DD Hub as a mid drive motor pros and cons?

[macribs]

There is a new mid drive frame coming this year. Using QS 205 as mid drive. Motor is attached to the swing arm very close to pivot point. Axle is stationary and the carcass spins. Front sprocket attached to motor side covers. Moto 420 chain/sprockets. Max RPM @ 72v about 1300 RPM.

What can be expected by running a 205 as a mid drive? If volted up to 22s max RPM close to 1500 RPM. So almost three times the speed of the rear wheel at 50 mph. If geared for 50 or 60 mph top speed how will the take of torque and acceleration differ from a regular 205 hub motor in rear wheel? What about mid range, how will mid range acceleration be different?

Sprocket ration > 2:1 should be possible. How does that translate into torque on the ground? I mean how will the motors max torque play out via chain and sprocket? Will acceleration become faster yet with a lack of top speed? And how will FOC play out? I am thinking both in terms of the amount of extra heat field weakening put put, but also in terms of what can be expected in extra top speed. I remember Axel raised top speed with around 28% on his bomber with FOC.

I guess such a mid drive will climb much steeper then a 205 in the hub? And with less heat as well. Also the weight of the motor is removed from inside the hub, giving more centralized mass. I can't think of any cons of running a hubmotor as a mid drive. But given that it is not commonly done I might be overlooking something?


I know I've seen a build or two here on ES that had a dd hub used as a mid drive but I can't seem to find back to those. If your search query are better then mine pls share link to builds that are similar as I would like to tread up on this subject.

[spinningmagnets]

Its in the non-hub stickey index:

"The Mid Drive Hub Motor Media Group"
viewtopic.php?f=6&t=38553

"Paul Brodies eBee custom build"
https://www.electricbike.com/paul-brodies-ebee/

"Simon's Specialized Supercharged" (ES member Tench)
https://www.electricbike.com/specialized-super-charged/

[Overclocker]

someone give the guy a pipe bender

well a DD hub has poor thermal path to outside air but the lower kv means less extreme sprocket ratios. just look at some of the ridiculously large rear sprockets on some of the builds.

[RIPPERTON]

Big advantages with a mid drive hub motor especially for low speed dirt bikes. That rock climbing attack torque and rocket boost out of turns but will need high voltage to get some of th top speed back. Just be satisfied with 70kmh and you will have a real enduro weapon.
BUT this particular build has one fatal flaw. The motor is mounted in the swingarm. BIG MISTAKE.
The torque reactions in the hub motor axle will extent the rear suspension under accel and squat under regen effectively playing havoc with the rear suspension action.
Needs to be changed.

[tommie]

^ Correct!!

The torque produced by the motor coming out of a corner for example will dig the rear wheel in and lift up on the front forks giving little or no traction to the front wheel.

[DanGT86]

BUT this particular build has one fatal flaw. The motor is mounted in the swingarm. BIG MISTAKE.
The torque reactions in the hub motor axle will extent the rear suspension under accel and squat under regen effectively playing havoc with the rear suspension action.
Needs to be changed.

RIPPERTON,
I'm inclined to believe you because I'm familiar with your work and experience. However, I don't understand how there would be torque reaction. Wouldn't the motor axle need to be the pivot point for it it to exert a pull up or down on the swing arm? Is the torque reaction you speak of occurring because the motor is acting like a gyroscope such that its mass would counter rotate the arm down when the motor accelerates? Not trying to argue. I'm actually really curious about the physics at work here. Bike and motorcycle suspension is very mysterious to me.

Non RIPPERTON specific,
As far as hub motor as a mid drive goes:
Does the rotational inertia of the motor increase with the square of the radius?
Does motor torque increase with the square of the motor radius?

What I am getting at is the possibility that a large motor might resist acceleration due to inertia 4x more than a motor half its size while only providing 2x the torque. I don't have time to hunt down the formulas at the moment.

I do see a huge advantage in how cheap and simple a setup like this would be for most builders compared to an equally powered high reduction mid drive. All the simplicity of a hub build with the ability to change gears to suit different needs. Loosening 2 axle nuts to change a sprocket is way easier than relacing a wheel every time! I love where this is going. Hope its affordable.

[RIPPERTON]

Wouldn't the motor axle need to be the pivot point for it it to exert a pull up or down on the swing arm?

nope you could apply torque anywhere on the swingarm and cause it to turn. In fact I believe a regular rear wheel hub motor would produce suspension rise and squat because the hub motor axle is connected to the swing arm. I dont have much experience riding hub drives so the more astute ES'ers could chime in here.

Is the torque reaction you speak of occurring because the motor is acting like a gyroscope such that its mass would counter rotate the arm down when the motor accelerates

Not because of any "gyro" effect. Simply because the motor stator is driving the rotor and there is always an equal and opposite reaction between any 2 parts where one is causing the other to move. The rotor represents a load to the stator because it resists movement (because you are sitting on the bike) so the stator will be forced to turn in the opposite direction of the rotor and because its bolted to the swingarm, it will cause the swingarm to turn or extend through its suspension stroke.
This bike would be annoying to ride because the ass end would be pogoing up and down in unison with throttle movements.

[nutnspecial]

Ripperton you've lost me in the dust. Also, I think you might be full of it

Seriously, I've no idea what you're trying say. The most virigin torque forces on suspension would either be hub motor or (EXACTLY THE SAME) a swingarm mount motor. Maybe I could see an argument for beneficial suspension action from a properly designed/mounted BB or motor elseware, but that's it.

[nutnspecial]

Right, for swingarm mount (or hub motor) It's simply the 'push' force of the wheel and how that acts on susp. But I didn't think it was so much the motor placement / chaingrowth that is used to control suspension under power - I thought it was the more advanced suspension geoms.

I will say that I expect squat to be fine on a swingarm mounted Q76 height pivot. I still don't fully understand it but my higher pivot urt klein mantra is really a treat and it really digs in, like a rocket.

[nutnspecial]

Wow thanks, you know geometry! (and physics). Just a sidenote- alot of people think the Mantras 'lockout' when standing, but they were designed to only 'lockout' when pedal force is involved AND you are seated. Otherwise, because of that design, the pedals just experience less (maybe 40-50%) travel than the seat.

[macribs]

Ripperton and craig you lost me. I have no idea craig how to even read your pic.

There have been a handful of builds here in ES with motor mounted on the swing arm. I can not remember any of them complain about harsh ride. I would think by the little I know of physics that when motor is moved out from the hub of the rear wheel and placed as close to the swing arm pivot point as possible the forces should be considerable less then with a dd hub motor. Length times arm etc.

Will you say this frame is useless because of the motor placement, or that it will be hard to ride? If the latter is the case, when and where will the motor impact ride quality and handling? Going WOT on the straight? Exiting sharp bends while on the throttle?

As much as I would love to have a theoretical perfect geometry on frame and every little thing placed in the most optimum position is that really feasible as long as I don't have the knowledge of how to design such a frame myself? I think for now e-bikes will be a compromise. Batteries are too large with too little power. Motors are too big with too little punch. Real estate is premium for e-bikes and within those restrictions one must work to find a way to make a frame that is a little better then last year models.

If this frame turns out to be not ridable at all, I guess the only thing left will be to move motor inside the frame. That will reduce battery capacity. So even with motor in very best placement for very best result there will in the end be compromises.

[ScooterMan101]

In Today's Modern World of Open Sourcing , the answer is ... Yes !



[quote="macribs"]

As much as I would love to have a theoretical perfect geometry on frame and every little thing placed in the most optimum position is that really feasible as long as I don't have the knowledge of how to design such a frame myself?

[macribs]

Thx for taking the time craigsj. Much obliged. Btw I don't post this to argue or because I doubt what you are saying, it is purely to try and learn more.
I don't have the tech specs for the frame, but I do remember the rear shock eye to eye could anywhere from 190mm up to 300mm. That wide gap of shock length is bound to impact the geometry of the frame as well as head angle and the rake of the fork? Up front you can use from 6-8" DH forks, or even 10-12" mx forks.

If one would know what to change I guess it would be possible to redesign both the head tube and even the pivot point and suspension to adapt to a linkage system in the rear. As this is steel frame a grinder and a welder goes a long way. But first one must know what to change and then what those changes are - what are the correct angle for head tube. And how does one make a better rear suspension?

I've noticed on motorcycles streetbikes got more steep head tube and fork angle. Dirt bike a little more relaxed. Could it be this frame is targeted primarily for street riding? I will try to read up on frame geometry, but I already got a back log on e-bike specific for the battery build. And for each new thing I try to learn about for e-bikes there seems to be so much information needed. How do you guys find the time? And not to mention the stamina to power thru stacks of books as ordinary day to day life takes it tolls?

Scooterman I don't have enough basic knowledge of frame design, suspension linkage or even general physics to start an open source project for the "one go to mid drive frame". Belive me if I did I would already have a project going. The front man needs to cover at least the basics in order for other to follow along. Otherwise the team will feel they do all the work and the front man is just harvesting the fruits without any elbow grease.

[John in CR]

With motor on the swingarm, whether in-wheel as a hubmotor or near the pivot like in the OP, it provides anti-squat force during acceleration and anti-dive during regen, but unlike Ripperton I see these as positive effects. It does feel different and I'm surprised the topic gets so little attention. To lessen the effect simply lower the pivot point so the forces have less leverage on the bike as a whole. With a high pivot point I guess I can see some potential for problems, though I think it would be more because it feels so different from a traditional moto.

With a hubmotor out of the wheel, it's especially important to keep it on the swingarm. That's because the low rpm creates the need for larger drive sprockets, so if the motor was on the frame it would act with greater leverage on the rear suspension and if you tried to address the reaction during acceleration with chain routing you would only make regen braking issues worse.

The biggest problem I've found with using a hubmotor is sourcing appropriately sized drive sprockets for the low gear reduction required.

I'm with Craigsj, that head tube angle looks awfully steep.

[macribs]

So here is the frame geometry.

[macribs]

Hi John. Thx for taking the time. Much appreciated. I guess you are among the ones here with most ride time with a hub motor on the swing arm? I seem to remember you did a custom swing arm with one of the monster motors close to the pivot. So you words carry weight.

Yeah about the fork angle, 66 seems a bit high but I reckon it could be countered with maybe mx fork and 21" wheel?

As for the rear shock angle craigsj the quickest fix I can see would be to change the swing arm a tad. Put another circle on top of existing one. Then make another steel tube run parallel to the what used to be the top tube of swing arm. This accomplishes two things. Strengthen the swing arm as well as giving the rear shock a slacker angle.

[John in CR]

Yeah, it looks fine on the drawing. It's something about the pic that looks too steep... maybe just an illusion.

[macribs]

Yeah I am not gonna be reading up on Tony Foale's Motorcycle Handling and Chassis Design any day soon. At that price I would only pay if I got a 2 hours session with the author thrown in. Asking price 340 £ or 340 $ plus shipping. When one see prices like that it becomes very tempting to use alternative sources to get the book instantly and free of charge. Oh well, I stop by the library one of these days, maybe they got a copy.

[Warren]

I had the paperback decades ago. Looks like it is all available on his site for free.

http://www.tonyfoale.com/Articles/RakeEx/RakeEx.htm

[DanGT86]

This thread has gotten super interesting. I've been wanting to understand these suspension mechanics for a while but never made the time since I wasn't building my own frame.

Just to check my understanding so far, are these statements true?
-Under acceleration on any bike, weight would transfer to the rear causing squat.
-The chain line and front sprocket location can be used to counter this squat effectively canceling it out or even lifting the rear and extending the suspension under power
-A motor mounted on the swingarm or a hub motor has no way to impart anti-squat because its only mount is the pivot. It would need another anchor point like the chain to pull on.

Craigsj-
You have mentioned linkages and their importance for a progressive spring rate. I'm curious if this progressive spring effect is why air shocks are so common on mountain bikes. Also a buddy of mine who is very into motorcycles told me that complex shock mount linkages on modern motocross bikes are necessary to provide traction over a huge range of travel but trail and trials bikes do better with single point shock mounts. Does this sound correct?

That Tony Foale book sounds like a must read. Looks like it's downloadable for 50euros on this site with a credit for the paperback version on the next run.
http://www.tonyfoale.com/Main.htm

I will check the library too. In the meantime I am going to go remove the shocks on my bikes and see which way the swingarms go when I apply torque to the chains.

[Warren]

Tony Foale has done some wild stuff.

http://www.tonyfoale.com/Articles/2WD_2WS/2WD_2WS.htm

So has Bob Horn.

http://rohorn.blogspot.com/

[John in CR]

...-A motor mounted on the swingarm or a hub motor has no way to impart anti-squat because its only mount is the pivot. It would need another anchor point like the chain to pull on.

With the motor torque anchored to the swingarm, whether motor axle is in the pivot point or on then end like a hubmotor is the same (other than the effect on unsprung weight), there will always be anti-squat force on acceleration or anti-dive during regen. The vector of the force is between the tire contact patch with the ground and the pivot point, though I'm sure things get interesting with multi-link suspensions. Think of the tire at the contact patch clawing it's way forward and pushing the bike via the pivot point. That's why the effects are increased with increased a higher pivot point.

I've tried to bring this topic up a few times related to mid-drives, but it never gained traction. The only time I've seen it brought up by someone in the know was Toolman2, who mentioned a "jacking effect" during acceleration that was noticeably different, but it was easy to become accustomed to it. He mentioned the same positive "digging in" effect as Nutspecial mentioned with his hubmotor(s) earlier in this thread.

I'm really surprised that racers like Ripperton haven't experimented with motor placement in this manner, because all I see are positives. It's probably just a bit too much carryover from gassers, where the motors are too big and heavy to anchor their torque in the swingarm.

[macribs]

The funny thing is that one of the first things I noticed was a pdf of the book. Pdf is legit. But as my days with sea legs and eye patch are behind me I didn't cave. Gonna trust library with this one.

[macribs]

I had the paperback decades ago. Looks like it is all available on his site for free.

http://www.tonyfoale.com/Articles/RakeEx/RakeEx.htm

I will check it out, if he is giving away pdf versions who am I to refuse?

[macribs]

The more I think about it, squat is probably more important to remove or minimize on a pedal only bike? Because squat will take away some of your "leg power", making pedaling require more power to keep momentum. Especially on accelerations and climbs I think this squat motion will be very much noticeable, correct?

Should we really worry to much about squat for an electric e-bike build? I mean lets be honest, those pedals are most for show and legal issues. For me anyway. And for power and torque, we got abundances of it. We can afford to be a little nonchalant about it. Those who pedals electric bike for the exercise might not care about squat either, as they can use throttle to adjust for squat.

I don't doubt the feeling of the squatting bike will take some getting used to, but do we really need to put much work into avoiding the squats in the first place? We've already pulled the hub away from the rear hub, resulting in more centralized mass and better polar moment of inertia. In short centralizing mass and better COG will already pay dividends with better handling, faster turn ins and less stress on the rear wheel, and suspension. Tire should last longer, less prone to puncture or pinch flats. Lighter rims/spokes can be used, even enhancing the effects of moving motor away from hub.

The centralized mass has given us more controllable and predictable handling bike when mid air after jumps and it will let that rear wheel get good amount of traction during tight turns on loose surfaces like gravel roads. And on top of that we got increased torque when deciding to go for a mid drive, letting the motor stay cooler. I'll think the design of this mid drive is worth to try.


Man those Evil Bike's frames look nice, small rockers that steal little real estate. That is contemporary art, nothing less.