The five rim dents on BMW motorcycle wheels and rims
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 that have five dents. A few stories have surfaced to explain these dents. I will reveal 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 the purpose.
When a tire blows out, it isn’t unknown for it to climb off the rim. Now you have the tire jamming in the swingarm 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.
Some 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. The bead is a metal ring covered by rubber so that you don’t see it. 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 loose enough to get over the edge of the rim. When a 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. One was enough. That was a foolish way to solve the problem, and I refused to do another. Ask anyone who has changed a tire with rim locks about how hard it is. BMW came up with an easier way. They decided to plug up the “drop center” area. They punched five dents into the rim directly across from the valve stem. Between the five dents and the valve stem, the tire bead is prevented from getting into the center well enough for the other side to have sufficient clearance to climb off. One must push the valve stem into the tire space to remove the tire. Now about 2/3 of the bead is free to move into the center. The bead on the other side, near the middle dent, can be levered up and over the rim.
At the bottom of the photo above is a line marking the valve stem’s hole. The five black triangles mark the five dents. Notice that they are spaced directly across from the valve stem hole?
The black magic marker points to a close-up of one of the dents.
Removing and replacing a tire on one of these rims takes special care. The owner’s 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 tough to get the tire over the edge of the rim. To see how to do it, 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 loose 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 to allow the tire bead to drop all the way in.
5. Use the tire irons directly across the valve stem and pry the first bead up and off the rim.
6. Remove the tube.
7. It is time for the bottom bead to be levered up and off the rim. Be sure to start directly across from the hole for the stem. Stand the wheel vertically (valve stem hole down), 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 were soaped up well on the inside of the bead, it would now come off by hand. It is hard to believe unless you have seen it done in person.
BMW advertised widely, at the time, about the wonderful way that they kept the tire on the rim in a blowout. Various tube manufacturers use different masses for the valve stem, so it helps the balance.
Updated 11 July 2022