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enter image description here

Every train car I can remember has its axles arranged like the photo. Two are put very close together in one chassey, and two more the same way at the other end.

But why do that? If you want 4 axles, isn't it better to spread them out evenly like I drew? Or if you just want axles at the ends, why not just have 2 total instead of 4 unevenly spaced ones?

So why are there unevenly spaced axles? If you want only 2 pivot points, why not just have 2 axles total (which would be spread evenly if they are equal distances from the ends of the car)? If you want 4 axles (presumably for load bearing), why not spread the axles evenly?

I could not find this answer via googling. It might help if I knew the exact terminology of that chassey holding two axles.

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    $\begingroup$ Bogies. Each par of axles is on a small articulated platform, which can self steer, and helps smooth out bumps. en.wikipedia.org/wiki/Bogie $\endgroup$ – Brian Drummond May 4 '17 at 8:28
  • $\begingroup$ @AndyT Why are there unevenly spaced axles. Will edit. $\endgroup$ – DrZ214 May 5 '17 at 1:35
  • $\begingroup$ @DrZ214 Good edit, the question is now clearer. And SolarMike answers it: for whatever reason you need 4 axles rather than 2 (be it bearing capacity or smooth riding) you can't have them evenly spaced as then it can't turn corners. $\endgroup$ – AndyT May 5 '17 at 8:12
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If you spread them out evenly then taking curves is more difficult - it's the turning circle : as the carriage goes round a curve the axles in the middle will need to move sideways in your diagram.

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  • $\begingroup$ Yes that's true. The wheels are already bit conical for that sideways movement. However, in your case we would just have 2 axles instead of 4. I didn't see the point of clumping 2 axles together. It would seem 1 would suffice. $\endgroup$ – DrZ214 May 4 '17 at 8:48
  • $\begingroup$ @DrZ214 "in your case" why my case? - you supplied the diagrams... $\endgroup$ – Solar Mike May 4 '17 at 8:54
  • $\begingroup$ Yes, I meant in this case, but it's not the same as my diagrams. That case I mentioned was a total of 2 axles, one at each end, but both diagrams show a total of 4 axles arranged in two different ways. $\endgroup$ – DrZ214 May 4 '17 at 9:09
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    $\begingroup$ @DrZ214, you did not say two axles, you said "If you want 4 axles, isn't it better to spread them out evenly like I drew?". So, clearly not two but 4 axles spread evenly... Which is how I answered your question. $\endgroup$ – Solar Mike May 4 '17 at 11:05
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    $\begingroup$ @DrZ214 Because each axle (pair of wheels) has a maximum weight capacity, so to carry more weight then more wheels / axles are needed - leading to a chassis (bogie) with two axles fitted one at each end giving the necessary carrying capacity. $\endgroup$ – Solar Mike May 5 '17 at 4:52
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Bogies. Each par of axles is on a small articulated platform, which can self steer, and helps smooth out bumps.

When a wheel hits a bump. the other one dips, both their springs share the load. The central pivot point halfway between them doesn't move much, giving a very smooth ride.

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  • $\begingroup$ Interesting but I don't see why each axle can't have its own shock absorbers and its own steer pivot. Maybe they're trying to collectivize things and save some deadweight but I can't see how its much of a savings which almost certainly does not make much difference for a heavy train car. $\endgroup$ – DrZ214 May 4 '17 at 8:47
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    $\begingroup$ na, if you have three bogies along one car the middle bogie would need to move laterally relative to the train in a curve - very hard to do. Draw a curve (tracks) and a straight line (train car) across. The two intersections are where the bofies sit. The third intersection is where a third bogiw would sit. there is no third intersection. $\endgroup$ – mart May 4 '17 at 9:18
  • $\begingroup$ The second sentence explains why a bogie is a big improvement over two individually sprung axles. $\endgroup$ – Brian Drummond May 4 '17 at 15:26
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It is all about the turning radius of the train. To understand it you must look from above. Scroll down a bit on the link below to see a model train on a curving track. If there were wheels in the center then they would have to slide way off center when the train is rounding a sharp bend or turning onto a siding.

http://www.ngaugelayouts.com/n-scale-layouts.html

The wheels are positioned close to the coupling between trains to allow for the curving track. If you look closely at modern trains they have reduced the number of wheel trucks by pivoting 2 rail cars on one wheel truck. If you look closely there are 6 axles on this section (yes I realize this is lego but it's the best I could find). There are 2 axles at the front and back, but only 2 instead of 4 between the cars. I have seen groupings like this that were 5 or 6 cars long, reducing the weight of the train by one whole 2 axle wheel truck assembly for each car.

http://www.eurobricks.com/forum/index.php?/forums/topic/82085-moc-ttx-intermodal-for-7939-container-size/

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