Question about radius

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Selector

Well-Known Member
If it is designed and assembled properly, a six-axle diesel in HO should be able to handle 22" curves. I don't know if this one will handle 18" curves...if it does, it won't be at much speed...like walkiing speed in scale.

If you have room, try for 24" and above, and you can run the diesel through the curves at any speed.
 

TomR

Member
The biggest problems I have are not with the locomotives themselves, but with the car following the locomotive. :)

The coupler gets out over the rail and pulls the car following the locomotive off track.

My smallest radius is about 20 inches, and I can run full speed though that radius but only if I watch what car follows the locomotive. Otherwise the car gets pulled off to the outside of the track. It can even give me problems on larger radius curves, my largest radius curves are 24 inch or so.
 

flh80

Member
Back in the begining, 18"r was the norm for sectional brass track, but today with everything getting more detail and more scale, you have to go bigger curves or run shorter loco's and cars. I have problems running some Walthers Pass equipment and a Proto C-Liner, those need at least 22", I have some #4 turnouts they don't like.
 

Selector

Well-Known Member
The biggest problems I have are not with the locomotives themselves, but with the car following the locomotive. :)

The coupler gets out over the rail and pulls the car following the locomotive off track.

My smallest radius is about 20 inches, and I can run full speed though that radius but only if I watch what car follows the locomotive. Otherwise the car gets pulled off to the outside of the track. It can even give me problems on larger radius curves, my largest radius curves are 24 inch or so.

The car trailing the engine...does it have a truck-mounted or a frame-mounted coupler? If the former, that is probably your problem. The leading truck on the car wants to turn with the sharp curve, but the coupler wants to stay mated to the coupler on the frame of the locomotive which, as you say, is jutting way out over the outer rail on the curve.

Without actually seeing what we are dealing with, it is hard to tell.
 

jeffrey-wimberly

Dr Frankendiesel
What is the smallest radius curve that an SD60 can run on? What about an SD40-2?
I've run Athearn SD40-2's on 18" radius without a problem. The problem you mention about the car behind the loco is most likely that the car has a short shank coupler. I had this problem with a tank car that was in behind an Athearn SDP40F which is as long if not longer than the SD40-2. The front of the tank car was being pulled off the track when the train went through a #4 turnout. I put a long shank coupler on the front of the tank car and that solved the problem. Now it your car has truck mounted couplers you're out of luck. Get those couplers body mounted.
 

bigB

Active Member
You can do 18" radius with 6 axle diesels but there are a couple caveats--first, your track has to be near flawless (no humps/bumps at the joiners or unlevel elevation laterally) and as has been noted, youll have to watch what types of cars you pull behind it thru these curves.

The kadee coupler #26 is a good long-shank, center mounted coupler to use.
 

Tasco26

Defendin Freedom Since 99
I have the same problem when I run my SD35 with a few cars tailing. They come undone and/or derail. So instead of pulling them, I now push them. Seems to work a lot better.
 

Outback

Low Budget & Old School
Curious, has anyone every done a list of loco to curve relationships. ie: what types of locos can handle what radius curves? Something like that might be a handy referance tool for planning and pre-operations purchasing.

Just a thought.
 

jeffrey-wimberly

Dr Frankendiesel
You can do 18" radius with 6 axle diesels but there are a couple caveats--first, your track has to be near flawless (no humps/bumps at the joiners or unlevel elevation laterally)
Not necessarily. My track is far from smooth. There are plenty of rises and dips. The tracks is however level laterally.
 

TomR

Member
I just built something, although I'm sure it has been done before.

I super glued a 6 inch piece of rail centered on a straight track to a three axle truck . When I push it through my curves and turnouts, I can see how far it moves off center.

I re-radiused several spots to smooth and minimize the check rail going off center, including some turnouts I was having problems with. Now I can run the same long cars and locomotives through the whole layout WOT. This includes in and out of a problematic siding.

To calibrate this thing, I set up a 18" radius track and marked the indicator rail with a marker where it crossed rails on the curve. I should have done this weeks ago.

When I built my layout, I used a screw on the curve center and a string to hold the radius. It's amazing what a lateral move of only about 1/4 to 1/2 inch in the right spots will do.

Tom
 

ossroy

New Member
Superelevating (banking) your curves will effectively reduce your radius somewhat and should improve performance.
 

Selector

Well-Known Member
Superelevating (banking) your curves will effectively reduce your radius somewhat and should improve performance.

I would be curious to see a definitive proof of this claim. I have never seen a mathematical demonstration that the actual radius of curvature that the trucks must contend with in azimuth relative to their frames' major axes differs when the rails are superelevated. The apparent path in profile drawn on paper below the curve would possibly appear to be slightly less curved, but from the point of view of the cars, their trucks, and their couplers, nothing changes. To see what I mean, imagine going to the extreme and eliminating the curvature entirely by tilting the curve onto its ends so that the rails were completely vertical. Looking down at them from overhead, you'd see level, uncurved, rails face-on. Would the cars negotiating that section of track now have trucks aligned with their frames, couplers centred, and so on? I don't think they would...they'd still be stuck on curved track with the very same radius as before.
 

galaxy

Member
I posted a similar question on another forum about what radius 6 axle locos would handle, Wanting to know about SD's specifically as I was interested in getting one.
I was warned that they MAY negotiate 18R" curves, especially the smaller SD's, but may have problems.
For better luck it was suggested I have at least a minimu of 20R" or 22R" for better workability.
I have yet to purchase an SD40 or so, so I can't say if they willnegotiate my 18R" curves or not.
 

bigB

Active Member
That's probably the most accurate answer. The problem you are more likely to encounter is that what is being pulled or pushed behind/in front of your 6 axle loco may derail as you go around the curve. It largely depends on how far out the front/rear of your loco juts out over the track as it goes around. My experience has been that sd40-2s do ok, then dash 8/9s/ac4400s, and sd70macs/aces the least well. but thats largely subjective on my part. may also depend on manufacturer.
 

cuyama

Member
Superelevating (banking) your curves will effectively reduce your radius somewhat and should improve performance.

Not correct, although often posted on forums.

Superelevation doesn't "reduce" the radius in any way. In fact, it can make things worse in tight curves by increasing friction and the tendency to stringline.

The physics of the model are completely different than the physics of the real thing. And even on the real thing, superelevation doesn't effectively "reduce" the radius -- tight curves are still tight and stiff equipment will still have problems. It does allow curves to be negotiated at higher speeds.

Easements (such as spiral curves) make tight-radius curves more tolerable.
 

Selector

Well-Known Member
What an easement does is to cause a lateral, or radial, acceleration toward the radial center of the curve at any one point along the curve. Instead of lurching into a fixed radius from tangential tracks, the curve starts very gradually, and then accelerates inward until a defined rate of curvature allows the rails, with the symmetrical easement out of the curve, to meet the next tangent.

It keeps passengers and crew, particularly galley cooks, from being thrown quickly into a fixed radius. Everyone stays on their feet.
 

frog

Member
I have Athearn SD40-2's and SD45's that run good on 18 inch track. The problem I think with running them on a smaller rad is not in the trucks but the allowance the trucks can turn when attached to the frame.
 




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