TOPIC:

650ed wrote: The 120 is the perfect fit for the 2.15 rear rim - no problems at all.

Umm, a 120 on a 2.15 wheel is pinched in at the sidewall. It's doable but far from perfect. Search around on the internet and you will find some variations in the information but typically a 120/90 is "best" fit with a 2.75"wide rim.

www.scribd.com/doc/253007466/Motorcycle-...re-Size-Chart#scribd

swest wrote: I've been cutting pieces for my frame so when I have the head off, I can weld them in. I'm tired of the "frame flex" too.
Steve

The hardest ones to fit are the 2 under the tank.

Nessism wrote:

650ed wrote: The 120 is the perfect fit for the 2.15 rear rim - no problems at all.

Umm, a 120 on a 2.15 wheel is pinched in at the sidewall. It's doable but far from perfect. Search around on the internet and you will find some variations in the information but typically a 120/90 is "best" fit with a 2.75"wide rim.

www.scribd.com/doc/253007466/Motorcycle-...re-Size-Chart#scribd

I am aware that different charts recommend different sizes, and different folks have different opinions. I am speaking from years of personal experience using 120 tires on the 2.15 rim. I found them to work perfectly fine on the 2.15 rim. I'm not alone in this belief; the folks that manufacture Continenal motorcycle tires also recommend a 120 tire for the KZ650-C (2.15 rim). Ed
Ed

There is no way in the world a 120 tire on a 2.15 rim would be considered a "perfect fit". Yes, you can use that size combo, but it's far from perfect. Like a fat chick in a bikini.

You said, "a 120 is a bit big on a 2.15 wheel" but what size wheel do I actually have on my '78 KZ650 B (B2? Not sure) with spokes?

The current front tire is 3.25x19. Is it the wrong size?

My recommendation is to use a tire that is approximately the same size as the original tires. If the exact size cannot be found the tire that may be a bit wider is preferable to one that is narrower. In the case of the KZ650-C1 (and may other models) the original rear tire is 4.00-18 and he closest metric size that I have found is 120/90-18.

Various tire / rim size charts do not necessarily match up with with each other (or sometimes even with reality) in a meaningful way. They can be contradictory not only to the manufacturers recommendations, but sometimes even within their own charts.

Here's an example. The two excerpts below are from the same chart (at www.steelthundercc.com/tiresizes.html). The size of the original, rear tire that came on the KZ650-C1 from the factory 4.00H-18, and it certainly was not a low profile, so it was Series 90 or possibly even a bit greater than a 90 profile. Notice in the top section of the chart (circled in blue) that the metric equivalent to the 4.00/4.25 Series 90 tire is listed as 120/90. Now notice that (circled in pink) the metric equivalent for the 3.50 tire, which is quite a bit narrower than the stock tire, is listed as 110/90. So based on that portion of the chart is appears to be obvious that the 120/90 tire would be the size to replace the stock 4.00 tire.

However - even within the same chart there is a contradiction. In the next section of the chart it shows the appropriate rim width for the 120/90 tire as being 2.75 (circled in red) and the overall width of the tire as being 4.8. If that is true, either some of the data in the charts is bogus or Kawasaki should never have fitted 4.00 tires on 2.15 rims on the many thousands of KZ's they sold.




The chart in the link Nessism provided (next image) also has issues. For example, it shows (pink outline) the STD rim for the 4.00 tire as being 2.5, but many different Kawasaki models came with 4.00 tires on 2.15 rims. Even worse, it shows (yellow outline) a 90/90 size as the STD size metric tire for a 2.15 rim, but that tire is far too narrow for use on the rear of a KZ. Anyone following that recommendation will quickly discover the tire is not wide enough for the application.



One other thing to bear in mind is that tire width sizes are often not much more than crude estimates. They may give a rough idea of the width of a tire, but often they are not even close to being accurate. Here's an example. Below is an image that shows 2 Michelin 19" front tires that I had in my tire rack as the same time. Both tires are size 100/90-19. Notice, however, that the Michelin A48 tire is far wider than the Michelin 50E Macadam. I have used both types on my bike and even when mounted on the rim the A48 is far wider than the 50E. Ed

From one Ed to another, holy crap man, you're splitting nuts here.

A 2.15" wide rim is narrow. Most OEM's installed 4.00 tires on that size. You can still find a decent selection of 4.00-18 tires BTW.

Use your 120. I'm using one on my 750. I ain't call'n it a perfect fit though.

BTW 2, look at your rear wheel and take note of how the sidewall angle flares out to accommodate the tire. The tire sidewalls should be much straighter, less outward flare. You can talk about variation in tire width all day long but how much the sidewall flares is the bottom line. On both our bikes the tire is being pinched in at the sides, which causes the tire profile into a more rounded pattern than ideal.

I agree that the 2.15 rim is narrow, and it would be nice if the bike came with a wider rim. Please understand, I'm not trying to start a crusade; I'm just trying to lay the facts as I know them on the table.

When metric size tires became available and in essence became the standard, Kawasaki switched from inch to metric sizes. In doing so, I'm sure they would have selected the best fitting metric tires for their stock rims, because it would have been disadvantageous for them not to do so. So when they began installing metric tires on their new bikes what did they use? Here's the documentation I found -

The documentation I have shows the stock rear tire on the 1982 Kawasaki KZ750-R1 (GpZ) was the Dunlop K427 tubeless 120/90-18 and the stock rim on that bike was 2.15. That bike used a Dunlop F8 100/90-19 tubeless front tire on a 1.85-19 rim. Both of those rim sizes are the same as my KZ650-C1, although mine are not tubeless. Since Kawasaki mounted those tires as standard equipment on those rim sizes I believe they are the best metric sizes available for those rims.

I know the same 120/90-18 rear 100/90-19 tire sizes were used on the 1983 KZ750-L4 and possibly other models, but I haven't pulled the documentation to prove what size rims they used. Ed

KZ750-R1 Rear Rim


KZ750-R1 Tire Specs