For this past off season, we build a tracked robot where we could pull the tracks off and run a 4 wheel drive with treaded wheels which ran great. Thinking more about it, I am trying to understand why you wouldn’t just go with a 4 Wheel drive over a 6 Wheel drive (without a drop center). My thought is a 4 wheel drive is one less wheel, meaning less sprockets, chain, tensioners… Is there something I’m missing that makes doing a 6 wheel drive better than a 4 wheel drive?
By going to the 6 wheel you reduce your wheel base and reduce the amount of scrub you experience with a wider(front to back) wheel base. Typically if you have to wide(front to back) drive base with only 4 wheel you get so much scrub that you actually bounce as you try and turn.
4 wheels works fine when you are wider left to right than front to back
There is something that you’re missing. Two things, actually! A flat 6wd turns more easily than a 4wd because there is less weight on the extreme corner wheels. A 6wd generally turns more predictably than a 4wd, I believe for the same reason. My first point can be demonstrated with a number of available drivetrain calculators. The second point is purely qualitative and based on my experience. I am assuming ‘all else equal’ and ‘all traction wheels’ here.
An increasingly common 4wd setup is 2+2, with two traction wheels on one axle and two omni wheels on the other. This drive setup is light, easy to turn, turns very predictably, but arguably sacrifices pushing force for this additional maneuverability.
Wheelbase is the distance from your back-most wheel to your front-most wheel. Track is the distance from your left wheels to your right wheels. For this example, lets assume the width stays the same.
The longer your wheelbase gets, the harder it is to turn. The shorter your wheelbase gets, the less stable your robot is. If your wheelbase is short enough, using 4 wheels is perfectly fine (assuming you have the proper gear ratios to give you enough torque to turn).
Sometimes, a long wheelbase is desirable or unavoidable. In this case, you can break up your wheelbase into two sections by putting a drop-center wheel in the middle. This gives you the best of both worlds in terms of stability and turning ability.
I do not suggest a 6 wheel drive without a drop center. As you stated, it adds more complexity without much added benefit.
I’ll weigh in here. The drop center for a 6 wheel drive helps the robot pivot a lot easier and helps take stress off of the gearboxes by reducing the scrub from the front and back wheels. For the 2016 season, in our rookie season, our team ran 10" pneumatic wheels only on the front and back and a basically nothing on the middle wheel with no drop center. Being Rookies, we didn’t know any better and didn’t realize the importance of the drop center. So we essentially were running 4 wheel drive. At the beginning of the season it was great and everyone loved it. By the end of World Championship, however, our gearbox suffered a lot of wear and tear and would often bind up. We decided this was due to the scrub from the front and back wheels stressing the gearbox. For off season, we redesigned it with a drop center with 6, 10" wheels, (Yes it was as awesome as it sounds) and it made all the difference in the world. We will be doing 6 wheel drive with drop center from here on out.
Not true. A 4WD and 6WD can have exactly the same wheelbase. OP specifically asks about 6wd without a dropped center.
For the sake of clarity in general, lets use ‘track’ and ‘wheel base’ to describe wheel layouts, as that is an industry standard.
I wouldn’t say that 2+2 has additional maneuverability. It’s pretty much the same dynamics as any other differential drive system except for the fact that it takes up significantly more space when turning (since it doesn’t turn about the center of its chassis).
I’ve read that 2+2s allow drivers to do drifts, slides, and other dynamic driving maneuvers that aren’t practical/possible with 4 traction wheels.
If you design it right, it will turn about its center.
“Right” in this case is the “opposite corners” configuration, with omnis on RFront and LRear (or LFront and RRear) and tractions in the other spots.
But you can play other games with the system as well–like picking which end to pivot around–that may help play the FRC game better.
We’ve modified one of our H drives to be a 2+2 and plan on running it at off season events this year. It’s nice, handles well, and seems pretty responsive. If I were to go from the base up I would still do a 6 WD though, even if using omnis on one or both sides for maneuverability’s sake.
One nice thing about a 2+2 is if you use Vex clamping gearboxes you can get it packaged very modularly. I would hesitate to recommend it unless packaging is a serious issue though.
I don’t know your whole set-up, but I’d caution that the issues you experienced with your gearboxes may not be entirely due to your turning scrub.
What type of wear were you experiencing? What was the mechanical cause of the binding? What gearbox were you using? What material were the gears made out of? What type of grease/lubricant did you use?
I’m poking a little bit at this because I think the fact you used 10" wheels in the first place might be part of your issues. You need a lot of reduction to use 10" OD wheels in a drivetrain that operates at a reasonable speed. Depending on which gearing solutions you used to get that reduction, you may have pushed things beyond their limit. Many of the COTS gearboxes developed for FRC over the past several years have been mostly intended for 6" or smaller wheels, and can be pushed close to or beyond the limits of 7075 aluminum gears when used in very high reduction scenarios. That’s also, presumably, why VexPro stocks 14t and 16t 4140 steel gears.
That being said, higher turning scrub can also help contribute to higher loading on the gearboxes on a regular basis (ie not just stall conditions), which can certainly contribute to these accelerated wear issues.
Why? For the sake of discussion it would be useful to hear your reasoning.
^
We ran 2+2 this season and what you get is the ability to pull off thatlarge spin around defending robots making it harder for someone to side swipe you into a t-bone pin. What you lose is the ability to pull of tight turns meaning you take up more space while making those moves like Ty said and someone hitting you from the side can throw you into a spin. You also have to pay attention to your weight distribution to keep a good portion above the traction wheels giving you less freedom in your design to take advantage of 2+2. There are pros and cons to the system, but its not a more maneuverable drive than 6wd or 8wd.
OP, like you said there are some financial and weight benefits to driving one less wheel, but its a small amount. Taking the time to design a system that eliminates any hassle in implementing chain tensioners would be wiser considering the advice in this thread about handling in 4wd vs. 6wd.
As others have said in the thread it mostly comes down to the center of rotation of the robot. I think it’s generally easier to navigate a field with a robot that turns about its center than its front or back. Getting out of a tough defense situation when you’ve got to turn about the side that is being blocked can be very difficult. When you’re able to turn at center it gives you some more options on the field.