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Hub motor wheelbuilding for ebikes



This page is to help you make a hub motor wheel for your bike, starting with a hub motor, a rim and some spokes.
Lacing a wheel is a key skill in making ebikes. Even if you buy a kit with a built wheel, there are a lot of badly made wheels in these kits so it’s worth knowing how it’s done.
Most of what you need to know about building ebike wheels is standard wheelbuilding. My favourite online wheelbuilding teacher is Sheldon Brown, whose excellent advice on wheelbuilding can be found at http://sheldonbrown.com/wheelbuild.html

The steps

Presumably you have a hub to start with. Now you can:
  • Measure the hub
  • Choose a suitable rim
  • Measure the rim
  • Decide on spoke cross number
  • Calculate spoke length
  • Get suitable spokes
  • Get other bits and pieces: rim tape, grease and perhaps some spoke washers
  • Lace it all together
  • True the wheel

The first few steps are focussed on getting the information you need to calculate your spoke lengths. You need to get this right!
The measurements required are:
Number of spokes
  • Effective rim diameter
  • Width from hub centre to left flange
  • Width from hub centre to right flange
  • Flange diameter, left side
  • Flange diameter, right side
  • Spoke hole diameter
  • Cross number

Measure the hub

If you’re lucky, you may have already have reliable measurements of your hub. Don’t trust measurements in web-shops! You can usually trust motor drawings – if they are drawings of the same motor. Do some checking just in case.

This is the Bafang SWXK front motor. Note how the power cable exits from the side plate

Bafang BPM rear hub motor
If you need to measure it yourself, read what Sheldon has to say here:
Also watch my video on measuring the hub:
You need to end up with these measures in millimetres:
  • Width from hub centre to left flange: this is the distance from the left flange
  • Width from hub centre to right flange
  • Flange diameter, left side
  • Flange diameter, right side

Choose a suitable rim

Most importantly, your rim needs to be the right diameter, and have the right number of spoke holes. Every hub motor I’ve ever seen or heard of has 36 spokes – but count yours to make sure! (36 spokes is good: strong and universal)
You also want a rim that’s strong and wide. Wide rims are stronger, easier to put tyres on, less likely to pinch a tube, and your tyre is less likely to get damage from rim brake pads. Especially for rear wheels (that do the hardest work), I recommend Sun Ringle Rhyno lite or similar.
Especially for front wheels, I often use the cheap Chinese rims from my hub supplier. I’ve never had a problem with them, but I keep spares in case of trouble (from crashes or wearing the sidewalls away with rim brakes).
It is good to have spare rims of the same type you use. It is so much easier to swap rims on a wheel to a new rim of the same type and dimensions, instead of re-calculating and re-lacing to a different rim.
I suggest give deep rims a miss – they won’t help you and they make it hard to put the nipples on.
A used rim in good condition is fine – first check it is true by giving it a spin before unlacing it from its previous hub. Also, especially if you are using rim brakes, check the condition of the side walls. Rim brakes gradually wear away the side walls of the rim, which will eventually start to split off (and eventually tyre air pressure will blast the wheel apart). Don’t use a rim with badly worn side walls.

Measure the rim

As long as you have the right number of spoke holes (got that?), you only need one measurement of the rim to calculate spoke length. That is: effective rim diameter (ERD).
ERD is the diameter of the imaginary circle which would touch the outer tips of all the spokes around the wheel, at the outer ends of the nipples. To measure this you use a couple of nipples, screwed backwards onto a couple of spokes, placed into opposite holes in the rim. Use a metre rule to measure the inside distance between the nipples, and add on the lengths of the nipples to get the ERD.
This video shows how:

Decide on spoke cross number

As a generalisation, it’s good to have as many crosses in your spokes as you can. This reduces the forces on the hub flange, and makes the spokes more effective at transmitting the torque (driving force and braking force) from the hub to the rim. Radial spokes aren’t a good way to go, as they are more stressful to the hub flanges and aren’t good at taking torque forces from hub to rim. However sometimes a large hub in a small diameter rim can’t fit any crosses and radial spokes are the only way to lace the wheel.
This 26" wheel with large hub motor has radial spokes (0-cross), it might have been able to fit 1-cross
Normal bike wheels with small flanges can accommodate 3 or 4 crosses, making a very strong wheel.
[3 or 4 cross wheel]
With hub motors it’s harder to use higher crosses, because the hub flange diameter is so big. Most motors can only use 1 or 2 crosses. More crosses would give the spokes too acute an angle at the rim, which bends the spoke where it enters the nipple.
I tried 2-cross on this BPM in a 26" rim, but the angle of the spokes was too acute at the rim, bending the spokes where they meet the nipples
Same hub, same size rim, 1-cross. Spoke angles are fine
Many Chinese hub motor suppliers (e.g. Greenbikekit.com or Bmsbattery.com) can supply spokes and rims. The spokes usually come (roughly) the right length to do 1 cross. This is a quick, easy and good enough way to lace your wheel, and I’ve never had any problems with doing it this way – but I do check the spoke length first myself.
20" wheel with 1-cross 2.3mm spokes on Bafang SWXK hub

Calculate spoke length

To calculate the spoke length, I like to use the “Spocalc” Microsoft Excel spreadsheet, which you can download from http://sheldonbrown.com/rinard/spocalc.htm
My preference is to use the version called “Spocalc Express”, which is a no-frills version without a database of hubs and rims (I haven’t ever found a hub or rim I was using in the database). This way there is less opportunity for mistakes. Spocalc Express only calculates one side of the wheel per spreadsheet, so for wheels that aren’t symmetrical (usually rear wheels) you need to use 2 spreadsheets.
This is what Spocalc Express looks like with the measurements entered:
Remember this is for only one side of the wheel, and the other side could be different if the wheel is not symmetrical. As you will read elsewhere, rear wheels (especially with derailleur gears) tend to have shorter spokes on the sprocket side of the wheel, so that the rim can stay in the middle of the wheel, despite the sprockets taking up a lot of width on the right hand side of the hub.
This is called “dishing” and makes the wheel weaker, as there is less bracing by the spokes on the flattened side of the wheel. Some hubs are better designed in this regard, e.g. Bafang rear hub motors require very little dishing, while Cute rear hub motors require a lot.

Get suitable spokes

Once the spoke lengths are calculated, you will need to get 18 spokes of the correct length for each side, or 36 spokes the same if you’re lucky.
Getting the spokes is easy if you can get them from your Chinese hub motor supplier. These are generally thick 2.3mm (13 gauge) spokes, which are strong and fit the hub motor holes well. I’ve often found them to be significantly different in length from the calculated length (usually a bit short), which can result in showing spoke threads below the nipples. Again this usually works out, but it’s important to check.
If you don’t get your spokes from your motor supplier, you need to buy them the right length elsewhere, or cut and thread them yourself. Both of these options tend toward using 2mm (14 gauge) spokes, as they are more common and cutting and threading them yourself is easiest with 2mm. Because hub motors have such a big flange diameter, the spokes are an unusual length, so you may need to look around. Lots of hub motor wheels have spoke lengths which are unavailable retail. Sometimes you can increase your options by using a different cross number.
If you use 2mm spokes (the most common size) in a hub motor, it is often good to use washers under the spoke head (not the nipple). Chinese hub motors are made for 2.3mm spokes and the spoke holes in the flanges are large diameter (~3.2mm). There is little risk of the spoke head pulling through, but the elbows of the thinner 2mm spokes may not sit snug against the flange, which can allow flexing at the elbow and increase the risk of breakage. Washers can help pull the elbows in so the wire bears on the flange around the elbow bend.

Cutting and threading your own spokes

I cut and thread a lot of spokes myself. I even reuse spokes in good condition from old wheels (heresy!), and also have lots of new spokes which are not the right length for any particular wheel. This makes it quick to make a new wheel without having to find the right length spokes online (often costing a lot) and waiting for them to arrive.
The key to making your own spokes is having a spoke threading tool. I use and recommend a Hozan spoke threader, available from Ceeway, at this link: http://www.framebuilding.com/Hozan%20Tools%202.htm
This rolls a thread onto the end of the spoke, squashing the thread into the spoke without removing any metal. It’s a lovely tool.
Some tips on making spokes:
  • I recommend keeping coloured spokes (especially black anodised) away from threaders as they can damage the rollers.
  • Non-stainless galvanised spokes are fine, especially if the bike will be kept in dry conditions (I’ve used plenty of gal spokes).
  • The Hozan threader only threads 2mm (14g) spokes.
  • Some fancy spokes have varying thickness along their length. These are called “butted” and they help to shave grams off racing bikes. They aren’t suitable for ebike wheels. Only plain gauge spokes (the same thickness all the way along) are good for cutting and threading.
Check out this video of how I do it:

Nipples

You need nipples to go with your spokes, of course. Most spokes are sold with nipples included, but often enough you need to buy them separately, so check.
Get brass (usually chromed) nipples, they are stronger and less prone to corrosion than aluminium.
If you have a choice, it’s good to get longer nipples. This gives you more leeway in case you make a mistake in spoke length and they are stronger.

Get other bits and pieces

Spoke key

Also called a “spoke wrench” in USA. This is your most important wheelbuilding tool (everything else can be improvised – even this can be home-made). I recommend getting a one-size spoke key which fits the nipples you are using. Multi-size spoke keys are common, and worth having in the toolbox for truing the odd wheel, but when you’re wheelbuilding it will waste a lot of time finding the right sized slot each time you pick it up, and you will risk damaging nipples by using the wrong size.

Truing stand

A truing stand is good. I use the cheapest Park truing stand, because my brother-in-law gave it to me, and it’s fine. I don’t miss having the fancy features such as dial indicators etc..
It would be easy to fabricate your own truing stand – make it adjustable for different width hubs. People also successfully build the odd wheel using a pair of forks or a frame as the stand.

Grease

I believe it’s important to lubricate the spoke threads where they go into the nipple. This helps reduce friction while you build the wheel, but most importantly it reduces the risk of corrosion over time. Many old wheels become hard to true because the nipples have corroded onto the spokes. I use waterproof marine grease.

Brass washers

As described above, it’s good to have brass washers if you’re using 2mm spokes in a Chinese hub motor with 3.2mm holes. I had a lot of trouble getting them, until my brother in law sent me a lifetime supply from Germany in an envelope. Use Google.

Nipple stick

Double wall rims, especially deeper ones, make it hard to get the nipple onto the end of the spoke when first lacing the wheel. It’s easy to make a little tool from an old spoke, a nipple and a stick, which can help you a lot. It is used to place the nipple through the rim onto the spoke end and give it a few turns to start the thread.

Get an odd spoke of the same diameter, cut it short and grind, file or forge a taper on the non-threaded end. Find a nice stick in the garden, drill a tight hole in the end, and push in the tapered spoke end. If the stick splits, try again. You can always drill oversize and glue the spoke in with epoxy.

Rim tape

I use 19mm blue Schwalbe rim tape. It’s strong, has good glue, and is cost-effective in rolls of 50m. I get it here: http://www.pushys.com.au/schwalbe-rim-tape-19mm-x-50m-adhesive.html
If you’re just doing one wheel, you might need to find a single wheel roll.
I’ve used gaffer tape (fibre reinforced tape) when I haven’t had anything better. I used a woodworking marking gauge to mark a parallel strip on the tape roll, then used a sharp knife to cut through a few layers before unrolling. It worked okay but I’m glad I have the Schwalbe tape now.

Dish stick

Sheldon Brown says you need a dish stick, but I’ve never used (or seen) one.

Lace it all together

This is the fun bit. Get all the stuff together, smear a little grease on the thread of each spoke, put your nipples in a small dish nearby. Wear clothes you don’t mind getting grease on (that blue waterproof grease is impossible to wash off). Get the Sheldon Brown wheelbuilding page open on your computer. 
Watch my video:
Some tips:

  • It’s easy at first to make a mistake in your lacing. Be ready to undo all or part of the lacing – it’s a learning experience!
  • Stop and look at the whole wheel at the end of each stage. Your eye can usually spot mistakes from the irregularity in the pattern.

Front disc brake

If you’re using a Bafang SWXK front hub motor and plan to use disc brakes, you have some challenges. There is very limited width between the left flange and the brake disc. I have successfully used disc brakes with these motors with special lacing and calipers which are thin on the inside (from Greenbikekit.com or BMSBattery.com).
Here is how I have laced SWXK motors to provide more room for a disc brake caliper:
Bafang SXXK front hub motor, laced for a disc brake
All spokes on the left side are on the inside of the flange (inserted from the outside), so the caliper doesn't run into the spokes. 
Bafang now make the SWXK5 front hub motor, which has more room between the brake disc and the left flange.

This is the Bafang SWXK5, which gives some more room between the brake disc and the left flange

True the wheel

Once laced, put your wheel in the truing stand. 
Bafang BPM being laced 1-cross to a Sun Ringle Rhyno Lite rim
Here it is easier to do the first, rough tensioning of the spokes, firstly with a screwdriver in the ends of the nipples. The more evenly you can do this first step, the easier it will be to get the wheel true.
Most bikes centre the rim in the middle of the axle – to be more specific: halfway between the outside locknuts on the axle. This makes it easy: when you take the wheel off the stand and put it back in swapped left to right, the rim should be the same distance from your pointer (the adjustable thing that touches (or not) the rim). At the start of truing, swap the wheel back and forth, aligning the valve hole with the pointer, so you’re centring a single point on the rim.
Here I’ll leave you to the experts like Sheldon, to help you true your wheel, then stress relieve it (I use a wooden spoon handle).
While you have the wheel in the stand, it’s easy to put the rim tape on. Put the hub in the stand so that you can freewheel the top of the wheel away from you (the hub gears can make it hard to turn the wheel backwards). Start a couple of inches before the valve hole, tape across the hole and right around the wheel, keeping the tape carefully central in the rim.
Cut the tape so that it overlaps couple of inches each way over the valve hole. That way the valve goes through both ends of the tape and prevents the tape from creeping.
After the tape is in place, carefully find the valve hole under the tape (don’t accidentally find a spoke hole!). Take a small, sharp knife and cut the tape out of the hole. If your knife takes a shaving of aluminium from the edge of the hole, that’s good – makes the edge of the hole less sharp.
Now you have a hub motor wheel! And you have laced a wheel – bike wheels won’t ever look the same to you again.

Tube and tyre

All you need now is a tube and tyre. My strong recommendation is for Schwalbe Marathon Plus tyres and Schwalbe tubes. Both have shown exceptional reliability and durability – and if there’s one problem you don’t want, it’s flat tyres.
The back Schwalbe Marathon Plus tyre on my folding bike (with front motor) has gone close to 10,000km with only one puncture through the tyre. The back tyre on my (rear motor, often heavily loaded) cargo bike has gone nearly 6000km, has never had a flat and is starting to show blue rubber through the tread.
I currently get my tyres from Chainreactioncycles.com



4 comments:

  1. I want to lace a electric hub motor to a rear fat bike rim, 26 inch...36 spoke..single cross pattern....can you tell me which video of yours will guide me properly ?

    ReplyDelete
  2. Thanks for sharing the information. That’s a awesome article you posted. I found the post very useful as well as interesting. I will come back to read some more.
    carbon track wheel

    ReplyDelete
  3. Excelent blog I will start working once my hub motor arrives and keep you posted. Thank you from the other side of the world (Argentina).
    Best
    Federico

    ReplyDelete
  4. MANY THANKS for sharing all this information!!!
    I am also a happy client of BMSBattery but I find difficult to find locally a mechanic that can help me with spokes. They simple don't know how to cut spokes to specific needed size :-(
    I will do it myself and buy the tool if needed.

    Many thanks!!

    ReplyDelete