I was going over LEGO Power Functions train motors, and I was not satisfied with the speed. Speed seven is fast enough to derail the train on the curve, but not fast enough realistically. I am thinking that if I open the motor and put in bigger cog wheels, it will hopefully make my train go fast enough (I'm trying to get at least 10 mph.) Will this work? Where can I get bigger cog wheels?
Mathematically, the largest driver wheel you can buy for LEGO is Big Ben's XL driver, with a diameter of 36.8mm. To reach 10 mph, that wheel will have to spin:
36.8 mm diameter -> 115.6 mm circumference -> 7.2 x 10^-5 mi/revolution -> 2320 RPM (!!)
This means that you have to gear up a motor to run at 2320 RPM. An XL PF motor with no load spins at about 220 RPM. I just don't think that's going to happen.
And if your goal is to use the smaller wheels, there's no way to accomplish what you want with LEGO.
Don't forget, the TGV (and other high speed trains) are incredibly complex and well engineered machines. It requires a specific track layout, curve radius, rail width tolerance, surface roughness tolerance, and we haven't even reached the train itself.
Honestly, accomplishing a scale-equivalent speed using only LEGO bricks would lead to a revolution in transportation design, because if you can do it with LEGO at L-gauge without blowing up, you could easily do it with steel, plastics, and composites at people-scale.
10 mph (16km/h) is a crazy speed at that scale . Chances are quite high that your car will derail even on straight lines.
Now, is it possible to reach that speed using lego motors and lego pieces ? I'd say no, but I think you can get quite fast, in the magnitude of 10-12 km/h (6-7.5mph).
As stated in another answer, using the bigger train wheels available, you need to reach 1740RPM loaded (for a 7.5mph target).
Note: all electrical motor facts are from: http://www.philohome.com/motors/motorcomp.htm
The first option coming to my mind is using a (couple of) pull-back motors (http://brickset.com/parts/6024100)
The torque is quite high, the rotation speed is crazy, but:
- it'll totally unwind in about 8-10 meters, going freewheel
- it'll probably slip at startup
- it's not RC
The second option is using the PF train motor, which rotate at ~1900RPM unloaded. You can get extra power (up to 1.9W) by powering it with 12V batteries (at your own risks). Changing the gearing on this one won't get you far. Sure you'll get higher unloaded RPM, but your performances loaded will be really poor.
I've seen cars using 2 of those reach 12km/h, so why not trains ?
Still not fast enough ? Try the mighty RC Buggy motor (http://brickset.com/parts/4177239)
You will have to gear it up a bit (unloaded RPM is 1700), probably at 2x. You might even have to power-it up to 10.5V, but I've seen technic cars reaching 12km/h using this motor, so why not trains ?
But it's already time to slow down, you're approaching the first curve.
Aside from the mechanical enhancement, you could also apply more electrical power to the motor. Some LUGs achieve record-breaking train speed by doing just that, although they usually use custom rails as well as transformers.
Of course, overpowering motors isn't usually recommended; and for a PF one you'll have the issue of embedding a larger battery; but considering the PF motor isn't very expensive and may be able to sustain excessive power quite gracefully, it might be worth a try.
Other than that, yes, using other motors than the train one might be a good solution. Even with regular trains, you can build a mock motor which can then be driven by another PF motor in the cabin. Not sure it would sustain high speeds though.
If the original train motor is not fast enough, you could build your own engine using normal PF motors and use gearing that is suitable for your needs. Then drive your train via stand-alone train wheels:
Yes, this train is not going at 10mph. The 12 kph from that video is about 6-7 mph. Closer to 5. You could try rewinding the coils in the motors of the wheel system as to increase the rpm of the motor.
Look this up if you are unclear on anything.
Also, try switching out the motor for that of higher RPM