Places to save weight on scooter conversion?

Chall Acustica

100 µW
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Feb 5, 2023
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New to building electric scooters. I am trying to convert a Yamaha Janus to a plug-in hybrid to improve fuel efficiency. The weight of having the generator (which will be modified from the original engine) will be a significant increase over just having the electric power train, but if I can use a light enough controller and hub motor, the lighter weight of petrol should outweigh the weight of going full electric at the same weight. (charging is also very hard to do here, so i am trying to avoid that)

As such, I am trying to find ways to find lightweight motors, controllers as well as trying to reduce the weight of the frame. I will be printing and machining my own mounts for most components. Any advice on this? The target weight is below 100 kilograms, since I don't think I can handle a heavier bike.
 
I doubt you can reduce fuel consumption by switching to gasoline-electric. Even if you get to run the motor at a happier RPM so it has improved gross efficiency, the conversion losses will cost you more than whatever you saved.
 
Ditto to the above; it's unlikely you'll get enough better efficiency this way to make up for the losses.

If you had a bigger vehicle you could have a battery in to hold sufficient amount of energy, you could use a smaller generator with a smaller engine that does not have to supply the entire peak power of the system, and would be lighter and more fuel-efficient. But I don't know if you have room in a scooter like this.




Chall Acustica said:
As such, I am trying to find ways to find lightweight motors, controllers as well as trying to reduce the weight of the frame. I will be printing and machining my own mounts for most components. Any advice on this? The target weight is below 100 kilograms, since I don't think I can handle a heavier bike.
What does it weigh now (since it will be heavier when you are done)?

Have you already calculated how much power you will need for it to perform as required under your specific conditions? (speed, acceleration, etc) And how many Wh/mile (or km) it will need for your conditions and riding style and performance? If not, the http://ebikes.ca/tools/simulator.html may be helpful to figure that out.


It will be lighter to use a faster RPM motor with a chain or belt reduction to the wheel than a hubmotor in the wheel, for the same power capability of motor. (plus you can regear it if you need a different top speed, or more low speed torque, which you can't directly do with a motor in the wheel).
 
The engine on this bike is 125cc, it can make 9.5 hp. The vehicle currently weighs 95 kilograms. I'm not really sure how much power iI'm going to need, since I don't know what the realistic end weight is going to be. I will certainly look into using a higher RPM motor, as well as look at that link, thank you so much!
 
Quick question on cooling. I'm not sure what motors are possible for this build, as well as what I'll need, so I was wondering about this cooling setup. The original Janus engine has a side radiator on the swing arm, as such there is no cooling in the bike itself. Would adding an intake scoop to collect ram air be a sufficient cooling solution? The drawing isn't to scale. (pink is airflow)
 

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Have you considered leaving the petrol motor in tact and adding a direct-drive front hub motor? See John in CR's fork sleeves for an idea: https://endless-sphere.com/forums/viewtopic.php?f=10&t=91615#p1336084

Upsides:
That would give you 2-wheel drive, which adds redundancy and probably better acceleration despite the added weight. More importantly, you would get regen braking, which will improve efficiency in stop-and-go traffic (regardless of whether you were using the petrol motor or the electric one for propulsion at the time).

Downsides:
You would need to figure out where to put the battery, and it couldn't be too big (for space & weight reasons, because you are only adding weight, not removing ICE components).
Also, you would need to practice not spinning your front wheel in loose conditions - just go easy on wet pavement.
 
thradad said:
Downsides:
You would need to figure out where to put the battery, and it couldn't be too big (for space & weight reasons, because you are only adding weight, not removing ICE components).
Also, you would need to practice not spinning your front wheel in loose conditions - just go easy on wet pavement.

This seems like a very appealing idea. Do you think it's possible to run that motor off a minimal battery by adding a larger alternator to the original engine? Although I suppose the issue with that is that now the engine is both running with variable RPMs as well as the extra load of the motor, which is going to be very inefficient.
What kind of battery would you recommend? LiPo? I am a bit concerned about expansion and fires with those, since this bike will be used in an environment that goes through both monsoon and hot summers.
 
There is nothing to be gained by running an electric hub motor off an alternator that is powered by an internal combustion engine. Just drive the scooter like normal and use the petrol engine to turn the rear wheel.

You wouldn't even have to connect the two systems by wires to use your petrol motor as a generator: you could engage mild regen braking with the front hub while letting the petrol motor push the scooter. The hub motor would be fighting the petrol motor, generating waste heat and hurting your efficiency, but you would be (slowly) charging your battery. But the energy you put into your battery would be considerably less than the energy you wasted burning excess petrol to charge it.

There is always waste when you convert energy from one form to another (usually waste heat), so the more times you convert, the more efficiency is lost. So charging a battery (or running an electric motor) off a petrol motor is always going to be less efficient than just using the petrol motor to turn the wheel. In stop-and-go traffic, a hybrid vehicle burns less petrol than an equivalent traditional vehicle because the traditional vehicle wastes 100% of its braking energy (heating up the brakes) while the hybrid recovers some of that braking energy in the form of regenerative braking (using the electric motor & controller to slow the vehicle while prodicing electricity to charge the battery).

Given your constraints (no large battery, not easy to charge on the road), I doubt you will be able to go much farther by adding an electric motor to your scooter. If you really want efficiency, you would be better off with an ebike that weighs a quarter if what your scooter weighs and with a battery you can detach and take inside to charge. But if you want a cool hybrid petrol-electric scooter, I think a front hub motor would be the most straightforward and robust solution.
 
thradad said:
Given your constraints (no large battery, not easy to charge on the road), I doubt you will be able to go much farther by adding an electric motor to your scooter. If you really want efficiency, you would be better off with an ebike that weighs a quarter if what your scooter weighs and with a battery you can detach and take inside to charge. But if you want a cool hybrid petrol-electric scooter, I think a front hub motor would be the most straightforward and robust solution.
I see, that does make sense. What kind of batteries could get about 200 km of range with lower weight? Currently this scooter has a 4.1L tank and a mileage of 56 kmpl (not particularly good). If the hybrid system will be less efficient I would be happy going full electric. If it helps, the current weight of the bike is 95 kg, and I weigh 45 kg. I am expecting to hold up to 80 kg extra though, in case I carry a passenger or two, or cargo, but certainly not going at full speed with extra weight.
 
Chall Acustica said:
What kind of batteries could get about 200 km of range with lower weight?

Imaginary ones?

Holding 50 km/h on flat ground (with no wind) on your scooter takes more than 1000W at the wheel. That's just to maintain speed; accelerating, climbing, riding into the wind, and negotiating rough road surfaces use more power than that. But let's say you ride around a smooth indoor track for 200 km at 50 km/h. That's more than 4 kWh at the wheel, so maybe about 5.5 kWh from the battery if everything's working just right.

The most energy-dense battery module I use contains about 2 kWh and weighs close to 12 kg. In a protective case and with its cables etc, it's more like 15 kg. If you had that same energy density in a 5.5 kWh pack, it would weigh about 41 kg.

And that's just enough battery capacity for you to ride 200 km on flat smooth ground at a steady 50 km/h speed. Add stops, climbs, wind, and rough roads, and you'll either have to go much slower, burn through the battery in much less than 200 km, or else carry much more than 41 kg of battery.

Do you ever run through an entire tank of petrol from full to empty all at one time? If that's not normal for you, then maybe you don't need 200 km range per charge.
 
Chalo said:
Chall Acustica said:
What kind of batteries could get about 200 km of range with lower weight?
Do you ever run through an entire tank of petrol from full to empty all at one time? If that's not normal for you, then maybe you don't need 200 km range per charge.

I do not, but the tank holds more than 200 km worth of petrol. For daily commute, it is unnecessary to have such a large tank, but occasionally I will be making longer trips. The battery weight is why I was thinking of the petrol/electric system mostly.
 
Chall Acustica said:
Chalo said:
Chall Acustica said:
What kind of batteries could get about 200 km of range with lower weight?
Do you ever run through an entire tank of petrol from full to empty all at one time? If that's not normal for you, then maybe you don't need 200 km range per charge.

I do not, but the tank holds more than 200 km worth of petrol. For daily commute, it is unnecessary to have such a large tank, but occasionally I will be making longer trips. The battery weight is why I was thinking of the petrol/electric system mostly.

It seems to me that you could meet your two different needs (local commuting and longer road trips) better by keeping your stinky gas scooter for longer trips, and using a lighter, more agile but shorter-ranged e-bike for around town. Trying to roll both purposes into one machine will make it worse at doing either of them.
 
Chalo said:
It seems to me that you could meet your two different needs (local commuting and longer road trips) better by keeping your stinky gas scooter for longer trips, and using a lighter, more agile but shorter-ranged e-bike for around town. Trying to roll both purposes into one machine will make it worse at doing either of them.
I did consider that, but I'm not sure if the e-bike will be able to carry extra weight. Currently for short trips I use a manual bike when I don't need to carry anything, so I don't really need to replace that. Will an ebike be able to seat a passenger or two? I am not really sure what the capabilites of ebikes are, sorry if I'm asking stupid questions.
 
Chall Acustica said:
Chalo said:
It seems to me that you could meet your two different needs (local commuting and longer road trips) better by keeping your stinky gas scooter for longer trips, and using a lighter, more agile but shorter-ranged e-bike for around town.
I did consider that, but I'm not sure if the e-bike will be able to carry extra weight. Currently for short trips I use a manual bike when I don't need to carry anything, so I don't really need to replace that. Will an ebike be able to seat a passenger or two?

I'm 203 cm tall, and I weigh about 150 kg at the moment. Any one of my three e-bikes could be rolling with 200 kg gross weight or more depending on the load I'm carrying. I'm sure at various times I've carried more than 250 kg on two wheels, or a lot more when towing one of my trailers

IMG_20200924_203630860~2.jpg

There are plenty of e-bikes that carry passengers, and of course any pedal bicycle that carries passengers can be converted to electric assist. Search "cargo e-bike" to give you a sense of how people have addressed the problem.

maxresdefault~2.jpg
 
Chall Acustica said:
I see, that does make sense. What kind of batteries could get about 200 km of range with lower weight? Currently this scooter has a 4.1L tank and a mileage of 56 kmpl (not particularly good). If the hybrid system will be less efficient I would be happy going full electric. If it helps, the current weight of the bike is 95 kg, and I weigh 45 kg. I am expecting to hold up to 80 kg extra though, in case I carry a passenger or two, or cargo, but certainly not going at full speed with extra weight.

If you use that simulator to guesstimate power usage (peak / continuous watts) and wh/km consumption under your various riding conditions, at least for average and worst case conditions (including with and without the extra passenger/cargo weight), you can then determine how many Wh your battery will have to have to give you that 200km of range under the worst of those conditions that you will still require that range, *and* the worst-case wattage the batteries must be able to provide.

Knowing the Wh tells you the "size" of the battery, and knowing the W tells you how capable the cells in it must be.

You'll want to add some percentage (I'd go with 25%+) of Wh to account for degradation due to aging (and if you have colder weather, allow for less responsiveness from that), and for detours or headwinds, etc., that decrease your range so you still have the full 200km available even under those conditions.

So if your calculations came up with say, 4kwh, you'd want 5kwh or more.

Some example calculations based on made up data, but you can substitute your own data once you have it:


Let's say your W (watts) worst case was 10kW; you'd need batteries capable of doing that for however long those conditions exist on your ride--safest to just get ones that can do that continuously.

Using the ancient (and less capable/heavier/etc than modern cells) EIG C020 20Ah 5C cells I've got on my SB Cruiser trike as an example, along with those made-up numbers above (5kWh, 10kW) a 72v pack, 20s (20 series cell groups), would take about 70Ah to get 5kWh. (V x Ah = Wh)

At 20Ah each, you'd need 80Ah to get 70Ah, or 4p. So you'd have a 20s 4p pack, or 80 cells total.

"5C" means that the cell can supply 5x the amps that the cell has in capacity (Ah), so 5x 20Ah is 100A, continuous. (twice that for a few seconds at a time). So 4p of 100A cells means 400A continuous, or 400A x 72v = 29kW, so no problem with power output.

Each cell is about 410g, so about 33kg for cells, not counting interconnects, casing, compression hardware, etc. Call it 40kg for the pack weight (could be less).

They're about 216mm x 130mm x 7mm each, x 80 cells, which can be arranged however would fit in your available space, but you can figure out the total volume, and guesstimate if that would work without adding side panniers, or building a new underseat/overwheel enclosure that would replace the existing one that has the gas tank/etc in it, to hold the cells).


Modern cells are either better specs for the same size or smaller for the same specs, etc., so it would be a little smaller / lighter than the above for those specs, or more capable for the same size/weight. But it still wouldn't be really small/light.
 
Chall Acustica said:
I did consider that, but I'm not sure if the e-bike will be able to carry extra weight. Currently for short trips I use a manual bike when I don't need to carry anything, so I don't really need to replace that. Will an ebike be able to seat a passenger or two? I am not really sure what the capabilites of ebikes are, sorry if I'm asking stupid questions.

An ebike can do anything a bicycle can do if it is well-designed for the purpose. If it's a factory-made ebike, then it depends on the purpose to which the factory created it for. Most of them are like most bicycles--created to be sold, rather than used. :( But some are well-designed for some purpose or set of purposes actually useful to people that ride them ;) ; you'd just have to see which are best suited for your specific purpose(s).

Or, you can pick a bicycle that does what you want, and convert *it* to electric, with assist enough to help you do what you can already do with it, just more easily, by amplifying your pedal power (rather than having the electrics do all the work).
 
OP: As others have said, you're not going to get any efficiency gain trying to make your scooter a hybrid.
Maybe you should consider two removable batteries.

My EV-converted Vespa weighs about 90kg as I bought it from the ES member JimVonBaden. He equipped it with a 2.7kwh battery that weighs about 14kg, which is easy enough to carry inside to charge. The battery is connected with an Anderson SB175 (upgraded from the SB50 it had when I bought it) connector.
I have built a second battery of the same dimensions, though chose slightly lower Ah, but, higher current cells (in retrospect, should've used the same cells). I now have about 5kwh of total battery capacity, and only have to carry 14kg at a time to bring inside to charge (though, I just charge them on the scooter). Total weight once I have the bracket made to mount the second battery under the right rear fender will be about 110kg.

It has a 4KW QS motor that can do bursts up to 15kw. Performance is pretty impressive, just quicker than the original petrol motor I'm sure. Top speed is 100 kph.

Range with the original battery is about 50 km if I keep speeds around 72-75 kph average. I'm expecting range to be around 110 km with both batteries as there will be much less heat loss in the batteries due to lower amps per battery.

The 15A charger I just purchased only weighs a few pounds and is small enough to take with me if I need to charge at the office, which I won't need to do with the range I'll be getting (plus, I'm hopefully changing jobs soon and will be 100% WFH anyway).
 
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