swapping trucks around between atlas and kato chassis

[Robbman]

One thing to keep in mind with this is to NOT limit yourself to proto dimensions when trying to find a mechanism with the right lentgh you want, especially for models with six-axle trucks.  Most rarely have their mounting/pivot points where the prototype bolster is, and tri-mounts will only obfuscate that further.   

[peteski]

One thing to keep in mind with this is to NOT limit yourself to proto dimensions when trying to find a mechanism with the right lentgh you want, especially for models with six-axle trucks.  Most rarely have their mounting/pivot points where the prototype bolster is, and tri-mounts will only obfuscate that further.

Are you (of all the people) advising others not to build accurate models?!?!   

[Robbman]

Nope, just pointing out that model pivot points (the gear tower) rarely match proto pivot points on six-axle models (what you'll find dimensions for)... so using proto dimensions to find a mechanism with the right wheelbase for another project, regardless of scale, is limiting.  Four-axle models rarely have this issue, as the gear tower is almost always centered, and the proto bolster is also almost always centered. 

[scaro]

Agreed.  Some of the below is guessing, but I think it's right.

Here we go.

If you look at the plan for a C628 it says the distance between bolsters is a whopping 46'5" - far and away longer than a comparable EMD SD40 or SD45, for instance.

BUT ... the distance between truck pivot points (ignoring the asymmetrical centre axles ) on the Atlas chassis is I believe 41' 5".   

Unlike the real loco, the gear tower on the Atlas C628 truck is centred -it is in the exact middle of the truck.   In terms of portraying the prototype accurately, I don't think it matters a fig where the bolster is on the model truck. 

But this means that in order to calculate the distance between midpoints of the trucks, you have to take that 46'5" on the plan and subtract 22" from the truck at each end to get from where the bolster is to the asymmetrical centre axle, and then an additional 8" from there to the midpoint of the truck itself.  the axles are spaced  5'7" + 6'11" = 12'6" - the midpoint is at the 6'3" mark. 

that is 41'5" which you multiply by 1.9 for N scale, which gives you 78.7mm.

divide that by 2.54mm (that's what a TT scale foot is) and it gives you 31'". 

That's bang on for a TT RS1, RS11 or RS27 and probably others.   That's why I am sniffing around this mech with the intention of doing awful things to it.