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Everyones favourite subject - Tyres

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Everyones favourite subject - Tyres

Postby Brian Stevenson » Mon Mar 04, 2024 2:24 pm

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.
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Re: Everyones favourite subject - Tyres

Postby TimV » Mon Mar 04, 2024 2:55 pm

What is the reason you can't run 18" wheels in the SOAR series? Because they allow the wider 17s? Hopefully other's in P3H will chime in with their experience with the 160s.
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Re: Everyones favourite subject - Tyres

Postby Brian Stevenson » Mon Mar 04, 2024 4:13 pm

Hi Tim.

Yes, SOAR allow 18" wheels. The problem is, as always, cost and availability. Its easier just to stick on a 180 and have a chicken strip. Later Suzuki wheels are great as all the sprocket carriers, disks and rear calipers and caliper holders just swap between rims regardless of the width. I were just doing VRRA I would think about the 18's

They also don't have wet tires in 18's and as you know if you don't have wet tires its going to rain all week end but if you do have a set of wet tires it will never rain again :D

I think the 18" tires would also be fine for this frame as the original wheel sizes in 1981 were 18"x3.25" and 18"x6.5". I do have an original 18"x6.5" rear wheel but there is no chance of getting a tire for that...

...Unless I go to the dark side. My wife's car has 18x6.5 rims. Hmmmm.....
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Re: Everyones favourite subject - Tyres

Postby desmobill » Tue Mar 05, 2024 12:57 pm

Hi Brian,
From Classic Racer Magazine, Jan\Feb 2018, the Team Classic Suzuki Katana Endurance race bike. This bike is ridden by racers like Guy Martin, Pete Boast and Jamie Witham. It rolls on 18" wheels, in 3.0" and 4.5" widths with Conti radial tires. Its got over 150 hp and over 95ft\lbs of torque. If these pro racers thought it was a dangerous combination they would not be on them. Their rule book forbids rain tires so they must work in the wet as well as dry.
I have bought a lot of stuff from England and generally it arrives quicker than parts from the US so the not available here comment doesn't really apply.
As for the cost of 18" wheels there are so many Japanese bikes that had them (Early GSXR to modern CB1100) that a quick check on EBay would find you something.
Decisions, decisions, decisions...
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Re: Everyones favourite subject - Tyres

Postby Brian Stevenson » Tue Mar 05, 2024 3:36 pm

Thanks Bill.

Its just easier using the Suzuki 17" rears as the only real difference is the actual width of the rim its self. The sprocket carrier, disk, caliper holder and caliper, cushions, spacers and 20mm axle are all the same. I just have to change the rear wheel depending if its SOAR or VRRA. You can also just get the tires from Wayne at the track and he will even install them for you.

I can also share wet tires between the Suzuki, Kawasaki and Yamaha as well as they all run those Suzuki rims and they are $40 each on eBay if I ever need any more.

The 18" Pirelli's are now the spec tire for the Classic TT and Classic Europe Endurance Cup so I'm pretty sure they are good tires and will easily take the weight of the bike. They are just hard to find in Canada.

Its also $400 for a set of Pirelli's from Wayne or $900 for Conti's shipped from the UK or Florida. I run the Conti's on the CB350 and they seem OK but come in weird sizes. Pirelli does have a 90/90 18 now which would be better on the 350.

They did advise not to run the 160 on a bike that weight but again, it could just be for liability purposes.
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Re: Everyones favourite subject - Tyres

Postby livetoride21 » Wed Mar 06, 2024 1:15 pm

Brian, I think we're beating a bit of a dead horse here.

Many, many riders are running 165 slicks on 4.5" rims in our club, myself being one of them.
Whether the bike is 350 or 400lbs shouldn't really make a huge difference, but may required you to run a little more tire pressure.

I'm also VERY surprised that any of the companies you mentioned would tell you information that conflicts with their rim size charts as none of their charts recommend a 180 on a 4.5" rim.

There really should not be any safety concern here and 17" rims in p3 are already a compromise for tire availability and cost.
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Re: Everyones favourite subject - Tyres

Postby Wayne McKinnon » Sun Mar 31, 2024 12:23 pm

Cool video
World classic endurance. The must keep 18" wheels to remain authentic.
This is the series that brought the new 18" race tires into existence.
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