Largest Rear Tire on a 18" Mag with Disc

[cinco]

What is the largest rear tire possible on an 18" mag wheel with disc?

Background: Looking at maybe swapping my wheel set-up from spokes to mags. I have on hand a 1979 18" rear disc mag and a 1979 disc swing-arm on the way.

- Current rear set-up: 2.15 x 16 front wheel & 120/90-16 tire & drum (1980 Special II)

- Possible mag rear set-up: 1979 rear disc swing-arm & 1979 2.15 x 18 mag


Resources: I've been using the following reference guides to try and get a better idea of fitment:

- "Rim to Tire Application" (Thanks Drewpy!): http://www.dropbears.com/u/utilities/tyrerim.htm

- "Motorcycle Size Conversion Chart": http://www.kgmotorcycletires.com/size_conversion_charts.htm

- "Motorcycle Tire Diameter Calculator": http://www.rbracing-rsr.com/tirediamcalc.html

- "Millimeters to Inch Conversion Calculator": http://www.metric-conversions.org/length/millimeters-to-inches.htm

Tire Comparision: From using the above resources here's what I've found...

- 120/90-16 Tire = 24.5" Diameter & 4.72" wide

- 110/90-18 Tire = 25.8" Diameter & 4.33" wide

- 120/90-18 Tire = 26.5" Diameter & 4.72" wide

What I've seen others talk about using:

Drewpy & Others - 4.00x18 = roughly 110/90

What I'm thinking:

- My existing 16" rim using the 120 wide clears my chain just fine - others have said a 130 wide on the 16" rim hits the chain - which looks correct.

- Theoretically a 120 wide tire on an 18" mag should also clear the chain - BUT - may hit on the inboard side of the swing-arm due to the increased diameter and the way the shape of the swing-arm is "stepped".

- Drewpy reports the 4.00x18 (110/90x18) fits. A 120/90x18 would mean a diameter increase of .7 inches and a width increase of .39 inches.

Whew - Just trying to provide some resources while explaining my thought process. Thus, to make a long quesstion short... Anyone actually using a 120/90-18 on a disc mag/swingarm?

And thanks for your time and advice Oh yeah - cold beer on me when we meet!

 

[drewpy]

I got an xs250 a few years ago, and it had a 120/80 x 18 on it. there was some rub marks on the wall from the chain case!

 

[cinco]

Hey Drewpy! I knew you'd show up here sooner or later Thanks for the info - I figure if anyone would have insight it'd be you

I'm due to get the matching 1979 18" disc swingarm tomorrow so I hope to compare my 1980 16" drum swingarm to it.

I think I'd read on a post that had info from the German site mentioning swingarms were changed in 1979 to be longer than the '78 arms (by 30mm IIRC) - so I'm curious if there are maybe more than two swingarm versions out there (for example '78 short 18" disc arms, longer '79 18" disc arms, '80+ 16" disc arms and '80+ 16" drum arms)

Dunno - maybe it'll just clear or a little grinding/clearancing to make a 120 fit? Also don't know if its worth it for a fraction of an inch in width - I don't have a 110 to compare to my existing 120/90-16. But daamn the stock 3.50-18 tire that came with the mag looks too skinny.

Thanks again - I appreciate your willingness to help me

 

[OliverB]

Well, my son's '81 has the 130/90x16 on the rear and it does not hit or rub anything. Has good clearance all around.

I, too, may go with an 18" rear disc mag setup. Already have most everything on hand except the tire and brake pedal shaft from a rear disc setup. My plan has been to use the 120/90x18 rear tire. My replacement swing arm is also from a '79. You have an interesting point. Can't add any more and will follow this thread to see what happens.

 

[eggcob]

Just to add a little more to this. I have two types of swing arms.

A wheel with a 120 tyre is fine on the 81 Special drum swing arm, plenty of clearance, but rubs on the actual swing arm of an earlier disk model.