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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 $\endgroup$
    – user_1818839
    May 4, 2017 at 8:28
  • $\begingroup$ @AndyT Why are there unevenly spaced axles. Will edit. $\endgroup$
    – DrZ214
    May 5, 2017 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, 2017 at 8:12

4 Answers 4

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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, 2017 at 8:48
  • $\begingroup$ @DrZ214 "in your case" why my case? - you supplied the diagrams... $\endgroup$
    – Solar Mike
    May 4, 2017 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, 2017 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, 2017 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, 2017 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, 2017 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, 2017 at 9:18
  • $\begingroup$ The second sentence explains why a bogie is a big improvement over two individually sprung axles. $\endgroup$
    – user_1818839
    May 4, 2017 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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The best kind of structure for riding along rails is a pair of axles that are held parallel to each other, and are separated by a distance roughly comparable to the space between the rails. While a single rigid axle containing two conical wheels would self-steer to follow curves if the rails were uniform and free of defects, it wouldn't take much of a defect to knock an axle sufficiently far away from perpendicular to the rails that one or both wheels would cease to be supported by the rails. If there are two axles that are held parallel to each other, and the distance between them is much smaller than any turning radius, such a thing can't happen absent some truly gross defects in the rails.

One could design rail cars with two rigid axles. Such cars were and still are used in Europe. The distance between axles, however, must be small relative to the minimum turning radius the cars will have to traverse, and consequently rail cars that only have two axles will need to be short enough that the portions of the rails near the front wheels are close to parallel with those near the back wheels. Otherwise, if the rails aren't parallel to each other, there would be no way for them to be perpendicular to both axles.

Longer rail cars are constructed with a pair of bogeys that will each ride the rails in the same way as would a very short two-axle rail car. The body of the car is then connected to the center of each bogey. Although the body of the car will of course have considerable mass an inertia, it will not prevent the two bogeys from following the tracks the same way as they would if they were independent vehicles.

The fact that rail cars have four axles is not generally necessitated by the need to distribute the load, but rather by the need to ensure that each axle is held parallel to another axle which is close enough that the portions of the rails near each axle will be nearly parallel. Unless it is necessary to ensure that a very heavy load is spread out e.g. between bridge trestles, increasing the space between the axles on a bogey would make it work less well, without offering any compensating advantage.

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