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Help with Avon tire choice

jim snyder

jim snyder

Member
Member
I am looking at the Avon AM-26 Roadrider and noticed they sell a 120/80-18 front. Now I know that most buy 110/80-18's, but my question is, since the OEM front tire was a 120/70-18, wouldn't the 120/80-18 be closer to the OEM size ?
 
I think the 80 is the aspect ratio and means the height is 80% of the width. I am guessing 80% of 110 is less than 80% of 120. And so the 110/80 should not be as tall.

120 * .7 = 84
110 * .8 = 88
120 * .8 = 96

So the 110/80 wont be quite as wide ( 10mm ) but will be a little bit taller. The 120/80 will be the same width but be a lot taller.  How I think it works.
 
Sly has it right. You run into the possibility of an interference issue with the fender.

The first number is the tire width. The second number is the aspect ratio (EG: height as a percentage of width) and the third number is the rim diameter.

You can determine overall tire and wheel diameter by dividing the aspect ratio into the width then multiplying it by two (both sides) then using 457.2mm (equal to 18" x 25.4mm) add the aspect ration x2 plus the rim diameter then convert the whole thing back to SAE numbers by dividing by 25.4mm to get your inch height.

The 120/70-18 has a 24.61" diameter + 0.00" (stock 94-06 models)
     
The 120/80-18 has a 25.56" diameter + 0.95"  (might be too tall for fender clearance)

The 110/80-18 has a 24.93" diameter + 0.32" (stock pre 94 models and fits fine on 94-06 models)

Don't forget that tires will expand under speed and with the heat generated by friction so that .95" is a static height increase.

Also IMHO having run lots of the 110/80-18's I feel they steer quicker, so..........

Hope this helps.
 
Jim Snyder said:
I'm running a 130/70-18 now so how could the 120/80-18 cause fender clearance problems ?
Click to expand...


Do the math will ya! Geez!    :-[

Your 130/70-18 is 25.16" tall therefore it is .4" shorter than a 120/80-18 would be.

70% of 130 = 91mm x 2 + 457.2mm = 639.2mm/25.4mm = 25.16"
80% of 120 = 96mm x 2 + 457.2mm = 649.2mm/25.4mm = 25.56"

I did not say you WILL have interference, I said "You run into the possibility of an interference issue with the fender" AND "Don't forget that tires will expand under speed and with the heat generated by friction so that .95" is a static height increase.

Also my numbers are based on exact and not "nominal" measurements. This is to say that the stock Dunlop K701 is listed as having a 24.68" diameter and not the lower 24.61" that I calculated. 

Here's a link to their fitment page for the K701 front in the stock 120/70-18 size:  http://www.dunlopmotorcycle.com/tire-catalog/sport-trackday-race/sport-oe-replacement/k701/

Therefore if you are still contemplating a 120/80-18 you are best served to measure twice before you "cut once".  ;) ;)
 
Ok, I found the dimension chart on Avons website. the 120/80-18 is 25.7" tall,
the 110/80-18 is 25.2"tall. Looks like I will be ordering the 110.

Thanks 2linby, sorry for being a butthead.
 
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