Seeking info on 842’s suspension system from 2016
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On their website they have a contact email coachfredi at hotmail.com. You might have some success emailing them and asking directly.
Ok. Thanks!
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There is some information about it in this thread
It was our way of imitating a rocker bogie type of suspension with fewer moving parts. The rocker part of the drivetrain (on the left side in the image below, click for larger) is free to move, while the trailing wheel on the right side has its own suspension arm and gas strut acting as a spring/damper.
http://i.imgur.com/hhlbEpk.jpg](http://i.imgur.com/ukCkmuJ.jpg)
All of the wheels on one side are powered by a single transmission, with a belt to transfer the power through an idler that sits on the rocker joint. If we were to do this again (we probably wont) we would likely use chains instead, as there was enough flex in the mechanism for the belts to skip under heavy load. The belt on the rocker mechanism itself was a pain to design around, since it needs a lot of wrap around the middle idler pulley.
We also needed to use a pneumatic cylinder to lock the rocker into a “lowered center wheel” mode, otherwise turning in place in high gear was impossible. You can see this in the image below, using a Spectra rope that goes under the electronics board to the front of the robot, to lift the front end of the rocker off the ground.
http://i.imgur.com/IfAlD32.jpg](http://i.imgur.com/Ntas7R7.jpg)
It was a neat drivetrain, but we’ll probably just stick with the normal 6 wheel drive setup when we can in the future, they are much faster to build. Let me know if you have any other questions about it.
Neat idea to keep in mind for obstacles!
It seems that it would it have been easier if the pivot point were at the center wheel; you would not have needed idler pulleys, just one to feed forward and another to feed back, WC style. Is there something about the offset pivot point that is particularly advantageious?
If there is, could you have put the gearbox shaft along that pivot axis, and again had fixed-length belts, though no direct-driven wheels?
A rocker type of suspension gives a lot of travel for all of the wheels relative to each other, which is really important when you are trying to make the robot drive over large things. If the middle wheel is unable to move up or down, then it can’t tuck up out of the way when traversing something like the Rock Wall from 2016. The idea is to keep the robot motion to a minimum, and let the wheels all move around to conform to the ground.
The other huge benefit of a rocker suspension is that the wheels all naturally split the weight of the robot. Whatever shape of ground you put under the robot, each wheel carries 1/6 of the robot weight and they all have traction. You can imagine that if you have a fixed middle wheel, with a wheel on a spring at the front and back, you will have a lot of trouble picking a spring with the right travel and spring rate to get what you want. If it’s a stiff spring, you’ll keep the wheel planted to the ground and have traction, but you won’t have any travel for the wheel anymore! Likewise, if it’s too soft, then you’ll have lots of travel, but no downforce on the wheel.
We have bumper rules to follow, too! The rocker lets the wheels move a lot without affecting the frame height. If you lift one of the rocker wheels 1" off the ground, the frame rises (at the pivot point) by only 1/2". It’s easier to stay in the bumper zone while still maintaining a lot of wheel travel.
Here is some more footage of the suspension going over obstacles. You can see how the wheels on the gas struts don’t always go up and down like we’d like, since it’s hard to tune the spring force. I kind of wish we were able to make a true rocker bogie setup like on the Mars rovers instead of using the cheaty trailing wheel on a gas strut. It would have been smoother and cooler. But we probably would have never finished the other mechanisms on the robot if we did.
Having the transmission at the rocker pivot point would have run us into some pretty bad ground clearance issues.
In retrospect, what we really should have done was just make a normal 6WD with pneumatic tires like all of the good teams did. 
You lost me on this point. Am I looking at this incorrectly, or isn’t the rocker pivot pont above the wheel axles? In any case, I was referring to putting it at that pivot point which was a constant distance from each of the three wheels on that side.
The rocker pivot is right in the middle of the rocker assembly:
http://i.imgur.com/JJcg3IV.jpg
We located it there because it was easier for us to build the rocker assembly if it was essentially a straight tube. It could be located higher, which would need a different design for the rocker and change the paths that the wheels move along, but it would still function. It would be a neat design challenge, but you could locate the transmission at the rocker pivot location (if it’s higher up, to get ground clearance).
I don’t think you would want to put the transmission output shaft, the rocker pivot, and the trailing arm pivot all on the same axis though. That would require 2 different frame members to pivot on the same shaft, that you happen to be transferring all of your drivetrain power through. You would also have to come up with a way to keep the chassis from rotating freely around that axis, probably similar to the cross beam that pivots on the Curiosity rover.