Mini Dyno

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A few minutes with a spreadsheet.

Besides, I’ve noticed the forum does not display the HTML…

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Chris – just did a quick’n’nasty workaround to display the tables.

GREAT WORK!

Derek

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Well, all this information is all very nice but again, it’s all theoretical. By those calculations a 3.8 motor with 8.25 gears would go12.15 Kilometersper hour or 7.25 Miles an hour. I’ve had this set up on my WRX before and there is no way it was going anywhere near that fast.

If you wanted to go my theory, I made a dual cell bit charge that has double the voltage going to the motor wich means double the rpms at the motor which means double the speed. So, theoreticaly my WRX goes 24.3 Kilometers per hour or 14.5 Miles an hour? NO WAY, not even close. I’d guess my dual cell bit (which is twice as fast as my regular bit) might hit 4 Miles an hour just by guessing.

The friction of the gears without bearings (just plastic gears over metal shafts), friction of the motor rotor without bearings (just brass bushings), friction of the tires, friction of the axle in the plastic housing (again without bearings), friction of the front wheels without bearings (just plastic wheels over metal axles) all adds up in a BIG WAY. If a full sized human carthat was made without any bearings like these things are and with the sloppy tollerances magnified to larger scale you wouldn’t even be able to push it downhill. If you had a huge engine in it youwould probably only go 20miles an hour and all the points where there should be bearings would all seize up or melt away within minutes. Not to be funny but just to bring some reality to these numbers.

Finally, when the motor rotor decreases in resistancethe RPMs increase but the torque does not. That means the torque to rpm ration decreases. IF this ration stays the same you have a somewhat linear increase in overall power but when this ratio decreases, the friction becomes a greater factor in top speed. This means a car with a 1.0 motor might only go 50% of theoretical speed and a car with a 3.0 motor might only go 40% of theoretical speed (these are guessus of course but you see what I mean)

I’ve been trying to think of a way to make some sort of infared timing devise to set up and drive my cars through at full speed and calculate actual speed. I’m interested to see what actual speeds are.

Anyway, it’s always fun to crunch the numbers, Theoretical or Actual.

-Peter

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hey prabbit. how about this. it seem skinda easy if you could find a certain part.

-a tomy lap counter counts laps, but we want a straight line. -what about measuring a distance that a prabbit special car could run (without having to much speed wobbles. maybe a 5 foot distance (or how ever long the break is in between the tomy lap counter counts laps, so if it needs 2 seconds between lap counts and your car does 1 foot/sec, then it has to be at least 2 ft long) – now for the fun part, open up the counter and find the sensor wires and run a second sensor with it, selectable with a momentary switch. -of course you need to make the wires as long as the measureddistance is going to be.keep the original sensor in the counter at the finish line. -put the other sensor in a housing at the start line. -keep the switch somewhere another person can easily hit.

bottom line. you hit the go button on the counter(with the switch making the 2nd sensor at the start the one that works when pressing the momentary button). then after the start beep from the timer, you start the car way before the line so that when you hit the start, the car is maxing out. someone watches the timer and super super quickly takes note of the time when you pass go and collect $200. as soon as the car passes the start, the switch person lets go of the momentary button so that the original sensor at the finish is now functional (as long as the switch between the sensors does not trigger anything which would nullify my long idea that many are getting bored of reading already). now, the time person records the time when the car passes the finish line. subtract the start line time from the finish time. now you got your seconds it took to cover that distance. now do the calcs to get your desired foot,miles/sec or kilometers/sec. blah blah.

-of course ifthelap counter allows you to use 2 sensors without a switch, this would be even easier to do.

-if this don’t work, the idea 2 is buy 2 lap counters, set them up a yard/meter apart put a yard/meter stick across the top of the counters and push down the yard stick to start the 2 counters at the same time. this should be more fool proof, but you gotta make sure the beeps are at the same time to ensure that they are N-SYNC!!!!!! haha.

-area of concern, make the reflectors as narrow as possible to make this accurate.

b

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PRabbit is right about the factors affecting the theoretical RPMs.

The stated purpose for Questformadness.com’s RPM Equivalency Table is to help provide amoreeasily quantifiableunderstanding of the theoretical gains/losses with diff motor/gear combos. The resistance factorsPRabbit refers to that adversely affect RPM, are constant if the only factors you change are the motor/gear combos. Therefore the theoretical gains/losses depicted in the RPM table are unaffected. Simply put, applying the same resistance to the all RPM results, will just yield lower RPMs in the same proportions. So the information about theoretical gains/losses is still accurate. Hope this helps clear up how to read the table.

Also, converting the end-value RPMs to speed, whether scaleor actual,is still theoretical, because as mentionedearlier by Deejay40,this table does not take into account torque or resistance issues. So sorry if I got your hopes up guys, but your BIts arent THAT fast! hehe. J/K 🙂

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Derek – I would love one of those dynos for my Tin-Car, if you were to get a shipment. If they were under ~$50.

[Admin]

Thanks for the support KFM. You’re right, the same resistance, being a constant, applied to all theoretical speeds will yield a good grasp of what different motors and gear rations will change with respect to eachother, just not accurate speeds.

b_drift, I HAD THE SAME IDEA! We must both be crazy! I would prefer to use the lap counter with a second sensor but I’ve never seen a lap counter and don’t know what it uses for a sensor so I don’t know if I’d be able find another sensor without having to but another counter. Do you know what kind of sensor it uses? I was thinking it probably uses an infared sensor since you say it has areflector. I also thought I probably wouldn’t need a switch. If you just wired in another sensor (in parallel if normally opened sensor or series if normally closed sensor). Again, the trick being finding another sensor. This is just my guess based on never seeing a counter’s circuitry.

I think this would be the most accurate way. Maybe I’ll order a counter and see what I can come up with.

-Peter

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