In my opinion, no, it does not need to be shaved, but can be depending on user preference. More detail below.
The reasons for shaving a tire are either:
1) Because the tire is slower with full tread.
2) Because the tire will “heat cycle out” before running out of tread anyway, so you might as well get the optimal part of the tire.
3) Some tires will chunk or blister when racing on full tread
In summary, none of these are a significant issue with the Dunlop Z2 when racing.
Heat cycling out is when a tire will only undergo a certain number of heat cycles, or times that it gets hot, before undergoing compound changes that make it harder and slower. Some tires end up getting slower before using their full tread. Some are more durable chemically and usually go through all their tread before this starts to happen. In my testing, the Dunlop Z2 does not heat cycle out, even when used unshaved and run all the way down to the cords.
And as for whether it’s slower with full tread, my testing has also shown that the tire is not slower with full tread. I’ve run back to back tests at a few sites where I compare bald Z2’s (not shaved but just worn out) with lightly used Z2’s, and the times are identical. The one caveat to this is if you have a lot of gravel/broken up-ness, or obviously water. Then more tread actually helps. But at most national autocross sites with good quality pavement/concrete, or in the dry, they’re pretty much the same.
This proves that both 1 and 2 above aren’t an issue. Also I experienced no chunking or blistering so that takes care of 3. They are a bit different in feel. There are pros and cons to each.
- Very low tread Z2’s
- Feel very direct. Awesome feedback. No tread squirm
- Are less forgiving to excessive slip angle
- Will be slightly lighter due to less tread. We’re only talking a pound or two per tire but…this is autocross we’re talking about.
- Seemed to not get overheated as badly, I think both because they had less rubber to heat/cool and because the water I was using between runs was more effectively cooling the rubber. But this was a small difference.
- Horrible in rain.
- Full tread Z2’s
- Are more forgiving
- Seem to have just slightly more ultimate grip although it’s not enough to really be able to tell from the data if that’s the case
- Have some tread squirm – need to turn a bit earlier and look ahead more
- A bit harder to manage temperature in the heat.
- Don’t have to worry as much about rain, since you can do very well without having to change tires
- Seem to be better on looser/rough surfaces, but how much is debatable, haven’t done really objective testing here.
But at the end of the day, I was running the same times with each. The bald ones were a bit more consistent in terms of having no bad runs, but the full tread ones the flyer runs were just a hair faster (like a tenth or so if I remember right). This testing included Lincoln NE, the home of the autocross national championships.
One thing to keep in mind with the Z2 is that they do need to be scuffed in to be absolutely ideal. Something on the order of 5-30 autocross runs will do it. But shaving doesn’t seem to change this requirement, even if you have to shave you still have to scuff in. The full tread tires above were scuffed in.
So in summary, you don’t need to shave Z2’s to win, at any level. But I could see some people doing it if they really value how the tire feels at lower tread depths.