Dimensions and Clearances for the TT:120 scale

[RB51]

My Silver King has just returned from the menders [Hornby] as it was constantly derailing and apparently needed the front bogie bracket to be adjusted. Running this morning on an R3 curve [previously problematic] it ran perfectly for about 6 circuits and then I noticed that the bogie wheels where lifting and falling - giving the appearance of a slow bounce - before leaving the track. I am using Hornby track, so confirm the 12.1 inner rail measurement but it looks as though the b2b is a bit out. This looks to me to be 10.2. Am I reading that correctly?

 

[ntpntpntp]

@RB51    that 10.2mm you're measuring, that's between the inside backs of the flanges?     Is the derailment happening through point frog/checkrails, or on curves?

If the b2b is too narrow the wheels will jam up through point frogs and check rails.  If the b2b is too wide the flange will catch the tip of the frog V and may ride up on curves.    Those are the only two tests I've ever used.  I've never bothered measuring b2b against a stated standard nor against a b2b gauge, as long as the wheelset passes freely through the points on my layout it is correct 🙂 

Edited May 7 by ntpntpntp

[RB51]

Yes, inside back of flanges. There is a noticeable “clatter” going through points, but the bouncing and eventual derailment is on a R3 curve.  I think from what you’re saying I should try to ease the wheels together a tad. R-

[ntpntpntp]

@RB51  hmmm I've just checked my 08 and a Hornby point (only items I had to hand).  I wouldn't go tighter than 10.2mm b2b as that seems to be the measurement of the frog and checkrail clearance.  My 08 seems to vary from 10.25mm to 10.4mm.  I've not played with the 08 much as yet.

I suggest you look more closely at exactly what happens  when it "clatters" through points and when it derails on the curve.   Could be the bogie pivot is still stiff or something is catching?   Is it possible the track isn't properly level on the curve, eg. if missed with the hammer and caught the rail when pinning down, or a joint has missed the fishplate?

Edited May 7 by ntpntpntp

[RB51]

 

Front bogie is awry somehow, not sure what is causing this but I think the adjustment Hornby have made has caused a further problem. This is on a R4 curve and the front bogie can be seen to lift and make contact with the first driving wheel, only visible from inner rail. On R3 that causes a derailment. On R4 it clicks back down after a few seconds and off it goes for another circuit. I can see nothing wrong with the track and there is nothing caught on the underframe. R-

Edit: no adjustment has been made to any b2b.

 

 

[moawkwrd]

My A3 has something similar that I need to investigate - the tightness of the screw changes the angle that the front bogie is held in at relative to the chassis. Too tight and it bends upwards meaning it can move laterally slightly, loosening the screw solves that but I think it’s causing me other issues as I still get occasionally derailment in R3 curves. I think I need to bend slightly upwards (or downwards depending on how you hold it) the metal from the screw hole to where the bogie spring/bolt sits so that it doesn’t bend upward when screwed in correctly (as it isn’t too tight that causes the issue). Might be worth checking?

Edited May 7 by moawkwrd

[RB51]

I think the bend up/down procedure is what they did last repair, when I got frequent derailments on R3 curves. It is odd that they always sign off the job with “ tested - ok “. R-

[Silver Fox 17]

The bogie arm that is fastened to the chassis needs to be tight and dead straight. If it is loose or angled slightly then the sliding pivot can allow the wheels to catch the drivers or cylinders.

That is why Hornby had the chassis block and bogie holding arm altered with a pin cast in the chassis and a hole in the holding arm to keep it straight.

[Silver Fox 17]

1 hour ago, RB51 said:

 

IMG_1864.mov 18.02 MB · 4 downloads

 

Front bogie is awry somehow, not sure what is causing this but I think the adjustment Hornby have made has caused a further problem. This is on a R4 curve and the front bogie can be seen to lift and make contact with the first driving wheel, only visible from inner rail. On R3 that causes a derailment. On R4 it clicks back down after a few seconds and off it goes for another circuit. I can see nothing wrong with the track and there is nothing caught on the underframe. R-

Edit: no adjustment has been made to any b2b.

 

 

Strangely I got this for the very first time last night. It was a second hand ebay purchase "supposedly new" and when I ran it I had the same on corners and points. The cause was the chassis to body holding screw was half way out catching the bogie. I simply screwed it in and it is like all the rest of my locos, no problem now.

 

[RB51]

If you mean A it looks like Hornby have replaced the black tape on a screw that looks as though it is fully home. Or did you mean B?

[RB51]

Right, so the tiny screw at the back of the arm. Yes, tight and arm straight. R-

[Fishmanoz]

@RB51, your measurement looks like 10.3 to me, not 10.2.

[Rallymatt]

@RB51 you were reading the vernier gauge correctly however to be accurate (hard to tell on picture as angle could distort it, looking again after Fishy’s post, I could say it could be 10.28)  all measurements are made on the work piece (don’t use the small lock screw to hold the slider) You put the jaws between/either side of the part to be measured, using the thumb grip you apply pressure and keep the pressure on and read simultaneously. 
As @Silver Fox 17 says, bracket perfectly straight and fixed. Pull the bogie forwards to gain as much clearance between leading edge of driving wheels and trailing edge of rear bogie wheels, when the bracket is not at correct angle or position they can interfere. 
I have found anything 10.35-10.45 eliminates almost all the running and shorting issues on Peco track. Some of the axle sets don’t allow much more than 10.35 on the B2B before you run out of engagement between axle and the wheel hub. Plus make sure the wheels are fitted square on the axle. 
This can all sound a bit of a faff but once set up these run silky smooth and reliably 😁

Edited May 7 by Rallymatt
Additional information

[RB51]

Thanks all. I have just re-measured and the b2b is definitely 10.2. I imagine the safest way to adjust would be to remove the bogie? Just to be clear I am using Hornby track.

[Silver Fox 17]

56 minutes ago, RB51 said:

If you mean A it looks like Hornby have replaced the black tape on a screw that looks as though it is fully home. Or did you mean B?

Screw A, the black tape has no reason to be there, even the second hand one yesterday had it but lower down as the screw was undone so obviously it is not there to keep it in. SK did say he had no idea why it is there as it serves no purpose.

[Rallymatt]

The Hornby track  seems to be a little more forgiving on B2B than Peco. 

[RB51]

We’ll see, it seems more challenging tbf. I have made an adjustment to 10.4. I’ll report back tomorrow. R-

[RB51]

As reported in this thread [where I will continue this conversation] the loco is now quieter over points, but the inner wheels still lift on curves. R-