.need help with tires

..so exhausting .what is really the difference between a 3.25 and the 100/90? ..front tire mount on xj650 1982 seca with 19 inch rim...

 

inches and millimiters

 

thats how the numbers convert

3.25 is the old style of numbering .

http://www.maxxis.com/other-motorcy...tire-school#motorcycle_tire_conversion_charts

 

..but why does 3.25 seem so different to 100/90?

 

(Modern/Metric rating) 100/90 means the tire is 100 mm wide and the height is 90% of the width.
so in this case 90% of 100mm = 90mm tall (from bottom of the bead in the rim to top of tread.)
Hence the lower the series # (90 in this case) the lower profile the tire.
Kids in their cars are always running the lowest series they can find. (man am i getting old).

The old fashioned "imperial" rating is 3.25 which only tells you that the tire is 3.25 inches wide... that's it.

Of course the other/last number would be the rim size in inches.

TT means tubetype
TL means tubeless (I ran a tube in my tubeless when I pooched my rim on an old bike, ran fine)

And there is also speed ratings for max speed on tires.
Q 99 mph
S 113 mph
T 118 mph
U 124 mph
H 130 mph
V 149 mph
W 168 mph
Y 186 mph
Z retarded fast.

There may be an R or B after the speed rating.
R means radial
B means belted bias (probably wont see this one)

There's also Alphanumeric code instead of the 100/90 type. It's still metric but in a code.. just to be retarded.
eg MH90
M means motorbike
H is width
90 is series (ratio)

To make it more retarded, the code is different for the front tire and back tire.

Front tire width codes
H is 80 mm
J is 90 mm
M is 100 mm
N is 110 mm
R is 120 mm
T is 130 mm

So MT70 means M (motorbike) T (130 mm wide) 70 (series; height is 70% of width; or 0.7 x 130 = 91 mm tall)

There's also date codes.

 

3.25 and 100 both being section width. I think the real question is if 3.25 = 82.55 mm and 100mm = 3.97 inches how can they be the same??
its just the way it is. the tire mfg matches up sets for you. mine were sold to me as a suggested set.

 

do you want L50's or L60's for your GTO?.....................

 

In our sizes you'll see a lot of bias ply tires offered.

 

..my back is avon am 26?s 120/90/18?... and what ihad involved removal of entire rear end thing was so well stuffed in between...probably best that i never took bike past 45mphs..not sure what would have occured...

 

I believe the old imperial system measured the width at the rim where the metric measures the widest part of the tire.
I tried to do the math when buying new tires and it gave me fits. Trust the chart, it will work.
Also, and this will start some controversy but as tempting as it is to go to the wide side, I know it looks cool, wider isn't always better. As a rule narrower handles better and wider wears longer. It is a compromise between competing forces,

 

Sorry for hijacking this thread, but PO did put on a the rear wheel of my Black Seca900 a tire narrower than stock, a 120/70-18 instead of a 120/80-18. I understand that I should expect shorter tire life, but is ti the only downside to a narrower rear tire?

 

You didn't go any narrower. 120 is the width. /70 and /80 are the section, which is used to calculate the sidewall height. . / 70 is 70% of the width, and /80 is 80% of the width. You fitted a shorter tire, which reduced the overall diameter and also increased your speedometer error by a tiny bit.

Tire longevity is a function of several factors. Width can play a role, however the tread compound, load rating, road quality, and riding style are all greater determiners of longevity than width is.

 

I understand that the height is the difference, thanks for the precision. It is the rear tire, so it will not affect the speedometer, though, unless the front tire is smaller too.

What about straight line stability? IS it mainly ruled by the front tire?