[WIP] Super Fast Speedcar - current top speed: 32.4km/h

[mocbuild101]

I have decided to make a WIP topic for my tests of my speedcar, which is a car that I hope to get up to 40km/h or more.

But I don't want this to turn into a discussion about wheels, I just want it to be a place to show my progress.

 

This is where I will be doing most of my tests: - it's about 80m of very smooth car park + 15m of footpath and 10m of gravel.

 

Here is my first car: 5:3 gearing, 725gm

Here is the results of the tests:

Yes, I know - only 24.3km/h, but it's more than I would have ever expected a few months ago (when I thought that the top speed for Lego was 27km/h).

I have some work to do...

Edited July 6, 2017 by mocbuild101

[TechnicSummse]

Nice to see finally some of your results :D

I think evryone of us startet with something like 20+ kph. Its pretty ok as first result. Dont you have a steering yet?

 

How you you make such a clean picture with white backround?

[Marxpek]

Good job! i'll be looking forward to see your progress here.

How does your steering setup work? Same system as mine isn't it?

that sportswatch looks good on the car ;D Good luck on your attempts!

[mocbuild101]

Just now, TechnicSummse said:

Nice to see finally some of your results :D

Don't you have a steering yet?

Just now, Marxpek said:

How does your steering setup work? Same system as mine isn't it?

I forgot to mention that, here's a quick video of it: http://bricksafe.com/files/mocbuild101/super-speedcar/Car1-steering video.mpg

 

Just now, TechnicSummse said:

How do you make such a clean picture with white backgound?

I have a shelf (about 50x60x40cm) that I covered the inside with white paper, and lit with 2 super bright LED lights, then I put it through GIMP to adjust the brightness/contrast.

It might seem a lot of work, but I've got quite good at it - so it only takes a couple of minutes.

[Marxpek]

Just now, mocbuild101 said:

I forgot to mention that, here's a quick video of it: http://bricksafe.com/files/mocbuild101/super-speedcar/Car1-steering video.mpg

Yes, exactly the same system based on caster just with 2 wheels, great minds think alike ;) you could take out the liftarms in the front and replace them with halfbeams easily to reduce friction in the front.

[mocbuild101]

Just now, Marxpek said:

great minds think alike ;)

How right you are! - not only did I think of the same steering setup, but I thought of that same way to reduce friction!

[mocbuild101]

Today I thought I'd try a couple of things that hadn't been tried yet: balloon tires and a pullback motor for a boost at the start.

So here it is: - 2:1 gearing, 780gm

Here is the results of the tests:

So as you can see, it didn't work! - only 22.7km/h

The pullback motor did give a boost at the start, but the added weight and friction slowed it down - a lot!

 

Well, to quote: "failure is always an option"  (I think some of you will know where that's from)

 

[Jurss]

I'am planning to build pure pullback thing (i have some 4 new and 3 old type), but have other projects at the moment, so justy don't have time for it.

[mocbuild101]

31 minutes ago, Jurss said:

I'am planning to build pure pullback thing (i have some 4 new and 3 old type), but have other projects at the moment, so justy don't have time for it.

That's a good idea, pullback motors certainly can go very fast!

But don't forget, pullback motors produce a lot of toque - so you will need very grippy wheels.

[TechnicSummse]

1 hour ago, mocbuild101 said:

Today I thought I'd try a couple of things that hadn't been tried yet: balloon tires and a pullback motor for a boost at the start.

So here it is: - 2:1 gearing, 780gm

Here is the results of the tests:

 

So as you can see, it didn't work! - only 22.7km/h

The pullback motor did give a boost at the start, but the added weight and friction slowed it down - a lot!

 

Well, to quote: "failure is always an option"  (I think some of you will know where that's from)

 

The pullback-motor will just shorten your required track. It is pretty much the same, then using a gearbox in our case.

You could use the pullback-motor as bearing. As you mounted it now, it produces extra friction at the axle.

But ( i know you didnt want to talk about :D) the biggest problem are the wheels. They have just a terrible rolling resistance.

[mocbuild101]

3 minutes ago, TechnicSummse said:

But ( i know you didnt want to talk about :D) the biggest problem are the wheels. They have just a terrible rolling resistance.

Yeah I know, but I thought I'd try them because they are quite soft - so it is kind-of like suspension.

I will be using the motorbike wheels from now on.

6 minutes ago, TechnicSummse said:

You could use the pullback-motor as bearing.

Hmm...

Here's another view of how I mounted it - if you are interested:

[TechnicSummse]

12 minutes ago, mocbuild101 said:

Here's another view of how I mounted it - if you are interested:

Oh, i thaught you mounted it on the wheel-axle. But its mounted on the motor-axle.

But still... the 3x5 L-beam should not touch the axle, thats what i meant. You can put the Pullback-motor to your chassis, and use only its axle-hole as bearing.

Edited July 2, 2017 by TechnicSummse

[Marxpek]

Like i mentioned before: great minds think alike, it's really fun to see you are trying the exact same things as i have.

3 hours ago, mocbuild101 said:

a pullback motor for a boost at the start.

I tried this pullback motor idea in the very early stages of my first racer, and found the exact same results as you have: friction and speed loss.

Here is a idea for you (that i have not tried) : make a 2 stage system, the first stage being a pullback car which can break loose from the rc racer, this will allow you to use a higher gearing in the rc racer since you can start the motors when they already have some speed, while causing no extra drag or weight.

1 hour ago, mocbuild101 said:

Yeah I know, but I thought I'd try them because they are quite soft - so it is kind-of like suspension

Don't worry about suspension too much, in your front setup the liftarms will provide some suspension with the flex they have mounted like this and i can tell you that my attempt with the suspended front wheels was a failure, ill update my topic today.

The wheel and bearings need a bit more thought, but i'm sure you will manage!

Looking forward to your ideas and attempts! Keep it up!

 

[mocbuild101]

2 hours ago, Marxpek said:

Here is a idea for you (that i have not tried) : make a 2 stage system, the first stage being a pullback car which can break loose from the rc racer, this will allow you to use a higher gearing in the rc racer since you can start the motors when they already have some speed, while causing no extra drag or weight.

Interesting, I might try that - but not yet, because I have already made another car to test (which I have very high hopes for!).

 

2 hours ago, Marxpek said:

Don't worry about suspension too much, in your front setup the liftarms will provide some suspension with the flex they have

Yeah, I was thinking the same thing... again!

[TechnicSummse]

3 hours ago, Marxpek said:

Like i mentioned before: great minds think alike, it's really fun to see you are trying the exact same things as i have.

I tried this pullback motor idea in the very early stages of my first racer, and found the exact same results as you have: friction and speed loss.

Here is a idea for you (that i have not tried) : make a 2 stage system, the first stage being a pullback car which can break loose from the rc racer, this will allow you to use a higher gearing in the rc racer since you can start the motors when they already have some speed, while causing no extra drag or weight.

You could just psuh the car by hand at the start... would bring the same result (shortening the required track until topspeed). This will save the pullback motors weight ;)

[mocbuild101]

4 minutes ago, TechnicSummse said:

You could just push the car by hand at the start...

Not when I have to walk halfway down the track before I start - to stay in range.

[TechnicSummse]

1 minute ago, mocbuild101 said:

Not when I have to walk halfway down the track before I start - to stay in range.

I'm using the same method for my runs ;)

Sometimes i tried to give the car a little push... if you start running full speed immediately, this should work.

But if you use a 2-stage-system, the pullback-car wont affect the rc-racers weight... it just makes things more complicated... you need to take care, not to overrun the pullback-car.

And you will also need to pull back and launch the car... if you dont launch it with the auxilary-ouput (wich would mean additional weight for the mechanism) you will still need to start running parallel to the car ;)

[mocbuild101]

1 hour ago, TechnicSummse said:

But if you use a 2-stage-system, the pullback-car wont affect the rc-racers weight... it just makes things more complicated... you need to take care, not to overrun the pullback-car.

And you will also need to pull back and launch the car... if you dont launch it with the auxilary-ouput (wich would mean additional weight for the mechanism) you will still need to start running parallel to the car ;)

I was thinking of making a separate pullback car with it's own wheels, connected to the main car via a loose axle-in-pin-hole connection.

As for launching it, I was hoping to be able to make some kind of mechanism that will release the pullback motor when the main car starts moving. 

[Marxpek]

1 hour ago, TechnicSummse said:

You could just psuh the car by hand at the start... would bring the same result (shortening the required track until topspeed). This will save the pullback motors weight ;)

I feel that would be cheating.. all power should come from Lego, that way i could drag it behind the bike to 30 km/h then engage the motors with an insane gearing, and with this setup the pullback motors are not included in the actual record setting car which breaks loose from the pullback motor pusher car to get the initial speed.

5 minutes ago, mocbuild101 said:

I was thinking of making a separate pullback car with it's own wheels, connected to the main car via a loose axle-in-pin-hole connection.

As for launching it, I was hoping to be able to make some kind of mechanism that will release the pullback motor when the main car starts moving. 

This is exactly what i had in mind, good luck!

[mocbuild101]

Just now, Marxpek said:

I feel that would be cheating.. all power should come from Lego

I agree, the method I'm trying to keep doing is: car not moving -> start motors via RC -> top speed.

2 minutes ago, Marxpek said:

This is exactly what i had in mind, good luck!

I can't believe how often this is happening! 

[TechnicSummse]

2 minutes ago, Marxpek said:

I feel that would be cheating.. all power should come from Lego, that way i could drag it behind the bike to 30 km/h then engage the motors with an insane gearing, and with this setup the pullback motors are not included in the actual record setting car which breaks loose from the pullback motor pusher car to get the initial speed.

This is exactly what i had in mind, good luck!

By using the pullback-motor you just store the pushing-energy. There is no difference if you give it a little push by hand, or if you pull up the pullbackmotor BY HAND...

At the end it is not a full RC-usable car anymore. You will allways need a special setup to start it.

You could use a little downhill ramp at the start, you could use a pullback-motor, a gearbox, a little push by hand.... all this wont change anything at the top speed, but help in acceleration.

 

Even if you would pull the car with your bike up to 30 kph... the topspeed would be the same then if its drivin from start on, with his own motors. The only thing wich would matter here... is the battery freshness. 

Whith a high gearing you suck alot of power from the batteries at the start. If you are doing a flying start, you need less power in total, but the friction, air resistance, rolling resistance... evrything stays the same... so there is no difference at top-speed... again... just a shorter track would be needed.

[Marxpek]

35 minutes ago, TechnicSummse said:

Whith a high gearing you suck alot of power from the batteries at the start. If you are doing a flying start, you need less power in total, but the friction, air resistance, rolling resistance... evrything stays the same... so there is no difference at top-speed... again... just a shorter track would be needed.

even if we use such a heavy gearing in which it cannot pull away on its own and then lets that work when its up to speed?  then why do we gear it up at all? and even if this all is no use, i would love to be able to use a shorter track, i still did not managed to get a full speed run from my car 

[mocbuild101]

1 hour ago, TechnicSummse said:

At the end it is not a full RC-usable car anymore.

Yes, but even the cars that we have made so far can't be used like a normal RC car.

 

35 minutes ago, Marxpek said:

even if we use such a heavy gearing in which it cannot pull away on its own and then lets that work when its up to speed?  then why do we gear it up at all? and even if this all is no use, i would love to be able to use a shorter track, i still did not managed to get a full speed run from my car 

The key is to get the balance of acceleration and speed just right, which would be much easier with a longer track.

[TechnicSummse]

54 minutes ago, Marxpek said:

even if we use such a heavy gearing in which it cannot pull away on its own and then lets that work when its up to speed?  

Well... your car can easy reach ... lets say 25 kph.

Now lets say your car is driving along the road with 25 kph, and accelerates more...  why do you think there is a difference if it reaches the 25 kph point by its own, or if you pulled it with your bike to those given 25 kph. The result is the same... the car is driving with 25 kph, and its going faster from there by its own motorpower.

 

The start is something different. You need way more power (torque) to start the car, compared to it allready beeing in movement. 

54 minutes ago, Marxpek said:

then why do we gear it up at all? and even if this all is no use, i would love to be able to use a shorter track, i still did not managed to get a full speed run from my car 

We are gearing up, because the motors output rpm is lower then our wanted wheel rpm ;)

 

Long time ago, when i started this project, i checked all so far records, and the gearing-setup there. I calculated speed + wheel diameter + gearing at all those models... what i endet at, was a output rpm of 900-1100 on each of them; meaning the limit is at about 3/4 of the maximum unloaded motor-rpm.

I set my wanted speed to something between 35-40 with my given numbers, and got a usefull gearing-range as result. something between 1,66666 and 2,5 (at the high speed output).

One times i used the pretty much same frame-setup with 3 different gearings. 40:20 / 36:12(at the low speed output) and 40:16  => this is 2,0:1   2,2:1    2,5:1.

The topspeed was exactly the same at all 3 gearings -> ~33 kph

 

At the end higher gearing will mean even lower speed, since there is not enough torque left to overcome all the resistances.

The aim is, to hit the perfect gearing fitting to your chassis resistances.

[Marxpek]

2 hours ago, TechnicSummse said:

about 3/4 of the maximum unloaded motor-rpm.

Most dc motors have their maximum power at 75% of their maximum rpm, so that sounds about right.

2 hours ago, TechnicSummse said:

why do you think there is a difference if it reaches the 25 kph point by its own, or if you pulled it with your bike to those given 25 kph

It is like a gearbox, me cycling would be like first gear and the motors on the car are like the 2nd gear, the car cannot drive away on its own in the 2nd gear because it needs more torque, which is doesn't have available. Should i bring it up to speed with a bike it will need less torque to get the gears turning since it already is in motion, this is the whole reason we are using gearboxes in cars right? to get a car moving up to a certain speed then use another gear ratio and repeat. 

But who knows maybe i'm looking at it wrong, of course there is a top on the power any motor can deliver and resistances will grow with speed..