Increasing top speed by overvolting?

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05silgto   1 mW

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Increasing top speed by overvolting?

Post by 05silgto » Jun 26 2020 5:21pm

Hello, I recently converted a fatbike to a fat-ebike. However, top speed isn't quite where I want it. My top speed is about 27mph on flat ground. I don't think there is a speed limiter, When I lift the motor wheel and hit the throttle, the speedometer shows about 34mph, so unless watts is being limited at higher speed, there isn't a speed limiter. It was a 1500w brushless motor kit with a 48v kt controller 22A continous, 45A Peak. Seems like my top speed should be higher with this setup but maybe not.

I was thinking about getting a 72v kt controller so that it's compatible with my lcd display and keep current the same at 45A by limiting max current through my display. The only problem is I already ordered two 48v 14ah 18650 battery packs from China a while ago and they should be arriving soon(I have been using 4 8ah lead acid batteries connected in series for the meantime). So, if I want to go to 72v, I would either need to disassemble the battery packs and reassemble them as 20s, or get two dc voltage boost converters rated for 1800w and connect them in parallel which is easier said than done due to having to implement load sharing, otherwise one would take all the load and burn up. Going the converter route would also be less efficient, take up more space, I wouldn't be able to monitor voltage on my display as voltage would be kept constant and brake regen won't work. I also plan on installing a temp. sensor in the motor.

Any ideas? Any feedback is appreciated.
Fyi, my end goal is around 40mph, this motor doesn't have to get me there alone as I plan on adding a 2nd motor in the future but it needs to have enough rpm to hit 40mph which it probably will after overvolting it.

05silgto   1 mW

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Re: Increasing top speed by overvolting?

Post by 05silgto » Jun 26 2020 5:27pm

What if I connected my two 48v batteries in series?
I just came across this link. https://electricbike.com/forum/forum/as ... ah-battery

What would I need to look for in the controller to see if it can handle 96v? Capacitors, and what else?

05silgto   1 mW

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Re: Increasing top speed by overvolting?

Post by 05silgto » Jun 27 2020 4:48am

I think I'm just going to order a 72v kt controller and reassemble my battery packs.
Only problem I have now is my kt lcd8h display is only rated for 36v/48v setups. I don't know if I can directly hook up 72v. I could use a voltage divider/voltage regulator to power the display but then I'm not sure if the display would communicate correctly with the controller. Any thoughts?

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Re: Increasing top speed by overvolting?

Post by docw009 » Jun 27 2020 12:42pm

I'm pretty sure the Kt displays contain an onboard voltage converter to power the LCD circuits and the onboard uart for serial data.

If you drop the voltage to the display, the unit will return that voltage to the controller. where it's used tp power all the lower voltage circuits in the controller. It in effect, turns on the controller. Someone reverse engineered a 6FET KT controller and showed this. The power to the MOSFET's is always there, by the way. Schematic attached.

In my opinion, your scheme for reducng the battery volage to the display would work, if you used an active circuit to lower the voltage, like another LM317T. A resistor divider might not pass enough current to run display and also the controller.
S06S_schematic.pdf
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05silgto   1 mW

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Re: Increasing top speed by overvolting?

Post by 05silgto » Jun 27 2020 4:26pm

Cool, thanks for the info. I guess I can use a voltage regulator, I wanted to avoid that at first so the voltage at the display isn't held constant making it easier to tell how much range is left and have the controller cut off under low voltage. I suppose I could just wire up a voltage divider to a Arduino, and then to a lcd to display remaining range. I could also maybe set up a jumper on the controller so a display isn't necessarily needed and add a relay with a regulator so all I need to do is flip a switch when I want to change options and the controller will perform normally otherwise.

I don't understand why kt sells a 72v controller but none of their displays are listed as being compatible with 72v. Kinda makes me want to try it but I don't want to risk frying my display.

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Drunkskunk   100 GW

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Re: Increasing top speed by overvolting?

Post by Drunkskunk » Jun 28 2020 4:47pm

Your speed Unload VS loaded is about right for a 48V kit. You should pick up a bit more speed from a Lithium battery over your lead acid battery, because of less voltage sag under load, but not much. Running both 48 volt batteries in series would get you over 45mph.
But 40mph takes roughly 3 times more power than 25mph. About 2300 watts of power vs the 800w or so you use at 25mph. You will also need a high voltage controller. Two 48V 13s lithium batteries fully charged in series is 109.2 volts.

You're also going to have another problem. Most Fatbike tires come apart at high speeds. I've had many simply fall apart in a couple miles above 45mph, and often the best are worn down to the thread around 100 miles. I'm not talking about cheap tires either. I've run Surley, Maxxis, Vee Rubber, 45NRTH, Kenda, Sterling, etc, etc, etc.

It's not a lack of quality, it's that the construction and rubber compounds used are optimized for slow speeds and high flexibility, but the trade off is high speed rigidity, stability, tolerance for heat, and ability to shed heat.
Buy the ticket, take the ride.
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pullin-gs   100 W

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Re: Increasing top speed by overvolting?

Post by pullin-gs » Jun 28 2020 5:30pm

I'm a big fan of overvolting. Be careful though because you can fry your electronics&motor if you overdue things too much.
Familiarize yourself with a motor analyzer tool where you can input motor specs, battery, controller information and test "what if" overvolting scenarios. I use ebike.ca's motor simulator tool.
My 36V motor is rated for 500 watts and controller is rated for 1000W.
I overvolt with a 12S pack (50 volts off the charger) and dump as much as 800+ watts into it carefully monitoring heat and run-time.
With stock 10S 36V pack (42V off charger) I get 22 MPH max.
Overvolted I get over 26 MPH.....not too shabby for an e-trike on a "500W" motor.
ebikes_CA_Motor_Simulator.jpg
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20200527_122815.jpg (198.18 KiB) Viewed 225 times

maanebedotten   10 W

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Re: Increasing top speed by overvolting?

Post by maanebedotten » Jul 04 2020 9:42am

You can get a little bit more speed by using a controller with field weakening capabilities. I have no experience with this so I dont know how much speed you can gain but it might be worth to dig for some info on it.

I would definitely not feel safe going 40 mph on a fatbike though :D They are usually not constructed for high speeds. I ride a heavy steel cruiser frame with mc rear wheel/tire and heavy steel fork with 26" 3.5" wide street front tire with beefy 200mm discbrakes, and anything above 30 feels a bit sketchy even for this bike with a much more stable geometry for speed than a fatbike. To feel safe at 40 mph with bicycle components I would need a beefy downhill front fork with dual 200mm discs and beefy downhill wheel. What do you think will happen when you hit a pothole at 40mph with your fatbike front wheel? Best case it bounces and you have a chance to bring it under control. Most probably your wheel will instantly deform throwing you off the bike and you wake up in the hospital.
I love high speed ebikes, just make sure you have components that can handle the forces involved.

05silgto   1 mW

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Re: Increasing top speed by overvolting?

Post by 05silgto » Jul 04 2020 6:01pm

maanebedotten wrote:
Jul 04 2020 9:42am
You can get a little bit more speed by using a controller with field weakening capabilities. I have no experience with this so I dont know how much speed you can gain but it might be worth to dig for some info on it.

I would definitely not feel safe going 40 mph on a fatbike though :D They are usually not constructed for high speeds. I ride a heavy steel cruiser frame with mc rear wheel/tire and heavy steel fork with 26" 3.5" wide street front tire with beefy 200mm discbrakes, and anything above 30 feels a bit sketchy even for this bike with a much more stable geometry for speed than a fatbike. To feel safe at 40 mph with bicycle components I would need a beefy downhill front fork with dual 200mm discs and beefy downhill wheel. What do you think will happen when you hit a pothole at 40mph with your fatbike front wheel? Best case it bounces and you have a chance to bring it under control. Most probably your wheel will instantly deform throwing you off the bike and you wake up in the hospital.
I love high speed ebikes, just make sure you have components that can handle the forces involved.
I don't plan on going 40mph constantly, just want the ability to go that speed and use it when necessary or when it is ideal to do so.
I usually stay at 25-30 right now. I connected another lead acid battery in series to make 60v until my lithium batteries get here. I can now hit around 35mph fully charged even with about 10v of voltage sag. When I get my 72v controller and lithium batteries set up I should be able to hit 40mph easily.
http://www.bikesdirect.com/products/mot ... sprung.htm
This is the bike I'm using but with a 180mm rotor in the front I believe and a 26'' 4.5 wide inch tire in the front and about a 5 inch in the rear. As long as you can lock up the brakes then you have enough rotor, then all that matters is how much grip and weight you got that determines how fast you stop. Feels pretty stable with this bike all things considered. The front springs are pretty incredible, 120mm of travel and it just floats over everything. Then again all my weight is in the back until my lithium batteries get here. I haven't encountered a pothole yet on the bike trails I ride but it is possible in the rare times I ride on the street to go from bike trail to bike trail. I feel like its safer going 40mph with traffic than going 20mph, I'll just have to keep an eye out for potholes vs getting hit by an idiot driver. I live in one of the surrounding communities near Houston so drivers are the real hazard out here.

maanebedotten   10 W

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Re: Increasing top speed by overvolting?

Post by maanebedotten » Jul 05 2020 2:31pm

That looks much better suited for speed than most fatbikes I have seen. Decent fork that probably has good enough damping, through-axle hub that should be able to take a beating. I have tried a similar tektro hydraulic brake though, and no way could I lock the front at high speed with 180mm rotors, going to 200mm helped, but I still ended up going back to my TRP Spyre mechanical brake. I have not done proper "scientific" testing, but it feels more powerful and controllable to me than the tektro did. It also seems to keep better power when heating up on long descents. My bike is a lot heavier than yours, I dont have the strongest grip and prefer using 2 fingers so I probably need more bite and heat shedding than most though.

How do you plan to add the second motor btw? I have never tried it myself, but the general advice is to avoid putting a motor in a suspension fork so your options are limited. Considering the complexities of a dual motor setup it is definitely easier and maybe even cheaper to just buy a properly sized motor for the power you want to push.

05silgto   1 mW

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Re: Increasing top speed by overvolting?

Post by 05silgto » Jul 05 2020 11:36pm

maanebedotten wrote:
Jul 05 2020 2:31pm
That looks much better suited for speed than most fatbikes I have seen. Decent fork that probably has good enough damping, through-axle hub that should be able to take a beating. I have tried a similar tektro hydraulic brake though, and no way could I lock the front at high speed with 180mm rotors, going to 200mm helped, but I still ended up going back to my TRP Spyre mechanical brake. I have not done proper "scientific" testing, but it feels more powerful and controllable to me than the tektro did. It also seems to keep better power when heating up on long descents. My bike is a lot heavier than yours, I dont have the strongest grip and prefer using 2 fingers so I probably need more bite and heat shedding than most though.

How do you plan to add the second motor btw? I have never tried it myself, but the general advice is to avoid putting a motor in a suspension fork so your options are limited. Considering the complexities of a dual motor setup it is definitely easier and maybe even cheaper to just buy a properly sized motor for the power you want to push.
Thanks, It handles high speed pretty well. Feels pretty decent compared to my friend's Ripcurrent S. Weight distribution is horrible right now though with 25lbs of lead acid batteries on the back rack. On the plus side I can do a wheelie pretty easily 8) .
The 2nd motor will be added in the future if I ever do it. Not sure if it is necessary now. As long as I can maintain a decent speed and hit close to 40 I don't see a reason to add one. It would only hurt efficiency and the only benefit would be 2wd. I don't think I will ride in enough mud to make up for the drawbacks. If I did, my ideal setup would be a geared motor in the back and a brushless motor in the front. As long as all components share a common ground I should be able to use the same throttle and pas for both motors/controllers.

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