This article is primarily aimed at guiding a less experienced bike mechanic through rim replacement on an electric bike hub motor. It also can be used as a guide for those wishing to do their own wheel build on a conversion project.
I don’t pretend to be a wheel expert. For detailed information on the subjects in this article, go to the Sheldon Brown website. This is a wheel building clinic for hub motor electric bikes.
If you’re reading this, and you have an e-bike with a bent rim, there are several approaches to fixing it. One of the simplest with a direct drive motor is to swap rotors. Simply disassemble the motor with a bent rim, and replace the part with the magnets, spokes and rim with one from an identical brand motor. Some of us have melted a hub motor in the past, and have the part sitting there on the shelf. Or you might find a melted motor for sale cheap.
Another option if you need to replace a bent rim is to buy a spoke set and rim from the place you bought the motor. This might be more expensive than sourcing a rim and spokes elsewhere, but does have the advantage of being sure you get the spoke set that fits. The downside of this method is you don’t get to upgrade the rim quality or the spokes quality. If you really want an improved wheel, it could be worth every penny to just send the motor to a shop that has a reputation for building a high quality custom hub motor wheel. Or order high quality rim and spokes from a shop that is expert at building motor wheels.
On the other end of the difficulty scale, you may have bought a bare motor, and need to start from scratch. The problem then is figuring out what length spoke to buy. First you need to select a rim. Once you get the rim, you will need to get the data to enter into one of the many online spoke calculators. You might be able to get the measurements for effective rim diameter and rim width from the seller, or look up the specifications for a rim that bears a brand name label. If not, the tricky part is measuring the ERD (effective rim diameter.) To do that, put two nipples snugly into the rim holes, exactly opposite of each other. Then measure from the tip of the nipple to the tip of the other nipple.
Then add the length of the two nipples. That is your ERD. Repeat the measurement a few times, in case the rim is slightly out of round. Grin cyclery has a spoke calculator that has some common motor types built in, or you can input raw data if the motor you are using is not one on the list. Once you have used an on line spoke calculator to get your length, there are many sources for spokes. Typically the china laced-hub motor wheel comes with a 12 gauge spoke, but higher quality spokes in the normal bike gauge may be stronger than cheap low quality 12 gage.
For the rest of this article, I will assume you are using the typical 12 gauge spokes, and are just replacing a rim. I will show a typical single cross lacing pattern.
I do not recommend the reuse of old spokes from a wheel that got bent. By the time you really bend a rim all your spokes are old and have been stressed by other hard knocks, or perhaps been slightly stripped at the nipple or spoke threads. If you are on that tight a budget, you still need to get some new nipples. Some of the old ones are going to be mashed or stripped.
Once your rim arrives, check it for warps. If the rim has been damaged by the shipping you will never get it to true properly. The rim should lay very flat on a good flat surface. Finding a truly flat surface might be tricky. You might have to look at your driveway or garage floor with a straight edge to find a spot without dips and humps. Look for obvious large flat spots, or defects in the joint. Why bother to lace and true a junk rim?
Now that you have a straight rim and the spokes that fit, the next problem is finding a spoke wrench that fits. The Park tools red wrench typically fits regular bike wheels. The Park tools blue wrench may be just right for a hub motor spoke nipple, but often I have found it to be too big. One option is to file a red wrench to a custom size for your nipple size. Another option is the multi size spoke wrenches. Generally you can find one size on the ring that fits well. The ring type spoke wrenches are a bit more awkward to use, but can work fine.
Now it’s time to lace it up. I will be describing a single cross lace, which is the typical lace for a 26” rim hub motor.
The smaller 20” rim is generally laced radial. With no crosses, a radial lace is so simple it needs no explanation.
I am also assuming you will not need to dish the rim much if it is a rear wheel. Older model hub motors used lots of dish to keep the rim centered, but newer models generally have little to no dish. I will show how to lace the wheel 4 spokes at a time on both sides, which is easier for a novice but more time consuming. Pros will build the wheel differently, lacing all the spokes that lean one way first, then the ones that lean the other way next. Then they flip the wheel and repeat the process on the other side.
It looks very daunting and complicated at first, but the key thing is to just focus on a group of 4 spokes at a time. The single cross lace pattern is based on 4 spokes, and then that repeats till the rim is full. The 9 continent hub motor in the pictures has 36 holes in the hub flange, so you want 36 holes in the 26” rim. 9 groups of 4 spokes will complete the wheel. It will help a lot to have a good picture of a single cross lace handy. Refer to the picture to get the first set of four done correctly, and then just repeat that pattern for each of the 9 groups of 4 spokes.
One very important thing about getting started is to start at the correct place on the rim. You want the set of four spokes to start at the valve stem hole on the rim.
This way the valve lands between sets of 4 spokes and you can get a pump on the valve much easier. In order to avoid bending spokes to reach the proper hole, lace each set of 4 spokes into the rim at the same time, then start making your crosses and screwing on the nipples. Once one spoke is screwed to a nipple, it can be hard to insert the next spoke and make the cross. In the picture, tape is used to mark the spoke holes, and a big piece of tape marks the valve stem hole. The tape is just for photographic clarity, so the spoke hole locations shows in the picture. The procedure is to insert 4 spokes, and bring them straight on the side of the hub they go on so they won’t tangle.
Then start making the cross on the two spokes on one side and installing the nipples. One spoke , either the left one or the right one will cross over the top of the other. Set a pattern which spoke is the outside facing spoke and keep repeating that pattern.
Next, flip the wheel over and repeat the process. Each set of 4 has two spokes that cross each other on one side, matched by a pair on the other side. Once you start thinking about just two spokes at a time, the pattern becomes quite simple and easy to repeat on each set of 4 spokes.
On the first set of 4 spokes, you will set the pattern, deciding how to thread the spokes onto the hub. Traditional bike hubs lace one spoke with the head facing in, the next facing out.
However, the motor in the pictures for this demonstration has all the spokes laced so the heads are on the outside of the hub. My best guess is that the shorter spokes on hub motors may have been cracking the flange on the hub. Looking at my collection of motors, about half are laced this way, and the other half laced the traditional alternating method. One old model 9 continent rear motor in my garage has a lot of dish, and for that hub all the spokes face in the direction that increases the dish effect. So on the dished wheel, one side of the hub is laced facing in, the other facing out, moving the whole hub over 2 mm relative to the rim.
Once you have 4 spokes laced on the hub, the two on one side of the hub should be brought to the same side of the rim. The two on the other side are brought to the other side. Find the valve hole on the rim. The left spoke on the side of the hub facing you will be crossed over to the first spoke hole left of the valve hole.
Screw the spoke onto a nipple, just enough to hold it. You will need the slack later to ease the starting of spokes on the last few spokes that complete the wheel, so don’t screw it tight yet. Decide how you will cross the spokes, which one over and which one under. Match what it was on your picture of how the wheel came originally. After crossing the spokes, the second spoke will go in the third hole left of the valve hole, and loosely attach a nipple. Skipping a hole is the key. If you look at the wheel, every other spoke is from the same side of the hub. So one spoke pulls the rim to the left, the next spoke pulls to the opposite side. It’s very easy to get confused and forget to skip every other hole on the rim when lacing just one side’s spokes.
Now flip the wheel over, and do the same thing on the other side. After flipping the wheel, what was the left in the picture is now the right. Don’t let it confuse you, just match the pattern you set on the first two spokes.
One spoke goes in the middle hole you left empty, and after the cross, the other into the next hole. When done, it should look like this. 4 spokes, each pair crossed, and no spoke crossing over the valve hole.
Backing up just a bit, inserting the nipples can be tricky on double wall rims. For the first few spokes, you can just shove the spoke way into the hole, and easily put a nipple on, then pull the spoke and nipple back into place. But that won’t be possible as you finish up the wheel. One trick is to screw the nipple onto an extra spoke backwards.
This gives you a nice handle for poking the nipple through the rim wall, and into place on the inner rim hole. Then you can use that slack I spoke of earlier to get enough space to start the actual spoke on the nipple. Once the spoke is threaded on the nipple, the extra spoke can be unscrewed to reuse on the next nipple. If you don’t have any extra spokes, you have to find a way to do the last one, like using a piece of wire and a bit of tape. A magnetized tool might work, since really cheap spoke nipples are iron instead of traditional brass.
Now that you have a pattern set for the first four spokes, just repeat that exact same pattern over and over till you finish the wheel. At some point, you may see the pattern well enough to stop flipping the wheel every two spokes, and just finish one side, then the other. If you do that, insert all your spokes into the hub on both sides, and pull them to the correct side first. Otherwise you will have trouble getting the other side laced, and have to bend spokes to get them into position.
Once you have completed the wheel, you can start tightening up the nipples. You might be able to use a screwdriver on the back of the nipples on a single wall rim, but it tends to be difficult on a double wall rim because the nipples are recessed into a hole.
Start by tightening all the spokes to the same length, by screwing in the nipples until the threads on the spokes are just covered by the nipple. Once the last thread vanishes, stop turning the nipple so the flats on the nipple align at right angles to the rim. This way a spoke wrench is always placed on the nipple at the same angle as the rim, or 90 degrees to the rim. Lining up all the nipples makes it easier and quicker to fit the wrench to each spoke as you go through the wheel truing process.
At this point, it becomes important to be tightening all the spokes the same number of turns. Very likely the wheel is still quite loose, and you want to snug it up some, but not start making the wheel even more egg shaped or warped. If the spokes are quite loose, tighten them all up one full turn of the spoke wrench. Start at the valve hole, so you know when to stop. Check the spokes again, and if still very loose, repeat. If nearly tight, you might tighten every spoke just a half turn. Repeat until the wheel is beginning to lose the slack that was allowing the rim to shift back and forth. When done, the spokes should not be very tight yet, but just snug enough to keep the rim from flopping around anymore.
Now it’s time to mount the wheel on a bike, and start the final truing. Chances are you don’t have a wheel truing stand. I sure don’t have one. One option is to take the wheel to a bike shop at this point. But that costs more money, and by now you have done the hard part. Finish it yourself, it’s not hard if you just use some patience, and follow the same routine so you don’t start loosening spokes you want to tighten. There are many ways to improvise a homemade truing stand, but the method I favor most is to use zip ties attached to the frame to make feelers to help you true the rim. In the past, I have also used bits of baling wire in a similar way. Most simply if you have caliper brakes you can just flip the bike upside down and use the calipers as a gauge.
First you need to get the rim as round as possible. Using a zip tie attached to the frame, find the spot on the rim that is the highest.
Tighten 2 or 4 spokes in that area ½ turn. The idea is to tighten spokes in sets that pull the rim towards the hub, but do not affect the side to side wobble. So doing a pair of spokes will have each spoke in that pair pulling from the opposite side of the hub. From now on, resist the temptation to turn spokes any more than half a turn. As you get closer to perfectly round, you will not turn any nipple more than a quarter turn. Be very patient and don’t try to get all of the bend out of the rim in one adjustment. Go round the rim as many times as it takes, always starting at the valve hole so you know when you have finished one revolution of the wheel. Keep finding high spots and tightening spokes. Using either ½ turn or ¼ turn keeps the nipples oriented the same, so you always know the angle to put the wrench on the nipple, and you stop at the same place every time, either the wrench is aligned with the rim, or 90 degrees to the rim. As you find the high spots and remove them, you can start moving the zip tie closer to the rim, and finding the tiny high spots you couldn’t see before. The high spots will contact the tie, and make a scraping sound. If you find a flat spot, you may need to loosen spokes at the center of the flat spot, and tighten the spokes on either side of the flat spot. Get the rim as round as you can, but the final touches will be done at the end of the process, when all the spokes are tighter.
Now you need to find the point where the rim is centered in the frame. Measure the space between the rim and the frame on both sides.
If you have tightened all the spokes the same so far, the rim should be very close to the center. Once you locate a spot where the wobble in the rim is centered, set your zip tie to that spot by cutting it just even with the rim, while the rim is at the spot where it centers nice. Slide the zip tie down, so the cut end will now rub the side of the rim.
Look at the spokes, and see how every other spoke pulls the rim the opposite direction. Look at the sets of 4 spokes in the pattern, and note how two pull one way, the other two the other way. Start slowly rotating the wheel, and when you see a gap between the zip tie and the rim, tighten the spokes that will make the gap smaller by ½ turn.
You will be looking at the gap from the side you have put the zip tie on, so you will only be tightening the spokes that pull towards you. Go around the wheel this way once or twice, till the gaps are going away. As you do this, there will also be places where the zip tie rubs the rim. Ignore them at first, and don’t confuse yourself. Work on pulling the rim straighter on one side. Then switch to tightening the spokes on the opposite side. Again, go around the rim, but this time, you concentrate on the spokes that will pull the rim away from you, and stop the zip tie from rubbing. Again, never turn a nipple more than ½ turn, and get a rhythm going where you stop having to think so hard about how far to turn the wrench. By aligning the nipples all the same, you can fit the wrench and turn it exactly half a turn quickly and accurately.
You could put a zip tie on either side of the rim, and work on both directions of the rim wobble at the same time. But as a novice wheel builder, I think you will find truing the rim much easier if you break it down to one bit at a time, and don’t walk around to the other side of the bike. Once you move to the other side to look at the feeler on that side, instantly you get confused again and start loosening spokes you meant to tighten . So just use one zip tie, and work from one side of the bike. For most of the process, you will only be tightening nipples, and you will not get as confused about if you are tightening a spoke or loosening it.
Once you get pretty close to having the wheel straight, it’s now time to start thinking about your spoke tension. If you think about it, you have been tightening spokes all this time, and some of the spokes will now be fairly tight, and others still quite loose. You also have some tensions in the spokes that may need releasing. Start pinching each pair of crossed spokes.
Again, start at the valve hole so you know when you have done a complete revolution. As you feel the spokes, a crossed pair that is tight will often be matched by a pair on the other side that is loose. Tighten the loose side spokes by ½ turn if they are very loose. After one revolution, go back to the same process as before, to get the rim straight again. Then repeat the checking of spoke tension again until the wheel is both straight and all the spokes are getting similarly tensioned. You can judge how tight by how they feel when you pinch them, and also by plucking a spoke and seeing what note it plays. The spokes should not be singing soprano, nor should they sing bass. They should all be a nice tenor note when you pluck them. Along the way, keep an eye on how round the wheel is, and if it starts to get egg shaped again, stop and re do the process of getting the rim pretty close to perfectly round. As you get tighter and tighter on the spokes, you may have to tighten a few spokes on one side of the wheel, then go loosen the same set of spokes 180 degrees opposite the spokes you just tightened. You don’t have slack in the spokes you had when you did the initial setting of the rim to round shape.
At some point along the way, you will be able to tell you are getting very close to done. At this time, you should start turning spoke nipples only ¼ turn at a time, and you might find a spoke here and there that is singing soprano will need to be loosened. Slow back down now, take your time, and don’t get all confused which way tightens a spoke and which way loosens one.
The final step is to go back and get the rim perfectly round one last time, then one more time getting the last bit of rim wobble out of the rim. At this point, every set of spokes should be at similar tension, with no over tight or over loose spokes in the wheel. If you cannot get the rim straight without over loose or over tight spokes, it usually means the rim was warped too much to start with.
Once the hub motor wheel is put in service, you will need to make some spoke adjustments in the first few hundred miles. So I leave the zip tie in place for use later. Just rotate it out of the way, and rotate it back when you need to adjust the wheel. Generally, a newly laced wheel will need some adjustments to the spoke tension after the first 50 miles or so. If the wheel makes a ton of creaky noises the first ride, snug up every spoke ¼ turn and try it again. I like to set my wheels just a hair on the loose side. But not so loose the wheel talks to you the entire ride. Overly tight spokes tend to break spokes, rims, or the motor flange. So avoid too much tension. The spokes should not sing a high c note when plucked.
This story was submitted by Dogman.