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Correct Tire Sizes

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  • Correct Tire Sizes

    One question that comes up from time to time is 'what metric size tire do I use to replace the old inch sizes these came with?' Another one is 'how big a front/rear tire can I fit?' Both good questions, here's the answers...

    The multiple 'conversion charts' out there for changing from the 'old' to the 'new' sizes aren't always accurate, so you need to pay attention. Most charts do note that you also need to check overall diameter, width, and most importantly, load rating, but not everyone looks that far. So a few facts are in order.

    The OEM tires fitted to these bikes were load rated at 420 lbs front, 670 lbs rear. Compared to modern tires, these numbers are rather low today for correctly-sized replacements. Also check aspect ratio; a lower-profile tire in the 'same' size will have a lower load rating, i.e. a 130/80-17 tire will carry less weight than a 130/90-17. The main thing to think about here is don't go less than the stock numbers; you will be compromising your safety! Another thing is the newer metric sizes tend to be smaller than the old inch sizes, even the 'equivalent' sizes; below is a pic showing the size differences in the various correctly-sized rear tires offered since these bike were new.

    Click image for larger version

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    Left to right, you have: a 4.50H-17, a new 160/80-16 (more on that later), a 130/90-17, and a 130/90-16. Now, the 4.50 has about 60% tread left (but is over 25 years old!), and the two 130s are pretty much worn out. But even if the latter two were new, that would increase their diameter by less than 1/2". As shown, there's nearly 1" difference between the 4.50 and the 130/90-17, and almost 2" between the 4.50 and the 16" tire. That's a lot in terms of circumference, and will make a noticable difference in engine RPM on the highway. The smaller tire will also wear faster, all else being equal.

    So, which size tires are acceptable replacements? Listed below are the sizes that will fit with no issues; if you try another size, you'll need to carefully check for clearances and load rating. Also mentioned are possible oversizes, but careful checking on your bike will be needed to be sure it will fit. The load rating can be critically important, so don't go less than the original numbers. If fitting an oversize tire, be aware that there's enough size variation between tire manufacturers and even tire models from the same manufucturer that you may find that not all tires in a given size will fit.

    Front: Use a 100/90-19 for the front. 'Standard' load rating on this size is right around 500 lbs, so that's more than adequate. Some owners have fitted a 110/90-19, but clearance issues have been reported, particularly fender interference when installing/removing the tire.

    Rear, Special: This one is simple; the Special came with a 'modern' metric 130/90-16 tire, replace with the same size. Load rating on this size is about 690 lbs. Some owners have fitted a oversized 140/90-16, but that reduces sidewall clearance at the shaft side of the swingarm to near-zero. Maximum tire width cannot exceed 5.45" (and carefully verify clearance once installed) in most cases. White-lettered tires in this size will rub....

    Rear, Standard: This is the one that causes all the confusion. Plugging the 4.50 size into most 'conversion' charts will result in a 120/90-17 size. THIS IS TOO SMALL! Beside being almost 2" smaller in diameter, this size is load-rated about 50 lbs below the OEM tire. The correct size is a 130/90-17. Again, load rating on this size is about 690 lbs. One problem with this size is limited availablity in many tire models/brands, many owners swap to the Specials' 16" wheel to get a wider tire selection. Also be aware that there are other aspect-ratio tires available in this size, but again, their load rating is less than the needed 670 lbs. There is no oversize tire available for this rim size.

    Two other things worth mentioning. First is rim width; the stock rims are 1.85" for all the fronts, 2.5" for the 17" standards, and 3" for the 16" Specials. You'll find 'recommended' rim widths for various tires and they may or may not match the stock widths. This isn't a problem if you use the above sizes. The other thing is the '78 and early '79 standard bikes came with 'tube' wheels; the later bikes were all sold with 'tubeless' rims. DO check, even if you have a newer bike as parts have been swapped around on some bikes; the tubeless wheels are stamped 'suitable for tubeless' in one of the spokes. For more info on the wheels, look here:

    The 160/80-16 shown above? As you can see, this is pretty close to the same size as the OEM 'standard' tire (about 3/8" smaller if the 4.50 was new), so that's my choice for a rear on one of mine. But this needs fairly extensive mods to the swingarm to make it fit due to it's greater width (6.15") so it's not an easy install.
    Last edited by tplank; 11-08-2020, 06:35 PM.
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    '78E original owner - resto project
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