|Why balance a BMW motorcycle wheel? Because it is very important to balance motorcycle wheels. Wheel wobbles are often related to wheel balance. 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 crash.
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 tapered 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 safety.
How far should we go? Zero resistance would be good, but that 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 balance becomes more important. 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 of 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 a 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 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.
The balancer supplied by BMW
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 the 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 balance.
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. After all these years, that is still my favorite balancer.
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, but I understand that they are 15, 10 and 5 grams.
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 from late 55 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. To get more sensitivity, I have used my electric tool etcher, by Craftsman, to vibrate the bearing assembly. Once you have balanced your wheel you should test the sensitivity of the whole rig. Tape a dime 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.
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 ” neutral steering” was lost to some extent.
In the 70’s, 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 balance.
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 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 manufacturer’s 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 re-thinking 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, usually on the order of one weight 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.
A tube typically comes with two nuts. Never put one nut on the inside of the rim. Both belong on the outside. Move both up to midway on the valve stem and then tighten them against each other. This keeps them from moving.
By keeping a space between the nuts and rim, the valve stem is allowed to be cocked off to one side and you will see it at an angle to the rim. This is evidence that the tire may have slipped on the rim. If it slips far enough, it can rip the stem from the tube. We call that a blowout.
Why does the tube even need a nut in the first place? That is because it is used in the tire removal procedure. It keeps the valve stem from disappearing into the tire when pressed into the tire to allow the bead to go into the channel for the drop center aspect.