Mamba Max on 5 cells = WOW!

[GSMnow]

I have been racing my Losi XXX-T MF2 with a Mamba Max 5700 for almost 4 months now. My local track only alows NiXx cells so I have been using 6 cell IB4200 packs. The performance is fantastic, but it is hard to control, especially when the track gets dusty. Speed is never an issue, control is. I have a 4 year old Sany RC2400 pack that is noticeably slower, but the softer punch makes it easy to drive, but it just can't make race distance. Fast forward to a week ago. One of my really good IB4200 packs dumped on me, this never happened before. I checked it out, and sure enough, one cell went reverse polarity. Instead of 7.2 volts, I was at just 5 volts and the low volt cut was kicking in. I failed to revive the cell, so I did a cell-ectomy. I planned on just getting another cell and making it a practice pack, but it turns out a few top drivers are using 5 cells for tracks with less traction, so I gave it a try.

WOW! again. Top speed is down a little from a 6 cell IB4200 as expected, but it is actually still faster than the RC2400 6 cell and acceleration is great, with smoother control. I even left my low volt cut set at 5 volts and had no trouble making over 7 minutes of hard driving. It can still pull wheelie, but it is much easier to keep it from doing so. I will try the 5 cell for a qualifier tomorrow and see how it goes in traffic. I have to make another spacer to keep it from shifting fore/aft though. It's lighter and allows more balance adjustment too. I may go up 1 tooth on the pinion for speed, and even a tick softer acceleration, but they did just change the layout, so I will have to see. I may need to re-gear anyways. If it feels this good in the qualifier, I will have to decide if I want to go 5 cell for the main.

Serious power is great, but control is needed too. on the old latuoy, I ran 16 to 86. I could get the same top end as the 5 cell by going to 14 to 90 but it will actually have more torque available. I guess I will figure it out on course tomorrow. I should get an hour of practice time. The new layout at Hot Rod Hobbies looks like alot of fun. This is the course they will run for the big shootout race, so I want as much track time as I can get before that even in 2 weeks.

[scoob]

That's interesting. I've found out lately that control is usually more important than power. A truck can look slow but be clicking off faster lap times. Sometimes it's mind-blowing how close the stock trucks can come to the mod laptimes.

[splode]

I found the same result running my XXX-4 on 4s A123 cells. When I dropped back to 3s the car had greater acceleration and better lap times overall. This makes perfect sense because I did not change my motor (a 5700 mamba) or the gearing. I believe the motor does not even draw more current when you do this. All you loose is a little top speed.

[GSMnow]

My fastest laps at tonights race were almost identical with the 5 cell geared 16 to 86 and the 6 cell geared 15 to 86. The 6 cell at 16 to 86 may be a tick faster, but I kept spinning out with worn tires. I also tried going down to 14 to 86 on 6 cell and it was almost as nice to drive in the infield as the 5 cell setup but the buggies walked around me down the fast straight. In theory, that should still be faster then the 5 cell but it wasn't on course, unless the buggies also changed gearing?? I had alot of trouble in the qualifiers and got burried starting 7th in the main. I went back to old faithful 6 cell and drove to a 3rd place behind a Losi team sponsored driver and an Associated team sponsored driver. Not bad for an old guy on his own money.

[Cain]

Good observation. You probably can acheive a similar result with the 4600 motor if the 5700 is too much on your track.

I have heard of guys using those 6.6V packs and loving it with the 5700.

[GSMnow]

A 2S2P A123 pack would probably be ideal on the 5700 for 1/10 stadium truck racing. That should be a bit more voltage under load than the 5 cell NiMh and a bit more mah as well. Speed wise I don't have a problem. It is the crazy torque these motors can make. Using the lower voltage and gearing it back up for the speed softens the torque a tiny bit, but the lower top speed when I didn't change the gear just makes it so easy to drive on the technical course. The torque is about the same, but the lower max speed makes it so you have more range of throttle trigger motion between stopped and 25 mph so you have finer control. When I did gear the 5 cell up to about the same speed as te 6 cell, the control was very close and the run time was at least 20% shorter as expected. The one very odd thing I observed was that even when I left the 6 cell gearing in the truck, I ended up sucking the 5 cell pack down faster. The only thing I can attribute this to is the fact I was on the throttle further and longer while using the 5 cell. Since I was not going as fast, I found I could get on the trigger sooner and staying on longer to make up for some of the speed I was lacking. I even found I was pulling full trigger on the back table, then in the tight infield after the drop off the back table, and again after the pitcher's mound. I was also on full trigger on the pit straight all the way to the sweeper with just a tiny lift at each step down. I was never able to use that much full trigger on 6 cells. Going to the 4600 motor will lower the rpm, but it also makes more torque. Dropping the cell off on the 5700 lowers the rpm, but the torque stays the same. At nominal 1.2 volts per cell, the 4600 x 6 (33,120 rpm) is very close to the 5700 x 5 (34,220). I will probably buy a 4600 as a backup and tuning option. It gets me into a better gearing range. I am running just a 15 tooth pinion now. I could run 18 to 19 teeth with the 4600.

[Cain]

Do you have any pics of your setup on your truck? I would like to see how these packs fit. Also, what is the mah rating on the packs you are using?

May be interested in these packs for next year, especially since these are the ones that are supposed to be really safe?

[GSMnow]

Here is a link to a site selling the A123 2S2P pack with all of the specs. 4 cells ends up about the same size as a 6 cell sub-C stick pack. It should fit perfectly.

http://www.horizonhobby.com/Products/Default.aspx?ProdId=AQR400063

It is rated at 6.6 volts at 4600 mah. That is 30.36 watt hours. 6 IB4200 cells works out to 30.24 watt hours, almost the same, but the A123 is about 30% lighter. 310 grams vs 450 grams for an IB4200 pack.

When I run the 5 cell side by side in my XXX-T I have a block of wood to take up the room of one cell for a nice tight fit. And even the 5 cell weights more than the A123 2S2P at 350 grams.

[tcolesen]

2s2p A123 is going to be about 6-8mm (1/4"-5/16") wider than 6cell NiMh stick packs, and about 4-6mm (3/16"-1/4") wider than side by side packs.

[ElectricThunder]

2s2p A123 is going to be about 6-8mm (1/4"-5/16") wider than 6cell NiMh stick packs, and about 4-6mm (3/16"-1/4") wider than side by side packs.

Werd.:cool:

At RCX they had different pack designs that would fit where a normal 6 cell pack would fit (though I haven't heard anything about 'em since then....hmph!).:wave:

[GSMnow]

I just measured the tray in the XXX-T chassis at just 47.75 mm wide. The A123 pack comes in at 53mm, but it is oval shaped. The walls are just over 11mm high for most of their length. The widest point of the pack will be a tick above the side walls. It is going to be a tight fit and a little shaving might be needed to make it sit flat against the floor. I think this could be done without killing the chassis strength. At $119 a pack though, OUCH!

[tcolesen]

I don't know why A123 charges so much for their prebuilt packs. You can get the Dewalt 36v packs which use the same A123 cells for ~$120. Those packs contain 10 cells, though. It would be easy enough to make a 2s2p pack, heck I have two each 3s2p and 2s2p stick packs I made with a technique I came up with on my own :). They are used as two 5s2p sets for my brushless Revo, and have worked flawlessly for quite some time now. I even stopped balancing them a few months ago.

What I'm getting at is, provided you buy a Dewalt pack, I could make you a pack for only $20 if you don't have the equipment to do so. That'd bring the total cost to almost half of what A123 charges :). Also, I'm in Huntington Beach, near SoCal RC Raceway, so we're pretty close (I've been thinking of heading up to Santa Clarita anyways to visit some RC buddies who live there).