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Old 04-15-2009, 11:19 AM   #31
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CCristo-I agree. Note the work on the stock motor is free of the active timing effects for the most part as is the 13.5 on four cell.

I have requested a loaner GTB speed control from Novak for testing on this dyno. The request has been seen and forwarded. We will see. I only have a 4 cell GTB at the moment. One of the items guys wanted to see was the effect of speed controls on the motor. If I get a GTB I will run this same LRP 3.5 on it and maybe we can see some difference.
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Old 04-15-2009, 02:40 PM   #32
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Oval versus Road Racer Runlines

I attach a sketch of a slightly shorter Oval runline and John's road racer runline on the same track map.

This is a nice illustration of Racing Theory in that the Oval guys carry more average speed on the longer line, the Road Race guys carry less average speed on the shorter line, and both get around the track in the same elasped time. The Oval motors run hot compared to the Road Race motor temperature.

I suspect the car that can stick everywhere at full throttle later in the run may have greater drag coefficient plus downforce, a battery with less volts due to pulling a larger equivalent flywheel inertia, and a hot motor with less peak power, thus it can stick at all times but sacrifices 0.2-0.3 seconds in elapsed times at the end of a run. This should be on the Oval guy line.

John I appreciate your time, effort, and insight on this project. If you could provide some data on the g-rate of acceleration in the tight turns, and the mass of the cars (does it differ for 17.5, 13.5, and 3.5 installed), I'd like to do a Road Race study of the gear ratio and acceleration rate off the turns.
Attached Thumbnails
Dyno, Homemade, Using a Novak Sentry Data Logger, Continued, The Experimental Thread.-ovalrunlines00.png  
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Old 04-15-2009, 03:49 PM   #33
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We were running at 41 ounces. Traction was good enough to traction roll cars with the overly tall oval wing at times. Both 1/10 and 1/12 with Nascar bodies were subject to traction rolls. I suspect this happens at about 3.0 g's with that tall heavy wing. The sketch is just OK at present. The oval guys line can be made fatter. It will be only a couple of inches from the inside board in the turn on a good lap, however. In other words the two racers lines will overlap at the left and right ends. The 173 feet line was walked with a wheel but it appears from other data and your drawing that the oval line is shorter if a tight line is made at the boards. It is fat and has the large radius turns. At one point we widened the track a bit and had a slightly longer oval. This may be the reason for my error in the 173 foot line. I think it was shorter on the early oval.
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Old 04-15-2009, 06:43 PM   #34
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These two dyno runs were obtained by using two different flywheels on the motor, one a Fantom Aluminum flywheel which is 1/2 the inertia of a second, Fantom Steel Flywheel.

I have noted previously not to use this steel flywheel with powerful motors. There is strong evidence of overheating with too heavy a flywheel. (equivalent to overgearing in the car). The 14 gauge leads from the speed control to the motor are too hot to hang onto even though they are insulated.

On the left is the Aluminum flywheels output. The motor puts out 536 W Peak. If you look at the yellow efficiency line, it is high. The second run with the heavy flywheel result in 522 W peak. Everything is quite hot. Amp draw is huge, but some of this energy has gone into heating things up. These two Maximum powers are only 2% different, but I think that 14 W is actually a real loss (as opposed to random error) from Overgearing (overloading) the motor.

click one more time on the graph to enlarge it then use your full screen box for best view. (on 1024 x 760 resolution machines)
Attached Thumbnails
Dyno, Homemade, Using a Novak Sentry Data Logger, Continued, The Experimental Thread.-lrp-3-5-aluminum-flywheel-2s-lipo001.jpg   Dyno, Homemade, Using a Novak Sentry Data Logger, Continued, The Experimental Thread.-lrp-3-5-steel-flywheel-2s-lipo001.jpg  
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Last edited by John Stranahan; 04-15-2009 at 10:13 PM.
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Old 04-16-2009, 09:36 AM   #35
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Joe gave me a set of numbers for the Encino Velodrome in California. I'll work up a set of minimum gear ratios for a couple of the motors after a while. I'll put that 10.5 on the dyno to work that motor ratio out.

Our Velo is approx 820 ft. in the race line. (It's a 250 Meter rated track)

Lap Times for various classes are

2cell LIPO (ORION 3200) - Novak 21.5 Motor = 10.0's, fastest in the 9.96 range

2 cell LIPO (ORION 3200) - Novak 17.5 Motor = 9.2's on average, with high 9.1's the fastest.

2 cell LIPO (ORION 3200) - Novak 10.5 Motor = Low 8's last time this class was run, but it's been roughly a year.
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Old 04-16-2009, 11:28 AM   #36
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Velodrome Suggested Gears 17.5, 8200RPM power peak

820 foot run line/9.2 second average lap = 89.1 foot/second (61 mph)

89.1 foot/second x 12 inch/foot x 60 second/minute = 64152 inch/minute


2.5 inch tire x 3.14 = 7.85 inch/revolution circumference

(64152 inch/minute)/(7.85 inch/revolution) = 8172 rev/min (axle)

8200/8172 = 1.00 spur pinion ratio. Note the pinion is equal to the spur.


69 tooth 48 pitch gear/1.00 = 69 tooth pinion


I would guess there is little forward tire slip at this speed so I am not going to multiply by a 5 -10 % fudge factor. If the motor runs too hot at this gear then you have to reduce the number of teeth on the pinion. Now if I am lucky Soutwest Tour will tell me what they actually use and the considerations for choosing it.

I am hunting the parts to accomplish this. I know there is an adaptor that mounts a spur to the motor shaft. I could not find it for sale anywhere.
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Last edited by John Stranahan; 04-17-2009 at 11:51 AM.
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Old 04-16-2009, 07:32 PM   #37
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Attaching a new sketch of Oval versus Road Racer Run Lines

Estimated Flat Turn Velocity and Time
R - turn radius in feet
S - segment length in feet
aC - centripital acceleration in g's
g - 32.174{ft/s/s} standard gravity

v = SQRT(aC*g*R) velocity {ft/s}
t = S/v {s} time through constant radius segment at specified aC

Oval Turn Study
R = 15.81{ft}
S = 49.6{ft}
ac = 3{g's}
v = 39.06{ft/s} = 26.56{mph}
t = 1.27{s}
2t = 2.54{s}

And if speed is constant 39.06{ft/s} in the straights
then Total Lap Time = 3.76{s}

If the line geometry is correct and this lap time is too small, aC is too large.
Some time would be added for braking segments.
Some time would be subracted for acceleration segments in the straights.

Road Racer Turn Study
R = 10.99{ft}
S = 24{ft}
aC = 3{g's}
v = 32.57{ft/s} = 22.15{mph}
t = 0.74{s}
2t = 1.48{s}

This car must punch out of the slow corners more aggressively.
An acceleration model requires a Dyno curve matched to the driveline.

John, it's not clear to me the power available on the track versus Dyno for 17.5, 13.5, and 3.5 motors. I'm assuming all curves above at 8.2{V} but some of the motors run 4 cells on the oval?
Attached Thumbnails
Dyno, Homemade, Using a Novak Sentry Data Logger, Continued, The Experimental Thread.-ovalrunlines00.png  
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Old 04-16-2009, 08:52 PM   #38
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Dyno Test Novak SS 13.5, Amp meter validation

Below is the dyno output for the Novak SS13.5 on the 2S LiPo. I ran this motor on the oval first with a 4 cell pack. The dyno ouput I used for gearing, I posted previously. Later we changed to 13.5 2sLiP but numbers had already dwindled under the 4 cell rules. We never revovered the numbers and then the track went under construction. The car is quite lively with 13.5 2S LiPo. Not quite enough straightaway speed for the full road course but plenty for the oval. Again the major result would be the RPM at full power. This can help you gear this motor. I geared this motor the same on road course and oval.

I tested the ammeter segment of the Novak Sentry by using the Competition Electronics turbo thirty (charger discharger) to discharge 30 amperes of current through our amp sensor harness. The sentry was within .75 amps and always read a little low. -2.5% error. This is sufficiently accurate.

Note that when the amp meter is pegged at 108 amps then the efficiency reading is higher than it should be. You just have to take this as a know error.

In order to eliminate this error I tried using the volt sensor over a short piece of 12 gauge shunt. The voltage drop could then be used to calculate amperes. Although the sentry records voltages down to a milliVolt it is apparent that it has a minimum threshold that I did not reach. The idea failed.

Matt- I have simplified my RPM vs time model to a cubic Polynomial. This eliminated some waviness that was causing the far right side of the curves to rise.
Attached Thumbnails
Dyno, Homemade, Using a Novak Sentry Data Logger, Continued, The Experimental Thread.-novak-ss13-5-7-4v-heavy-steel-flyweel-4-16-09003.jpg  
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Old 04-16-2009, 08:58 PM   #39
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system theory-
"If the line geometry is correct and this lap time is too small, aC is too large."
I agree. the 3 g was just a rough estimate of maximum g available at traction roll with the tall wing. Some driver error is involved here too causing a snap type roll from too much steering input. Cornering would be smoother and a a lower rate. Try 2.8 which is what my touring car was measured at in a corner. It would be better to use the real lap time average 4.3 seconds or fast lap 4.1 s and use that to estimate the g's attained in the turn. You are starting to get some interesting looking numbers.

My car was certainly able to pull out of a slower corner better due to a more advanced 3 link rear suspension.

the sketch is very good now and decribes accurately the difference in our line. The oval guy run line is now shorter than it was previously at 147 feet. I think they should have used more pinion teeth but not 5-6 more. They were probably a few teeth over geared.

This racing and descriptions occured with 13.5/ 4 cell

John
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Old 04-16-2009, 09:20 PM   #40
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John, when you are ready I will include the new mathematical model in the openoffice version of the spreadsheet

-Matt
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Old 04-17-2009, 02:26 AM   #41
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John
On the drome we use 2.5 inch tall tires. If I remember correctly we used a gear ratio of 1.5 on the 17.5 motors.

Steve
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Old 04-17-2009, 11:47 AM   #42
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Steve-Thanks. Tell me if you remember are the motors overheating or are you able to select the ratio at will. I made corrections above to the tire size. I still come up with a very tall ratio of 1.0 for best performance. This might be helpful on one of your speed runs to come.

I have ordered a couple of sensors that may allow us to measure up to 200 amps on the advice of System Theory. Chances are this sensor has a split path with a built in shunt. 1/2 the current is measured with the same sensor. We can apply the correction in the spreadhsheet.

When we finish getting the little kinks out of it, Matt will repost the spreadsheet for anyone interested in tinkering with it.

http://search.digikey.com/scripts/Dk...me=620-1115-ND
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Old 04-17-2009, 12:15 PM   #43
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suittable gear for the high speed run of the stock motors.

http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXEY18&P=7

http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXWLK7&P=7

The pinions go up to a 62 tooth.


some precedent


"At our VELODROME race a couple weeks ago, using the 21.5/LIPO combination the gear ratios went OVER THE TOP as Nic Case ran his combo using a OVER DRIVE Ratio

Running the small 73 & 76 t spur gears in both positions (using a spur gear adapter on the pinion shaft)"


I could not find a spur gear adaptor for sale.
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Old 04-17-2009, 12:30 PM   #44
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John
Most of us use PRS gears, pinions to 65t and spurs down to 72t in 64 pitch.

Spur gear adapter
http://dynotech-racing.com/partDetail.php?RefNum=4006
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Old 04-17-2009, 04:14 PM   #45
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John & Velodrome Tuners,

Check my logic here. Pushing air is usually the dominant factor on big ovals.

Top speed on most cars is limited by the power available to push air. On a long stretch (salt flat) the lower gears are used to accelerate, and the top gear is used to match the engine power to the driveline loss plus the power invested in pushing air. More gear beyond that makes you go slower, since it loads down the engine with a greater reflected load torque under friction.

Power = Force*velocity, P = F*v

Drag Force:

D = 0.5*rho*Cd*Af*v^2

rho - air density
Cd - drag coefficient
Af - frontal area
v - velocity

Drag Power:

Pd = D*v = 0.5*rho*Cd*Af*v^3

If one ignores the driveline friction (not very reasonable) then a formula for the top speed:

v = Cube Root of Pmpk/(0.5*rho*Cd*Af)

where Pmpk is the peak power output on the engine Dyno curve.

Find this velocity and use the tire radius to estimate the angular speed of the axle. Identify the angular speed of maximum power and match gear ratio G.

Brush motors generally overheat when run at peak continuous power, but in RC racing, manufacturers may make their motors more thermally robust. A brushless motor dissipates heat better for the can size because the copper coils are on the stator, not the rotor, and can achieve lower thermal resistance (better heat sinking).
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