Drivetrain Sprocket

I’m a freshman and I am doing a cad of a drivetrain. I’m doing a west coast 4 pneumatic 6 inch tires. I don’t know what size sprocket or chain to use. I am designing this for the 2016 game

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First question is, how much manufacturing slop are you likely to get with the chain? If the chain runs can be made (and kept) very close to straight, #25 chain is a valid option. But if not, #35 is more forgiving of misalignment.

On the other hand, Stronghold. #35 chain would be my gut call, and I’ve seen Stronghold break that for a variety of reasons.

First, why 4 wheels? 6 or 8 would be what most people expect on that type of drivetrain.

Now, to answer your question, #35 would be the safe choice, you may be able to get away with #25 but it would give you a low factor of safety.

For 6" in wheels I’d recommend 18 or 22T sprockets @ #25 chain.

4901 used Team 221 double 17T #25 sprockets with 8" pneumatic wheels (hubs were AndyMark, tires were an Amazon find), center dropped 1/4" on 1x3 tubing. That worked well once we got past some master link teething problems, but it’s important to note we supported both ends of the axle with bearings and didn’t cantilever them. Could we have gotten away with cantilevered axles? Maybe, but we knew after the Team Cockamamie Ri3D build that we wanted a robot that could be driven fearlessly.

A lot of people have already said this, but as far as chain goes, use #35. Especially since its off season. You can get away with #25 if you know what you are doing but you will be constantly fighting it. For example, if your chassis deflects(as all chassis do) then the center to center distance on the axles(and therefore the sprockets) will either get bigger or smaller. When this happens, #25 chain is much more likely to jump than #35. If you are clever and manage that problem you can use #25, but it needs to be taken into careful consideration. IMHO it is not worth it.

As to what size sprockets, it all depends on what gear ratio you want. I would use the JVN gear ratio calculator and put in the gearbox you want to use etc. to determine the best sized sprockets for your application. The important thing is to get enough wrap around the sprocket and to have the correct ratio so that you don’t burn out motors.

Just make sure the chain is tight enough, so it doesn’t do this.

We have always used #35 chain for drive train and tend to use 22 tooth sprockets on the wheels. This gives plenty of teeth while still being small enough to fit on a 4 inch wheel. Our size of choice for general wheeled drive trains. Last year being the exception as we had treads.

For WCDs, your options are 18T or 22T for #25 chain, or 12T for #35 chain.

The bigger the sprocket, the less chain “stretch” (pin wear) you have to deal with, and the easier it is to keep in tension. The tradeoff is more weight and less clearance.

It is honestly not that hard to keep 25 chain aligned and tensioned if you plan ahead and don’t build sloppy drivetrains. The challenges are often overstated. I would generally recommend starting with 25 and switching to 35 chain if you have any problems with the 25 chain - the weight penalty is pretty big.

This year, especially if you had larger pneumatic wheels, the rough driving over defenses led to more chain and sprocket damage than normal, making 35 chain a safer choice, particularly for big wheels which place more load on a sprocket and chain (all other things equal). For 6" wheels, you are probably okay with 22T #35 chain. This the size my team ran and we never had issues apart from having to tension it repeatedly. For 4" wheels, or 6" wheels on a flat field game, 18T #25 chain can work just fine too with a bit more clearance.

This does not work for west coast drivetrains - a 22 tooth 35 chain sprocket is bigger than 2 inches in diameter and will not fit within the profile of the tube. If there is a belly pan placed below the tube (as is common on most WCDs) the chain will not have clearance.

The chain could be run outside the drive rails.

Do you mean on the same side of the tube as the wheels, or just not inside the tube?

If one wanted to run a tradition WCD setup with 22t #35 sprockets, then they would have to cut into the bellypan to allow chain clearance, which reduces the strength of the pan. Otherwise, they could cut only small slots for the sprockets and assembly the chain around the bellypan, which is a pain.
12t #35 sounds pretty good to me. We horribly screwed up a 2013 offseason WCD, but because of 12t #35 sprockets it still drove just fine.

The same side of the tube as the wheels.

I understand what you and Chris are saying. I’m just saying that if a team had some legitimate need to use sprockets with a larger diameter than the width of the tubes, the chain can be placed on the outboard sides of the drive rails (between the drive rails and the wheels). You sacrifice some of the spatial economy of a WCD (particularly those with gearboxes designed for interior chain runs), but it would facilitate the use of larger sprockets/pulleys.

1712 used 12t #35 chain in both 2015 and 2016 and never had a single drive chain issue.

If you did that, couldn’t you go with dead axles on the outer wheels? Otherwise, I’d be concerned about putting the wheel so far out from the bearing supports.

One wheel on each side would presumably still be powered directly off the gearbox shaft, so that wheel would still need to be live axle. The others could potentially be switched to dead axle.

I’d like to emphasize I don’t really endorse this design choice in general. I’d much rather just use smaller sprockets and be able to protect my chain inside my drive rails and above my bellypan. This is only if, for some reason that I cannot currently contemplate, a design bound you to using sprockets larger than the width of your drive rails AND bound you to a “west coast” style drivetrain.

I have to respectfully disagree - in my experience, it is indeed quite difficult to use #25 chain reliably if you’re limited to chop saw/drill press-type construction. It is really quite sensitive to alignment. I know I’m not the only one who has come to this conclusion, as well.

If your team is very good at machining, go for it, but I think telling teams that “the challenges are often overstated” is likely to cause people a lot of undue frustration.

It’s not significantly different loading in terms of bending for a live or dead axle here - the support is the same distance away in either case. There are good reasons to make the axles dead if you’re going with the sprockets out method, though. It does increase the cantilever but not necessarily to the point of failure. I’ve heard of teams with plenty of success doing this sort of thing.

Not my first choice though.

Hmm, you could probably do something like 2451’s 2013 drivetrain but put the sprocket on the wheel side of the tube. that’d actually make an extremely solid set up.

EDIT - But I’ll echo your sentiment and say not my first choice.

If you want to look at the math of the kind of loads chains or belts take and design the system appropriately, you can take a look here:

I stepped through the math to help with understand the loading. It all depends on the drivetrain setup. #25 chain may work for one style of drive and not another. Belts may work, or you may need #35 chain, just take a look at the math.