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"Back in the day" when we were dealing with bias ply tires, there was not so much rotational mass (weight) on the outside diameter of the tire. The most of the weight of the wheel was the wheel itself which by definition was closer to the center of rotation. The wheels were narrow - frequently 4.5 to 5 inches wide. Roads were rougher and speeds were lower. Suspensions were probably less sensitive to transmitting road feel. Balance became more of a problem when we went to bias belted tires in the early 70's and then radials with steel belts on the outer diameter creating a much different weight distribution within the tire / wheel assembly. These also came with wider wheels (6 to 7 inch). And roads were improving as well as car suspensions. Speeds were increasing. Now think of an individual tire - the reason we balance them is that they are imperfect with respect to weight distribution - there has to be a splice in each ply where the carcass is built up. This results in a local area where there is overlap and non uniform weight. The rubber mixture may also be less than uniform density - after all it is a mixture that is extruded into sheets that are also spliced. So if a tire is perfectly uniform, it is unusual. And the rims may be imperfect also as steel rims are frequently welded together and aluminum wheels may have nonuniform density due to casting. A tire can be modeled as a rotating circular disc - but now think
not of disc representing the tire geometry but the disc
representing its mass (weight) distribution. It you tilt the disc
to where the disc runs thru the center of mass (weight), the heavy
spot causes the plane of the disc to tilt from the disc
representing the center of geometry of the tire which is still the
center of rotation. But the tire wants to rotate with the plane
representing the center of mass and hence wobbles creating the
vibration we feel from imbalanced tires. In rotational bodies there is a resonant frequency which is a
function primarily of outer diameter and rotational speed. When
imperfect objects rotate they create forces due to the imbalance,
but these forces may be relatively small at speeds not near the
resonant frequency. Typically for older vehicles this was roughly
60-65 mph so you would feel the worst vibration at that speed. The
frequency is purely a function of revolutions per minute and hence
tire diameter and vehicle speed. The vibration will be worst at
this speed - and you may actually be able to reduce the severity
of the vibration by increasing or reducing speed - though it will
probably still be present. We balance tires and wheels to make the car think the assembly is
perfect and the planes of rotation and mass are the same. But how
do you do that when heavy spot may be on inside or outside of tire
and there may be more than one at different placed around the
tire. The first fix was to add a weight opposite the heavy spot - this
worked at low speeds but failed at the resonant frequency
(specific vehicle speed). In reality, a single point balance or
even a 2 point balance on the back only has a 50-50 chance of
aggravating the problem - If you are unlucky and the heavy spot is
on the front (outside) of the wheel and the balance weights are
opposite and on the back (inside), you have increased the effect
of the tilt of the plane representing the center of mass. We then went to a "4-point" balance to try and better distribute
the heavy areas compensating for the imbalance. In this method you
put 2 weights on top and 2 on bottom spaced apart so that there
were really 3 heavy spots spaced around the tire - this worked
pretty well as speeds increased with bias tires. But the first belted tires with more weight and more splices on
the outside diameter required more sophistication - hence the
development of on car rotational balance and eventually computer
spin balance. Sorry for the history lesson and the tech-speak, but I tried as
best I could to make it understandable. Hope this helps. Edward Mills
Antique Tractors 1930-1960
Antique Cars 1960-1985
On 7/29/2019 1:05 AM, 'Rich Barber'
c300@xxxxxxx [Chrysler300] wrote:
__._,_.___ Posted by: EMills_ATC <millserat@xxxxxxxxxxxxx> To send a message to this group, send an email to: Chrysler300@xxxxxxxxxxxxxxx To unsubscribe from this group, send an email to bob@xxxxxxxxxxxxx or go to https://groups.yahoo.com/neo/groups/all/manage/edit For list server instructions, go to http://www.chrysler300club.com/yahoolist/inst.htm For archives go to http://www.forwardlook.net/300-archive/search.htm#querylang __,_._,___ | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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