Author Topic: Scratch built giant rc car  (Read 13235 times)

Online steamer

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Re: Scratch built giant rc car
« Reply #45 on: November 24, 2018, 06:55:03 PM »
I know that will be far too small  ( 10 holes at 1mm diameter) so just keep opening the holes up a little at a time until they work on a fairly smooth surface with 35 wt.   Then you can go up and down with oil viscosity as a tuning aid
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Offline AlexS

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Re: Scratch built giant rc car
« Reply #46 on: November 25, 2018, 05:47:46 PM »
Hi KB. Interesting construction project!
As others indicate, the vibration in the steering could be remedied by using a damper or changing springs or weight of the wheels.

It is difficult to determine whether this fully follows the natural frequency.
I saw this passing by and thought, maybe I can help somehow to draw up a small model. In order to map out the natural frequency in a simplified manner of the control device. And on the basis of this change parameters such as spring strength and or application of a damper.
This might create some insight.

As an example I have made a model in which a certain mass and corresponding radius get a corner rotation. This mass is connected to a spring (2nd model + damper) over a certain distance from the center of the mass.
This model gives an outline of how the rotation of the mass would take place with the use of only a spring or with a damper.

I find it interesting to look at this. For this I do need the masses and dimensions of the components of the steering system. And any spring constant of the mounted spring.
Let me know if this seems like something to you!

Offline Vixen

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Re: Scratch built giant rc car
« Reply #47 on: November 25, 2018, 06:40:40 PM »
If it is smooth by itself and only becomes twitchy when you add the weight and inertia of the road wheels, then it sounds like a steering damper cylinder may help.


By steering damper cylinder, I meant a hydraulic shock absorber, similar to a suspension shock absorber, between the steering arm and the chassis. I believe this would kill the inertia and overshoot and system resonance from the front road wheels. The steering servo saver is there for an entirely different purpose.

Mike
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Online steamer

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Re: Scratch built giant rc car
« Reply #48 on: November 25, 2018, 06:47:53 PM »
Hi KB. Interesting construction project!
As others indicate, the vibration in the steering could be remedied by using a damper or changing springs or weight of the wheels.

It is difficult to determine whether this fully follows the natural frequency.
I saw this passing by and thought, maybe I can help somehow to draw up a small model. In order to map out the natural frequency in a simplified manner of the control device. And on the basis of this change parameters such as spring strength and or application of a damper.
This might create some insight.

As an example I have made a model in which a certain mass and corresponding radius get a corner rotation. This mass is connected to a spring (2nd model + damper) over a certain distance from the center of the mass.
This model gives an outline of how the rotation of the mass would take place with the use of only a spring or with a damper.

I find it interesting to look at this. For this I do need the masses and dimensions of the components of the steering system. And any spring constant of the mounted spring.
Let me know if this seems like something to you!

Also you can check out a RC vehicle dynamics analysis program on the market called "RC Crew Chief"   It's quite detailed and gives good predictions on handling changes.

Dave
 
"Mister M'Andrew, don't you think steam spoils romance at sea?"
Damned ijjit!

Offline AlexS

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Re: Scratch built giant rc car
« Reply #49 on: November 25, 2018, 07:02:59 PM »
Dave, looks like an interesting site and analysis program.

Offline KB

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Re: Scratch built giant rc car
« Reply #50 on: November 25, 2018, 09:40:26 PM »
Alex, thank you for your time and insight. Fascinating. I do have most of this modelled in CAD which can give me mass, but something like spring constant is beyond me. I appreciate your time. Let me try some simple things first.

Mike, thanks, I see what you mean. Last night I made a longer servo arm which didn't seem to help much but I turned up the frame rate on the radio system and that seemed to help some as well as a load on the steering knuckle. But I need to be 100% throwing this kind of weight around.

Dave, thanks I'll check that out.
This is the radio I have. http://www.competitionx.com/pdf/radio-equipment/radiopost-ts-401-manual.pdf
I have just started playing with it. What can I adjust to make the steering less sensitive at centre? Negative expo?
I bought this radio a while ago so that I could set the endpoints and have an extra channel for a kill switch.
Also do you know if something like the super bee failsafe would work on a four stroke?

Thanks all!

Online steamer

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Re: Scratch built giant rc car
« Reply #51 on: November 25, 2018, 09:43:21 PM »
The link comes back forbiden.  Cant open it
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Offline KB

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Re: Scratch built giant rc car
« Reply #52 on: November 25, 2018, 11:08:13 PM »

Offline KB

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Re: Scratch built giant rc car
« Reply #53 on: December 02, 2018, 06:29:29 PM »
I ended up switching to a wiper motor and it still behaved strangely. I indexed the feedback servo horn away from what I thought was centre and it smartened right up! I think it was bouncing around in a dead zone in the pot..?



Offline Vixen

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Re: Scratch built giant rc car
« Reply #54 on: December 02, 2018, 09:36:39 PM »
Looks good.  :ThumbsUp: :ThumbsUp:

What happens when you add the weight and inertial of the two road wheels? Is it still nice and smooth, or does it overshoot?

Mike
It is the journey that matters, not the destination

Offline Florian Eberhard

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Re: Scratch built giant rc car
« Reply #55 on: December 02, 2018, 10:06:48 PM »
Hi Kevin

Looking very good! Just one thought - I think your servo is maybe a bit small for the size of your car.
Do you know about the large size servos that are used in 1/5 scale rc cars?

Florian

Offline KB

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Re: Scratch built giant rc car
« Reply #56 on: December 03, 2018, 11:24:21 PM »
Thanks Mike, I haven't tried it with wheels on. The frame jig needs to be lifted for that, (I did build a rotisserie attachment for when it comes time to weld it all out.) But it seems pretty solid.

Thanks Florian. When I started this adventure high power servos were quite expensive, they have come down in price. A quarter scale, double servo set-up might work but I would call this just about half scale. I also like the DIY component of this route.

I put a fish scale on the end of the axle, pulling more than I could hold! Could crush a finger for sure.


Online steamer

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Re: Scratch built giant rc car
« Reply #57 on: December 04, 2018, 12:45:19 AM »
Seems to be working now.   Was it a tuning issue or was the servo too small?

Dave
"Mister M'Andrew, don't you think steam spoils romance at sea?"
Damned ijjit!

Offline AlexS

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Re: Scratch built giant rc car
« Reply #58 on: December 04, 2018, 10:20:58 PM »
Great work! If wheels add inertia and friction has also come with some overshoot, you should also try an old trunk damper.  Mount this on the steering arm. Maybe make a long steering arm with multiply holes, so you could change the radius to attache the damper so also the speed of damper movement. So you could change this to try not to much damping or over damping.
No thanks for the model. I had it already. I though it could be possible to 'copy' the steering movement shown in your video. Just to add an extra damper in the model and tune this to get the same sway movement. And then modify the second damper to get it right.
If you want to try this. I am open to try this. If you want to share your 3D modeling.
Alex

Offline Florian Eberhard

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Re: Scratch built giant rc car
« Reply #59 on: December 05, 2018, 07:08:29 PM »
Hey Kevin

Yes, the torque of your servo seems to be quite enough. I rather thought about the gears of the servo than its power - in case you have an impact. But maybe I am just a bit too cautious..  ;)

Florian