7XL gearing, ~45mph ? - RC-Monster Forums

zeropointbug · **7XL gearing, ~45mph ? -** 04.01.2007, 01:44 AM

Hey, i'm just wondering what the real world gearing i should use to get 40 - 45mph top speed with a 7XL running 5 cell A123 pack @ 16.5 volts. I have used Brian's speed calculator, but it doesn't seem to be right, to me. sorry Brian! :010:

It gave me 51T 18T Mod1 gears, that's single speed conversion. Is that right?

MetalMan · 04.01.2007, 02:23 AM

I geared my Revo for ~42mph (theoretical, based on the BrianG's calculator) with the 7XL on 5s2p A123, and the motor never goes above 150 deg., and is usually 140 deg. That's with the Wanderer 7XL kv, 2.8v/cell for the A123 cells (in a 2p setup), and some slight ballooning of 5.5" tall tires. FDR is 13.91:1, max. motor RPM is ~30,000.

zeropointbug · 04.01.2007, 02:38 AM

soo... what was your real topspeed, do you know? what do you think is an appropriate spur/pinion combo for me then?

Nick · 04.01.2007, 10:10 AM

18/51 should push it there.

MetalMan · 04.01.2007, 11:02 AM

Quote:

Originally Posted by zeropointbug

soo... what was your real topspeed, do you know? what do you think is an appropriate spur/pinion combo for me then?

I haven't gotten around to putting the GPS on it yet, but it looks to be going around 40mph.

I also get 18/51 using the calculator, assuming E-Maxx diffs, 2nd gear, 6" tires (0.25" ballooning), 7XL, and 2.8v/cell.

crazyjr · 04.01.2007, 12:10 PM

I'm not running A123's but i am running 5 maxamp 8k lipo's and i'm getting around 40mph(not radared yet) with a 13/40 mod 1 slipperential setup. When i go to my ue diffs i know i'll lose a few mph

Finnster · 04.01.2007, 06:29 PM

What setup is this? I use a 15/51 sometimes a 16/51 on my Revo w/ Emaxx tranny and a 8xl. For the same setup I would goto a 14t to start as a guess, depending on weight, yada yada.

The calc too gives me speeds that are a bit off.

BrianG · 04.01.2007, 06:48 PM

Quote:

Originally Posted by Finnster

The calc too gives me speeds that are a bit off.

"Off" in which way? This seems to be a recurring theme for high speed setups. I might simply calculate in a derating factor for anything over 40mph for MT. It won't be terribly accurate, but hopefully better than it is now.

zeropointbug · 04.02.2007, 02:27 AM

Thanks guys

Brian, what if you implement a few voltage drops into the calculator, such as A123's, the average LiPo, the FlightPower's (i hear they are very strong?).

Well, if i stick to 32P, then i have to buy a 22 or 24T pinion. So I will have to buy some more gears anyway... I don't know :(
It would be a shame to waste 4 brand new 32p pinions, (14, 16, 18, 20)

BrianG · 04.02.2007, 10:30 AM

Quote:

Originally Posted by zeropointbug

...Brian, what if you implement a few voltage drops into the calculator, such as A123's, the average LiPo, the FlightPower's (i hear they are very strong?)...

I do have that... sort of. You can select the number of cells and then the volts/cell. If you want to approximate a heavy load, choose a lower voltage per cell. The list has values from 2.7v all the up to 4.2v in 0.1v increments. I suppose I could add some lower values (like 2.5v) for the A123 cells...

Also, if the user enters in the motor resistance and approximate amperage drawn, the calculator DOES compensate by dropping the kv. Most people don't use that however and are calculating based on whatever raw kv values published, which results in a faster calculated speed than the real-world speed.

Finnster · 10:47 AM

Quote:

Originally Posted by BrianG

"Off" in which way? This seems to be a recurring theme for high speed setups. I might simply calculate in a derating factor for anything over 40mph for MT. It won't be terribly accurate, but hopefully better than it is now.

For me its a bit too low a speed from what I measure on the radar. To get the approx speed close I have to raise the voltage drop to very low levels (too high V under load really) or make obscene amt of ballooning.

Maybe its more on than it seems, but w/o knowing what the real #s are I'm just making reasonable guesses and getting low answers.

Eg: for my 5S setup, I was getting 48 mph reliably, 49 even a couple times, but the values below give me 47mph.
16/51 gear, emaxx 2nd gear, Revo diffs, 5cells@ 3.2V, 8.25" tires (w/ ballooning, these are the stock talons.)

These values get it close, but IDK if the tires get that big, or if the lipos hold higher V. They are a 8Ah 10C 5s pack from TrueRc. Maybe I'm just discounting the batts too much given the C rating, but IDK... don't have an eagletree.

This is pretty good tho, when I was using NIMH packs it was not so close.

BrianG · 10:55 AM

If that's the case, then it must be because of the "loaded" value some motors provide. Who knows what load they are using? I suspect at top speed, the motor isn't loaded to anywhere near what it would be if it were using a prop. I really want to get unloaded values for all the motors and adjust the kv ratings accordingly. If I could get that, I'd have the program automatically fill in the motor resistance. All the user would have to do is enter the approximate amperage.

Gotta remember too that the main v drop happens when the motor is accelerating. The faster the acceleration, the greater the draw. At top speed, the motor isn't accelerating anymore, it's just holding that speed and overcoming whatever resistances there are to maintain it, so the current draw (and v drop) isn't as much.

glassdoctor · 04.02.2007, 11:15 AM

The current load drops off at top speed. I have had cars average peaks 70-90amps on acceleration but once up to speed it drops to about 20 amps or so.

I have even seen the amps drop lower than that I think with my buggy...

This will vary depending on specific motor and gearing, etc... but the load does drop off a lot. My example was for a low-geared truggy... only topped out in the 30's. Gearing for speed runs will put a lot more load on the car at speed (gear ratio/wind resistance...)

At any rate, it makes guessing the voltage drop kinda tricky. Guess I need to strap the micrologger in for a few speed runs sometime...

Finnster · 04.02.2007, 04:58 PM

Hmmm, yes, all very good points. I guess this just shows just how difficult calc'ing these things are. For what its worth, the truck is usually still accelerating as I'm holding the radar, but is topping out as its getting too close to radar. The hard part is finding a long enough runway to really see where it tops out, but its close.

The calc is very good tho, don't get me wrong, great job on it. Its very handy and seeing the gear ratios in one place is great. I guess its that one can't get too wrapped up in the exact top speeds as there are so many variables.

zeropointbug · 04.02.2007, 06:36 PM

true, true... I am going to post a test of one of my 5s1p A123 packs.

I just used my old dual 820 dewalt motors to give it a workout, i just kept blipping the throttle to 100% and back down (brakes).

The results are actaully better than i expected, for a 2.3Ah pack @16.5 volts Nom.

The current peaks at 91 amps (39 C), that's alot for such a small Ah rating.

-Also, the highest current spike, the voltage went down to 12.6 volts.

-I will be running a 5s2p pack, so the current will be half that...

Here is a link to my Picasa album: Picasa Album