I know this is as contentious as an oil post but…. I’m in the process of building a P3H bike for next year and have been struggling to build it within the rules + the manufacturers recommendations for Tyres and Brakes.
The bike will end up being around 180Kg and per the P3H rules the maximum size wheel I can run on the rear is a 4.5”.
I’ve been back and forth with both Pirelli and Metzler in Europe and have been speaking with their engineers so I’m posting this information in case it is of use to anyone else. I have lots of documentation from them about tyre construction, compounds, recommendations which I’m more than happy to share.
Generally speaking, the correct tyre fitment for a 4.5” x 17” rim is a 160/60 radial. However, after speaking with the engineers they recommend NOT doing this. The carcass on the 160 was never designed for a 180kg bike and has been constructed for a bike weighing less than 158Kg .
Both Pirelli and Metzler recommend running a 180 tire, even on the skinny rim. It will pinch so will lose some of the contact patch, but the 180 carcasses have been developed to take up to 220Kg. They said its not a question of if the 160 will let go, but when – maybe they are just covering their ass for liability reasons.
The other option would be to run 18” rims as both Pirelli and Metzeler make 18” x 150 race tyres.
The big difference between these 18” race tyres and the 17” race tyres is that these are Bias Ply while the others are Radials. This means that although the carcass is much narrower, the Bias Ply construction is much more robust and can take a lot more weight. In fact, these 150’s can support up to 325 Kg.
This is why you never see Radial tyres on a Semi or a Bus. They just can’t take the weight or heavy loads.
In my case I can’t run really 18’s as I’m building the bike primarily for SOAR so it’s not easy to keep swapping between 17’s and 18’s. It’s not only swapping the wheels, but the brakes, forks and all of the other bits and pieces that go with the wheel.
They also had concerns over the compound. 160’s only come in soft compound and a heavy bike will generate too much heat = excessive wear. They recommend running the harder compound that is only available in the 180+. The 18” race tyres come with the hard compound by default.
My plan is to squeeze a 180 tire on a 4.5” and run a 6” rim at SOAR. For the Philip Island race at Mosport, it will probably have the SOAR setup.
The intent of this post is not to cut into the current rules, I simply wanted to document what both Pirelli and Metzler told me as it may be of use to someone else down the road. Even though its counter intuitive, their thought is that the 180 is a better tyre for this particular use case.
