Baja 5B Electric roller for $345 should I do it?

[Horatio]

Crap I just realized that a 780KV motor will go above 30,000RPM at full throttle which is higher than the maximum rated RPM for that motor. The max rated RPM s 30,000 and 12S would drive the motor at about 39,000RPM on a full pack or about 34,5000 on a depleted pack. Looks like I will need to return it and get the 580KV version.

Anyone know what the maximum pinion size is for a stock 57T spur gear and 55mm motor?

It likely wouldn't hit those RPM's under load though, so you should be fine. Or maybe consider replacing the motor bearings for something with a higher speed rating?

 

[Rotauq]

Crap I just realized that a 780KV motor will go above 30,000RPM at full throttle which is higher than the maximum rated RPM for that motor. The max rated RPM s 30,000 and 12S would drive the motor at about 39,000RPM on a full pack or about 34,5000 on a depleted pack. Looks like I will need to return it and get the 580KV version.

Anyone know what the maximum pinion size is for a stock 57T spur gear and 55mm motor?

30k limit is just a number
As Horatio already mentioned you will never reach the theoretical max rpm under load. 90%+ efficiency would already be beyond my believe and then there is also voltage sag.

I've now put a 650kv in my rofun and I will be running it on 14S as well. Which would be 38k rpm. In below run at 12S is saturates at 146A, meaning it can take a lot more than it's current load, but it's not hitting 30k rpm i think (i still need to double check why HW doesn't accurately show the rpms)

 

[Atomic Skull]

It likely wouldn't hit those RPM's under load though, so you should be fine. Or maybe consider replacing the motor bearings for something with a higher speed rating?

The issue isn't bearings it's the rotor potentially exploding.

They have a chart for that motor that shows the 780kv is for 40V and less, the 570k is 53v and less. But you're right it's probably not going to be an issue and I imagine the RPM limit has been derated somewhat as well.

EDIT Also I'm pretty sure the Surpass 56112 is the same motor as the Hobbystar one.

How do you calculate the transmission rate on a Baja 5B? I have not been able to find any information on the diff pinion and diff gear tooth count other than the overall transmission ratio of 3:1

Would just setting the diff pinion and diff gear to 10 for both and then inputting the overall transmission rate work in this calculator?

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

Also, Baja 5B rear tires would make great Kagama 6S tires just need 24mm wheel hex adapters. And because they are beadlocks new tires become cheap and easy once you have a set of wheels.

 

[danl]

While I wasn't able to find any videos on this specific servo I have found testing videos on youtube of other DS Power servos and the specs are more or less accurate.

I forgot to mention there's a trick to keeping self leveling electronics silicone from flowing down around the PCB and sticking to the bottom of the case. I put a couple strips of teflon plumbers tape across the bottom of the exposed PCB where I'm going to apply the goop to make barrier and then put the bottom cap on over it and apply the silicone to the top side. It won't stick to the tape and it can be peeled off once it cures. If I need to add a bit more to the other side on the main lead I do that after the top side application has cured.

Applying it to the leads on the potentimeter can be tricky, non slumping stuff is easier to work with here but I don't like keeping a tube of it around for just one purpose. 95% of the time when I need electronics silicone for something I need the semi flowing stuff because it creeps around and under components and wires while non slumping would leave gaps.

Any reason you can’t use hot melt glue to hold things in place for areas you are concerned about the silicone running into places it shouldn’t?

 

[Horatio]

How do you calculate the transmission rate on a Baja 5B? I have not been able to find any information on the diff pinion and diff gear tooth count other than the overall transmission ratio of 3:1

Input 20
Crown 48

48/20

1:2.4

Pinion 17
Spur 57

1:3.35

Overall 1:8.04

Obviously, I use gearing to calculate engine rpm and top speed, prior to real world testing. My spreadsheet is as accurate as it gets and works across all my rigs just by plugging in applicable numbers.

My Baja 5B has a rollout of 64.42mm/rev. My real world speed was 49 mph,

So from that I know that my engine speed was 20500rpm.

Would just setting the diff pinion and diff gear to 10 for both and then inputting the overall transmission rate work in this calculator?

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

I don't use anyone else's calculator, because they weren't flexible enough.

If people would like to see the maths here, I'm happy to share.

Also, Baja 5B rear tires would make great Kagama 6S tires just need 24mm wheel hex adapters. And because they are beadlocks new tires become cheap and easy once you have a set of wheels.

Yes, I've often toyed with that idea. Albeit most Baja tyres aren't belted, so you need to be careful not to shred them on a 6S brushless rig.

 

[Horatio]

 

[Horatio]

The issue isn't bearings it's the rotor potentially exploding.

The rotor exploding is a potential outcome of exceeding the specified rpm limit by over volting, but it would be highly unlikely to 'grenade' unless it was being allowed to 'free rev'. 30k rpm isn't that much in motor terms. For some context, my HW V10 5.5t motor often exceeds 44k rpm, under-load, during racing conditions.

The limits that govern the maximum safe rpm of a motor are:

• The mechanical integrity of the motor - this includes both the bearings AND the rotor construction
• Thermal limits

If the motor begins to exceed thermal limits due to bearing failure, the consequences of that will be subsequent rotor failure, often due to excessive vibration and friction. The rotor can certainly grenade when that happens.

Given the numbers you've stated and the application, I am very confident that you will not hit 30k rpms.

If you did, on stock gearing and street tyres, your rig would be doing over 72mph, which in a Baja is off-the-charts, insanely fast. Good luck with that!

Keep us posted!

 

[Atomic Skull]

Ok, if I use the stock spur and pinion that comes with the Rovan Baha 5B electric (57T spur and 14t pinion) about what speed will I get with a 780KV motor?

Stock electric 8S RTR has a 750Kv motor and they claim 60mph so I think I either need to return the 780Kv and get a 580KV or use a smaller pinion.

 

[Horatio]

Ok, if I use the stock spur and pinion that comes with the Rovan Baha 5B electric (57T spur and 14t pinion) about what speed will I get with a 780KV motor?

Stock electric 8S RTR has a 750Kv motor and they claim 60mph so I think I either need to return the 780Kv and get a 580KV or use a smaller pinion.

Personally, I would not be wanting to lower the gearing, which will reduce the load and encourage the motor to spool up to higher RPMs, potentially exceeding the envelope. If anything, I would gear up some, which would actually increase the theoretical top speed. But that's ok.

Another issue is that you can't run a 14t pinion with a 57t spur - it wouldn't mesh. You'd need a 60t spur - if they even exist.

On 12s 50.4v - 780kv = 39k rpm

Gearing: 17/57

64.42 mm/rpm (based on

39000 x 64.42 x 60 / 1000000 = 150kph
93.6 mph

On a purely hypothetical 14/57

That's an FDR of 1:9.768

Tyre circumference (based on 165mm diameter)
mm/rpm = 518.43mm / 9.768
mm/rpm = 53.074

39000 x 53.074 x 60 / 1000000 = 117kph
73mph

So whilst you are reducing the top speed of the rig with this ratio, a) it's not possible to attain given the Baja's gearing restraints and b) it would actually be making it easier to over rev the motor.

Gearing options for Baja should always add up to 74.

I know some people take liberties with this and force different ratios with an extra tooth (creating a tighter mesh) but I don't recommend this.

If you go up in teeth on the pinion, you need to go down the same number of teeth on the spur and conversely when you go down teeth on the pinion, you need to up the same number of teeth on the spur, because with the standard arrangement, engines/motor position is fixed.

Hope this helps.

 

[AngrySection8]

Personally, I would not be wanting to lower the gearing, which will reduce the load and encourage the motor to spool up to higher RPMs, potentially exceeding the envelope. If anything, I would gear up some, which would actually increase the theoretical top speed. But that's ok.

Another issue is that you can't run a 14t pinion with a 57t spur - it wouldn't mesh. You'd need a 60t spur - if they even exist.

On 12s 50.4v - 780kv = 39k rpm

Gearing: 17/57

64.42 mm/rpm (based on

39000 x 64.42 x 60 / 1000000 = 150kph
93.6 mph

On a purely hypothetical 14/57

That's an FDR of 1:9.768

Tyre circumference (based on 165mm diameter)
mm/rpm = 518.43mm / 9.768
mm/rpm = 53.074

39000 x 53.074 x 60 / 1000000 = 117kph
73mph

So whilst you are reducing the top speed of the rig with this ratio, a) it's not possible to attain given the Baja's gearing restraints and b) it would actually be making it easier to over rev the motor.

Gearing options for Baja should always add up to 74.

I know some people take liberties with this and force different ratios with an extra tooth (creating a tighter mesh) but I don't recommend this.

If you go up in teeth on the pinion, you need to go down the same number of teeth on the spur and conversely when you go down teeth on the pinion, you need to up the same number of teeth on the spur, because with the standard arrangement, engines/motor position is fixed.

Hope this helps.

While this is true for the gassers, I do believe the electrics have a motor mount with slotted holes to allow adjustment where as us gas guys are fixed in that department.

 

[Horatio]

While this is true for the gassers, I do believe the electrics have a motor mount with slotted holes to allow adjustment where as us gas guys are fixed in that department.

I think the newer conversions have slots, but I'm not sure what Rofuns come with.

Either way, I wouldn't be putting 14/57 if running 12S. Not unless AtomicSkull turned the punch down to about 25%

 

[Atomic Skull]

Another issue is that you can't run a 14t pinion with a 57t spur - it wouldn't mesh. You'd need a 60t spur - if they even exist.

It literally comes with a 57t spur and a 14t pinion:

When he got to the soldering part I was half-shouting at the screen "WHY AREN'T YOU USING FLUX!??" You gotta use extra flux for wire that big that what's inside the solder just isn't enough.

 

[Horatio]

It literally comes with a 57t spur and a 14t pinion:

When he got to the soldering part I was half-shouting at the screen "WHY AREN'T YOU USING FLUX!??" You gotta use extra flux for wire that big that what's inside the solder just isn't enough.

14/57 isn't possible with the Gassers, so I suppose the newer rigs have totally revised mounts etc.

The older converted ones basically used the gasser parts where gearing is fixed to:

17/57
18/56
19/54
20/53
21/52

Either way, 14/57 on a 780kv motor on 12s is encouraging your motor to over-rev, by reducing the load due to under-gearing.

The rollout, ratios, and theoretical top speeds are still applicable though.

 

[Atomic Skull]

Ok so after studying that video it seems the diff pinion is 30t and the diff ring gear is 48t, there is also an extra 20t gear between the spur and the diff pinion plugging the diff gear values and a transmission rate of 1.5 for the extra 20t gear and the 57T spur and 14t pinion into this calculator with a 780kv motor:

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

Shows a speed of 79.5 mph at 50.4v. So the 780KV is a no-go I need to return it and get the 580KV.

Also the stock 750kv motor in the RTR on 8S shows 50.96mph on a full 8S pack.

Dropping to a 580KV motor with the stock gearing shows 59.12mph on a full 12S pack.

 

[BashingBrian]

I’m running 8s and went up 2 teeth on the pinion if my memory serves me right, that’s with max5/800kv 56118, works fine as the electric Baja has slotted mount. It’s plenty fast enough for a huge 2wd car..
I keep 12s for my 4wd controllable cars

 

[Horatio]

Ok so after studying that video it seems the diff pinion is 30t and the diff ring gear is 48t, there is also an extra 20t gear between the spur and the diff pinion plugging the diff gear

You have this arse backwards.

The input gear is 20t.

The idler gears is irrelevant.

The diff gear is 48t. Why are you telling us this when you are the one that asked?

Gear box ratio is 48/20 =

1:2.4

values and a transmission rate of 1.5 for the extra 20t gear and the 57T spur and 14t pinion into this calculator with a 780kv motor:

The input (drive gear) is 20t

The diff gear is 48t

The reduction is 1:2.4

1:2.4

The idler gear does not alter the transmission ratio in the slightest, so you're plugging incorrect numbers into a generic calculator. Needlessly so - because essentially, you've completely ignored accurate numbers already in front of you.

You and only you will know what pinion and spur you want to use. I calculated it for you, even though I erroneously thought that it is not possible to achieve that ratio. Given that newer electric versions have adjustable motor mounts, 14/57 is possible, but you wouldn't want to use that ratio for the reasons already stated.

14/57

Is 1:4.07

4.07 x 2.4

FDR is 1:9.768

This has already been posted above.


What are you trying to achieve here?

Establishing whether the 780kv motor will exceed the performance envelope in an 8S Baja, if you run 12S with the smallest pinion?

Answer: Possibly.

But why would you run such a low gear ratio with all that surplus torque/HP on tap in the first place?

Again, it's a solution in search of a problem.

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

Shows a speed of 79.5 mph at 50.4v. So the 780KV is a no-go I need to return it and get the 580KV.

Also the stock 750kv motor in the RTR on 8S shows 50.96mph on a full 8S pack.

Dropping to a 580KV motor with the stock gearing shows 59.12mph on a full 12S pack.

The actual, most accurate theoretical speed for 50.4V on the 780kv motor using 14/57 is 73mph Facts. (Real world after losses and sag would likely be closer to 70mph - if you could hang on to it, which is highly unlikely).

8S = 33.6 x 750kv is 25200rpms

14/57 = 4.07
FDR = 9.77
165 tyre diameter
518.43 circumference
Mm/rev = 53.06

25200 x 53.06 x 60 / 1000000 = 80.23 kph
49.85 mph (Based on 165 mm diameter tyres)

(Which coincidentally is almost exactly the same speed as my Fuelie 26 running 17/57 and hitting 20500rpms)

Take home.

Problem exists between keyboard and chair.
ID 10 T error.



Conclusion:

If you plug in numbers that are wrong into a general RCTalk calculator, you'll get 'wrong' out.

But also, if you confuse trying to reduce top speed with preventing a motor over-revving, (by lowering the gearing), you're going to encourage the outcome you're attempting to prevent.

If you don't want to over rev your full-size car engine - don't drive around in first gear for the entirety of your journey.

 

[Rotauq]

If you gonna return the motor, maybe take a different brand that doesn't sit so low on max rpm....

My 650KV SSS motor has a limit of 45k rpm

 

[Atomic Skull]

It's not that easy to visualize how the transmission works when you're not holding it in your hand and are just looking at pictures of gears and it's different than anything you have used before because you've never owned a 2WD R/C car. Well maybe it is for some people, but not for me.

Now that I "get it" the solution still seems to be:

Exchange the 780KV for the 580KV motor (I have at least a month till the slow boat delivers the car so plenty of time)

Go up on the pinion by 1 tooth if the stock pinion is too low (no harm in real world testing first)

Is this right?

 

[Atomic Skull]

Yes, I've often toyed with that idea. Albeit most Baja tyres aren't belted, so you need to be careful not to shred them on a 6S brushless rig.

Gorilla Ultimate tape is amazing, on 140mm tires a double layer eliminates 95% of ballooning even on very soft tires like the soft truggy tires I recently did. Unlike normal gorilla tape is has a crosshatch of very strong filaments and twice the amount of adhesive, it will not tear and has to be cut with a knife or scissors.
I tried a single layer at first but it still allowed the tire to form a "D" shape at high RPM to some extent though it was not terrible. Doubling it eliminated this.

Also, Gorilla Ultimate is NOT gaffer tape, the adhesive seems to bond permanently to rubber (it is a rubber based waterproof adhesive) and if you forcibly pull it off (not easy) it will leave the adhesive behind.

 

[Horatio]

It's not that easy to visualize how the transmission works when you're not holding it in your hand and are just looking at pictures of gears and it's different than anything you have used before because you've never owned a 2WD R/C car. Well maybe it is for some people, but not for me.

Now that I "get it" the solution still seems to be:

Exchange the 780KV for the 580KV motor (I have at least a month till the slow boat delivers the car so plenty of time)

Go up on the pinion by 1 tooth if the stock pinion is too low (no harm in real world testing first)

Is this right?

The solution would be keep the 780 motor and use it to the death, but use appropriate gearing. Ie - not 14/57.

You'll then be very unlikely to hit 30000 rpms unless you're running at ROSSA.