Drop Center Distance

[cscottm]

Hello Chief Delphi

I am asking what teams did/do for a drop center in their drive train. We did 1/8" this year, and we still had a decent amount of hopping/skipping. So, what do some of you guys do for your drop center distance?

[Jon Stratis]

You did 1/8"... but what type of wheels did you use? When you use harder wheels (KOP traction wheels, AM Performance wheels, Colsons, etc) you can get away with less drop, but when you use wheels with more give (like the pneumatic wheels that were so popular this year) you need a larger drop. You need to take into account wheel compression, otherwise your center wheel will compress, allowing the outer wheels to touch the ground!

[dmorewood]

We used a 1/8" drop this year. In previous years we have always used either a 1/8" or 1/16" drop and been fine. For most games though I would recommend going with a 1/8" drop.

[cscottm]

Originally Posted by Jon Stratis

You did 1/8"... but what type of wheels did you use? When you use harder wheels (KOP traction wheels, AM Performance wheels, Colsons, etc) you can get away with less drop, but when you use wheels with more give (like the pneumatic wheels that were so popular this year) you need a larger drop. You need to take into account wheel compression, otherwise your center wheel will compress, allowing the outer wheels to touch the ground!

We used 8" pneumatic wheels.
I realize that I should have said that.

I suppose I am wondering what other teams that used them used as their drop center.

[Taylor]

Originally Posted by cscottm

We used 8" pneumatic wheels.
I realize that I should have said that.

I suppose I am wondering what other teams that used them used as their drop center.

It also depends on how you inflated your 8" pneumatic wheels.
If they were at 35psi (max) you'll find much different handling than if they were at 10psi.

I know my team used the KOP chassis drop, our wheels were around 20psi, and the robot handled pretty smoothly (the little red machine toward the bottom of the screen).

[SenorZ]

We did a 1.5" raise on our front wheels (8" pneumatic), and had the middle and back level. So during a "tip" we had 0.75" drop effectively.

Post season we shifted to a more traditional drop of about 1/8" and we had a lot of trouble turning.

[cscottm]

Originally Posted by Taylor

It also depends on how you inflated your 8" pneumatic wheels.
If they were at 35psi (max) you'll find much different handling than if they were at 10psi.

I know my team used the KOP chassis drop, our wheels were around 20psi, and the robot handled pretty smoothly.

We had them fully inflated.
(I seem to be forgetting a lot of information, sorry)

They were on a 30" side rail, spaced out evenly, driven off the front wheel.

[nuclearnerd]

We used a kit 6wd chassis with 8" pneumatic wheels. The kit has a 1/8" drop iirc, but we supplemented that by using different tires for the center wheels than the corners. The larger center wheels probably added another 1/8" of drop, which we found was necessary to turn such a long wheelbase.

[Billfred]

4901 ran a 1/4" drop, if I remember correctly, and played some air pressure tricks on the corners too.

We wouldn't normally go that crazy with the drop, but we were running pneumatic tires, with a narrow track width at the corners, and expecting this frame to get beat to hell over the season. So we played it safe.

[Peyton Yeung]

We ran a 1/4" drop on the 4 inside wheels. 6" pneumatic wheels at 30 psi and about 7.5" center to center.

[Steven Smith]

3005 ran a 3/8" drop with 8" pneumatic wheels in an 8WD tank, WCD style config. Would not recommend that much drop

Originally we had a 1/4" drop (rev 1) and had some amp draw issues that were more associated with our gearing and construction quality, but were misdiagnosed as excessive scrub. The 3/8" drop certainly didn't hurt, but the rocking, especially on short/fast movements as the driver tried to make fine adjustments, wasn't great.

That was also on a lowbar robot, obviously the problem would have been magnified on a taller robot.

[D.Allred]

We did 1/4" with eight 6" pneumatic tires and played with the air pressure. We ran the outer wheels almost flat to dampen shock crossing defenses.

Also, the 6" tires were much harder and less "bouncy" by nature than the 8" wheels I saw.

We built an offseason robot with six 6" pnematic tires and 1/8" drop with no turning issues. Our wheel base was very short though. The whole robot was only 24" long.

David

[GeeTwo]

We had 0.78" of drop this year and a combination of performance and kit wheels, and were quite stable for making shots.

We had 8 8" wheels (plus 2 4" wheels that only saw duty on a few obstacles).

Even with 6 wheels, you can improve stability for shooting by having your CoG either clearly ahead or clearly behind the center axle.

[Chris is me]

Drop centers are a function of a few variables

1. Wheelbase length and width
2. Wheel material (rubber, tread, pneumatic, tracked)

Good rules of thumb:
Pneumatic wheel - 1/4"-3/8"
Treaded wheel - 1/8"
Rubber wheel - 3/32"

Your mileage may vary. Keep in mind all of the "conventional wisdom" for drop centers was determined when wheelbases were usually 38x28, or much longer than they are wide. Drive bases closer to square shapes may need less drop, and wide bases may need no drop at all. But a little too much on a 6WD isn't the end of the world.

This year 228 did 1/8" drop on a 10WD, with the outer wheels raised up 1". This was unnecessary, we didn't need any drop at all with how short our effective wheelbase was.

[Golfer4646]

Team 4646 (my team) used an 8 wheel drop center drivetrain. All wheels were 8 inch pneumatic tires inflated to a firm feel, (if I remember correctly, that was between 30 and 35 psi).
We were running with a 1/2 drop on the center wheels.

It performed well with no major issues, and was only felt unstable when we quickly changed directions. If we had a chance to redo the drivetrain, we would probably do a copy of our original, because it worked well enough. (If anything, the drop would have been reduced slightly, probably to no less than 3/8.)