Speed Calculator

[BrianG]

Hey, I just created a little javascript that calculates the speed of an R/C vehicle based on some input. Not sure if it's of any use to anyone else, but I've used it a bit when I was picking out various parts to buy.

Anyway, here it is: http://www.scriptasylum.com/rc_speed/index.html

If anyone has any other presets to add (you'll see what I mean), let me know and I'll add them. Also, please let me know if there are any accuracy issues. It does not account for wind resistance or weight.

[MetalMan]

I checked it against this program:
http://www.waits.net/public/rcgears/
and found that for the setup I will be using in my Rustler a difference of about 0.5mph came out between the two programs. I don't know which one is more accurate, but I am just pointing that out.

[Gustav]

The Lipo voltages are out by 1 cell BTW.eg 8s says 33.3 etc.

Very usefull though.according to this,my 8 cell gp2200 packs are falling to about 7 volts under load.(says it would be pulling like 56 mph at 9.6 volts when it's pulling around 45mph according to GPS)assuming it's reaching max. rpm and the GPS is accurate,i guess the real voltage is somewhere in between.

[BrianG]

Originally Posted by Gustav

The Lipo voltages are out by 1 cell BTW.eg 8s says 33.3 etc.

Very usefull though.according to this,my 8 cell gp2200 packs are falling to about 7 volts under load.(says it would be pulling like 56 mph at 9.6 volts when it's pulling around 45mph according to GPS)assuming it's reaching max. rpm and the GPS is accurate,i guess the real voltage is somewhere in between.

Yeah, the script assumes "ideal" values; IE: no voltage losses. Unfortunately, there is no way to guess what the losses would be because it depends on the quality and current capacity of the batteries, and how much current the motor is pulling. The drop also varies with speed and is probably most prevalent during a standing start and top speed. Maybe just subtract percentage of the ideal voltage from the presets to get a more reaslistic value? Something like 5-10%?

The battery issue has been fixed and I used the real PI value instead of a simple constant. There was no rounding issue, so I don't know that the .5mph difference is about. All tranny and diff teeth were counted and calulated so that's not the issue. Anyway, the updated version has been uploaded.

[BrianG]

Poo. I did a Google search and didn't find any such programs, that's why I made mine. If I had known, I wouldn't have bothered.

As to the .5 mph difference, I have no idea since there is no source code to compare to. Since they are so close, I'd say it's a rounding error or something equally simple. I'll check mine over and keep the precision high until it needs to display the output.

Similar to the executable version, in the source of my version are provisions to add other ratios, except I've split it out a little more by adding seperate differential, transmission, and spur/pinion ratios in case you just change a tranny gear or use a different diff than stock.

I may also add unit conversion. Seems like a decent feature.

Thanks for the feedback MetalMan!

[squeeforever]

no offense by anymeans, but i dont think its to accurate. i had a e-maxx with a xl3100 on 12 cells and its was doing appoximately 50mph and your calculator says 35?!?!?!?

[BrianG]

Originally Posted by squeeforever

no offense by anymeans, but i dont think its to accurate. i had a e-maxx with a xl3100 on 12 cells and its was doing appoximately 50mph and your calculator says 35?!?!?!?

No offense taken.

What spur/pinion and diff were you using? What tires were you using?

The cell voltages are assumed to be 1.2v. If you have really good cells with not much voltage drop under load, or if the load isn't really high, the cell voltage could be up to 1.3v per cell. That alone could account for up to 10-12% increase in mph. Also, tires ballooning increases the ratio as well. A 5.5" tire ballooning up to 6" can make a 10% difference. MetalMan said his calculation came to within .5 MPH, so it works. There just might be other factors at play.

The point here is that there are a lot of variables, which tend to be skewed especially at higher speeds. Tire ballooning alone is a function of tire rpm, tire composition, tire temperature, truck weight, etc. If you want more accurate results, take the things I've mentioned into consideration using best guesses and enter those values. I bet the answers will be VERY close.

The script simply uses the following formulas:

Total motor RPM = kv x battery voltage
Total Reduction = spur ratio x tranny ratio x diff ratio
Axle RPM = Total Motor RPM / Total Reduction
Tire Circumference = TT x Tire Diameter
Wheel speed = axle RPM x Tire Circumference.

Convert from inches per minute to MPH, which simplifies to:
MPH = Wheel Speed / 1056

That's it. If there is something wrong here, please let me know.

holy cow that isnt even close maybe there are a few bugs that need to be worked out
also a xl3100 in an emaxx wouldnt that heat up pretty fast

[MetalMan]

squee, tire diameter does have a lot to do with it. With my Rustler's setup, a ballooning vs. non-ballooning setup gives a 25% top speed difference (4.5" vs 6"). That equals almost 20mph!

that is a big differance what kind of top speed are you getting

[MetalMan]

I am hoping for 60+ mph. I can't calculate the actual speed, since I don't know the exact size of the tires when they balloon, or the voltage drop of the batteries, or the kv drop of the motor.

what motor are you using?

[squeeforever]

well my tires didnt balloon or at least noticably. and yes, the xl3100 did heat up a tad but the power was insane!

Originally Posted by squeeforever

well my tires didnt balloon or at least noticably.

what tires were you using

[squeeforever]

proline standard size roadrages.