Am I hosed? (Build question)

slm9s

100 mW
Joined
Jun 29, 2021
Messages
45
The bike is a Kona Dawg Supreme with a 20" frame. I have a BBSHD mocked up with a Luna 42t ring on it. With enough spacers on the drive side of the BBSHD to just BARELY clear the chainstay, the motor is shifted to the drive side enough that the motor bracket bolt "mount" is hitting the downtube before the middle of the motor does. This is in addition to the poor chainline this situation is giving me. The chain is inline with 2nd outer-most gear on the cassette. I'm new to this, is that going to give horrible performance? I see my options as being:
A. Build it up and ride it with a spacer between the motor and the frame so that bolt "mount" doesn't dent my frame.
B. I could grind just a little off the top corner of that bolt mount and the motor would hit the frame as it should
C. Get a different donor bike.
D. A smaller chain ring would let me move the motor back toward the non-drive side, but then my chainline would be even worse as the smaller ring wouldn't have the offset this one does.

Thoughts? Advice?
Thanks.
 

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If I were in your situation, I'd make a conformal spacer to go between the downtube and the motor body. Like some epoxy putty in a small zipper bag, squished between the parts while the resin sets. That's better than the point contact you get from a typical installation anyway.

Don't use a rubbery material (e.g. Sugru) for this. You want the motor to be completely immobilized. An elastic spacer material would allow a small amount of back and forth movement that could cause the lockrings to loosen.

Be aware that the BBSHD will sack and plunder your frame pivots much faster than normal pedal power. As the miles accumulate, your bike will kick its tail end to the right every time you apply power.
 
I just remembered I paid $4 for a spacer for the ring. If I used that instead of the BB spacers it would push the motor back towards the non drive side.
I was smart enough to order it, but not smart enough to remember that I ordered it. Must be getting old.
Hopefully that will fix the motor/frame interface, maybe I'll need a small spacer. The chainline might still be an issue tho.
 
Most people end up spending 95% of their time in the 2 outermost gears anyway, if that helps. The HD makes enough power you don't need to gear down too much unless you're on serious climbs.
 
The first time I built a BBS02-equipped bike with a similar chainline problem, I just had three gears 11-17-28 spaced 8-speed on the end of the cassette. Worked great for me since they were the only gears needed with the motor. AIR (it was six years ago), the 17 and 28 were the ones in use most of the time since the off road terrain here is hilly, so either ascending or coasting down.
 
slm9s, I think we've talked before, but can you clarify what you're going to use this bike for? If you're going to do any serious trail riding, I think you're going to want that 42 chainring with something on the cogset at least up to 34 or so. The BBSHD does indeed have a lot of torque and can take pulling a pretty high gear without burning out at the drop of a hat. But, for off road mountain biking there is a real benefit of matching motor output to gearing in a slower, technical terrain section where you don't really want a high gear pull. The motor works amazingly well in a decently low gear at low speed if programmed smoothly. If I'm picking my way through a line of problematic rocks, ruts, or obstacles, the low gear, low speed, and smooth power delivery really keeps your control way more manageable. I find if I've gotten lazy or distracted as I ride into a more challenging and slow section, if I'm up a gear or two too many, power delivery is sluggish to kick in, and then when it does kick in it may be a little aggressive when I don't want it to.

People are different and riding locations, conditions, and bikes are different. I don't think most folks on these forums are riding a mountain bike for actual trail use on a challenging trail. I'm not talking about world cup enduro/DH stuff...just stuff that makes you select good lines and work the bike to get over and around stuff on a trail. This is where the gearing and power delivery needs to be smooth and predicable. I have 11-50 on the rear with a 42T Luna Eclipse up front like yours. I will admit that I'm almost never in the 50T, but the cogs in the 30's and low 40's are of great benefit in the application like I mentioned. You could just get spacers and stack the necessary cogs in a more beneficial spot on the cassette body...but you may run into derailleur clearance/position issues there too.

If you're not doing any of the mountain biking I described, then disregard most all of what I mentioned here. I was just considering the kind of bike you have there and what your possible pursuit with it might be.
 
Thanks everyone. I actually went to the hardware store and ended using the spacer as well as two very thin washers on longer bolts. It moved the chainring just off the chainstay and I only had to use 1 thin spacer on the motor.

TNC, yes you have given me advice already, thank you again. I will indeed be using this for actual mt biking. I took my first foray this evening, but just a 10 minute singletrack ride. I robbed the small battery off my wife's bike, and its probably 20a max continuous but at least I know everything works.

A couple things, I like the 5" travel on both ends. I was wondering if a short travel XC bike would be better, but I could tell right away this was going to work well and even more would be welcome as long as the fork angle isn't too slack.

However, I'm worried this frame is too small. I'm 6'5 and its a 20" Kona Dawg Supreme. They made them in 22", but they are hard to find. In fact all big frame long travel bikes are hard to find.

The chain skipped gears on the cassette a couple times. I kind of expected this and have a new chain on hand, but not a cassette as I was thinking i would make a custom setup with only 4 or 5 gears as many people suggest.

My first bike was a hub drive (Specialized SX Trail - the BBSHD wouldn't fit this frame) and this mid drive seems to take a second or more to turn "off" when I stop the grip throttle. I'm sure it will just take a little getting used to.

There is definitely more torque. I've noticed more pedal/crank strikes on roots with these two 26" bikes (vs my 29er Stumpjumper) and in some places I'd just use a little throttle and keep the cranks level. Again, I'm sure I'll get used to it.

This bike is lighter than my hub drive Trail SX, but after just a 10 minute ride I'd say its tougher to pedal unassisted on level ground, does that sound right?

Thanks again everyone for your help, I'm really enjoying this new hobby!
 

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slm9s, when you mention that the motor doesn't turn off as quickly as you'd like, I'm guessing you haven't programmed the Bafang at this point?...perhaps? That overrun can be softened up to a large degree. For most mountain biking you want a little overrun power, but that varies a lot with preference. Regardless, that can tuned to better suit your riding needs.

On your pedal strikes, I don't usually like to make generalizations, but I think Bafang sourcing 170mm cranks is a bit silly for these BBSHD's. With the kind of torque this motor produces, they're just not needed. Now, for the usual recreational riding or commuting and such, it won't even be that noticeable if at all to most riders. However, any serious mountain biking will result in frequent pedal strikes in some gnarlier terrain. I always ran 165 cranks on my mountain bikes, and I've gone to 160's on my Nomad/BBSHD. Long travel MTB's make this matter worse as they go to the ground a bit more than an XC or hardtail MTB.

One issue on our Bafang MTB's is that you want straighter crank arms because of the Q-factor the motor introduces. Again as with the crank length, with pedal-assist and throttle the Q-Factor isn't as big an annoyance as our pedal-only bikes. Still, as the width grows, so does the frequency of pedal strikes when you're trying to squeeze between rocks and such.

I was able to get a set of Pro Max BMX crank arms at 160mm to replace those ridiculous, long, 170mm sticks on the BBSHD. I work at a shop and was able to buy two pairs with the intent of drilling out one of the left side pedal threads and installing a proper thread direction for a drive side application...like you do when you repair a stripped pedal thread on a customer's bike with an insert. However, after inspecting these Pro Max crank arms and doing some measurements, I literally just hacked off the chainring spider and went to town on the grinder.

Surprisingly this has worked perfectly, and I still have a spare set of cranks. I haven't even painted the ground down portion. This is a mountain bike, and I'm not that concerned about magazine picture finish. Heck...in this component market climate, I was only able to find red 160mm BMX Pro Max cranks and was only able to get a purple Luna Eclipse chainring at the time. Took what I could get...LOL!

These cranksets are available on ebay for around $60 and are way better than the cream cheese Bafang crank arms. Through the shop I works at, these crank arms were dirt cheap, hence why I picked up two.
 

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