pic: 4464 Preseason Drive WIP



Here’s our partially-completed preseason drive. Don’t mind the (apparent) lack of output shafts on the gearboxes - they’re cut short and end in hex couplers concealed inside the framing members. Design (slightly out-of-date, but mostly correct) can be viewed here:

http://www.chiefdelphi.com/media/photos/40523

We’re very pleased with how it’s come together. The only thing so far that I’d like to change in the final version, if possible, is the size of the inner framing members - currently they’re 2’‘x1.5’’ tubes, to give the aforementioned hex coupler a half inch of purchase on both hex shafts. If we could downsize that to 2’‘x1’’, it’d save a bit of weight and space and be somewhat easier to manufacture, but I’m not sure if the reduced amount of coupler purchase would be OK - if anyone has any input here, I’d greatly appreciate it.

We also may have a belly pan on the final version, though I don’t know what material would be optimal (plywood? thin aluminum?).

Nice design. I like the couplers which will make it easier to remove the gearbox.

Why did you choose to put the CIMs above the wheels? Is the belly pan space worth the significantly higher center of mass?

.25" birch plywood is a very good material for a belly pan. It’s stiff, light and easy to cut or drill holes in.

It appears as if by delegating that much room to your wheel wells as well as supporting them on both ends that you are offsetting the advantage of the inverted CIM setup, and in the end are saving less room than possible. I’d actually argue that you would open up more chassis space just by having cantilevered wheels and non-inverted CIMs, which will also bring a lower CoG, and make maintenance far easier.

To answer your question, 1/4" plywood makes for a quick and excellent bellypan for those who want to save time and/or do not have the resources for a machined aluminum one. It can be argued that the aluminum ones provide more stiffness, but if you design your frame right the plywood will be perfectly acceptable.

Another question I will have to ask is why you chose 6 CIMs. Do you have mathematical proof of the advantages of the other two motors that you personally have done, or are you going off the anecdotes of other teams? I’m not saying 6 CIMs are a bad thing, but having once been a part of the hype that is built around them I now only design 4 motor drivetrains because after doing the math countless times I haven’t found any advantages that outweigh the potential disadvantages.

Possibly true on the points about space and lower COG. Not so much for easier maintenance.

I have two major gripes with WCD setups: belt/chain access and gearbox removal. This design specifically aimed to make both of those as easy as possible. The coupled output shaft design would be much harder to do with cantilevered wheels, as would keeping the belts outside of the gearbox.

The center of mass for this gearbox really isn’t that much higher than for a stock WCP 3-CIM shifter, actually - the gearbox is only about an inch taller.

I really agree with the maintenance comment. Being able to swap a gearbox in less than five minutes is a huge advantage.

IMO, both of these tradeoffs shouldn’t cause a redesign that throws many other benefits of WCD out the window. In your design, the wheels are much harder to remove and it is far easier to mis-align the axles which then causes chain/belt woes. Your manufacturing and wheel decisions may make these concerns less of an issue, however.

Gearbox removal is definitely an Achilles heal of WCD. Yet it can be mitigated by making a more bulletproof gearbox. We’ve experienced a bit of this both years we’ve done WCD, and this year will probably do steel output shafts as a result.

WCP’s WCD blocks make chain access for chain maintenance irrelevant (assuming proper sprocket spacing). Even replacing an axle with limited access isn’t an issue.

I wouldn’t call it “much” harder. For the outer wheels, you remove a nut and pull the bolt. For the inner wheels, you loosen one shaft collar and pull the shaft. It’s not as nice as just pulling the wheel off the end of the shaft, but it’s hardly a problem.

and it is far easier to mis-align the axles which then causes chain/belt woes. Your manufacturing and wheel decisions may make these concerns less of an issue, however.

We manufactured this frame to very good tolerance, so misaligned axles really a problem at all. Wheel spacing is done with the stock VexPro delrin spacers (of which I am a huge fan), which remove any chance of having mis-aligned pulleys.

Gearbox removal is definitely an Achilles heal of WCD. Yet it can be mitigated by making a more bulletproof gearbox. We’ve experienced a bit of this both years we’ve done WCD, and this year will probably do steel output shafts as a result.

WCP’s WCD blocks make chain access for chain maintenance irrelevant (assuming proper sprocket spacing). Even replacing an axle with limited access isn’t an issue.

I will agree that WCD is a very good solution (449 used it last year, and in all likelihood will use it next year).

This design trades a bit of convenience in wheel access and a bit of space for much easier gearbox and belt access. We think it’s a worthy trade-off.

An aside: The wheel wells are as big as they are so that this design can fit either 1.5’’ VexPro traction wheels or Colsons. We’ll decide which we like better as we test the thing. I do wish that VexPro sold a live-axle colson hub with a standard bolt circle for mounting pulleys, though, as it would save us one hex versahub and one hex bearing per side in that configuration. It’s a minor gripe, though.

I have to thank you again for the hex coupler idea - it’s a wonderfully simple and elegant solution.

I just realized I never posted the CAD for this, which I should do if we’re going to use this (or any derivative of it) during build season.

So, here it is. This is a slightly earlier version than the one that we actually built - the only major difference is the size of the wheel wells, which since been enlarged to allow us the option of using Colson wheels and to move the motors a bit back from the edge of the frame.

http://www.mediafire.com/download/t8zv5l10h1p858e/Chassis_0.1.1.stp