Wheelbases
 

I've been working on an 8wd for next year, and found our robot has a wheelbase of about 11". It uses 4" wheels. My question is, around which length of a wheelbase is best to use, optimizing both turning performance and stability?

Thanks!

Re: Wheelbases
 

For optimizing turning ability, you want a wheelbase of zero. For optimizing stability, you want a wheelbase of 38 inches.

It's all a compromise....

Re: Wheelbases
 

A wheel base of 38 inches would mean wheels of radius 0 inches..

Re: Wheelbases
 

Or wheels that are square.

EDIT: I suppose triangles would sort of work also, but both would give you a changing wheelbase that is not always 38" and you may violate the single appendage rules.

Re: Wheelbases
 

We actually have these in production and will start offering them as COTS items to teams next season;)

Re: Wheelbases
 

If the wheels are parallel to the robot, but not if they aren't.

But you did get the point about the wheel size affecting the maximum wheelbase.

Re: Wheelbases
 

To accurately answer your question, we'd need a few more specifics - mainly track width.

In the case of long based robots, where your track (distance between wheel centers from one side to another) is around 22"-26" I've found that a wheelbase of somewhere around 14"-16" is optimal - this is around where most dropped center 6WD's fall in terms of effective wheel base.

When building long based 8WD's (track width of 22"-26"), I'm partial to doing unequal wheel bases to increase the resistance to turning a bit. Most Common 8WD's have an effective wheel base of 10"-12" and I've found that they're often prone to over rotating. To combat this, I'll either increase the center wheelbase to somewhere around 14"-16" or change the wheel drop arrangement to get a similar effect.

When looking at wide based robots, you can usually get away with having as long of a wheelbase as physically possible with no ill-effects. This is because a wide based robot often has a track of somewhere between 32"-36". Personally, I prefer as long as a wheel base as possible to avoid over rotating while turning when building wide bases, though some teams have found success this year with dropped center drives on wide based machines.

In the end, it's really a matter of driver preference too. I'd suggest building a prototype drive train where the wheelbase is adjustable without too much effort and see what your drivers like best and is best for the robot it's being built for.

Re: Wheelbases
 

We discovered this year that we could get too much traction on a wide base robot, so it could not turn while not moving forward/backward. We ended up changing wheels for a bit less traction, but we could also have changed the drive ratio to fix the problem.

There are lots of variables here! so there isn't one answer.

Re: Wheelbases
 

If you want to optimize turning performance and stability (but not traction) I would suggest a 6/8wd with high traction wheels in the middle, omni wheels on the corners, and no appreciable center drop :eek:

Re: Wheelbases
 

Note that the maximum centre of mass you can have before tipping (assuming CoM is in the middle of the bot) is (wheel base)/(2*tan(max angle)). So, if your wheel base is 11", and the greatest angle you'll see is ~21degrees (going over the bump, arcsin(4/11)) you won't be able to put your CoM any higher than about 14" off the ground without fear of tipping! Of course, the bump is specific to this year -- but keep it in mind.

On the note of turning performance, we were highly recommended against using 4WD with all solid (non-omni) wheels -- but at a wheelbase of 17", we haven't noticed any issues. It does skip while turning of course, but not enough for the drivers to be able to notice; especially on carpet. All in all it has been very successful for us so far.

Re: Wheelbases
 

I thought wide base was easier to turn (all other factors being equal) than a long base. For the same torque per wheel, you get more turning torque (a longer moment arm about the center of desired rotation). Also, the shorter wheelbase gives less inherent resistance to the turn (a wheelbase of 0 would be very easy to turn, and an infinite wheelbase would be impossible to turn if the wheels have non-zero sideways traction).

Re: Wheelbases
 

It is easier to turn than a long 4 wheel robot, but it's not always real easy to turn...the combination of 18" wheelbase, high traction wheels, and not much drive reduction resulted in not being able to spin in place.

You need to adjust things to make it work right, to meet your game playing priorities

Re: Wheelbases
 

Our robot this year was an 8wd with 6 inch performance wheels and blue rough-top tread from mcMaster. Our wheels were evenly spaced, 10 inches apart, with the middle wheels .125 lower than the outer wheels. The robot cannot turn in place very fast without skipping, however it can turn with a radius much less than the width of the robot. The track width is 27.75 inches, with less than 1/6 inches of clearance to the bumpers. I would recommend this drive-train to anyone, as it has worked extremely well, our gearboxes are geared for 16.4 and 6.6 ft/s

Re: Wheelbases
 

You just made me want to start designing ANOTHER chassis. (chassis #11) Hmmmm.... What should i design this time? Any suggestions?

I have a few questions.

What kind of chassis are you designing? Sheetmetal? WCD? etc

Is it going to be wide or long chassis?

what gear boxes (and geared for what speed) and wheels are you using?

Re: Wheelbases
 

It will be a WCD, with the wheels as far out as possible.

It'll be long

It will be custom transmissions, and though the speeds haven't been set yet, we're thinking about using no more than 16fps, and no less than 6fps.


If you do make chassis #11, do show us! If I like anything, it's a well-built chassis. :)

Re: Wheelbases
 

This was our first year with a wide 4-wheel robot and we had issues with motor load during turning (2 super shifters driving four kit wheels). We had to switch to omni wheels in the front to make turning easier. We have also had stability issues on the bridge and during very fast direction changes.

To be honest, one of the best tank wheelbases we've had was a long 2-wheel drive with ball casters in the front. Obviously, this is not good for climbing obstacles, but it allows high friction wheels to be used with no scrubbing during turning and gives great maneuverability when the driver gets used to the turning point. Our 2003 robot climbed the mesh ramp and pushed robots off the top with standard casters and big, soft rubber wheels in the back.

I wouldn't really advocate our current design in the future. There's better climbing setups, and its only a mediocre floor-only design. The issue with compromise is that can often produce results with no advantage either way.

Re: Wheelbases
 

We're 10 wheel wide this year, with about 27" wide base and the center 6 dropped, making it 9" long. Seems to do just fine.

The whole 10 wheel thing was to get over the bump while keeping everything low, but that hasn't worked out yet for other reasons....