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Old 06-21-2015, 01:46 PM   #1
thetubespoke OP
Gnarly Adventurer
Joined: Dec 2014
Location: Bay Area, CA
Oddometer: 108
/5 tire and rim sizes

Hi everyone,

I have a '71, SWB R50/5. I've been thinking about tire width and rim width a lot lately.

I have Bridgestone S11s mounted, 90/90-19 and 110/90-18. The 110 is definitely a tight fit and bulges a bit. The 90/90 looks about alright.

I've noticed some things. The S11 front has a center groove and I feel like it fights me a little bit on slow turns as you have to push/pull the rubber halves. It's very minor and probably helps with wear resistance, but does this seem likely to anyone else? Once it's turned it's fine and it doesn't matter when I'm moving, only slow speed turns of the bar.

The original tires were Continental RB2 and Continental K112, 3.25-19 and 4.00-18. The rim sizes are 1.85-19 and 1.85-18. BMW eventually switched to a wider rear rim, 2.15. I believe the tires stayed the same. From what I've read, this happened before the LWB switch, but I wonder if they went together. The smaller rim should pinch the tire to a flatter center patch and slow handling. The larger rim should be more correct and sharpen the handling. This would make sense, sharpening the handling with the rear tire to help balance out the longer wheelbase. Does it seem plausible to you?

The common metric alternative recommendation is 90/90 and 110/90 for a /5. The second one seems completely wrong if you have a 1.85 rim. Why would the width of the tires not be the same if it's the same size of wheel? If you have a 2.15, a 110 may work.

To back me up:

Most it says you should run on a 1.85 is a 3.6. For a 2.15, max is 4.00. Granted these aren't perfectly precise and it will vary by the tire, but I think it's a step in the right direction.

With my current setup, the bike feels pretty good leaned over. In a longer, faster sweeper it's a little wobbly but doable. In the short and tight turns I can really lean it down. In a straight line on California's glorious interstate system, the rear really seems to bounce around. I think it's because the flat section is too large and a taller bump on the right side of that patch will pull it one way, and vice versa. It's a small patch it'll roll over and bump, not bump sideways. These effects may be small and I'm sure there's a dozen other contributing factors, but it would explain a lot of iffy behaviors I feel on rain grooves, where they tend to wear unevenly.

Has anyone tried 3.50-19, 3.50-18 on these bikes? It seems like it would make the most sense to me. That, or 90/90 in front and rear. I'm thinking about trying a 90 S11 rear to see if it helps with the weaving, or maybe going to another set of tires altogether.


thetubespoke screwed with this post 06-21-2015 at 01:55 PM Reason: Submitted too soon
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Old 06-21-2015, 02:30 PM   #2
turd polisher
Joined: Jan 2011
Location: Midcoast, Maine
Oddometer: 1,648
Currently, I'm running a 110 on a 1.85 rim on the back of my /5. I used to have a /7 rear wheel, and had a 110 on that too. The wide tire on the narrow rim gives a rounder tread profile, which seems to make the bike feel nimbler. I like it.
I wouldn't bring her home to Mama, but Mama ain't home tonight.
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