Improving the SD70 Power Pickup
Making the Center Wheelsets Sprung

Click on images for larger view

If you look at the image on the right closely you will see that the middle wheelset on the SD70 does not touch the ruler therefore the track. And the rubber tire wheelset does not touch the track much electrically either if at all.

This means that there are only 4 out of 12 wheels that can conduct power to the loco reliably.

Most of the time this is not a problem but with turnouts that have insulated frogs like American Models this can be a major problem especially in a ladder track or crossover situation.

Here is the bottom side of the SD70 truck.  

With the cover removed you will notice that the wheels on the left side protrude above the truck frame due to the springs under the journals and the middle set does not.
Here the wheels are removed and you can see the springs in the left set and none in the middle journals.
I did a modification to the middle journal pockets that adds a set of springs. I drilled holes for the new springs in the journal pockets. The springs I used are  Lionel part number 6408036055 They are about 1/16" x 1/4" and are a bit larger than springs used in the left journals 
The cover plate needed some modification also .
Note the clear shims on the left holes. There are 4 of them total. Only two are shown in the picture.

If you want to eliminate those clear plastic .006" shims Lionel used, mill or file the cover plate as shown on the picture on the right for that wheelset.  

On the middle wheelset I milled out a pocket in the cover .012" deep for the new sprung journal's travel.

The Micro Mark Mill makes easy work of the milling.
While the cover plate was off I chucked up the rubber tired wheelsets in my lathe and trued the rubber tires a bit to help eliminate some wobbling. 

Once these modifications were done the stalling of the loco was eliminated and the middle wheelsets now provide added power and traction.