phorensic

After a bunch of searching, I found a formula a couple weeks ago where I can calculate the top speed of my Mini-Baja. I was also lucky enough to find the tranny gearing ratio and tire diameter for my foamies. However, a complication arises when you take into account the voltage of my Li-Po pack. I have a 3s 1320mAh lipo that on the first charge held 12.7v. Actually, that was pretty high, considering the excellent documentation that came with it (Thunder Power) said that it should be holding about 12.6v max. I have a feeling people are doing calculations based on the nominal voltage of battery packs and not the peak charged voltage, because Castle Creation's own "how fast will my mini-t go?" PDF lists a top speed near what my calculator lists for 11.1v, and not the peak of 12.7v.

Now here is where it gets tricky. Everyone knows that battery packs drop in voltage when a huge load is placed on them. How much is the question? I can certainly throw a multimeter in there and give the throttle a stab and see the drop, to get a more accurate calculation of the top speed, but with no load on the drive train that's not very accurate. To be more accurate, I suppose you could place the car on a slick surface and do a sort of burn out, but even that is not as accurate as the load given during acceleration on dirt or asphalt. But you can't just strap a multimeter to the car, it's too big, and it's really heavy--changing the load even more! So does anyone have a relatively accurate estimate of what the voltage drop would be? IIRC have I seen little on-board telemetry boxes that measure this??

Top speed for my setup should be 42mph with a 15t pinion, 5400 mamba, and 3s lipo assuming no voltage drop. Not bad! What is more impressive is the wheelie at any speed ability! 1/2 ounce of lead on the front should help with that and net me faster acceleration next time out. Did I mention I love tuning? lol

Link to calculator (sorry, crappy host): [link]http://upload2.net/page/download/HP1mzI5tXHhNKAy/bajagearing.xls.html[/link]

nik77356

nomadio has a telemetry system for that. you could get their react or sensor radio. it comes with a sensor that you plug in on the transciever and you solder the other end to the battery wires and it shows you battery voltage on the radio screen

pbmax

I've developed a spreadsheet that calculates theoretical top speeds based on gearing and brushless motor kv for all of my minis (mini-t, rc18, photon, vendetta, KMI). There are 2 big uncertainties: the one you hit on (voltage droop) and accuracy of motor kv rating.

Castle Creations' web site uses 3.5v per lipo cell under load for its estimates. This seems to be pretty accurate given my measurements, perhaps a little conservative.

As nik77356 said, a telemetry system would do the trick, but these are pretty darn pricey. I bought a rc watts up meter that captures minimum voltage droop and maximum current draw for a run (a run defined as from when you plug the watts up meter into the battery). This is handy, but not as accurate as the telemetry system because you don't know when that min droop / max current occurred. Likely, it happened during acceleration to top speed, rather than top speed itself. The way I've tried to work around this is by doing VERY slow acceleration runs to top speed. Downside is it takes a lot of space...especially since you have to be very careful with the brakes.

I have a little garmin foretrex 101 GPS that I strap to the top of my cars now to measure top speeds and it's very close to the theoretical numbers when using 3.5v / cell under load.

phorensic

So if I am understanding you correctly, pbmax, your GPS is reporting top speed close to your calculations at 3.5v per cell? Isn't that a huge drop from my pack's peak voltage of 4.2v per cell?

pbmax

Peak voltage is just that, peak voltage. Operating voltage is 3.7v per cell, so 3.5v per cell isn't unreasonable at all. With my permax-powered vendetta, the variance was less than 1% (this was the closest my numbers came).

Check out this discharge plot for Cellpro 2100mah 18C cells:

[link]http://www.fmadirect.com/tech_data/2100mAh.htm[/link]

As you can see, under decent load the voltage drops to about 3.5v pretty quickly.

phorensic

Oh, wow, that graph has opened my eyes. Thanks man!

phorensic

More tuning ramblings...

Well, I forgot to unplug my 3s lipo and it's dead! Yep, $40 down the drain.

Back to the 2s lipo. Remember I said I was going to try 1/2 ounce of lead on the nose to tame the wheelies?? Well, it's wayyy too much for a 2s 5400kV setup. Really hard to get it to wheelie now, which could be good, but it just feels sluggish. However, I have the start power at medium and timing turned all the way down and the temps never got above 140F, so I'm gonna turn up the wick and see what happens. I have a feeling that with 2s I'm gonna have to cut the lead down to 1/4 ounce, and when I get another 3s pack that I will probably need a full 1/2 ounce.

pbmax

You might be able to partially revive your 3s lipo with a good balancing lipo charger. Mine has a "low voltage restore" mode for those kind of situations (cellpro charger).

Got a wheeliebar yet?

phorensic

Nah, no wheeliebar, I think they look dumb.

Yeah I don't think a balancer can revive a 3s pack with a total voltage of 2v. Ha!

phorensic

Rather than create yet another thread, I thought I would continue in here with Yet More Tuning Ramblings.

Alright, turned the wick up on the mamba to try and get some of the 3s mojo back with just a 2s lipo. Start power on high and timing on high. Motor never got above 155F which I am happy with and acceleration and top speed are noticeably better. Nothing at all like 3s, but fun enough while at the same time being easy on the parts.

As I suspected, 1/2 ounce of lead is too much for 2s 5400kV. Wheelies on demand from a stop are easy, but anything over 5mph and it just won't do one. Not that that's a bad thing, but it's just so easy to control and not do a wheelie from a stop, that I fear I'm sacrificing mid range acceleration with too much weight. Also, when flying off my jump, it wants to fly seriously nose down even with full throttle. I'm going to go down to just one weight at 1/4 ounce. Luckily, one popped off at the end of the run for me, lol.

So....next problem. The motor plate is bending. Yeah, my jump is too big I guess and the motor plate steel has yielded and will no longer stay straight after I straighten it back out. This creates a whirring noise between the pinion and spur because the mesh becomes too tight. Dammit. Either my jump is too big (approx 3-4 feet of air) or I just still don't have the shocks stiff enough....because it is still bottoming out. With the current 50wt oil and relatively stiff silver (stock) springs on the Losi racing shocks, I just really don't want to go any heavier on the oil or springs for fear of losing handling on the street. What do you think? I also have 2 hole pistons in front and 1 hole pistons on the back (1 step stiffer on each end from stock). Must I step up to something like 70wt, which I have seen other guys run? Will running 70wt with such restrictive pistons and moderately stiff springs compromise on road handling? Am I being stupid and it will still be softer than a touring car setup?

I still don't like that the motor plate bent, so I ask you: has it happened to you? What did you do to solve it? Rear aluminum heatsink motor guard thingy, aluminum motor plate, or both? I have a feeling the aluminum motor plate is going to bend even easier, but the one's I have seen DO have ribs on them to act as heatsinks.. hmmmm

One thing I did notice was that with the shocks set up the way I have them, 20% drag brake on the mamba, and 1/2 ounce of lead on the front (most important), the thing turned like a DREAM compared to stock. It tended to oversteer a lot, but was so controllable, almost like a drift. Hopefully I will keep this characteristic with 1/4 ounce lead and keeping the 20% drag brake.

pbmax

You might be able to get some use back out of it...doubtful, but possible. You should have a balancing charger anyway for lipos.

Wheeliebars may look dumb, but it's pretty cool when you can pull a wheelie all the way down the street

jaejw1 plugs 1 or more shock piston holes with CA and keeps it with fairly low weight oil (below 50 I think) - this keeps his vendettas from bottoming out after jumps. You could also try 100wt instead.

No idea about compromising road handling... I can't stand my mini-t personally - all my other minis out-handle it easily.

phorensic

Yeah the design of the mini-t is absolutely horrible for handling. No weight on the front, a TON of weight on the back, no long enough, etc. A vendetta or rc18b will just blow it away just because of their basic design. I just bought the mini-baja because I race with a Class 5 off-road race team and it looks like the car our race car is based off of.