External BEC needed for RX8 high voltage/Mech brakes? - RC-Monster Forums

Aceldama · 09:40 AM

I'm having a strange issue with my truggy setup at the moment maybe someone could help diagnose:

I'm running 6s lipo and mechanical brakes.

Under hard braking it seems that the steering servo takes a second to 'recover' as if it's not getting enough voltage. I can steer excellent off throttle/brakes. But if I"m using brakes/throttle (mostly brakes) during steering, or try to steer immediately after I don't get the full range.

Anyone else seeing this problem that has a similar setup to mine? This behavior is easily observed on a high grip surface like a parking lot. My turning radius is easily double for a second or two after hard braking.

himalaya · 05.14.2009, 03:04 AM

maybe you could share some info like what model you are talking about ? RX8 ?

Aceldama · 05.14.2009, 09:40 AM

Good point! It's an RX8 combo and I've edited the title.

TekinTeamMgr · 05.17.2009, 08:12 PM

I also run mechanical brakes and dont' have this issue. Have you tried another brake servo?

You should not need a seperate bec for this application. I have had a JR servo take a LOT of extra current right before it gave up ...

Aceldama · 05.18.2009, 09:41 AM

Quote:

Originally Posted by TekinTeamMgr

I also run mechanical brakes and dont' have this issue. Have you tried another brake servo?

You should not need a seperate bec for this application. I have had a JR servo take a LOT of extra current right before it gave up ...

Out of curiosity are you running on 6s lipo like I am? I thought maybe since the BEC has to work harder to switch down from 25v it might be a contributing factor.

For the record here are the servos I'm running:
Steering Servo
Brake Servo

Maybe I could get a smaller brake servo because I can reproduce this problem each time. Can't get full range out of the steering with the brakes on.

TekinTeamMgr · 05.18.2009, 12:04 PM

I don't run 6s often but have tested with it and didn't have this issue. It's entirely possible that your is an isolated issue. I'll get with the guys and test it. I'll see if I can find some servo's with those specs....

BrianG · 05.18.2009, 12:07 PM

Could have simply been a stalled servo. Just like any motor, if the serco was somehow jammed (rock or other heavy load), lots of current can flow. I'd be more interested in knowing what kind of current limiting there is on the BEC or how it handles high currents (does it shut off, cycle on/off like many switching circuits do, burst into flame

, etc).

Aceldama · 05.18.2009, 01:11 PM

Quote:

Originally Posted by BrianG

Could have simply been a stalled servo. Just like any motor, if the serco was somehow jammed (rock or other heavy load), lots of current can flow. I'd be more interested in knowing what kind of current limiting there is on the BEC or how it handles high currents (does it shut off, cycle on/off like many switching circuits do, burst into flame

, etc).

I'm thinking it might be heavy load on the brake servo that causes the issue. I have my EPA set to a reasonable level but I'm going to try tweaking it a bit to see if I can make the problem go away. I can reproduce it every time with my current setup.

Some tests I might try:
-Disconnect brake servo completely, make sure my full steering range is there.
-Connect brake servo but disconnect the brake linkage
-Reduce the EPA on the brake servo
-Change the brake servo to a weaker one.
-Maybe even try running my existing setup on 3s to see if it's an issue with switching down from 6s.

Maybe my brake servo is just extreme overkill for this application, it didn't look like it on paper.

BrianG · 05.18.2009, 01:15 PM

Actually, changing to a weaker one may be worse. A stronger servo will be able to move the linkage easier, while a weaker one may be stalled and actually pull more current.

Running a switching BEC on lower voltage makes for reduced efficiency. I've never tested the Tekin BEC specifically, but all other BECs I've tested (Koolflight, CCBEC, DimensionEngineering, etc) tend to operate more efficient (and cooler) with higher voltage.

Aceldama · 05.18.2009, 01:27 PM

Quote:

Originally Posted by BrianG

Actually, changing to a weaker one may be worse. A stronger servo will be able to move the linkage easier, while a weaker one may be stalled and actually pull more current.

Running a switching BEC on lower voltage makes for reduced efficiency. I've never tested the Tekin BEC specifically, but all other BECs I've tested (Koolflight, CCBEC, DimensionEngineering, etc) tend to operate more efficient (and cooler) with higher voltage.

Interesting, now I have no idea what the hell to do. lol.

I was thinking of running a seperate BEC for the steering/brake but I'm not sure how that would interact with the existing BEC on the RX8 since I don't think it can be disabled.

TekinTeamMgr · 05.18.2009, 09:44 PM

If you were to run a seperate BEC simply remove the red wire from the RX8 harness.

George16 · 05:49 AM

That's the biggest problem I had with those Bluebird servos. They are slow to recenter and tend to get stuck momentarily on each turn. I had relayed this to the manufacturer from Taiwan. I'm on the ship so I can't access my home email to check his address. By the way, I returned about 3-4 BMS660MG, BMS 661MG, and 630MG's. I now used Hitec/Futaba/KO Propo servos on my trucks.

They are cheap and you get what you pay for.

Aceldama · 05.19.2009, 10:12 AM

Quote:

Originally Posted by George16

That's the biggest problem I had with those Bluebird servos. They are slow to recenter and tend to get stuck momentarily on each turn. I had relayed this to the manufacturer from Taiwan. I'm on the ship so I can't access my home email to check his address. By the way, I returned about 3-4 BMS660MG, BMS 661MG, and 630MG's. I now used Hitec/Futaba/KO Propo servos on my trucks.

They are cheap and you get what you pay for.

I'll keep that in mind but that's not the problem being described in this thread.

Aceldama · 06.02.2009, 10:32 AM

Just an update:

I haven't replaced the servo, I've tried changing the linkage up a bit but the problem still persists. I went out to a big parking lot yesterday to really wind it out and test this problem and here's what I came up with:

-Top speed run
-Heavy braking
-While braking, turn steering in either direction. The steering radius and response is drastically reduced.
-At this point you can release the brake
-Throttle has no response for a second or two and then kicks back on, almost like an LVC. Same with the steering.

I can reproduce this over and over. It feels like the LVC is kicking in, which is odd since I have mine set to 19v.

If I disconnect the brake servo this problem goes away (obviously).

I'm going to try leaving the brake servo connected but I'm going to disconnect the linkage and try again. I'll report back.

BrianG · 10:42 AM

You might want to check/replace the servo. I had a Hitec steering servo that would operate fine on the bench, but in the vehicle, it would sort of jam sometimes. Inside the servo case, there are several gears, and they ride on "axles". Well, it turns out that one of the axle holes had elongated and was causing a bad mesh internally, but only under a decent load. Since it was a MG servo, I just replaced the servo case and it worked fine thereafter.

Not saying this is exactly the case here, I'm just saying it could be the servo itself for whatever reason. If the servo is jamming and/or stalling, the high current flow could be dragging down the BEC voltage to the point where it is causing odd behavior in the ESC brains.