pic: Concept 8wd Drivetrain

This is the drivetrain I have been working on for the upcoming season. It is entirely wood and plastic with the exception of axles, bearings, gears, and bolts. It will be constructed from 12mm meranti or possibly okoume plywood and possibly fiberglasses. It is 8wd with 1.5" wide cutom hdpe wheels. It uses modified andymark 2 speed transmissions. The frame has approximately a 40 degree approach and departure angle perfect for ramp climbing. Tensioners are built in to the frame. The total weight for the drivetrain with transmissions, motors, wheels and chain should be ~37lbs. Let me know what questions you have.

O and it should be able to float for that water game :wink:

Couple of questions:

1> HDPE is rather slick, are you planning on covering it with roughtop?
2> Are the center 4 wheels lowered? If so, by how much/. If not, how are you overcoming friction?
3> Whats the diameter of the wheels and whats it’s expected speeds?

I’ll let sdcantrell56 answer officially for himself,

but I’m pretty sure there is a tread on the wheels already (If you look closely). I think he meant that the hub itself was made out of HDPE. The two sets of center wheels also appear to be slightly lowered by my eye, but I could be fooling myself. The wheel diameter looks to be 4".

Quick Question, why not uhmw for the wheels. Just wondering, I’ve heard of teams having success with them.


Nice work on the approach and departure angles. I really like that. Every year we’ve ever had something to climb, it pains me to see teams with huge wheels that still can’t do it, because the wheels are not positioned well in relation to the frame.

Robots don’t always need large wheels or high ground clearance to climb things. This design is a fine example.

UHMW and HDPE are two very similar materials. They do have differences though. HDPE stands for High Density Polyethylene which is long strands of ethylene monomers. UHMW-PE (Ultra High Molecular Weight Polyethylene) is similar, but has even longer monomers, generally making it stronger with better wear properties. HDPE will hold it’s shape a little bit better and some argue that it can be machined easier. UHMW tends to be a bit more expensive than HDPE depending on the scenerio.

Honestly, when it comes to something like a wheel, using one or the other shouldn’t make a big difference. It probably comes down to what you can get your hands on easier.

Thanks for clearing that up, I wasn’t sure what the difference was. Does anyone know how long those wheels last and nice job on the drawing and I totally agree with Sanddrag.

Ok I will try to answer everyone. First of all I was planning on UHMW for the wheels. Something about having 2 hours of sleep made my brain function…well not function at all actually. Also the center wheels are lowered .175 inches for easy turning. The wheels are 4.5" diameter with roughtop tread. One of the nice things about using uhmw is we can buy enough material for 20 wheels for right around $70 dollars. From what I have heard UHMW makes a very durable wheel. It is stronger in impact situations and easier to attach tread (sheet metal screws). I will post up a picture of the wheel assembly with sprockets later but the entire thing with rim, tread, 2 sprockets, and 3 bearings weighs .5lbs.

 Right now the speed is set right at 5fps and 14fps although that can easily be changed by switching the drive sprockets.  Needless to say this drivetrain will not be getting pushed around.

Sandrag thanks for the compliments. I’m thinking there will be some type of obstacle next season and I hate big wheels so this is the result.

besides traction, what does a 8WD offer that a 4 or 6WD doesn’t offer? To me it just takes up extra weight, but that’s just me.

If UHMW is buoyant (which I have absolutely no clue), it would aid in keeping it afloat in next year’s water game…

Other than that, it’s pretty much traction. Also, there is less “rock” to the chassis as 2 sets of wheels are lowered instead of just 1. This could make the turning a little more stable, but I don’t really think it’s too big of an issue.

I would recomend going with 6wd, with a couple idler wheels that don’t contact the ground to avoid rubbing on the bottom of your frame on a ramp (that is, if there is a ramp.) 8wd is really just a design choice. There is an added coolness factor as well which I think should be factored into any design decision.

well i know that 1251 had an 8 wheels and they did great but they were pretty easy to move, maybe not the wheels, and maybe the weight of their robot, or the distribution of their weight towards the front but i think most robots have this problem unless the have a good weight distribution over the right places

having lots of traction dosent matter if you have all the weight in the wrong places

Keep in mind that increasing the number of wheels will no gain your more traction. You’ll get the same traction out of a 6wd that you will out of an 8wd. At least theoretically.

You may gain some traction if using wedgetop tread due to it’s ‘stickiness’. You may also gain traction from using roughtop tread which will actually cling to the carpet. These situations are both relative to a similar setup with less wheels.

The tread you are using looks like standard rubber, unless you are using something else and just didn’t model it, which is completely understandable. If you are using this tread, you will likely gain little to no advantage in traction over a 6wd.

well i also dont see why teams are usig such small wheels all the time there cant be any benefit except maybe weight

Exactly, why use larger wheels when you can save 2-3 pounds and have the same functionality? Not to mention money.


well do you think its really worth it i mean 1557 was 119.1 pounds and that was mostly in our frame and scissor lift

and with smaller wheels wouldent you have less traction because of less surface area

Smaller wheels also let you set your whole robot down lower to the ground (low center of gravity, yay!) and they allow you to make your wheelbase about 2" longer than one with 6" wheels (less tippy, yay!). There’s lots of good reasons to use them.

We put 4" wheels on our robot last year since it didn’t have to do any climbing, and I didn’t notice any difference in performance between it and our previous year’s robot (with 6" wheels). Both were set up with 6 wheel drive and wedgetop tread.

I’ve also been kicking around a wooden eight wheel drive base in my head for a while, but it looks quite a bit different… I like the eight wheel drive concept, it has several things going for it that make it preferable over six wheel drive (IMO). Having four center wheels instead of only two slows down tread wear, and as someone else stated doesn’t let your robot “rock” while it’s driving around. It also gives you a bigger contact patch on the ground to avoid being spun. By bigger contact patch I don’t mean more traction, but traction where you need it. Six wheel drive bots spin pretty easily when pushed on a corner since all their weight is on their two center wheels. But an eight wheel drivebase has four center wheels that carry the weight, and they’re all distributed further out from the center.

And the obvious reason for using an eight wheel drivebase is that it allows you to climb over all those ramps on the field without high-centering.

No, because the overall robot weight is the same. If the surface area goes down, then the concentration of weight on the area goes up. Traction does not change.

Small wheels provide less cost, less size, and you need less reduction before the wheel. The only thing they lack is provided ground clearance. But that can be worked around.

but were would one get four or 5inch wheels? without custom machining

http://www.robotmarketplace.com/products/colson_wheels.html comes to mind quickly, Banebots also has some.

IFI has 4", Andymark has 4", 5", and will have 4" plaction soon (as far as I know). Banebots has 3", 4", and 5". Colson makes smaller wheels as well I believe.