Center Wheel Drop?

I have almost finished CAD for my teams drive base and have been able to figure out everything out from other CD posts but I put a 3/16th inch center wheel drop. As I finish up CAD I am starting to question my self that is too much of a drop. Just so you know it is a 4in wheel with a ball shifter geared for 17.1fps High and 7.54fps Low. It is also a 6 wheel drive tank drive.

What is your wheelbase and type of wheel? Also how are you measuring your drop?

We are using the AndyMark 4in high grip wheels

Drive Base is 25in wide x 30in long

Taking the center of the wheel and moving it down 3/16in

The HiGrips are smooth enough that you could go down to 1/8" for a long configuration or even less for a wide configuration.

Yup, I think it mainly has to do with the type of wheels. If you use something a little more rough like blue nitrile, you’ll probably need something like 3/16th. But I think 1/8 should work for those wheels that you’re using.

3/16" may be a little too much, with 1/8" drop it gets a bit horsy…wobbles a bit too much, this is what we have experienced.

The right answer is a function of many things. How stiff is your robot frame? Stiffer frames can get away with less drop. How high is the center of mass? A high CoM exacerbates rock when accelerating or decelerating. How willing are you to frequently replace wheels/tread? The middle wheel generally wears quickest, reducing the effective drop over time. Being proactive about replacing it (or rotating wheels) lets you get away with less drop. Driver preference also plays a role in this. A little bit of scrub is helpful for tracking straight and for doing controlled turns, while more rock makes the robot drive more smoothly (but requires greater driver skill or software help to control).

Some of these factors are difficult to estimate beforehand. As a general rule, especially with the smaller robots used the past two seasons, most teams will be happy with 1/8" of drop.

1/8 inch is not enough drop. The robot sinks into the carpet about that much. The high-grip wheels also wear like crazy and the center wheel wears more. With 1/8 inch drop you’ll eventually have no drop at all.
3/16 might be a bit too much, .150 is the number I always use.

I’ll echo this.

We just did our first 6-wheel drive last year and did a 5/32" drop. It worked out pretty well for us, but there was a little more rock then we liked initially, especially when we used the W-tread. When we switched to rough top, it felt a lot better. We had a fairly stiff sheet metal channel drive. I’m not sure if 1/8" or 5/32" would work out better. We haven’t tried the 1/8" yet. For sure we would if we knew we were using the W-tread.

This is a good answer, anyone in here throwing out specific numbers is playing a dangerous game.

The last two years of sizing rules have really changed all this. Not many true “long” bots still exist, and it seems the square-bot is almost standard.

This along with more diversity in wheels being used, has greatly changed the game.

It used to be simple enough back in the days of 28x38 and almost everyone running kop or roughtop to say 3/16", but it isn’t anymore.

You’re doing people a disservice by stating a number as an absolute on here.

Just to illustrate this point: Black is the perimeter size, Blue is the actual wheelbase.

Edit: words cuz ether

I think you’d find 3/16 to be too much at first, but it would improve with wear. 1/8 would be good at first, however, you’ll probably need to replace wheels due to center wheel wear.

Based on what I’ve heard, 5/32 works well for a square drive with blue-nitrile wheels.