Plan for MTB to E-Dirt Bike 8kW Dual Motor Conversion - looking for advice.

[JoltJockey]

Hello everyone,

I have been daydreaming, sketching and revising this Idea since about a year ago (but please feel free to give critical feedback, that's what I'm here for).

The idea:

The MTB: This LUTU aluminium 6061 full suspension frame will get a 200mm DH fork, front/ rear 29' 27.5*2.4in wheels. The welds look good at least from the outside and it is made with hydroforming. With 3.9kg on the heavier side but that is of little concern to me. I just hope it is strong.

Mounting: I want to do all mounting without drilling into the frame to avoid structurally weakening it. The battery pack will take up all the space in the main triangle only leaving space for the rear shock. A 14S 8P Samsung 50S battery is packed into a molded carbon fibre clamshell that has nylon straps embedded and "suspends" the pack from all sides. Since the clearance between shell and frame will only be ~1cm I expect this to he stiff enough + give the pack some extra isolation from impact forces.

Drivetrain: I plan to use brackets made of 3d printer carbon-nylon filament modeled after (plaster) prints of the frame. The brackets along the top tube will have cutouts that allow for titanium rods to be inserted which can be used as mounting points. Right behind the head tube where you can see the top tube and down tube welded together I want to have one big clamshell type bracket that will be the main support for the motor plate mounts. One Flipsky 63100 + 75200 vescs on each side. The drivetrain consists of three stages.
1st Stage: The motors (parallel to the frame) connect to a titanium shaft via a HTD 5M belt/ pulley system. The peak torque of the motors (18Nm combined) will be transferred onto the shaft (1.5:1 reduction = 27Nm total).
Stage two: The shaft ends at a jackshaft mounted before the seat tube reinforcement with a module 1.5 helical bevel gear to another bevel gear on the jackshaft with a 4:1 reduction (108Nm). Finally on this jackshaft is a 420 9T sprocket mounted - peaking 2-3 cm over the top tube with a chain going to a a 36T chainring mounted where the 6-bolt brake disc mounting point is. Along the way will be spring loaded tensioners (top and bottom) and chain guides to give a correct alignment with the rear sprocket and clear the seat-stay/ chain-stay.
(All drivetrain components aswell as the motors/VESC will have ventilated enclosures/ guards.)

My biggest worry: With this final reduction the peak torque along the chainline should be (24:1 total reduction) 432Nm in theory. Of course traction would break well before that but I feel very unsure about what magnitude of force I am exposing the seat tube/ beginning of top tube - area to. If this is would be within the design limits of a Trail/DH frame or the forces are simply too much and from a direction that MTB frames are not designed for.

So tat's where I am at the moment. I expect things will break during testing but as long as it is limited to clearance, overheating, mounting I think I'll find solutions. When it comes to the load on and limits of the frame I'm still in the dark.

Note on rear braking and clearance: I plan to put a 142TA hub on this 148mm dropout. shifting the hub to the right so that the left side now has 6mm of extra clearance. If this isn't enough the XD hub due to it's compact size should allow me to grind away a couple extra mm from the right hub cap. To center the wheel I will adjust the spoke tension accordingly and the exposed Thru Axle space will be occupied by washers.
For braking I plan to use motor braking + either modifying a hydraulic brake to be able to clamp onto the chainring or mount a V-Brake that can grip onto it. Ideally I can make enough room to only have to adjust the brake mounting point and be able to have it function with the disc brake as it should.

If you have ideas, inputs, critical feedback that you can share I'm super thankful. Or maybe someone knows of a very similar successful DIY project out there that I just haven't found yet.

Have a great day, I wish you all smooth riding!

 

[scianiac]

That is quite a complicated drivetrain that will probably be quite noisy. I don't really see the advantages here over a 2-3 stage belt/chain setup. You can for instance install a much much larger chainring on the rear which will do a lot of the reduction and put less stress on the chain. From there a single belt reduction before it or two depending on your target speed and torque wold be more reliable, easier, lighter, efficient, and quieter.

The hydro brake on the chainring is a bad idea for a number of reasons, even if you wax the chain it's still going to get contaminated at some point, and about a dozen other issues. You can fit the brake rotor and a sprocket side by side, I've seen if done a number of times. Contamination is still an issue but with a waxed chain or a belt drive you may get away with it.

I'm a little confused where the suspension movement is taken up, is it a sliding driveshaft and the 9T cog is mounted on the rear triangle? I guess this is an interesting way to solve that constant issue of rear suspension and regen although depending on how you are using the bike I don't know that having regen is worth the added weight and complexity. Or is that done with those spring tensioners? If done with tensioners that mounting the front sprocket in line with suspension pivot is a way better idea.

Also two motors will be worse than one larger motor, what motor would be better than two 63100s I'm not sure, but something like an 80100 would be a start, have to do the math on stator volume between those but there are bigger motors than 80100s, matching the stator volume to two 63100s will give better performance per weight and complexity.

Based on the amount of power and torque you are talking about I would just drive it from the right side and either make it not pedable at all or at a pointless ratio. Then like many tried and true designs you can just put the front chainring free spinning around the BB which will limit chain growth.

The power and torque really depends how you are riding the bike, I've found unlike a dirbike dumping insane amounts of torque in a bike like this does not result in just spinning the rear wheel, it normally results in a power wheelie. Now I don't know if this is because I don't weight very much and my bike is not super long and a bit taller due to all the suspension travel (2010 Santa Cruz V10 with a Lightning rods small block at around 5kw currently). You want to gear the motors so you have reasonable torque at the speeds you are normally traveling, gearing it down too much means you have more torque then you need from a stop and then it drops off quickly. The real thing about a high power bike like this is how and where you are riding it, I just cannot use the power my bike has 99% of the time, like full throttle maybe once a ride on a long straight bit and then I have to let off before I'm going dangerously fast but I'm riding on trails, on double track dirt roads that's a different story.

 

[JoltJockey]

Yes, it surely would be noisy. The helical teeth should be quieter than straight ones but still, I imagine it would sound like a GT - car which could be either cool or really annoying.

About the belt setup: Yes, if possible I'd prefer to only have to deal with belts and the chain instead of the axial thrust of the bevel gears. This decision was mainly made because I was under the impression that HTD 5M belts might be too weak once the power goes above ~25Nm/cm width. What are your experiences with what load they can handle? I honestly did a lot of brainstorming with Chat GPT/Deep Seek/Claude AI. They famously are often confidently spewing out incorrect information - so please correct these numbers if possible. My only real-life experience with belts has been a ~ 2.5kW E-Skateboard. Never had one of the 2cm belts snap only belt skipping probably due to me not tensioning it enough. However, there is quite a bit of visible wear and deformation on the aluminum pulley.

After writing the initial post a solution that is a bit simpler came to me. But as you suggested maybe the spur gears are not needed after all.

About the 9T sprocket. Like in the previous concept it would be mounted under the seat tube on the second jackshaft with tensioners dealing with chain growth. I have also thought that it would be good the put the sprocket in line with the center of the dual pivot suspension. However, I'm unsure of how I'd transfer power there. If belts are strong enough after all to handle some more reduction I'd definitely consider it.

The bigger rear sprocket would be great since like you said it would reduce forces on the chain as well as the frame/ mounting point. I haven't purchased any components including the frame yet so I have a hard time estimating how much clearance there will be.

Alright, I'll do the brake disc/ sprocket mount side by side in that case. Maybe there is more space than I guessed especially with some modifications.

Good to hear your bike is handling the power well. About riding style: I primarily want the power for climbing mountains as well as quick rides through flat forest paths/ gravel roads. Would the ideal efficiency and torque for a BLDC be around 20% of its max rpm? I think that is what the dyno-graph from the manufacturer shows.

I did some quick searches for a single-motor solution and came upon this motor. With the KV of 50 instead of 140, it would already take care of a lot of the reduction.

Thanks for the detailed Response!

 

[scianiac]

How much power and torque you can put through a belt is highly variable but belt wrap and the size of the smaller pulley are key. With good belt tension and wrap you can put a lot of power through these belts, supposedly more as well if it's an advanced profile and not just a basic HTD profile. Those LLMs are probably correct for the manufacturers ratings but those are probably extremely conservative. Modern belts are astonishingly efficient and capible. While obviously done at a very high RPM the belt on the front of a top fuel dragster that runs the blower is moving around 1000HP through it, think about that.

This that new setup seems awfully complex still. I don't think you really can have any real rear suspension travel and not have the front drive sprocket somewhat inline with the pivot. The chain growth will just be so high it will be a nightmare to make a tensioner work. Have you looked at other drive setups, generally the idea is motor below downtube or inside triangle with belts or chains either a chainring on the BB or through a jackshaft first if 3 total stages of reduction.

I think two reduction stages is generally all you need. For reference the LR drive on my bike has a 20mm wide belt reduction from the motor to BB, that is 18T/90T, then a 22T chainring to a 58T chainring on the rear. This is already geared lower than anybody I've ever seen run one of these motors because I don't ride very fast, normally under 10mph. Now the bike will do 45mph on pavement with knobby tires at low pressure but that is at 72v and MTPA. Motor is 67kv and I'm "only" running it at about 130PA instead of the 180-200 peak it can take because I don't know what I would do with more and this cheap VESC won't run stabiliy any higher. I mostly worry about blowing up the freehub even with some attempts to protect it as best I can and it being I think one of the stronger freehubs made.

So I have about as much reduction as you're setup with just two stages, no gears using a 20mm wide 5m gates belt and a regular 7speed bike chain. I actually had a 60T on the rear but that was too much, I have a 54T and the only reason I have not changed it is the motor is very slightly more efficient it seems at this higher RPM as are most motors. All fits nicely in front of the downtube with maybe a very slight reduction in ground clearance over a 34T chainring but that's basically never an issue in spite of me riding through rock gardens continuously. Of course credit for most of this has to go to Lightning Rods that made the drive.

The 420 chain I think is way overkill, 219 if you're worried about bike chain and using 219 for the first reduction can also make a very compact high ratio setup although noisier than the belt.

That being said a direct comparison also depends on the motor and even the 80100 will want to spin faster to make real power and be at all efficient. Peak power will should be lower RPM than peak efficiency.

Just based on the math I have maybe 200-250Nm at the rear wheel (26in wheel) and I've certainly never found a hill I was power limited to climb and trust me I've tried.

You can calculate the rough torque you'll have by converting the Kv to Kt and with that and the phase current you'll have ideal motor torque. It won't of course be that high in real life but closer enough for broad comparisons and estimates.

 

[Tony01]

I skimmed or rather just plain read for about ten seconds then scrolled hard. Dude you’re never going to get it right the first time so stop trying. The whole multiple systems and gears and jackshafts is a waste of time, you will never achieve better efficiency than a single motor to rear wheel.

That is my pro builder opinion. Should be end of topic and go build something finally…. But it won’t be… this thread will continue eating people’s time of their life or many small portions of it…

 

[bananu7]

With 400+Nm at the wheel, you're unlikely to add anything on the climbs. With 7kW+, you're unlikely to add anything on the straights. Why bother with a pedal bike frame in the first place?

 

[scianiac]

With 400+Nm at the wheel, you're unlikely to add anything on the climbs. With 7kW+, you're unlikely to add anything on the straights. Why bother with a pedal bike frame in the first place?

I mean that 7kw of that 80100 is peak probably at saturation, it's not going to put out anywhere that much continuously even with quite aggressive forced air cooling. Perfectly reasonable amount of power for an enduro or DH bike that is to say in the 3-6kw range. But I agree 400Nm even assuming 29in wheels is still more than is possible to use unless he's doing a hill climb competition.

 

[JoltJockey]

Right, my reasons for the low gearing were 1st: as mentioned not having to worry about motor temps when climbing especially in tricky steep terrain where speed can get low. 2nd: making the motors more responsive at low speeds/ avoiding "stutters" and using a just a bit of "ramp up" in vesc tool to not make it too twitchy at the same time.
The 3rd reason was pretty benign and probably too simplistic: Since I set 50km/h as the max speed I'd allow myself to go I assumed I'd waste the motors performance if I geared for 70 or 80km/h. But @scianiac already hinted at the fact that I'd be using the motors in a inefficient range well before getting to 50 as I understood.
What ballpark area would you place your gearing if you 50 is all you wanted and you have motor power to spare? ~70?

 

[JoltJockey]

With 400+Nm at the wheel, you're unlikely to add anything on the climbs. With 7kW+, you're unlikely to add anything on the straights. Why bother with a pedal bike frame in the first place?

Apart from reasons stated:
- Having a way to get home that is easier than pushing the bike once something breaks.
- In steep terrain, slow speed, where grip is at the limit breaking traction/ feeling the limit with the pedals.
- Having the ability to get some exercise in
- Maybe at a later point try to integrate a torque based PAS

 

[JoltJockey]

I skimmed or rather just plain read for about ten seconds then scrolled hard. Dude you’re never going to get it right the first time so stop trying. The whole multiple systems and gears and jackshafts is a waste of time, you will never achieve better efficiency than a single motor to rear wheel.

That is my pro builder opinion. Should be end of topic and go build something finally…. But it won’t be… this thread will continue eating people’s time of their life or many small portions of it…

I'm not looking for the easiest way to get an E-Bike - otherwise I'd get a hub drive or even just buy one since it wouldn't be much more expensive and it would work without any issues. To me it's a hobby, I love to tinker, try out ideas, challenge myself and sure things will break on the way there but once they do work out it's a joy that a off the shelf bike wouldn't give me even if it technically performed better.

No one is forced to reply to my post obviously and anyone who does I appreciate a lot for taking the time out of their day to do so.

 

[bananu7]

I've seen values mentioned as high as 75% max rpm, but that would depend on the entire system. I'd assume you'd want to mostly stay out of the FW range during cruise.

Apart from reasons stated:
- Having a way to get home that is easier than pushing the bike once something breaks.
- In steep terrain, slow speed, where grip is at the limit breaking traction/ feeling the limit with the pedals.
- Having the ability to get some exercise in
- Maybe at a later point try to integrate a torque based PAS

1) you'll need some sort of extra freewheeling mechanism for that, I'd assume? Doesn't seem very plausible to pedal against all the chain and gears and motor easily.
2) that's what I alluded to with the massive torque produced; I'm afraid the force you'll generate will be negligible in comparison, but maybe that part is worth verifying
3) I think that bit, at least exercise from pedaling, is easiest on the flats with even cadence. Otherwise it's very hard to put any power down as the motor is dictating how quickly you're going and you need to adapt to it, very quickly. Unless...
4) you indeed do this and rely only only pedal input for motor control. How precise is that going to be compared to not pedaling and just using throttle? I can imagine, but would still love to see it in practice

 

[scianiac]

I feel like I'm very similar to you in that I really enjoy the building and tinkering as much or more than the riding and I build bikes to be hyper optimized for their use case, which sounds like for my V10 is exactly the same use as you are aiming for. I had long planned to start with a large outrunner drive pretty similar to what you are describing but after much research I decided to just buy the LR small block kit instead because I really wasn't sure I would be able to make anything better on my first attempt and knew I would have plenty of work to do to improve the bike regardless. And I'm really glad I went that route, while I could have maybe made a good drive myself on my first attempt there was also a chance of not finishing the project or ending up with something that was worse or had more problems and this way I've still learned a ton and have spent all the development building and improving on an already good system to make a bike that is unbelievably capable. As in this bike is way way better than I have the skill to ride it and I've learned so much my planning for an eventual replacement is far more refined.

I guess I'll give some of my general bullet points for dirveline what I'm thinking for a future high power trail bike, maybe give you some ideas:

- DH frame - why not? only downside is a little weight and battery placement but I've solved that before I can do it again.
- Big Outrunner with fan forced air cooling running though a filter with a PID controller. - efficient and quiet with potential for very high power to weight ratio
- Ideally a frame with a pivot concentric to the BB (very rare) or as close as possible (not as rare)
- Belt drive from motor mouted under downtube to concentric cog on BB, to rear on right side via bike chain or belt if frame allows with large reduction, maybe can manage regen braking but not critical
- VESC + encoder motor control
- other motor option is a smaller IPM motor if I can find one, so either way less motor than I have now.
- Other option would be maybe a high pivot bike and the reduction moves up into the frame.

Right, my reasons for the low gearing were 1st: as mentioned not having to worry about motor temps when climbing especially in tricky steep terrain where speed can get low. 2nd: making the motors more responsive at low speeds/ avoiding "stutters" and using a just a bit of "ramp up" in vesc tool to not make it too twitchy at the same time.
The 3rd reason was pretty benign and probably too simplistic: Since I set 50km/h as the max speed I'd allow myself to go I assumed I'd waste the motors performance if I geared for 70 or 80km/h. But @scianiac already hinted at the fact that I'd be using the motors in a inefficient range well before getting to 50 as I understood.
What ballpark area would you place your gearing if you 50 is all you wanted and you have motor power to spare? ~70?

My comparison won't be exact since the motor is different, yes you want low enough gearing for that low speed control but once the motor is spinning VESC can handle it pretty well, I've only had my admittedly garbage quality VESC loose motor position on extremely slow and technically climbs maybe twice and I'm working on switching from halls to an encoder which I think will solve that. If you mean a throttle curve in VESC then yes that is highly recommended, the low gearing, high torque motor, throttle curve and domino throttle I have basically perfect torque control on slow technical climbs.

As for ideal motor gearing that is tricky without being able to test the motor but I'm pretty sure those 80100s have been used a lot so you might be able to find some numbers to use. I would mostly be guessing as to where the motor starts to run into high RPM based loses. If I were to just guess I would say 80% unloaded RPM targeted at the high range of your normal speeds. You have to remember part of the math here is how much time you spend at any given phase current. So if the bike has a lot more power than you normally use you don't spend much time at high phase current so the ideal efficient RPM will be lower due to lower RPM based magnetic losses because resistive losses are lower since you aren't using max phase current that much.

I guess I would say flexible gear reduction is always nice to have and I think you can get enough with two belt/chain stages with a very large final cog. Also a bike with a smaller rear wheel like a 26/27.5 will help the gear ratio out a nice bit.

Apart from reasons stated:
- Having a way to get home that is easier than pushing the bike once something breaks.
- In steep terrain, slow speed, where grip is at the limit breaking traction/ feeling the limit with the pedals.
- Having the ability to get some exercise in
- Maybe at a later point try to integrate a torque based PAS

- Better to make the bike reliable, easy to service and have the tools to cobble it back together with you (done this a few times).
- I've never felt the need to use the pedals to feel the grip, pedals are terrible at this because their torque input is in big pulses, a good throttle you have plenty of feeling.
- That's what I said when I built my first bike, for some I guess this works but you have to keep in mind the compromise, lots of added weight, complexity, unsprung mass, etc.
- Torque PAS is useless at this power level which means dropping the power way way down which again means compromises.

Not saying it's a bad idea to have pedal and this much power but personally I would rather build the bike to be really good at one thing and not try and make it do everything and comprimize the whole time. Better to build a second bike with a low power mid drive that has torque sensing for exercise. My bike technically can be pedaled but at such a high cadence it's pointless and I'm considering disabling that just to I make it quieter.

 

[JoltJockey]

I've seen values mentioned as high as 75% max rpm, but that would depend on the entire system. I'd assume you'd want to mostly stay out of the FW range during cruise.


1) you'll need some sort of extra freewheeling mechanism for that, I'd assume? Doesn't seem very plausible to pedal against all the chain and gears and motor easily.
2) that's what I alluded to with the massive torque produced; I'm afraid the force you'll generate will be negligible in comparison, but maybe that part is worth verifying
3) I think that bit, at least exercise from pedaling, is easiest on the flats with even cadence. Otherwise it's very hard to put any power down as the motor is dictating how quickly you're going and you need to adapt to it, very quickly. Unless...
4) you indeed do this and rely only only pedal input for motor control. How precise is that going to be compared to not pedaling and just using throttle? I can imagine, but would still love to see it in practice

1) I hope to only need that feature mainly for the first couple of longer test rides. My plan is to just carry a multi tool and disconnect the drivetrain somewhere between the motors and the rear wheel.
2/3) Ah I see what I meant was for uphill terrain where as you said cadence is not even I was thinking to have a current limit - VESC-preset triggered by a switch that provides just enough power to overcome drive train resistance + a little extra to make the bike feel lighter but not so much as to have the bike gain speed without pedalling (I read a switch can be implemented without extra programming). And by breaking traction with the pedals I meant I like the instant reaction and being able to "feel" the wheel and if it is about to slip you have when using the pedals compared to being "disconnected" when only using the throttle. - I'll need to test and I see if this viable.
4) Yes it would be cool if I managed to find a way, the cadence based PAS systems that are quite easy to implement have been called useless by everyone who I heard trying it. Again if there is a way to implement a torque based system via chain tension/ bottom bracket sensor I'd again use it together with a switchable preset that limits power since it obviously wouldn't make much sense to have 8kW of "assist" for ~300W of input when pedalling really hard.