Why balance a BMW motorcycle wheel?
It is very important to balance motorcycle wheels.
Wheel wobbles are related to wheel balance in some cases. A properly
balanced wheel makes the small shakes go away and the rider will feel more
secure at speed. An unbalanced wheel may lead to uneven tire wear and/or a
BMW motorcycle wheels
Read all about BMW wheel bearings. Since the wheels have bearings already, why not use them to balance the wheel?
From 1955 to 1985, the BMW motorcycle used roller bearings. A roller bearing has
more resistance than a ball bearing. Our wheel bearings are greased up and that
further adds some resistance. The seals are rubbing on the axle and that also
adds resistance. By this time, there is so much resistance that we aren't even
close to being able to balance them on the existing bearings. We just don't have
the sensitivity that is required. We must use an exterior device (tool) to help
reduce the rolling friction. Don't ever think that the many "gimmick"
balancers that use the wheels existing bearings will be "good enough" for
How far should we go? Zero resistance would be good, but is impossible. How
close should we get with the weights? Perfect would be good, but is also
impossible. Some error will always be present, but it should be less than 25% of
the desired spec. If our goal is 8 grams, we should have a system capable
of about 2 grams of accuracy, or better.
The balancer for a BMW motorcycle wheel
Wheel balancing can be divided into two types: static
and dynamic. Static is where the wheel just rotates till the heavy part
is down. It gets weight added in the right place to get it to stay anywhere that
it is stopped. Dynamic is where the wheel is spun at some speed to show where
weight is needed and actually prove that it is finally correct. If a wheel were
of no width at all, such as a rigid sheet metal disc, static and dynamic would
be the same. In the real world we don't have any such thing. As a wheel gets
wider, the accuracy of static balancing decreases and dynamic increases. A car
or truck wheel has great width compared to a motorcycle, and the industry
standard is dynamic balancing. So where does a motorcycle wheel lay on
this continuum? For most uses the static balance is "good enough." In some
special cases, it just isn't good enough.
Many years ago I found an old timer who explained balancing car wheels on a
static balancer and I learned some useful things. Once static balance would show
where and how much weight was needed, it was split up in two places on the rim. Equal amounts were attached at points on the rim about 8" apart. The weight was
further divided between both sides. So the total weight was in 4 parts and
places. This knowledge becomes useful when today, most balancing of motorcycle
wheels, seems to be done with stick-on lead weights. Sometimes we found that a
statically balanced wheel would still cause a wobble. We would remove the single
weight and use two of 1/2 weight and move them about 6" apart. Sometimes this
made the needed improvement. We didn't have the "stick on" weights then, so we
couldn't divide them by side. If 4 equal stick-on weights don't work then changing it to
2/3 on one side and 1/3 on the other side sometimes may help. Then try
swapping the 2/3 and 1/3 to the other sides. For speeds
under 120 mph, the static method seemed to work.
With the wheel off of the bike, it is obvious that we need something to stick
through the center of the wheel to hold it up. If we stick a 1/4" rod (axle)
through, then the wheel is already way out of balance. The rod is too small and
the wheel drops down, below "center." Already more mass is below the center than
above, so the wheel rolling will be very insensitive. The center of the "rod"
needs to be exactly that same as the bearing center. The easy question is "Why
not use the axle?" That would take care of part of the "center" problem. We
could use the axle, and some do it that way, but it requires a machined sleeve
between the nut and bearing to keep the cone seated. The ID of the sleeve must
be a tight fit on the OD of the axle.
Tapered bearings are in two parts. They have an "inner part" called a cone,
or inner race, because it is the shape of a cone. The "outer" matching part is
called a cup, or outer race. The cone sits in the cup. If the cone is allowed to
creep out of the cup, a bit, then the wheel drops down a bit. Now we are back to
the same problem again, only to a lesser degree. The seal keeps the cone from
falling out totally, but won't keep it fully seated in the cup. The cone
must be fully seated in the cup.
Now we know that we need something to hold the cones fully seated. We also
know that we can't trust the internal bearings to be free enough of resistance
to use them for balancing. We need "external" bearings. I
have seen this done in a couple of ways, but the most common is to have a
bearing system on each side of the wheel to hold up the "axle." I will include a
picture of two discs on bearings with some overlap, to cradle the axle, as soon
as someone sends me one.
This spot needs a picture, a close up of the two discs
on a balancer
The balancer used by BMW dealers
See the two cones that center the wheel? This balancer needs to be mounted in
a vise. It is of medium quality.
One commercial balancer uses two tapered cones that will slide along the axle
to keep the bearing cones in place. The axle is smaller than any wheel, but the
cones center the wheel and keep pressure on the cones to keep them seated.
That set up will then sit on two pairs of discs on clean ball bearings to show
Better than cones for providing the preload necessary to keep the tapered
wheel bearings seated is to use spacers. The cones can still be slightly off
center. A motorcycle wheel balancer of the type sold by Marc Parnes uses the
proper spacers. I am not sure of the type and quality of the bearings on
the ends of the axle used, but they should be very good.
We used steel motorcycle wheel weights. As I remember, the smallest weights,
of three sizes, was 5 grams. They were round weights, with a slot, that slipped
onto the spokes and then were lightly hammered down on the spoke nipple. This
meant that the weight was, more or less, in the center of the rim. This system
seemed to work well and we had little problem with it. See below.
I have no experience with the stick-on lead weights, but they allow a greater
variety of balance options. They also make balancing more complicated. They are
on one side of the rim and that may require an offsetting amount on the opposite
side to keep the total weight centered. It may require more tinkering to
get it balanced.
The two weights on the left are not BMW, but were seen on some bikes and I
have no idea where they come from. The three on the right are all BMW or the
very close copies supplied by Flanders. I don't have a scale to measure and get
the actual weight. For us it wasn't important, we had small, medium and
Their weights are all for the common /2 and /5 nipple. They won't fit the
nipple for the 3.5 mm spoke that was used up to mid 57. Go to
this page to read about the many parts that
were unique to the first two years of Earles fork BMWs.
If the balancing tool is sensitive, it will show a 5 gram change easily. Once
you have balanced your wheel you should test the sensitivity of the whole rig. Add a dime, penny or 5 gram weight to the wheel in any spot and the wheel should
easily rotate until your "new" weight is at the bottom. If it doesn't, then you
don't have the needed sensitivity and may not have the wheel balanced. You may
need to clean the bearings of the balancer. To get more sensitivity, I have used
my electric tool etcher, by Craftsman, to vibrate the bearing assembly.
Here is the
balancing tool I prefer, but there are other good ones too.
The motorcycle tire painted "light" spot.
In my shop we used almost exclusively Metzler and Continental tires. Once in
a while a customer would have some special need and we would install "his" tire.
In every case using an "off brand" tire, the much desired "BMW motorcycle
neutral steering" was lost to some extent.
Thirty years ago, the tires that we used had a spot of paint on the tire that
was supposed to be the "light" place on the tire. They accepted that a perfectly
balanced tire was unlikely to be made and just marked the light place with a
spot of red paint. Since the "heavy" spot on the tube was the valve stem, one
could match them up. All of this assumes that the wheel and tube are perfect. Not so fast. All are suspect.
So now we were dealing with three things that should be assumed to be out of
One day my mechanic came to me and asked me to take a look. He had installed
a new tire and it required 3 weights to balance it. It was seldom to be so far
off. He had then deflated the tire and broken the bead in order to move the
"spot" to another place. His first attempt was worse, but his second got it
nearly perfect. This was proof that the manufacturers spot was way wrong, or the
wheel/tube combination was a disaster. He dismounted everything and started by
measuring the wheel. It was quite good, so that meant that the tire/tube was
off. This started us to thinking about the balance issue.
Over the next weeks we tried several experiments. We checked several wheels
with no tire or tube. They weren't perfect, but alloy was the best and the steel
rims the worst. None were really far off, say two weights, usually on the order
of one or slightly less. How does one measure the balance of a tube? Only by
substituting them could we sort of get an idea. We would need to start with a
perfect wheel and tire to make meaningful measurements. Since it was a lot of
work and we had no real way to quantify them, we decided to ignore the variable
of the tube. One must remember that we were in business to make money too. All
of this investigation was quite interesting, but we couldn't expect the paying
public to subsidize our research hobby on the balancing of motorcycle wheels.
While we weren't charging for this time, still in the end the customer is paying
for everything that we do, one way or another.
We sort of started by moving the tire around the rim on wheels that required
two or more weights. We would first rotate the tire 90 degrees one way and
balance it again. If we then used only one weight, that was good enough. If we
needed more than it had with the paint spot at the valve stem, we knew that the
tire was either marked perfectly or we went the wrong way. We would move the
spot to 90 degrees the other way. Sometimes we even tried the spot directly
opposite the valve stem and that would "fix" it up. This meant that the spot was
more or less on the completely wrong side of the tire. That was seldom the case,
but roughly 1/3 of the time the spot was somewhat off. This caused great
confusion with some customers.
In saying that the spot was off, didn't necessarily mean that the spot wasn't
at the lightest point on the tire, but the combination of wheel, tube and tire
required the spot to be moved for best balance. We proved that sometimes
the spot was incorrectly marked, but we suspected that most of the time our
combination of wheel, tube and tire just needed to be altered for best balance.
In actual practice we found that the more weights needed to balance the
finished wheel, the less likely it was to fix a low speed wobble. That told us
that while it was statically balanced, it was still dynamically off. At that
time a dynamic balancer was very expensive and rare. Many times we fixed a low
speed wobble by just changing out the front tire. A front tire that is down to
half tread depth is completely worn out. So, half of that is only 1/4th "worn
off" tread. We could take one that was only 1/4th worn and balance it, but it
would still wobble. Then install a new tire and all was OK again.
For a good article on
building a /2 wheel.
What are those 5 dents in the rim for?