belt drive VS chain drive

[Sam_Mills]

Originally Posted by VPpanther

my first guess would have been that chain would be more durable because

Steel is more ductile than the fiberglass or kevlar that you will find in a lot of timing belts. Which is "more durable" or "stronger" depends on material properties which is an entire science in and of itself.

Just know that chain will stretch with use, and is harder to replace for someone who has no experience if you are just handing the drive off to a separate group. More information about the specific use of the bot would be helpful if you want the best possible answers.

As for wheels, Colsons all the way. They wear slower than Hi-Grips and have a higher friction coefficient with carpet. If the robot isn't competing you may want even higher durometer wheels than Colsons though, so that they wear even slower. Colsons can last 100+ matches on carpet, but if its on pavement at a demo, you will eat through an 0.125" drop before you know it.

[asid61]

As far as general drive trains go, I would recommend designing a WCD (cantaleivered wheels, drop center, 6 wheels, shifting gearboxes). We made a nice one in the offseason. For this year's bot, however, we simply applied the concepts learned from WCD (slots, tensioning, bearing blocks) and made a standard 4WD that has independently controllable wheels for switching to mecanum or "regular". The WCD gave us a lot of experience for the effort.

[roystur44]

Belt has a much better damping effect than chain. Try spinning a chain system and a belt system at a high rate of speed.

Belt is easier to keep aligned over a long distance between axles.

[String]

Originally Posted by VPpanther

and I'm not sure if we could do a drop center chassis because the rocking back and forth would severely impact what we're trying to do. The robot would probably only have to face level surfaces most of the time with the odd bump now and again (its actually for a sponsor to take to tradeshows and events to advertise). Would it be bad to just have all 6 colson wheels touching the floor at ounce?

Based on this, I'm not entirely sure you understand the complete purpose of drop center. Without drop center, using Colsons, and given the dimensions of the robot you specified earlier, your robot will have an awfully hard time turning. Drop center increases the ratio of width of wheelbase to length of wheelbase, decreasing wheel scrub while turning. A 1/8" drop is pretty much unnoticeable, but if you really can't have any rocking, consider using omni wheels.

[EricH]

Originally Posted by String

Without drop center, using Colsons, and given the dimensions of the robot you specified earlier, your robot will have an awfully hard time turning. Drop center increases the ratio of width of wheelbase to length of wheelbase, decreasing wheel scrub while turning.

Technically, you are correct in the reasons. However, you need to rethink the robot dimensions, specifically the length/width ratio (assuming 34" wide, 40" long).

They're 34" x 40". 1.17 length/width. A typical FRC robot that's difficult to turn has tended to be a 1.35 length/width ratio (28"x38"), and before that a 1.2 ratio. They'll have an easier time than most FRC robots, particularly given that the actual wheelbase will be shorter than 40", probably more like 36" which will give nearly a 1:1. If the ratio of length/width (actually, wheelbase/trackwidth) goes under 1, turning suddenly gets easier without a need for a dropped center. Ask Ether for the physics on how that works sometime, I'm a touch rusty.

There's a reason you rarely see wide-bots using drop-center--it doesn't really help their turning, and it adds to their overall "oops, I fell over again" factor.

[Scott Kozutsky]

Originally Posted by EricH

Technically, you are correct in the reasons. However, you need to rethink the robot dimensions, specifically the length/width ratio (assuming 34" wide, 40" long).

They're 34" x 40". 1.17 length/width. A typical FRC robot that's difficult to turn has tended to be a 1.35 length/width ratio (28"x38"), and before that a 1.2 ratio. They'll have an easier time than most FRC robots, particularly given that the actual wheelbase will be shorter than 40", probably more like 36" which will give nearly a 1:1. If the ratio of length/width (actually, wheelbase/trackwidth) goes under 1, turning suddenly gets easier without a need for a dropped center. Ask Ether for the physics on how that works sometime, I'm a touch rusty.

There's a reason you rarely see wide-bots using drop-center--it doesn't really help their turning, and it adds to their overall "oops, I fell over again" factor.

Outside dimensions =/= wheelbase. If you're running a west coast drive you've likely got a slightly larger track width and shorter wheelbase than an otherwise equivalent sheet metal drivetrain for example. Other factors like wheel size and center of gravity are major factors in both turning and tippyness, mainly because of how they affect the wheelbase.

From my experience on 1310 (2013), 4wd colsons even in a wide config can cause turning problems. Keep in mind that our high gear was too fast for a 4 cim drive that year but turning was still an issue. (our wheelbase must have ended up at like 2:1 wide but I can't remember the actual dimensions unfortunately)

I have yet to hear a team that used 4wd all traction tank say that they liked how it drove.

Most competitive wide robots I know of used 6wd drop center. 254, 2056 in 2014; 254, 1114, 469, 33 in 2013.

[EricH]

Originally Posted by Scott Kozutsky

I'd really like to see where you got those stats. I haven't heard of many teams running 4wd long, let alone teams doing it effectively. At least without omni or some other non-tank solution.

You haven't been in FRC for 12+ years. Trust me, 4WD used to be the most common drivetrain around, maybe with the exception of 2WD. This was before about 2005, when the 6WD drop center really started gaining ground; 2004 was when the WCD really hit the FRC world.

[Scott Kozutsky]

Originally Posted by EricH

You haven't been in FRC for 12+ years. Trust me, 4WD used to be the most common drivetrain around, maybe with the exception of 2WD. This was before about 2005, when the 6WD drop center really started gaining ground; 2004 was when the WCD really hit the FRC world.

That's fair but similarily these things evolved away from that for a reason. I've been told that casters were once very popular but I haven't seen one compete at all.

Sorry about rewording my comment. I just figured i could get my point across better with different wording and examples but that was after I already pressed submit.

[Chris Fultz]

Check out this -

http://www.chiefdelphi.com/media/papers/2216

and this -

http://www.chiefdelphi.com/media/papers/2246

[asid61]

Originally Posted by Chris Fultz

Check out this -

http://www.chiefdelphi.com/media/papers/2216

and this -

http://www.chiefdelphi.com/media/papers/2246

I saw that before, but I never found a test for 9mm belt, at least not on the load ratings of it. I don't like belts because thay are so wide.

[EricH]

Originally Posted by Scott Kozutsky

That's fair but similarily these things evolved away from that for a reason. I've been told that casters were once very popular but I haven't seen one compete at all.

Sorry about rewording my comment. I just figured i could get my point across better with different wording and examples but that was after I already pressed submit.

No problems there. I think I can counter your examples though. It might help your case to use teams from 2012 and prior; 2013 and 2014 were Frame Perimeter sizing rules rather than Fit in the Box sizing rules, which former tended to result in square-ish robots instead of longer rectangles like happened before the Frame Perimeter came to be used.

You see, there's a big difference between a 28" long robot that's 38" wide and a 28" square or some such similar size robot, something about a longer lever arm to turn the robot. I know I saw one really tippy one, back when the 28" by 38" box was first used in 2005, but I can't recall the drivetrain. (I do know they had wheely bars--saved them more than once! If anybody can recall, it was 393 competing at Sacramento, I think. No, they aren't on TBA--but I distinctly remember that regional having a bunch of non-West Coast teams.)

I have actually seen a few caster-bots competing, and I've actually seen one on Einstein, one of the last competitive ones. Folks have figured out that these things called omni wheels are better than casters, and if you don't have omnis, you really want a peg of HDPE or something like that before you reach for the casters. The HDPE peg might get snagged, but it won't try to control your robot for you!

[Scott Kozutsky]

Originally Posted by EricH

It might help your case to use teams from 2012 and prior

I have no real experience prior to 2010. Both 2010 and 2011 had no real reason to be wide so there weren't lots of wide bots then. The best wide bots I can think of from 2012 all had more than 4 wheels (though usually 8, probably for the bump). 33, 610, 341, 118, 67.
Since I didn't go to champs that year I don't really know some of the other good widebots though.

I understand that 4wd wide can work but I just think it's a suboptimal solution, especially with very high traction wheels like Colsons.