Boxerworks.com

My '82 R100 came to me in 1993 with metric size tires on it - - 100 front and 120 rear. Stock sizes are 3.25 and 4.00 but I've used the 100/120 combo since I got the bike and have always been satisfied. Now I want to try a set of Conti Attack Classic radials which only come in metric sizes. I can still do a 100 in the front but am undecided on the rear - - 110 or 120? The rear wheel is a snowflake in the original 2.50 size. 120's have always been a tight fit but Metzelers and Michelins have always cleared the swingarm. Not a whole lot of room to spare, but okay, so I'm thinking that 120 Contis will also be an okay fit although I realize that they MIGHT not be. So, two questions: (1) which rear size is preferable for overall handling with the 100 front tire? and (2) will the 2.50 rear wheel pinch the 120 narrower than it is designed to run, thus partially negating the benefits of the radial design?

Okay, so you're running a 2.15 19 front rim and a 2.50 18 rear, correct?

Conti shows this info:

Size_______________________Pattern_______________Standard Rims

Front

90/90 R18 M/C 51V TL____ContiClassicAttack___1,85x18 ; 2,15x18 ; 2,50x18
100/90 R19 M/C 57V TL___ContiClassicAttack___2,15x19 ; 2,50x19 ; 2,75x19


Rear

110/90 R18 M/C 61V TL___ContiClassicAttack___2,15x18 ; 2,50x18 ; 2,75x18 ; 3,00x18
120/90 R18 M/C 65V TL___ContiClassicAttack___2,50x18 ; 2,75x18 ; 3,00x18
110/85 R19 M/C 57V TL___ContiClassicAttack___2,15x19 ; 2,50x19 ; 2,75x19 ; 3,00x19

So, both front and rear tires you've specified would be on the narrow end of the recommended range of rims, but within the recommended range, so the rim shouldn't be any problem at all. As far as whether the Conti 120 will fit on the rear as well as the tires you've run, I have no idea. I can't find any actual width info on the Conti Attacks to use for comparison, so you'll be taking a chance. How much room did you have with those Metzelers and Michelins? if it was only 1/8" or so, then you're really taking a chance.

IF IF IF the nominal size is pretty close to the actual size on your rims, then, since original stock was 3/4" wider on the rear than the front, then a 20mm difference is a little closer to the original 3/4" than 10mm would be - as long as the tire fits with the swingarm.

Personally, I would choose a tire that gave me the option of closer to stock sizes, but if you really want to run the Conti's, and really want the 120, then you're just going to have to take a shot - unless you happen to find someone who has run that exact tire and size on a similar model year. And, even then, not every bike is exactly the same on those clearances.

Very useful info, Major. Yes, 2.15 on the front and 2.50 on the rear. I never found the tire/rim size data that you posted but it answers the tire/rim width compatibility question. Thanks! Plus, considering the 3.25/4.00 size difference, it would seem that a 100/120 combo would more follow the original front/rear size differential than a 100/110 combo. The question then comes down to how much lateral space a 120 Classic Attack tire requires. Right now, I have a good 3/16 inch at the absolute narrowest space between the Michelin that is mounted on the bike now and the swingarm. I know that BMW makes spacers to move the rear hub over enough to fit a slightly larger tire. Searching around on this site, I found references to the fact that '81 and newer twin-shock boxers were fitted with these at the factory. I think I read that correctly. If so, my '82 already has all the tire/swingarm space it's going to get.

I don't think that the spacer was standard......

I used to run 120s on the back of my 82, no problems with fitment, but could be difficult to remove and fit. I've gone back to using proper 4.00 tyres and I find that the smaller sized tyre makes the handling a bit quicker and generally more agile. With modern tyre compounds we don't really need the extra width.

According to the folks at the Conti Tech Line, the 120 measures 127 mm (5.0 inches) at the widest point when mounted on a 2.75 wheel. Don't know about a 2.50 wheel but it's gotta be close. Near as I can tell, the 120 Michelin that's on the bike right now measures 4 5/8 inches at the widest with a shade over 1/8 inch to spare to the swingarm, so 5.0 inches is out of the question. That leaves the Conti 110 which would be fine. But after thinking it all over in far too much detail (which is what I usually do), I've about decided to go with bias tires in stock 3.25/4.00 sizes. Pretty much settled on Michelin Pilot Activs.

CVA-42 wrote: Pretty much settled on Michelin Pilot Activs.

PA 4:00/18 is 116 mm wide, on a snowflake
PA 90/90/19 is 94 mm wide on a snowflake.