When I first built my Cafe' conversion....I did what almost everyone else does and put a 100/90-18 tire on a 2.15" wide front rim, and a 130/70-18 rear tire on a 2.50" wide rear rim.  It all appeared normal and rode fine....however it didn't take long for me to see that the tread on the rear tire was rolled over onto the sidewall....and a lot of tread was never going to be used no matter how far I leaned the bike over.  The unused tread (chicken strip) on each side of my front tire appeared to be about 3/8", while the unused tread on the rear was closer to 3/4"!  (If you look closely at the picture below you can see the width of the chicken strips, and how the tread profile on the rear tire rolls over onto the sidewall and doesn't match the front tire).



The center tread on the rear tire wore pretty quickly and I developed a wide flat spot in about 4,000 miles....about 2/3rd of the tread was gone in the middle while the sides were not worn.  This was a bit of a surprise as I don't ride on straight roads much and corner pretty aggressively.  As I began to look and see what tire options I had, I discovered that at 130/70-18 tire is not recommended or approved to be mounted on a 2.50" wide rim....it is supposed to be on a 3.50" wide rim and the narrowest rim it is recommended for is a 3.00".



So the next tire I ordered and used was a 110/80-18 rear tire.  Before I took the 130 tire off the 2.50" rim I measured the width of the tire tread...and it was 116mm - the 110 tire width was 109mm when mounted.  I also changed the front tire from the 100/90-18 to a 90/90-18 tire as it was a better size match for the smaller rear tire.

I rode on these tires for a couple of months last summer, and the smaller tires proved to be plenty of tire for the bike.  The front tire cross section didn't change much and the tire was just a bit narrower and less tall, the rear tread cross section laid flatter to the road.  I was able to get over to within 3/8" of the tire edge on both tires.  The tires appeared just a bit skinny compared to what I was used to.....however they seem to be perfectly suited to the weight and HP of the bike.

Last winter I did some more changes, and I wanted to explore the 130/70-18 tire on a 3.50 rim where it is supposed to be.  At the same time I decided to upgrade the front tire to a 100/80-18 for a bit more width.  The change went fine....although it is hard to find a tube to fit a 100/80 tire...Heidenau is the only tube I could find in that size.

This year I have been riding on the larger tires for a couple of months including a Dragon trip a couple weeks ago.  The tires work good and look OK...however the 3.50 wide rear rim and wide 130 tire have brought back a bit of the Cruiser look and less of the Vintage Road Racer look....old bikes don't have wide rear rims or tires!

I made some templates for the tire cross section/width, and here is what I found:

This sketch shows the comparison of the tires on the rims they are made to fit.  The 110/80 tire is on a 2.50 rim and the 130/70 tire is on a 3.50 rim.  The 110 tire has a tread width of 109mm and the 130 tire had a tread width of 128mm.  You will note that the tread shape is almost identical.....it is designed to have this shape on the recommended rim width.



This is a comparison of the 130 tire mounted on the 2.50" rim and the 3.50 rim.  You can see how the narrow 2.50 rim arches the tire out of the proper shape.



This sketch shows what the 130/70 tire looks like on a flat road surface, as well as on a 45 degree surface.  A street bike will be hard pressed to obtain this lean angle - but note how close the edge of the tire is to the 45 degree line that represents the road in a hard turn.



This photo shows what happens when you squeeze a 130 tire onto a narrow 2.50 rim.  The tire tread is arched and pulled away from the paving so that the tread touching the road has been narrowed.  The tire on the 3.50 rim has 4.2 inches of tread that can touch the road between the 45 degree lines in both directions...this tire only has 3.8 inches between those lines (The 110 tire has 4.0 inches between those same 45 degree lines!).  The difference might even be more as these profiles don't take into account the tread will flatten a bit when the weight is applied to the tire.



CONCLUSION - It is my belief that if you want the lightest and quickest Cafe' bike....you should go with a 90/90-18 front tire on a 2.15" rim and a 110/80-18 rear tire on a 2.50 rim.

If you want a bit beefier tire combination you can use the 90/90-18 front tire on a 2.15" or 2.50" wide rim.....You can also use the 100/80-18 tire on a 2.15" or 2.50" wide rim for a bit lower profile tire (although you will find that it is hard to get this size in some performance tires like the Battlax BT-45......they list it as being made but it doesn't seem to be imported into the US).  For the rear tire you can use a 130/70-18 if you mount it on a 3.50" wide rear rim (or even a 3.00 rim if you can find one) - but squeezing this size tire onto a 2.50" rim will actually get you less rubber on the road than you will get with the narrower 110 tire.

FOR ME......at this point I am sticking with the 100/80-18 front tire.  I am not sure which back tire/rim I am going to stick with....a summer of riding/testing is in progress!

Good write up Dave. I went thru this issue years ago with a track built SV650 I had. I noticed it was harder to get a 130 rear to push over than it was a 110 rear. Tire options were even more difficult to find and I was stuck with a Pirelli Diablo slicks because I had 17's and a 120/70 front was the smallest I could find for the rear tire. Man did it wanna lay over once you got leaned off the crown enough though!

Motorcycle mag, doctors office, discussing traction and tire deformation in corners. A picture showed the bike in a hard left. The contact patch looked huge, because the tire was so distorted. I wish I could describe it.

The deformation of the tire....and resulting contact patch area is one of the reasons I started this thread.  The current trend is for the Cafe' owners to stuff a "Too Wide" tire on a "Too Narrow" rim, and the result is the tire no longer has the contact patch that was designed into the tire.  Instead of  the arched oval pattern...the tire beads are pulled in a full inch and the result is a round pattern and the tire has tread rolled over onto the sidewalls.

Modern tires....especially the Sport Touring tire that are being used on the Cafe' bikes have tire cross sections that look like this:



And when mounted on the proper width rim....the tire contact patch changes with lean angle.  Notice that the maximum contact patch occurs at 45 degrees.....however when the 130/70-18 tire is mounted on a the 2.50 rim - the sidewall tread has been rolled over and the contact patch will no longer look like what is shown in the following diagram.



I started this thread as I had followed the tradition and mounted a 130/70-18 tire on my 2.50 rim....and there was nearly a full inch of the rear tire that was not being used (chicken strip) - while the front tire has only about 1/4" of unused tread.  It is my belief that when folks mount a 130/70-18 tire that is designed to be mounted on a 3.50" rim (and is listed as being recommended on a 3.00" - 4.00" rim) - they are forcing the tire to create a tire profile that is too rounded.....and it rolls the edges of the tread over onto the sidewall where it will never be in contact with the road.  The 2.50x18 rim is able to accept the 110/80-18 and 120/80-18 tire size....and will accept the vintage 4.00-18 size if you can live with the very tall tire diameter (most likely it will create a swing arm clearance problem).

NOTE:  This discussion is directed toward Cafe' and Tracker conversions that are using the common 2.50-18 rear rim.....this size rim uses Sport Touring tires that are designed as performance tires for somewhat aggressive cornering.....these tires can wear out in 6,000 miles.  The stock 15x3.25 rim can easily accept the commonly used 140/80-15 and 140/90-15 tires, as those tires are made to fit on that width rim and they are more designed for long mileage and acceptable performance in all weather conditions.....MMRanch has about 20,000 miles on his Michelin Commanders.  For best handling the Savage would very likely respond better to a 130/90-15 tire....but that is not the Cruiser look that we have come to accept.

I just stumbled across some photos of my 100/90-18 front tire on the 18x2.15 rim and the 130/70-18 rear tire on a 18x2.50 rim after a lap of the Tail of the Dragon, the tires are Pirelli Sport Demons.

This is the front tire, and the relatively narrow Chicken Strip that shows I am using most of the available tread.
 



This is the "Too Wide" rear tire on the "Too Narrow" rim, and you can see that the chicken strip is much wider.  The tire cross section is too round when you force the 130/70 tire onto a rim that is 1" narrower than the tire is designed for.



The rear tire would work much better if it was a 120/80-18, and that will be the size of my next rear tire!

Wow, you guys get techo, I'll stick with my Michelin Commander and Pilot Activ , but I'm only a wuzzle.

Thanks for the informative tire article Dave. This is something I've been mulling over of late before I tear apart the Savage.

Ok Dave.... leaning or turning into the curve?

I noticed in the first pic of your old 90's that your toe is still 4" from the pavement.
Is it because you "break loose" any further in the lean? or that you don't trust the tire to lean into it further or is it a semi-aggressive angle you are taking in the curve?
I understand the "angle of attack" you are talking about and the "patch" that touches as you approach your apex.
And the front is always going to be worn more than the rear due to steering into the turn, or corrections made while doing so.
Style of riding and wear have an impact one your remaining "unused" portion of rubber.
Each person will have to find the "perfect fit" for their riding style, but this is just my opinion... I suppose it is the reason for your post.
More info is needed!  guess I will have to do so with my next set of rubbers which is soon.