Taming SPUDs

Stu asks:I see that your Hellingly motive power uses SPUDs for power – I’ve heard that SPUDs aren’t very good runners – would you agree with this?

To be honest - yes. Sort of.

Tenshodo SPUDs (Self Propelled Underfloor Device) are a great way of powering a model railway locomotive without having to attempt the dark arts of chassis building.

Available in a wide range of wheelbases, if you want to power a railcar or DMU, they have a lot to recommend them. You'll also find some simple diesel kits suggest they are just the way to give your model some go.

And go it certainly will. The ease of use is countered by the unit's jack-rabbit running characteristics. The gears are tiny 28:1 sets driving each wheel. giving a top speed of warp 3 with acceleration greater than a BMW spotting a half car space in a line of traffic, yet I still find them suitable for producing locos. In fact both of the electric locos on the Hellingly Hospital Railway run on them and actually run pretty well.

As far as I can tell, there are a few tricks to getting a SPUD to behave itself. First, mount it properly. There is a bushed brass thing in the top which is where the device is supposed to hang when used as a bogie - this is where the weight should be taken, not on the ends of the axles, something I have seen a few times.

The biggest difference though, is how much weight you put on the SPUD. Lots and lots seems to be the key. The loco shown is a whitemetal kit. It weighs quite a bit (sorry, don't have a loco to hand to plonk on the scales at present) and this really makes a difference. I've built other locos and again found plenty of lead makes a big difference. You won't get a scale walking pace but a modest glide is perfectly possible.

Of course you still have to run the model in carefully - SPUDs do improve with use. Don't take the thing apart either or you can get dirt in the mechanism.

If you are building a DMU, take the chance to give your drive unit more pickups too. Tenshodo have handily provided some contacts on the top to allow this. Talking of pickups, if a SPUD plays up, check that the pickups are bearing on the back of the wheels properly, they can bend and flip over the top if badly mistreated.

So, a SPUD powered loco can be OK. It will never be as good as one built with a multi-stage gearbox but then it will be a heck of a lot easier to build. The choice is yours.