"Hopping" With tank style turning.

[QuantumNinja]

Alright. Our robot is configured to have all 4 wheels being driven, two motors on each side drive each sides wheels. To steer, it obviously slows down one side, or to spin on a dime, one side goes forward, the other backwards. The problem is... the robot seems to "hop", and the motors get really loaded when turning and especially when trying to spin. We are using the IFI Robot Wheels which are extremely grippy. I notice that alot of teams steer like this... and i'd like to know how they overcome the probelm we are having. Thanks.

[Ryan Foley]

There are a couple ways to do this:
1) use less "grippy" wheels. Since the wheels will slide easier, it won't hop
2) widen your wheelbase. If a wheelbase uses 4 identical wheels like you have, and it is longer than wide, it will hop. Typically, robots that are wider than they are long will not have this problem. Note that for this purpose, you measure from where the contacts the floor.

At this point in the season, #1 would obviously be easier for you to do.

There may be other ways around this problem, but this is all I can think of off the top of my head.

This whitepaper discusses this problem and shows you how to fix it.

Good Luck!

[Andy Baker]

Along with Ryan's advice, many teams use Omni-wheels to make their front pair* of wheels slide laterally with ease. You will still have good front to back pushing power. Also, your resistance to getting pushed will still be good on the end of your robot where the Traction Wheels are located. You will lose resistance to being pushed sideways at omni-wheel end of your robot.

Teams have been using this technique to steer better since '98.

#: or back pair, depending on which end is carrying the most weight. You want the traction wheels carrying the heavier load.

Andy B.

[dk5sm5luigi]

Another option is to replace all the wheels with mecanum wheels and drive them all with seperate motors. These wheels are similiar to Omni-Wheels but have the rollers at 45 degrees making it so you can drive holonomically. This idea is probably not feasable at this stage since it took us 4 weeks to make ours last year and a lot of programming to make them work perfectly.

[KenWittlief]

what are we up to now? Plan D?

Plan D: use different wheels on the front that are not as grippy. They will slide sideways easier. The back wheels will dominate the steering, and the front wheels will slide.

Plan E: make a pneumatic wheelie-castor that raises the front wheels off the ground using a castor and a pneumatic cylinder. When you want to turn easily you push the castor down, turn, then retract it.

[Not2B]

I'm guessing it's a bit late to go to a wider frame or change the drive around.

And if you don't have the money or machine tools to get omni wheels (which are a great idea at this point for ya), then you can do the cheap fix.

Wrap the wheels on the light side of the robot in something slippery - we use rip-stop nylon zip tied onto the wheel. Heck, last year we PLANNED on using replaceable rip-stop nylon wheel covers. They worked well, and cost about $2.

Omni wheels are, of course, better if you can afford them.

[Jared Russell]

The pneumatic pop caster has worked flawlessly for us - you get the best of both worlds: turning on a dime, and four high traction wheels on the ground when you need them.

[lukevanoort]

This is exactly what happened to us at VCU last year. See, with your robot's drive geometry it can't turn. What's happening is the insane torque of the drivetrain is twisting the chassis so one wheel is no longer in contact with the ground. Since that wheel now has zero traction and no real effect on turning, it becomes a 3WD that can turn. Then, the chassis flexes back, since you don't have the force to keep it up anymore, starting the cycle all over again.

Now, as to fixing it, we packed our treads with epoxy clay last year, and it worked like a charm. After a while, though, it started to crack and fall off (epoxy clay is brittle when dry), so we reapplied at the championship. It also looked way better than a bunch of zip ties wrapped around the wheels.

[coreyjon]

We had the same problem last year, too much traction, it seams can also be a bad thing. So we simply wrapped the two front tires in a bunch of zip ties (not the best looking) but elegant in it's suplicity. Hind sight, being what it is, you could have always gone for real grippy front or rear tires, and gone for some of thoose fancy omni wheels on the other set. Still provides great forward traction, and at the same time, allows you to turn without causing as much load on the motors and drivetrain.

[Wolf103FM]

Originally Posted by QuantumNinja

Alright. Our robot is configured to have all 4 wheels being driven, two motors on each side drive each sides wheels. To steer, it obviously slows down one side, or to spin on a dime, one side goes forward, the other backwards. The problem is... the robot seems to "hop", and the motors get really loaded when turning and especially when trying to spin. We are using the IFI Robot Wheels which are extremely grippy. I notice that alot of teams steer like this... and i'd like to know how they overcome the probelm we are having. Thanks.

We've used zip ties all around the rear wheels before,we have also cut grooves in the tires but i don't remember if that was for traction or turning

[Chriszuma]

We had that problem last year with our massive 1.5" brecoflex 4-wheel drive. This year we switched to a six-wheel drive. With the center wheels mounted about a quarter inch lower than the front and rear pairs, most of the weight is on the center wheel, which allows for insane turning, and insane traction at the same time. In fact, we just got the chassis driving last week and we were having so much fun just spinning the thing in high gear circles, something that wouldn't have come close to working last year.

So yeah, it's probably a bit late to add another pair of drive wheels to your chassis, but for next year, definitely go 6-wheel. Last year we partially fixed the problem by using the beige diamond-shaped tread on the back wheels and the super grippy black tread on the front wheels.

[gburlison]

Are your IFI wheels 8"?
You could try replacing the front or back wheels with the ktibot skyway 8" wheels. Since they have less traction, this would be a quick way to experiment and see if wheels with less traction would solve the hopping problem.

[Al Skierkiewicz]

Originally Posted by QuantumNinja

Alright. Our robot is configured to have all 4 wheels being driven, two motors on each side drive each sides wheels. To steer, it obviously slows down one side, or to spin on a dime, one side goes forward, the other backwards. The problem is... the robot seems to "hop", and the motors get really loaded when turning and especially when trying to spin. We are using the IFI Robot Wheels which are extremely grippy. I notice that alot of teams steer like this... and i'd like to know how they overcome the probelm we are having. Thanks.

Quantum,
As you see, this is a common problem with tank style drives. The side friction causes incredible (near stall) current demands on the elctrical system and the hopping is due to a variety of factors including robot frame flexing. The best alternative at this stage (late in the build season) is to use omni wheels. A variety of types are available including the types found here. http://www.andymark.biz/plastic-omni.htm which are easily available and designed for First robot use. Once installed you will find your robot is happier, your electrical system is happier and drivers are happier.

[Dick Linn]

Try to find some wheels of the same diameter with less traction, at least on one end. Even with that, you may need to bump steer. One year, our drivers had to reverse one side and go full forward with the other in order to turn worth a darn.

[JamesBrown]

The easiest way to cut down the traction on the fron wheels without spending too much more money is to buy th pool hose (like for pool filters) and cut it in half then zip tie it to your wheels. If you want to see what i mean find pictures or 121 in 2004 and look at their front wheels. Try http://www.Joemenassa.com