bdt0805

A quick measure using a ruler and a magnifying glass shows that the driving wheels a tad under 10mm diameter and the bogie wheels about 7mm but I need to find my digital vernier and check properly . The data for the M7 lists the driving wheels at 67"/1702mm and the bogie wheels at 43"/1092mm. Quick maths shows that the bogie wheels are scaled at about 1:150 (ish) but the driving wheels at about 1:175 - all very, very approximate but clear enough to suggest that the driving wheels are below scale as per ED-FARMS, IZZY & OFFTHERAILS suspicions. I reckon if Dapol had made the driving wheels closer to scale (using 1702/148 = 11.5mm diameter) then there wouldn't be any ride height issues at all. I understand compromises have to be made and there are moulded details such as brake blocks etc but why would Dapol do this? Is this intentional or has the factory misinterpreted the spec/drawings?

Here's a couple of photos of the bogie, showing the electrical pickups (referred to as "prongs" above for want of a better word). Straight from the factory - not exactly identical but it clearly shows the bend required to get the pickup correct and reliable. Now........I've just looked at my loco on a flat surface and despite what I said, the footplate is slanted from rear to front by about 0.5mm to 0.75mm. Its just about visible in the photo. The bogie is in close contact with the underside of the loco so that height is fixed. I can actually get a piece of paper under the rear driving wheel so strictly speaking it wouldn't be in good contact with the track. No amount of fiddling with the electrical pickups would solve this. Here's the interesting bit - if I put my finger on the rear of the loco to keep the bogie nice and flat, BOTH driving wheels lift up together and there's definitely daylight under them. Its a fraction of a millimetre but its there! So - one of two things, either the driving wheels are very slightly too small (which is unlikely) or the bogie needs to sit slightly deeper into the underside of the model. I've taken another picture which just about shows this. Dapol need to be challenged on this? As I said, the loco runs OK but unless all of the wheels have good contact with the rails, its not getting electrical pickup as good as it should?

Very interesting thread. I bought a DCC fitted BR early crest 30673. It was a very poor runner. I did some tests and there was no electrical continuity from one side of the rear bogie. I removed the bogie (which is very easy) and one of the fixed contacts inside the loco was "springy", the other totally flat. I'm sure the bogie had been lifted slightly and checking the driving wheels carefully, I reckon the rearmost driving wheels were not quite touching the track. Now.....thinking this was the ride height issue, I did the Dapol "tweak" but of course, too much bending on the electrical prong from the bogie means that they never touch the internal contacts and definitely not the flattened contact. Anyway, after confirming this was a fault due to the flattened internal contact, Rails kindly replaced the loco with a new one. The new one runs better and there's no ride height issue. It looks like slightly earlier M7 suffered from a pronounced ride height issue but the Dapol DIY fix seems a bodge to me. The internal contacts need to be springy and the prongs from the bogie need to be just right so once the model is placed on the track, the weight pushes the bogie onto the loco frame / body and the electrical pick up should be OK (allowing the spring of the internal contacts to sit on the prongs). Based on the experience of the first model, if the prongs from the bogie are straight, its obvious that they are likely to push against the internal contacts and potentially flatten them. Similarly, they might force the bogie way from the loco body and hence push the back of the footplate up causing the ride height issue but I'm not convinced that's the whole story. If the internal contacts do get flattened then they are unlikely to give electrical continuity. There has to be some flexibility here. Anyway, my replacement loco sits level on the tracks and seem to be OK. The bogie has some "spring" to it due to the fixed contacts inside the model but once on the track with the bogie in touch with the frame it looks fine. The only other "moan" is the factory fitted decoder - its poor and no messing with CVs will help the abrupt starting and stopping so I've actually replaced it with a Zimo MN170F. Very fiddly to solder the smaller diameter decoder wires to the existing wires in the loco but the difference is worth every penny - superb low speed performance. Its transformed the model. Highly recommended. Andy