pic: Drivetrain Concept

[Tom Line]

Originally Posted by s_forbes

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I would be a little concerned about frame-torsion. Your design looks like you intend to screw into the wood, and over time the twisting of the frame may well loosen it up, especially since you're screwing into end-grain. I'd consider L-brackets with through-bolts to avoid that.

Also, unless there is a pressing reason for 8 wheels - I would avoid it. Unless you drop the middle 2 sets, you'll have trouble turning, and making 8 wheels the correct heights is really difficult. There's a reason stools have 3 legs rather than 4 (wood warps and twists over time). You can avoid that in part by using plywood, but that has problems all it's own if you plan on screwing into the end of it.

[Andy Baker]

Originally Posted by Tom Line

Also, unless there is a pressing reason for 8 wheels - I would avoid it. Unless you drop the middle 2 sets, you'll have trouble turning, and making 8 wheels the correct heights is really difficult.

I love the simplicity of this design. I too would suggest that the wood be made from plywood.

Another suggestion... if you make the 4 interior wheels have a slightly bigger diameter, that will work too. What I would do is put treads on the 4 interior wheels so that they have a diameter that is 0.2" larger than the outside wheels. This may be easier than varying axle heights.

Andy B.

[JesseK]

Originally Posted by Andy Baker

Another suggestion... if you make the 4 interior wheels have a slightly bigger diameter, that will work too. What I would do is put treads on the 4 interior wheels so that they have a diameter that is 0.2" larger than the outside wheels. This may be easier than varying axle heights.

Andy B.

If all 8 wheels are driven, wouldn't that cause the outer wheels to fight the inner wheels since their smaller circumferences cause slower linear translation than the middle wheels? Is it too minor to worry about?

[AndyB]

Originally Posted by JesseK

If all 8 wheels are driven, wouldn't that cause the outer wheels to fight the inner wheels since their smaller circumferences cause slower linear translation than the middle wheels? Is it too minor to worry about?

Yeah, the difference is too minor to worry about. I've seen teams use this method successfully.

[sdcantrell56]

I like it in general. It looks very similar to my design I posted. I too would switch to plywood, particularly a good 1088 meranti or baltic birch plywood as it is much stronger and more stable than a solid chunk of wood. Also u should look into epoxying the wood to the fiberglass plus screwing. If you do that you will have an indestructible joint.

[Alan Anderson]

Originally Posted by JesseK

If all 8 wheels are driven, wouldn't that cause the outer wheels to fight the inner wheels since their smaller circumferences cause slower linear translation than the middle wheels? Is it too minor to worry about?

Even if it weren't too small of a difference to actually make a difference, there is a wee bit of theoretical benefit to a minor mismatch in speed. If the corner wheels are any less grippy than the specially-treaded middle ones, they'll be slipping slightly. The way friction works, they'll thus slide sideways more easily than if they were in solid lock-step with the rest of the wheels.

But it's not an issue. Andy Baker's suggestion is a proven one. The TechnoKats 2008 robot uses six-wheel drive with in-line axles and extra tread on the center wheels, and it drives great.

[gorrilla]

does anyone use the kit wheels anymore? they work great for all kinds of stuff not just as drive wheels

at TNT i only saw 4 teams that used tham

[M. Mellott]

Originally Posted by Tom Line

Also, unless there is a pressing reason for 8 wheels - I would avoid it. Unless you drop the middle 2 sets, you'll have trouble turning, and making 8 wheels the correct heights is really difficult.

Another option is to use omnis on the 4 corners--that way, all eight wheels can contact the ground for stability, but still be able to turn smoothly...and I second the idea of adding L-brackets or some additional support to the 4 primary corner joints.

Question for anyone on the aluminum belly pan: what is the minimum sheet thickness one would use on such an application?

[gorrilla]

yes use L-brackets the dont add to too much weight for all there worth

and if the ends are fiberglass why not just glass it all its not very hard and that way you would have all the great performance of wood and the added strength of the fibergalss but it would still be able to give a little allowing for a smoother ride then a metal frame


the only problem with fiberglass is its kinda messy to work with and if it gets on a tool or something you might have to throw it away


and I second what andy said about the inner wheels

[s_forbes]

Originally Posted by Tom Line

I would be a little concerned about frame-torsion. Your design looks like you intend to screw into the wood, and over time the twisting of the frame may well loosen it up, especially since you're screwing into end-grain. I'd consider L-brackets with through-bolts to avoid that.

Also, unless there is a pressing reason for 8 wheels - I would avoid it. Unless you drop the middle 2 sets, you'll have trouble turning, and making 8 wheels the correct heights is really difficult. There's a reason stools have 3 legs rather than 4 (wood warps and twists over time). You can avoid that in part by using plywood, but that has problems all it's own if you plan on screwing into the end of it.

Good points, I forgot to cad in the top corner brackets that would reinforce those joints. It could be constructed in a very similar way to our 2008 drivetrain which was able to go through three competitions with no problems. I am not sure whether wood screws would be enough or if through bolts would be needed, that's something that may need to be tested.

Oh, and the middle four wheels are all dropped 1/8" on this model.

Originally Posted by M. Mellott

Question for anyone on the aluminum belly pan: what is the minimum sheet thickness one would use on such an application?

Last year we used a .050 or .060 sheet, I believe... not sure on the actual thickness (squirrel might know?). It held up great and provided a solid mounting surface for all of the 'guts'. Adds about five pounds to the weight, but it compensates for additional bracing and a sturdy electronics board.

Originally Posted by gorrilla

yes use L-brackets the dont add to too much weight for all there worth

and if the ends are fiberglass why not just glass it all its not very hard and that way you would have all the great performance of wood and the added strength of the fibergalss but it would still be able to give a little allowing for a smoother ride then a metal frame


the only problem with fiberglass is its kinda messy to work with and if it gets on a tool or something you might have to throw it away

We're a little hesitant to take on a project like that... like I said this particular model was designed with manufacturing taken into consideration. That means that we cut a length of wood, drill some holes, and we have a side rail. The fiberglass ends come as a pultruded channel already, so we just hack off the correct length and bolt it on. (We have this problem of being lazy and trying to avoid too much labor )

Some of you also mentioned using plywood, and I'm curious as to how it would best be implemented. One team member suggested laminating several sheets of plywood together to make the siderails...


One more thing I forgot to mention before (and why I like this design so much): Since it is constructed with siderails that require just a proper sized hole for a wheel module to fit, we could really stick wheels wherever we want. So, if it turned out that we didn't want an eight wheel drive robot after all, we could just drill another hole in the center of each side and we'd have a functioning six wheel drive robot. The difference between a 6wd and an 8wd bot with this frame is just two extra wheels and chains.

[MrForbes]

I've been discussing this with some of the students and Steve, here are a few more comments. The plan for the sides was to use a hardwood such as oak. I'm sure we'll get the "that's way too heavy!" comments...that's ok...last year our robot with it's way-too-heavy drive base was still 10 pounds lighter than it could have been, and it never wanted to fall over. Hardwood has the advantage of being dense, so it won't let the bearings open up their holes too much, and it should hold screws better than a lighter wood (although putting the screws in would be more of a challenge). We used .060" 3003 aluminum for the belly pan and corner braces last year, it worked fine, it's pretty easy to rivet to the fiberglass channel to make a strong structure.

One interesting point about this design is that it uses three quite different materials for the chassis structure, and each material is used in that place where it is most effective.

[sdcantrell56]

I do like the way that you are using multiple materials and your use of fiberglass has actually gotten me thinking about incorporating it possibly. I do think that fiberglassing the side rails and bonding the whole thing together with epoxy would be your best bet. Your frame would be indestructible. Also for all the naysayers, oak is much less dense than aluminum and stronger as well for the weight so go for it. I hope you guys get a chance to prototype this and post it up on here. Maybe we should see who has a prototype frame done first

[Jonathan Norris]

I think the wood will work fine for the overall stress of the base, but I worry about how about the stresses at the axels and how you are attaching the bearings. I could see big issues with however you attach the bearings for your cantilevered wheels (can someone rationally explain to me why everyone likes to have cantilevered wheels??? seriously...). I see the wood warping at the bearings (even hardwood), unless the bearings are very securely supported. For this reason it just makes more sense to me to go with two 1/8" plates on both sides of the wheels with cross-bracing.

[gorrilla]

cantilevered wheels have benefits that i am not aware of and i will let them explain




its like the wood trim in cars, noone knows what it is but its red so they call it mahogany

[sdcantrell56]

The 2 major reasons for cantilevering wheels is to ease wheel replacement, and to maximize your robot's footprint. Many teams are able to change a wheel on their cantilevered system in a matter of seconds by removing a snap ring such as 254.