by Patrick M.
They say necessity is the Mother of Invention. That was certainly the case when I decided to build my own hard battery boxes, custom-fitted to the three different e-bikes I have at home in California. I’d been looking for a company that produces this kind of box – a nice-looking, sturdy hard battery case that is custom-sized for a variety of different frames (especially mine) and fits like a glove. Through all my searching, I could find no company that produces this kind of box – only a few skilled artisans that produce them by hand, to order, for a specific bike, and they were pricey. So I took a deep breath, got my heat gun ready, and decided jump with both feet into the world of being a battery box sculptor and artist.
Let’s start with the cons
Yes, I agree these boxes look fantastic. And they’re much stronger than a nylon soft bag. This may seem strange, but now that I’ve lived with the three custom boxes I’ve made, some cons have become apparent and I thought I should mention them before digging into the build process. These, I hope, will help you in deciding if taking on this project is for you:
1. Very time-consuming and frustrating to build: At least for me, this is true. Each box I built took 2 full working days, which were filled with more swearing, frustration, high blood pressure moments, and fist-clenching that I’ve experienced in a very long time. Building these boxes is much more akin to being a sculptor and artist than a tech-geek guy – and I fall much more into the geek realm than the artist realm, so for me it was difficult. If you’re a natural-born sculptor, this may be much easier for you.
2. Convenience #1 you lose, compared to using the Luna soft bag: I realized that while the boxes I made look awesome, they do make you give up some conveniences that are taken for granted when using the Luna soft triangle battery bags. The first one of these is losing the side pockets for carrying stuff. I never realized how much I relied on these to carry my keys and cell phone while hard trail riding. Now that I’ve installed the hard boxes on all my bikes, I have to carry these things in my pockets or backpack; it was much more comfortable and convenient to keep them in the Luna bag’s side pockets.
3. Convenience #2 you lose: being able to swap out batteries quickly. This may not apply to most people, but I often found myself changing out packs in my Luna triangle bags. For example, if riding a 72v bike for an extended time, when the 72v pack was dead I could just easily unzip the bag and swap in a 52v pack, and keep riding. Not the case when you’re using one of these hard boxes.
4. Convenience #3 you lose: “Waterproof-ness”, for lack of a better word. Keeping your pack dry is a real, very important, thing – and the Luna soft triangle bag does a surprisingly good job of keeping water off of your pack. While I’ve never ridden my bikes in hard Seattle rain, I do hose them off after riding in dusty, sandy SoCal. And even though I’m careful, some water has sprayed onto the battery area. With the soft bag, if it’s zippered shut you get surprisingly good water protection. With the hard box, you have to do a very, very careful job siliconing all potential areas of water ingress to make sure moisture won’t be an issue – and even after doing that, I still have a gut feeling the soft bags gave me better protection from water. If you live in a wet area and often commute in the rain, I don’t know if this type of case is for you, unless you really go to great lengths to waterproof it.
5. Mounting – you need to really think seriously about mounting the box, how you will do it and where the attachment points will be, BEFORE you start bending the Lexan to shape. If your bike only has 2 water bottle holes on the downtube, you’re gonna need additional mounting to secure the pack – be it zip ties, black metal worm gear clamps, or billet aluminum clamps. And you’ll have to plan for the space those items will use, both outside the pack (between the box and inside of your frame tubes) and inside (allowing space for them between the pack itself and the box).
6. ADDENDUM – I just remembered another con while trail riding, the day after this was first published. These hard boxes definitely lessen the “stealth factor” of riding your e-bike – they make it look a little bit motorcycle-ish. While riding the Canyon DUDE pictured here at my favorite home trail, I crossed paths with a group of three spandex MTB riders, at the top of a big summit. Early morning, beautiful weather, nobody else around. Two of them smiled and said “morning”, to which I also smiled and replied the same. As they were riding away, the third yelled “nice motorcycle”, followed by some chortling and cackling from the group. I really wanted to reply with “nice spandex”, but alas, they were too far away. I am in complete agreement with Karl Gesslein of electricbike-blog.com, when it comes to having a preference for ‘stealthy’ riding. And if you’re using an under-seat Mighty Mini pack (the most stealthy), a Shark Pack, a backpack battery, or even a soft triangle pack — all of those are stealthier than these hard cases. So if you’re riding in an area where it’s best that you look un-powered, this might not be for you.
Let the swearing – I mean building – begin!
With those caveats out of the way, here goes the build process. These aren’t hard & fast rules, they’re the techniques that I found worked the best for me. You might find that another technique works better for you, if so please feel free to share it down in the comments. I will include links for all the items mentioned here, at the end of this article.
Let’s start with the list of materials and parts I used. In short, these boxes are fabricated from .093 Lexan sheet material, bent to shape using a heat gun. They’re held together with 1/2″ aluminum L channel, along with M5 bolts and special Lexan glue. The covering is automotive carbon fiber wrap with a self-adhesive backing.
I’ve experimented with several types and thicknesses of Lexan. Here’s the short summary: the .093 Lexan sheets that Home Depot sells are just about perfect for box building. That thickness is the minimum I’d use for making the box. Once assembled, with the brackets bolted in place, this thickness material is plenty strong. I wouldn’t go thinner, but if you have the space (meaning at least 12mm free space on all sides between your pack and the inside of the box), you can definitely go thicker – .125 material is also a great choice and will be stronger than .093. For the boxes shown here, I used the 4×3′ sheets of .093 material from Home Depot linked above.
3’x4′ Home Depot .093 Lexan is plenty for one box (two, actually)
I found that for all three of my bikes, 4′ was just barely long enough for the one single long piece used for the main center part of the box. Besides Home Depot Lexan, I also purchased some very cool-looking black .125 Lexan sheet material from a California company called ePlastics. I’ve gotten several different sheet materials from them for battery building; they have an amazing selection of stuff available. I really wanted to use the .125 black Lexan I purchased there for both my Canyon Strive and DUDE box builds, however, in both cases the batteries were fitted inside so tightly, there just wasn’t room to use the thicker material.
ePlastics sells this cool black Lexan
For brackets, I used the 1/2″ and 3/4″ Aluminum L Channel from Home Depot. I wanted to keep the bracket width as small as possible, and keep the bolt locations as close to the outside of the box as possible, so neither was impinging on space needed for the pack itself – or worse yet, sticking out into the pack. If your boxes are very tight like two of mine were, I’d choose the 1/2″ L channel. If you have plenty of space, the 3/4″ .050 channel will give you more strength and more thickness ‘bite’ for the side that will get tapped & threaded later. To hold it together, I got these slick-looking 8mm Black M5 bolts and Black M5 locknuts from Amazon. I’d highly suggest getting them in various lengths – 6mm, 8mm, and 10mm should give you the lengths you’ll need for all circumstances. You want to keep the bolts absolutely as short as possible so they don’t poke into your battery pack inside. For my cases, I found that 8mm bolts were ideal – they went through the thickness of the Lexan and the aluminum channel, ending up flush with the locknut perfectly – no bolt end sticking out. I found glue was needed for strengthening the main center housing’s butt joint – and learned that almost no regular glues stick to Lexan. I’ll save you the testing time I wasted – just get the Special SCIGRIP 16 Lexan Glue from ePlastics or Amazon, and save yourself some of my swearing and frustration!
Now, onto covering and mounting. I learned there are two options for self-adhesive carbon fiber-look automotive wrap: 3M (expensive and the best), and VVIVID (no, not an adult film company – much cheaper CF-look wrap). Originally I’d planned to use the 3M stuff. Then I calculated that I needed way more material than I’d first thought, did a cost comparison between 3M and the cheaper brand, and decided on VVIVID! I have to say, I’m totally satisfied with my purchase so far – it went on easily and I was able to get any bubbles out easily. The VVIVID material also went around bends easily and could be heated a bit for smooth radius corners on the box side panels. 3’x5′ should be more than enough for one box.
VVIVID carbon fiber wrap – much cheaper than 3M, works well
There are several options for mounting your box. First, of course, you’ll want to use all the water bottle screw holes you have available. But in many cases, those won’t be enough to hold the box and its 8-12lb. battery pack securely in place. There’s always the option of tapping additional holes for more M5 bolt locations, but I am very reluctant to put more holes in my bikes’ frames – especially as my type of riding is mostly hard mountain, MX-style trail riding. You can also use rivnuts to secure the box in more locations, but again, I preferred not to put any more holes in my frames. That leaves the ubiquitous nylon zip ties as an easy option, or, for something stronger and cooler-looking, there are two other possibilities. First are these Stealth black metal worm-gear clamps from Sure Motorsports. They’re available in a variety of sizes, and look much better than using the usual silver clamps from Home Depot. Just be sure to a thin strip of rubber between clamp and your frame, to keep it nice and scratch-free. Here’s the most important thing I found about using these clamps: make sure you have plenty of room inside the box – between box and your battery pack – for the gear & screw part of the clamp. They take up a good 1/2″ or so of vertical space within the box, and you’ll need to make sure you have this space free in order to use these clamps. I got a few of the Sure stealth clamps, only to be disappointed and realize that for two of my (really tight) boxes, I didn’t have space inside to fit the screw part of the clamp.
Sure Motorsports Stealth clamps
But perhaps the ultimate cool-looking mounting solution is a CNC’d billet aluminum clamp. Especially when anodized black, these clamps can give your box that perfect Terminator-like finishing touch. I spent many hours researching them, and here’s what I found. Pacific Customs has the best selection I could find of Billet Aluminum Clamps, in all different sizes. These actually come from the dune-buggy world – they’re mainly used for holding accessories on the tubular welded frames of dune buggies and sand rails. I got several sizes of their black clamps for my builds, including this 1.5″ clamp. During my research, I also found very cool-looking clamps by Power Tank on Amazon. When I got them, I realized they wouldn’t work for my build. The photos are misleading – it appears the lower part of the clamp (that goes between your box and the inside edge of the frame tube) is very thin – which I needed for my tight-fitting boxes. But when they arrived, I saw that section was a good (and beefy) 1/2″ thick – way too big to fit in the space between my frame and the box. That brings up the next important – very important – point: if you want to use these cool-looking billet clamps, I highly recommend you buy them BEFORE starting your box build. That way, you can determine exactly how thick the bottom part is, and allow for that when sizing your box to fit within the frame triangle. You’ll need to allow space between the box and the frame triangle that’s equal to the bottom thickness of these clamps, on all sides that you’ll be using them.
Billet aluminum clamps look great – but you need to allow enough space between frame and box to use them
Cardboard and Lexan and Heat
Now onto the real ‘how-to’ brass tacks. I think it might me easiest to do this in numbered steps, so here goes:
1. Make a template of your frame opening. I used a big sheet of cardboard – holding it carefully in place, I drew the outline of my frame opening with a Sharpie. Then, cut along this line (maybe a couple mm larger on all sides) to make a rough outline of your frame opening. Test-fit the cardboard piece within your frame and see how it fits, making adjustments and removing material where necessary. IMPORTANT things to keep in mind: remove all battery bottle screws before measuring. Allow space for the thickness of the inside part of billet aluminum clamps, if you’re using them (as explained above). Also look for any cables and cable holders that might interfere with mounting the box. For example, the cable holders along the top of your downtube – you may need to relocate the cables off to the side. My carbon fiber Canyon fat bike frame had cable holders (and cables) that interfered with the bottom of my battery box; I had to carefully grind off the holders. After you’ve got the right size template, then transfer that shape with a Sharpie onto a flat surface that resists heat well – this is where you’ll do the forming for the center housing. I had an extra slab of quartz that worked perfectly for the job.
Cardboard template for the box – also helps in designing battery pack layout
2. Rip the Lexan sheet material to make the main center housing. I used a table saw to slowly and carefully rip my big sheet of Lexan into two 10cm, and one 11cm, wide strips for the three boxes I was making. About that width – you may want to go thinner than I did. If you build it too wide, the box will feel like you’re straddling a motorcycle gas tank. The reason I built fairly wide, 10-11cm thick, boxes is because most of the battery packs I build have the BMS mounted on the side, and that makes them a bit thicker than usual. That, plus I wanted to allow space for plenty of padding and shock insulation on both sides of the pack, and keep the battery well away from the necessary locknuts and bolts that are near the edges inside the box. Here’s what I found: 10cm gives you plenty of space inside for padding and to avoid the pack hitting the box’s nuts and bolts. It feels just a tad wide – it’s about 1cm fatter than my fully-filled Luna soft bag – but is manageable. My 11cm box is starting to feel like you’re straddling a motorcycle gas tank. If you have a ‘normal’ thinner battery pack (one that doesn’t have the BMS on the side), you may well be able to get away with 9cm wide for the main housing, and be glad that you have a thinner, more comfortable, resulting box.
10cm wide box is a little fatter than a Luna triangle bag, but doesn’t feel like a motorcycle gas tank
3. Using a heat gun, form the main center section of the box. First, you’ll need to measure the perimeter of the template you made, and cut your ripped 10cm wide piece to that length. Next, prepare the tools you’ll use to make your bends. I used a combination of 2×3 wood pieces, along with some various sized cast iron plumbing nipples, to make different radius curved bends. For tight bends, I made a type of clamp using two pieces of wood, held together with screws. The Lexan is held between the wood pieces, then heated carefully on both sides where the bend is needed. This YouTube video does a good job showing the process of heating and bending the Lexan. (He does it while the piece is vertical on a vice – however, I find it best to bend the Lexan strip while it stands vertically on its side, long-wise, on the flat surface – that way the entire finished box will be nice and straight side to side.) Here’s the main thing I learned about heating and bending Lexan: the ‘bending point’ (good) and the ‘bubbling point (bad) are only a few seconds’ difference with the heat gun. I have a fixed temperature heat gun that can easily bubble the material if you’re not careful – a variable-temp model might help in setting the perfect bending point temperature for the Lexan.
My very advanced Lexan Bending Toolkit
You need to be very, very careful when heating up both sides of the sheet where your bend is going to be. It’d be a good idea to practice bending a few scrap pieces first, before trying it on you main center piece. What I found is this – once the Lexan gets to its bending point, you’ll see it wiggle a little bit back and forth. That’s the time to turn off the heat gun, grab your 2nd piece of wood, and carefully make the bend while holding both the wood clamp and the 2nd wood piece. I found my wood clamps work well to make nice, tight radius bends (like for a 90 degree sharp bend). For bigger radii curves, the cast iron plumbing nipples work well. Heat up where the curve will be located, on both sides, then once it softens you can bend the piece around the curved pipe to get a nice larger radius curve. This is really the ‘art and sculpture’ part of the whole process – it just takes practice to get a feel for when the Lexan becomes bendable (and not ‘bubbly’), and practice making the curves. The more bends you do, the more comfortable you’ll get with the process. That said, even on my 3rd box, as carefully as I tried, I still overheated a couple bends and got some bubbles in the Lexan. I sanded the areas to smooth them before covering with the CF wrap, and they are pretty much invisible on the finished product.
Bend the main center strip’s curves on a flat heat-resistant surface – this slab of quartz worked perfectly
4. Test-fit the housing inside your frame triangle, and make any adjustments necessary. This is where I had a heartbreaker – on my 3rd box, I thought I’d made a perfect double tight curve at the very front of the box, to fit perfectly inside my Mongoose Tyax frame. Then when I went to test-fit it, I realized that the box was about 2mm too high at the very front! No mater how I tried to shove it in, or move it around, it was just barely too big to fit. I had to re-heat that entire front bend section and make it into one big (and slightly bubbly) curve.
Heartbreaker – I thought this box came out great, until I realized the front part was 2mm too tall for my frame
When you are test-fitting – keep a couple important things in mind. First, your space inside the box is going to end up quite a bit less than it looks at this point – because you must allow for the space needed for the L-channels as well as the nuts and bolts that’ll be on the inside of the box. In general, I’ve found you need at least 10mm all the way around the perimeter of your battery pack (between pack and box) to have enough space for the nuts/bolts, aluminum channel, and most importantly – dense foam padding around the pack. Next, again, remember to allow for your clamps and mounting brackets – if you’re using those billet clamps you’ll need adequate space between the box and your frame.
Test-fitting box on my Canyon Strive – L-brackets and nuts/bolts will take up valuable space inside the box
5. Secure the butt joint with a 2nd piece of Lexan, then template and cut the left and right side panels. Once you have the right shape for the center housing, you’ll need to connect the butt joint where the ends meet. I usually try to put this in the least visible position possible, at the back of the box. What I did was cut about a 3″ long strip of Lexan that is at least 1 inch narrower than the width of your box (if you are using 1/2″ aluminum channels). This is because I always put a piece of channel on the back of the box, and you want your Lexan strip to sit between the left and right channel pieces, so it has to be narrow enough to fit between them. To make the butt joint strong, use theSCIGRIP 16 Lexan Glue. Apply it to both pieces to be joined, press together tight, then clamp it while drying for at least 4-6 hours. I also use very strong bi-directional filament tape to hold the piece in place while drying.
Butt joint is held using a 2nd piece of Lexan, adhered with special SCIGRIP Lexan glue
Once the glue has dried, you can move onto templating and cutting the left and right side panels. Like with the frame, I drew the shape of the box using a Sharpie – but this time directly on the Lexan sheet. Be careful to mark the left and right side pieces, because if you’re a really bad amateur sculptor like me, you’re gonna have some real variation in size/shape between the two sides! Also, you may want to check your shape outline with the center housing mounted inside the bike’s frame – if it’s tight in there, it may flex or deflect a little bit which will affect the shape needed for the side panels. Once you’ve drawn the templates, just cut the shapes on your million-dollar waterjet or fiber laser cutter. Kidding! If you’re lucky enough to have those, who bother reading a lowly DIY article in the first place?!? But seriously, as most of us aren’t lucky enough to have a full machine shop at our disposal, you’ll probably have to do like I did and carefully cut the two side panel shapes using a jigsaw and a fine, (24 tooth or more) and preferably new, cutting blade. After you’ve cut them, test-fit on the center housing. If you’re like me, there will be lots and lots and lots of fine corrections in the panel shape that need to be made. I found that a belt sander works great for removing a millimeter or two of material at a time on the side panels. Test-fit, swear loudly, belt sand, test-fit again, swear again, belt sand again. Repeat another 20 times or so and you should be good to go. Then it’s time for the next side panel! Man, cutting these pieces with some sort of waterjet or automated machinery starts looking really desirable about this time…
Side panels cut, more test-fitting, more cursing
6. Begin locating, cutting, and mounting the aluminum L channel brackets. Usually by about this time in the build, I’ve had enough of battery box making for one day. My throat is sore from loud swearing, neighbors are tired of hearing me, and I’m just about ready for dinner and some nice mind-numbing TV. If you’re able to do this entire process in one day, you have more stamina than me! Anyway, whether you keep pushing through or take a rest at this point, the next part of the build process is cutting and fitting the aluminum brackets.
Aluminum brackets cut to size and test-fitted on my Mongoose Tyax box. Remove before wrapping
You can see from the photos the general location and size I’ve used for the brackets. You can make them smaller if you like, I chose to make them fairly long to give more structural rigidity to the boxes since I do a lot of off-roading. After cutting and locating them, I tape them in place and then drill the holes for all mounting bolts. It’s very important to mark each bracket for its location, and which edge is side and which is top/bottom – because after locating and drilling them, you’re gonna remove all of them before covering with the carbon fiber wrap. Once the brackets are in place, I held them securely with either clamps or a block of wood beneath, and drilled for my mounting screws. Once you’ve done one side, it’s easy to transfer lines to the other side of the box using a small combination square, so that all your bolt locations line up nicely. Here are two very important points to keep in mind while drilling: First, be careful about the mounting distance of your bolts to the outside edge of the box. For my first box, I wanted the bolts/nuts to be as far to the outside of the box as possible, so they wouldn’t interfere with the tight-fitting pack inside. So I drilled every M5 bolt hole only about 6mm on center from the outside edges of the box. I didn’t realize until I started assembling it… (mechanics, you can already imagine what I’m going to say next)… that on the inside of the box, the nuts were touching each other (hate when that happens) and couldn’t both be tightened fully. I didn’t take into account that by putting both side and top/bottom bolts so close to the edge, it left inadequate room inside the box for the two nuts where top and side bolts met on the channel. I had to do some quick thinking to fix this problem without re-making and re-drilling new side panels. Moral of the story: either allow enough space away from the edge for your bolts (I found about 10mm on center, from the box edge, works well) – or have the side panel bolts and the top/bottom bolts slightly offset so their nuts don’t touch each other inside the box.
Determine which side will be your removable panel, and use a tap to make threaded top/bottom/back holes
Next important point before drilling: you’ll need to determine which side will be the fixed panel and which will be the removable panel. You need to decide this now, because the hole drilling is different for either side. For me, the logical choice was to have my fixed panel on the right (drive side) of the bike, and the removable panel on the left. Now, thinking ahead, you need to figure out just exactly how this panel’s gonna be removable, and how you are going to mount it. I realized early on that the aluminum channel on the left side would have to be mounted first with nuts & bolts to the side panel, and the holes on the outside part (top/bottom/back) of the channel were going to have to be tapped so they have threads for the M5 bolts to screw into. It must be assembled this way because if you mounted the channel first to the battery box, in most cases it will block you from having enough space to actually put the battery pack into the box once the box is mounted in the frame. The channel needs to be affixed to the side panel so you have space to slide the battery (and its padding) in first. So for the left panel, I first drill the holes for the side panel bolts. Then – after the carbon fiber wrap is applied everywhere (kind of as a last step) – mount the left panel into the box, and tap the holes for the threaded holes. I realized this must be done as a last step, because originally I did it before applying the CF wrap – then when it came time to put everything together my tapped holes never lined up with the holes in the center housing. I realized the thickness of 2 layers of carbon fiber wrap was causing the holes to get misaligned. So just remember – don’t drill at this time where your tapped holes will be. Save that for almost the final step.
Drilling holes for the M5 bolts – then brackets come off, and carbon fiber wrap goes on
At this time, you’ll also want to drill holes or slots for all the mounting hardware that you will be using. It’s much easier (and cleaner) to drill for your mounts now, than to think of it when when the box is wrapped and almost finished. Mark the hole locations for your water bottle screws, as well for any other mounts you are using: zip ties, metal hose clamps, or billet aluminum clamps. Also, determine where your battery and/or charge leads will exit the box and drill those holes as well. In the case of all three of my boxes, the packs within them used a BMS that has one pad for the charging and output connection. What this means is that for your output (XT90) connection, that can be used for both discharge and charging. To have a cleaner look and only one cable exiting the box, I chose to run only the larger discharge cable out of the battery case – so I charge the packs inside each box using the XT90 discharge connector. Note that this is ONLY safe to do if you know 100% for sure that your pack’s BMS uses the same solder pads for charge and discharge (P- and C- on the BMS). Many of them do not, so for them you can’t charge via the discharge port. Also, for charging, these boxes would be an ideal candidate for the magnetic Rosenberger connectors sold by Luna Cycle. I haven’t tried one yet, but I’m sure one would look and work great on these cool-looking battery boxes.
M5 bolts and fender washers secure the box at water bottle mount locations
7. Next, it’s time to do some faux carbon fiber wrapping. Remove the aluminum brackets from your box and side panels (again make sure you’ve marked them clearly). Clean everything very, very thoroughly – even one little dimple on the surface will show through on the carbon fiber wrap. If there’s any tape residue, you can use Goof Off to take it off the Lexan – even that will be visible through the vinyl. Once everything is clean, it’s time to wrap. I start with the hardest part, the main center box. Cut a long strip of wrap about 1″ wider on each side, and long enough for the perimeter of your box. Then pull it tightly and slowly work your way around the box, looking for air bubbles as you go and removing them as much as possible with a plastic squeegee. This, again, is more art form than tech geek stuff, and I found it took a lot of practice (and stress) to get a good result. After you’ve pulled it tight lengthwise and cut to length, then you can pull the sides tight and wrap them around the inside of the box. You’ll probably find that at curves, the material bunches up – you can solve this by cutting slits in the material where it goes inside the box. The slits run “side to side” if you’re looking at the box from the top. The same technique works for the side panels – when you have to wrap the material around the curve of an outside corner, cut slits in it where the material wraps around to the inside, allowing you to make a ‘fan pattern’ and bend the wrap around the curves. Also, for those outside curves on the side panels, I found that carefully heating them up a bit allowed me to then press the material into the curve, making a smoother radius without ridges. It’s a hard process to explain in writing, probably a video showing the process would be much better.
Carbon fiber-look vinyl wrap, carefully applied to center housing and side panels
8. Once you’ve got the carbon fiber vinyl applied successfully, you’re in the home stretch. Only a couple more steps needed and it’s all done. The next step is to re-mount all your aluminum L-channels, this time screwing everything together tightly. Even though it uses locknuts, for an extra measure of protection I also applied a drop of blue Loctite to each bolt in the box. First bolt together the fixed side of the box, then attach the brackets to the left (opening) side panel. Test-fit the opening panel into the box and make sure everything fits OK (it should be snug). Once that’s done, you can finally drill and tap for the threaded holes on the removable left panel. One thing to keep in mind – look at your bike frame and what’s near it before locating these. You will be screwing in these bolts at the very end, and I learned from experience that unexpected things can get in the way – like fat bike tires or a frame-mounted controller. So be sure that where you locate these bolts, you’ll be able to actually get to them once the box is mounted in place on the frame.
Box for my Canyon DUDE fatbike. 100 cell, 72v battery pack is a very tight fit
To tap the holes, I used an M5 tap along with thread cutting fluid. I mounted the left panel to the box and held it in place with tape. (Very important the panel does not move around a bit while you are drilling the holes – if so, your holes won’t line up when you want to assemble it.) Then I drilled a pilot hole that’s about 1mm smaller than the diameter of the M5 tap. After that, I slowly and carefully tapped each hole, with the power drill on slow speed, using the tap cutting fluid for each hole. After your holes are tapped, it’s almost time to mount the box. But before doing that, it’s a good idea to apply silicone sealant for waterproofing around the inside perimeter of the fixed side of your box. For the opening side, you’ll have to put a thin bead of silicone around the perimeter as a last step before finally screwing the side panel on (after the battery is installed). I found that this Black Silicone from Home Depot works well for this application, and sticks well to the Lexan. Also, it’s a good time to mount any shock-absorbing/padding material to this side panels as well – easier to get to it now than when the box is mounted in your frame. I used this dense black sheet stuff from Amazon for padding the side panels.
Box for my Canyon Strive, ready to be mounted in the frame
9. Finally, time to get that sucker in the frame! With the opening (left) side off the box, mount it inside your frame triangle and attach all needed mounting bolts and clamps. It’s also a good idea at this point to use the black silicone to plug any small openings you may have around holes for zip ties or other mounting hardware. Next, test-fit your pack and determine where it will be touching the inside of the box, and where the most stress is being put on those mounting points. Then plan your dense foam protection for the pack – of course, it’s most important to have adequate padding on the bottom and back side of the pack, where its weight would be contacting the frame/battery box. As I said before, ideally you’ll have at least 10mm all the way around the pack to allow plenty of foam padding to be stuffed between the pack and box, to protect it from vibration. But things don’t always turn out so lucky, and in some cases – like my Canyon DUDE fatbike – you have a very tight-fitting back (by design), with only a few mm of space around it for padding. Of course, this isn’t an ideal situation, but do the best you can and try to put as much padding protection between pack and box – especially where the pack’s weight is placed on the box.
Mongoose Tyax box was the ideal situation – huge space, relatively small pack, lots of room for padding
(note extra space above box – for billet aluminum clamps)
After the box is mounted, and the battery pack and foam are installed, it’s time to put on your opening (left) side panel. (Note in the photo above, for my Mongoose the opening side panel is on the right – that’s a mistake! It was supposed to be on the left, but I drilled regular holes for the left side panel before realizing I should’ve tapped them instead.) First put on a very thin bead of the black silicone all the way around the perimeter of the box. Then press the opening side panel on, and while pressing it, thread in your M5 bolts on the top, bottom, and back. If everything works out well and the battery box gods are with you this day, all your holes will line up and the bolts will thread in easily.
Tight battery fit in my Canyon Strive box doesn’t allow for much padding around the pack
If not (and I experienced this a lot), you’ll need to finesse how you push the side panel on and jockey it around a bit while trying to screw in the final bolts. The worst case scenario is that the holes can’t be made to line up, in which case you’ll need to re-drill or enlarge the problem holes on the main case frame, hopefully by only 1-2mm so the bolt heads will still cover the holes sufficiently.
Finished box on the Canyon DUDE
Once you’ve gotten all those last bolts screwed in, there’s only a couple things left to do. First, be sure to silicone around the hole where your power and/or charging cable exits the box. Also, double-check for any small holes or openings where water could potentially get in – look all around the perimeter on both sides and make sure any tiny gap between side panel and center housing are sealed up with black silicone.
Well, if you’ve made it this far (without losing your voice, hair, or sanity), congratulations! For me, each box was a very time-consuming and stressful project. I shudder to think what I would have to charge for one of these if my labor was paid by the hour. But as I mentioned, I’m definitely not a sculptor or artist. If your talents lean more toward that, you may find the process much easier than I did – maybe even therapeutic. There is definitely something rewarding about building something, fabricating some part or working thing that will be around a long time – it’s immensely rewarding.
Carbon fiber-look wrap and black hex bolts give the box a decidedly ‘mean’ look
I Hope you found this DIY useful. If you use my techniques to build your own box, be sure to write about it in the comments below, and post some pictures in the Fast Electric Bike facebook group.
Links to products mentioned in the article:
Home Depot Lexan Sheet
ePlastics Lexan Sheet
Home Depot L channel
Amazon black M5 nuts & bolts
SCIGRIP 16 Lexan Glue:
Billet Aluminum Clamps
About the Author:
Patrick M. is a A former coupon-book entrepreneur and travel agency owner, and Patrick developed one of the first consumer-review web sites way back in 1996. In the early 2000s, he worked as a journalist, serving as contributing editor for two home theater magazines. Now, Patrick splits his time between producing documentary movies and renovating homes – but his true passion has always been anything with wheels and ultra-high performance.
Written by Patrick M, October 2017