Hey I have an idea to use the 5" wheels as part of a 6 wheel drive. I have a few questions as to how it will perform and whatnot.
Would it be fine to just drive the middle wheel (5") and the rest just 4" omni wheels just idling to provide easy turning?
What would be the path of turning if both the middle and back wheels were the 5" wheels (both being driven) and the front being the 4" non driven omni.
Just like question 2, but with the omni being the back and the 5" being the middle and forward.
Please help.
PS: Don’t worry about the motor output and speed ;).
Would you design it such that the wheels are always (ie all 6 of them) touching the ground at the same time?
I only ask because it is possible to do it in other ways, and some teams have shown this to be a viable and effective strategy. Good to make sure we’re all on the same page before we discuss further.
-Just so you know you won’t be able to fit 3 5" wheels in a 17.5" long robot. If you wanted to use a 6wd then you would have to stagger them.
-Driving only the middle wheels in a 6wd is not a recipe for success IMHO. Drive all of your wheels, always and your drivetrain will be better for it (you loose tractive force for every wheel which is not driven.)
-The path of turning for what you described in #2 and #3 directly relate to how far apart the two 5" wheels are. When looking at how your robot will turn draw a picture of your wheelbase from the top. Then draw a circle from the center of your robot through all your wheels, this is the path your wheels take as the robot turns. If the wheelbase is longer than it is wide your outer wheels will literally be dragged sideways while you are turning. This makes for very slow and inefficent turning (if at all). This is why most people who use a 6wd drop the center wheel and most 4wd robots have omni wheels on them somewhere (very low sideways CoF.) When designing, make sure your wheels that are touching the ground have a wider wheel base then long and/or are omni wheels.
Thank you, yeah, I know that omni wheels are the best for easy turning but unfortunetly vex does not sell 5" omnis ;), which relates to I believe my first question, only driving the middle wheel.
Reason for using the 5" wheels is to put less stress? Or friction on the drive.
The Vex 5" wheels are marketed as the highest traction wheels available. Why would using the highest traction wheels put LESS stress and/or friction on the drive?
It Won’t. High traction tires will draw more current as manuvers are more violent (turning, forward/reverse slams) making it more likely that your breakers trip.
Consider looking at the 2.85" VEXplorer wheel if you have an opportunity to buy a set. We are running the 2.75" traction wheels on the outside and the 2.85" VEXplorer wheel in the center of our drive, which gives us a really nice .1" difference in diameter and mimics the ‘drop center wheel’ commonly seen in FRC.
If your heart is set on a large diameter wheel though, one thing we did with a previous robot was take a 30 tooth HS sprocket and insert it inside the 4" traction wheel rubber tread. It actually makes the wheel slightly larger than 4" and we didn’t have any issues with the tread slipping off the sprocket. Use a couple of bar locks to secure the shaft and you’re good to go (thanks to KTOR for this tip).
Assuming 5" and 4" wheels are actually that size, then you can mount the 4" axles one hole lower to make a planar drive train. Moving the axles by non 1/2" (or non 1/4") increments is trickier. Pillow blocks with washers are one of the easier answers.
Six wheels, 5" in center and new small omni in all four corners should be fine,
as long as the center wheel never gets lifted off the ground. There will be some trade-off between drop-center amount and tipy-ness when driving with the arm lifted.
Your plan to drive only the center 5" wheels reduces the following usual reasons why 5" are not so good. Good luck, and be sure to post how it works out for you.
Here are some reasons 5" wheels have generally lower drive performance for VRC bots:
larger = clumsier to fit
larger diameter = heavier = more rotational inertia = accelerate more slowly
larger = require more torque for the same gearing (although they provide more speed)
high wheel scrub when turning
larger diameter means balance point is farther from corners of robot = robot tips over more easily.
Most past VRC games have a foam flat field with no obstacles to drive over.
Large wheels (or treads) may be specifically better for other applications like BEST or FTC, where there are objects on the field that you need to straddle over, or climb over with your wheels.
This was a little unclear, you said that because I am driving only the middle 5" wheels and having the 2.75" double roller omni wheels that, (and this is here I got confused, sorry) I am eliminating the following “usual” reasons why not to drive 5" wheels? Correct?
I think, that ima keep the regular design and run a 5:3 gear trip with just the 4" wheels middle being high traction and the forward and back being omni, I would like to use the 2.75"/2.85" wheels, but the gearing to make it the same speed as the 4" is difficult, (using 6-8 motors.)
But if I did use the smaller wheels, this would reduce the chance of tipping, correct?
Small wheels can reduce tipping if you move the axles closer to the corners.
They also have a lower center of gravity, and promote building low CG frame.
You can still build a low CG frame with larger wheels if you try.
Nominal measurements:
A 4x5" wheel robot that is 18" long can have axles 13" apart.
A 4x3" wheel robot with axles same 13" apart has same tipiness based on wheelbase.
A 4x3" wheel robot that is 18" long can have axles 15" apart, which reduces the part of tipiness that is based on wheelbase length.
The other component to tipi-ness is CG (Center of Gravity).
Low CG prevents tipping, mounting batteries low and centered is one way.
Mounting tower motors lower and using gears or chain is another way.
Alright, Tomorrow at robotics I will be trying out some gear ratios and some double/triple gearing that will “apparently” relieve some stress off the motors with still roughly the same torque and speed.
I have decided to try and use the 2.75" wheels with what is becoming more common, the wheel-e bar. This might be a little difficult and may even be unnessisary due to the frame/drive shafts being so close to the ground. Within a month or so I will post a picture of my final Product. (Finished robot)
I was thinking about this the other day for a new 6 wheel drive.
three 4 in wheels per side
omni, traction, omni
36 tooth on axles, 84 tooth on motors … 3:7 speed increase
10 in between outside axles
4 393 motor drive… one on each 84 tooth gear for skills challenge… maybe two extra 269’s for normal competition.
We know that the traction wheel is slightly smaller than the omni which is the reverse of what we want but in general the load distribition would favor the front/center or rear/center two wheels so the effective turning wheel base would usually be closer to 5 in.
In theory, this would have an advantage over our existing 3:5 geared 4 wheel drive with an 8 in axle spacing between traction and omni’s in that it would be screaming fast and more stable due to the larger wheel base. It still fits nicely on a 25 hole chassis and only increases the total length from 12.5 in to 14 ins. Plus no chain is required.
Anyone tried something like this?
I noticed that 1064 is using 6 wheels in this config with a slower gearing.
Haha, yeah, we’ll in the design built now, I have that wheel config. With a more or less 5:3 (6:3 (sprockets)) with 1 393 and 2 269s on each side, and sadly with this, during the skills it’s definitely what I want, getting a little greedy and want to be a little faste. But… 2 sets of motors has burnt and it’s time for a redesign.
Unfortunetly vex is lame and is not selling the vexPlorer wheels anymore so I cannot go with the 2.75" rocker base. Hopefully the 2.75" 6-wheel omni and traction will be enough
You can do creative things to the small wheels to make them bigger, before putting the tires on. String, rubber, ziptie wraps, double tires, etc.
Or just use a spacer under a pillow block bearing to get the drop center you want.
