6/8 Wheel Extra Features
 

This year, we really like our 6 wheel drive. We wan't to improve it for next year, including increasing performance in general, and by maybe adding some extra features.We had some questions about these features, if anyone has used these before, including:

2 in thich Wheels

• Is there a notable increase in turning scrub?
• Is the increased traction noticable? In what ways?
• Have you found that it improves your defensive/offensive performance?

Drop down casters
(I heard that 987 has done this previously. Correct/incorrect?)

• How effective is it at getting out of t-bone pins?

Drop Down Outer Wheels
(I heard that 469 has used this previously. Correct/incorrect?)

• Is the increased traction noticable
• Have you found that it imroves your offensive/defensive performance?

Obviously, in qual matches, these would most likely have little to no effect on performance. But in elims, it seems like there could be a significant improvement in performance.

Re: 6/8 Wheel Extra Features
 

Yes we did. We used it quite a few times last year, extremely useful.
We would have one this year, if we had the weight. :P

Re: 6/8 Wheel Extra Features
 

2 inch wide wheels don't make as big a difference as tread material, and they can make turning a little screwy.

We used drop down omnis instead of casters and the drivers loved them. They did help against defense.

Re: 6/8 Wheel Extra Features
 

What I quoted above is one of the most important things a team can do. Setting a baseline, and improving from it each year.

On FRC2168, we did the same thing this year, and intend to improve again on it for next year.
Things we improved on from 2011-2012:
- Moved from a 6WD to a 8WD to gain some stability (and to go over the barrier)
- Designing for Center to Center distance between axles to have exact chain or belt length
- Designing the tensioners in as standoffs as well (A 1" nylon rod with an off center thru hole, rotate to add tension)
- We continue to use 1.5" wide Colson Performa wheels with a WCP hub and dead axles as we like the grip and the wear properties

Things we are looking to improve/add in the future:
- An Integrated braking system
- Belt Drive
- Reduce weight
- Increase efficiency (less gear reduction/better gearing selection)
- Attempt to decrease wheel diameter
- Improve packaging/modularity

About you question in wheel width, this topic has been discussed a number of times, with different answers. For us we have noticed that while our traction seems to be about the same, our tread seems to wear better with a wider contact patch. It's for this reason, and the amount of space they take up that we settled on using 1.5" wide wheels.

There are a ton of options out there in terms of wheel options, and even more for tread material. I strongly suggest exploring them.

Re: 6/8 Wheel Extra Features
 

I am confused on this. Can someone explain why having drop down casters would help with defense? Also, what do Drop Down Outer Wheels provide? Some pictures would help if possible.

Thanks.

Re: 6/8 Wheel Extra Features
 

It changes your center of rotation allows you to spin about 2 outer wheels rather than the middle wheels. This prevents you from getting t-boned by allowing you to spin out of it.

Re: 6/8 Wheel Extra Features
 

Traditional Skid Steer Drive trains are very susceptible to 'T-Bone'* pins. Because Most Skid Steers rotate around their center, or somewhere close to it, they cannot simply spin out of the pin - if you draw a simple sketch, you'll see that while you're rotating one end of your robot away from the defender, you're also rotating a side into the machine.

Dropping a caster/omni-wheel/skid on one end helps to move the pivot point of the drive to the opposite end, which allows the robot to simply spin out of the pin.

*T-Bone Pins usually refer to an instance where a robot is hit perpendicular to it's drive train's motion - these usually happen on open field, essentially pinning a robot without needing a wall/object.

Re: 6/8 Wheel Extra Features
 

I'll make a plug for WCD ("West Coast Drive") here. The concept of the frame setup, wheel blocks, live axles, cantilevered wheels and tensionless chains all cobmine to make it a great, versatile drive setup. Our team will do this next year since we proof-of-concepted the first sliding block in March. It took 2 hours to make the block on a manual mill, but we feel confident we can get that time down.

WCD itself isn't the answer to everything related to drive train -- yet it is the one "baseline" drive train that allows for the most design adjustments (# of wheels, wheel diameters, wheel types, tread types, which gearbox, etc) in the least amount of time. Adding extra features (like a drop-down caster/omni) is very easy as well.

Re: 6/8 Wheel Extra Features
 

Are you referring to turing scrub? Has anyone else used 1.5 in wheels with success?

By dropping your outer wheels, you theoretically are able to gain more traction when you are trying to push through defense or stop offense.

Re: 6/8 Wheel Extra Features
 

Some tips learned from our drive-train this year:

Hollow tubes make for great axles, These were tapped so that there were no collars on any of the drive axles, it also saved a ton of weight, we used .495 OD with roughly .1 walls.

Track width, our 6 inch performance wheels were less than .125 from the edge of the 28 inches, it made turning much more forceful.

Maximizing traction, blue tread from Master on 6 inch performance wheels has a LOT of grip.

Choosing the proper gear ratio, our high and low gears were 16.6 and 6.7 ft/s this is NOT traction limited in low gear, enabling us to push robots with lower gear ratios when they broke traction due to overzealous driving.

Drop down casters sound interesting, our driver never got stuck on other robots, but if he ever got close/parallel to a wall it was difficultly to quickly get UN-stuck.

Re: 6/8 Wheel Extra Features
 

If you look several posts higher you will find that someone mentioned 1.5in wheels.

Re: 6/8 Wheel Extra Features
 

Exercise caution in future years and apply the material for the year rather than using the "we always do it this way" way of thought. I'd be concerned that the outer axles would bend if it came off the bridge or bump incorrectly. Did you do any simulation or calculations to figure out the tensile stresses your axle could hold?

On a flat or predictable field like 2005, 2006, 2007, 2008, 2009, & 2011 this isn't a concern.

Re: 6/8 Wheel Extra Features
 

I saw. I was just wondering if the consensus was that 1.5 in wheels were the way to go.

Re: 6/8 Wheel Extra Features
 

As far as the width of the wheel, we on 973 noticed a big difference in traction with 2" wide wheels vs traditional 1" wide wheels.

In 2011 we used a wcd with 4" diameter, 2" wide weels, with a 3/16" center drop. The combination worked great. The drivetrain allowed us to switch from strict offensive play to strict defensive play with the ability to push every robot we came in contact with. Check out the cad for Titan in my signature. If you like what you see feel free to use our design(s) and pm us if you have any questions.

Re: 6/8 Wheel Extra Features
 

In 2011 team 33 had an 8wd with the back wheels able to articulate up/down. When they were down it picked up the back of the robot enough to lift the inner 4 wheels off the ground making it very difficult to turn us. When they were lifted up we were an 8wd that could turn on a dime.

Just adding to the mixing pot.
Bryan

Re: 6/8 Wheel Extra Features
 

What kind of wheels, and what kind of tread? And any reason for using 3/16 drop over 1/8 drop? Did you feel it had an advantage? If so, what was it? Any disadvantage?

Re: 6/8 Wheel Extra Features
 

My guess is that because they were using 2 in thich wheels, they had an amount of turning scrub that they didn't like. By increasing the drop, they decreased that amount.

Watch this http://www.youtube.com/watch?v=mhtdk...e_gdata_player video of us (45) and 111. We play defense so they cannot deploy their minibot. Interestingly enough, the T-Bone pin is crazy effective, and because we were soo good at pushing last year, they just could not get out of it.

Re: 6/8 Wheel Extra Features
 

Our drive was setup so that there was about 1/4 of an inch between the wheels and the bearing blocks. This allowed for the lighter axles, bending was never an issue, one match we even drove off the 24 inch drop made by an unbalanced bridge with no damage to the axles. Keeping the axles short was critical to maintain a robust system.

Edit:
We did test the axles on an our off-season WCD, they were flawless and bending was minimal. We beefed up the axle wall thickness a bit for competition to ensure they would never break.

Re: 6/8 Wheel Extra Features
 

The wheels are custom, we used blue rough top tread. The reason we went with 3/16" drop over 1/8" drop was to ensure better turning capability, less scrub on the raised wheel, etc. The advantages were that we could push every robot we came in contact with (even 469) without any fancy add-on's and without giving up offensive speed. There weren't any big disadvantages with the drive (the only thing that really gave us trouble was the 775 motors (which were removed before champs), if we use the design again we will use 550's in conjunction with the 4 cims).

Though for the moment, emperor swerve is working great for us, so it may be a while before you see 973 using the drive train again. ;)

Re: 6/8 Wheel Extra Features
 

This is where my team started with the development of our current drive trains that we use. We started with IFI sheet metal 4" X 2" wheels. We then went to custom 4"x2" wheels. After that we did the math on the physics and realized that the difference wasn't that big between 2" wide and going down to 1" wide. Also we had greater variation in traction with various tread materials that we tried out, which is why we currently use 1" wide AM performance wheels with the best traction of all materials we tested. It's one less thing to make and 1" wide wheels package better.

Re: 6/8 Wheel Extra Features
 

If only two axles are touching the ground, how does lifting the remaining axle further off of the ground help?

I'd bet it's more of a wheel wear concern.

Re: 6/8 Wheel Extra Features
 

Sometimes, the drop is slight enough that the center wheels can sink in the carpet a bit and the wheels which are "off the ground" will make slight contact which can degrade performance slightly.

Re: 6/8 Wheel Extra Features
 

We actually ran into this problem this year. We designed a .125" drop into our 6WD drivetrain, but found that we didn't like the turning once we actually got it running. Swapped out a pair of plactions for a pair of omnis on our competition and practice bots and it worked much better.
I'm not sure what exactly caused it to behave so poorly turning. My guesses were either the drop was insufficient and the wheels still scrubbed because the robot sunk into the carpet, or our chassis flexed, allowing all 6 wheels to contact the ground.

Re: 6/8 Wheel Extra Features
 

I'd buy the chassis flex argument, or maybe if the target 1/8" drop was not reached for whatever reason. We've never had any issues using a 1/8" drop with 1in wide plaction wheels in a 6wd setup over the past couple years.

In the past we've seen the center set of wheels (skyway wheels FWIW) wear down to the point of negating whatever drop they had, and that was an issue.