Speed Calculator

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

[BrianG]

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

[coolhandcountry]

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.

[squeeforever]

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.

[SpEEdyBL]

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.

[BrianG]

Originally Posted by SpEEdyBL

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?

[BrianG]

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

[stampy]

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

[BrianG]

Originally Posted by stampy

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.

[SpEEdyBL]

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

[BrianG]

Originally Posted by SpEEdyBL

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.

[Serum]

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.

[BrianG]

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.

[coolhandcountry]

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.