Why are the DaVinci drives used?

G

glenn0010

100 W
Joined
Jul 1, 2015
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117
Location
Mgarr, Malta
Hi all,

I have been trying to figure out why the DaVinci drives are so popular.

I have been looking on this form and on their website and all I've found is that they are used for some reduction. This however, is very confusing to me. The gearing seems to go from small to large, small to large and then to small again. How is the reduction achieved. Sorry if this sounds stupid but I seem to be getting rusty on my mechanical engineering. (Electronics and Control Degree Student).

Glenn
 
Assuming you are talking about Matts design?
It is a 2 step reduction to achieve a total reduction that is hard to achieve with one stage. (With the tech that is used)
They are quality components and uses a motor that has hard-to-beat power to weight ratio.
The engineering challenge lies in that the motor need a large reduction (it spins fast).

Another quality drive featuring these motors is the tangent ascent which uses different tech to do the gearing and can achieve the desired ratio in one stage.
Thus making a more compact design. But this type of gearing comes with a sound signature.

Both the davinci and tangent drives are low volume high priced drives.
Maybe popular (debatable, as most ebikers doesn't even know they exist) but not high volume compared to more typical ebike stuff.
 
Motors operate best at higher RPMs, and let's just say that 3,000-RPMs is a fairly normal average. A 26-inch wheel doing 28-MPH is turning somewhere around 300-RPM.

In order to get a high speed motor to drive that wheel, you'd need a 10:1 reduction.

Sprockets that have less than 12 teeth are unusually loud, so let's put a 13T sprocket on this theoretical motor. You would need a 130T sprocket on the final output to achieve a 10:1 ratio. It is difficult to find a 130T, and custom made units are expensive, plus they are hard to fit.

The design becomes easier to fit and much cheaper by breaking the reduction into two stages. If the first stage is 2:1, then the second stage only needs to be 5:1

In a 2-stage reduction, the first stage experiences higher speeds, but lower torque...the second stage experiences higher torque and lower speeds.

If you use a chain on both stages, then the high speed first stage will be loud, so...Matt uses a belt on the primary stage. You can use a belt on both stages, but a chain on the secondary can take more power.

Once you put together a dual stage reduction, you can immediately source the biggest reduction sprockets and pulleys that exist on the shelf (so you don't have to pay custom prices), and then...you can have more than 10:1 reduction (if you like). Lightning Rods kit has a 33:1 reduction, which seems to be the max possible.

At the other end of the scale, large direct drive hubmotors spin at the RPMs of the wheel. There are benefits and drawbacks to either method...
 
Maybe you mean NuVinci?

https://en.wikipedia.org/wiki/NuVinci_Continuously_Variable_Transmission
 
Hi all thanks for your great advice,

Wheazel yes they are quite high priced and bulky though the tangent is really compact and sexy.

Spinningmagnets thanks for the great in depth explanation on how it works especially on why belts and chains are used I did not know that.

Hi fetcher thanks for the reply, no I was referring to the DaVinci drives as the two gents explained above, thanks :)

So here is my issue now. I am building my first e bike and I am just completing my motor mountings. I have a 1kW 48V mid drive which spins at 3000rpm. It has an 8 tooth sprocket and I have mounted a 42 tooth sprocket at the rear wheel. Now I am realizing that I may have geared my bike too tall. In the Image below, you cans see my calculations, I think they are correct and 70kph is wayyy to much for a 1kW motor.



Any recommendations?
 
I think you've discovered the purpose of the dual stage reduction drive. 8T is a very small sprocket to begin with, it's going to make alot of noise and probably cost some efficiency while doing so. You could try to find a larger sprocket for the rear but I think you're still going to have trouble finding anything larger than 60T. You probably don't want to go with a smaller (20") wheel, as that's basically a different bike.

You need another stage of reduction, then you could replace the 8T motor sprocket with something a bit more reasonable and keep your 26" wheel. A simple jackshaft would do the job, but you'd need to flip the motor around somehow so the sprocket is on the other side. Your target wheel speed is probably somewhere around 350RPM.
 
When your system is loaded down with driving the bike at top speed, it will likely achieve about 80% of the unloaded speed so...70 X 0.80 = 56-km/h (34-MPH)
 
spinningmagnets said:
When your system is loaded down with driving the bike at top speed, it will likely achieve about 80% of the unloaded speed so...70 X 0.80 = 56-km/h (34-MPH)
Click to expand...

Hi spinningmagnets,

So I have checked the specs of my midrive again and it turns out its no load speed is 1600RPM (bought the motor a while back) so I think my gearing is spot on.

It gives a speed of 37 kph (~23mph) so under load (*0,8) should be around 30 kph or 18.4 mph.

This should be fine for a 1kW motor right? And shouldn't stress the battery and controller too much with high current draw cause the gearing sounds about right

Glenn.
 
glenn0010 said:
spinningmagnets said:
When your system is loaded down with driving the bike at top speed, it will likely achieve about 80% of the unloaded speed so...70 X 0.80 = 56-km/h (34-MPH)
Click to expand...

Hi spinningmagnets,

So I have checked the specs of my midrive again and it turns out its no load speed is 1600RPM (bought the motor a while back) so I think my gearing is spot on.

It gives a speed of 37 kph (~23mph) so under load (*0,8) should be around 30 kph or 18.4 mph.

This should be fine for a 1kW motor right? And shouldn't stress the battery and controller too much with high current draw cause the gearing sounds about right

Glenn.
Click to expand...

Yes 1kW usually is up to 50km/h capable for upright bikes. Fat tyre bikes and bikes with the aerodynamics of a barn door will achieve less of course.
And as you probably know, the curve is not linear. So at 30km/h expect less than 600W draw.
 
Yeah I know the curve is not linear, at higher speeds the power draw raises exponentially.

Perfect than at 30kph it should be perfect :)

Thanks all for the advice :)
 
Hi all,

My motor will be connected to the rear wheel directly and there are no freewheels. Therefore when going down the hill the motor will keep on turning with the wheel.

Will this be a problem? I bought an off the shelf controller and I don't know if it has regenerative braking (doubt it). If it doesn't, it shouldn't be a problem correct? As the battery should charge at 100% regen through the body diodes of the mosfets correct?

This is the controller
http://e-bike-diffusion.com/index.php?module=produit&prd_id=526&url_retour=http%253A%252F%252Fe-bike-diffusion.com%253A80%252Findex.php%253Fmodule%253Dcategorie%2526code_cat%253D27%2526page_start_num%253D2

Thanks
 
If the voltage of the motor as a generator exceeds the voltage of the battery at that time, then the controller will begin to act as a 3phase rectifier, pulling current from the motor and heating both the motor and the controller (at a greater rate than normal motor usage would) while it charges the battery, and slows the bike down.

As the speed drops, the current drops, and the drag on the motor drops, so there will be a balance where drag vs slope you're on reaches a certain speed, and the battery pulls current from the controller/motor at a certain rate depending on the battery voltage and pack's internal resistance.

Since it depends on those different factors, the balance speed will be different depending on charge state of the battery, the slope you're on (and any wind), etc.

Also, if the battery is already full, and you go down a hill steep enough to cause this generator rectification at sufficient current, it could overcharge the battery. If you know you will be going down such a hill before using up power from the battery, you'd want to start out at less-than-full by however much you'll get back out of it (experimentally determined).

If there's a BMS that cuts off charge at a certain voltage, this won't happen--but it will also cause you intermittent braking/no braking any time this occurs, and the response time of the BMS to re-allowing charge will determine the cycle time of any "shuddering" you might get out of it because of that.


Also, if the current flow is higher than your battery's max charge rate, and the BMS doesnt' prevent it, that could also cause problems (probably not immediate, but eventually).



BTW, if there's no freewheel, then also when pedalling (if you do) you'll also feel the "cogging" resistance of the motor even if not exceeding the speed taht causes braking. Depending on the motor's actual cogging resistance times the ratio of gearing between motor and road, it could be a little, or a lot.
 
amberwolf said:
If the voltage of the motor as a generator exceeds the voltage of the battery at that time, then the controller will begin to act as a 3phase rectifier, pulling current from the motor and heating both the motor and the controller (at a greater rate than normal motor usage would) while it charges the battery, and slows the bike down.

As the speed drops, the current drops, and the drag on the motor drops, so there will be a balance where drag vs slope you're on reaches a certain speed, and the battery pulls current from the controller/motor at a certain rate depending on the battery voltage and pack's internal resistance.

Since it depends on those different factors, the balance speed will be different depending on charge state of the battery, the slope you're on (and any wind), etc.

Also, if the battery is already full, and you go down a hill steep enough to cause this generator rectification at sufficient current, it could overcharge the battery. If you know you will be going down such a hill before using up power from the battery, you'd want to start out at less-than-full by however much you'll get back out of it (experimentally determined).

If there's a BMS that cuts off charge at a certain voltage, this won't happen--but it will also cause you intermittent braking/no braking any time this occurs, and the response time of the BMS to re-allowing charge will determine the cycle time of any "shuddering" you might get out of it because of that.


Also, if the current flow is higher than your battery's max charge rate, and the BMS doesnt' prevent it, that could also cause problems (probably not immediate, but eventually).



BTW, if there's no freewheel, then also when pedalling (if you do) you'll also feel the "cogging" resistance of the motor even if not exceeding the speed taht causes braking. Depending on the motor's actual cogging resistance times the ratio of gearing between motor and road, it could be a little, or a lot.
Click to expand...

Tahnks so much amberwolf for this really in depth description. This is my first e bike build so I'm trying to keep it as simple as possible. I will learn from it and hopefully make a new and improve mach 2.

No joke next to youtube this forum is the best thing on the internet

Thanks all
 
Wheazel said:
Assuming you are talking about Matts design?
It is a 2 step reduction to achieve a total reduction that is hard to achieve with one stage. (With the tech that is used)
They are quality components and uses a motor that has hard-to-beat power to weight ratio.
The engineering challenge lies in that the motor need a large reduction (it spins fast).

Another quality drive featuring these motors is the tangent ascent which uses different tech to do the gearing and can achieve the desired ratio in one stage.
Thus making a more compact design. But this type of gearing comes with a sound signature.

Both the davinci and tangent drives are low volume high priced drives.
Maybe popular (debatable, as most ebikers doesn't even know they exist) but not high volume compared to more typical ebike stuff.
Click to expand...

Is tangent a type of drive or is it a company?
 
"It's a company that uses a cycloidal reduction to have a large RPM reduction in a tiny and light package..."

Thank you Spinning Magnets, that's a really valuable piece of information. Cycloidal reduction drive is very interesting.

Well done, Sir.
 
Cycloidal drives are tiny and can have huge reduction ratios, but they're lossy. There's a lot of wiping going on in there.
 
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