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Speed/Gear Ratio Question

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Speed/Gear Ratio Question

Old 08-08-2015, 06:39 PM
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Hi guys. This is ny first thread. Here goes: I got a top speed of 43mph with: 1st gen Tekin 10.5 motor, 2nd gen Teken RS, 50c battery, 10 timing, 72/30 gear ratio in my SC10 2wd. I went to a 69/36 gear ratio and now I'm only getting 40. What am I missing?
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Old 08-09-2015, 10:30 AM
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Your acceleration is now slower and you're lugging the motor such that you're running out of room before hitting your (theoretically) higher top speed.

To use a full-scale car analogy, you're trying to merge on the freeway in 5th gear instead of 3rd - you need 5th to hit freeway speeds, you need 3rd to get there first. As such, with only a single gear ratio in play on an RC car, you will need to aim for a better compromise.
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Old 08-09-2015, 05:42 PM
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Thanks Mark. I understand that. But I'm running a good 7 houses length at full throttle. I feel like the acceleration would eventually even out. My slipper isn't too loose or too tight. Could you reccomend a better gear ratio maybe? I need a 100 tooth minimum for the gears to bind correctly, and want to take some heat off the motor too but it's not a must have. I'm not sure if I can go any more than the 43mph I hit the first time without a new motor. Your opinion?
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Old 08-09-2015, 08:16 PM
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Your starting ratio was already more or less off the chart for a 10.5 in a SC truck so...

Speaking of charts, I ran your ratios through RCgearhead (see attached screenshot below) and your starting ratio is already good for a theoretical 60+mph - that tells me that in the context of what you are doing for the length you are doing it (seven houses?), you are distance/acceleration limited and aerodynamically limited. The distance issue is one you can't solve (most speed run competitions are actually held at full-scale dragstrips or even private airfields) but to overcome your aerodynamic limit of using a short course truck for speed running combined with less than ideal distance you actually need more torque to reach as high a speed as possible in that distance - that is, less gearing should be more real-world results. Looking at the chart below again, I would try a gear ratio of 75/26 and hope that with enough traction, your real world speed gets closer to that theoretical one which would be higher than what you've achieved so far.

I guarantee you that your truck is bogged down at 69/36 - no SC truck is going to hit that speed on 2s with a 10.5 in the real world so you've hit the point where the rate of acceleration has diminished to create the appearance of it being maxed out.

To go faster than that you will definitely need more power (When you see those Traxxas ads for "60MPH!" they put the part about it being on 3s in the fine print), possibly a way to adapt a more aerodynamic body to your truck and a longer, high-grip place to try it.
Good luck!
Attached Thumbnails Speed/Gear Ratio Question-gearhead.jpg  
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Old 08-09-2015, 10:33 PM
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Wow, thanks a lot Mark. I think I'll be using RC GearHead in the future. I don't like how they don't have many options. It doesn't tell me what I should be getting for exactly what I have. I'll try 75/26 or something near that if I can't get exactly that. I'm not sure if RC GearHead knows what gearing is available for certain vehicles, and if it does it's probably not spot on like their motors. But your time, effort and help are much appreciated. Thanks again!
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Old 08-10-2015, 05:57 AM
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Getting down to number crunching... ((($J39*J38)/12)*60)/5280

That's the speed formula on my spread sheet for car set-up. It represents the maximum speed I can obtain, not considering aerodynamics and resistance. I race dirt oval so speed and feel are important. I shoot for the highest speed that doesn't create that dreaded point where I either cut initial acceleration where I'm geared too low so I can't power out of the corner or overheat the motor because I'm geared too high for top speed. So, the formula is basically (((motor RPM times (rear tire circumference divided by final drive ratio) divided by 12) times 60) all divided by 5280. Motor RPM is KV times 7.4 (for 2S lipo). Tire circumference is diameter times pi (3.1417). Final drive ratio is (spur gear divided by pinion gear) times transmission ratio.

The reason I went through all this is to show there are many variables besides spur to pinion gear ratio to consider, such as bigger rear tires, more RPM (timing, fewer winds, etc), more battery, etc. Couple that with the aerodynamics and the variables increase dramatically... have fun!

Oh, and yes, I am a retired "rocket scientist"!
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Old 08-10-2015, 10:28 AM
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Thanks! I'll add that formula to my notes...I also race my SC10 2wd on dirt. I've just run the gearing I've been reccomended and been doing fine with my AKA Grid Irons. Thanks again!
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