Width over frames. How wide should my frames be?

[bluestag]

Hey guys,

 

I am assessing my new LNWR 2-4-0 Precedent (Jumbo) kit in 7mm.

 

It is an old Modeller's World kit.    And like the two before it, it has the chassis at a crazy wide, near scale, width.   The frame spacers and the frames added together would come out to be 1.170" wide.  Sorry, work that out for mm if you must, I'm a yank.    My special tank, a very old kit with milled frames, is 1.015 thou wide.   It has a wheelbase similar to the Precedent, and gets around 6' curves fine.   And my newly acquired Zero Zephyr 2-4-0,, with a sophisticated chassis, is .996".    It is a bit shorter than the Precedent.   A recent product of Gladiator, the coal engine, is 1.034" wide.    It is about as long as a Precedent.   It has never picked a point.

 

That is about 3mm, or an 1/8th of an inch, wider than the three working locos cited.

 

Does anyone have a rule of thumb to work with?    I am going to set the two driving axles rigid in the frames, and spring the front axle.    The motor is going on the forward driver.   There is not room under the cab floor for a gear.

 

Way open for advice.

 

Kevin

 

[Simond]

My approach is to standardise on 28mm over the frames.  You can go a bit wider if you want, 28.5 is entirely possible and 28.8 might be, depends on the curves you want it to go round, how many axles, how long it is, and so on.

 

For some reason, probably to do with the potential use of coarse scale wheels, the frame widths of older models were typically around an inch.  There seems no excuse for this in modern finescale modelling, it looks wrong, even daft, and the S7 boys will be in hysterics.

 

For reference, the GW locos I have recently been working on had real frame spacings of 4 feet and some fractions between the frames, and frames about 1.25" thick - that scales out at 30.11 mm which obviously won't work with standard Slaters wheels which are 29.0 back to back.

 

It'll massivley improve your models if you can get the frames close to the wheels - that's what the real ones look like

 

atb

Simon

[Florence Locomotive Works]

On 17/10/2023 at 22:53, bluestag said:

Hey guys,

 

I am assessing my new LNWR 2-4-0 Precedent (Jumbo) kit in 7mm.

 

It is an old Modeller's World kit.    And like the two before it, it has the chassis at a crazy wide, near scale, width.   The frame spacers and the frames added together would come out to be 1.170" wide.  Sorry, work that out for mm if you must, I'm a yank.    My special tank, a very old kit with milled frames, is 1.015 thou wide.   It has a wheelbase similar to the Precedent, and gets around 6' curves fine.   And my newly acquired Zero Zephyr 2-4-0,, with a sophisticated chassis, is .996".    It is a bit shorter than the Precedent.   A recent product of Gladiator, the coal engine, is 1.034" wide.    It is about as long as a Precedent.   It has never picked a point.

 

That is about 3mm, or an 1/8th of an inch, wider than the three working locos cited.

 

Does anyone have a rule of thumb to work with?    I am going to set the two driving axles rigid in the frames, and spring the front axle.    The motor is going on the forward driver.   There is not room under the cab floor for a gear.

 

Way open for advice.

 

Kevin

 

When I was considering scratchbuilding a 7mm LNWR Jubilee (with cab extension) I decided on 1 inch for the frame width as that is what I thought was standard for 7mm. Then again that was on a bogie loco not a rigid chassis engine. 
 

If I were you, I would stick with 1 inch frames just to be on the safe side.

[bluestag]

Simon,

 

Yes, thanks.    Wider is better, up until the loco won't go around a curve.    This is a 2-4-0.   The two driving wheels will have next to nil sideplay, and the leading carrying wheel will have enough to get it around a curve.

 

I'm inclined to 26.25mm, as that is what my coal engine has, and it never hesitates on curves.   That and it has the same fixed wheelbase of a Jumbo.   But wider would benefit the cab interior.    With the Slater's wheels having a 29mm back to back, and my proposal to use a Slater's sprung hornguide on the front axle, I'll have about 2.5mm sideplay, or a bit less on the front axle.   Does that sound like a lot?

 

Kevin

[Gordon A]

Decide what minimum radius curves you wish your loco to negotiate, then work back from that to calculate how much side play you will need.

[ian@stenochs]

S7 back to back is 31.2mm so we can set our frames to scale or very close. You don't need much sideplay, especially in fine standard 0, unless you plan on using trainset corners.

[bluestag]

I have 6' curves, although the loco must get around a 5'10" curve on my mate's layout.

[bluestag]

7 hours ago, Gordon A said:

Decide what minimum radius curves you wish your loco to negotiate, then work back from that to calculate how much side play you will need.

I have NO IDEA how to do that.   I am running 6' curves.

[Simond]

I think for your 2-4-0 you can go beyond 28mm, but it’s a good compromise.

 

 I am currently rebuilding an older model of a GW small prairie  with 26mm outside the frames, I have fitted Slaters horns.  I’ll be putting spacing washers between the axleboxes and the wheels, but it means everything has to be mounted further outboard, brakes, sandboxes and cylinders, to get them where they need to be.  Not a problem as I can 3D print what I need, but it seems daft to go narrower than you must. (And the S7 guys know this.  Their compromise is the minimum radius of corners their stock can traverse.)

 

As regards calculation, it is possible, with an excel spreadsheet, to work it out, but I could never be bothered to build the spreadsheet, because it’s easy to draw.  Easier still on CAD.  Draw your coupled wheelbase with your wheels to scale, you need the flanges too.  
Cut a horizontal section at railhead height, you should have a pair of rectangular-ish shapes for each axle, the flanges are of course coned slightly, and have radii at the outer edge, and the root where the flange and trad meet.  Id ignore the radii for this exercise.

Then draw your minimum radius (maybe less a little tolerance) so it is tangent to the outside of flanges of the inner wheels at the middle of the wheelbase.  

Draw the outer rail concentric to the inner, and obviously to gauge.

 

if the outer rail interferes with the flanges (it’ll be the leading edge of the leading axle, and the trailing edge of the trailing axle) you need to allow sideplay.  You can measure it off the drawing.  Sideplay can be accommodated by having some on each axle, or all of it on the middle, or by doing clever tricks on two axles, and not on the third.  
 

on a x-4-x like yours, the sideplay inherent in 0gauge standards will be all you need.

 

I’d recommend 28mm.  More is possible but might be tricky, less is simply unattractive imo.

[Hal Nail]

24 minutes ago, Simond said:

Draw your coupled wheelbase with your wheels to scale, you need the flanges too.  

Cut a horizontal section at railhead height, you should have a pair of rectangular-ish shapes for each axle, the flanges are of course coned slightly, and have radii at the outer edge, and the root where the flange and trad meet.  Id ignore the radii for this exercise.

Then draw your minimum radius (maybe less a little tolerance) so it is tangent to the outside of flanges of the inner wheels at the middle of the wheelbase.  

Draw the outer rail concentric to the inner, and obviously to gauge.

 

if the outer rail interferes with the flanges (it’ll be the leading edge of the leading axle, and the trailing edge of the trailing axle) you need to allow sideplay.

Is this A level or T level?!

[bluestag]

Simond,

 

That went way over my head.   I'm relying on a 0-6-0 of similar length in setting the spacers at 26.35.   The extra tenth is just how they turned out.

 

Kevin

[bluestag]

Well, the wheels came in today. I had cut spacers at 26.35mm, and tacked together the chassis. The opening for the front sprung hornguides needed opening up by about 3mm upward. The front axle has about half a mm in movement up and some down. The chassis COULD have been wider, but I don't think the plunger pickups would have fit. It is not bad, the front axle has plenty of movement. Possibly too much. It is all a compromise.