pic: FRC558's modified VEXpro Drive in a Day



With all the recent custom drivetrains being posted I figured it would be good to post up a COTS option with some simple modifications. FRC558 added a few holes to run a 8WD in the 2014 Drive in a Day Chassis from VEXpro. We flipped the gearbox plates and bolted them into the rails where the holes just happened to line up nicely with the output shafts.

Basic info:
8WD unequal spaced wheels
6 CIM Single Speed (12:64 Gearbox Reduction)
4x2 Colson Wheels on WCP Hubs (15:22 Sprocket Reduction)
Dead axles (Grade 8, 3/8-16 bolts), #35 chain, plywood belly pan

Also you can read more about the robot on beyondinspection.org

I’ll be the first to attest that this drivetrain is a BEAST. For reasons I have yet to figure out, it could manhandle almost any bot on the field… Including 2 speeds.

Looks good, I like the purple.

Why did you opt for 3/8" solid bolts as dead axles instead of using 1/4"-20 bolts going through 1/2" tube axle?

The dead axle WCP Colson hubs are already bored out for 3/8" bearings.

You can use the 1/2" I’D x 7/8" OD bearings as well. Allowing teams to use tube axle.

Just curious, what is the benefit of that? To save weight? Smaller holes in the frame?

Truth this setup was a beast.

I’m curious if anybody has run these for a season. My gut would think the smaller balls of these bearings would reduce their load carrying capacity to the point where I’d be concerned about an ill-timed bearing explosion. But I haven’t tested it, run numbers, etc. so I’d love to hear some reports of teams having success with this setup.

The tube axle becomes a frame member and dramatically increases the rigidity of that chassis section. When using the 3/8" bolts you can’t tighten the bolts since that would bend the frame. With the tube axle setup you preload the bolts to achieve greater chassis strength.

Did you guys slit the colson wheels at all? Or run them stock?

Not to steal any thunder, but they ran them stock. 228 is/was the biggest proponent for slitting Colson wheels and I think Art Dutra might have posted numbers somewhere.

Personally, I don’t think the cost of implementation is worth the benefit of the added traction. Colsons are already up there in the traction world. They’re just under blue nitrile if I remember correctly and slitting them still doesn’t get them to nitrile’s level. Their wear characteristics far outplay any wheel I’ve seen in FRC, though.

If you were to use proper length spacers with the bolts, couldn’t you tighten the bolt too?

In my past experience, if the bolts were tightened (to the point of them trying to act as a stiffener) with spacers, the spacers would interfere with the bearings and lock them up, making the dead axle wheel hard to rotate.

Ideally in this configuration, you would use standoffs around the axle to provide stiffness. That adds more parts, more weight, etc. Not hard to do, but you are limited to using the mounting holes provided by the DIAD, chain routing, etc.

Anyone have thoughts on using 3/8" steel rod as dead axles? Tap both ends of the axle and it’d be close to using tube axle/bolt. I wouldn’t think a 1/4"-20 bolt would work…not enough thickness left in the rod. Something smaller like a 10 or 8…at which point you would have to drill all 4 holes in the DIAD sideplate, rather than using the pre-punched 1/4" hole for the outer wheels (of the 8wd)…but it could work.

We had great success with 1/4" steel rod that was put through a die on either end with locknuts in 2007. 2011-2012 we moved to 3/8" steel rod that was supported similarly. It was quite easy to machine - stick the die into a vice, stick the rod in a cordless 18V drill, use lots of tap magic and BAM 30 seconds later you have an axle. Just make sure the mount hole hits the solid rod and not the threads.

As for maintaining proper wheel alignment & spacing - I’d recommend any solid non-deforming material for standoffs. The dark-grey plastic from Andymark works great (it’s why it’s in the KOP, or was last time I used a KOP).

I agree that the axle itself should NOT be used to stiffen a frame. The only load you want on a wheel axle is normal to the floor so it rides correctly on the balls inside the bearing.

I should clarify. I was not looking to form 3/8"-16 threads (or similar) using a die…I was thinking about drilling and tapping the ends of the steel rod (using a drill and tap). The rod would become a stiffening dead axle.

You could turn down the ends of the 3/8" rod and thread them for 1/4-20. The major downside of this being that you’d need to remove the outside of the drivetrain if you wanted to change a wheel.

Ah, I see. If you mount through the bolt rather than the axle (i.e. the mount hole is 1/4" rather than 3/8" and the 3/8" axle is the stiffening rod) then I don’t know if there would be axial load on the bearings.

I would worry about the 1/4" bolt elongating the mount holes after a rare hit - like one that tips the bot up a bit, then the bot slams back to the floor, or like what happens when coming down off of a ramp/bump. We experienced some of this in 2007. When we went back to this style of dead axle in 2011, we used 3/8" rods and also used 1" angle brackets (1/8" thickness) to mount the wheels below the 1x1 frame. This gave us flexibility to change a mount out if we had problems. The “look” of the drive train where we mounted the wheels was similar to the old IFI KOP frame rails.

Thanks for the feedback, feel free to keep the questions coming. FRC558 was extremely happy with our drivetrain’s performance and reliability in the 2014 season. There is something to be said for being able to assemble during week 2-3 of build and not have to touch the drivetrain for 100+ machines.

I’ve looked into this, and would likely pursue it if we had the internal resources to machine them. At this time it would require relying on our mentors and sponsors to do work outside of the teams shop.

We run them totally stock, I don’t believe the benefit is worth the extra work, however if we need more traction we’ll look into it. We have never wanted more traction.

RC, have these bearings been tested for load? I’d love to run these but I have concerns about testing new things in our drives. If these are more then capable, we’ll switch over to them and the tube axle material for 2015.

do you guys think that there’s any chance that FIRST will move back to a max of 4 CIMs instead of 6 CIMs?

It’s a possibility. Or some rule limiting max power in drive.