Electric Vehicle C

rman
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Re: Electric Vehicle C

Post by rman » March 30th, 2009, 1:50 pm

I have been reading the series of questions and answers regarding uing a ball bearing on one end of a vehicle and wheels on the other and I had a question for the original poster. When you say "ball bearing" do you mean a single steel ball used as a roller, or do you mean a ball bearing of the sort that are used on rotating parts? If you are suggesting a single ball that is free to rotate in any direction then your steering will be based on the relative size of the front wheels, the relative traction of the two wheels, the accuracy of the axle and bearings, the runout (wobble) of the wheels, and the imperfections of the track surface. These same factors effect any vehicle in much the same ways, but with a free rotating ball on one end how do you adjust the steering?

If you actually meant that you would just use a standard ball bearing (a bearing used to support a rotating shaft or a stationary shaft with a rotating outer race) for a wheel, then the ball bearing simply replaces a wheel and you would need to adjust steering the same way as any other vehicle.

I am afraid that there is no magic bullet for making a vehicle that goes straight. As others have already pointed out the only "trick" is to make sure the track is as clean as possible. Make sure the wheels have no runout and have as close to the same friction as possible. Make sure any wheels connected together on the same axle are as close as possible to the same diameter, or allow each wheel to rotate independently. Then adjust the steering until the vehicle goes as straight as possible and try and keep the conditions as close as possible each time you run.

It may seem unintuitive, but when a wheeled vehicle (car, bus, "electric vehicle" etc.) is moving the wheels are always slipping slightly. The amount of slippage necessary to cause your vehicle to turn a cm or so, over 10 meters is incredibly small. 1 cm side movement over a 10 meter track is equivelent to 1 meter over a kilometer. Imagine trying to get a car to go down a kilometer of highway and stay within 1 meter of the center without any input fromt he driver, it's nearly impossible. If you scale up your electric vehicle and the tracks used in typical competitions you will see that the imperfections in the best vehicles and smoothest tracks are probably much larger then the imperfections in a decent car and nice piece of highway. The cracks between gym floor boards, when multiplied by 100 (the difference between our 10 meter track and my hypothetical 1 km highway) becomes a large bump in the road. Even the finish on the floor, or scratches, or pieces of dirt and dust, become major obstacles to a perfect striaght run.

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Re: Electric Vehicle C

Post by fleet130 » March 30th, 2009, 6:09 pm

Imagine trying to get a car to go down a kilometer of highway and stay within 1 meter of the center without any input from the driver,
Ah, but the wheelbase on the scaled-up vehicle would be somewhere in the neighborhood of 28-32 meters! Still, the tolerances needed to produce the desired results are extremely small!
Information expressed here is solely the opinion of the author. Any similarity to that of the management or any official instrument is purely coincidental! Doing Science Olympiad since 1987!

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Re: Electric Vehicle C

Post by rman » March 31st, 2009, 12:46 pm

fleet130 wrote:
Imagine trying to get a car to go down a kilometer of highway and stay within 1 meter of the center without any input from the driver,
Ah, but the wheelbase on the scaled-up vehicle would be somewhere in the neighborhood of 28-32 meters! Still, the tolerances needed to produce the desired results are extremely small!
An interesting point but I think that the notion of a longer wheelbase would somehow make the vehicle more accurate is possibly missunderstood. It is obviously true that as the wheelbase gets longer the steering angle required for a certain rate of turn decreases, but how does wheelbase change the effect of things like wheel runout, differences in traction from one wheel to another, differences in wheel diameter of coupled wheels, etc.?

By the way, I heard that someone got a perfect score at their regional competition this past weekend (200/200). Like most regions, they do not publish scores, so it is impossible to verify, but I did see the run and noticed that the event coordinator was looking very carefully and just shook his head and didn't even take out a ruler to measure the distance. As the kids doing the run walked away they said that it was absolutely perfect, zero time error, zero, distance error and zero finish line error (and obviously the centerline bonus as well). There first run was perfect on distance and time I believe, but I think was off several cm in aim, so it would seem they have to compensate aim for track variations (or they were just lucky). I guess the "no electronics" bonus wouldn't help with a perfect score. I also noticed in the rules that the tie breakers for that event are best distance and best time score, so there would be no way to pick a winner out of several vehicles with perfect scores.

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Re: Electric Vehicle C

Post by fleet130 » March 31st, 2009, 1:14 pm

I think that the notion of a longer wheelbase would somehow make the vehicle more accurate is possibly misunderstood.
I agree, but the misunderstanding is where the misunderstanding is!

Given that all other factors remain the same, the longer the wheelbase, the greater the turning radius. The greater the turning radios, the less the error from a straight line.
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Re: Electric Vehicle C

Post by nickfastswim » April 4th, 2009, 1:22 am

The basic formula for the turning radius is
where L = length of wheelbase (distance between the axles)
theta= angle the axle is turned with respect to the the center line

If we were to increase the wheel base L then according to this formula the turning radius R also increases.
Thus fleet130 is correct. :geek:
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Re: Electric Vehicle C

Post by ahage16 » April 6th, 2009, 7:10 pm

Thanks for everything on the last page gh! I really wish we had a computer science class at our school, or at least someone who knew about this kind of stuff! I've been pretty much trying to work this for the past year with nothing but google, which hasn't worked out too well. All I know for sure is that I am abandoning propeller because that hasn't worked at all for me. I guess this is what summer is for!

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Re: Electric Vehicle C

Post by rman » April 13th, 2009, 1:11 pm

I'll keep this simple. Has anyone come up with an explaination for the fact that most vehicles can't repeatedly go to the same aim point? We have great sights, nice bearings and wheels that are straight and true, nice bearings, no slop or wiggle, stiff chassis, etc., but the vehicle will do as much as several cm left and right between test sessions on different tracks. On a single session on a single track the variation is only mm, but when testing on different days on different tracks the aim point seems to mysteriously move. I hate wasting one of my two runs just to find out the peculiarities of that track, but maybe there is no way to go to the exact same point on different tracks?

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Re: Electric Vehicle C

Post by fleet130 » April 14th, 2009, 10:20 am

I believe the interface between the wheels and the track surface has a lot to do with this. Hard skinny tires don't perform as well in this respect as wider softer ones. Solutions to these problems are usually a "best compromise" after considering several other factors (such as rolling resistance).
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Re: Electric Vehicle C

Post by sachleen » April 18th, 2009, 6:41 pm

Had my states today. My EV was 2cm from the center of the target on the first run and 3cm away the second run (distance from the center of the target to the tip of the pointer). I don't know my time score yet but I think that's what caused me to get the 5th. Other teams' distance scores were about the same as mine, all within that 2cm radius range so either it was down to that last mm, or time (or both).

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Re: Electric Vehicle C

Post by starpug » April 19th, 2009, 9:26 pm

Yeah their device was pretty awesome
Last edited by starpug on April 20th, 2009, 5:15 pm, edited 1 time in total.
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