Lacing a wheel, rim offset, spoke sizes and lengths
This page is about the BMW motorcycles from about 1950 until they stopped using spokes in the 80s. BMW is again using a spoke wheel with an entirely different design. It is a far better way to do it.
This article is to help you build a wheel with the correct offset, know what spoke is correct for your BMW motorcycle, or help you identify a spoke.
Lacing a wheel and the simple trick to get the offset
Go ahead for those who want to lace and true a wheel at home. You will find that it is time-consuming for the first one but very satisfying. The rim has an offset; that is, the rim edge isn’t lined up with the edge of the hub. Before you take the old one apart, check the offset. Lay the “old” rim on your flat surface with the brake side down and the bearing access side up. Notice that the rim is about 1/4″ above the surface? Find three nuts that just fit under the rim and keep them for later use. The measurement is .220″. On the /2 and /5, the offset is exactly the thickness of a 13 mm nut. Far too much is made of this “offset thing.” If you are off by one mm, you may ignore it.
The rim has an arrow to show rotation. If you have it in the wrong direction, the spokes will need to bend around each other. If you don’t find the arrow, then maybe you have an aftermarket product. They are of questionable quality. Lace-up 4 consecutive spokes and nipples (and bushing for the /2) into place. The crossing spokes should barely miss each other. They must not curve around each other to get into position. In that case, the spokes are no longer “straight pull” and will slowly try to straighten out. That means that they will become loose. Remove the spokes, flip the rim over, and try again.
BMW has a stupid tool that is a template. My shop had them but quickly found that it took way too much time to keep holding it up to see if the offset was correct. The way described here is such that it just automatically comes out right.
This is a /2 hub with a steel rim on a flat surface.
These two photos are from Bob Erley, thanks.
Here you see a nut under the rim holding the wheel rim off the flat surface. The amount of offset is the same as the thickness of the nut. You have an automatic offset by using three nuts spaced equally around the rim. Now carefully start threading the spokes into place and start the nipples on each spoke. Plan to go around the rim a few times and tighten each nipple the same distance on the spoke. You are “sneaking up” on getting the nipples to all be threaded the same amount on the spokes.
Always start any procedure at the valve stem hole so that you know when you have gone the whole way. I run each nipple down to where the spoke is just starting to come up flush with the bottom of the slot in the nipple. When all are in, the rim must still be fairly loose. I check for the “tightness” of the rim by gently lifting it. It is not enough to lift it off the flat surface but enough to feel any looseness. If it is still loose, go around and give each one a small turn, but they must all be the same. Test it again and, if needed, go another small turn. It is far better to tighten up “too little” and have to do it again. If you over-tighten them, you might lose sync and not know where you started. When finished, the rim is mostly tight, and the offset is in place.
If the offset is off by .040″ (1 mm), don’t worry, it isn’t that important. If it is off by 1/8″ (3 mm), then fix it. Next, mount the hub to a stand and use a dial indicator to “dial it in.” Check each spoke to hear that it has a musical note when struck. They should all have a similar “ring.” The musical note indicates the tension on the spoke. If you find that a few go “thud,” that means that those spokes are loose.
I have confidence in the accuracy of the offset numbers on this site.
BMW motorcycle wheel truing stand
I used what was at hand. Since I started in the days of the /2, I used a set of Earles forks to hold the wheel. Forty years later, I suspect that few owners have an Earles fork swing arm lying around. Maybe try installing the wheel, without the tire, back on the motorcycle and attaching the dial indicator to a fork leg. If you can’t rig up some way to hold the wheel and dial indicator, then you won’t be able to tune it.
The photo above shows the dial indicator rigged to show the up and down error. When set to show the side-to-side error, the dial will jump around erratically due to the small “teeth” that hold the tire from slipping. Just average it out. For the side-to-side alignment, the smooth outer surface of the rim is more accessible to measure than the inside surface but is not accurate. Try to use the inside, if possible. The rim may not be the same thickness all of the way around. The greatest error is usually at the weld for a few inches on each side. You must ignore that wavy area because there is nothing that you can do about it.
With a dial indicator set up similar to this, one can tune your BMW motorcycle wheel rim quite easily. Expect your first one to test your tenacity. The second one will be much faster. The satisfaction of doing it yourself is well worth the frustration of the first one.
BMW used the “hooked spoke” type on all models up to, but not including, the R25/3, R67/3, and the late R68. About 1954, they produced a hub that allowed a “straight pull” spoke. All previous spokes were hooked on the head end. This allowed a place to hook it at the hub. The advantage of the hooked spoke is that the hubs are easy to make and, therefore, cheap. The disadvantage is that where the spoke is bent or “hooked,” it is weak. Over time, the pull on the spoke is such that the 90-degree bend wants to straighten out slightly. This is why motorcycles often need to have spokes checked for tension. It was common for one to check spoke tension, even between major services. The advent of the straight pull spoke eliminated this time-consuming procedure. With its tapered roller bearings and straight pull spokes, the BMW wheel made it the best wheel on any production motorcycle.
The photo shows the two lengths of hooked spoke for the /3 half hub wheel, so common on the R25, R25/2, R51/3, R67, R67/2, and the wonderful R68. All of these spokes had the hook at the head end. The R67/3, late R68, and the R25/3 had the full hub brake and therefore used the straight-pull spokes.
The photo shows how I made my measurements with the tape. At the head end, I measured to the long end of the curve. That means that I didn’t include the head. The total spoke length would be very slightly longer than my method.
BMW preferred to have two suppliers for all parts, and the spokes were no exception.
Most had the four-leaf clover mark on the head of the spoke. According to Mark Huggett, that is the trademark of Berg Union. Later they used Alpina, from Italy. They have a large “A” on the head.
You can see that the spoke is thicker near the head. Some BMW spokes had this, and some were the same diameter along the entire length. That shoulder is “swaged” or squeezed to make it thicker.
The spokes with an end 1 inch thicker were called “Dickend” (one thick end) speiche, the one with two thicker ends was called “Doppeldickend” (double thick end). This clarity is from Rolf Gehrke
This photo shows a NOS /3 long spoke with a plain head. I presume that it is from a different manufacturer.
The /2 BMW spoke bushings.
The brass bushing on the left is a stock item on the /2 BMW. The early ones were just plain brass, and the later ones were cadmium plated. The bushing on the right was removed from a BMW hub. It is slightly larger and has a cutout so that it can slide onto a spoke. I can’t remember the story.
This shows the spoke head and bushing.
The shoulders fit into the notches in the bushings.
BMW motorcycle spoke tools
The top spoke tool is common, and it is worthless. It will only grab the nipple on two corners, and the soft brass nipples used on BMW spokes will easily distort. The middle tool is by Matra, and it grabs three corners. It is far better, but I think that it is hard to find. The lower tool is homemade and works very well. It is just a modified screwdriver. The cut in the middle is to clear the spoke. Be sure to grind the tip to fit precisely in the nipple slot. One can tighten a nipple enough with this tool to do the complete job. Don’t waste your time finding a spoke wrench.
The 1970-71 /5 spokes that broke, and the fix
In 1970, with the advent of the /5, BMW changed the spoke method, and a serious problem developed. They eliminated the small bushings, used on the /2’s, into which the head of the spoke fit. BMW used the same spoke design.
Typical /2 spoke was also used on the /5
This meant that those shoulders had to dig into the aluminum hub and seat themselves. The shoulders I am talking about are about one mm in size. The symptom was that after some miles, the head of the spoke broke off and sometimes got between the rear end and the hub and gouged the metal. The damage wasn’t usually as visible on the final drive as on the wheel hub. It was rumored that sometimes the spoke would poke a hole into the inner tube, resulting in a flat tire. BMW had tape around the rim to protect the tube from spokes, so I never saw that problem. We found either that customers would hear or feel the grinding, or we would find a loose spoke during regular service. I never saw more than one or two broken at one time. We would just replace them. Sometimes they would break again soon, puzzling us greatly. Butler and Smith either sent out a service bulletin or told us in person about the fix.
When the spokes were made, the two shoulders were not precisely of even height. When the spoke got tightened, more pressure would be on one shoulder than the other. This “cocked” the head of the spoke off to one side, and it eventually snapped off. BMW recommended that each spoke head should be “set” with a hammer and pin punch. This drove the shoulders into the aluminum hub and seated both shoulders. A good whack to a tight spoke would often loosen it a bit, and retightening would finish the job. Putting the spoke shoulders into the old holes often wasn’t enough; they needed to be “set.” This put a heavy burden on the mechanic. The wheel had to be laced, tightened a bit, then “set,” fully tightened, and then trued.
In 72, they changed the spoke to have only a vestigial shoulder. They were no more than a few thousands in height, just enough to set with normal spoke tension. The tiny shoulder is just sufficient to keep it from spinning but not enough to cock it to one side. We would still set them, and they would loosen slightly. I consider it prudent always to set the spokes.
I have mixed and matched the types of spokes with no problems. Of course, I prefer the later type. If I were building a /5 wheel, I would only consider the later type spokes.
The “new” spoke for 72
I watched the /5 wheels being trued at the factory in Berlin with great interest. I was amazed that they didn’t use a dial indicator. I had used one for years and didn’t know that anybody would do it any other way. The guy was excellent and could true an already laced-up wheel in only 7 minutes. He did them all day long, but I did only about one a month. That is my excuse, and I am sticking with it.
BMW motorcycle spoke measurements
We typically call the spokes for the /2 either 3.5 mm or 4 mm. That is what BMW calls them. While I knew these sizes, I just thought of them as “fat” or “skinny” spokes. Recently when I put a micrometer on the two spokes, I was amazed. The 3.5 mm spokes are closer to 3 mm, and the 4 mm spokes are closer to 3.5 mm. I have no clue as to what that is about, but that is what I found.
I must add that these measurements are for common spokes. BMW made motorcycles to order according to the buyer’s specs. Some agency or larger customer could have ordered some special wheel with anything for spokes. BMW could have run out of the usual spoke and made up a few motorcycles with some odd spokes. You could own a BMW with original spokes, and they may not fit this list. I am not claiming this list to be complete or the “gospel,” either. I have seen too many “odd” things from BMW ever to be sure of what is original.
In cases where I have a number of spokes from the same model but different wheels, the variation in diameter can be .005″ or about .1mm. The length can vary too.
R51/2 hooked spoke, diameter .140″, no swage, length 5.290″ and 8.937.
R25, R25/2, R51/3, R67, and R67/2, hooked spoke, diameter .150″, swage diameter .170″, length 5.170″ and 8.530″ The length from the curve to where the swage tapers is about 1.2″ and varies .125″ or so.
R25/3, R26, and R27, straight spoke, 140 mm in length, 3.5 mm diameter at the thick end. Thanks to Rolf Gehrke for info.
R50, R60, and R69, from the first ones in 55 up to mid-57, when BMW changed the spoke pattern for more strength. The spokes are straight, and the diameter is .120″, length 5.545.
The diameter of the BMW motorcycles from mid 57 up to the end of 1969 is .136″, and the length is 5 1/2″ or 14 cm. Part # 36 31 4 038 222.
Rear wheel spokes for /5 and later, diameter is .136″ length 5 1/2″ or 14 cm. Part # 36 31 1 234 591.
Front-wheel spokes for the /5 and R60/6, part # 36 31 1 230 321 or part # 36 31 1 234 592, size unknown.
/6 with disc brake front wheel, diameter .136″ and length 6 15/16″ or 17.7 cm. Part # 36 31 1 234 593
The death knell for spokes, sort of
To some extent, the superior rigidity of a cast wheel and the popularity of tubeless tires signaled the end of spoke wheels. The fact that spoke wheels are lighter and therefore handle better seemed to have escaped notice. Some manufacturers made wheels of pressed and riveted steel. They were cheap but rigid, and they became popular. More manufacturers came out with some form of cast wheel that would use tubeless tires by the mid to late 70s. BMW introduced cast wheels in 77 (I think) and, within a few years, had almost totally replaced the spoke wheels.
Other related pages
Some balancing info specific to the BMW wheel
What about those 5 dents in the rim?
Some aspects of tire air pressure.
Updated July 15, 2022