Project: Flux XL

[Jahay]

i curretnly have 50k fluid in my centre diff, and it looks like i am going to have to put at least 100k in to stop the front wheels unloading. But maybe because of my longer chassis, i wont experience many wheelies so my front will stay down and maintain traction... hmmm wont know till i try it...

Truck looks great in the vid and seems to be performing nicely! any furthur plans for it apart from heavier diff oil for the CD?

[_dV]

I have been having a heat problem with the motor.

After the first decent run i gave the truck i thought the temps of the motor\ESC\battery where all good. I didn't see a motor temp above i think it was 38c on that run. Ambient was about 18-19. Slipper gearing was 16\58 (mod 0.8). scriptasylum calcs were
Max speed: 44.55 mph (71.56 km/h)
Total reduction ratio: 11.68056 : 1
At the end of the run i stripped the delrin mod 0.8 spur so subsequent runs are mod 1.

The next run (the one the video was shot on) i didn't check temps except by using my hand. Wasn't paying much attention but i can remember the motor feeling quite warm to the touch. Slipper gearing was 13\46
Calc
Max speed: 45.64 mph (73.31 km/h)
Total reduction ratio: 11.40171 : 1
The run ended with the pinion coming loose and somehow i managed to lose it

So next run was a short one after a day at work. Used the 14t pinion i had and checked motor temp towards the end as i though it might be slightly overgeared(ie too high a top speed). Motor temps were around 60c and ambient probably ~17c.
Max speed: 49.15 mph (78.95 km/h)
Total reduction ratio: 10.5873 : 1

So today i did 4 runs to try and find out what was happening. Ive always read that in an overgeared system (ie too high a top speed) the ESC will run hot and if its really overgeared motor\ESC\battery will all get hot. After today i'm pretty confused about that.

Motor: Tekin T8 4038 1550kV
ESC: Tekin RX8 with fan
Battery: HK Flightmax 5s 5800mAh 30C
Truck weight: 200-300gms lighter than a stock flux. About 3722gms without wheels\battery\body.

Run 1 - confirm heat issue with 14 pinion
Slipper gearing: 14\46
Ambient temp: ~18-20c
Motor temp max: 61c (stopped at this temp)
ESC temp: 38c (fan coming on)
Battery temp: 33c

Run 2 - change to 15 pinion
Slipper gearing: 15\46
Ambient temp: ~18-20c
Motor temp max: 62c (stopped at this temp)
ESC temp: 39c (fan coming on)
Battery temp: 34c

Run 3 - change to 20 pinion (obviously way overgeared)
Slipper gearing: 20\46
Ambient temp: ~18-20c
Motor temp max: 71c (got to that in about half the time of prev runs)
ESC temp: 41c (fan on a lot)
Battery temp: 36c

Run 4 - back to 14 pinion but with sensor cable pulled out of ESC ie forced sensorless
Slipper gearing: 14\46
Ambient temp: ~20-22c
Motor temp max: 61c (stopped at this temp)
ESC temp: 39c (fan coming on)
Battery temp: 34c

Some things i've checked
- drive train moves freely with pinion off motor.
- slipper mesh is good. spur can move slightly back n forth without moving pinion.
- thought maybe sensored mode was permanently engaged. have read sensored systems can run hotter. so did the forced sensorless run. AFAIK with the cable plugged in the system runs in dual mode, sensored at low speed and sensorless at higher speed.
- wheel\tire combo used for all runs was close to 300gm per wheel which is same as stock weight.
- highest slipper temp i checked was 41c. thought it might have been hot and transfering heat to motor somehow.
- ESC settings all seem ok, haven't changed timing from default. It's running in brushless delayed reverse mode with no current limit.

So i'm not sure what to try next. I not sure why temps were so good on the mod 0.8 run, i was taking it a bit easier and there was no grass.

Total reduction is quite low compared to stock but i am running a much lower kV motor. I would have thought gearing for around 45mph would have been reasonable for that motor with 5s?

Any suggestions\comments much appreciated, cheers.

[Jahay]

I know exactly what you mean! i thought that running a lower KV motor with a larger can would result in lower temps whilst running gearing for higher top speed. But i found this was not the case.

I am running 6s geared for around 53mph with my 1600kv motor and temps are just about fine. I will be running 11/46 and i will save 13/46 for only road use.

I was hoping to have speeds of 60mph on 6s with my more powerful motor! but this is not the case, and in order for me to reach those speeds, i will need o run 8s or more which is fine by me.

It does seems that you have slightly over geared the motor for the number of cells you are running... may start using 6s?

[suicideneil]

By many accounts, the tekkin motors arent as powerful as you might imagine, they would be working hard in a savage sized/ weight vehicle- 5s is a little low for 1600kv motor geared for the higher speeds there too; 6s would definately be better, but a change of motor might be in order too- 1518 1800kv castleneu being my choice..

[_dV]

Thanks for the comments guys. It seems that for a vehicle of a certain weight with a motor of a set physical dimensions there will be a gearing range thats viable and that range can't be shifted much by altering motor kV or battery voltage.

I was hoping that my lighter build with lower kV motor and higher voltage batteries would be ok with the lower gearing. The motor has no probs moving the truck, it accelerates great but apparently the gearing is just too low for a motor that size.

So what to do. I'm gonna try going back close to the stock Flux gearing, using the same motor and using a 6s battery to keep the speed up. Had been considering using 6s for more efficiency and there seem to be more readily available options than 5s. I can fit almost any battery and it should only way about 100gm more. There are a few nice Turnigy\FlightMax options. For the spur I'd like to try a 65t mod 0.8 spur but a steel one isn't currently offered. I've posted in the Slipperential forum to see if Mike might provide one.

Wouldn't mind trying a larger motor as neil suggested but i would have to rejig the TVPs for the longer motor as there virtually no room at the back of it atm. Maybe later on when i have the CAD stuff complete and i can get them cut instead of having to remake by hand.

Least i have the suspension fairly well sorted. I went back to the original pink springs with tubes and 400 centistoke(40 wt) oil. Maybe a touch soft for large jumps but they seem like good general offroad settings.

Also found that there are diff fluids available between 100K and 500K. 300K looks like a good candidate.

[_dV]

Finally received all the parts for the regearing.
- Turnigy 5000mAh 30C 6s LiPo
- Novak 10,11 and 12 tooth pinions
- 52t metal mod 1 spur gear for slipperential
- also a selection of heavy diff fluids

Can't say i was too impressed with the weight of the 52t slipper spur. Zero effort made to keep it light. It weighs exactly twice as much as the 46t metal spur. Oh well, at least it fitted in the truck nps, only had to widen the cutout in the diff mount plate a little.

Eventually decided to go with 500K diff fluid over the 300K and i'm glad i did. It can still turn the diff by hand. Requires much more effort than 100K but its far from being locked. First run i did the slipper wasn't tight enough so i couldn't tell what effect the fluid was having.

Temperatures were much better. Problem solved i think. Max motor temp was 44c, ESC was about the same, battery 23c. Ambient was around 18c. Used the 11t pinion which results in Total Reduction Ratio of 15.23 with stock Flux 20t pinion being 15.7. Speed calcs to 41 mph (65.85 km/h) and felt plenty fast enough for bashing.

Was very glad i had the skid plate on for that first run. Had some nice 5 foot or so jumps happening off a small mound of dirt but the landing area was a bit rough with large chunks of very hard dirt. Found a depression in the front of the skid plate after i'd finished the session. Must have landed square on a hard chunk of dirt.



My initial thought was that the MDf might be too soft, until i pulled it off the truck.




Must have been some impact to break that. I used plastic braces so they would flex but i'm still glad it broke instead of passing the force of that through to the rest of the truck.

[_dV]

I'm having a bit of down time atm with the truck due to a freak accident so I thought I would post up comments about how some aspects of this build have worked out in practice. I have about 15 full runs up on the truck in various environments including:- an offroad track, grassy parks, various dirt offroad locations and a couple of skate parks.

Comments are based on a running setup of:-
- 52t mod1 spur, 11t pinion. (total reduction ratio 15.2, max speed 41.26 mph (66.28 km/h))
- 500K slipper fluid, 50K front\rear diff fluid
- Turnigy 6S 5000mAh 30C Lipo
- Tekin T8 Truggy 1550kV, RX8 with 2.08 software

Shocks Tower Supports
Really wasn't sure how these would go. I was concerned they would make the towers too stiff and cause damage on impact. Was also anxious about how it would affect the suspension. Turns out that so far they are a highlight of the build for me.The tie rods pivots allow the tower to flex a bit in a few directions while preventing the whole tower from bending backwards. The tie rod ends also have a little flex in them. This seems to allow impacts to be absorbed ok. As an added bonus the top shock mount bolts don't bend anymore. These would usually bend after a few runs. I've had some pretty decent crashes and they looked ok. I wasn't convinced so i took one front and one rear out this morning and they were still perfectly straight.

On the suspension side of this I have had to dial back the shocks from what I thought I would need. The supports really make a difference to how a stiff a suspension setting feels. Handling feels sharper to me and
the truck appears to handle landings from height better.

I have been considering moving the receiver in behind the front shock tower to tidy things up a bit now that i have confidence in the supports.

Battery\Motor\ESC
Runtime is 30-50 mins depending on how I drive. For bashing its usually towards the latter.
Temps, running in mostly 22-25c ambient, motor temp max I have seen is 54c which is great. ESC fan comes on for short periods, temp on it is usually around 34c, battery mostly reads 32c. Happy with the temps atm, will see how they go in the Australian summer.

There were 2 things that annoyed me with the ESC. They were the braking strength and the long'ish delay time for the reverse but both of these have been fixed with the 2.08 software.

Custom Chassis
The 5083 H321 grade Al has held up well. Don't think it is as hard as the 6061 but it does the job. It's very stiff when all bolted up.

The chassis was center balanced when i was running 5S. The 6S battery is 93 gm heavier but the rear end of the truck handles better now so it seems a little weight towards the back is better for this build.

Motor Mounting
Had my doubts about the Tekno mount and if it was suitable. I thought the mounting bolts might slip being bolted on the side of the TVPs but this hasn't happened once. I wasn't too keen on only bolting the motor at the front to the mount but i was able to use M4 bolts and the motor fits snugly in the mount, no probs thus far. It seems well supported.

Maintenance
Truck is night and day to work on compared to a stock Flux. I haven't had to remove the TVPs since the final assembly. With the exception of having to remove the skidplate first, any component inside the TVPs can be removed by undoing the bolts that are easily accessible from the outside of the TVPs and then just pulling it out. Gear mesh is a breeze to adjust. I assembled\disassembled the truck a lot during the build and its much quicker than the stock Flux, no real fiddly bits or stuff you have to do in the right order etc.

MDF Skidplate
Holding up ok, definitely need one at skate parks. It's getting beatup a bit quicker than I thought. If I can find some 3 mm plastic\acrylic sheet I might try that at some point.

The grinding on the sides happens at skate parks. Yes that is part of the RX8 you can see, did i mention freak accident?

Steering
Have had no issues with the reversing of the steering link. Hitec 7955TG + bearings + FLM drag link + hellfire tie rods + HD servo saver spring has resulted in a strong and responsive steering setup. I was considering purchasing an all Al steering replacement before but not now.

I wouldn't mind a slightly better turning circle. Might try the RSC kit or grinding existing parts at some stage.

Wheels
Stock Flux tires on inner bead of Rulux half offset halfups. The half offset works well with the lower profile, no probs with the truck wanting to roll over sideways a lot. Grip seems a bit better in some situations than the stock tires on normal sized wheel but worse in others. I like the diameter\weight of these tires but not the grip so I will be trying something else. On the plus side of less grip, on a loose flat surface I can hold a pretty good power drift with them and that's quite fun

Air Control
Not quite as responsive as stock length chassis but good enough. No probs at all getting it to land on all fours. I suspect this is due to the low weight. Running weight with 6S is still under stock Flux with 4S. On the outing of the accident I did my first few backflips off a short curved ramp at a skate park. Ramp was only a couple of feet high and I wasn't getting that much air but it was enough to get the backflip out. Keen to try for a double now on a larger ramp

Diff Fluid
50K front\rear, provides for nice turning circle but feels a bit light in loose offroad conditions. Somewhere in between 50K and 100K would be nice to try though I might have to mix it up myself.

500K center diff, this gives a similar feel to the stock flux direct front\back drive but not as sharp. It was interesting to play with but I have no real interest in using the center diff to keep the front wheels on the ground. I'd rather have the sharp 4WD response and use throttle control. I might experiment with Ofna diff lock fluid or a different center diff with a locker inside to see how that feels. Pretty happy with how it is atm though and I can hold long wheelies without too much trouble.


Post is already way too long, maybe some more later. Having run the truck a fair bit I now have a heap of ideas for tweaks\improvements. Looking forward to trying them out.

[Jahay]

thanks for reporting back on your setup...

One thing that caught my eye is the 500k centre diff oil weight.
At the moment im using 100k and i feel i am unloading too much to the front. Do you know where i could get 250k? i cant find weights this heavy?

Thanks

[_dV]

Kyosho make some heavy stuff, 100K, 300K and 500K. Amain

Ofna make a 'Diff Lock Lube'. I have some but haven't tried it. It looks like normal silicon diff fluid but it's viscosity is like sludge. It's heavier than 500K ..i would guess at around 1000K.

[thzero]

What we really need is for some genius to come up with a reliable LSD for the RC vehicles.

[Jahay]

an LSD would be awesome and would make even the biggest trucks fun to drift and handle extremely well!

How hard would it be to do something like that?

[Bmr4life]

You all referring to a center diff?

[Jahay]

centre diff would benefit from an LSD, but i think just finding an appropriate diff oil for the centre would be fine...

but LSD for front and rear would be great... Stop the inside tyres unloading when turning

[thzero]

Nah, LSD for all three would be ideal. That way the power is getting sent to the tires with the most resistance, i.e. traction usually. The LSD is basically the inverse of the current diffs current diffs, which are simple viscous diffs, sent power to the tires with the least resistance, i.e. diffing out.

Locking a center diff is the same as running a Savage gearbox; 50/50 power split between front and rear. Just the center diff is more compact and doesn't reduce the gear ratio as much. The higher viscosity of oil you use the closer you get to a locked diff.

LSDs are fairly complex, so I don't know how difficult it would be. The issue would not be minaturization itself I would suspect, but getting a smaller scale version that can handle the power outputs (which are NOT scale) that we are delivering these days. Also would be manufacturing costs vs resale costs. The simple diffs we use are very easy to manufacture and assemble (for RTR purposes).

Originally Posted by Jahay

centre diff would benefit from an LSD, but i think just finding an appropriate diff oil for the centre would be fine...

but LSD for front and rear would be great... Stop the inside tyres unloading when turning

http://compare.ebay.com/like/3004124...=263602_304662

LSD for 1/8 applications ;)

your welcome ;)