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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?

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    10 mph means that train is going really fast. In scale, that's approximately 380 mph!!!
    – Nick2253
    Commented Oct 30, 2014 at 2:54
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    Yes, but I like to think of it as 574.8 km/h (just like the real TGV!)
    – Taylor Garrett
    Commented Oct 30, 2014 at 3:01
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    Even if speeding up the motor were possible (which I'm ambivalent on), I doubt that you'd be able to use it at that speed on Lego track. Once again, issues of scale come back to bite you. A small .1mm lip would be the equivalent of almost 4cm lip in real life. Which is well outside the rail specs and would put the train at high risk for detailing. Though Lego has very tight tolerance, I don't think they are tight enough for that speed.
    – Nick2253
    Commented Oct 30, 2014 at 3:08
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    10mph is nearly 4.5 metres / second. You're going to need a lot of track...
    – Kramii
    Commented Oct 30, 2014 at 9:53
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    You also need to remember that there are really two scales at work with LEGO trains: One is the true "Minifigure" scale, and the other is the actual train build scale: Most real world trains are wide enough to fit two sets of two seats in a carriage (four people and an aisle), while LEGO trains barely manage 1 seat wide. The gauge of the track does not match the scale of the people.
    – Zhaph - Ben Duguid ♦
    Commented Oct 31, 2014 at 9:08

6 Answers 6

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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.

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  • Thank you for your answer. I already have the curve radius problem that you speak of completely eliminated using this technique: holgermatthes.de/bricks/en/smooth-curves.php.
    – Taylor Garrett
    Commented Oct 31, 2014 at 0:07
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    While that may be "smooth" from our perspective, that is incredibly tough from the trains perspective. The gaps and angles there are simply unacceptable for a train moving at 10 mph
    – Nick2253
    Commented Oct 31, 2014 at 0:17
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    if someone want to check the computation, or play with variables: wolframalpha.com/input/…
    – Stephane Delcroix
    Commented Oct 31, 2014 at 7:40
  • The stalling torque on an XL PF motor is 40 N.cm so to get 10x the speed, it would be 1/10 the torque, or 4 N.cm, or 0.04 Nm. Without knowing the mass of the wheel its hard to go further with those calculations though. In a perfectly efficient frictionless world of course.
    – user4139
    Commented Nov 6, 2014 at 16:32
  • The PF XL motor is a bad choice for a direct RPM comparison, as it's more built for axle-bending torque instead of pure speed (yes, you can use gears to change the two, but you aren't doing so in your comparison) - a faster motor such as the 2838 (old, 9v motor) reaches 4k RPM, the buggy motor reaches 1700 RPM, which is much more in-line with the 2k RPM of the train motors themselves. Even the E-motor is close to 800 RPM and makes more sense in a comparison if you want a fast motor RPM.
    – user2813274
    Commented Jul 10, 2015 at 13:58
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10 mph (16km/h) is a crazy speed at that scale [citation needed]. 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)

pull-back

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)

enter image description here

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.

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  • The reason those cars can reach 12km/h is because they are ridiculously overbuilt. Imagine a car that looked like that in real life, and you'd get something like a dune buggy with 2-3 ft wide wheels. However, it would be really hard to reach that speed with a scale-accurate vehicle because you've got relatively imprecise (in-scale) LEGO tolerances to deal with.
    – Nick2253
    Commented Oct 31, 2014 at 14:16
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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.

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  • Thanks for the suggestion! This really helps. I plan on purchasing an XL motor and some gears (like the ones in the picture in the link that you posted) and get my train going! Thank you! You have gave the answer that I was looking for! Installing this in my train will be not a problem at all.
    – Taylor Garrett
    Commented Nov 8, 2014 at 4:22
  • Also notice their special custom tracks and the way curves are made bent to counterbalance centrifugal forces. Also, for having witnessed it, a train crash at those speeds is spectacular, although maybe not so for the one who owns the train.
    – Joubarc
    Commented Oct 21, 2019 at 8:22
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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: enter image description here

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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

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Simple way is to buy mini faster engine on ebay and install it in, like 10000rpm one or 25000rpm one would then it would increase speed but the issue is quite complicated to install in but can be done then if rpm increase, speed would be about the same but don't think it would hold on the track you would need to redesign whole train add weight and then different wheels etc...

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    Hi Nuero, you seem to have a quite concrete suggestion here. Do you, per chance, have any experience with this kind of solution, i.e. have you done that yourself? If so, I'm sure OP and the community would appreciate if you shared your insights, results, potential pitfalls and recommendations.
    – zovits
    Commented Aug 16, 2021 at 9:27

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