, so the friction takes some of that forward motion and transfers it to more vertical motion. This means that a shot of 2500 rpms, spinning around 19.5 mph, is going to sit up more as grit is increased while it loses pace since quite obviously that shot is traveling faster than that. So yes, a grittier court that has the same bounce as a smoother court, when the ball is simply dropped with no spin, will increase the bounce and take off pace. The result will be a court that is much harder to hit through.
I suspect this is the main factor we are seeing, although an overall higher bounce is also likely - such as at Wimbledon.
Finally - and this I have no idea about - if the hardier rye grass grips the ball more than the old grass, that is going to slow down Wimbledon. And that, in combination with firmer ground under the grass which is making the ball bounce higher, would also make the ball sit up even higher and move through slower. I would assume cutting the grass a bit shorter or doing any of a number of things could reverse some of that. Does the grass grip more when there has been a lot of rain? Is the grass itself then hardier and perhaps courser?