6WD vs 8WD

[Hawiian Cadder]

well, as far as i understand, the goal of having the outer wheels raised is to make them act more like omni wheels. for the purpose of the argument imagine all we care about is drive systems with omni wheels on the corners. in a 6 wheel configuration, 1/3 of the weight is on the traction wheels, and there are 2 of them, in an 8 wheel configuration 1/2 of the weight is on the traction wheels and there are 4 of them. so if the goal of having the outer wheels raised is to make them behave more like omni wheels, in that they can scrub sideways with less resistance, then logic supports that an 8 wheel drive-train will be harder to turn, because there are 4 wheels, and they scrub sideways a little bit, and because there is more of the robots weight on those wheels at any given time.

[Madison]

Originally Posted by Hawiian Cadder

well, as far as i understand, the goal of having the outer wheels raised is to make them act more like omni wheels. for the purpose of the argument imagine all we care about is drive systems with omni wheels on the corners. in a 6 wheel configuration, 1/3 of the weight is on the traction wheels, and there are 2 of them, in an 8 wheel configuration 1/2 of the weight is on the traction wheels and there are 4 of them. so if the goal of having the outer wheels raised is to make them behave more like omni wheels, in that they can scrub sideways with less resistance, then logic supports that an 8 wheel drive-train will be harder to turn, because there are 4 wheels, and they scrub sideways a little bit, and because there is more of the robots weight on those wheels at any given time.

In six- and eight-wheel drive systems with lowered wheels, we examine the behavior of each as two or three four-wheel drive systems, respectively. The raised wheel isn't treated as an omniwheel, it's treated as if it doesn't exist.

[kramarczyk]

Originally Posted by Hawiian Cadder

... the goal of having the outer wheels raised is to make them act more like omni wheels. for the purpose of the argument imagine all we care about is drive systems with omni wheels on the corners.

You may want to look further into this assumption.

As you raise the wheel (or increase the center wheel drop) you decrease the load on those wheels. Yes, it decreases the sideways slipping force on the wheel, but it also decreases the tractive force in the same proportion. The first point is that it never really behaves like a omni wheel, just as a wheel with less traction.

The second point is that the load drops off across the distance of your wheel dropped (teams range from 1/16-1/4") which makes it hard to tune. I worked with an adjustable drop 6WD for a few years and we discovered that 2-3mm drop was what worked for us as optimum. Outside that sweet spot we began to get significantly different driving performance. We also discovered that a 1/2 day of matches was enough wear on gum rubber treads to get us outside this range. We eventually decided that it was not worth the effort and decided to let the wheel off the ground and allow the robot to rock between the two 4 wheel systems. It was much simpler in the long run and allowed us to apply our resources elsewhere. I know the 2011 kitbot configuration has enough lift to accomplish this.

You may want to spot check with teams to see if all of their wheels are actually on the ground or just really close. How might having one set of corners off the ground affect the logic you used above?

[Wayne TenBrink]

For Logomotion, we had an 8WD chassis with 6" wheels. The four at the corners were "kit" wheels. The four in the center were closely spaced pneumatics with adjustable drop. (I noticed in St. Louis that 330 used the same arrangement). We adjusted the tire pressure and center wheel drop so that the corners were about 1/16" off the floor on a hard surface, and had light contact on the carpet.

We were quite happy with the arrangement. It turned very well, since the pneumatic tires had better traction and higher loading than the corner wheels. It didn't rock or bounce during accelearation or turning since the corner tires were hard. During acceleration or in a pushing situation, all four pneumatics and one set of corner tires were in contact with the carpet. It was very difficult for others to push or turn us.

It was a good balance between the agility of a 6WD with center drop or omnis, and the pushing power of an 8WD with pneumatic tires.

[Triple B]

eight

[Chris is me]

Originally Posted by Triple B

eight

Why 8?

[Triple B]

From my experience, the 8 gives you more stability at higher speeds and better predictability of how the robot will handle. We also decreased the diameter of the outside wheels instead of offseting the axle height.
mike d

[IKE]

Originally Posted by Triple B

From my experience, the 8 gives you more stability at higher speeds and better predictability of how the robot will handle. We also decreased the diameter of the outside wheels instead of offseting the axle height.
mike d

Did you change the ratios to match teh diameter changes, or just allow for the inherent scrub to occur?

[Triple B]

no change of ratio, just let it ride.

[Alan Anderson]

Originally Posted by IKE

Did you change the ratios to match teh diameter changes, or just allow for the inherent scrub to occur?

Our 2007 "Rack 'n' Roll" robot was a six-wheel drive with no center drop. It had extra-thick tread on the center wheels and it drove like a dream. We never figured out whether the very slight discrepancy in wheel circumference made any difference one way or the other. There was some conjecture that a teeny bit of scrub on the corner wheels might actually result in better turning.

[Blake L]

Personally, I like 8WD, although my opinion might be a bit biased because I haven't driven a 6WD robot anywhere near as much as I've driven an 8 wheeled one.

This year my team used eight 4" diameter wheels geared 7:1 and I loved it.
Here is a video of us playing with the chassis a week into build season:
http://www.youtube.com/watch?v=inI40uMKuEE

If I remember correctly, the center two wheels are dropped 1/16". We later added another two motors to the drive train which gave us a nice balance of speed and power.