PF XL vs. M -- are my XL's broken or something?

[mdoupe]

Hi guys. I've been reading a lot over the last weeks, catching up on the lego world. One of the things that sparked my interest was RC type stuff.

So to add to my collection I bought a bunch of PF stuff, but ordered 2 XL's and one M motor, as I had read that M's are good for steering, and XL's are good for drive (as M's don't suck as much power while stalled)

building up my first car, I mainly wanted to go for speed, so I tried various configs using one and both XL motors, but I just couldn't get it to go very fast on carpet (the majority of the floor in my apartment :P )

for giggles, I tried using the M motor for drive, and it was MUCH MUCH faster than the XL. I know what you're thinking, they have different RPM and Torque, but using both Sariel's calc and Nico's TTools, I came up with:

XL - 40:8, giving 500rpm and 2.9 Ncm

vs.

M - 20:12, giving 308.4rpm and 2.18 Ncm

to make absolutely sure it wasn't a weight issue, I bolted both XL's on as dead weight with the M as the drive motor and it was STILL faster.

So am I missing something with these two motors, or should I just pick up a couple more M's if I'm going for speed?

In the pic: I messed up the angle, but there's an XL motor behind the 40t -- not attached to the chassis, just for visualization.

[Zerobricks]

It may be, the XL motor draws more current, which makes the voltage of the BB drop, therebye reducing power...

[mdoupe]

So if I'm going for speed should I pick up a buggy motor or two?

[Zerobricks]

So if I'm going for speed should I pick up a buggy motor or two?

Deffinately!

[Jurgen Krooshoop]

If you really want speed, the Buggy motor delivers best IMO, as you can read in my post about small racecars. Where my yellow racecar (driven by an XL-motor) doesn´t perform on carpet either, the red car doesn't seem to have many problems with it. It might have to do with gearing up the speed of the XL-motorm, while the buggy motor need gear-reduction, which is more efficient. I'm thinking about trying a setup with 2 M-motors.

[tomacwhite]

Jurgen, your red RC car is amazing, I love it!

One question though; where do you get those buggy motors from? Are they official Lego parts?

[eMHa]

On carpet you have a lot of rolling friction. The greater the wheels the more rolling friction you have. I got this while putting PFs into 8048. I used the M motor, because there's not much space inside.

I tested a few combinations and I found out that the less gearing you have, the more power you get to the floor. Putting the M motor near the drive axle with using only the gears http://www.peeron.com/inv/parts/32269 and http://www.peeron.com/inv/parts/32270 I got most power.

Putting the motor where the seats were and gearing with a lot of http://www.peeron.com/inv/parts/4019 and http://www.peeron.com/inv/parts/32072 (still using http://www.peeron.com/inv/parts/32269 and http://www.peeron.com/inv/parts/32270 to have the same gear ratio) from the axle up to the motor I lost a lot of power!

So my assumption is, that you loose power with the XL motor if you gear a lot up while with the M motor you don't have to gear much down to have a reasonable speed. (With 40:8 the friction of the axle where the 8-tooth is does slow down more, because of the gearing. With 20:12 the friction of the wheel axle doesn't slow down that much.)

Friction is really a important point while gearing up and down with PF motors.

Edited July 20, 2010 by eMHa

[DLuders]

@ mdoupe: Check out this Lego 9V Motor Comparison page by "Philo" (Phillippe Hurbain): http://philohome.com/motors/motorcomp.htm . The relative power and RPMs of the Power Functions (PF) "Medium" and "XL" motors are shown, along with many others.

@ tomacwhite: The RC Buggy Motors are shown on that page too. They are 5292 Electric, Motor RC Race Buggy .

[Jurgen Krooshoop]

Jurgen, your red RC car is amazing, I love it!

One question though; where do you get those buggy motors from? Are they official Lego parts?

I had one buggy-motor from my 8421 set.

[mdoupe]

I tested a few combinations and I found out that the less gearing you have, the more power you get to the floor. Putting the M motor near the drive axle with using only the gears http://www.peeron.com/inv/parts/32269 and http://www.peeron.com/inv/parts/32270 I got most power.

...

Friction is really a important point while gearing up and down with PF motors.

I will keep this in mind, thanks!

@ mdoupe: Check out this Lego 9V Motor Comparison page by "Philo" (Phillippe Hurbain): http://philohome.com/motors/motorcomp.htm . The relative power and RPMs of the Power Functions (PF) "Medium" and "XL" motors are shown, along with many others.

@ tomacwhite: The RC Buggy Motors are shown on that page too. They are 5292 Electric, Motor RC Race Buggy .

I had read philo's page. it just didn't make much sense to me that the motor that theoretically delivers 2.9Ncm of torque to the axle couldn't drive a simple frame as fast as one that delivers 2.18Ncm at fewer RPM's to the same axle, all other factors kept the same.

[tomacwhite]

I am currently in the process of adding motorisation to the Race Track 8041 and reading about the XL motor is interesting. I assumed it would be better due to the weight of the Race Truck, so with a bit more torque the motor should be good. But based on what is being said here I am not so sure.

I have added the M Motor just to try it out (with a battery, no IR or steering), just to see how fast it goes forward and its quite decent with the M Motor. I am tempted to order an XL Motor off of Lego to see how it compares (I have two in my Bulldozer but don't want to take that apart).

So even with the weight of the Race Truck, would you still recommend using the M Motor? I have been using a Double Conical Wheel Z20 1M gear which connects to the Double Conical Wheel Z20 1M gear next to the diff. Doing this allows me to keep the V8 engine intact which is nice.

[DLuders]

I am currently in the process of adding motorisation to the Race Track 8041 and reading about the XL motor is interesting.

BrickInside featured a motorized 8041 Race Truck: http://www.eurobricks.com/forum/index.php?showtopic=39175&st=0&p=699656&hl=8041%20%20&fromsearch=1entry699656 .

[mdoupe]

I am currently in the process of adding motorisation to the Race Track 8041 and reading about the XL motor is interesting. I assumed it would be better due to the weight of the Race Truck, so with a bit more torque the motor should be good. But based on what is being said here I am not so sure.

I have added the M Motor just to try it out (with a battery, no IR or steering), just to see how fast it goes forward and its quite decent with the M Motor. I am tempted to order an XL Motor off of Lego to see how it compares (I have two in my Bulldozer but don't want to take that apart).

So even with the weight of the Race Truck, would you still recommend using the M Motor? I have been using a Double Conical Wheel Z20 1M gear which connects to the Double Conical Wheel Z20 1M gear next to the diff. Doing this allows me to keep the V8 engine intact which is nice.

I'm the one that started this thread with the dumb question, so take my opinions with a grain of salt. ;)

it seems to me XL motors may be more useful for heavy lifting/torque applications such as large, heavy trial trucks where speed is not as much of an issue as getting over something without getting stuck. :)

M almost seems to be suited to drive for lighter vehicles, in addition to the steering that everyone else uses it for.

the buggy motor is very fast, but at the same low torque as the M motor. I'm betting it's an excellent RC motor until your vehicle gets too heavy. I don't believe it's noted here, but if you combine the buggy motor and the rechargable Power Functions battery (8878), and the Buggy motor stalls, the battery pack will go into "safe" mode, and won't turn on until you plug it into the wall. I don't think this is an issue with the AA PF battery pack. also, if you get the whole RC set, it comes with a remote, battery pack, and steering motor (I'm not sure if this includes RTC)