wallacengineeri

iTrader: (1)

So Im picking up my new Kraton in about a week or so. Im trying to figure out what 2 Robinson Racing Extra Hard Pinions I should buy for it (One for Speed Running on Belted Tires, one for bashing on 4S or 6S).

For off-road bashing, everyone says the optional speed pinion is okay to use, and Ive already seen a YouTube video where a 16T worked just fine and did not overheat the motor on a completely stock, out of the box V4 Kraton. As far as I understand, 13T is the stick included pinion and 15T is the optional high speed pinion, so I figure a 15T Robinson Pinion is the way to go. This way I should get about 40 MPH on 4S and 60 MPH on 6S, right?

Now the Speed running gear is where things get a bit complicated because Im going to be swapping to SweepRC Belted Road Crusher Tires which are smaller than the stock DBoots Copperhead 2 Tires and also shouldn't "balloon" at all. I want to aim for about 70-75 MPH Top Speed on 6S using the SweepRC tires. If I try to push past 80MPH, I feel like I could overheat the ESC or motor quite easily.

So, calculation of the theoretical top speed - Online calculators dont work as they require a spur gear input and as far as I know, the Kraton has no spur gear. The Motor Pinion Meshes directly up to the Center Diff as far as I know. So when I try to use values to bypass the spur gear, I end up with weird results like a weirdly low 18 MPH top speed or a rediculous 842 MPH Top Speed lol.

So Im trying to calculate the percentages myself to get a close estimate. The SweepRC Tire OD is 5.6 inches, which is a 13.85% smaller tire than the 6.5 inch CopperHead Twos.

So we start with a 13.85% ratio reduction from 60MPH on the 15T pinion. Now we take an Estimated Pinion (21T) and deduce that this is a 40.00% increase in ratio. Now we subtract the reduction from the increase and end up with a 26.15% overall increase in Motor RPM to wheel rotation ratio.

So if the 15T pinion yields 60MPH on 6S using 6.5 inch tires, then the 21T pinion should yield 75.7 MPH on 6S using 5.6 Inch Tires. This is the best estimate I can come up with, especially as I would have absolutely no idea on how to calculate the difference in ratio caused by the ballooning of the stock tires, which would therefore change my calculations entirely. Is there anything else I can do to more accurately estimate my top speed?

Dave_S

iTrader: (1)

The number of teeth on gear on center diff is what you use for spur gear.

I think it's 50t

wallacengineeri

iTrader: (1)

Well I tried this but it seems to be off. Its saying Stock Kraton on stock optional pinion (15T) will do 70MPH on 6S and thats WITHOUT calculation of tire ballooning.

Dave_S

iTrader: (1)

That calculation shows fully charged lipo voltage of 25.2, but under full throttle at speed your voltage will dip down and won't be at 25.2 volts.

wallacengineeri

iTrader: (1)

Oh yea I hadn't considered voltage drop under load. I wonder what it would drop to, maybe 24.0V?

Dave_S

iTrader: (1)

No idea.

wallacengineeri

iTrader: (1)

I don't think anyone knows for sure.

Dave_S

iTrader: (1)

Only way to know for sure is a datalogger.

wallacengineeri

iTrader: (1)

Well if we assume 0.20V voltage droop per cell, it makes math very easy and Actually seems to be accurate (8V = 2S, 12V = 3S, 16V = 4S, ECT.)

When you plug the Kraton V4 into the calculator with 15T pinion and 24V, you get 66 MPH. Subtract 3-5 MPH for wind resistance, ECT and you get ~ 62 MPH. Use the same math on the 21T and SweepRC Belted Tires and you get ~75 MPH after wind resistance.

Dave_S

iTrader: (1)

Assuming is kinda a crapshoot in this situation. Many variables.

wallacengineeri

iTrader: (1)

True but I don't have any way to log data, and the math does add up surprisingly well if you calculate using 4.00V per cell and subtract some wind resistance. It can't be too far off.

SIDENOTE 1:
I feel like Arrma has made a HUGE mistake on the BLX ESCs by not including 5S Voltage recognition in the ESC's Programming. 20V on a 15T pinion would be absolutely PERFECT on the Kraton for Bashing. A picture perfect middle ground between gentle 4S and crazy 6S.

SIDENOTE 2:
I was thinking I might do a crazy powerful 8S conversion later using the HobbyWing MAX6 ESC and a TP Power BYO 40x83mm 4250-4Y 1570KVmotor rated for up to 9S (38.0V)!

What's great about the setup is that 1570KV on 32V is only a tad over 1,000 RPM more than the stock 2050KV motor on 6S, so it wouldn't be some out of control speed demon, but I would swap for a 16T pinion offroad and a 24T for speed running since I know the new power system wouldn't even break a sweat.

Stick on some SweepRC Belted Road Crusher Tires, fit the 24T Pinion, subtract 0.20 voltage droop per cell and subtract 6:8 MPH for extreme wind resistance and you get an 85+ MPH Asphalt Monster with GOD-LIKE Horsepower and Torque levels that would probably snap a plastic wheelie bar right off at full throttle lol and you wouldn't even need a motor cooling fan! Hmmm... Maybe some day...

Dave_S

iTrader: (1)

50,000 rpm is a lot lol. It will be fast.

wallacengineeri

iTrader: (1)

I might make another thread to explain how to build the ultimate 8S drag racing Arrma Kraton/Talion with these electronics and suggested aluminum upgrade parts to prevent driveline damage.

Nortran11

I really wish Arrma would just leave their trucks alone......I literally just bought a Typhon and now a month later.....new version.

wallacengineeri

iTrader: (1)

That sucks ☹️ but look at it this way: Its still a nice truck 😃

Im getting the Kraton V4 because I just had a bad experience with the 1/6 Losi Super Rock Rey, found out that I should really stick with lighter 1/8 scale rigs that are more bash capable and then I hear about the V4 Arrmas, I already LOVE Truggies, and Ive already got nice, brand new SMC 7400mAH 149 Amp batts in both 6S and 4S, so yea...