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How round is "round enough"?
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I bought a new front tire for my CB900 last week. Since then I've been
noticing a small but distinct bouncing oscillation in the front end. I
don't feel it on the freeway, it's only apparent at street speeds (25-50
mph). Today I put the bike up, pulled the calipers, gave the wheel a good
spin, and watched.
This tire is not entirely round.
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...And the correct answer is: Manufacturing Defect.
The shop folks poked and prodded, checked the bead, the tire, the wheel, and
finally announced that the tire was, in fact, not quite round. The tech
thought it had maybe been molded incorrectly, and showed me what he
described as a weird mold line in the rubber.
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Wow. That is very, very rare.
I had it happen on a car, once, about 20 years ago.
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Upshot is that they replaced the tire free of charge, and even apologized
for the inconvenience. Very cool. That's the kind of service that keeps me
coming back to this shop year after year.
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It's not out of round by much, I'd say maybe 3mm. It's also not quite true,
wobbling back and forth by about 5mm. The wheel itself is fine, it's easy
to see that the wheel tracks straight while the tire doesn't.
Is that much deviation tolerable? My last tire sure didn't behave like
this, and it was a good deal cheaper at that (this new one is a Metzler
ME880). Is this bad enough that I should return it as defective?
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It sounds to me like it's not seated on the bead properly. It happens.
There's often a line running round the circumference of the tyre, close
to the rim, for you to check. If the line isn't equidistant from the rim
all the way round, that's the problem.
Quite often you have to blow tyres up to 50-60psi and higher to get them
to seat properly.
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That can also be caused by not using enough lube during mounting.
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I had to go to 90 with my recent front tire for my Concours... Yikes!
MAN, what a POP when that bead seated!!!
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I wouldn't go that high. A guy did that to a cage tire and lost part of
his calf when the tire blew past the rim.
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Jim sez:
I agree with TOG on this.
Except, If you find where the tire's bead isn't seated evenly on the
rim, I would deflate the tire, lube the bead and then air it back up
to get the bead seated properly.
Like TOG said, it may take 50+ psi to get it to come out evenly.
Then deflate it to the proper pressure.
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Ditto. If you didn't rip the cords apart when you were mounting it
that Metzeler 880 isn't out of round by 3mm, the beads aren't seated
right. I always use Ru-Glyde tire lubricant when mounting and
dismounting tires (which I just did last night, two new tars on my
ZG1000). It's amazing how quick and easy it is to swap tires when
you use the right tools and the right stuff. I usually air my tires
up to about 50 lbs to make sure they pop on correctly, one tire
popped in three places, so don't assume if you hear it pop once
that it's all done. I always look for the concentric circle that
TOG mentions, too.
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