It seems inevitable. When I build a bike, I go through front chainrings trying to get the gearing just to my liking. My Mongoose Envoy build has pretty much set the world record for tweaks in this regard. But gearing wasn’t the problem so much as chain alignment. Alignment is one of the most talked about issues with mid drives and up to this point I have not had to work too hard to get it right. This build, not so much but I think I finally got it (like $350 later).
While dealing with this I have fooled around with three different sets of crankarms (160’s, 170’s and 175’s). Not the subject here so if you notice the different crankarms in the pics, I am ignoring them on purpose.
Sidebar: When building the Surly Big Fat Dummy, I found exactly the same thing as I did here insofar as chain alignment is concerned. And used the same solution – the USAMade adapter listed as an Honorable Mention below got pulled out of the parts pile and put to use.
The Right Tool For The Job
The Mongoose build is a first for me in many ways. One thing in particular: the BBSHD fits the frame really well. Its a 68mm bottom bracket with absolutely zero chainstay obstruction for the secondary housing. So I can butt the motor right up against the bottom bracket. Further, its a lonnnng way back there so chain alignment and misalignment – an inevitable concern with an HD build – is a lot more forgiving since the angles are gentler thanks to the longer reach. On this bike, if I want I can even forego the offset non-drive side crankarm and the pedals are still easily centered under me. So the HD is a great fit here.
About That Job…
The Mongoose is a cargo bike. So it hauls heavy stuff (usually groceries). It has a secondary job as an unladen backup commuter, but primarily it needs to be optimized to start from a stop while the entire system – with me – weighs 400-450 lbs (180-204kg). I have really loaded it that heavily so this is not a theoretical exercise. So I want a big-ish chainring for when I am pedaling fast and light, and still need to be able to get to the big cogs in the back for when I am loaded up and chugging along like a two-wheeled freight train.
Plan A: Luna Eclipse (42T)
The Luna Eclipse is one of the best BBSHD chainring setups on the market, with a unique ‘wicked’ tooth profile meant to eliminate the possibility of a chain drop under extreme use. It also has the most extreme internal offset of any chainring option. This will do the most to overcome the grief visited upon the BBSHD builder by that drive’s secondary housing sending the chainring way out to right field.
Its also gorgeous. The gunmetal finish I chose matched beautifully with the dark grey frame. Unfortunately 42T (which is the standard for full-offset chainrings as any smaller and you can’t clear the secondary housing) was not large enough to keep me from clown-pedaling when riding the bike as a commuter. There was another problem: Chain alignment. Running that smaller 42T ring with the smallest rear cog resulted in, after only a few weeks, a whole lot of wear on the inside. This is why mid drive builds demand the most out of the builder in terms of thinking things thru. Time for Plan B.
The Eclipse is a proprietary chainring platform, but fortunately other sizes are available. the largest of which is what I tried next.
Plan B: Luna Eclipse (48T)
So Plan B was to swap in a Luna 48T ring onto the Eclipse center section to fix the clown pedaling, and to stay the hell off the 12T small rear cog to deal with the alignment issue (I am using a welded together steel cluster for durability and the 12T is alloy and not a part of the welded cluster, so its better to stay off it for the sake of longevity anyway). I thought that 48T/14T on this bike was the perfect sweet spot. A small front ring is best when its on cargo duty, and a large one is best when its a commuter. 48T, when used in conjunction with upshifts, gave me pretty much everything I needed.
Pretty much but not everything. First of all, remember the deep offset of the Luna ring? It moves the chain inboard 24.8mm which *usually* eliminates the damage the BBSHD does to chain alignment. Not on the Mongoose, whose narrow bottom bracket effectively papers over all of the sins committed by the motor (at this time I had not yet fully figured this out). So, as I found with the 42T ring, it was inset too far, even when I stayed off the smallest cog.
So Plan B helped, but it didn’t solve the problem. After only a couple weeks (I am now checking carefully and frequently) I saw the beginnings of the same wear on the inside of the chainring. Like the 42T, I had to retire this thing fast so I could use it on some future project.
Sidebar: A mid drive chain powered by a 1500w motor is a chain saw when it comes to components rubbing against it. That is just a reality of a mid drive and you have to deal with it as part of your design/build process. When you get it right, you are golden for thousands of happy miles. Get it wrong and you are sawing thru chainrings and cogs like nobody’s business.
Plan C: Lekkie Bling Ring (46T)
So now what? 42T was too small. 48T was more or less just right. And the chainring offset that lets me use the inner cogs at great alignment still needs to be reduced or I can’t use anything but the lower gears. Lekkie has a Bling Ring available in 46T. It has the same internal offset their 42T ring has and, since I use them on two other bikes I know they are top quality. At 18.3 mm its offset is quite a bit less than the Luna. So I got a 46T. I also added a 2mm spacer underneath it, further reducing the chainring offset to 16.3mm. That is a whopping 8.5mm less than before so I hoped I would be good on the smaller outer cogs and still let me use the big inners.
And, pretty much, it was. Chain alignment didn’t seem to be much of an issue, although it still wore down a bit more on the inside. I was also able to shift up to the biggest cogs in the rear for very low gearing options. Those are important on a full cargo load and if I am dealing with hills.
But… I flat out missed that 48T high gear for commuting. And I was still seeing – very slight but noticeable – wear on the inside of the chainring teeth from the chain, which was still visibly angling outboard a fair distance.
I decided to try an extreme option I had not previously considered. But on this bike, where all of the normal chainring offset stuff doesn’t seem necessary, it might actually work.
Plan D: Luna 130 BCD Adapter and Wolf Tooth 48T Ring
BBSHD chainrings are generally all proprietary to the platform. Not so in the cycling world, where chainrings are universal, needing only to match the proper Bolt Circle Diameter for the chainring bolts. Match the BCD between crankset and chainring and you are good to go. There are adapters out there in the world that allow a Bafang motor to use standard 104mm and 130mm BCD chainrings. The problem is they don’t give you anywhere near as much inward offset. But given my experience so far, maybe I can live with that. They should fix my alignment on my ‘commuter’ cogs, but will I still be able to use my ‘cargo’ cogs?
In addition to the LunaCycle 130 BCD adapter, I also chose the Wolf Tooth Drop Stop chainring as those rings are best-in-show for this sort of thing on a mid drive. Attachment to the adapter was a little different than the usual chainring-to-crank operation in that its backwards. The chainring bolts onto the inside. I was able to play some games to good effect: I reversed the chainring so it is logo-side-inward. Not as pretty, but doing that lets me take advantage of the countersunk bolt holes on what is normally the outboard side. The countersinking let me mount a bolt so it is almost flush with the ring, which in turn is butted up almost on top of the secondary motor housing. With the countersinking it now has plenty of clearance.
Plan D Results
FINALLY. Everything is working right. The reduced chainring offset means my 14T cog (still not using the 12T for the reliability issues mentioned above) lines up straight back. This outboard shift did affect my inner cog alignments but I can still get to all of them but the biggest 32T. I’m comfortable with the angles on all but the second-largest 30T for long term use, and in a pinch, that 30T will work fine. I just don’t want to stay on it for a week. So this 9-speed is now a 7-speed and as DIY mid drives go thats still better than a lot of builds can manage.
And worth mentioning, like a lot of what they do, the CNC-machined Luna adapter is freaking gorgeous, and very precisely manufactured. So much so it really stands head and shoulders above another adapter I got my hands on and was able to compare it directly to.
Honorable Mention: USAMade 130 BCD Adapter
I was surprised at how well this worked and how nicely it was made. The part only cost me $29.99 on Amazon. Still, it was Made in USA, well machined and rock solid. The only things I didn’t like about it was the fact it was machined a bit too heavily, which meant it placed the chainring a millimeter or two further outboard than was necessary, and in this game millimeters count. Further, as you can see above I was able to reverse the WolfTooth ring and take advantage of the bolt head countersinks. That didn’t work with this part as USAMade countersunk the outside edge of their part, which made the bolts too long to allow my trying the same trick on the inside, where I needed it. For a different build it might work fine so I am keeping it for my parts pile.
As for the Stone chainring seen on the USAMade adapter (scroll up to the title image at the top of the page), thats a Chinese Special that ran less than the godawfully expensive Wolf Tooth. Its noticeably lighter in construction than the WT and I’m not sure I am sold on the tooth profile. This ring will sit in my parts pile waiting in the wings as an emergency replacement.