Re: wide tire slips on narrower wheel

On Mon, 31 Mar 2008 12:02:50 -0700 (PDT), Chalo
<chalo.colina@xxxxxxxxx> wrote:

Jerry wrote:

I have an ATB with the usual 2" tires. I got hold of some used aluminum wheels
(Araya) that are narrower and put them on.

All is fine except that the tire and tube rotate on the wheel, probably during
braking. So the stem gets pushed over in its stem hole, and eventually will
tear from the tire causing an instant flat.

That's a problem I have only seen on snow or sand bikes, where riders
inflate them as little as 5psi for maximum flotation. It also happens
to bikes when their inattentive owners ride on almost-flat tires.

Make sure you are using at least a reasonable minimum pressure in your
tires. 30psi is usually OK for a 2" tire, but your bike may handle
and roll better with more pressure than that. If you ride mostly on
the street, I'd recommend about 50psi.

If your tires have a lot of soap, Armor-All, silicone, or other
slippery stuff on them, clean them off and remount. And if that's the
case, apply special attention to cleaning your rims and brake pads,
because they are sure to be contaminated.

Chalo

Chalo

Dear Chalo,

Another possibility is that the tire slipped on the rim because a
too-wide tire tends to pull its bead away from a too-narrow rim. This
leaves much less tire-rim contact than expected, and the tire can slip
because there's not enough contact area to provide the necessary
tire-rim friction.

With a narrow rim and a wide tire, hoop stress tends to pull the tire
bead away from the rim at low inflation because of the excessively
sharp angle where the tire turns and dives into the rim well.

That is, the pull of the casing on a wide tire seated on a narrow rim
is almost sideways and can tug the hidden lip of the bead away away
from the rim at low pressure.

A crude sketch:
http://i31.tinypic.com/fxf4h1.jpg

With properly matched rims and tires, trials motorcycles run 4-6 psi
rear tubeless tire pressure without rim slip, even with no rim locks,
~400 pound loads, and clutch-popping 10 horsepower accelerations.

This video shows how low the pressures go, with a trials tire
repeatedly rolling over a golf ball and scarcely disturbing it:
http://home.comcast.net/~jc-long/clips/trials-tire-test2.wmv

Cheers,

Carl Fogel
.

Relevant Pages

  • Re: H+Son rim failure, Mr. Brandt, your thoughts?
    ... other than Mavic) fail this way, regardless of inflation pressure ... Hoop stress at any cross section is proportional to tire cross section ... angle at which the tire casing departs from the rim. ... It's the hoop stress that matters, ...
    (rec.bicycles.tech)
  • Re: H+Son rim failure, Mr. Brandt, your thoughts?
    ... other than Mavic) fail this way, regardless of inflation pressure ... Hoop stress at any cross section is proportional to tire cross section ... angle at which the tire casing departs from the rim. ... It's the hoop stress that matters, ...
    (rec.bicycles.tech)
  • Re: H+Son rim failure, Mr. Brandt, your thoughts?
    ... other than Mavic) fail this way, regardless of inflation pressure ... Hoop stress at any cross section is proportional to tire cross section ... angle at which the tire casing departs from the rim. ... It's the hoop stress that matters, ...
    (rec.bicycles.tech)
  • Re: H+Son rim failure, Mr. Brandt, your thoughts?
    ... other than Mavic) fail this way, regardless of inflation pressure ... Hoop stress at any cross section is proportional to tire cross section ... angle at which the tire casing departs from the rim. ... It's the hoop stress that matters, ...
    (rec.bicycles.tech)
  • Re: Thumb test
    ... The net force of air pressure acting on the full width of the tire ... at the bulge is greater than the force of air pressure on the rim ...
    (rec.bicycles.tech)