Two wheel drive Beach bike

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Beach test today. After about four months parts acquisition and build time. 750 Bafang front drive with a Tongsheng mid drive. Hailong 52v battery. Hydraulic brakes. Fat bike frame from Alibaba with a specialized Fatboy fork. Grin torque arms on both sides with aluminum angle brackets to beef up the drop outs. Detailed parts list to follow.

 

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I was paranoid about breaking the fork ears, so I fabricated a couple of brackets to attach to the forks. I then drilled a second hole in the Grin torque arm to attach them in two places. The bracket on the right attaches with hose clamps. The bracket on the left attaches to the brake caliper mount.

 

[AussieRider]

Nice build mate! Good for the sand. Front drive pulls the front over the sand, mid drive pushes through.

AussieRider

 

[squared]

Nice build mate! Good for the sand. Front drive pulls the front over the sand, mid drive pushes through.

AussieRider

Thanks, that was my thinking and it proved to be true. The front motor is solely controlled by a thumb throttle. Mid drive has both manual and PAS. I have the front drive controller in a fabric bag, was concerned about cooling. I did manage to over temp it as it shut down a couple times and stopped providing speed. It seemed to return to normal operation after a battery power reset and some cool down time. I need to find or fabricate a suitable container.

 

[d8veh]

I built a few 2WD bikes and one with three motors. I always found the main problem was traction on the front because the torque from the back motor unloads the front one. That means that you need a way of modulating the power to the front one. Are you having any problems with traction on the front?

Normal throttles are speed control devices, so are not really suitable. You need something that controls the current. I found a good solution was to use a KT control with pedal assist. Each level of the pedal assist is a different power, so on level 1, it's only about 100w, and you can turn the power up when you're at higher speed or use the throttle then. You can use a single pedal sensor to work both throttles if the controllers share the same ground. just splice the signal wire. on the second controller, you don't need a 5v and ground - just the signal wire. It's the same with the throttle.

 

[scianiac]

I built a few 2WD bikes and one with three motors. I always found the main problem was traction on the front because the torque from the back motor unloads the front one. That means that you need a way of modulating the power to the front one.

I have also played around a bit with this and it seems to depend on conditions and what you are trying to do. I thought stopping the front wheel from spinning was always the best and used the VESC traction control to accomplish that but as it turns out that's not the case in some conditions. In deep snow I found actually letting the front spin like crazy provides more forward traction although you lose some control but if it means getting up the hill or not it's worth it. Might be the same in soft sand, probably not true in basically all other conditions.

Sand and snow are very weird, you're trying to strike a balance between not digging but also applying forward motion and it depends a lot on how soft the ground is, how much float (ground pressure) you have, and tire tread. I find it all very interesting.

 

[squared]

I built a few 2WD bikes and one with three motors. I always found the main problem was traction on the front because the torque from the back motor unloads the front one. That means that you need a way of modulating the power to the front one. Are you having any problems with traction on the front?

Normal throttles are speed control devices, so are not really suitable. You need something that controls the current. I found a good solution was to use a KT control with pedal assist. Each level of the pedal assist is a different power, so on level 1, it's only about 100w, and you can turn the power up when you're at higher speed or use the throttle then. You can use a single pedal sensor to work both throttles if the controllers share the same ground. just splice the signal wire. on the second controller, you don't need a 5v and ground - just the signal wire. It's the same with the throttle.

I Haven't had an issue with the front loosing traction, although it could be my BFA.

Your comment regarding the throttle control makes sense. I really would like a more exponential response with low gain initially and building more rapidly as I add throttle. I noticed it right off the bat, there's too much torque all at once. I'm really only using the front to augment, under acceleration and powering through sand. I'm also running on level 1 on both motors, riding for fitness, and low assistance. I'm running PAS on the mid-drive and manual throttle on the front. Not sure if the PAS on the mid-drive is fully functional. It's a "new" unit I picked up on eBay for $50.

 

[squared]

I have also played around a bit with this and it seems to depend on conditions and what you are trying to do. I thought stopping the front wheel from spinning was always the best and used the VESC traction control to accomplish that but as it turns out that's not the case in some conditions. In deep snow I found actually letting the front spin like crazy provides more forward traction although you lose some control but if it means getting up the hill or not it's worth it. Might be the same in soft sand, probably not true in basically all other conditions.

Sand and snow are very weird, you're trying to strike a balance between not digging but also applying forward motion and it depends a lot on how soft the ground is, how much float (ground pressure) you have, and tire tread. I find it all very interesting.

I think you're right. I've ridden dirt bikes in sand and there's a point where it goes from fighting the handlebar to skimming across the sand. Works great in open landscape, on a narrow forested trail... not so much depending on your comfort level