Gear Ratio help....plZ..someone???

[Dick Linn]

Instigator,

We're trying something similar, but only on one end, assuming that 2 wheel drive is good enough (time will tell). 4WD is heavy and 6 is even heavier. Plus 4WD can be less maneuverable if it is a simple scrub-steer 'bot. We're using the drill motors in low, so we aren't faced with the problem of getting a 40:1 reduction in speed at the end.

Try to make the wheels drive at the same ft/sec, so carefully measure the diameter ratios of the two types of wheels, under typical load conditions, and select the sprockets or gears accordingly. If you drive the smaller wheels off the axle shaft that the large wheel is mounted on, you can add a small sprocket there and a slightly larger one on the 10 inch wheel to get things evened up. That may reduce the number of large gears (read weight) that you need. If the wheels are exactly 12.5 and 10 inches, a 15-tooth on the 12.5 driving wheel axle connected to a 12-tooth on the 10-inch driven wheel axle should do it. A 12.0 inch tire would require either a 12 & 10 or 18 & 15 tooth sprockets. If the wheel is about 12 1/4, you are in between a 16:13 and a 17:14. These are assuming you're using commonly available sprockets for 3/8 (#35) chain.

[Matt Adams]

Originally Posted by Dick Linn

assuming that 2 wheel drive is good enough (time will tell).

There's no need to let time tell you- many many teams have gone down this path before.

If you're using two wheel drive, you should loose almost every pushing match against and four wheel driven robot, simply because half of your weight (approximately) will be dragging instead of being pushed. I can not stress enough that you need to have all wheels in contact with the ground be powered if you plan on using a simple tank style drive and wish to be competitive.

The pneumatic wheels are very light, and drivetrain is by far the most important part of your robot. Don't cut power to two wheels because of weight unless you plan on doing something so specacular with the other 5 or 8 lbs that if offsets the immense disadvantage you'll be at from having half of the pushing/pulling force.

I plead that you'll reconsider having unpowered wheels if you haven't finallized your design already.

Matt

[Dick Linn]

Originally Posted by Matt Adams

There's no need to let time tell you- many many teams have gone down this path before.

If you're using two wheel drive, you should loose almost every pushing match against and four wheel driven robot, simply because half of your weight (approximately) will be dragging instead of being pushed. I can not stress enough that you need to have all wheels in contact with the ground be powered if you plan on using a simple tank style drive and wish to be competitive.

The pneumatic wheels are very light, and drivetrain is by far the most important part of your robot. Don't cut power to two wheels because of weight unless you plan on doing something so specacular with the other 5 or 8 lbs that if offsets the immense disadvantage you'll be at from having half of the pushing/pulling force.

I plead that you'll reconsider having unpowered wheels if you haven't finallized your design already.

Matt

Who said pushing was part of our strategy? We've built both 2 and 4 wheel bots in the past, and this years can be converted to 4 WD in about a day if need be, so no worries there. Weight distribution, tire patch on the ground and gear ratios are equally important. We'll try maneuverability for a change. Thanks for the concern.

[Matt Adams]

Originally Posted by Dick Linn

Who said pushing was part of our strategy? We've built both 2 and 4 wheel bots in the past, and this years can be converted to 4 WD in about a day if need be, so no worries there. Weight distribution, tire patch on the ground and gear ratios are equally important. We'll try maneuverability for a change. Thanks for the concern.

Dick, I wish you guys the best of luck. I guess my fear is just that pushing isn't as much a stratetgy as a fact of life- you can get in pushing matches and pinned by other teams even if you don't want to be.

I think it's also fair to assume that unless you have a crab or swerve style drive system, the manuverability increase isn't too significant between a tank style 2 and 4 wheel drive system. This is what my experience has shown, hopefully yours has been different.

Again, good luck!

Matt