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Help with building an 8x8 vehicle


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#1 Paul202

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Posted 29 October 2012 - 02:12 AM

I recently started trying to builds again, after a hiatus of a couple years. I am going to attempt to build a BTR-80, a Russian 8x8 APC. However, my building skills are a little rusty, so I was wondering if anybody has any advice to give me before I started building. I have looked around on the web, but not that many people have built RC 8x8 vehicles, excluding tatras. I have bought and am halfway through Sariels new book, which I hope will provide some good tips. I want to make it full wheel drive on all 8 wheels, and I am going to be using the motors from the new 4x4 rock crawler set which I just purchased yesterday on Bricklink. I have used Sariels model scaler to figure our the dimensions, and I plan on using these wheels:
http://www.bricklink...P=41896c01&in=S
However I need some help on designing things such as the chassis, suspension, gearing, frame, and bodywork. I was wondering if any body knew of some sources or had some useful advice on their own to help me.

#2 Alasdair Ryan

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Posted 29 October 2012 - 02:54 AM

That should be a very simple model to make. :laugh:
I would start by making four axles using the common linear actuator bracket as a steering hub that should allow you to get a decent steering angle.
Make a frame to fit on top of each axle and have the motors connected to it.I guess something like this will have live suspension so you could have the drive coming up to the pivot point of each axle.
In-between the too front and rear axles you can fit your servo motor for steering and have a drive shaft to each side,if you really want to push the boat out you could add a gearbox so you could have different steering modes ie crap,front,rear....

Edited by Alasdair Ryan, 29 October 2012 - 02:59 AM.

Posted Image

Updated 02/08/12
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#3 jorgeopesi

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Posted 29 October 2012 - 03:19 AM

May be something like this...



I have a few pics on my brickshelf page. I tried independent suspension and pendular, better with independent.

#4 legomuppet9

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Posted 29 October 2012 - 09:12 AM

This might help: http://www.eurobrick...50#entry1418838

#5 Paul202

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Posted 30 October 2012 - 12:09 AM

http://s1174.beta.ph...e6a36c.jpg.html

This is my first prototype of the front axle. In the final version, the shocks will be replaced with hard yellow ones, but I need to order some from Bricklink. I am obviously using independent suspension, and I have tried to model it after the real thing. I decided not to have multiple forms of steering, as from my research I see no sign that the real life BTR-80 has multiple types of steering.

#6 Paul202

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Posted 30 October 2012 - 09:01 PM

In the process of designing a BTR-80, I have begun to make the rear axles. However, after further inspection of my distinct, I have realized that when the wishbones rise and the suspension compresses, the axle that holds the bevel gear in the differential reclines, and the bevel gear pops out. Furthermore, I think there is some friction on the axle, as there is resistance when I turn it. I think it may be possibly related to the way the suspension is set up, but I want to keep it like it is because the suspension is the same as on the real BTR-80.
Pic:   http://s1174.beta.ph...e6a36c.jpg.html

BTW, I have replaced the shocks with some stiffer yellow ones, and that might affect something.

Edited by Paul202, 30 October 2012 - 09:02 PM.


#7 D3K

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Posted 30 October 2012 - 10:49 PM

Looking forward to seeing the finished project!
On topic however;
It's a bit difficult to see from the image you posted, but when making a double wishbone setup, it's really important that the wishbones and the u-joints bend "as one", that is so that the u-joint is neither further out nor further in than the pivot point on the wishbones. Or else the geometry will misalign as the suspension travels.
An alternative could be to use CV-joints instead, as they are essentially frictionless where you put the axle, and allows for some travel (about 1,5 L I think?). However, for an 8 wheeled soviet APC you would need eight of them, or maybe even 16, as well as the "female" CV-joint-axles.. and they are not especially cheap..
Hope this helps! :classic:

I  made a quick model to show you what I meant by the allignment:
Posted Image
This shows the concept, however it would not work, as the arms (in this case (that's not even a wishbone)) are touching the axle

Edited by D3K, 30 October 2012 - 11:12 PM.


#8 dhc6twinotter

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Posted 31 October 2012 - 12:14 AM

D3K hit the nail on the head.   Sounds to me like the u-joints don't lign up with the pivot points.  This will cause the axles to slide out and/or cause excess friction on the axles.

A side note:  It's possible to build a suspension with the u-joint offset from the pivot points, as long as it's offset equally on the oppposite pivot point.

#9 Paul202

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Posted 31 October 2012 - 02:20 AM

Well apparently for some stupid reason I had put a friction pin in between the wishbones, and removing it cleared the problem of friction right up. However I realized that when I push the suspension up, the U-Joint and attached axle push out utill the U-Joints form a straight line, and at his point the axle recedes as you push the suspension farther up. I think it is because unlike the common double-wishbone Independant pieces of the 8297, I am not using a CV joint, a joint that allows an axle to move into it 1.5 studs. I am using a universal joint p, which does not have this feature, and none of the pieces in my collection have this feature. I think some pieces were include with the new Supercar that allow you to do this, but unfortunately I don't own the new supercare. Short of buying new pieces off Bricklink what would be the best, if any, way to solve this problem.

#10 dhc6twinotter

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Posted 31 October 2012 - 05:59 AM

Hey Paul,

If the geometry is setup correctly, you don't need the axle to slide in/out of the CV joint.  If you don't have any special hub pieces or CV pieces, it is still possible to build an idependent suspension with just u-joints.  It may not be as compact, but it's still doable.  

Do you have any pictures of your suspension setup without the tires?

#11 Lipko

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Posted 31 October 2012 - 09:03 AM

The steering will be the tricky part. I'm struggling with it at the moment. If you go for rack-and-pinion steering, make sure the rack is guided from all sides. I mean guide it from the toothed side too somehow. If you don't do this, the 12 tooth gear itself won't be enough, and the steering will be loose (you can push the wheels towards each other, because the rack can move a little vertically).

#12 MarkusOSX

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Posted 31 October 2012 - 10:33 AM

I´ve had the same problems with gearrack steering on my 8x8. This is not very tigh as you can see.

So i´ll use linear actuators now.
Posted Image
Greetz

Markus

Edited by MarkusOSX, 31 October 2012 - 05:13 PM.




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