Not an expert, but glad to share what I know.

► The major concern is overall outside circumference. Vintage type 4.00-18 "inch" tires have a height-to-width ratio of 100%. (That is, a 4" wide tire is also ~4" tall.) Metric tires typically have a number like 120-18/90 which means the height is 90% of the 120mm width, or ~108mm. That in itself leads to 2 concerns when using "metric" tires....
1) If we had a chain on our final drive, we could simply fit a new rear sprocket with 1 sprocket tooth difference, and the gearing for the final drive would remain about the same. But since BMWs use a geared final drive, re-gearing is out of the question. Fortunately, some "metric" brands of tires are known to be larger than others. So what you really want to know is "What is the mounted and inflated outside circumference of this new tire?", because you don't want drastic changes in the gearing or speedometer reading.

2) Tire height also affects other areas, like cornering clearance and possible trouble with hoisting the bike onto the center stand. BMW does an excellent job of tucking the stands up into the bike, so it's not like the side stand is going to start grinding in every corner. But you still need to think about this idea.

► Handling is generally improved with metric tires because they have less sidewall flex during cornering and a more rounded profile. If you live in western NC and cornering is important to you, then the right metric tire will help. If you live in west TX, where the roads are straight, then possibly not so much.

Other notes....
• Before you do anything, measure and record your present tire circumference !!
• Equivalent size metric tires will require about 10% higher air pressure when replacing inch types. Some pressure testing is required.
• Most metric tires are "tubeless". If your rims aren't, then simply use a tube like you always did. Just know that a 3.50-18 tube might be a better fit now.
• Lots of /90 tires will work on your bike, just shy away from /80 and /70 tires for the reasons mentioned above.

Hope this helps.

Wobbly is right about 4.00 x18 being the oem size. I like to go slightly smaller at 110/90 to make it easier to squeeze the wheel between the FD and the swing arm.

Thanks so much. I did not understand the ratio difference
Tim

Here is an interesting chart made by Heidenau Tires showing circumference. Now Heidenau makes both inch and metric tires so this chart will give you some basic understanding of the relationship between the 2 standards...

http://www.heidenautires.com/tire_spec_sheet.pdf

Realize though that not all tire manufacturers start with the same carcass size, so overall circumference will vary by manufacturer. This is one of the reasons for using the same manufacturer for both front and rear tires.

Hope this helps.

With the monoshock, BMW went to metric size. My '86 had a 90/90 front and I don't remember what rear from the factory. It was tubeless and the pressures specified were for those tires. You can run metric sized tires as specified by BMW

Bob

With the monoshock, BMW went to metric size. My '86 had a 90/90 front and I don't remember what rear from the factory. It was tubeless and the pressures specified were for those tires. You can run metric sized tires as specified by BMW

Bob

With the monoshock, BMW went to metric size.

Then I wish someone with a later bike could tell us the OEM sizes and tire pressures from their owner's manual.

Owner's manuals get lost or destroyed, but under the seat of every twin I've owned is some good information printed on the rear fender. From my '86
R65 which has the same frame as the larger bikes:

Front 90/90 H18

Rear 120/90 H 18

Pressures

Solo- 2.5 BAR (36.2 PSI) 2.2 BAR (31.9 PSI)
Dual-2.9 BAR (42 PSI ) 2.4 BAR (34.8 PSI)
Stock rims on these machines were tubeless rims and were factory supplied with tubeless tires.
There is also a list of approved tires, but those tires are probably no longer available.

There is also a warning not to exceed 130 KPH with saddlebags mounted. Information on proper oil for temperatures and even on the '86 timing and carb adjustment instructions.

Bob

Owner's manuals get lost or destroyed, but under the seat of every twin I've owned is some good information printed on the rear fender. From my '86
R65 which has the same frame as the larger bikes:

Front 90/90 H18

Rear 120/90 H 18

Pressures

Solo- 2.5 BAR (36.2 PSI) 2.2 BAR (31.9 PSI)
Dual-2.9 BAR (42 PSI ) 2.4 BAR (34.8 PSI)
Stock rims on these machines were tubeless rims and were factory supplied with tubeless tires.
There is also a list of approved tires, but those tires are probably no longer available.

There is also a warning not to exceed 130 KPH with saddlebags mounted. Information on proper oil for temperatures and even on the '86 timing and carb adjustment instructions.

Bob

Actually I got original manual with this 1993 R100RT. This is part of what confused me. Every referrence I found was in inches.
Tim

Thanks, guys !