Tommygun

My Feedback: (2)

Hi all, first post in this section. Normally I'm exclusively into airplanes or old school Tamiyas, but the other day I bought a Redcat Sandstorm TK brushed for my daughter to play around with in the back yard. It works pretty well, but the stock brushed 550 gets scolding hot! Saliva will sizzle off of the case, and the heat comes all the way through the bottom pan. Often a longer run is accompanied by an electrical burning smell, which is never good. My question is, what sized KV motor should I be looking at with a 4x4 truggy running on 2S Lipo on thick (but mowed) grass to avoid such overheat, and get as long a run times as possible? Pinion is 17t, spur is 64t for a final drive ratio of 3.76. Not looking for racing ability here, just something that lasts and gives decent run times. I was looking at the Turnigy Trackstar 550 SCT motor, but it's 3750 KV. Most of the 550s seem to be in this KV range as well. Not sure if that's too much or if I should be looking <3000KV for my application. Any input would be much appreciated!

Maj_Overdrive

My Feedback: (11)

Going brushless and with a bigger 550 size motor definitely puts you on the right track. Lower kv does have more torque all things being equal, but motor size has more effect on torque. And your Kv choice should be made with voltage and gearing options in mind. In fact after making some calculations I think a 3750kv 550 motor with stock 17/64 gearing would be close to ideal and fine on temps. It'll definitely be faster so I'd get a 15t and 16t pinion just in case (your grass is really really thick and lush making) temps end up too high or you want to slow it down. FYI top speed with 3750kv on 2s with 17/64 gearing is around 35mph, 15/64 would drop it down to 30mph and would definitely be safe on all but the thickest and/or too tall grass.

Tommygun

My Feedback: (2)

Thanks Overdrive, I appreciate the input. It does seem that the lower KV 550 would be the ticket, though such a system as a complete sensored motor/esc set seems to be a rare species indeed- everyone seems to be replacing the brushed 550s with brushless 540s. (The one 550 I was looking at is sensorless, after reading the differences I think I need a sensored setup.) I did try dropping the pinion down, and it worked okay for a while. Unfortunately, for some reason made the stock brushed 550 ran even hotter- smoke started pouring out from under the body tonight. I switched it back to the higher pinion and it stopped smoking. The mesh was fine, no binding or anything, just more gear noise like you'd expect from lower gearing. I'll give the stock motor one thing, despite running really hot and smoking it keeps going, though I'm sure it won't for too much longer.

Maj_Overdrive

My Feedback: (11)

For the size of your truggy a 540 would be fine as well. Like a prop on a plane you just want to make sure it's geared right so the motor can handle the load. A 4 pole 540 would be enough as long as you don't gear for over 40mph. A 4 pole 550 is just longer so it should have more power and should run cooler compared to the same kv 4 pole 540.

Sensored vs sensorless is a matter of preference and intended use. For bashing around sensorless is fine. Sensored comes into its own when precise throttle/brake inputs and smooth startup are required, like racing and rock crawling. I always ran sensorless until I put a Tekin system in my 8ight-t 2.0 and can now see its benefits for racing but I still don't think I need it for bashing around. Sensorless reads the EMF coming back through the motor wires to determine the motor's position and when to fire each phase. The motor needs to be spinning at least a certain rpm for enough EMF to be generated that the esc can read so the esc uses a generic startup algorithm to get the motor up to speed. This startup algorithm can be a little abrupt depending on the esc, the motor, gearing, weight, etc. but once the vehicle is moving its plenty smooth. The biggest thing the average basher will see is that with light throttle input a sensored system will move from a stop smoother and may have a lower minimum speed while the average sensorless won't want to go slower than about walking pace (depending on motor and gearing) because of the minimum motor rpm needed for EMF. The decision is yours of course, those are just the real world implications.

Maj_Overdrive

My Feedback: (11)

Just a heads up on the pinion and spur gears. That Redcat model uses mod .6 pitch gears. If you tried using the far more common mod.8 or 32 pitch gears they may have seemed to match up but if you look really close you'll see they're off just slightly. This would cause more gear noise and increase motor temp as more drivetrain resistance is introduced.

Gears will always make some sort of noise. How much depends on the material of the pinion/spur and also the mesh. Metal pinion and metal spur make the most noise which is part of the reason I like plastic spurs. Mesh can also affect the amount of noise produced particularly of the mesh is too tight. The paper trick always ends up too tight for me, I just set it as tight as possible while also having just a little bit of play between the gears.

Tommygun

My Feedback: (2)

I also find the paper trick to be useless- I have to use it as a starting point only. I did buy 48 pitch pinions for it, but maybe something doesn't match up. Today I bit the bullet then ordered a Hobbywing 65Amp 3000KV 13T brushless setup. I'm hoping that with the included programmer, I can set it on the lowest timing and the heat will stay down on the grass. This cheap car certainly got expensive fast...

Maj_Overdrive

My Feedback: (11)

If you go to the parts lookup on Redcat's website and find the pinion and spur they're mod .6 pitch. I believe Robinson Racing has some mod .6 pinions. I forget the exact order of largest to smallest gear pitch as I've been in the land of mod1 for quite awhile but 48p doesn't match mod .6 and will wear out. I only saw 17t and 21t on Redcat's site but I didn't look too hard and I'm sure there are other vendors out there with mod .6 pinions too.

I ran some calculations and with your new 3000kv motor you'll be at 28mph with 17t pinion and 33mph with 21t pinion. I highly doubt you'll have a problem with heat at either of these speeds so I wouldn't mess with the timing at all.

Tommygun

My Feedback: (2)

I finally got the Hobbywing motor today, and installed it. It's just way too fast after installing the new motor, and scorching hot after a run on grass. Halfway into a charged pack I think it's shot, lot's of electrical burning smell, and so hot saliva sizzles off of the motor case, plus now it's studdering during starting and braking. Oh well, that was a short lived, $90 down the drain in 10 minutes.
In closing, thank you for your help Overdrive, I really do appreciate you taking the time to repost those sites and give advice. Not too many have your patience to deal with noobs these days, it was a humbling experience to deal with this truck after being an expert at airplanes for 30 years. I think the combination of the grass, the type of vehicle, it being driven by a child, and it being an electric were a recipe for disaster, so I'll chalk it up to a very expensive learning experience. In hindsight, we probably should've just bit the bullet and gone with an Emaxx or something larger with twin motors that can "float" over the grass with large tires.

RustyUs

I've been following threads here and there, and I was cringing when I saw the "RC running on grass" subject come up. The only thing I can think of that is safe to run on grass exclusively.... is like you said, something that will float on or have a chassis not touching the grass. Even my 1/8 scale truggy with 6" tall tires had a hard time in normal grass when it went off the track.

Tommygun

My Feedback: (2)

Not to be outdone by this car, I think I figured it out, so here's what I did for archives sake. We went through two motors, (and puffed a lipo) one brushed, one brushless, but finally installed this 1250KV rockcrawler motor, along with a steel spur. I think this may be the answer. While the grass is shorter for the winter, I put this motor in and ran it around for a while in the back yard. Motor was almost cool to the touch! The speed is of course way down, probably about what the brushed motor ran, which is actually perfect. The torque is amazing. Yay.

EXT2Rob

My Feedback: (1)

<cringe> 😫 is exactly what I do when I hear this. LAWN is not a good running area for your average RC. You really need to get an infrared thermometer to monitor your motor temps. What brand of Lipo did you puff?

Tommygun

My Feedback: (2)

Well, the lawn is the area I currently have available to run it on, so the car needed to be set up accordingly. It was a Venom 5000 LiPo that puffed. But the point I was trying to make is I found a solution- use a very low KV motor. The new motor runs quite cool, so the fix for running on grass appears to be the rock crawler motor for anyone in a similar predicament.