Re: Why such large wheels?
 

Lets see... as I've mentioned in an earlier post, we used the six of the 8" KOP wheels (with SBR Roughtop conveyor belting on them) for budget reasons. They worked extremely well, we never came close to tipping, we got insane traction (although we did leave some rubber on the carpet during our more "spirited" pushing matches), and our ramp climbing was among the smoothest at VCU. (We didn't have to charge it or anything, just drove up it normally) For many cases it shouldn't really matter much, if your center of gravity is high enough that you need smaller wheels, then you have bigger problems. That said, I happen to think that those little wheels on 696's machine look really stylin' and am looking into Colson wheels for our drivetrain next year. (There are other reasons why small wheels will be desirable in a couple of the many drive configurations I'm looking into)

Re: Why such large wheels?
 

We use 6" skyways turned down to 5" and rivet on wedgetop belting. So.. we use six 5.5" wheels. Sanddrag has a point, you dont need very big wheels to get up the ramp. We used skyways because they're like, 5 bucks each, have precision bearings, weigh next to nothing, etc. So our team bought twenty. We have 1" of clearance below the chassis, but we get up the ramp smooth. We have strategically placed rollers on the underside of our chassis, so we never beach.

I say, the bigger the wheel, the harder it is to fit and the harder it is to get a reduction that big. However, the smaller the wheel, the more your tread dies because there is less of it. I think 5-6" wheels is the perfect balance for what my team needs.

Re: Why such large wheels?
 

In our configurations 2.75 was enough cuz the front of the wheel hit the ramp on the slope before the frame bottomed out, however since then we've raised it for other reasons ( namely ball intake and jumping problems )

Also some teams using old robots with .5" clearance getting up, its just not very consistant from what I understand unfortunately.

We used the 8" wheels for clearance and they've worked great.

Re: Why such large wheels?
 

We use 6 8" wheels. The front and back set of wheels are 1" wide and the center wheels are 2" wide because they will always be on the ground.

We use 8" wheels because we need clearance for the the ball to travel under our robot and to our harvester.

Re: Why such large wheels?
 

My team initially had a tank tread design. It was optimal we figured because we could keep ourselves very low to the ground and easily get up the ramp. We had about 3/8" to 5/8" inches of clearance on our chassis depending on where you measured. However at our first regional this year our treads all sheared so we were forced to convert our robot into a 2WD set up. We wrapped the tread around the one driven pulley and put some smooth material on the front pulley. We just assumed at this point that the ramp was out of the question. However in one of our matches an opposing team gingerly pushed us up their ramp and plopped us on the top of their ramp. It got us thinking and we took our robot over to the practice field. Turns out with 2WD and less than half an inch of ground clearance we can still scrape our way onto the top of the ramp. Its not pleasant and once we peak at the top our drive wheels hang in the air, but its far from unreasonable and its perfect for last second ramp point scoring. So onc can get up with basically no clearance that means. We still have about 6" wheels but thats just due to the nature of the tank tread drive train. At our regional next week we're going to modify the drivetrain again to either a 4WD or possibly a 6WD set up. 4WD won't help us get up the ramp, but a 6WD set up would make it even easier. Our other idea is put delrin or HDPE slides on the bottom of the chassis to make it glide more easily over the top.

Re: Why such large wheels?
 

We have always used 8in. wheels and they have always worked for use. One thing though is you stated that larger wheels will take up space. this in a way kind of puzzling. how many times have you ever put something near your wheels? personally through previous experiences we try to avoid putting anything near a moving object, especially a wheel (except chains, axles, etc..). also design comes into consideration. maybe you need x amount of clearance and those 8in. wheels are looking like the easiest way to get to that Clearance. anyways everyone has there own considerations so some times it just happens that you have to use those big wheels. in the future your team might even consider to use large wheels for similar issue or facts. anyways have a great year at comp. and good luck.

Re: Why such large wheels?
 

Your clearance over the ramp will depend greatly on your wheel placement, not only the size of your wheels. Robots who have their wheels placed near the edges of the 38" space will need greater ground clearance than those who are mounted further inwards. The reasoning to space them further out being a more stable drive base in terms of center of mass while climbing the ramp (although less stable if taking impacts from the sides). Additionally it matters how your wheels are mounted to your frame (how far beyond the bottom they extend, as it really isn't the size of the wheel that matters, it the ground clearance, and the two factors are not the same).
Additionally, the relationship between ground clearance and wheel size is altered by the number of wheels you have (as this will change the contact points as you travel up the ramp).
Also, as mentioned before, a larger wheel results in less rpm on the drive shaft to acheive the same speed, which in turn means less stress on the bearings.

Re: Why such large wheels?
 

Big wheels are pimp.

We've been doing 8 inch wheels for years and it works for us.

Re: Why such large wheels?
 

One word...

Preference.

Re: Why such large wheels?
 

6 inch wheels are what we have gone with for the last 2 years. We like them because they allow for climbing easily, and are generally nice to design into as frame. As for what we use for the wheels, we had our own made by our CNC sponsor. you can see them here: (http://www.chiefdelphi.com/media/photos/22758)

One of the main reasons we pick this size over others is the fact that we've used it many times before, and it has been reliable and consistent. We perfer to stick with something we know works, and only go out on a limb for a new design occasionally.