RC Car Speed Calculator

[WoodiE]

Hey guys! If you're looking for an RC speed calculator, take a look at the one we just built on RCTalk.

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

The calculator should useable across all devices. Plus I've reached out to a few of the members here for initial testing and so far, the feedback has been positive and very helpful. In fact, we've already added features to it.

If there are improvements you'd like to see, or you have ideals or requests for other calculators, please let me know!

 

[Doom!]

I haven't even had a chance to try it out, looks good though.

 

[Cereal_killer]

Sweet! That’s great! Thanks!

 

[Diem Turner]

Tire ballooning modifier would be nice. The difference in diameter between a GRP at rest and going 140mph is no rounding error. That will put an additional 7-12mph on your top speed.

This calculator does it by adding to the radius of the tire (which may not be the most intuitive way of going about it...I got to my 0.1" by reverse engineering because I can't measure the radius delta of my tires at 130mph). Maybe you guys can come up with an easier to approximate value than added radius in inches.
https://scriptasylum.com/rc_speed/top_speed.html

 

[Donovan]

Nice!

 

[EJunlimited]

Interesting! According to my data I put in with my setup, it comes to 131 MPH.
My best so far is 111 MPH. LOL

 

[Dan B.]

Interesting! According to my data I put in with my setup, it comes to 131 MPH.
My best so far is 111 MPH. LOL

If you subtract 20% from your kv to allow for voltage sag, that’s probably very close to the ballpark. Good for you and that 111!

 

[EJunlimited]

If you subtract 20% from your kv to allow for voltage sag, that’s probably very close to the ballpark. Good for you and that 111!

Well I thought the 3.4v would already account for voltage sag

 

[Diem Turner]

Well I thought the 3.4v would already account for voltage sag

This is true. But your motor also undergoes what I refer to as KV sag (which is really just the efficiency falling - only on a free standing motor with nothing attached will you reach the KV value on the can, +/-10%). As you increase the load, the efficiency of the motor drops off. With your more typical street basher gearing like 15/46-20/46 you're probably somewhere in the 90% vicinity. With taller gearing like 37:28 and lower, you should be somewhere in the 80% range. Somewhere around 78% you're scraping bottom. At that point you've, essentially, pulled as much from the motor as is useful and taller gearing won't net you any further gains. These are all rough figures. Exact values will vary from case to case.

So, just to be clear, you need to account for both voltage sag and loss of KV.

 

[EJunlimited]

This is true. But your motor also undergoes what I refer to as KV sag (which is really just the efficiency falling - only on a free standing motor with nothing attached will you reach the KV value on the can, +/-10%). As you increase the load, the efficiency of the motor drops off. With your more typical street basher gearing like 15/46-20/46 you're probably somewhere in the 90% vicinity. With taller gearing like 37:28 and lower, you should be somewhere in the 80% range. Somewhere around 78% you're scraping bottom. At that point you've, essentially, pulled as much from the motor as is useful and taller gearing won't net you any further gains. These are all rough figures. Exact values will vary from case to case.

So, just to be clear, you need to account for both voltage sag and loss of KV.

Good to know. Now I know my solution is more power. Lol

 

[King blitz]

Hey guys! If you're looking for an RC speed calculator, take a look at the one we just built on RCTalk.

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

The calculator should useable across all devices. Plus I've reached out to a few of the members here for initial testing and so far, the feedback has been positive and very helpful. In fact, we've already added features to it.

If there are improvements you'd like to see, or you have ideals or requests for other calculators, please let me know!

Thx woodie

 

[WoodiE]

Requested on RCTalk, added field to enter a transmission gear ratio (if applicable).

 

[vwturbowolf]

Hey guys! If you're looking for an RC speed calculator, take a look at the one we just built on RCTalk.

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

The calculator should useable across all devices. Plus I've reached out to a few of the members here for initial testing and so far, the feedback has been positive and very helpful. In fact, we've already added features to it.

If there are improvements you'd like to see, or you have ideals or requests for other calculators, please let me know!

Nice photo at the top of the calculator @WoodiE
I think after cross checking the calculator with some of my most constant cars, one runs 158-159 everytime and the other 170-175 I notice that it may be reading a % high, even if we adjust the voltage down to make the rpm’s match the rpm’s of the run itself.

However I think this would likely keep folks from overgearing their setup initially which is a very common problem with new speed runners.

If you want to see my data comparisons send me a message and I can show you what I mean

 

[WoodiE]

If you want to see my data comparisons send me a message and I can show you what I mean

I am interested and I'll certainly message you for more info.

Right now I've just added a conditional message is the RPM gets to high.

@Diem Turner has me working on trying to figure out how to calculate speed with tire ballooning.

Really love the feedback and suggestions so far!

 

[WoodiE]

Tire ballooning support has been added.

 

[phildogg]

Nice photo at the top of the calculator @WoodiE
I think after cross checking the calculator with some of my most constant cars, one runs 158-159 everytime and the other 170-175 I notice that it may be reading a % high, even if we adjust the voltage down to make the rpm’s match the rpm’s of the run itself.

However I think this would likely keep folks from overgearing their setup initially which is a very common problem with new speed runners.

If you want to see my data comparisons send me a message and I can show you what I mean

I think alot of the calculators are usually off to the positive mph. I always calculate with 3v per cell also. but drag/wind resistance is a big challenge to overcome in real life.

 

[WoodiE]

Re-wrote parts of the calculator and corrected the km/h speed as well. Looking better.