Speed Calculator
 

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.

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.

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.

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!

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.

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?!?!?!?

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

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.

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

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?

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

what tires were you using

proline standard size roadrages.

huh i have those same tires but they seem to balloon a little bit at about 40mph

Just an update to say that I added the stock Revo transmission presets (for wide, close, and standard ratios), tire ballooning compensation, carried higher precision in the gear ratios, and a note about voltage drops. Same URL...

cool

at 55 mph my tires balooned pretty good. I have tried a couple different ones. What kind of gearing and stuff was you running squee.

it was my lightmaxx with some 4 year old, 6 cell sanyo packs, lehner xl3100, 9920, locker in the rear, cvds, powerstrokes, and the rtr weigth was about 7 1/2lbs with the road rages.

It may be necessary to consider rpm drop of the motor under a load in the calculations.

RPM drop of the motor (per amp) is the unloaded kv x the resistance. Rpm drop will always be the same no matter what voltage. Ex. a 5000 kv motor with .006 ohms will drop by 30 rpm per amp. So if you're running it at 7 volts and it's drawing 20 amps, it will have 34400 rpm. At 1 volt and 20 amps, it will have 4400 rpm.

That's nice to know, except many motors don't specify the resistance, and you don't really know the current it is pulling unless you simply use worst case numbers. You can't generalize either because the current draw depends on too many factors. I'll see if I can add that, but not make it required so the calculation will still work if current and resistance are not available.

Anything else to add to make the script more accurate?

OK, yet another update to add the feature SpEEdyBL mentioned. At this rate it will be accurate enough to be exact! (OK, maybe not):

http://www.scriptasylum.com/rc_speed.html

isn't there 2 big things missing from that: weight and wind resistance

Well, yes, but if you read my first post on this, it specifically says these are not included. Wind resistance would be all but impossible to figure; you'd need the wind speed and direction, truck surface area, and (the hardest) aerodynamic calculation based on the shape of the truck. Ugg.

However, with the last update, it indirectly addresses weight (sort of). If the vehicle weighed nothing, then the motor would pull very low current to get it going. Just the fact that a lot of current is drawn means it is pulling some weight around.

The point of this is to provide a reasonable estimate of what the final speed will be given various readily available parameters. It also can be used to compare various final reduction ratios for different spur/pinions, trannys, etc.

www.feigao.com has the spec sheets for the feigao motors.

Yeah, I have that chart already, but it is specific to Feigaos. The BK site doesn't have some of that information. Every motor brand should have at least that kind of information available.

Most motors, (lehner/plettenberg/wanderer) use a loaded kv value.

Nice calculator Brian! That balooning, i don't know what to think about it.

with a 2wd it can be included in a calculation, with a constant 4wd (no centerdiff) balooning gives a huge stress on the drivetrain. (front wheels are bigger than the rearwheels (differs an inch with ease) i experienced less topspeed with balooning larger tires than with non balooning smaller tires.

Thanks!

Yeah, that's what I mean, there is no consistency in values between brands. BrandX offers an unloaded RPM so you'd have to take that into consideration, while BrandY already specifies loaded RPM (but exactly how much load??). What's a newbie to do? You've got all this jargon that you're trying to make sense of, and just when you think you have it nailed down, some other variable comes into the equation. Argg. Personally, just give me all the parameters (in a consistent format) and let me do the calculating!

As far as the ballooning figure goes, leave it at zero and it won't even be figured into the calculation. Although, as long as all 4 tires are ballooning the same (which they should be), there isn't any stress on the drive train. You are right; any difference in tire diameter between the front and rear would cause issues.

Even if the calculator's output speed isn't totally accurate, at least it will give you an idea of the relative differences between two spur/pinion and/or tranny gear ratios.

Why would the front tires baloon more than the rear? They are turning the same rpm so what is the difference. They should baloon the same. With a center diff they usually have a slight difference in speed for the different ballooning.

coolhand, i was wondering the same thing but i just thought of it. think of it this way, the front tires are gonna balloon more than the rear because all the weight is gonna be transfered to the rear when accelerating.

I think I see what you're saying. It's hard to guage, but when I saw my tires balloon on a straight run, they looked like they were all ballooning by the same amount. Of course, if the truck is turning or there's a change in the ground traction (loose dirt etc), that will cause the left or right tire to balloon more because of the front/rear diff. Unless there is some "give" somewhere; like a center diff, a clutch of some sort, or there is some tire slip in the front or rear, they will all balloon the same since the tranny is driving both sets of wheels at the same time.

OK, new version. Changed the battery voltage selector so you can select cell count and the voltage-per-cell seperately. This should allow you to calculate "best case" speed with a fully charged battery, "worst case" with a almost discharged battery, and various spots in between. This was mainly done for Squee (I hope you're happy ;)).

Also added a motor selector that lists the BK Wanderer, LMT, and Lehner Basic motors and puts the kv value in for you. I didn't realize how big Mike's list was, and I didn't even put all his motors on there!!!

Same link as before: http://www.scriptasylum.com/rc_speed.html

Oh, can a mod change the title of the thread so it can be found easier next time? Something like "Speed Calculator" or something?

i tried it and it seems right on the money for my truck. 44mph is close for my gearing, 15/51.

thanks brianG, this is a really useful tool for everyone at this forum!

Great Brian...Make me feel bad.... lol. It still only says about 39 MPH but its close than it was. I guess my packs were charging very high voltage like 1.5 volts per cell +.

It always worked for me and i test my speeds with GPS and it's usually very close to the calculated speed.I never had a tested speed turn out to be higher than a calculated speed.I just put in the nominal voltage and 6" for a 5.75" tyre.

Thanks guys (and thanks for changing the title). Suggestions for future "releases" are welcome! :)

Squee: lol. I have no idea why yours is so far off. I don't think the packs charged quite that high, but there has to be something to account for the discrepency...

what a nice tool you put online!!! all thanks guy! :)
What could be nice is a warning signal for overgearing i.e...
I don't know if it's possible to do, but I would like to be prevented of thermalings or overgearings... i think the type of motor and the tires diameter should be enough to prevent this??? I realy don't know, just suggest...

And you could also add Feigao motors on your list...

You can be proud of your work BrianG!!!

Thanks!

I could add an overspeed warning for the motors, although I only know the ideal rpm range for the Wanderers to be about 40k rpm. The LMTs says 65k. Is this the ideal limit, or the physical limit? What about the Feigao's (I assume they are like the Wanderers)? And, what is the limit for the Lehner basics?

As far as adding an overgearing (or undergearing) warning, that is dependent on weight, drivetrain drag, and other hard-to-quantify variables. It is really up to the user to monitor temperature to make sure gearing is OK. If anyone has an idea on how to implement this, I can do it as long as it is fairly accurate and not simply a "fudge factor".

Also, you can save the page and add whatever options you want. If you view the source, you'll see there are sections where you can easily add options to present lists.

Brian, some of the motors are rated for more RPM's but there ideal range is almost always in the 35 to 40,000 RPM's.