What’s the advantage of a 8 wheel to a 6 or is there any major advantage switching to a 8 wheel drive
It looks cooler
We did 8wd in 2020 to make the robot more stable.
It allowed us to have a drop center with 2 wheels rather than 1. This makes it so the robot doesn’t rock. This in theory made our shots more accurate.
I’ve also heard that it makes turning more predictable to have good autos.
It may help with getting over large obstacles as well.
In 2016, many teams ran 8-wheel drivetrains to overcome obstacles without being beached in the middle.
It also allows you to shorten your wheelbase relative to the robot’s overall length more than a 6 wheel, which generally means smoother turning. But the center 4 drop is my favorite part, since it both (mostly) eliminates rock outside of heavy acceleration and puts the robot’s turning center at its geometrical center rather than halfway forward or back.
You can accomplish pretty much the same effect with a 6 wheel drive by smart selection of wheel spacing, which wheels are dropped, and controlling your center of mass. For example, purposely offsetting your COM in front or behind of the center drop will ensure you are on a consistent set of wheels whenever stationary, and should only rock onto the other set during acceleration. You can also achieve this by having your “center” drop offset from center if you can’t move your COM. Or, rather than just dropping the center set, you could drop the front set and center set (or rear set and center set) to give the same affect.
It’s all about effective wheelbase. Switching between 6wd and 8wd affects not only the location of the effective wheelbase, but also the dimensions of it. Use both of these to your advantage depending on the game.
In 2019, using the 6wd allowed us to hang almost half our robot off the HAB, allowing all 3 robots on our alliance to start on Hab2 for extra points. This wouldn’t have been possible with 8wd. In 2022, we chose the 8wd because a 6wd with a balanced COM is less predictable which way the robot would tip, and we were worried this might affect our shot.
Another advantage of 8wd, at least with the KOP, is that it allows all wheels to be dead axles. This can make wheel swaps much easier. 8wd also comes at the cost of added weight.
In 2019, we did a 6 wheel with no drop center. The omni wheels on the outside helped with turning since we didn’t do a drop center. We double stacked the wheels for a “dually” type setup to help with traction/defense. Worked out really well for us and we had no issues driving it. This felt a bit more agile than our 8 wheel we did in 2018.
Going to preface this post with “Welcome to the realm of FRC Pseudo Science.”
Typically the major advantage of an 8WD over a 6WD is how well (or poorly) the drive will transition over an obstacle. If we assume that the robot Center of mass is approximately centered, an 8WD will always move smoother over the “Crest” of a bump / obstacle because it will be able to “Straddle” that crest (as if it were a 4wd) - while a 6wd will have a tendency to abruptly tip over the crest since it’s going to pivot over the center wheel.
Beyond that, there are dynamic benefits to both chassis, again depending on where exactly the mass is and how you’d like the robot to drive - typically 8WD’s will rock less if the mass is approximately centered, and they will almost always pivot between the two center wheel pairs (assuming the center (4) wheels are dropped) - by comparison a 6WD with a very centered Center of Mass will typically rock quite a bit, which will shift the point of rotation of the drive fore and aft of center. I personally prefer 6wd robots to have a biased COM for this reason, as it means that the robot is always on the same pair of wheels.
There’s also the “More wheels = More Better” dynamic when on carpet - for wheels that dig into the carpet any amount, the more wheels in contact with the carpet, the more traction you’ll have, (up to a certain point, long story) - basically this means that in theory, for some configurations, you will see more traction with an 8wd than a 6wd, assuming the drop is reasonable and the wheels are relatively small.
When we have used Tank drive we have found 8wd with the following configuration to work best for us:
4in diameter wheels; 2 center traction wheels with dropped center and omni wheels on the outside. As some have stated before the benefits we have found are:
- Reduce rocking by a large marging
- Reduce wheel size (with 6wd due to stability, obstacles wheel center distance 6in wheels often work best)
- Allow for more stable and better distributed robot weigh which helps a lot with turning.
- More traction to prevent o fight off distance
- More versatily on drivetrain gearbox placement, you can even drive each pair of traction and omni wheels independently as in a 4x4 truck.
A final note this is all very game dependant but in most “hard obstacles free” games where you decide you are doing tank I highly recomend a 4wd.