Low end torque weak on Stealth Bomber clone bike

[ebikeFan1234]

Hi folks,

I have an 8 KW Stealth Bomber clone bike. I love many things about the bike. The part that I don't like as much is the low end torque. The acceleration from 0-15 mph is very weak, esp in comparison to similar powered bikes like Talaria Sting R. I have the 2.5T motor with 21" inch wheels, see pics for more details.

Are there any options to improve the torque in 0-15 mph torque or make the throttle more responsive, possibly by tweaking controller setting?
Or do I mainly need a new motor kit with 4T/5T kind of motor - this will be an expensive option.

From the pic, do I have the QS273 or QS205 motor?

Pls advise

 

[neptronix]

Typically this is a matter of tuning the controller to use higher phase amps than stock. has it always had a low amount of initial torque?

what voltage / amp hours battery do you have?

 

[ebikeFan1234]

Yes it’s been like this from the first mile on the bike
Battery is 72v, 40 ah

I bought the bike new and assembled from a person who isn’t available for support. I don’t have the history of how they put it together. I imagine it’s a standard stealth clone bike

Also these motors have regen function right? I would like to install regen brake on the handlebar if it a simple enough project

 

[ebike4healthandfitness]

From the pic, do I have the QS273 or QS205 motor?

View attachment 357584

That is a QS273 40H. It is rated at 4000 watts continuous power and 8000 watts peak power.

2.5T winding on a 21" moto rim at 72v is very tall for something as unaerodynamic as a stealth bomber.

So yeah, you are sacrificing quite a bit torque by using that winding yet your chassis is not able to make full use of the top speed that winding and tire diameter (edit: 27.3" diameter not 26") is capable at 72v. At 8000w peak your stealth bomber should top somewhere around 60 mph yet that hardware combo you are using is targeted at vehicles capable of going much faster than that at the same 8000 watts.

 

[ebikeFan1234]

That is a QS273 40H. It is rated at 4000 watts continuous power and 8000 watts peak power.

2.5T winding on a 21" moto rim at 72v is very tall for something as unaerodynamic as a stealth bomber.

So yeah, you are sacrificing quite a bit torque by using that winding yet your chassis is not able to make full use of the top speed that winding and tire diameter (est. 26") is capable at 72v. At 8000w peak your stealth bomber should top somewhere around 60 mph yet that hardware combo you are using is targeted at vehicles capable of going much faster than that at the same 8000 watts.

Thanks for the very insightful answer!! I am assuming I have a Savoton 150A controller since I have seen a peak power of 9.6KW on my bike's SW900 display. Note: I am not savvy to open the frame and examine the hardware. I have a local mechanic who can do that.

My goal is to have a fast accelerating bike in 0-30 mph range like in 3 seconds (Talaria Sting R with 8KW peak does that). Plus I need strong power delivery in the 0-55 mph range. My current bike is pretty strong after 20 mph. But the initial start is very slow, anemic at best.

What options do I have to reach that goal with the least cost?

Option 1 - Tune the existing controller to deliver more phase amps at startup? As I mentioned above battery is capable of 150 amps approx.

Option 2 - Buy a beefier controller

Option 3 - Swap the current 2.5T motor to a smaller rim like 19" (and a wider tyre - I like such a look!)

Option 4 - Buy the same motor with a higher T like 4T (5T won't be desirable due to 45 mph top speed). Or step up to 8000W nominal (12 KW peak) with 4T. And use the existing 21" rim

Option 5 - Option 4 + smaller 19" rim

Please advise!

 

[ebike4healthandfitness]

5T won't be desirable due to 45 mph top speed

That is the unloaded speed at 72v nominal with 24" tire. (Kv is 8.855 for 5T). Your 80/100-21 tire measures 27.3" in diameter so unloaded speed at 72v nominal for 5T would be 51.8 mph. When your 20S (72v nominal) battery pack is fully charged to 84v unloaded speed would be 60.43 mph.

Of course, under a load mph will be slower than what I wrote above.

P.S. Kv of your 2.5T motor is 17.71 (I.e. double the Kv of a 5T motor)

 

[ebikeFan1234]

Thanks for the insights! What would be your recommendation for my goal - fastest acceleration in 0-55 mph zone.

Can I get more low-end torque out of the current QS 273 4KW 2.5T motor by tweaking the controller. The motor with the current settings feels anemic in 0-15 mph zone.

OR buy a new motor like QS273 4KW with 4T/5T

Here's a Youtuber who moved from 3.5 to 5T and says 5T is great for 0-25 mph acceleration and then it tapers off. After watching the video, perhaps 4T is a better fit for my needs

 

[Chalo]

Electric vehicles with only one gear ratio must live with the power curve they have. For optimum initial acceleration and efficiency, you decide what's the lowest to speed you can live with, and you set up the motor kV (RPM per volt) and battery voltage to deliver only that much top speed when everything is at full blast. Typically that's an unloaded speed about 25% higher than the top speed you want.

Then you configure the controller (if it's configurable) to deliver the maximum phase amp ratio that it/your motor cables will tolerate. Also, use a battery that can dish out more current than you ever ask of it, so the voltage won't sag more than necessary when you stomp on it.

The moral of the story is the more unused speed range you have on top, the weaker acceleration and low speed climbing will be.

 

[E-HP]

Thanks for the very insightful answer!! I am assuming I have a Savoton 150A controller since I have seen a peak power of 9.6KW on my bike's SW900 display. Note: I am not savvy to open the frame and examine the hardware. I have a local mechanic who can do that.

My goal is to have a fast accelerating bike in 0-30 mph range like in 3 seconds (Talaria Sting R with 8KW peak does that).

These peak values are based on battery current, so don't directly correspond to the phase current required for acceleration.

 

[neptronix]

Most motors run a 3:1 ratio of phase amps to battery amps, IE 90 phase amps, 30 battery amps

The ratio will determine how much power you have from a stall.

A 3:1 ratio will result in a torque curve that's extremely peaky and looks like this:


You run a 2:1 ratio and you get a ton of the initial torque lopped off..



If you have anemic response and a stuttery start, the reason might be because the motor is in sensorless mode. Sensorless controllers limit initial startup torque a lot vs wheat settings you inputted.

If you don't have a stuttery start, the battery is maybe not large enough to deliver the current and it was detuned to account for that, or if you're very lucky, the phase amp to battery amp ratio is 1:1 and it is just poorly programmed.

 

[ebikeFan1234]

To make this more objective, it would be great to have published numbers for 1/8 mile run, with metrics being 0-30 mph time, total time and top speed of the run. I have seen some Youtubers do that, esp for Talaria and Surron.

Back to the Bomber, changing the battery would be very intrusive option. I will be more inclined to change controller or motor, and approach it in a step by step manner.

Would changing the motor to 4T (from 2.5T currently, 4 KW nominal, 8 KW peak) achieve a big portion of my goals? Everything else remaining unchanged

I assume my bike has a Savoton 150A controller. Would a beefier controller help? I understand battery may be a limiting factor but I don't have detailed specs other than 72v, 40 ah

 

[neptronix]

Yes, when you go to a higher winding count ( that's significantly higher ), the result will be lower speed but proportionally higher torque per amp.

If you do have a sabvoton, your best route is to program it, and they're not rocket science to program.

If you could tell us what cells your pack is made of, we can make a good estimation of what the ideal maximum amperage for it would be.

Too bad you don't have detailed specs. We'll help you build the next bike so that you know what you're actually working with.

 

[ebikeFan1234]

Great tips, I really appreciate this forum!!

Btw are the specs written on these batteries, otherwise what is a way to find out? What about BMS max current? Someday I will open the frame to look at the battery or have my mechanic do it. I understand I should know the battery and controller specs. And then our discussions will become much more precise.

 

[neptronix]

You can find a cell model number by opening the pack. Would be easier to ask the vendor or perhaps hunt down the original sales listing - hopefully it included some specifications.

 

[ebikeFan1234]

It's clear from this thread that there are various approaches to hit my goals, with different levels of efficacy

for a moment, let's assume I want to install a new stronger motor on my rear wheel. I have several questions related to it

1. My existing bike (4 KW) has a dropout of 152 mm. If I buy the 8 KW motor, its spec indicates 200 mm dropout. Does this mean this motor won't work on my bike?

2. Where would you buy these motors from, qsmotor.com, nbpowers.com or aliexpress/alibaba. I don't have experience with either of them.

3. I see some eBay listings for QS273 8KW version, but there's limited choice of Ts - 3.5T and 5T only. I was looking for 4T. Does this mean its best to contact the manufacturer directly and hopefully they will be more flexible

 

[neptronix]

50mm is way too far to stretch dropouts!

I would guess that the existing motor could handle a lot more power, your problem is in the controller/battery. Most motors can take 2-3 times their rating. Your motor is probably very underpowered and if it runs barely warm/cool, you're not pushing it enough

Having a screen that shows voltage / wattage in realtime can help us find out what power level is appropriate for your bike. Observing the voltage drop from a standstill and then calculating how much it dropped at full throttle can tell us how much power your existing battery can handle.

 

[ebikeFan1234]

This is what my display shows (SW900). During rides, I have seen the wattage climb to 9.6 KW tops for few seconds, and it reaches 6-7 KW easily. At such KW, the speed builds up real fast and when it's 45+ mph, usually I back off.

From standstill when I apply throttle i see initial numbers like 500w, 1.2KW and it grows gradually. I guess this is the crux of the problem

The display shows Mode = ECO. I wonder whether there is a SPORT mode as well

 

[neptronix]

This is what my display shows (SW900). During rides, I have seen the wattage climb to 9.6 KW tops for few seconds, and it reaches 6-7 KW easily. At such KW, the speed builds up real fast and when it's 45+ mph, usually I back off.

Bummer that you don't have a display of voltage. We can't figure out your battery's capacities then.

From standstill when I apply throttle i see initial numbers like 500w, 1.2KW and it grows gradually. I guess this is the crux of the problem

Sounds exactly like not enough phase amps, but could also be a problem that you're running sensorless mode.

 

[ebikeFan1234]

What should I do next to root cause the problem?

Should I install a different display that has more metrics

 

[neptronix]

I'd get the appropriate programming cable if you confirm you have sabvoton and change those battery / phase amp values.

I would buy some kind of realtime voltage meter so you can determine the voltage drop of the battery so we can see what it can handle.

More expensive, but very good: CA3-HC
Cheap but might not be able to handle your voltage: Turnigy 180A Watt Meter and Power Analyzer

 

[shap08]

you could also check your charger and see what voltage it shows

 

[ebikeFan1234]

This is the charger

 

[AGS]

You should be able to get well over 70mph with blistering acceleration from your bike if the controller is set up properly and you should be blowing Surrons and Talarias away with ease.

You need to confirm what controller you have, both make and model.

It looks like you are running in eco mode all of the time, which is limiting both acceleration and top speed.

If you have a Sabvoton controller you need to check the wiring on your controller to see if it has 3 wires for a 3 speed switch. Some Sabvoton suppliers provide the wiring for the 3 speed option and some don’t.

The 3 speed switch wiring on a Sabvoton should be a pink, black and transparent wire connected to a 3 pin connector. Pink connected to black will be high speed boost mode, Transparent connected to black will be eco mode. Nothing connected will be normal mode. So if you find a 3 pin plug with transparent linked to black, pull it out and see if it runs better.

If you don’t have the wiring then the 3 speed mode can be disabled in the settings. But you will need a Bluetooth dongle or a pc programming cable to adjust the controller settings. These should have been supplied with the kit to the person who assembled the bike for you.

Presumably you set it to the highest pas level as soon as you switch on. Or you can just disable pas in SW900 display and run on throttle only, which may be more appropriate on your bike.

Be careful about buying a new display. Different Sabvoton suppliers have their own firmware and only the display they supply will work with their controller, so it’s not a good idea to try to mix and match displays and controllers between different suppliers.

 

[thebes]

Im surprised that the person who put together the bike opted for a 2.5t with such a puny controller (assuming that its a 72150 as you stated in your other post and that its running at its limit of 350pA and 150bA) as the 2.5t has a very high kv (as was stated previously, ~17.71) which translates to something around a hundred mph at 72v with your wheel size which youre very unlikely to hit as the controller just cant push enough power for that.

This combination of a high kv and not very many phase amps is the main cause of very little torque in the low to mid end. My two cents are if you want more torque you have two options, either swap out your current 273 for a 3.5t or 4t, it shouldnt drop your speed much if at all as youll be constrained by power before you are by the wind, or, option two, upgrade your controller/battery to something that can dump way more phase amps, fardrivers seem to be what most people opt for today...something like an nd72850 would probably do the trick.

Before anything though, as it was already mentioned, would probably be best to identify exactly what controller and battery youre running to see if anything can be done there.

 

[dominik h]

The ND72850 can do >30kw and is huge. I do not know if the 10mm² phase wires from the 4kW QS273 can handle 850A
I would vote for the ND72680. It will fit where the SVMC72150 was installed. I melted my 13.3mm² phase wires with the ND72680.

First pic SVMC72150 and ND72680.
Second pic SVMC72150 SVMC72260 and ND96850 same size as ND72850