pic: 6CIM WCD

[Joe G.]

Originally Posted by Adrian Clark

looks great!

But it looks like you have some problems with your chain interfering with your gearbox standoff and possibly your belly pan too. I would suggest you downsize to 16T sprockets instead of the larger ones you're using.

I would not recommend this. Smaller sprockets load your chain more, and make your drive less tolerant to variable tension and alignment inperfections. We did a similar drive with #25 16t sprockets, and were throwing chains left and right. There should be plenty of room on a drive like this for bigger sprockets.

[HammadB]

Originally Posted by Joe G.

I would not recommend this. Smaller sprockets load your chain more, and make your drive less tolerant to variable tension and alignment inperfections. We did a similar drive with #25 16t sprockets, and were throwing chains left and right. There should be plenty of room on a drive like this for bigger sprockets.

Agreed. We try to stick to (if #25 chain) at least a 22t sprocket on the drive. Some rough tests I've done have shown this to be the smallest tolerable size. And when I say tolerable I mean it works pretty darn well.

However, if you're aligned perfectly the smallest of sprockets won't give you an issue. But are you really going to be perfect?

[AdamHeard]

For comparison sake;

We traditionally have run 22T sprockets for 3.5-4.5" wheels, and have a lot of runtime on that. It's bulletproof.

We ran 16T sprockets for 4.3" wheels for the start of the season, was bulletproof.

We then ran 16T sprockets w/ 6.3" wheels, and threw the same chain 3 times. I assume some sort of minor misalignment existed that was exaggerated by the very high chain load. This was late in the season and only on the comp bot (practice was fine with massively more runtime) so we never investigated the issue.

I'd say 16T sprockets are fine if your chains are all lined up properly for even 6" wheels, and are bulletproof for 4" wheels.

I know this exceeds the rated working load of the chain depending on how you look at it, but whether or not that situation actually arises is unknown.

[craigboez]

Originally Posted by AdamHeard

For comparison sake;

We ran 16T sprockets for 4.3" wheels for the start of the season, was bulletproof.

We then ran 16T sprockets w/ 6.3" wheels, and threw the same chain 3 times. I assume some sort of minor misalignment existed that was exaggerated by the very high chain load. This was late in the season and only on the comp bot (practice was fine with massively more runtime) so we never investigated the issue.

Slightly off topic, but what were your reasons for switching to larger wheels in the middle of the season?

[R.C.]

Originally Posted by craigboez

Slightly off topic, but what were your reasons for switching to larger wheels in the middle of the season?

They wanted to be able to drive over frisbees!

-RC

[AdamHeard]

Originally Posted by R.C.

They wanted to be able to drive over frisbees!

-RC

Yup.

Our ground clearance was awkwardly too high to keep them out, and too low to drive over them.

Looking at skirts, etc... The easiest solution was to go to larger wheels.

[sanddrag]

Originally Posted by AdamHeard

Yup.

Our ground clearance was awkwardly too high to keep them out, and too low to drive over them.

Looking at skirts, etc... The easiest solution was to go to larger wheels.

Funny. We went to larger wheels for fear of not being able to make it over the "bump" with our original 3" wheels. At that point, a standard zip honestly would not fit under our robot. Oops.

[DampRobot]

Originally Posted by sanddrag

Funny. We went to larger wheels for fear of not being able to make it over the "bump" with our original 3" wheels. At that point, a standard zip honestly would not fit under our robot. Oops.

Wow, 3" wheels? I know 254 has run 3.5"ers, but 3" is really really small! Were they that small to allow for less reduction, for compactness, or something else?

Originally Posted by DampRobot

Wow, 3" wheels? I know 254 has run 3.5"ers, but 3" is really really small! Were they that small to allow for less reduction, for compactness, or something else?

3" wheels aren't really that small. I've gotten some decent clearance from 3" colsons on some CAD models on WCDs and other drive variations. If it weren't for colsons being the only source of 3" wheels, I'm sure we'd see a lot more of them in competition.

[Jon Jack]

In 2012 we ran a single row of rivets on the underside of the chassis siderail. When we were welding the chassis we noticed that there were a couple of areas where the siderail was not standing vertical and the top had pulled in slightly. This year, instead of running a single row of rivets, we put the rivets in a 'zig-zag' pattern and the issue went away.

Normally we run 22T sprockets so that the chain clears the chassis cross member. With this year's chassis not needing a cross member we went with 16T sprockets and had no trouble with them all year. Tensioning is huge with 25 chain, even more so IMO than alignment. In the 8 years we've used 25 chain, the only time we've had an issue with throwing chains was when the chain was not properly tensioned.

[Chris is me]

Originally Posted by Andrew Lawrence

3" wheels aren't really that small. I've gotten some decent clearance from 3" colsons on some CAD models on WCDs and other drive variations. If it weren't for colsons being the only source of 3" wheels, I'm sure we'd see a lot more of them in competition.

I would call 3" wheels "really that small". On a 2x1 frame, that leaves you with a large span to high center on, as well as at most .625 inches of ground clearance right at the center wheel. We've run 4" wheels on 3" tube in 2011 and while it did clear the bumps in the field, anything other than almost perfectly flat would not have worked for that drive.

Basically - there's a reason they aren't a super viable product. The teams that can handle and wish to use sub 4 inch wheels on their robots are the teams already making their own wheels. I wouldn't assume they'll work in the general FRC case just because it looks nice in CAD.

Originally Posted by Chris is me

I would call 3" wheels "really that small". On a 2x1 frame, that leaves you with a large span to high center on, as well as at most .625 inches of ground clearance right at the center wheel. We've run 4" wheels on 3" tube in 2011 and while it did clear the bumps in the field, anything other than almost perfectly flat would not have worked for that drive.

Basically - there's a reason they aren't a super viable product. The teams that can handle and wish to use sub 4 inch wheels on their robots are the teams already making their own wheels. I wouldn't assume they'll work in the general FRC case just because it looks nice in CAD.

In my models every wheel was moved down around 1/2" on the 2x2 to provide enough clearance, and it was sufficient.

[Gregor]

Originally Posted by Andrew Lawrence

In my models every wheel was moved down around 1/2" on the 2x2 to provide enough clearance, and it was sufficient.

Sufficient to what standard?

[Adrian Clark]

Originally Posted by Joe G.

I would not recommend this. Smaller sprockets load your chain more, and make your drive less tolerant to variable tension and alignment inperfections. We did a similar drive with #25 16t sprockets, and were throwing chains left and right. There should be plenty of room on a drive like this for bigger sprockets.

I can't say I've had any similar experiences. We ran 16T sprockets with untensioned chain and it lasted us two regionals and champs with absolutely no fails.

You are correct about the chain loading and for that reason I would say that it's best to run the largest sprocket you can on your drive. However on this drivetrain it looks like the 22T sprockets OP is using are too large and the next smallest size is 16T, that is the reason I suggested using 16T sprockets. Also, I'm pretty sure 16T is standard for WCD's, and I've never heard of WCDs popping chains.

[Nate Bloom]

Thanks for all the input. I'm planning on replacing the 22T gears with 16T. Since I dropped the middle wheel 1/8'' rather than raising the corner wheels 1/8'' or doing 1/16'' on both, the 22T sprockets make the chain run into the bellypan in the current config.

I'll also a 1/16'' aluminum bellypan, and I'm going to keep to gussets and rivets for now, although I'll try to get some people interested in welding since that can still be useful. Are there any other suggestions on improving the drive?