REAR TIRE SIZE

[ADEEL MIRZA]

What is the maximum 18" rear tire size (90 or 100 aspect ratio) that can be fitted on CB750 Four (1976) with standard swing arm.

[shinyribs]

120/90-18 is pretty much the max. Some companies tires run a bit small,or even a little different profile,meaning my current Shinko 120 rear tire is a tad skinnier than my previous Dunlop 120,but still 120 is about all you can fit.

[Lucky 1]

I am running a 130/80-17 Avon. 1978 CB750K Slightly longer swing arm for that year.

What matters is the outside diameter of the tire. Mine is 23.2 inches OD.
There is a formula for figuring it out.

cross section 80mm X 2 =160 devided by 25.4 +wheel diameter = OD of wheel with tire.

[ADEEL MIRZA]

shinyribs, although your response is quite clear, I just wanted to confirm if the side walls of a 140/90-18 tire would touch the swing arm or not. I am asking this for the reason that I do not like the look of a thin rear tire. I want to go to as fat a tire as possible.

[jonnyLow]

I am running a 130/80-18 Kenda K761 in the rear and 100/90-19 in the front on my '76. I feel like I could squeeze a 140 in there...

[rrgunslinger]

ADEEL MIRZA wrote

shinyribs, although your response is quite clear, I just wanted to confirm if the side walls of a 140/90-18 tire would touch the swing arm or not. I am asking this for the reason that I do not like the look of a thin rear tire. I want to go to as fat a tire as possible.

If you go with a 80 instead of a 90 you might get away with it. You could also make the chain a bit longer and that would allow some adjustment.   I have a 120\90-17 on my 73 and it's way to small.  Yea I know a 73 comes with a 18 inch wheel but thats another story....

[Lucky 1]

The question is would that size tire be recommended on that size rim ...safely?

[Lucky 1]

What really matters is the overall OD of the wheel with tire.
If you go with a 80mm instead of the 90mm It will look fatter and the OD would be the same or close.

The taller 90 will LOOK skinnier because it is taller.

The 120/90-17 is 24" O.D.
The 120/80-17 is 23.2" O.D. because it is FATTER.

[shinyribs]

ADEEL MIRZA wrote

shinyribs, although your response is quite clear, I just wanted to confirm if the side walls of a 140/90-18 tire would touch the swing arm or not. I am asking this for the reason that I do not like the look of a thin rear tire. I want to go to as fat a tire as possible.

When I had the Dunlop 120/90-18 on the back I had about 1/4'' clearance from the tire to the welded seams of the swingarm,but it was less than an 1/8'' from the brake stay rod for the rear drum brake. A disc brake may give you more clearance.

A 130 on a 17 is shorter and will not be as far in to the narrow part of the swingarm.So yeah,you could probably go 130 on a 17'' rim. Actually that is what is on the '78K parts bike I have now. It does look like a nice beefy tire,but it also looks very short.IMO. It's a 130/80-17.

I wanted to let you know the size/brands I was running because different brands measure different in the real world. Maybe you could find a listing of tire sizes with real world, accurate measurements somewhere.But whatever a Dunlop Elite 120/90-18 measures out to is all you can get on a bike without hitting the rear brake stay arm.I can testify to that. But you could also change the arm,too. All depends on how much you might wanna modify.

[rrgunslinger]

Here's a chart for tire sizes.

https://steelthundercc.com/tiresizes.html