Touring car gearing Q. MM 6900, Maxamps 6000 lipos.

[Volumex]

Hi, I'm in the process of building a TC5 for bashing use and the motor/battery setup is:
MM 6900 kw, Maxamps 6000 mha 2s lipos.
This is my first touring car in a long time and the very first since brushless and lipos have been around.
I was wondering, what kind of gearing should I look for, so that I can keep everything cool temperature-wise and still be able to achieve very fast speed? I'd like to keep this car/setup reliable and avoid thermals.
Thanks guys.

[burnineyes]

I dont know the gearing of the TC5 but I have a TC3 with a MM7700. I always stayed with 72 spur. I often tried for some extremely fast speed runs with a 30 tooth pinion. 30 tooth is a bit much for reliability, anything over a 26 tooth and I would check temps constantly. I could put in a 24 and run pack after pack for hours on end, reaching speeds close to 50mph.

[chilledoutuk]

I assume you have a tc5 manual in that case tell us the final drive ratio and we can tell you what sort of ratio to look for a top speed of about 40-45mph.

[whiteman]

I just bought TC3 of ebay that included MM6900. I would like to know what is the best setup for Speed? I assume it can handle 3s lipo. I also noticed, the more mah, the more speed you get>?

Thanks

[ElectricThunder]

It's not reccomended nor covered under warranty to run that motor on 3s lipo.

Speed is based on a couple things. Torque is necessary to pull the larger gearing for speed runs. RPM is needed to turn that large gear ratio at a decent pace. A good setup for speed is a lower kv motor, such as the 4600 motor, running on 3s lipo geared relatively agressively. (More efficient than a high kv motor and lower voltage)

More capacity in a pack means a higher discharge rate, which means more current available to the motor, so it has the potential to make more torque and thus, you can gear it higher safely without killing the battery.

The 6900 is a fast motor on 2s lipo, but you're pushing it very hard and spinning that rotor very fast on 3s lipo. Centripetal forces are probably going to blow the rotor apart sooner rather than later. Amp draw will be higher under the same gearing as well.

[GSMnow]

No, the 6900 is not rated to run at 3S. You may get away with it for a while, but the RPM is just too high to be reliable. The ESC will take 3S all day long, but the 5700 KV motor is the highest rated for 3S operation.

And the original thread topic...
I also agree, gear it for 40 - 45 mph as a starting point and check your temps. Wide open track can take a bit more top end. A tight technical track with alot of braking and accelerating will require a little lower gearing. The Mamba Max systems are actually very forgiving when it comes to gearing as they make more torque than you can use over such a wide rpm range. Ideally, you want to gear it just slow enough that you can actually pull full trigger and have it top out on the fastest part of the track. Gearing any higher is a waste of power out of the batteries and more heat stress on everything. Touring cars are a bit different than offroad, but most of the gearing rules will still apply. You should do a motor temp check just to be sure.

Racing on a dirt track with a 2WD truck, I did try gearing a bit too fast for the track to soften the torque.... IT DOES NOT WORK! Even geared for 50 mph I had power to spin the tires and/or pull wheelies all over the place, it just had a much shorter run time and the motor hit 205F in under 7 minutes. If it is too fast to hold full trigger for over 1 second, use a smaller pinion. I am geared for 38 mph now, and it tops out about 15 feet before I have to lift for the sweeper. I could go a little higher, but it certainly is not going to make me drive any faster. My motor temp now hits the 170-180F range when I am hooked up and able to use the power. It runs cooler when traction is poor since I am not on the trigger as much. With just a 3000 LiPo, I am getting solid 10 minute run times at full race pace. 12 to 14 minutes when grip is off.

[whiteman]

Looking at CC website, 5700 with 3s can go faster than 6900 2s. Man that is not good. How can we make it go 60+ like 5700 using 2s on 6900? Thanks

[GSMnow]

Sorry to tell you, but 2S with any motor is just not going to make the power you need for reliable 60 mph runs. You need 50% more current at 2S to tmake the power of 3S and that extra current just heats everything up. The 6900 motor (and 7700) are really just for when you are voltage limited (usually due to rules) and need the MOTOR rpm to make the desired wheel speed with the available gearing range. 45 to maybe 50 mph is really pushing it for ANY 2S setup. Search the web all you want, and you will find that all of the top speed runners were using higher voltage, even as high as 8S. This is to lower the current which reduces heat and the losses that come with it.

[burnineyes]

I have to agree here. With 8 NIMH cells(9.6V) on my 7700 I can do over 60mph, but I have to check temps very often. Im sure when I use my best batteries I am pushing what this motor can handle for RPMs, and it is stressful on the batteries. Not counting 2 or 3 breaks to let everything cool I only get 5-8 minutes on 4200mah at that kind of speed. But I have to say I have played with the gearing alot and it still builds speed every time I have upped the pinion. Man all this talkin makes me REALLY want a 4600 and a big LIPO!

EDIT: Make that a Monster Max, really big LIPO, and a 40 tooth pinion!!!lol just dreaming