This page is about the BMW motorcycle models with spoke wheels: R26, R27, R50,
R60, R69, R50/2, R60/2, R50S, R69S, R50/US, R60/US, R69US, R50/5, R60/5, R75/5,
R50/6, R60/6, R75/6, R90/6, R90S, R60/7, R75/7, R80/7, R100/7, R100S, R100RS.
Since WWII, and up until the cast rims, BMW has used rims with 5 dents in
them. A few stories have surfaced to explain these dents. I will explain the
truth and debunk one myth. If your /2 doesn't have them, then it isn't an
original BMW rim. Few seem to notice them and even less know what they are for. I will try to explain it. When a tire blows out, it isn't unknown for the tire
to climb off of the rim. Now you have the tire jamming in the swing arm or forks
and the metal rim is on the ground. You are in for an exciting ride.
It is time to review the way that tires are manufactured, mounted and
removed. Way back in the old days the rims came apart in some fashion. The most
recent were called "split rims" and were known to be very dangerous. The latest
and safest version is called a "drop center" rim. It is that "place" where the
rim diameter is the smallest, in the very center where you find the spokes. Now,
a note about tire construction. The tire has something called a bead. Inside the
bead is a metal ring. It keeps the bead from stretching like a rubber band.
If the bead stretched, it could climb off of the rim.
To dismount a tire the bead must "drop" into the center enough that the other
side of the tire bead is far enough over to get over the edge of the rim. When
the tire goes flat, the bead can get into the center enough for the other side
to climb out. To prevent this the center must be plugged up somehow or the
bead must be captured so it can't get into the "drop center" area.
To keep a tire bead on the rim, many motorcycle manufacturers decided to
capture the bead in place. The way that they do it is with something called a
"rim lock." I know little about them as I only did one. Once is enough. That was
a really stupid way to solve the problem and I refused to ever do another one. Ask anyone who has changed a tire with rim locks to hear how hard it is. BMW
came up with an easier way. They decided to plug up the "drop center" area. They
punched 5 dents into the rim directly across from the valve stem. Between the 5
dents and the valve stem the tire bead is prevented from getting into the center
well enough for the other side to have clearance enough to climb off. To remove
the tire, one must push the valve stem into the tire space. Now about 2/3 of the
bead is now free to move into the center. The bead on the other side can
be levered up and over the rim.
At the bottom of the photo is a line to mark the hole for the valve stem. The
5 black triangles mark the 5 dents. Notice that they are spaced directly across
from the valve stem hole?
A close up of one of the dents.
It takes special care to remove and replace a tire on one of these rims. The
owners manual describes how to do it. Failure to follow these exact instructions
can result in a broken bead. In the best case it will be very hard to get the
tire over the rim. Here is the short version of the way to do it.
Long version with pictures, go here.
1. Deflate the tire and leave it sitting in the sun to warm up.
2. Remove the valve stem nut and push the stem into the tire.
3. Step on the tire and get both beads off of the rim. Lubricate
the beads with thick soapy water.
4. At the valve stem, step on the tire to allow it to drop into the center of
the rim. It is very important to have the stem pushed into the tire so
that it allows the tire bead to drop all of the way in.
5. Use the tire irons directly across from the stem and pry the first
bead up and off of the rim.
6. Remove the tube.
7. Now it is time for the bottom bead to be levered up and off of the rim. Be
sure to start directly across from the hole for the stem. Stand the wheel up
vertically (valve stem hole down) and stick the tire iron in, at the top, from
the wrong side and lever it over. You may be able to stick the second iron in
there too. If it was soaped up well on the inside of the bead it will now come
off by hand. It is best seen done in person once.
One myth is that the metal/mass in the 5 dents is shoved away from the center
of the wheel to offset the extra mass of the valve stem on the other side. It is
a balancing technique. The fact that the resulting mass is changed by punching
these dents further from the center will help. The valve stem adds weight to the
rim. To offset it would require weight be added to the other side. Punching them
in is the same as adding weight to that side. Here are the problems with
The metal is actually punched closer to the center of the wheel. In
effect that makes it "lighter" on that side, or heavier on the other side, which
ever way you want to think about it. Adding weight to the side of the rim
with the valve stem and the extra weight of the valve stem only makes it worse,
not better. So, in fact, the "myth" is opposite of what happens.
1. BMW advertised widely, at the time, about the wonderful way that
they kept the tire on the rim in a blow out.
2. Various tube manufacturers use different masses for the valve stem, so
while it helps the balance, the dents would need to be calibrated for one type
of tube. That would be very "German," but they didn't do it for that reason.