Julian Roberts wrote:
Tony Wilkins wrote:
If the BB had been set at 17.77mm the figures would add up and perhaps explains why using the max permitted dimension of 17.75mm gives smooth running results.
I have 4 BB gauges and they all have different measurements ranging between 17.67mm and 17.87mm which is slightly worrying. As modellers we depend on our suppliers to apply adequate quality control to achieve consistent results, so it is always worth checking that the gauges we have meet our requirements.
On the full size check rails on curved track are intended to prevent the outside wheel flange from climbing the rail not to take all the side force as it would with the P4 min BB setting.
My engagement with this thread arose from replying to this on P2. I showed a model train of mine (which I enjoyed making very much, thank you, Terry) running on a video absolutely fine on a 2ft radius curve propelling wagons through
pointwork. My general gist was what you are all saying more recently, P4 works just fine without worrying about things to the nearest 0.08mm
To reiterate what I said there I made several locos with fairly inaccurate BB. As they all err narrow there is no problem. One has wobbly front wheels that measure approx 17.47 but still it goes fine through pointwork.
My suggestion that a more accurate check gauge tool might be easily made arose regarding previous discussion on about P3 or 4. Where the BB is at its max of 17.75 it might be good to have a similarly exact CG tool, perhaps. At 17.75 the flangeback kisses the checkrail as the other flange is against the railhead. Above 17.75 - nose nibbling. Hot stuff. Somewhere beyond, ecstacy for the 00 cynic, derailment.
It is a perfectly valid question in view of Tony's remarks to ask what dimension a BB gauge is.
But I despair of the Forum being a place for sensible discussion when plainly people haven't bothered to read the thread, and make a travesty of some of what I said, coupled with not uninsulting suggestions as to how I should spend my time. Compared with an email from Steve Hall couteously answering my (same) question to him immediately.
I regret that you feel that this Forum is apparently mot a place for sensible discussion. Six pages and 139 posts, discussing/debating wheel and track dimensions would, I think, suggest otherwise. Having followed the topic, but not partaken in the mathematical debate. I became rather frustrated that this didn't seem to be getting anywhere, although Quentin and Martin now seem to have suggested what may be a pragmatic solution.
My frustration was brought about by this concentration on very small variations in "standard" measurements, when other factors can have an impact greater than the variance being discussed. That is why I raised the question of construction "variables" which will invariably exceed the established track and wheel standards and which, in my view, would provide a greater practical gain if they could be reduced. Redefining track standards and manufacturing different/more accurate gauges (which is what I think you suggested at one point) doesn't address the difficulties we have with wheel sets if they are less than totally accurately set up. We are in the hands of commercial suppliers for nearly all of the products we use (I exclude the Society's stores from that categorisation) and must make the best of what is available. For me, wheels, especially larger diameter loco wheels represent the biggest challenge in achieving "error free" running when building a model. .001" variation at the hub axle mounting on a 6 'loco wheel becomes .008" at the rim. That is .02mm (if I have my maths correct). B2B changes during running, especially with carriage and wagon wheels, is another. How often do you read that, when a vehicle derails, then the B2B is the first thing to check.
It was also suggested that the commonly available B/H track varied in manufacturing tolerances, which could possibly have an impact. However, as the track dimensions we work to are inside, rather than outside, that doesn't seem to present an issue.
What this topic has done, for me at least, is to again highlight the need for more accurate and consistent wheelset assembly. The range of Exactoscale carriage and wagon wheels have largely addressed that for rolling stock, but steam loco wheels can still present a problem (I have no experience with diesel and electric prototype models). The Exactoscale and Ultrascale ranges don't supply what I need and with the demise of Sharman Wheels that only leaves AGW. I don't share Philip's view that the problems with AG wheels (or any other plastic centred wheel) are solely due to the tyre/centre relationship. Whenever you distort a plastic centre, particularly a non uniform one (i.e. one with a crankpin hub) by forcing in an axle to get an interference fit, then some distortion is possible.
If I could get totally true running wheels, 100% of the time, then I could worry about whether or not established P4 standards need redefining.