How to snap necks for cheap

[Smayonaise]

Say I wanted to build a gokart or ebike where range and a high top speed didn't matter and I only cared about the cheapest and easiest way to get the hardest acceleration from 0 to ~20mph. I'm talking really hard acceleration, like at least 1G. It won't be driven on the street and only needs enough battery to launch a few times, so like a few miles worth. I'm probably going to use an existing chassis because that's easier but we'll see. I'm mechanically inclined, I've got a welder and plenty of tools and I have experience working with electronics. I don't have any deep experience with batteries and electric motors, or gear ratios lol. I'd want to do this build in the most budget/diy way and I'm not scared if it's a little sketchy. Here's the questions that I've got.

What are some of the most common motors and batteries I'll find on fb marketplace or ebay (or anywhere else you might suggest) that will work for my requirements?

What size battery would I need to sustain full power for at least a mile or two?

I'm thinking I don't need an esc because it's either going to be launching at full power or braking. So I'd use something like a contactor or hv switch here as the gas pedal? Without a brain what's the best way to charge something like this? If I go with lead batteries that's easy enough, but lithium I'm not sure.

Are there any good sources for this stuff that I can pull out of something else like how the ev car guys use forklift motors.

What kind of gear ratios would be the best for this task? I'm probably going to be using wide grippy tires to get better acceleration.

Anything else I didn't think of, thanks!

 

[E-HP]

You need to quantify what your goals are. "Really hard acceleration" doesn't really say much. What's the elapsed time that you're targeting to get to 20 mph?

You need an ESC if you're using a BLDC motor. I guess if you get an old school brushed motor you might be able to get away with no ESC.

Once you put some numbers to your wish list, then that would be something to work with. Why only 20mph?

 

[neptronix]

I've never built a go-kart but i know you want a really high C rate cell as an essential ingredient.
RC Lipo is dangerous but it's a cheap way to get a 20C or higher rated cell, which you're going to need to push some amps.
Read this before proceeding:
RC Lipo pack inspection, series/parallel charging, and lifespan/safety tips

You probably also want a 5-10kw rated motor with chain drive.

Here's the scale for seat of the pants feel on an ebike. Consider that your vehicle is going to be a bit heavier.

 

[Smayonaise]

You need to quantify what your goals are. "Really hard acceleration" doesn't really say much. What's the elapsed time that you're targeting to get to 20 mph?

You need an ESC if you're using a BLDC motor. I guess if you get an old school brushed motor you might be able to get away with no ESC.

Once you put some numbers to your wish list, then that would be something to work with. Why only 20mph?

Apologies, I wasn't sure what numbers you guys would be interested in so I put 1G in my post which translates to roughly ~~ 1 second to 20mph. I picked 20mph arbitrarily, because I understand there's trade offs between acceleration and top speed. And from the builds I've seen on the forum 30-50mph is considered pretty fast, and achievable without spending $1000s.

And yeah I was thinking more of an old school motor because they can produce high torque too, they're relatively common, and wouldn't need an esc. So it'll easier to find one cheaply and easier to setup.

 

[Smayonaise]

I've never built a go-kart but i know you want a really high C rate cell as an essential ingredient.
RC Lipo is dangerous but it's a cheap way to get a 20C or higher rated cell, which you're going to need to push some amps.
Read this before proceeding:
RC Lipo pack inspection, series/parallel charging, and lifespan/safety tips

You probably also want a 5-10kw rated motor with chain drive.

Here's the scale for seat of the pants feel on an ebike. Consider that your vehicle is going to be a bit heavier.

View attachment 383302

Awesome, thanks for the links! RC lipos are the most mysterious batteries to me lol so I'll definitely do more research on them.

 

[Smayonaise]

I've never built a go-kart but i know you want a really high C rate cell as an essential ingredient.
RC Lipo is dangerous but it's a cheap way to get a 20C or higher rated cell, which you're going to need to push some amps.
Read this before proceeding:
RC Lipo pack inspection, series/parallel charging, and lifespan/safety tips

You probably also want a 5-10kw rated motor with chain drive.

Here's the scale for seat of the pants feel on an ebike. Consider that your vehicle is going to be a bit heavier.

View attachment 383302

Would 5-10kw be the power needed without gearing? Because I'm hoping to take advantage of ratios to be able to use a cheaper motor

 

[jonescg]

Just apply some physics - 1g acceleration is 9.81 m/s/s so for a combined rider and bike mass of say 125 kg, that's a force of 1226 N.
The power required to achieve this acceleration up to a particular speed (in a straight line) is:

P = F × v

and if v is 50 km/h (13.89 m/s) you need just over 17 kW. If you're happy with 20 mph (32 km/h, or 8.89 m/s) you only need about 11 kW.

As to how you gear it, well that depends on the wheel diameter and shaft torque. Power is the product of torque and angular velocity, and because rolling motion links angular velocity and linear velocity, we can say:

P = ω × τ where ω is in rad/s and τ is in Nm.

ω = v/r where v is the linear velocity in m/s and r is wheel radius in m. A 700C wheel has a radius of 0.35 m.

Solving for τ:
τ = P / ω
τ = P / (v/r)
τ = 11,000 / (8.89 / 0.35) = 433 Nm

Now most motors don't have that much torque, but 100 Nm is quite common. So a ~4:1 reduction gear will deliver the required torque. All this will depend on the final mass of the bike and rider, but this is literally how you do power and torque calculations if you don't have a dyno, but you do have a straight road, a tape measure and a stop watch.

 

[E-HP]

Apologies, I wasn't sure what numbers you guys would be interested in so I put 1G in my post which translates to roughly ~~ 1 second to 20mph. I picked 20mph arbitrarily, because I understand there's trade offs between acceleration and top speed. And from the builds I've seen on the forum 30-50mph is considered pretty fast, and achievable without spending $1000s.

And yeah I was thinking more of an old school motor because they can produce high torque too, they're relatively common, and wouldn't need an esc. So it'll easier to find one cheaply and easier to setup.

What is your experience in launching a bike that can do this. It takes a fair amount of skill and lots of practice to keep the front wheel down and the rear from losing traction, even on a motorcycle. I watched my friends cousin on a little 650 twin totally beat a guy on an 1100 in a parking lot race, all from the launch. With electric motors, where peak torque is at 0, the launch is even more critical and difficult to do consistently.

I chose 0-30 times because 0-20 times are so dependent on launch that the times varied more. With the tiny contact patch of a bicycle tire, it’s even harder. I posted a thread a few years ago on this. 0-20 times varied by 10% or so, but 0-30 was way more consistent. You have to keep your front tire within a few inches off the ground so most of the torque is moving the bike forward, so finding the right position to place your body in is the key. Also, any voltage sag will kill your times, so don’t just get any old lipos, get high performance ones like graphenes. I picked up 0.3 seconds on 0-30 by paralleling more lipos to drop my sag at launch from 7v to 1.5v.

You can use an app called gps race timer to test with. It works well without hassle so you can concentrate on the test. I have a fair amount of experience in launching motorcycles that can do what you want, but you’re not going to do it on bicycle tires without a lot of practice. I switched to 0-40 for testing which produces even more consistent and useable data, since I’m more focussed on measuring bike performance than my skill level.

Edit:
The 3 bikes discussed in this thread are running at 7kW (mine), 13kW (eastwood’s) and 20kW (dogdipstick’s). I don’t think any can do 0-20 in 1 second because of the launch; maybe dogdipstick’s might. I was varying around 2 seconds when I switched to 0-30 to get more consistent data. Im at 200A phase, Eastwood is at 500A phase, and has a 90/80 16” Shinko moped tire out back. He’s not losing traction, which is one less thing to deal with, but still needs to keep the front wheel down, so hard to go all out.

0-30 MPH with bicycle tires

 

[AdR]

Get the biggest engine ever built starter motor. Lol

 

[hillslayer@]

Saw a starkvarg motor in a gocart.

 

[neptronix]

Would 5-10kw be the power needed without gearing? Because I'm hoping to take advantage of ratios to be able to use a cheaper motor

You can use really steep gear ratios to achieve much more torque, but the choice of the motor's continuous rating is essential.
Taking gearing into account the smallest motor you probably want is rated for 2.5kw cont, and expect to put in excess of 7.5kw into it for short periods of time

It should be pretty fast but it may or may not meet your definition of neck snapping.
It may be more like being shot out of a cannon or less.
So you might want a 5kw rated motor - to put up to 15kw into peak

 

[neptronix]

ps, the more power, the more of a wheelie problem you'll have.
Wheelies will probably start around 7kw from a stall.

I'd only use a FOC controller so that you can gradually bring up the torque. A regular cheap sine wave or square wave controller will not have enough modulation of the torque to be safe at high power. These cheaper controllers would be dangerous because the margin between wheelie-ing and not wheelie-ing is a few millimeters of throttle motion too soon, whereas with FOC, it is like being able to finely control the speed of the cannon launch.

I would also take a hint from RWD super sport cars and put the human up front as much as possible to act as ballast against wheelies.