pic: FRC488 - Fall Project, Rev. A

Based on everyone’s great feedback (http://www.chiefdelphi.com/forums/showthread.php?threadid=118840), I reworked our fall project chassis. It’s not complete yet, but it’s close.

.090" sheet metal everywhere.
10 4" Versawheels
Gear drive; no belts, no chains.
34 lbs.; a little heavy, perhaps.


Now that I’ve got a bit more time, I can write a bit more about the changes I made between the first iteration of this design and this one. Most of the changes were directly influenced by feedback I received in the other thread. I appreciate everyone’s feedback and questions.

Wheel Modules:

  • Added vertical flanges along the front and back edge to tie into the
    belly pan.
  • Replaced 3/8" hex standoffs with 3/8" round spacers and 10-32 bolts inside. I stayed with 3/8" here rather than 1/2" since the VexPro gears’ 1/2" hex bore can rotate freely about a 3/8" spacer and probably won’t do so around a 1/2" spacer. We could open the bore on the gears up on a drill press since concentricity of that hole is not a concern, but I am not sure the switch to a 1/2" spacer is worth the effort. It’s possible to change my mind.
  • Haven’t yet decided between a long spacer or a shaft collar to locate the idler gears axially.
  • Swapped the 35 tooth idler gears for 48 tooth gears; this allowed me to move the idlers upward some, creating clearance below to extend the belly pan to the outer face and adding torsional stiffness to the module via the now offset wheel / idler spacers.
  • Eliminated the hole-slot pattern for a simpler hole pattern. The hole on top will get shifted toward the outer edge; I see now that I moved them the wrong way yesterday.
  • Reworked ball shifter mount and added captive nuts to the inside face of the module to accept the transmission bracket. The transmission can couple to any of the three inner wheels.

Belly Pan:

  • Extended the belly pan to outer face of wheel modules and added short flanges to couple the outer module face to the belly pan with rivets.
  • Removed taper from front and rear flanges.
  • Our electrical system layout is not in my sphere of influence, so I will need to provide several mounting options and they will give me a board of their own design to couple to the belly pan.
  • Added battery box. I will redesign this tonight; I am not happy with it and it can be one part instead of two.

Hi Madison,

The battery box can also hold the breaker and provide a base for easy access to the breaker. It can also hold a data access port for connection to the router. How many times have you seen teams struggled to gain access to the router data ports? Maybe even hold the sidecar for easy access.

Don’t forget the mounting holes for the manipulator and the bumpers brackets.

Make sure there is adequate bend relief where the angled belly pan and the wheel housing meets. The parts will align easier if tolerance on the tab is loose.

Using pins to hold the wheel assemblies make it easy to drop a wheel out of the assembly. Using dowel pins also helps align the transmission gear to the drive assembly. Bring the gear closer to the transmission bearing keeping the distance to a minimum.

I feel the corners need some triangular gussets to reinforce the corners should you get a big hit.

Whats the drop on the wheels? Driving more than six wheels means you get a lot of wheel scrub. The outer wheels will scrub down to nothing if there is a lot of turning and the drop is minimal.

Is this a 2012 or 2013 rules drivebase?

  1. It’s just about 28" x 28".

I’m not sure if you had considered this, but cutting out part of the bellypan where the gearboxes will be above can make maintenance a lot easier. We didn’t do that this year and it was a nightmare trying to repair our gearbox before eliminations.

This times a thousand. We built a bellypan and then realized we needed to and cut it out after the fact, which didnt look pretty, but it was glorious. That and it saved close to a pound.

Maybe you should think about having the belly pan Plexiglas or clear plastic. It saves weight and you could put some cool LEDs. Also, you could make it removable to put electronics or make it easily accessible if you need to fix something

No, you want the metal for rigidity. Plus, CNCing out all the material into that pattern saves quite a bit of weight already.

You are right, maybe it should be just a middle piece cut out. The reason being that this metal underbody is just added weight that can and should be removed

I have to say, this is a really beautiful design. It looks just like 971, but with gears instead of belts. I’m a huge fan.

Just a few questions, perhaps they’ve been addressed elsewhere. Any reason you’ve chose 10 wheels? I don’t see any advantage for 10 over 6, and having more wheels carries cost and weight (not to mention potential turning) disadvantages. Also, I’d assume these parts are being laser cut? Waterjetting or turret punching those parts would take a lot of sponsor time.

I’m guessing because designing for ten allows for configurations of 6 and 8. Correct me if I’m wrong please.

If you’re still going to power the corner wheels, a gear has to occupy that space anyway to act as an idler. Why not run 10 at that point?

Done properly, the weight gains are marginal and the added strength and rigidity are quite useful. In a sheet metal drive like this, the lightened metal belly pan is what holds the entire design together (literally and figuratively).

“metal is weight and weight is bad” isn’t by any means necessarily true.

Not really. You need something there, unless you like frames being non-rectangular parallelograms or other interesting shapes. We’re talking 0.090 sheet metal here, with a weight reduction pattern–can’t get much thinner and lighter than that without needing some extra support. If that bellypan is more than 2 lb, I’d be shocked; I wouldn’t be surprised if it was closer to 1 lb or under. Removing more weight won’t exactly help in this situation.

You would have to remove 80% of the material in a 1/16" pan that is 28x28 to get it under a lb.

Have you tested or analyzed that cantilevered gear design? Even small deflections could result in poor performance and/or damaged/destroyed gears.

I like the idea of adding a fixed data port somewhere along the outer frame. I think it’d get me a lot of brownie points with the electrical and programming teams. :slight_smile:

Don’t forget the mounting holes for the manipulator and the bumpers brackets.

Bumpers have been a weak point for us for the last two seasons, so I’m going to give them some real thought this time around. I haven’t gotten to them yet, though.

Bring the gear closer to the transmission bearing keeping the distance to a minimum.

I’m looking at ways of managing this. Currently, the transmission is offset from the clamshell by 3/8" to allow access to the wheel mounting hardware on the inside face. I’m considering using captive nuts here instead (to accept the bolt inside the wheel axle spacer) and moving the transmission flush with the inside face. That’d put the gear 3/8" closer to the bearing.

Whats the drop on the wheels? Driving more than six wheels means you get a lot of wheel scrub. The outer wheels will scrub down to nothing if there is a lot of turning and the drop is minimal.

The outer two wheels on each side are 1/8" higher than the inner three.

As it stands now, the gearboxes are removed by lifting them upward. Since they’re sealed, I’m not sure how having access to the bottom will be helpful.

Billfred’s got it right. I could’ve designed this for 6 or 8WD as well and included more idler gears. Maybe in competition, that’d be a bit more likely, but for a fun project, why not go with 10WD?

And yes, the parts are being laser cut.

The middle chassis piece is 4.84 lbs and is made from .090" sheet. It’s not just a belly pan; it integrates the front and rear cross rails.

We ran a similar arrangement in 2013 and it was the only thing that didn’t fall apart after the frame got all wonky. I’m sure, by the end, it wasn’t as efficient as it should’ve been, but it was working. I’m working on some alternatives, though, to see if we can’t make it better.

Yes, that would get you brownie points. :slight_smile:

Perhaps consider removing from the bottom since there isn’t anything that can go below the chassis yet you will (likely) have a superstructure and removing the transmissions through that tends to get hairy.

It looks like the corner joints in this case wouldn’t make this very easy, but you could also consider using a few bolts to attach each side rail assembly to the central frame, so that you can remove an entire drive side, without needing clearance out the top or bottom, by just removing those few bolts. Maneuvering the gearbox with its output shaft out the guts either upwards or downwards can be difficult, so it can be easier to leave everything intact until you have it in open air to work on.

Edit: I just noticed with the belly pan coming under this time around, this would be even less feasible :rolleyes: But maybe if the side rail stands alone as its own rigid box…