My single that's as fast as a dual cell......

ph2t · Post by **ph2t** » Sun Apr 11, 2004 9:15 am

I know I've been slack lately but I've been so focused on the mini-z madforce and all things mini-z.

Anyways, a big thing in the Z world is to have motors that generate extreme amounts of torque but the top speed is a % of what it could be. Now this is achieved using neo-magnets. Whilst the torque increases is like 200-300% the top speed is reduced by up to 20%. Why is this good, well you then run a realy low gear ratio to get a higher top speed. These high torque motors work really well under the higher load a low gear ratio presents

So I though why can't I do this to a bit?

The first thing was to find a gear ratio that was soo low that it was near impossible to power. The SmartLand tanks that are available from Toyeast come with gears that are just insane! The ratio on these gears are 3.95:1. Now I know that the BitRacer gear set are comparable so maybee you could use them as well. I don't know where to get these gears so I choose the ones out of the SmartLand tanks...

3.95:1 Gear ratio

Now I need the power to drive them. Even if you were to use the lower end motors that have the greater torque (1.0, 1.6, 2.2) the startup torque needed is still insane. This is where the mosfet mod is soo good. The mosfet mod will give me the power I need to get the wheels moving under such a high (low geared) load. Of course a single fet stack is not enough. At least a double stack is required. I've found my favorite to be the triple stack. Works well in this instance and even driving on carpet with this gear ratio is a breeze.

Triple fet stack for increased torque

The rims on the front are stock, the rear rims are the ones of the SmartLand tank series. I used these rims for the sole reason that the gear wouldn't come off easily. So I didn't want to wreck the rims I left them on!

And here she is in racing reds:

After putting it all together and going for a test run the first thing I noticed is that the acceleration is great! Even with the ultra low gear ratio the power the fets help deliver is really evident. At top speed this car give a daul cell a run for it's money.

I put this car together especially for the meet at the 1/10 racing track tomorrow. It will be interesting to see how it performs on an ideal racing surface....

ph2t.

crazydave · Post by **crazydave** » Sun Apr 11, 2004 9:57 am

So you've determined, what I've been coming around to slowly realize. FETs give you enough power to pull you're fastest gear ratio.

I like running higher gear ratios now, because it actually helps keep the front wheels on the ground, but I don't have anything as high as the bit-racer gears yet.

Are you really seeing much improvement with the triple stacked FETs? I was comparing stacked FETs, by stacking the forward FETs, and leaving the reverse. Double stacking seems like a slight improvement, but nothing like just putting FETs in there to begin with. Triple stacking seems to make the motor whine louder off the ground like there was more speed, but I didn't see any improvement on the ground.

...but honestly, every time I run low on FETs, I steal the FETs off the cars that have stacked FET, so I haven't played with stacked FETs that much, that's why I'm asking about your experience with them.

filelicker · Post by **filelicker** » Sun Apr 11, 2004 10:35 am

I really want to try a fet stack but I have one problem-----how do you know what fets to buy {part #} are they different from clones to ms to bit ?I got they soldering skills , I just don't know what fet I need ,I have looked at the stock ones with a magnifing glass and no info on them at all.
I am sorry this sounds like a serious noob question .

betty.k · Post by **betty.k** » Sun Apr 11, 2004 10:42 am

there's all sorts of fets you can use, but the most common type is irlml2502tr. you can find lotsa product info at www.onsemi.com . dunno if it's a good place to buy from.
and check out crazyd's thread on fetting http://www.bitpimps.lixlink.com/pages/p ... php?t=1778

crazydave · Post by **crazydave** » Sun Apr 11, 2004 11:27 am

That link is assuming you're familiar with the FET mod. It should be made clear that the FET mod involves replacing the N-channel as well as the P-channel transistors, with upgraded N-channel FETs, irlml2502 available from digikey.com. Since we're using all N-type FETs 2 will need to be flipped, the diagrams in the link above are to show you which 2 will need to be flipped on various boards.

ph2t · Post by **ph2t** » Sun Apr 11, 2004 5:05 pm

crazydave wrote:So you've determined, what I've been coming around to slowly realize. FETs give you enough power to pull you're fastest gear ratio.

I like running higher gear ratios now, because it actually helps keep the front wheels on the ground, but I don't have anything as high as the bit-racer gears yet.

Are you really seeing much improvement with the triple stacked FETs? I was comparing stacked FETs, by stacking the forward FETs, and leaving the reverse. Double stacking seems like a slight improvement, but nothing like just putting FETs in there to begin with. Triple stacking seems to make the motor whine louder off the ground like there was more speed, but I didn't see any improvement on the ground.

...but honestly, every time I run low on FETs, I steal the FETs off the cars that have stacked FET, so I haven't played with stacked FETs that much, that's why I'm asking about your experience with them.

Dude, the improvement in stacked fets is there. But you're right, that first jump from the normal BJT transistors to fets is still the gratest jump performance wise. What took me to appreciate the increase in torque a double/triple stack provides was this simple test:

Put on your rear wheels the standard orange gear ratio. Now (as a sinlge fet stack) hold the rear wheels in you fingers (with the front of the car facing the ground, vertically) and press forward. You will noticed that the car swings a bit but prolly not much more than a 10-15 degrees angle. Now make it a double stack. Do the same test. I now get about 30-50 degrees spin, more torque is definately evident. No do a triple stack. The car spins the full 360 degrees, no Bullshit.......

cheers,

ph2t.

z-beam · Post by **z-beam** » Mon Apr 12, 2004 1:23 pm

that flip test sure sounds solid. pity no ones worked out how to fet a sbcg

leonli17 · Post by **leonli17** » Thu Apr 15, 2004 12:16 pm

For the while. I think SBCG doesn't need FET. What it needs is DUal cell which cannot be done either.
If a SBCG and a BCG FET drag together with same specs on a smooth surface. THey will be same because a SBCG does deliever more current to the motor than a BCG. THe torque difference between a SBCG and a BCG FET only shown on a rough surface and a incline.

Post by **CaboWabo** » Thu Apr 15, 2004 6:20 pm

I had no idea the SBCG couldn't be dual celled, I take it the pcb is limiting power?

ph2t · Post by **ph2t** » Thu Apr 15, 2004 7:38 pm

nah, it's a low voltage drive (<3 volts) to the drive transistors that causes this limitation. The fets use (as most fets everywhere else) need at least 3V (ideally 4.5Vmin) to turn on. The SBCG pcb doesn't do this accoring to the bible of microamps, new testament of course!

ph2t.