Topic: Rear tire sizes -difference between 120-80-18 and 120-90-18 - CB750

[andy750]

I did search and found some great info but this is an additional question...

Using this chart:

http://www.cb450stuff.com/tires/

and reading the FAQ

http://forums.sohc4.net/index.php?topic=368.0

I think I am right in saying I can use a 120-80-18 on the stock rear wheel of the  CB750 and the difference from 120-90-18 is in the tire height. My question is, is there any downside to a tire with a lower height?

Here is the reason for asking...I am looking to buy these tires and they only come in 120-80-18:

http://www.motorcycle-superstore.com/2/29/393/15130/ITEM/Kenda-K761-Dual-Sport-Rear-Tire.aspx

Not going to be cornering like a racer with this tire but more for use on bad roads/dirt roads with a loaded bike.

thanks
Andy

[Gordon]

It will have an effect on your final gearing, basically the same as going with a slightly smaller rear sprocket.  The total effect depends on the actual difference in tire height when mounted on the rim.  Considering that a 120/90 is already a shorter tire than the stock standard tire, it might be a fairly big change from the stock gearing/tire set-up. 

[Jerry Rxman Griffin aka MuthaF'er]

Better acceleration = shorter tire life = happier quicker rider 

[camelman]

Try a google search on the subject.  I've found a few articles stating large handling differences with just small changes in profile.  You might never notice the difference, but it wouldn't hurt to know what you are in for.

Another point though, is tire mass.  You'll have less mass to absorb the heat (and less surface area to transfer it away) if you run a smaller sidewall tire.  If you are running a heavy motorcycle with a load on rough roads, then good heat shedding ability might be noticed.  It would be noticed in longevity of the tire and handling characteristics as the tire ages and experiences deeper heat cycles.  Again, it probably wouldn't make too much of a difference, but it wouldn't hurt to know.

Camelman

[MCRider]

You can use this gearing calculator to see if it makes much difference gearing wise. You'll have to look up the final drive ratio yourself, I've misplaced mine for now.

Going from a 17/48, I dropped to a /70-17 tire from a /90-18, which required that I go to an 18/48 to maintain the same overall ratio. One tooth on the front roughly equals about 3 on the rear. (2 and a fraction) I doubt that your change would make more than a rear tooth difference and that's in the favor of a dual sport application. I think, anyway.

[Gordon]

Better acceleration = shorter tire life = happier quicker rider 

I think you're thinking of a taller tire or sprocket.

[sparty]

Andy,

I am running a 130-90-18 in the rear and a 110-90-19 up front.  In the curves the handling is HEAVY.  Will go back to stock when they wear out.  But I did the 130 rear for the drag strip, only problem is I haven't run on the strip in a while.

I recommend going stock tire sizes.  Look at other tire options since they don't make that tire in stock size.

Art