Advanced Brushless Motor Vehicle Speed Calculator

[Lance Carbuncle]

Hey Guys,

New to RC cars and been lurking and studying for a few months now.

I thought I would try to make my first post a contributing one.

I decided to go to a brushless setup and was trying to figure out the right kV motor to get and after digging around and doing the calculations over and over on a bunch of combinations, I just decided to make a spreadsheet that automatically calculates the vehicle speed based on your vehicle's Specs.

It uses tooth count inputs for pinion, spur, transmission in/out, transfer case in/out, over/under drive ratio, and ring and pinion.

You also input the kV of your desired motor, The voltage of your desired battery pack, your throttle position, and your tire diameter.

Change any tooth count, kV, voltage, tire size, and throttle position, and it instantly gives you your speed at throttle position.

Note that for tire diameter I used "at speed" because I think a 4.5" tire at full speed with a 4s LiPo probably stretches to 5" when spinning that fast due to centrifugal force.

I couldn't find anything out there like it, so hopefully someone else will find it a valuable tool.

If I missed anything, or anyone has ideas to further develop it, let me know.

Light green cells are for input and bright green are results.

It probably needs some instructions and a legend with basic stock gearing tooth count info.

I don't see a way to attach a file, so you can download it from my site at

click the link and check your download folder

Directory /repashy_rc_downloads

Cheers, Allen

Here is a screen shot of a test I was doing for a TRX 4 with a 2200kV motor on 3S

[TheLetterJ]

A member here made a similar calculator that has proven invaluable: R/C Calculations

I'm still not sure if I should use my front OD ratio, rear UD ratio, split the difference, or what to figure out my effective gearing. I guess by the time we get down that far, it's probably best to fine tune based on motor temp. Thanks for your efforts too.

[lonleycreeper]

I read in there that you are ballooning a tire at crawler speeds. The reality is that the tires don't expand much, if at all until you start spinning them at 25-30 mph.

Then you have listed tire sizes. The actual tire size may be dramaticly different from their posted specs. Eg I had a tire listed at 5.5" tall barely make 5".

[Lance Carbuncle]

Originally Posted by TheLetterJ

A member here made a similar calculator that has proven invaluable: R/C Calculations

Oh man, that's a great tool and much more in depth than mine as far as having the drop down menus.

No real advantage to mine other than you can download it on your computer and save each car profile as a spreadsheet so you can tweak on it later or compare and share it. I thought I was making something unique, so apologies to the creator of the link you sent. I didn't know that existed, but there are definitely similarities.

I already changed it to be able to add not just an over/underdrive combo, but also portal axles with input for tooth counts to provide additional gear reduction.

[Lance Carbuncle]

Originally Posted by lonleycreeper

I read in there that you are ballooning a tire at crawler speeds. The reality is that the tires don't expand much, if at all until you start spinning them at 25-30 mph.

Then you have listed tire sizes. The actual tire size may be dramaticly different from their posted specs. Eg I had a tire listed at 5.5" tall barely make 5".

Thanks man. I put my SCX10 II on a stand and spun the tires and even with a 12mph top speed, I can see the tires ballooning quite a bit.

Definitely agree on listed tire sizes. My input is for measured tire size.

Cheers, Allen

[Lance Carbuncle]

I have made a bunch of upgrades to the calculator to accommodate most any crawler setup.

2 speed Transmission
2 speed Transfer Case
Portal gears

Input the gear tooth counts or gear ratio and it will calculate all possible shift combinations available.

I would enjoy some feedback and feature requests.
Directory /repashy_rc_downloads

[Lance Carbuncle]

Fixed some locked cell errors and uploaded the new version 1.47 to the download page.

Directory /repashy_rc_downloads

Not sure if anyone has even downloaded it but hoping someone finds it useful.

Here is a mockup build showing the potential with a 2 speed transmission, 2 speed transfer case, and portal reduction axles.

[whitrzac]

I'm just going to leave this here.

R/C Calculations

[Lance Carbuncle]

Originally Posted by whitrzac

I'm just going to leave this here.

R/C Calculations

Thanks Captain Obvious. ..... Obviously you didn't actually read the thread where someone else already posted the link, and did it with respect because they understood that I spent more than 30 hours developing this myself because I am new to the hobby (2 months) and didn't realize that online tool existed..... and that I posted it to share because I thought it might be of benefit to others.

Its comments like yours that kill places like this and keep people from trying to do things that actually make a contribution. My first post on this site was a contribution..... not a newbie question, not a stupid question. I wanted to add value to this site with my presence.... but people like you make me want to just keep to myself.

My calculator is also quite a bit different...... it allows actual tooth counts and you can save the sheet on your site for reference and catalog your cars. It also has many features not in the online calculator and that one has a lot of options that mine doesn't.

I also added two more versions over the weekend. One that is similar to the first one, that adds side by side comparison so you can change any parameter and see in a glance what it changes.

I also added a "Dummy" Version that removes the tooth counts and just lets you put in ratios and returns the basic details. It is also a side by side version because I have found that I am constantly trying to compare cars and its easier than doing it on two separate sheets.

Enjoy the Hobby.

[Ironhide13]

Originally Posted by Lance Carbuncle

Thanks Captain Obvious. ..... Obviously you didn't actually read the thread where someone else already posted the link, and did it with respect because they understood that I spent more than 30 hours developing this myself because I am new to the hobby (2 months) and didn't realize that online tool existed..... and that I posted it to share because I thought it might be of benefit to others.

Its comments like yours that kill places like this and keep people from trying to do things that actually make a contribution. My first post on this site was a contribution..... not a newbie question, not a stupid question. I wanted to add value to this site with my presence.... but people like you make me want to just keep to myself.

My calculator is also quite a bit different...... it allows actual tooth counts and you can save the sheet on your site for reference and catalog your cars. It also has many features not in the online calculator and that one has a lot of options that mine doesn't.

I also added two more versions over the weekend. One that is similar to the first one, that adds side by side comparison so you can change any parameter and see in a glance what it changes.

I also added a "Dummy" Version that removes the tooth counts and just lets you put in ratios and returns the basic details. It is also a side by side version because I have found that I am constantly trying to compare cars and its easier than doing it on two separate sheets.

Enjoy the Hobby.

Don't waste your breath on him, big guy. But you're gonna need a little thicker skin to hang out in forums like this one. Otherwise, the trolls smell blood and start circling!
I do agree with you though, forums do take a beating from the few that make it hard to enjoy the exchange of information.

Cool set up, though. I'll probably wind up using your calculator when I go to brushless. Are there any that can be used for brushed motors (turn count vs. kV)?

[Lance Carbuncle]

I created a side by side comparison version of what I am calling the "RC Drivetrain Configurator" and think it is a much more valuable tool than the original version so I will continue to develop this version.

I have added a bunch of calculations that show differential factors and change percentages, as well as a visual check to see what values are different in your sheet. Here is a screenshot of the new configurator comparing a Bomber with differences in pinion and motor kV that result in a similar top speed.

[Ditchrat]

Originally Posted by Lance Carbuncle

Note that for tire diameter I used "at speed" because I think a 4.5" tire at full speed with a 4s LiPo probably stretches to 5" when spinning that fast due to centrifugal force.

That doesn't matter in the calculations. The reason is that the calculations are supposed to be based on radius, not diameter. Because of the formulas diameter is often used as a short cut, instead of calculating it out using the formula.

Any way the radius should remain relatively unchanged at the point in which it contacts the ground due to the opposing forces.

It is good that you identified that as a potential issue, but if you ever look at slow speed videos of tire defections you will see what I am talking about.

If anything the tire might actually be smaller then you are figuring because of the weight on the rig compressing the foams.