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Building a dual drive fat bike

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    Building a dual drive fat bike

    I have a Motobecane Boris that I have installed the Bafang Mid-Drive Kit to. I use it in the desert where I come across long stretches of deep sand at times. It bogs me down to a stop at times and I have to walk a short stretch. I am thinking about installing a front drive hub assembly to have available to use on demand with only a thumb throttle. Has anyone done a similar build? Comments or suggestions?

    #2
    How do you have the Boris' mid-drive geared? Is it the BBSHD or the BBS02?

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      #3
      I'm running a BBSHD boris with 4.7 x 26" tires and ride in the sand all the time with no issues at all and frankly would never consider a front hub drive and don't see it helping since it will add so much weight to the front and likely hurt more than help. What pressures are you running? When out in deep sand I'm running 5-8psi...

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        #4
        Originally posted by BK Xray View Post
        How do you have the Boris' mid-drive geared? Is it the BBSHD or the BBS02?
        It is the BBSHD

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          #5
          Originally posted by AZguy View Post
          I'm running a BBSHD boris with 4.7 x 26" tires and ride in the sand all the time with no issues at all and frankly would never consider a front hub drive and don't see it helping since it will add so much weight to the front and likely hurt more than help. What pressures are you running? When out in deep sand I'm running 5-8psi...
          Thanks for the reply. I run the boris with the stock rims and 4" On-One Floater tires. I have had the inflation at about 12psi. I could take it down and that would help. It is mainly the deep sandy uphill stretches that stop the bike. I agree it would make the bike heavier with a front mount motor hub, but I think the two wheel drive action would overcome that. I am a heavy guy, 220 lbs and the bike is quite heavy as is, so I don't think a little extra poundage would make much difference. Ideally I would like a small 48v hub motor that would only be 500w as I think that would be more than enough for short bursts of usage as long as I'm using my mid drive motor. But I have not seen any available for less than 1000w.

          Comment


            #6
            I have no first hand with doing a 2WD but from the posts I've read I doubt that's the approach that will give you the outcome you're looking for.... but admittedly I'm no expert.

            I would go much lower than 12psi. If you went to 6psi the difference would be tremendous. It's just half the pressure. Think of taking the pressure in half in any higher pressure pneumatic tire. A 70psi MTB tire at 35psi? A 32psi car tire at 16psi? I'd try that way before going down the 2WD route regardless.

            From my moto days I've got the instinct to get my weight to the back so try that too. I used to ride very large motos in the deep sand and gravel washes which are often worse than the sand and while it literally took years and many miles to get any good at it there is technique too.

            I did a long ride with only 6psi in the tires last weekend and the only downside outside of increased rolling resistance (who cares when you have electrons along for the ride? ;-} ) and more tire wear was a hard surface handling concern of the tendency of the bike to fall into turns more - I found it easy to adapt to though.

            YMMV

            Click image for larger version  Name:	ricky.gif Views:	17 Size:	1.3 KB ID:	75447
            Last edited by AZguy; 10-05-2018, 11:02 AM.

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              #7
              Originally posted by AZguy View Post
              I have no first hand with doing a 2WD but from the posts I've read I doubt that's the approach that will give you the outcome you're looking for.... but admittedly I'm no expert.

              I would go much lower than 12psi. If you went to 6psi the difference would be tremendous. It's just half the pressure. Think of taking the pressure in half in any higher pressure pneumatic tire. A 70psi MTB tire at 35psi? A 32psi car tire at 16psi? I'd try that way before going down the 2WD route regardless.

              From my moto days I've got the instinct to get my weight to the back so try that too. I used to ride very large motos in the deep sand and gravel washes which are often worse than the sand and while it literally took years and many miles to get any good at it there is technique too.

              I did a long ride with only 6psi in the tires last weekend and the only downside outside of increased rolling resistance (who cares when you have electrons along for the ride? ;-} ) and more tire wear was a hard surface handling concern of the tendency of the bike to fall into turns more - I found it easy to adapt to though.

              YMMV

              Click image for larger version Name:	ricky.gif Views:	17 Size:	1.3 KB ID:	75447
              Yeah I will definitely try the lower pressure first on a good test run nearby. I usually keep them at 15psi on hard packed surfaces, but took it down to 12 knowing there would be sandy spots on my last run. I obviously did not take the pressure down low enough. I'm still learning riding on the sandy stuff. Thanks for your input!

              Comment


                #8
                Before you go to a 2WD system, try lowering the tire pressure :-)

                With that said, I can tell you that 2WD done right will effectively make all traction problems go away. It is not a dramatic iimprovement until you try your same maneuver on a RWD only bike and realize what you just powered thru effortlessly is actually something to worry about. That is pretty much what the USAF said about the AWD motorcycles they put into service.

                I've run in deep dry beach sand with this behemoth. It was built for the steep hills of Monterey CA area and it is demonstrably better at that than my single BBSHD lightweight carbon-wheel'd fatty. Byu better I mean that I can put 250w to the front wheel and go up steep hills faster but not eat so much power I regret doing it. I can push up to 2Kw to the front and its overkill on the street well before that.

                Doing AWD right is not a given. You want PAS running on both motors. You want brake cutoffs set up to kill both motors. think your battery solution thru. Much better to have a single with hi amp output than duals. Slow start on the front motor if doing it with a mid drive maintains control while eliminating the shock on the drivetrain that a mid drive hands out. I have crossed 1100 miles on this bike, I stick almost exclusively to the 11T in back and I am still using that original 11T cog, I never downshift at stoplights. Neither motor gets hot because they help each other out. But this was no built-it-in-an-evening project.

                mid-drive + hub for hills. hub+hub for flat ground. On my twin hub 2wd bike I have the soft start turned off and I take off like greased lightning. PAS and 75-85 rpm pedaling yields 34 mph.


                Open the pic in a new window to embiggen.

                Last edited by MoneyPit; 10-05-2018, 01:57 PM.

                Comment


                • AZguy
                  AZguy commented
                  Editing a comment
                  You've got me thinking a bit different about 2WD...

                  It's clear from the picture we have very different "missions". What's the front chain ring? I do a lot of rough surface very steep hill climbs so I've got very different gearing. Looks fast and fun =]

                • MoneyPit
                  MoneyPit commented
                  Editing a comment
                  Yeah what works for me is great for what I use it for. I am almost purely a city dweller with brief excursions off road onto trails, and a fe trips from the road to the beach trail and across the deep dry sand in Monterey, CA where this bike was designed to live. Mostly for me though I am running errands around town up/down steep hills. So the HD is the main player, with the front motor helping it out so a) it doesn't work so hard and b) the drivetrain gets a LOT less punishment. I think I may have already mentioned I do not downshift at intersections. From a standing start I hit the soft-start *front* throttle and re-mount the bike. By the time my feet are on the pedals and positioned I am going about 10 mph and then my pedal cadence brings on the HD. At this point I lift off throttle and I have two motors powering me pretty fast (soft start on front motor has become full power after about 2 lanes' distance) down the road. No shifting off of an 11T required and at present it has 1100 miles on it with no skipping.

                  The front chainring is a 42T Lekkie Bling Ring. 42T has best offset. Luna 42T ring has too much to fit my frame. Lekkie is perfect. I have dead center chain alignment on the middle cog on this frame so I am well sorted for 11T on the hi gear and IIRC 36T on the low gear. That alignment is only possible thanks to the titanium frame's chainstay design. I have a build thread here on this bike showing it. Somewhere :D. This bike and the black/white BBSHD Stormtrooper I also wrote up here make up the two bikes I use at home in the hills. The more traditional but lightweight trooper is an around-town runabout while this 20ah bruiser is better suited to longer rides to a set destination... The AWD means I grab it any time I don't have to leave it locked up outside at my destination.

                • AZguy
                  AZguy commented
                  Editing a comment
                  When I looked at the photo it looked more than 42t but I didn't try and count them LOL

                  I'm also running 42t into a 11t top gear cog with 26 x 4.7" tires so we're pretty much even steven with top gear. I can't pedal all that fast but am cozy up to about 25mph pedaling. I've manged low 30's pedaling but that was on very smooth surface, straight and level.

                  My 42t is a narrow-wide on a bafang spider shimmed as close as it can get to the motor without interference and I have a really good chain line

                  Where mine departs is that my first gear is 46t and I routinely climb 80+% grades that are loose and rough surface... Even in the picture I'm pretty darn sure your first isn't anywhere near a 46t =]

                  I'm too scared of those great big chunks of metal hurtling along at the crazy speeds they do around here to mess with the streets... I guess I'm more scared about the loose nuts behind the wheels and out here they are decidedly not bicycle friendly and some are distinctly out for blood... For real... I dealt with them in my moto days... I'll take my chances with the rocks, thorns, rattlesnakes and scorpions...

                #9
                I gotta give you credit MoneyPit - that thing is a brute in so many of the ways that work for me...

                Comment


                  #10
                  As long as we are talking AWD, I just finished this one, which is a rebuild of my Central California (table-flat terrain) daily driver commuter bike. So I still have some components left over from before I figured stuff out... like two separate batteries. This is a truly purpose built flat land street bike. The rear cluster is 11-30T 9-speed and I never expected to get all 9 gears usable. I got 5 which was a surprise and so far after two days of riding it (JUST got it on the road) I have yet to downshift once thanks to the terrain, the awd and the fact these are hub motors. Front ring is 46T and putting a ring that big on a fat bike is no fun at all. In the near future I am changing the bottom bracket and doing some surgery on the crank spider to get better clearance from the stay.

                  Acceleration from twin hubs is surprising. 52v batteries, twin 35a controllers. I have only a whisper of soft start on the front motor on this bike to do nothing more than counter almost (!) all of the tendency to do a 1/4-turn front wheel burnout when using either full PAS or full throttle from a standing start.

                  At 46x11 this bike is geared to cruise at max speed on the street where I can pedal and the bike cannot run away from me. That means 32 mph and 75-85 rpms cadence. Its working like a champ and I'm hauling ass and getting a good workout.

                  And thats a big frame - XL size Chumba Ursa Major in chromoly. The triangle holds 29.5ah worth of 25R and 30Q batteries... bigger than it looks.

                  Front forks are Surly Ice Cream Truck 135 QR's whose brakes are spaced for a rear hub. I had to modify the brake mount - take about 2mm off of it nice and even and parallel - to get the brakes to fit a normally spaced hub.

                  North of 30 mph, these hubs start to hit their max rpms and they use less and less power as you go faster. I have found that running full blast PAS and pedaling hard I am actually going faster and using significantly less power. Thats true even if I don't pedal, but double bonus if I am working off the cheeseburger from the night before.

                  Plan on doing a more thorough writeup on this when I'm finished.






                  Last edited by MoneyPit; 10-11-2018, 09:44 AM.

                  Comment


                  • fattiejack
                    fattiejack commented
                    Editing a comment
                    You truly seemed to have mastered the awd build! It would be great if you could document "how to" instructions, maybe starting from already having a BBSHD system. I have not found anything on the web and your pics certainly show you know what you are doing!

                  • MoneyPit
                    MoneyPit commented
                    Editing a comment

                  #11
                  @MoneyPit: I did read your thread earlier. It's a pretty good write-up. I skimmed it again and noticed you mentioned "The BBSHD kit is from Luna, as well as another of their bare Bafang fat front motors. As far as I know only Luna sells these fat front motors." That is no longer offered at Luna and an exhaustive google search has failed to find it or anything comparable. That's most unfortunate.

                  Comment


                  • MoneyPit
                    MoneyPit commented
                    Editing a comment
                    To find that front motor, you need to talk to volume Bafang dealers directly. I know of one one who told me she could get them. I would absolutely ask Luna first though.

                  #12
                  WOW!!! mr. money pit your bjkes are works of art. questions.... how fast and how far and how much ?

                  Comment


                  • MoneyPit
                    MoneyPit commented
                    Editing a comment
                    How fast: The twin hub bike peaks at about 34 mph, but I got that number before I got the gearing you see in the orange bike above so I am going to guess I can push it to 35 mph with a full 58.8v charge, no panniers and a bad attitude.

                    How far: Honestly I go for speed not for distance. I could probably push it oujt to 50 miles if I sipped at the dual PAS rather than going full bore on both motors. However right around 30 mph you start getting to that motor's max rpms and it starts pulling watts big time. Not to slow you down but because it doesn't need them to keep spinning. So I have found now that I am geared to run full bore, I use full bore and now I am actually using less power at PAS5/PAS4 (rear/front) than I would be at my formerly normal PAS3/PAS2

                    How much: Not cheap. I am buying quality parts. USA made titanium frames aren't sold on AliExpress (at the same time I got a ridiculous deal on it). The Chumba just above is also USA hand made. Again I got a smoking deal but *again* compare that to Red China and you're in the deep end of the pool. By contrast I have an awesome Motobecane Lurch BBSHD build on display here as well. That frame was $150 on Fleabay and a $60 strip and powder coat turned that frog into a prince. I get deals on stuff. Got the Blackburn racks on clearance too. Beyond that... I'll let you zoom in and count up the parts cost yourself :-) You definitely want to build these yourself and pay internet prices for parts as they are all easily $10k+ if someone is selling them out of your friendly LBS.

                  #13
                  heh, i was just gonna make a post seeing if anyone had done a 2WD bike as i hadnt heard of it, and here some are...

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