Here is Dunlop’s tyre marking explanation page and another very simple summary of tyre specifications – including why wider is not necessarily better.
The greatest tyre challenge with the GPz is the 18″ rear rim and the 16″ front rim of 1st gen bikes.
In 1984 all GP bikes were using the 16″ front so this simply translated across to the sports bikes of the time. Interestingly there are more matched tyre sets for these bikes than the later generation!
I haven’t found any definitive reason for a 18″ rear but this was certainly the standard for earlier bikes and as such there would be no logical reason for Kawasaki to try and change the wheel (pun intended) here. Also the limitations of bias-construction may be a factor here, radials only arrived in 1987 for motorcycles so perhaps the additional rubber of the larger tyre was a physical benefit (heat?) for the tyres. 17″ rims front & back are the modern norm, so even 2nd gen bikes with a 17″ front have limited tyre choices especially if you wish for matched sets.
Having greater tyre options is an often quoted argument for changing the rear rim.
As mentioned although the rim sizes vary on the two generations of bikes the calculated diameters of tyres for all version is virtually identical. Given the bikes potential performance I strongly suggest V-rated tyres (150mph) would be the minimum specification for safety & insurance, there is even an unexpected Z-rated (168mph) option for +A7 bikes (curiously an adventure not sports tyre!). However there are far more tyre choices available if you include H-rated (130mph) tyres that some owners choose to fit, so I’ve also listed these for interest.
Tyre Pressures (cold)
The service manual specification (1990-93 model radial tyres) is to run F: 36psi & R: 41psi. IMHO you need to take these with a grain of salt, because in 2020 with my Dunlop RoadSmarts I cannot run anywhere near that pressure at the front – it feels like you are riding on a solid piece of rubber. Might be fine on the race-track but on the road I drop to 29psi, as anything below that the front begins to get ‘squirmy’. I then set the rear by the same 5psi offset, so 34psi. And that is still so firm I had to drop the rear shock pre-load!
V vs H-rated tyres
The minimum specification for the GPz900R is a V-rated (149mph) tyre, which is logical for a 150mph bike and of course higher speed rated tyres (Z or W) would be perfectly suitable as well. However….many riders use tyres that are only H-rated (130mph) such as the 130/80/18 Pirelli sport Demon. I would personally not use anything rated less than V, however others may choose to do so based on their riding. So I have included these for interest.
I would also strongly recommend confirming using a H-rated tyre doesn’t negatively affect your insurance!
Smaller diameter tyres
Smaller tyres reduces clearance, shortens the gearing (note speedo runs of front wheel) and changes the geometry of the bike to be even more tail heavy. If you haven’t done it already you can rotate the rear chain adjusters to raise the rear axle.
Folks who modify to the ‘easy swap’ ZZR600 rear rim may need to consider more significant modifications (such as longer rear shock length) to correct the geometry and gearing effects of the even smaller 160/60/17 tyre (∅623mm).
Note: +A7 bikes have no ability to lower the triple-clamps on the front forks.
There’s handling arguments against wider & lower profile tyres, the most obvious is that the greater profile radius means the bike won’t naturally turn in as easily. But that is potentially balanced by the ‘pinching’ effect you would get putting this onto the smaller rim and the original profile of the tyre itself – which can vary significantly between products. The greater contact patch also potentially creates ‘push’ especially when cornering – analogous to understeer in a car.
On the positive side a wider tyre typically is softer riding due to more material.
Provides a few different matched set options for +A7 owners.
Maybe. For +A7 owners (MT3.5″ rim) if you use this rear size that gives you many more Z-rated (even W) radial tyre options.
The general consensus of public opinion is that a rim can run up & down one standard size – manufacturers might think differently of course. As there is minor dimensional variances with tyre manufacturers specifying in the middle is the correct choice – or better yet specify a particular product that fits perfectly, which is exactly what Kawasaki did. One group member mentioned how he used a Dunlop Track Endurance way back in 1985 – the 130 looked as wide as other brands 150’s! Of course none of the OEM spec tyres are available now, so it’s possible that a ‘slim’ modern 160 tyre might fit better than another ‘fat’ modern 150 – I simply do not know.
The only way to confirm would be to test different 150/70 & 160/60 tyres on different width rims and compare the tyre profiles. Kinda difficult…
Note: to fit even wider tyres/rim usually need an offset front sprocket and many replace the swing-arm, this is discussed in more detail here.
FYI the 4.5″ rear rim of the ZX-10 Tomcat (ZXT00B) can run this tyre which is identical in diameter to OEM, however not in the stock swing-arm. I find it difficult to believe that this rim/tyre combination is lighter but have some feedback is that it actually is.
Bias (cross-ply) vs Radial tyre construction
Although radial construction is a ‘newer’ design, many modern bikes like cruisers still have bias tyres as OEM. I would certainly NOT recommend bias-construction tyres on a modern superbike, however it’s not an apples-vs-apples comparison, as variations of weaves & materials all mean a different result. Of course each manufacturer considers their design to be the ‘best’, but then again over 50% of drivers think they are better than average.
Link discussing the two types of construction.
Also the early 1980’s were a significant time in tyre development. Michelin introduced radials into racing in 1984, and released the first road bike tyers in 1987. Although the A1 had the widest tyres ever fitted to a Kawasaki, from the service manual specifications it looks like it was only with the 2nd gen bikes (+A7) that radial tyres (Michelins) tyres were specified as OEM. So the GPz was probably designed to use bias-construction tyres, maybe a factor as to why the geometry is ‘slow’ compared to modern bikes.
The first vehicle to have radial tyres fitted as standard was the Citroen 2CV in 1948 (or 49) – a cool car but hardly ‘sporty’!
It stands to reason that not only have radial tyres developed a lot further since then (so have greater influence on modern bike engineering) but also that the GPz900R engineering would probably still be quite happy with modern bias-construction tyre, which would *hopefully* be much better than what was available in 1984! This is a ‘score’ of the original rear tyre Dunlop K727 – hardly confidence inspiring numbers!
Although still bias construction the ‘ancient’ Bridgestone BT-45 (only superceded in 2020 with the BT-46) is one of the universally recommended tyres for A1-A6 bikes and I have personally run the 120/70/17 front without any adverse handling issues at all.
As such you can do your own research and decide which tyre construction suits your riding style – IMHO radial gives sharper handling but a much firmer ride.
Lastly it is definitely NOT RECOMMENDED to mix the two tyre constructions, although curiously in the early days sometimes racers actually did just that. Just goes to reinforce how the most important factor in motorcycling is the rider.
Need to do homework
One of the complexities when searching manufacturer or retailer sites is that they do not always recognise the different generations of bike or even the bike full stop!
For example when searching (Nov 2020) www.bridgestone.co.uk and choosing the GPz900rR you cannot select a year, and the only tyres returned as suitable are the BT45 with no listings at all for the either the BT46 or the BT016, despite these being available in Australia for both 1st & 2nd gen bikes. So if a major manufacturer from a major country can’t get be bothered to get their online site right for our bikes – that gives insight into the challenges of hunting down tyres!
Other sites like Pirelli only offer searching by bike make & model, so if there isn’t OEM tyre sizes available you won’t get any matches.
Local online sites
I’ve added the manufacturer marketing ‘blurb’ for interest and for comparative pricing I also have snapshots from an Aussie online supplier motorcycletyrewarehouse.com.au. Personally I purchase tyres directly from my local Adelaide tyre fitter – dcmotorcycles.com.au