Question About 4-Wheel Drive
 

I'm doing some robot designing in my free time over the summer, and I came to a question.

I'm making a 4-wheel drive chassis using the Kit transmission. To do this you can do it with two sprockets on the output of the gearbox, as seen in the attached picture.

Now to make a very reliable drivetrain with this, I think the wheel's sprocket should be aligned with its corresponding sprocket on the output shaft of the gearbox. Doing this would need me to stagger the wheels one way or the other.

The question is:
Which way is most beneficial to stagger them?
Front wheels inward and rear wheels outward or visa versa?
Does it even make a difference?

Re: Question About 4-Wheel Drive
 

Why not just put the sprocket on the other side of the wheel and keep the wheels pretty close to being inline? So you have two wheels with the sprocket outside and two with the sprocket inside.

Re: Question About 4-Wheel Drive
 

Not the kit-transmission, but this photo illustrates Sanddrag's point pretty well.

http://www.firstrobotics.uwaterloo.c...etrains111.jpg

Re: Question About 4-Wheel Drive
 

You don't want your wheels to not be inline. Go with what Sanddrag said, or get two separate sprockets so that you can slide them on the output shaft to align them with the wheels, instead of being stuck with the double sprocket you've pictured.

Re: Question About 4-Wheel Drive
 

What's shown in that illustration is not a 'double-sprocket', but two 21 tooth sprockets placed back to back. This arrangement was suggested for the 2005 Kitbot as an easy way of implementing four-wheel drive, but results in the wheels at one end being offset from those at the other. The picture Karthik linked to is effectively the same thing. The offset will vary with the length through bore of each sprocket.

The difficulty in customizing the spacing of chain on the provided transmissions comes simply from the limited length of the custom-made output shaft. The arrangement shown in the first illustration will probably do the best at getting the wheels as close to flush with one another as is possible without facing off some of one sprocket.

I don't think you'll see much benefit in placing one set of wheels closer together over an another, as we probably spend equal amounts of time going in each direction throughout the course of a match.

Re: Question About 4-Wheel Drive
 

That is exactly how 1147 did their drive train this year. Attached is a shot I just took of it to illustrate that the wheels are in alignment and the drive chains are straight.

Re: Question About 4-Wheel Drive
 

Oops. That's what you get for looking at it without opening it :rolleyes:

you could also use spacers between the wheel and sprocket to get them in line, if removing material off the hub isn't an option.

Re: Question About 4-Wheel Drive
 

Wow! You mean the sprocket spacing on the output shaft is such that when a drive is configured like yours... the wheels are already in-line???

You'd think it was designed that way... ;)

Re: Question About 4-Wheel Drive
 

Funny how things turn out when they are designed well, and then used as designed! :D

Re: Question About 4-Wheel Drive
 

Don't forget to implement some type of chain adjustment or tensioning in your design. This year for time and simplicity we didn't do this and had one of the longer chains stretch allot. There was never time between matches to deal with it. We where lucky it didn't get bad enough to derail. Next year we will deal with it. A commercial chain tensioner isn't cheap. However it's not too hard to make your own.

Re: Question About 4-Wheel Drive
 

we made a longer output shaft thingy for out kit tranny if u can u should try that out and have fun!

Re: Question About 4-Wheel Drive
 

Thanks for all the input guys! I'm taking from this that inline is the best way to do things. However, this again poses a slight problem to me.

I should have mentioned that I won't be using those narrow wheels from the kit too. :p I'm going to be using something closer to 8" diameter, 2" wide pneumatic wheels. That is why I was having a rough time working with the two 21 tooth sprockets on the output shaft of the kit tranny. Their spacing doesn't quite allow for such a simple solution.

I suppose that the best way to solve this problem may be a simple use of spacing on the wheel shafts, leaving the sprockets at different positions on the shafts, but the wheels inline. I'll probably use keyed wheel shafts to make things a little easier.

Re: Question About 4-Wheel Drive
 

You could always just use one sprocket on the output shaft of the transmission and then send that directly to one of your wheels. On this wheel you would place another sprocket on the opposite side of the wheel from the chain coming from the tranny. If you ran a chain between the two wheels like that they would end up being inline. The only bad thing is you use more chain this way.

Now if your comfortable with using smaller chain, which I know some people would yell at you never to use smaller chain in the drivetrain, but we succesfully use the set up I just described on our robot this year without a single problem.

Re: Question About 4-Wheel Drive
 

My current design only left 6 inches in between the gearboxes. Whoops.
If I use only one CIM in each Kit tranny, would a set up as pictured in the attachment cause any problems?

I'm limited in the dimensions of the chassis to very close to what it currently is.
22" wide, 24" long. So I can't make it wide enough to fit two CIMs in each tranny.

Re: Question About 4-Wheel Drive
 

In the picture of our bot, even though you can't see it, there is only one CIM per side. We were going to run two per side, then we did some rough calculations and decided 2 HP to run the bot was overkill. Running one per side was just fine. We never drained a battery in competition and had plenty of speed and power.

Re: Question About 4-Wheel Drive
 

FYI: This isn't a competition robot design, it's going to be off-roading and other fun things. :cool:
I think that one CIM per side will still be enough if it was good in competition.

Re: Question About 4-Wheel Drive
 

Another downside is that if you somehow manage to derail the chain from the output sprocket, you've lost power to all wheels, instead of just one.

Re: Question About 4-Wheel Drive
 

Not necessarily, for the last two years we have taken #35 chain from the transmissions to the back drive wheels and then from #25 chain to connect the front and back wheels.

We have only had a few problems with the #25 chain for example last year while climbing up the steps at an angle we snapped the #25 chain on one side, but we still made it up the step with 3 wheel drive!

Just make sure that the chain connection from the transmission to the back wheel is properly tensioned and correctly aligned. You should not have a problem if you do that right.

I have included a picture of this years drive-train, it is not the clearest picture, but it will do.

Re: Question About 4-Wheel Drive
 

We never had a single problem with this chain set up at all this year, but just in case, tension was checked before each match and we color coded all the master links for very quick access in case of emergency. Attached are pictures of the front and back wheels. Ignore the little bit of damage to the back wheel, that post next to it took a very big hit. Can you say two robots accidentally backed up full speed into each other? And just for clarification, the back wheel is the smaller wheel.

Re: Question About 4-Wheel Drive
 

If by any chance you choose to get rid of the second driver sprocket, you can still power both wheels on the one side by setting up the tensioners and/or idlers in just the right spots. We used the idler sprockets this year and had no problems whatsoever with our drive train.

Since you're concerned about space constraints, this method saves a few inches width-wise because the wheel sprockets are on the same side and there is only one driver sprocket.