What rear tire should I have for my R90/6?

44006 said:

"OP report (so far):

The rear shocks (sorry they are original) were set to different heights. Fixed. "
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"This marginally stable weave thing really has me spooked."

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Are you saying that even after you reset the shocks which had been at different load setting each side the bike still weaves at speed? Mismatched rear shock load settings would have applied forces trying to twist the swingarm assembly when going over humps and bumps which could have easily made the bike behave strangely.

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I realize there could be a coupling from vertical input to steer yaw when the rear shocks forces are not similar, but I experienced a very heightened (i. e. peaked) yaw response to handlebar inputs at ~ 4 Hz with no vertical (jounce) input, very much like I have seen in videos of race accidents. I have never felt that before on this bike. Something is definitely wrong.

Maybe the snow will melt (yeah....) enough for me to try if the new tire works. Will post.

R90/6 tires and stability

1-Get the steering, swingarm and wheel bearings absolutely right and replace them if at all questionable. Steering bearings too tight will give spooky weaving at moderate speeds, but at high speeds it seems to be OK- I think the input energies are higher so the friction is overcome.
Too loose and the best you can hope for is to fill your shorts, the worst is to fill your coffin.
2-Also, check the front springs for sag. They do sag, and that steepens the steering angle. I have found up to a 2" sag, and that bike handled really scary. Rebuild the front forks is necessary, and
3- MOST IMPORTANTLY, be sure the forks are aligned. See w6rec.com (Duane's site). Misaligned forks can fill your shorts as quickly as can steering bearings.
4- Once that is done:
I run Spitfire S11 tires front and rear and really, really like the handling. They are on an R90/6, am R60/6 and a SWB R75/5. I run 90/90-19 or 100/90-19 on the front (I have tried both and both feel OK- but the 90/90 is on the SWB R75/5 so a direct comparison is not possible.), and 110/90-18 on the rear. I don't know who said that was too small, but it is not- it is a bit bigger than original but is the smallest easily-obtained S11 rear. Any bigger and you will have clearance issues. The 100/90-19 on the front just misses the fender brace, but not by much, on the /6. Pressure is about 34-36 solo, maybe 38 rear. I weigh 200.
5- As for shocks, please get rid of the originals. IKONs work, I have seen some OEM look-alikes (Bob's?), but don't know how well they work. I have Konis on the /6 bikes, and YSS on the /5. The YSS shocks were the best handling and comfort mod I ever made to that machine, and I have owned it since Feb of 1976. Ted Porter sold them to me. They weren't set up just right- the eye to eye length was a bit long (it should be 13'' exactly- at least that was what my OEMs and the Konis were) and the damping was too hard for me, but that was so easy that it did not matter. It took almost no time to set it right, he walked me through it, and the damping can be adjusted on the bike.

With a decent set of shocks, your bearings absolutely correct, S11s front and rear, it should put a really big smile on your face. Not a pucker you know where.