pic: ABS-122, shifter-in-tube chassis

[asid61]

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[Cash4587]

It doesn't look like you have any form of chain tensioners on this setup. Although you can do without them, I would still really recommend putting them on even in the case of using large tooth count sprockets. We used a setup similar to this in terms of chain, and really wished we had designed a way to tension the chain during the season.

[asid61]

Originally Posted by Cash4587

It doesn't look like you have any form of chain tensioners on this setup. Although you can do without them, I would still really recommend putting them on even in the case of using large tooth count sprockets. We used a setup similar to this in terms of chain, and really wished we had designed a way to tension the chain during the season.

We used 16t #25 sprockets all season with only a single jump at SVR (and that was after another competition in Utah) and that was after the chain got very loose. It's possible to add tensioners in the form of versablocks or even WCP sliding bearing blocks, but I like the set-and-forget of direct C-C. Are you certain that your sprockets were correctly aligned? I'm guessing that they were and you guys just did much harder driving than us, but it never hurts to check.
With massive sprockets such as these I'm not as worried about the chain coming off as much as I am about the chain picking up tape. I could swap these out for #35 chain sprockets, but the extra weight isn't worth it to me. In-season if it became a problem switching to #35 chain would be my first choice.

[Cothron Theiss]

I'm really impressed with how compact this is. What size are those sprockets, by the way? I think you'll be fine leaving it at exact C-C, but since the chain is outside of your tube, it'd be easy to add a static tensioner after your first or second competition. Just for peace of mind you might want to print something that fits around or on the top of your motors to ensure the chain doesn't rub against the motors in the worst situations.

[asid61]

Originally Posted by Cothron Theiss

I'm really impressed with how compact this is. What size are those sprockets, by the way? I think you'll be fine leaving it at exact C-C, but since the chain is outside of your tube, it'd be easy to add a static tensioner after your first or second competition. Just for peace of mind you might want to print something that fits around or on the top of your motors to ensure the chain doesn't rub against the motors in the worst situations.

Thank you!
The sprockets are 38 tooth versasprockets from Vex with a minor diameter of just over 2.8". They are inline with the CIMs, so I'm not too worried about the chain hitting. In the past 115 has used polycarbonate chain guards, so if there was a problem I would stick those in.

[Gregor]

I like this. I think it would also be a neat way to do a single speed drive with a PTO, not super clunky like most PTO setups.

[asid61]

Originally Posted by Gregor

I like this. I think it would also be a neat way to do a single speed drive with a PTO, not super clunky like most PTO setups.

I was thinking about making a PTO option of this. With the Versadog it becomes a lot easier too.

[Cash4587]

We never had issues with it slipping or jumping or breaking or ever coming off. We just find it very hard to write fast, accurate and repeatable autonomous modes with so much slack in our chain. It gives semi un repeatable results in our vision code when only the front 2 wheels move while the back 4 don't having to make up the distance in chain slack. It would have been much better to just run tensionors from the start.

[asid61]

Originally Posted by Cash4587

We never had issues with it slipping or jumping or breaking or ever coming off. We just find it very hard to write fast, accurate and repeatable autonomous modes with so much slack in our chain. It gives semi un repeatable results in our vision code when only the front 2 wheels move while the back 4 don't having to make up the distance in chain slack. It would have been much better to just run tensionors from the start.

I see, that's a good reason to run proper tensioning.
How did it cause errors/to what extent? If two wheels are moving, theoretically the whole bot should move anyway. Would direct driving the center wheel help?

[Cash4587]

Originally Posted by asid61

I see, that's a good reason to run proper tensioning.
How did it cause errors/to what extent? If two wheels are moving, theoretically the whole bot should move anyway. Would direct driving the center wheel help?

So this year our robot had about 60% of our weight on the front wheels. Direct driving the center wheel or any wheel might have helped. We ran a single gear reduction 12-66t and then a 12-22 #35 chain reduction to the wheel. (We later switched from 12-22t to 12-28t and is much better) In our case, direct driving any wheel may have helped but we really enjoyed this setup. We would get no movement on our robot for about 1/8th of a turn on the back wheel to catch up in chain slack. The whole robot wouldn't move because we had tons of turning scrub also so I guess a few things would have helped. But in any case, the chain slack and slop in the hex shaft really was a pain when trying to tune our code. I guess that's why 971 goes through so much to get all of the slack out of everything and makes custom hex.

[GeeTwo]

Originally Posted by Cash4587

So this year our robot had about 60% of our weight on the front wheels. Direct driving the center wheel or any wheel might have helped. We ran a single gear reduction 12-66t and then a 12-22 #35 chain reduction to the wheel. (We later switched from 12-22t to 12-28t and is much better) In our case, direct driving any wheel may have helped but we really enjoyed this setup. We would get no movement on our robot for about 1/8th of a turn on the back wheel to catch up in chain slack. The whole robot wouldn't move because we had tons of turning scrub also so I guess a few things would have helped. But in any case, the chain slack and slop in the hex shaft really was a pain when trying to tune our code. I guess that's why 971 goes through so much to get all of the slack out of everything and makes custom hex.

If 60% of your weight was on the front wheels, I would recommend (other things equal) that you drive the front wheels directly; this is ultimately the same reason that West Coast drives the center wheels directly - to make the drive to the most dependable wheels invulnerable to a broken chain/belt.

On the other hand, if 60% of my robot's weight were on the front wheels in a static configuration (and presuming I had at least six wheels), I would be VERY worried about the robot falling on its face in a braking maneuver.

[MichaelBick]

Originally Posted by GeeTwo

On the other hand, if 60% of my robot's weight were on the front wheels in a static configuration (and presuming I had at least six wheels), I would be VERY worried about the robot falling on its face in a braking maneuver.

That is completely dependent on CG height and, given 4587's short robot, probably not a major risk.

[Cash4587]

Originally Posted by GeeTwo

If 60% of your weight was on the front wheels, I would recommend (other things equal) that you drive the front wheels directly; this is ultimately the same reason that West Coast drives the center wheels directly - to make the drive to the most dependable wheels invulnerable to a broken chain/belt.

On the other hand, if 60% of my robot's weight were on the front wheels in a static configuration (and presuming I had at least six wheels), I would be VERY worried about the robot falling on its face in a braking maneuver.

We had #35 chain direct driving the first wheels. Given the strength of 35 chain, and the fact that we put the chains in line, as well as calculated the direct c-c we had no issue other than chain stretch which is to be expected. We had all 6 drivetrain motors above those wheels accounting for the rest of the weight minus the catapult and battery.

With this being said. After 2 regionals, Champs, and an offseason already we never replaced 35 chain, or 25 chain. We were never worried about it breaking either. It is just something that doesn't occur that often if chain is used correctly, which we did, just without tensioners. Which in our case worked very, very, very well, and in many cases would be fine for most every other team in FRC not wanting to do some of the precision complex things things we need to get our code really dialed in.

[chrisfl]

Looking at the size of those tubes, would you be able to put the wheels inside them to maximize space used in the tubes? That would also allow you to move your side rails out more to create more space in the center of the frame.

[pwnageNick]

Originally Posted by chrisfl

Looking at the size of those tubes, would you be able to put the wheels inside them to maximize space used in the tubes? That would also allow you to move your side rails out more to create more space in the center of the frame.

The two wheels directly driven by the gearbox have to be outside the tube as the gearbox is inside the tube already there. The other wheels could be put inside the tube, but that doesn't really do anything for you.