RC Vehicle Speed Calculator

[biggman100]

Guys we've added an RC Vehicle Speed Calculator to the front page of RCTalk, with more tools hopefully to come.

https://www.rctalk.com/rc-tools/rc-car-speed-calculator/

Check it out and share your thoughts on anything that needs to be improved upon.

If you have any ideas for more calculators please let me know!

How does this work for 4WD? Do you just input the same info, such as ring and pinion for the diff, as you would a 2WD? I tried it using my Slash 4X4, and either i put something in wrong, or, it doesn't calculate for 4WD, because the result was that it would only do 9MPH. Another thing i noticed, it asks for motor KV, but, not motor size, and, i thought bigger motors had more torque and/or speed.

 

[Jamr]

I think may have misunderstood "ballooning diameter" - it works now at least. Thanks for adding it!

How do we measure that?

 

[Lukedavis]

How do we measure that?

I have that same question

 

[tudordewolf]

How does this work for 4WD? Do you just input the same info, such as ring and pinion for the diff, as you would a 2WD? I tried it using my Slash 4X4, and either i put something in wrong, or, it doesn't calculate for 4WD, because the result was that it would only do 9MPH. Another thing i noticed, it asks for motor KV, but, not motor size, and, i thought bigger motors had more torque and/or speed.

The calculator assumes perfect conditions - that the motor isn't over-geared, that the battery voltage won't sag below 3.7V, etc, and it just works out how fast the wheels would turn.

Weight, torque, etc, doesn't need to be simulated to determine theoretical max speed, just the max RPM the engine is expected to produce and the transmission ratio to the wheels, and how large the wheels are.

2WD and 4WD makes no difference for that reason. What values did you enter?

I have that same question

I think the best one can do for ballooning value is guess. It's worth noting that the calculator isn't asking for "final ballooned diameter", but additional diameter over base value when ballooning.

 

[biggman100]

The calculator assumes perfect conditions - that the motor isn't over-geared, that the battery voltage won't sag below 3.7V, etc, and it just works out how fast the wheels would turn.

Weight, torque, etc, doesn't need to be simulated to determine theoretical max speed, just the max RPM the engine is expected to produce and the transmission ratio to the wheels, and how large the wheels are.

2WD and 4WD makes no difference for that reason. What values did you enter?




I think the best one can do for ballooning value is guess. It's worth noting that the calculator isn't asking for "final ballooned diameter", but additional diameter over base value when ballooning.

I used the info I got from Traxxas for the Slash 4X4. 3.92 transmission ratio, and, I think it was 13/37 for the diff gears. The motor is a 4700KV 3656, and, the tire diameter is 115MM, on 2S. With a 15/52 spur and pinion, it said 9MPH.

 

[tudordewolf]

I used the info I got from Traxxas for the Slash 4X4. 3.92 transmission ratio, and, I think it was 13/37 for the diff gears. The motor is a 4700KV 3656, and, the tire diameter is 115MM, on 2S. With a 15/52 spur and pinion, it said 9MPH.

Transmission ratio is redundant in this case, that's what you're entering with the pinion/spur values. Some vehicles (like the Revo I believe) have an additional gearbox between the clutch/spur pair and the pinion/diff gear pair. You can use the values you entered above with a transmission ratio of "1".

The calculator was dividing your result by an extra 3.92x

 

[Lukedavis]

The calculator assumes perfect conditions - that the motor isn't over-geared, that the battery voltage won't sag below 3.7V, etc, and it just works out how fast the wheels would turn.

Weight, torque, etc, doesn't need to be simulated to determine theoretical max speed, just the max RPM the engine is expected to produce and the transmission ratio to the wheels, and how large the wheels are.

2WD and 4WD makes no difference for that reason. What values did you enter?




I think the best one can do for ballooning value is guess. It's worth noting that the calculator isn't asking for "final ballooned diameter", but additional diameter over base value when ballooning.

Oh it is?
@WoodiE does it ask for additional ballooning or final ballooning combined with the regular tire size?

 

[tudordewolf]

Thanks again for making these, I use them regularly!

I have an idea for another electric calculator: theoretical acceleration. Given the motor's KV rating, saturation current, and the vehicle's weight, you could calculate the maximum theoretical acceleration an RC should be capable of.

You'd have to create a value called the "torque coefficient", which you get by multiplying the KV by (Pi/30) (reference), which describes the torque per amp the motor creates in N*cm.
Then you multiply that by the transmission ratio and divide it by the radius of the wheel, and you've got force on the car in grams.
Divide that by the weight of the car and you've got acceleration in G's (assuming perfect traction)

Would be fun for drag racing or just comparing that value to a measured one.

 

[Doom!]

Thanks again for making these, I use them regularly!

I have an idea for another electric calculator: theoretical acceleration. Given the motor's KV rating, saturation current, and the vehicle's weight, you could calculate the maximum theoretical acceleration an RC should be capable of.

You'd have to create a value called the "torque coefficient", which you get by multiplying the KV by (Pi/30) (reference), which describes the torque per amp the motor creates in N*cm.
Then you multiply that by the transmission ratio and divide it by the radius of the wheel, and you've got force on the car in grams.
Divide that by the weight of the car and you've got acceleration in G's (assuming perfect traction)

Would be fun for drag racing or just comparing that value to a measured one.

I think you just broke my brain!

 

[tudordewolf]

Still getting regular use out of these, really appreciate them!

I notice that the "rollout" on the nitro RC calculator doesn't seem to be returning a value, would it be possible to implement that?

 

[Lukedavis]

how do we calculate for 2wd?

 

[Lukedavis]

Nitro RC calculator is now available here, https://www.rctalk.com/rc-tools/nitro-rc-speed-calculator/

How would we see how fast a 2 speed car goes?