Dealing with drop outs Yuba Mundo build

As is common after riding for a while I just had to do an upgrade. I started out with a early 1990's Giant MTB and added a 600 W rear BMC v2 with a 35A controller and a 36v 20 AH Ping pack. I loved it and put about 500 miles on it. I pulled 230 pounds of me plus 40 pounds of bike and another 26 pounds of battery and hub motor and 100 pounds of kids and 50 pounds of trailer plus groceries some times... That's 450 pounds . often up 5% grades (pedaling heavy in granny gear of course).

I'm still learning a bit about C rates and all that stuff. I don't want to hammer the battery or let the smoke out. So I set the CA to allow maximum of 24 amps, Typical cruising on flats with trailer and kids etc was around 7-12 amps. On hills Watts would max around 1100. I had the BMS kick in once when I pulled too many amps going up a hill before I regulated with the CA but aside from that no real issues. Performs like a champ.


So back to my point. I got really tired of lugging around the (poorly designed)trailer (and 100 pounds of kids really is getting to be a little too heavy for a crappy entry level trailer) so I bought a Mundo that I'm going to transfer all the ebike bits to. I have just a couple of questions.

The major one I have is an issue with the Mundo dropouts. The rear drop out is a horizontal 15 mm dropout... the BMC is 14 mm axel. I don't think I have to explain the problem with that any further. I do have a torque arm that I will be installing (do I need a second torque arm for the other side? Should I be looking at finding someone to braze something on to shrink the drop outs? I've ridden the bike sans power and the ride is uber smooth and I'm really looking forward to electrifying.

So hope someone here will have an idea on how to deal with the dropout issue so I can get riding in e-assist style again soon.

I'll post some pics when I get the rear deck built. Hopefully I'll get that done tonight. I wanna be on the road this weekend (knock on wood).

Kai here has a Mundo. Perhaps he can help?

Pics will help. Maybe you can carefully enlarge the dropouts. .5mm on each side is not frightening to me.

Edit = oops, misread.

I tried installing the torque arm last night and it's just not going to work at least not on the drive side. When the torque arm is inside the frame It pushes the wheel too far to the non drive side that it rubs the frame. The rear deraileur gets in the way with the torque arm on the outside of the frame. I could try the torque arm on the non-drive side but then I'd have to source a new five-pin female connector because I'll have to cut the phase? wire. I'm wondering if the keyed washers with the little lips will be enough to avoid a spinout? Maybe Justin's torque arms would be a better fit than the one I have?? I'll try and snap some pics tonight.

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the standard motor is 10mm on the flats, so at 15mm, yeah, you won't be able to secure the motor to the frame. And since the BMC make a substantial amount of torque, Yes, you need torque arms on both sides. And good torque arms, since instead of supporting the dropout, they will have to carry the full torque load of the motor.

Shim it? Along with the dual arms of course..

Because you can't fit torque arms, the only solution is to have the drop outs modified, not shimmed. Needs to be better than close enough, if you know what i mean.

chet said:

Because you can't fit torque arms, the only solution is to have the drop outs modified, not shimmed. Needs to be better than close enough, if you know what i mean.

Click to expand...

It would be nice to have some input from experienced welders for this modification.
The drop-outs are likely forged and brazed to the stays. Filling the gap with bronze rod would be too soft for the axle.

In my admitted ignorance, I'd try stick welding a bead all the way around the inside of the slot and then break out the files.

FTR, I've been running my X 5304 rear (48V 48A) in the Xtracycle without torque arms. Frequent nut checks are my insurance. Usually two after having the wheel out and then the nuts don't get loose again so the axle is in no danger of spinning out. Were I to try getting some regen braking, I'd put on two torque arms in opposite directions.

The Yuba Mondo comes with a 15mm axle because it's designed and built to carry loads. It's also running 40 spokes back there.
The most sensible upgrade for an Xtracycle or Kona Ute intended for heavy hauling would be a rear wheel built for a touring tandem with a drag brake.

someone here in portland, who i have forgotten the name of now, brought their mondo with C'lyte 5304 to the ebike101 session of the OHPV electric assist tutorial.

but he had the 5304 on the front. that actually make more sense to me since you want the rear axle to be mega monster strong for carrying the airconditioner or concrete mixer back there.

traction should not be a problem on front since there is usually a lot of load just from the weight of the bike.

maybe the trick to fill in the space is to not use welded filler on the dropout, but on a small torque arm or torque washer like on the goldenmotor, but add the metal filler to the torque washer so that it would fill the space inside the dropout.

you can machine the extra metal built up on the side of the torque washer with a mill so it is a tight fit in the dropout, and the entire width of the dropout. then the torque washer flats will lock the axle.

then weld on an extension or arm to the torque washer to line it up with the frame to clamp it as a useful torque arm.

Have a couple steel shim pieces machined out for the hub, not the frame.. then braze them on to the axle.

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the perfect solution would be to have some plates cut from 3/16th" or 1/4" steel, then drilled for a 10mm axle dropout, then weld them over the old dropouts. You could even braze on the plates to the frame with one of those $40 map/Oxy torches they sell at walmart.

And if welding them on is too much destruction on your new frame, it should be possable to bolt them on

Drunkskunk said:

the perfect solution would be to have some plates cut from 3/16th" or 1/4" steel, then drilled for a 10mm axle dropout, then weld them over the old dropouts. You could even braze on the plates to the frame with one of those $40 map/Oxy torches they sell at walmart.

And if welding them on is too much destruction on your new frame, it should be possable to bolt them on

Click to expand...

And there you have the best advice.

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Drunkskunk as usual, has it exactly right. The best solution is a custom made torque plate, that will essentially become a substitute and superiour dropout.
Shims could be built into the plate, so it fits snugly into the existing dropout, and to remove the wheel to fix a flat, you would first unblolt the torque plate from the frame, then the wheel could be dropped out. There are other solutions, but that one would be the most bomb proof. Too bad more motors aren't built more like the heinzmanns, that have a really cool integrated torque arm and need no flats on the axle at all.

No, you don't have to weld, cut,paint. Here is what I did.
I had 2 torque plates, as I call them , laser cut from 6mm plate. I had them zinc plated to stop them going rusty.
They slide over the axle and react against the frame tube. Designed to be just the right size so that the weight from the bike keeps them against the frame.
However, I wish I had made them longer to give more leverage, not that they have ever moved from where they sit. 72v 48a 5305 big hills 2 kids.
I include a drawing and a .dxf file you can take straight to your local lazer cutter, (sheet metal workshop) of the new longer version.

Nice adapter plates Kiwi!

I'm a bit suspicious about that thinner axle.

My Yuba Mundo will get a front wheel motor from my bmx-bike. I'm dreaming about ping battery, but why should I buy it when I can carry a load of sla's?