What would an acceptable tolerance be for wheels along the circumference?
I just got a set on new Dunlop write wheels, I noticed a definite
front wheel shimmy.
The bearings and front suspension is A-ok with respect to angles and
tightness, and all wheels except the spare have brand new Michelin tyres,
and have been balanced woth the right WW gear. The rack is on solid
mounts, and is a 2.5 turn-to-turn high ratio manual type, so the steering
wheel is very sensitive to things happening at the front axle.
I measured wheel straightness using a dial indicator, and found one wheel
to wobble 2.5 mm sideways along the circumference. This was not very
difficult to see, either. The other one had 1 mm. I replaced the worst
wheel with the spare, and found that this too had 1 mm variation along the
rim.
A long test drive now indicated the vibrations through the wheel to be
significantly less, but with one very interesting variation: Driving in a
straight line at a steady 80 km/h (seems to be the worst speed) there is a
cycle of 3 to 4 seconds where the steering wheel quite distinctly goes
between a state where no vibration whatsoever can be felt, to a point
where the vibration where easily felt through the fingertips.
The solution behind this mystery was that while my four tires are all 15"
185/65, the spare is a 15" 195/65. A simple calculation reveals that
while the larger wheel rotates 33 times during a period of 3 seconds at
this speed, whereas the small wheels rotate a full 34 times. So over
this period, the 2 wheels goes from a mode where their sideways motion
goes from a state of their sideways movement cancelling each other to a
state of amplifying each other.
To summarize, then, my questions are:
1. How much tolerance, with regards to sideways and lateral movement
along the wheel rim, should one allow? I mean I've heard a figure
of 1 mm, but it seems that this does not guarantee a "silent"
steering wheel.
2. How easy is it to correct? Bicycle wheels are easy enough to
straighten with good results, and since the angle between the
inwards and outwards pointing spokes is so much greater here, it
would seem that these wheels would be even more controllable.
Egil
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Email: egil@kvaleberg.no Voice: +47 22523641, 92022780 Fax: +47 22525899
Snail: Egil Kvaleberg, Husebybakken 14A, 0379 Oslo, Norway
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