|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.
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. 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 far/loose enough over 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 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 about 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, near the middle dent, 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 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 so that it allows the tire bead to drop all of the way in.
5. Use the tire irons directly across from the valve 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 understood if you see 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 while it helps the balance, the dents would need to be calibrated for one type of tube. That would be very “German,” but thank goodness they didn’t do it for that reason.