Kraton Kraton 8s EXB Motor Selection

[Slo1]

I recently purchased a Kraton 8s EXB (new old stock) and I’m looking into the ideal motor. I started with the Firma 160A/1250kv combo, but began getting concerned about the longevity of the 5mm shaft. I’ve seen posts where the shaft twists off. Given that I don’t want to destroy the motor, I purchased a Firma 800kv to pair with my setup.

At first, I wanted to replace the whole setup with a Castle MX8s combo, but I didn’t want to spend too much. Additionally, cool running temps are critical (under 150F) to me so I thus avoided the 1100kv Firma. Running the 800kv on 28/47, I’m surprised I can get my temps up to 155F with 85F ambient temps. I was street bashing with hard WOT pulls and drag braking down the street (on Bandito MFT). At this point, I’m a bit unsure if something is wrong. People are running 1:1 gearing and seemingly getting very cool temps (under 130F). Should I get an even lower Kv motor? What if I move up to the 1100kv Firma?

Additional Notes:
I will add that my truck cannot wheelie. I’m unsure if this is due to the lack of grip from the Bandito MFT, or if my truck genuinely is just “weak”.

Also need to add that the center diff was rebuilt with fresh 20 million. The factory fill was low to say the least.

 

[SrC]

I don't have an 8s rg....
But it seems the 800kv motors run great/best with that platform. Perhaps gearing should be fine tuned better? And Not all 800kv motors run the same. Actual Kv can vary from one to the other. Advertised Kv Specs can vary by brand, size, and mass of the motor Can and Stator. When I test BLX motors on my Motor analyzer, they do vary. Even with 2 identical Motors. Food for thought.
You generally need taller gearing the lower your KV's are. If your setup appears "weak", you might just need a diff. pinion. You need to find the best RPM/Torque range for your 800 Kv motor. Having a range selection of pinions on hand is best. Changing up or down 2T at time. Finally you can tweak it best with just 1T change Up or Down.
Your temps appear to be in the ball park Ok.
If you say others are running cooler, no setup is identical to the next guy. And anothers Ambient temps also make it subjective to what cool actually is.
A motor Fan setup may be in order, IDK.
But in general, 800kv motors should run cool without one needed. But again , ambient air temps dictate. And Higher humidity , I find even runs hotter. From my exper.

 

[dure16]

800kv on 28/47

Your motor is heating up because you are geared too low.

Install the 39T spur in the bag with your instruction manual. A 39T pinion would be ideal but your ESC won’t be able to handle that, so I’d start with ~34T pinion.

And get some fans on the motor.

 

[Slo1]

Thanks all, picked up the SkyRC dual motor fan. As far as gearing, I guess I’ll gear up? I’m used to gearing down for cooler temps, but since I’m so far off from everyone else’s gearing.

 

[Moirae]

Thanks all, picked up the SkyRC dual motor fan. As far as gearing, I guess I’ll gear up? I’m used to gearing down for cooler temps, but since I’m so far off from everyone else’s gearing.

That's how it normally works but the reverse is true if you're geared so low you spend all your time wot. With more gearing you will be faster without pushing the motor as hard as less wot to get the same speed. I think that's how it works anyway

 

[Slo1]

I prefer acceleration and wheelies more then top speed, but I know gearing too low will cause issues for those too. I’ll switch over to a 30/39 gearing setup tonight. Fingers crossed, I just want to get this truck dialed in.

As a side note, was my concern for the 5mm shaft sheering off valid?

 

[dure16]

As a side note, was my concern for the 5mm shaft sheering off valid?

Kind of. It was common enough for Arrma to make a change and start including the 8mm version in their RTR trucks.

It might've worked just fine for you though. But you'll enjoy that big motor.

 

[Slo1]

Did a quick run with a higher gear ratio (30/47). Moderate 7minute run and motor temps were ~135F. We seem to be moving in the right direction! I’ll swap out for a 30/39 tonight and try again. GPS speed test show the setup tops out at 44mph. Ambient temps 83F overcast.

Admittedly I was pinning the throttle and brakes a lot less, perhaps that had to do with the lower temps? I also switched back to the CH2 tires. I still can’t pull full on wheelies. The front will blip up for a second and fall back down. An additional thing I noticed is the brakes aren’t very smooth it’ll pull nicely and suddenly jolt (cogging?).

 

[SrC]

Thanks all, picked up the SkyRC dual motor fan. As far as gearing, I guess I’ll gear up? I’m used to gearing down for cooler temps, but since I’m so far off from everyone else’s gearing.

If you are way too undergeared you are maxing out the RPM's way too soon and sustained. Hence heat.
Find that sweetspot pinion. Find the best torque range.
No need to max out a motor's RPMs/ over reving it constantly when bashing. You generally lose torque as you approach max rpms. Best torque is generally somewhere in the middle RPM range. Not a hard and fast rule, but generally holds true for many motors.
With an 8s pack/33.6v, that 800 kv motor would max out at 26,880 RPM's. (at no load) There is plenty of grunt/torque available if you gear it well.

Edited.

 

[Slo1]

If you are way too undergeared you are maxing out the RPM's way too soon and sustained. Hence heat.
Find that sweetspot pinion.
No need to max out a motor's RPMs/ over reving it constantly when bashing.

Admittedly, I’m used to setting up my rigs to where I can pin the throttle all the way down. It’s been fine for me thus far, but perhaps 1/5 rigs may be a different beast?

I setup my rig on 28/39 tonight and it’s bat …crazy. We had night temps of ~73F and I had to run in my backyard. Overall, temps topped off at ~120F after a 10minute run. Now this is much more like it. Though with the rig so fast I could never more than just blip the throttle. Once the motor fan comes in, hopefully I can sustain this temp.

Thank you all again
Photo of the rig in question

And of my past bashing rig, 45+mph all day long with 140F max temp even in 90F+ weather. Wheelie for days and admittedly much more lively then the K8S, understandable considering how much lighter it is.

 

[Moirae]

Admittedly, I’m used to setting up my rigs to where I can pin the throttle all the way down. It’s been fine for me thus far, but perhaps 1/5 rigs may be a different beast?

I setup my rig on 28/39 tonight and it’s bat …crazy. We had night temps of ~73F and I had to run in my backyard. Overall, temps topped off at ~120F after a 10minute run. Now this is much more like it. Though with the rig so fast I could never more than just blip the throttle. Once the motor fan comes in, hopefully I can sustain this temp.

Thank you all again
Photo of the rig in question
View attachment 318489

And of my past bashing rig, 45+mph all day long with 140F max temp even in 90F+ weather. Wheelie for days and admittedly much more lively then the K8S, understandable considering how much lighter it is.

View attachment 318490

I bet you can wheelie now!

 

[Slo1]

Also this video while in Korean, provides great insight into the specs and performance of the new K8S EXB. Considering the new K8S/O8S EXB run a 23t pinion w/ the 1100kv motor, the trans is out driving at ~5,700 rpm on 8s. With my gearing of 28/39, my trans outdrive rpm is now ~6,000rpm closely matching the new stock setup and should hopefully be doable by the larger 800kv can. It’s all coming together now!

 

[SrC]

I also like to set up my rigs where I can Utilize the full Throttle trigger range as needed. I don't like that hair trigger power myself.
The 8s line are a 30+ pound platform. Getting the most torque out your motor matters here.

 

[Slo1]

I also like to set up my rigs where I can Utilize the full Throttle trigger range as needed. I don't like that hair trigger power myself.
The 8s line are 30+ pounds. Getting the most torque out your motor matters here.

Agreed motor temps and overall performance come first. I’ll likely set and EPA limit to prevent myself from getting into trouble

 

[SrC]

What works for you.
Makes sense.

 

[Slo1]

Ok, the wheelies are here (at least on grass)! I upped the gearing last night to 32/39 and it wheelies off the line which is great. In 95F ambient temps, the motor (no fan) was 125F, batteries 105F, and ESC 135F. I ran for 8minutes with no body on so temps will rise once I add the body.

My goal now is to lower the gearing as much as possible while keeping wheelie capability. Perhaps also look at a better fan for the esc

 

[Moirae]

Ok, the wheelies are here (at least on grass)! I upped the gearing last night to 32/39 and it wheelies off the line which is great. In 95F ambient temps, the motor (no fan) was 125F, batteries 105F, and ESC 135F. I ran for 8minutes with no body on so temps will rise once I add the body.

My goal now is to lower the gearing as much as possible while keeping wheelie capability. Perhaps also look at a better fan for the esc

haha man you're funny, those temps look great to me. Especially with no fan?!?! if you added 2 fans to that motor it probably wouldn't have been over 100-110 in the same time frame.

 

[Slo1]

haha man you're funny, those temps look great to me. Especially with no fan?!?! if you added 2 fans to that motor it probably wouldn't have been over 100-110 in the same time frame.

I know my weakest link at this point is the esc so just trying to keep it happy. Though yes, overall, I’m happy with the temps . I was about to throw in the towel yesterday and just buy the 1100kv. Seeing the new EXB reviews, it appears the 1100kv is quite peppy on 23/47 (stock gearing), but it appears it has less headroom to gear up. While I’m unlikely to (~45mph is plenty for me) it gets hot where I live. Summer temps normally stay at 90F+ during the day with hot days topping out at 110F.

Another reason for my lower gearing target is apparently you can puff your lipos on 1:1 . I’m running mid grade CNHL 6200 100C packs and they aren’t top shelf lipos like the ones you guys run.

 

[Moirae]

I know my weakest link at this point is the esc so just trying to keep it happy. Though yes, overall, I’m happy with the temps . I was about to throw in the towel yesterday and just buy the 1100kv. Seeing the new EXB reviews, it appears the 1100kv is quite peppy on 23/47 (stock gearing), but it appears it has less headroom to gear up. While I’m unlikely to (~45mph is plenty for me) it gets hot where I live. Summer temps normally stay at 90F+ during the day with hot days topping out at 110F.

Another reason for my lower gearing target is apparently you can puff your lipos on 1:1 . I’m running mid grade CNHL 6200 100C packs and they aren’t top shelf lipos like the ones you guys run.

If it makes you feel any better, I probably gear higher than 99% of the people on this forum on average and I have like 7 1/5th scale rigs that can all do 60+ mph and I've never ever ever ever never ever puff'd a lipo in a rig. I run CNHL in a lot of them too. A lot of people here with arrma 8s run 800kv and 1:1 gearing and have never puff'd a lipo. That's definitely not something you have to worry about with a hobbywing or firma motor.

You are good!!

 

[Slo1]

If it makes you feel any better, I probably gear higher than 99% of the people on this forum on average and I have like 7 1/5th scale rigs that can all do 60+ mph and I've never ever ever ever never ever puff'd a lipo in a rig. I run CNHL in a lot of them too. A lot of people here with arrma 8s run 800kv and 1:1 gearing and have never puff'd a lipo. That's definitely not something you have to worry about with a hobbywing or firma motor.

You are good!!

Just super excited to really drive this truck! It’s been over a month since I’ve bought the roller with just small test runs. This weekends gonna be fun

Thanks again Moirae and all the AF members.