# Why the giant wheels on old locomotives? **URL:** \ **Category:** Factual Questions\ **Created:** [February 5, 2017, 6:26pm UTC](https://boards.straightdope.com/t/why-the-giant-wheels-on-old-locomotives/779079 "2017-02-05T18:26:32Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Chefguy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chefguy/32/138_2.png) [@Chefguy](https://boards.straightdope.com/u/Chefguy)\ **Post date:** [February 5, 2017, 6:26pm UTC](https://boards.straightdope.com/t/why-the-giant-wheels-on-old-locomotives/779079/1 "2017-02-05T18:26:32Z")
We were at the local train museum yesterday. They have some old locomotives that are enormous. The wheels on them are as tall as I am. Why were they so large? Was it a casting problem or because of an inferior alloy that would overheat a smaller wheel? Or some other reason that escapes me? I was also impressed with the engineering skills it took to design those behemoths. ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [February 5, 2017, 6:41pm UTC](https://boards.straightdope.com/t/why-the-giant-wheels-on-old-locomotives/779079/2 "2017-02-05T18:41:11Z")
The wheels on a steam locomotive were directly driven with no gearing. If you make the wheels too small, and as the train goes faster, you exceed how fast the engine can do its thing without coming apart, and you lose high end speed. Make the wheels too large, and the torque required to get the train moving from a stop becomes excessive if you are hauling a large, heavy load. So the wheels that you see are a good balance between low-end torque and high-end speed. Engines designed to haul heavy freight tended to have smaller wheels, where engines designed to haul passengers (higher speed, less weight) tended to have larger wheels. ---
**Author:** ![MonkeyMensch](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@MonkeyMensch](https://boards.straightdope.com/u/MonkeyMensch)\ **Post date:** [February 5, 2017, 7:16pm UTC](https://boards.straightdope.com/t/why-the-giant-wheels-on-old-locomotives/779079/3 "2017-02-05T19:16:37Z")
Given your location I’m guessing you maybe got a peek at SP 4449, in red and orange Daylight colors. If so, its 80" drivers were just about as big as locomotive wheels ever got in the North American rail systems. There might have been a couple of 84" drivers somewhere, but I know what you mean about tall wheel sets when compared to the 40" and 42" wheels you see on big locos today. Good for you! ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [February 5, 2017, 7:44pm UTC](https://boards.straightdope.com/t/why-the-giant-wheels-on-old-locomotives/779079/4 "2017-02-05T19:44:49Z")
The [GNR Stirling class](https://en.wikipedia.org/wiki/GNR_Stirling_4-2-2) with the 8 ft 1 in drive wheels was probably the most extreme example. I think the drive wheels became smaller after that, as technology advanced and they could build pistons/cylinders that could reciprocate at a higher rate. ---
**Author:** ![Bert\_Nobbins](https://avatars.discourse-cdn.com/v4/letter/b/aeb1de/32.png) [@Bert\_Nobbins](https://boards.straightdope.com/u/Bert_Nobbins)\ **Post date:** [February 5, 2017, 7:53pm UTC](https://boards.straightdope.com/t/why-the-giant-wheels-on-old-locomotives/779079/5 "2017-02-05T19:53:32Z")
> [@scr4](#): > > The [GNR Stirling class](https://en.wikipedia.org/wiki/GNR_Stirling_4-2-2) with the 8 ft 1 in drive wheels was probably the most extreme example. No, indeed. There was a 9-footer: [http://www.douglas-self.com/MUSEUM/LOCOLOCO/9footer/9footer.htm](http://www.douglas-self.com/MUSEUM/LOCOLOCO/9footer/9footer.htm) and a 10-footer, though that was definitely a freak. [http://www.douglas-self.com/MUSEUM/LOCOLOCO/hurricane/hurricane.htm](http://www.douglas-self.com/MUSEUM/LOCOLOCO/hurricane/hurricane.htm) ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [February 5, 2017, 7:57pm UTC](https://boards.straightdope.com/t/why-the-giant-wheels-on-old-locomotives/779079/6 "2017-02-05T19:57:45Z")
p.s. Turns out there’s an even more extreme example - the [Bristol and Exeter 4-2-4T](https://en.wikipedia.org/wiki/Bristol_and_Exeter_Railway_4-2-4T_locomotives) with their 9-ft diameter drive wheels. ETA: Ninja’d! By the way, it’s exactly the same reason why early bicycles ([penny-farthings](https://en.wikipedia.org/wiki/Penny-farthing)) originally had large wheels. The legs can only turn the wheel so fast. The larger the wheel, the faster you go. (Though in case of the bicycle, an equally important factor was that a large wheel makes the ride smoother. The invention of the pneumatic tire and the chain drive removed both of these factors.) ---
**Author:** ![Chefguy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chefguy/32/138_2.png) [@Chefguy](https://boards.straightdope.com/u/Chefguy)\ **Post date:** [February 5, 2017, 9:18pm UTC](https://boards.straightdope.com/t/why-the-giant-wheels-on-old-locomotives/779079/7 "2017-02-05T21:18:24Z")
> [@engineer\_comp\_geek](#): > > The wheels on a steam locomotive were directly driven with no gearing. If you make the wheels too small, and as the train goes faster, you exceed how fast the engine can do its thing without coming apart, and you lose high end speed. Make the wheels too large, and the torque required to get the train moving from a stop becomes excessive if you are hauling a large, heavy load. > > So the wheels that you see are a good balance between low-end torque and high-end speed. > > Engines designed to haul heavy freight tended to have smaller wheels, where engines designed to haul passengers (higher speed, less weight) tended to have larger wheels. Good stuff; thanks. **MonkeyMensch** : yes indeed. Very impressive up close. We’ve ridden the Christmas train when they roll it out for a couple of weeks each year. ---
**Author:** ![Snnipe\_70E](https://avatars.discourse-cdn.com/v4/letter/s/b5a626/32.png) [@Snnipe\_70E](https://boards.straightdope.com/u/Snnipe_70E)\ **Post date:** [February 5, 2017, 9:24pm UTC](https://boards.straightdope.com/t/why-the-giant-wheels-on-old-locomotives/779079/8 "2017-02-05T21:24:45Z")
I was told by an old steam train engineer that the size of drivers determined the speed of the engine. A rough estimate of the top speed of a steam engine was found by taking the diameter of drive wheels and multiplying by 10. Gave you the speed by MPH. To go faster would shake the engine apart. ---
**Author:** ![Melbourne](https://avatars.discourse-cdn.com/v4/letter/m/b5e925/32.png) [@Melbourne](https://boards.straightdope.com/u/Melbourne)\ **Post date:** [February 5, 2017, 10:53pm UTC](https://boards.straightdope.com/t/why-the-giant-wheels-on-old-locomotives/779079/9 "2017-02-05T22:53:37Z")
> [@Chefguy](#): > > We were at the local train museum yesterday. They have some old locomotives that are enormous. The wheels on them are as tall as I am. Why were they so large? Was it a casting problem or because of an inferior alloy that would overheat a smaller wheel? Or some other reason that escapes me? > > I was also impressed with the engineering skills it took to design those behemoths. Additional factors for very early engines was the stess put by the wheel on the rail, and the slip when starting up. Larger wheels have a larger contact patch, which meen they are less likely to break the rails. If you watch the old film clips of steam engines starting up, you’ll see the drive wheels slip. This is because the tourqe is not constant - it pulses. Faster engines with smaller wheels have faster pulses, which only works when the whole engine-wheel-rail system is better designed. Conversely, larger wheels put the axle higher up, which is a bad thing. ---
**Author:** ![Toledo\_Jim](https://avatars.discourse-cdn.com/v4/letter/t/34f0e0/32.png) [@Toledo\_Jim](https://boards.straightdope.com/u/Toledo_Jim)\ **Post date:** [February 5, 2017, 11:27pm UTC](https://boards.straightdope.com/t/why-the-giant-wheels-on-old-locomotives/779079/10 "2017-02-05T23:27:43Z")
The rule I heard is that he driver diameter in inches equals the running speed in miles per hour, which is close to what Snnipe 70E said. The top speed would be a bit higher, limited by the effective piston speed. You just can’t get steam into and expanding and out of the cylinder fast enough beyond a certain point. Some designs were limited in sustained speed by the boiler capacity, there was only so much steam available. Running beyond that point started to pull down the boiler pressure, essentially strangling the locomotive. Erie RR had two giant locomotives with three sets of driving wheels. Which meant three pairs of cylinders, etc, the third set of drivers was under the tender. The boiler couldn’t mke enough steam to supply all that except at a very low speed and load. The Virginian RR had similar triplex locomotives, all of them (both rrs) had a short operating life. ---
**Author:** ![Mangosteen](https://avatars.discourse-cdn.com/v4/letter/m/6a8cbe/32.png) [@Mangosteen](https://boards.straightdope.com/u/Mangosteen)\ **Post date:** [February 5, 2017, 11:56pm UTC](https://boards.straightdope.com/t/why-the-giant-wheels-on-old-locomotives/779079/11 "2017-02-05T23:56:20Z")
In the early 1980’s I spent about 6 months in India riding the old steam locomotives from place to place. Watching them steam into the train stations was one of the highlights of the trip. Even at that time they were being phased out. Sadly they are probably all but gone today. ---
**Author:** ![RavensLunatic](https://avatars.discourse-cdn.com/v4/letter/r/cab0a1/32.png) [@RavensLunatic](https://boards.straightdope.com/u/RavensLunatic)\ **Post date:** [February 23, 2017, 3:47pm UTC](https://boards.straightdope.com/t/why-the-giant-wheels-on-old-locomotives/779079/12 "2017-02-23T15:47:51Z")
a related question - the locomotive wheels had large solid pieces along the arc. are these to balance the wheel (seem too large for that) or did it provide some other benefit? ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [February 23, 2017, 3:56pm UTC](https://boards.straightdope.com/t/why-the-giant-wheels-on-old-locomotives/779079/13 "2017-02-23T15:56:07Z")
> [@RavensLunatic](#): > > a related question - the locomotive wheels had large solid pieces along the arc. are these to balance the wheel (seem too large for that) or did it provide some other benefit? The piston rods and connecting rods (rods that connect the multiple wheels) are hanging from the wheels. The solid part of the wheel is on the opposite side, to balance the weight of the connecting rods and their supporting structure. ---
**Author:** ![Quercus](https://avatars.discourse-cdn.com/v4/letter/q/7ab992/32.png) [@Quercus](https://boards.straightdope.com/u/Quercus)\ **Post date:** [February 23, 2017, 4:34pm UTC](https://boards.straightdope.com/t/why-the-giant-wheels-on-old-locomotives/779079/14 "2017-02-23T16:34:02Z")
> [@Melbourne](#): > > Conversely, larger wheels put the axle higher up, which is a bad thing. Is a higher axle a bad thing just because of the difficulty of fitting the other locomotive parts around it, or something else? ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [February 23, 2017, 4:49pm UTC](https://boards.straightdope.com/t/why-the-giant-wheels-on-old-locomotives/779079/15 "2017-02-23T16:49:17Z")
The higher the axle, the higher all the rest of the loco sits above the ground. Recognizing that rails are kinda close together, you get a big machine that’s tall and narrow and has lots of weight up on top. Which means it’s tippy. All else equal you’d rather have a wider lower-slung machine to go faster around corners. Part of the reason train tracks have such wide curves is just avoiding tip-overs. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [February 23, 2017, 5:05pm UTC](https://boards.straightdope.com/t/why-the-giant-wheels-on-old-locomotives/779079/16 "2017-02-23T17:05:09Z")
Specifically, the boiler needs to sit above the drive wheel axles. The boiler is the largest part of the locomotive. The drive wheels must be connected with solid axles - even if you could design one-sided wheel supports, it wouldn’t work because the right/left cylinders will go out of sync. ---
**Author:** ![Melbourne](https://avatars.discourse-cdn.com/v4/letter/m/b5e925/32.png) [@Melbourne](https://boards.straightdope.com/u/Melbourne)\ **Post date:** [February 23, 2017, 8:25pm UTC](https://boards.straightdope.com/t/why-the-giant-wheels-on-old-locomotives/779079/17 "2017-02-23T20:25:20Z")
> [@scr4](#): > > Specifically, the boiler needs to sit above the drive wheel axles. The boiler is the largest part of the locomotive. The drive wheels must be connected with solid axles - even if you could design one-sided wheel supports, it wouldn’t work because the right/left cylinders will go out of sync. I think you could probably sync the cylinders. I think that the problem is that the train would drive off the rails. The wheels are self-centering, but the self-centering is driven by the wheel at the other end of the axle. You can feel this happening on a train. The characteristic sway of a train is synchronised to the self-centering action of the wheels on the track. If you go too fast for the train/wheel/track system, the train sways right off the track: this sets a maximum speed for a train, even on a straight level track. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [February 23, 2017, 8:37pm UTC](https://boards.straightdope.com/t/why-the-giant-wheels-on-old-locomotives/779079/18 "2017-02-23T20:37:36Z")
> [@Melbourne](#): > > The wheels are self-centering, but the self-centering is driven by the wheel at the other end of the axle. :smack: You’re right. The self-centering relies on the wheels on both sides being rigidly connected to each other. ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [February 23, 2017, 9:22pm UTC](https://boards.straightdope.com/t/why-the-giant-wheels-on-old-locomotives/779079/19 "2017-02-23T21:22:17Z")
MPSIMS that has nothing to do with the GQ. [Tehachapi Loop (article)](https://en.wikipedia.org/wiki/Tehachapi_Loop) [Tehachapi Loop (video)](https://www.youtube.com/watch?v=xjwjoXChVvI) ---
**Author:** ![Chefguy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chefguy/32/138_2.png) [@Chefguy](https://boards.straightdope.com/u/Chefguy)\ **Post date:** [February 23, 2017, 11:30pm UTC](https://boards.straightdope.com/t/why-the-giant-wheels-on-old-locomotives/779079/20 "2017-02-23T23:30:27Z")
> [@Johnny\_L.A](#): > > MPSIMS that has nothing to do with the GQ. > > [Tehachapi Loop (article)](https://en.wikipedia.org/wiki/Tehachapi_Loop) > > [Tehachapi Loop (video)](https://www.youtube.com/watch?v=xjwjoXChVvI) Very cool, thanks. [Next page](https://boards.straightdope.com/t/why-the-giant-wheels-on-old-locomotives/779079.md?page=2)