LRM brushless gearing/FDR question

[pmathews924]

So my brushless set-up finally showed up and now I'm even more confused about applying it to the car...I went to the local track to pick up a longer sensor cable and asked around to see what guys were running for an FDR and the consensus seemed to be around 6.5 or so. Most of the cars out there are newer AE buggies, so I asked for FDR instead of trying to figure out the rest of the math. I don't know why, but that seems super low to me unless I was trying to go 50+mph. Here are the options I'm working with, 86t spur...

17t = 11.03 FDR
18t = 10.42 FDR
20t = 9.37 FDR
24t = 7.81 FDR
28t = 6.70 FDR

So, basically...I'd have to throw the biggest pinion I have in the car to get close to what they're running, but I can't help but feel like I'm comparing apples to oranges (new vs. vintage tech). Here's the rest of my set-up...

GensAce 7.4v 4000mah 2s LIPO
17.5t Speed Passion V3.0 Competition Motor
SP Reventon Pro ESC
- Timing is @ 18.75, but I can crank it up to 26.5, or drop it back down to zero
- Throttle response is cranked up to 9...most aggressive on/off setting available
- Throttle neutral range...not even sure how this would impact it, but if you moved the max throttle point closer to zero I would think it would make somewhat of an impact

Do I want to run the timing lower or higher? If I'm understanding this stuff, lower timing would give me more torque (more punch off the line), a cooler running motor, but I would sacrifice top end speed and RPM. If I'm gearing the motor higher (say 86/28) I'd lose torque and increase motor heat, so wouldn't you want to drop timing to offset that?

Or...should I jack the timing all the way up to max out the power from the motor, and compensate for the loss of punch with a higher FDR?

Or...did I just buy a crappy motor? I've read story after story of guys dropping a brushless motor in their vintage ride and it pops wheelies like crazy. I haven't pushed the motor as far as it will go yet, and if I dropped lower and lower pinions in there I'm sure it would get there but I'm not seeing the crazy power.

[DerbyDan]

The thing to remember with a brushless motor is that by being effectively three phase, they can can output more power with more load applied to it, so to a certain extent they react in the opposite way to how a brushed motor with respect to gearing. This torque characteristic means that they are very flexible & gearing is less critical especially since the advent of the more efficent brushless motor & high capacity LiPos means that the art of gearing your motor so that it gives the best performance whilst lasting the duration of the race is a thing of the past.

Brushless motors on the whole can 'pull' a higher gearing than the 'equivilant' brushed motor - for example when we first went from running 27t stock brushed motors to running 13.5 brushless (considered equivilent at the time on non-timing advance ESCs) we were running with pinions about 4-5 teeth larger than with the old brushed motor.

On my JRX2 i've been running something like a 22 tooth pinion on an 86 spur (but i'll have to check for you) with an 8.5 Orion brushless motor - it runs great & with a slipper clutch doesn't wheelie all over the place! I first ran the car with a 13.5 on a 28t Pinion which seemed like a pretty good bet.... on that basis i'd try something like a 30-32t pinion, you might struggle squeezing this on, running out of adjustment on the slotted holes - so you might have to play around with smaller spurs?

With regard to the settings on your speedo - run it to start with on zero timing & 'default standard' for the other settings.... then go from there.... i've seen so many people fiddling around with ESC settings not really knowing what they are achieving, its always best to start with the out-of-the-box settings then when you make changes do hem incrementally so you know what effect different settings make (just like the suspension set-up on the buggy)

HTH

[pmathews924]

Thanks for the input Dan.

I definitely figured out what I was doing wrong. The Reventon Pro firmware has several options, and I started with one of the Mod programs because I thought that would give me the most tuning options. After doing a bunch of reading, I figured out the Mod1 software only allows you to manipulate timing and punch...if you want to add 'boost' you have to use the Stock1 software.

So I downloaded it, and after about 3 punch offs on the street the car was so powerful it literally unglued my rear tires and shot the pinion gear off the motor. I had the slipper very loose to start, and the car was still lifting in the front with a 7.81 gear ratio...and I don't have the boost turned all the way up, only at 80% with 12 degree timing advance. I think I could go down a 29t pinion (6.46 FDR) and still have the punch I need based on what I just saw...or I could move to a 20t and get crazy, lol.

After I glue the tires back on, I'm going to play with it for a bit and see what happens. Now I understand what the guys at the track were talking about...with a properly tuned and boosted esc it will definitely be possible for me to get too fast, and that's kind of what I'm looking for...make the car too fast, then tune it down so it's controllable, and then go run a few laps and tune it down more if it's getting too hot.

[pmathews924]

Oh...and put the gear cover on first so I'm not walking up and down my street looking for another lost pinion, lol.

[DerbyDan]

Just to say that I checked my gearing on my JRX2... i'm on a 26 pinion with the green 88t spur from a XXX-SCT.... & thats with a 8.5BL which comes off just 'warm' after a 5min race! - On that basis i'd be looking for the smallest spur & largest pinion I had in the box!

Btw are you sure that you have a 17.5 & not a 7.5 motor? A JRX2 with a bit of boost should drive like a 'lady'? I would suggest that your pinion fell off because it wasn't tightened up correctly (grub screw pinching on the raduised shaft & not on the 'flat' perhaps?) not because of the brute power of a 17.5?

BTW timing & boost settings won't really affect the initial 'punch' off the line... unless you've got some wild curve with masses of timing coming in at low RPM?

[pmathews924]

It is a 17.5t, and I do have the boost coming in pretty low...4000rpm I think.

I received a couple pinions in a deal I made with a guy and I've had some issues getting them to stay on, but a dab of clear nail polish on the set screw usually fixes the problem.