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Old 06-15-2004, 08:59 PM   #1
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Default Question about amp draw between cars

I've got an interesting situation here. One of my buddies has a Tc3 and XXX-S. Both cars have free drive train, with all the tricks applied to both drivetrains to free them up. Blueprinting the Tc3 gearboxes, black belt on the XXX-S, etc. My friend is using the same motor in both cars with a final drive ratio within 0.1 of eachother between each car. He's using the Integy Indi-Dyno running at 3 volts to power each motor.

Here's the thing we can't figure out. The XXX-S should have more drag compared to the Tc3 (the whole belt vs. shaft arguement). But when running on the Dyno, the losi is pulling about 9.5 amps, while the Tc3 is pulling almost 10.7!

with the higher drag of the XXX-S drivetrain, shouldn't the XXX-S be pulling the higher amps, not the Tc3?

Sorry if this is a repost. A quick search didn't pull any results specific to this.
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Old 06-15-2004, 09:04 PM   #2
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We were thinking it's due to the higher static weight of the Tc3 drivetrain compared to the XXX-S drivetrain, but we though we'd post it on here and get some other opinions.

Thanks for anybody who can provide some feedback!
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Old 06-16-2004, 02:01 AM   #3
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Default Re: Question about amp draw between cars

Quote:
Originally posted by cartmen34
The XXX-S should have more drag compared to the Tc3 (the whole belt vs. shaft arguement). But when running on the Dyno, the losi is pulling about 9.5 amps, while the Tc3 is pulling almost 10.7!
That's all belt v shaft is - an argument.

If belt wasn't up to scratch, why was the Reedy race won by a belt-drive Tamiya 415, with the runner up a belt-drive XRAY?

The result doesn't mean much though - how often do you race the car with a 3 volt battery pack, and the wheels off the ground?

You would learn a lot more about the current drain in the car and the motor performance with something like the Eagle Tree Systems Car Data Recorder which records current, volts and RPM as you go round the track, then you can download it to your PC and graph the results (we're the UK distributor). It's like an in-car dyno.
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Old 06-16-2004, 02:23 AM   #4
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I'd try the same experiment, but only wit the trannies... ie measure current after "un-hooking" everything that's doward from the diff outputs (ie remove UJ's)... Before that, clean/oil all bearings thouroughly.

this could become and interesting thread...

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Old 06-16-2004, 07:59 AM   #5
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Default Re: Re: Question about amp draw between cars

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Originally posted by sosidge


The result doesn't mean much though - how often do you race the car with a 3 volt battery pack, and the wheels off the ground?
I respectfully disagree with that arguement. How often to F1 cars drive in a tunnel with a gigantic fan in it, and without the engine running or a driver even in the car? They never do, but teams spend millions of $ on wind tunnel testing as it provides them with valuable data.

Our situation isn't the same. No $ involved, but the theory still applies. There are one of a dozen ways to check for drivetrain/car performance, I'd say this is as valid a method as any of them. Although impossible to simulate the car actually on the track, it still provides you with information. All the variables are controled, the only difference being the actual car.

I was afraid this would turn in to a belt vs. shaft arguement. Starting another one of those is not my intention. I guess you cannot answer my question without directly answering "belt vs shaft" as well.
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Old 06-16-2004, 09:49 AM   #6
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cartmen.

There could be a difference between the rotation mass between both cars.

The older ( no offence ) TC3 might have some heavy rotating parts inside which might need more power to make it turn.

On the other hand, a XXX might have a lighter rotation mass and thus making it consume less amp.

The argument about Belt v/s Shaft will never stop. Shaft cars are smooth but alot of people tend to forget that rotation mass inside the car also plays alot of difference in the performance of the car.

Cartmen. If you have a chance to test out the Xray T1FK, let us know k ? I wouldnt be surprise if it has the same or better performance then the XXX (again no offence).
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Old 06-16-2004, 10:26 AM   #7
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I think if you were to use some sort of data acquisition-or your method for your testing, but use a treadmill lets say for a control device-you could really come up with some interesting data I think.

For example-if you set the teadmilt to run at 15 MPh (do they go that high?) and throttled the car to keep the car at a steady pace on the treadmill-you would simulate a more realistic load.

There would of course still be a certain percentage of error, but it would be more than close enough for our needs.

Ray
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Old 06-16-2004, 10:49 AM   #8
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Roatating mass will not affect a static current draw measurment. The rotating mass will affect a measurement that is taking during acceleration. after the mass is up to speed, it will not consume more power to keep it rotating.

Lonestar's idea of "unhooking" everything is a good idea. At the very least take the wheels and tires off of the cars and make sure none of the wheel bearings are jammed up at all. I noticed this winter while running on carpet, that a carpet jam in a bearing could take minutes off of the run time of a sedan, and cause weird pulls that looked like tweaks. Since I have owned both of the cars you have tested I would be suspicious of the results that you have gotten. Maybe try several different voltages and measure the voltage accurately at the motor terminals with a handheld voltmeter. The integy motor dynos use FET's to adjust their voltage waveforms and those could get hot and change resistance. Which order did you test the cars and how much time elapsed between the tests? For a true measurement you really need a hand held volt meter on the motor leads and a clip on current probe for a descent current measurement. Anything that uses a shunt (resistor) to make current measurement can have a large error depending on temperature. since the integy dyno is not that expensive, I kind of doubt they use a Hall-Effect type of device for current measurement and therefor the measurement may have a significant error from measurement to measurement. Just some ideas, hope you figure it out.

Almost forgot, I agree that the car with the higher drive train friction should draw more amps. This may be very hard to measure with all things considered, especially if the two are pretty close to the same.

Last edited by kufman; 06-16-2004 at 10:57 AM.
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Old 06-16-2004, 11:48 AM   #9
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cart men are you testing both cars with the same motor and how reworked is the tc3s drive train. also is there oneways,spools, or regular diffs in both cars. also is the gear mesh the same! are you letting the motor cool down between tests to room temp to get a controled type test. also do you have wheels on the car if you do are you mounting and dismounting the tires for use on both cars!
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Old 06-16-2004, 02:01 PM   #10
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Quote:
Originally posted by kufman
Roatating mass will not affect a static current draw measurment. The rotating mass will affect a measurement that is taking during acceleration. after the mass is up to speed, it will not consume more power to keep it rotating.

Lonestar's idea of "unhooking" everything is a good idea. At the very least take the wheels and tires off of the cars and make sure none of the wheel bearings are jammed up at all.

We did this. All the bearing are butter smooth and no wheels on the cars at any time.

Quote:
Originally posted by kufman

The integy motor dynos use FET's to adjust their voltage waveforms and those could get hot and change resistance. Which order did you test the cars and how much time elapsed between the tests? For a true measurement you really need a hand held volt meter on the motor leads and a clip on current probe for a descent current measurement. Anything that uses a shunt (resistor) to make current measurement can have a large error depending on temperature. since the integy dyno is not that expensive, I kind of doubt they use a Hall-Effect type of device for current measurement and therefor the measurement may have a significant error from measurement to measurement. Just some ideas, hope you figure it out.
This is a good idea. I'll let my friend know to test that. (He doesn't have internet access)
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Old 06-16-2004, 02:10 PM   #11
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i think to achieve 100% correct result it's near to imposible.

the motor have to be same, com size same brush same which u can't do it on 2 car even the same motor u used on the 1st car the 2 nd might run on poorer condition higher amp

other, bearing friction, belt tension (tend to increase tension when load on), ratio, gear meshing and etc.
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Old 06-16-2004, 02:15 PM   #12
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Testing the drivetrain with wheels off the ground (=no load) is absolutely useless. Do you dyno your motors with no load? No. Why would you do that with cars?
There is a huge difference how gears vs. belts behave under different load conditions. Gear transmission has losses that are pretty much proportional to the load on them. Belt transmission losses don't depend on load that much. Hence, gear transmission (=shaft) is generally more effective with small loads, but belt transmissions are more effective with big loads.
You have to build a roller dyno, if you want to test drivetrain efficiency
Earlier posters were right saying that rotating mass makes no difference in stable conditions (constant speed), only during acceleration.

Comparing wind tunnel with "freewheel testing" is IMHO not very appropriate... wind tunnel is actually a very useful thing that approximates real life aerodynamic situations quite well. Same can't be said for the freewheeling tests.
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Old 06-16-2004, 02:19 PM   #13
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Quote:
Originally posted by speedxl
cart men are you testing both cars with the same motor and how reworked is the tc3s drive train. also is there oneways,spools, or regular diffs in both cars. also is the gear mesh the same! are you letting the motor cool down between tests to room temp to get a controled type test. also do you have wheels on the car if you do are you mounting and dismounting the tires for use on both cars!
same motor, and he is letting it cool down via a fan between runs.

Drivetrain is as similar as it can be. diffs on both cars. No wheels, not even wheel nuts. Same brand/style pinion gear. finaldrive is near 0.1 of a difference.
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Old 06-16-2004, 02:23 PM   #14
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yes i agreed with u olev, have to test full equiped and run on rollers. if u want to test the amp
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