New to fast electrics, selecting a motor/drive/battery?

[Roog]

I have a slightly unusual requirement, I would like to build an electric tethered car, the issue Is, there isn’t much of a precedent set, and those that have built them seem to be aiming for speeds much higher than I can safely run. I.e. 200 mph. 100mph is fast enough for me.

the car doesn’t need to run for a long duration, say 1 to 2 mins, but shouldn’t self destruct!

wheel diameter is likely to be 80mm, torque isn’t such a big issue but it needs to get up to speed on a smooth concrete track within 10 seconds.

I have read all of the brushless motor guides I can find but I am at a bit of a loss as to what motor/drive/battery to buy.

I have two trains of thought, inrunner motor with right angle drive reduction gearbox or two outrunner motors bolted back to back, direct drive to the rear wheels.
I'm not sure if the outrunner motors would have sufficient torque to get moving but it would simplify the build. DIY gearboxes present a challenge.

Any sources of self contained gear box with rear axle (no diff required) would be helpful.

any ideas welcomed.

 

[tudordewolf]

I have a slightly unusual requirement, I would like to build an electric tethered car, the issue Is, there isn’t much of a precedent set, and those that have built them seem to be aiming for speeds much higher than I can safely run. I.e. 200 mph. 100mph is fast enough for me.

the car doesn’t need to run for a long duration, say 1 to 2 mins, but shouldn’t self destruct!

wheel diameter is likely to be 80mm, torque isn’t such a big issue but it needs to get up to speed on a smooth concrete track within 10 seconds.

I have read all of the brushless motor guides I can find but I am at a bit of a loss as to what motor/drive/battery to buy.

I have two trains of thought, inrunner motor with right angle drive reduction gearbox or two outrunner motors bolted back to back, direct drive to the rear wheels.
I'm not sure if the outrunner motors would have sufficient torque to get moving but it would simplify the build. DIY gearboxes present a challenge.

Any sources of self contained gear box with rear axle (no diff required) would be helpful.

any ideas welcomed.

I don't know as much about outrunners or mounting them, but I have a few ideas about the inrunner...

For the axle, here's a metal version of the HSP crawler solid axle, which uses the same ring & pinion gear as their other 1/10ths, so it's good to at least 7k rpm in my experience. With 80mm tires you'd be spinning it at 11k, but with the metal upgrade I bet it'd hold together. There's a few more parts that go with it, 1, 2.

That gives you a 13/38 gearing for a reduction of 2.92.

With a Hobbywing 10.BL.120-G2 (easy to remember name...) and a 2350kv 4268 sensored motor on 4S, with a quality lipo of ~5000mah, you should hit 100mph pretty easily, and pretty quickly... You could probably get by with a smaller overall build at still hit 100mph, but it would be more complicated with more gearing... a big sensored motor will have enough power to get started on 80mm wheels and relatively shallow gearing.

The speed calculator function on the forum is very useful for working out theoretical speed run builds: https://www.rctalk.com/rc-tools/rc-car-speed-calculator/

 

[Roog]

I don't know as much about outrunners or mounting them, but I have a few ideas about the inrunner...

For the axle, here's a metal version of the HSP crawler solid axle, which uses the same ring & pinion gear as their other 1/10ths, so it's good to at least 7k rpm in my experience. With 80mm tires you'd be spinning it at 11k, but with the metal upgrade I bet it'd hold together. There's a few more parts that go with it, 1, 2.

That gives you a 13/38 gearing for a reduction of 2.92.

With a Hobbywing 10.BL.120-G2 (easy to remember name...) and a 2350kv 4268 sensored motor on 4S, with a quality lipo of ~5000mah, you should hit 100mph pretty easily, and pretty quickly... You could probably get by with a smaller overall build at still hit 100mph, but it would be more complicated with more gearing... a big sensored motor will have enough power to get started on 80mm wheels and relatively shallow gearing.

The speed calculator function on the forum is very useful for working out theoretical speed run builds: https://www.rctalk.com/rc-tools/rc-car-speed-calculator/

Thank you @tudordewolf for your thoughts, and in particular the link to the speed calculator its a bit slicker than my spread sheet efforts.

I may have found a reasonably sturdy looking 1:1 rear axle/gearbox, just add wheels and motor coupling, but to get to my target speed that would leave me with motor revs as low as 12000 rpm at full power. Are there motors that can operate at this speed? They all seem to be much higher.

tbh, low rpm is good as I would worry about my machining to be adequately balanced to operate much above 20000 rpm :0(

 

[tudordewolf]

The challenge with such "low" RPM's is that the motor is going to have to make up for lack of gear reduction with raw torque.

From those sensored motors I linked, the biggest, lowest-KV one, a 1400kv 4274 on 2S, would spin at around 12k RPM, and it may very well have enough torque from a standstill in a more or less direct drive configuration - that's the main reason I'm recommending a sensored motor, you're likely to encounter cogging with such a high rollout between the motor and the wheels.

 

[Roog]

Thank you, I was going ask about 'sensored' and 'non-sensored' motors, from what I read sensored improves control.

Actually we are allowed to push start IC cars so I guess a little assistance to get an electric car would be Ok too!

I can see that i am going to have to do some calculations, (not my strong point) regarding torque.

 

[ahr43]

Interested in hearing and reading more about your tethered events Roog. A good option for speed events when long straight line venues are unavailable.

Recall a few years back there was a tethered RC movement in Southern California USA. Never followed up on it other than knowing it happened.

 

[Roog]

Interested in hearing and reading more about your tethered events Roog. A good option for speed events when long straight line venues are unavailable.

Recall a few years back there was a tethered RC movement in Southern California USA. Never followed up on it other than knowing it happened.

I can see the appeal, you can focus on just one easily measured thing, mph!
Bit like free flight airplanes, I have always pulled my friend's leg about launching his beautifully built, uncontrolled, high performance airplanes and being surprised when it flies off and he loses it! :0)

 

[tudordewolf]

Thank you, I was going ask about 'sensored' and 'non-sensored' motors, from what I read sensored improves control.

Actually we are allowed to push start IC cars so I guess a little assistance to get an electric car would be Ok too!

I can see that i am going to have to do some calculations, (not my strong point) regarding torque.

As I understand it, KV has a counterpart, KT, torque constant, which expresses torque generated as newton-meters per amp. KT = 9.55 / KV (because 60/2pi...)

So, a 1400kv motor would have a KT of .0068, which multiplied by its max current of 88 amps, gives just about 0.6 newton-meters.

Dividing that by the proposed 40mm radius of your wheel, you get 15 newtons of force, or about 3.3lbs-force pushing on the car. Divide that by your car's weight to get a theoretical acceleration value, assuming the motor can "saturate" with the voltage you're giving it.

There's a lot of assumptions of perfectly efficient physics in the above summary, but it's a place to start.

 

[Roog]

As I understand it, KV has a counterpart, KT, torque constant, which expresses torque generated as newton-meters per amp. KT = 9.55 / KV (because 60/2pi...)

So, a 1400kv motor would have a KT of .0068, which multiplied by its max current of 88 amps, gives just about 0.6 newton-meters.

Dividing that by the proposed 40mm radius of your wheel, you get 15 newtons of force, or about 3.3lbs-force pushing on the car. Divide that by your car's weight to get a theoretical acceleration value, assuming the motor can "saturate" with the voltage you're giving it.

There's a lot of assumptions of perfectly efficient physics in the above summary, but it's a place to start.

Thank you @tudordewolf, nice explanation

 

[Roog]

Hello again, sorry to raise this again, but doubts remain in my head about my plan to go with a 'Low rev' motor solution in my car.

Despite visiting my local model shop for a chat and reading around the topic a little I remain concerned that a 1400KV motor driving through a 1:1 gear box on to 80mm dia. wheels will not deliver sufficient power.

I have simple mechanical calculations which show that to achieve my target speed of approximately 100mph the motor needs to turn at around 10,500rpm and the car load at that speed, which I have assumed will mostly be in overcoming air resistance will be in the order of 650W allowing for mechanical losses etc. I want to aim for nearer to 1000W motor power. I am not sure that the above motor, a 4274 1400KV will deliver this at a battery voltage aligned with the required revs at close to rated output say 11.1V ?

Is there a way that I can assure myself of what the motor power is likely to be?

 

[Greywolf74]

Those Rocket 4274 motors are good up to 2800W, up to 50k RPM, so if you max out your gearing then just up the voltage. You could also just choose a little higher Kv motor if your have to run it on 3S. My concern here is that your talking about building a 100+MPH tethered car with 1/8scale electronics. This means you're liable to end up with a 5-8lb rig doing 100MPH armed with a 3S LiPo. If that thing wrecks it could turn in to a high speed bullet and even if it doesn't hit anything important, if that LiPo goes up in flames then youve just lost your rig. Just

 

[Roog]

This is true @Greywolf74 and i agree, the risks are not insignificant. The plan is to run it up steadily and iron out the behavior of the chassis by adjusting the spring rates and dampers. The car will have double tethers and the track is fenced off. I agree the Lipo does add a risk if it rolls.
My current nitro tether car weighs 2.2 kg or 4.8lbs

I can appreciate that increasing the gearing will get me closer to the Brushless motor operating sweet spot but I am struggling to find bevel gears which will get me to the desirable 3:1 ratio.

I have found a neat self contained 1:1 gearbox complete with decent sized drive shafts all integrated, but this would require the motor to be happy producing 650W to 1000W at 10500 rpm for my target speed.

Call me lazy but this would reduce my machining for the build quite a bit. So my issue is find suitable gears or a motor/controller/battery combination which is able to deliver 1000W drive at 10500 rpm for 2-3 minutes, (probably less), I'm not sure that this exists?

I have considered tooth belt drive too,, (nice and quiet) but a sideways mounted 4274 motor would make the chassis a bit wide and most of the powerful cars are shaped like a spear.

 

[Lukedavis]

This is true @Greywolf74 and i agree, the risks are not insignificant. The plan is to run it up steadily and iron out the behavior of the chassis by adjusting the spring rates and dampers. The car will have double tethers and the track is fenced off. I agree the Lipo does add a risk if it rolls.
My current nitro tether car weighs 2.2 kg or 4.8lbs

I can appreciate that increasing the gearing will get me closer to the Brushless motor operating sweet spot but I am struggling to find bevel gears which will get me to the desirable 3:1 ratio.

I have found a neat self contained 1:1 gearbox complete with decent sized drive shafts all integrated, but this would require the motor to be happy producing 650W to 1000W at 10500 rpm for my target speed.

View attachment 174903

Call me lazy but this would reduce my machining for the build quite a bit. So my issue is find suitable gears or a motor/controller/battery combination which is able to deliver 1000W drive at 10500 rpm for 2-3 minutes, (probably less), I'm not sure that this exists?

I have considered tooth belt drive too,, (nice and quiet) but a sideways mounted 4274 motor would make the chassis a bit wide and most of the powerful cars are shaped like a spear.

Can you post some pics of your nitro tether car? I have never seen one before and I think that this is a cool project.

 

[Roog]

Can you post some pics of your nitro tether car? I have never seen one before and I think that this is a cool project.

Sure, I posted a pic here:

this chassis is based on parts from a recycled Mardave Marurder, it is not typical of tethered cars from the 40’s & 50’s as the Mardave parts drive the layout.

https://www.rctalk.com/forum/threads/so-who-burned-some-nitro-today.113309/post-1398027

I have carried out a few mods since and it still needs a body, I am planning on making a Lotus 49 shell, which might look a bit like this:

but with thin wheels! My chassis works out at approximately 1:7 scale.
After going over the options for several days, countless CAD drawings later, I have decided to order a selection of gears and axles to try!

I believe the right path is to go with the common theme of a high reving motor with gearing which will open up the choice of appropriate motors too!

 

[Lukedavis]

Sure, I posted a pic here:

this chassis is based on parts from a recycled Mardave Marurder, it is not typical of tethered cars from the 40’s & 50’s as the Mardave parts drive the layout.

https://www.rctalk.com/forum/threads/so-who-burned-some-nitro-today.113309/post-1398027

View attachment 174932

I have carried out a few mods since and it still needs a body, I am planning on making a Lotus 49 shell, which might look a bit like this: View attachment 174931but with thin wheels! My chassis works out at approximately 1:7 scale.
After going over the options for several days, countless CAD drawings later, I have decided to order a selection of gears and axles to try!

I believe the right path is to go with the common theme of a high reving motor with gearing which will open up the choice of appropriate motors too!

That is awesome!

 

[Roog]

Thank you for your kind words @Lukedavis, I was more or less gifted the Mardave Maruder for its parts so I felt I had to make use of it, the next one, well that needs to be purpose built!

 

[Lukedavis]

Thank you for your kind words @Lukedavis, I was more or less gifted the Mardave Maruder for its parts so I felt I had to make use of it, the next one, well that needs to be purpose built!

Cool!

 

[Roog]

I have found a bunch of motor ESC combos, just one thing, they all seem to be sensor less, is this likely to be an issue?
I have tried to read up on the pros and cons of sensored motors and it seems that the three key reasons why sensored motors are favored is

Slightly better starting torque
Less cogging
Additional protection under failure modes

Whilst I like extra protection I don't believe the other two to be a concern for my 'round the pole' racer

What do you think please?

 

[Greywolf74]

It shouldnt be an issue. Sensored has a smoother start up so for certain things, like crawling or racing for example, its much more important to have very smooth and controlled torque at start up. Sensorless is more efficient however and most (if not all) sensored ESCs switch to a sensorless mode once they get to a certain point. Given how you're going to be using the car, I dont see a problem.

 

[Roog]

Many thanks @Greywolf74 i have watched a video demonstrating the differences and I can see why scale truck models would benefit. Certainly better control at start up and when running at low speed