Tractive effort is the force that a locomotive can apply to pull a train. Real locos pull from a few thousand pounds to several hundred thousand pounds depending on the locomotive design and its weight.
Tractive effort can be measured with special test cars, but the usual method was to simply measure the weight on the locomotive's driving wheels on a special scale and to divide by four. This worked well for steam locomotives and older diesel locomotives. Newer diesels with sophisticated traction control systems can exert up to 40% of their weight in tractive effort under ideal conditions.
Model locomotives work in much the same way. Provided that the motor can handle it, a heavier loco will outpull a lighter one. With some models, the actual sliding friction of the wheel to rail can be modified with a rubber or plastic traction tire. Some Lionel "O" scale locos have a feature called MagnaTraction where the wheels themselves were magnetic and adhered to the steel rail better that weight alone would allow.
There are two methods commonly used to improve the tractive effort of Large Scale locos by increasing the traction. These methods are traction tires and/or additional weight.
Most Large Scale locos have more motor power than they can use so that, with some care, the traction and therefore tractive effort can be improved beyond the box stock condition at a small risk to the loco. More traction allows a loco to pull a longer train without wheel slippage. At first blush, this would appear to be good, but there can be a serious darkside to improvements in traction. Some locos won't tolerate increases in traction at all.
A loco that is "improved" to increase traction can be made to work harder before the wheels slip. Wheel slippage is a protective mechanism for the loco. If the weakest link is the wheel to track connection, the load on the motors and gears can only rise so high before the wheels slip and limit the load. This is an obvious condition that the operator can then remedy by reducing the load.
If wheel slip is prevented or delayed, the loco will work harder before its self protection mechanism (wheel slippage) occurs. As the loco works harder, the motors will draw more power and run hotter. The load on the gears will be higher and gear wear will increase. These increased stresses will tend to decrease the life of the loco. These increased stresses may not be immediately obvious to the operator and the loco may become physically damaged before the operator realizes that irreversible damage has occurred.
In some cases, the reduction in life can be dramatic and immediate. If the loco was lightly built, like a Bachmann Big Hauler, additional force may push it right over the edge and strip out the gear train virtually instantly.
Some locos are a little light on motor power in relation to their traction. The earlier version of the Aristo Pacific is one such example. Increases in weight can increase the traction sufficiently so that the motor will actually lug and maybe even stall under load. This will cause the motor current to increase dramatically and can result in very rapid motor failure.
If weight is added to improve traction, then the loco should be tested for slippage before it is allowed to do any extended work. The best test is to put it on the track and simply restrain it by holding onto the coupler. Turn the power all the way up and the drivers should slip furiously. The motor should have the ability to turn the drivers at or nearly at full speed while slipping. If the driver speed lugs down, take out some weight.
If the improvement was gained by adding weight, the forces on the axle bearings will be higher all the time and the bearings will wear faster. If the loco is equipped with ball bearings, bearing wear is not as serious a problem. In any case, proper lubrication is critical for an "uprated" loco.
[ Top ]
Traction tires are rubber or plastic bands set into grooves on one or more driving wheels. They work by sticking to the track better than a bare metal wheel. Many locos are supplied with traction tires on one or more driving wheels. Traction tires do work to increase traction.
Traction tires are inexpensive to produce and don't add to the loco's weight. Manufacturers don't like to add weight because weights cost money to install and the more weight that they add, the more that they have to pay to ship the loco.
In my humble opinion, traction tires aren't worth the powder it would take to blow them up. For track powered locos, they virtually prevent that wheel from contacting the track and contributing to power pickup. The tires tend to wear quickly and sometimes they rot off the wheel. Rubber tires are especially prone to degradation due to exposure to ozone so that in smoggy environments they fall apart quickly.
In every case but one, I've replaced the wheels that came with traction tires with wheels that don't have them. I've recorded decreases in traction but also noticed improvements in power pickup. See my Tractive Effort Tips page for the data on the GP9 and the Aristo Doodlebug. Only my Aristo Rogers still has tires because Aristo doesn't provide replacement rear drivers without them. The Rogers sits in my basement in the mountains and isn't exposed to smog so that the tires have held up fairly well. On the Aristo Doodlebug, the tires literally fell off in less than a month. The plastic tires used by LGB seem to tolerate wear and smog better than rubber tires.
Due to the problems that traction tires cause, changing out existing regular wheels for ones with traction tires doesn't seem to be a good plan to increase traction beyond the box stock condition.
[ Top ]
The other popular method is to add weight. I've done this on several locos with good results. Only an Aristo Pacific has suffered consequences. In trying to make it pull five heavyweights on my 1.6% grades, I overweighted it and nearly toasted the motor. It got so hot that I couldn't touch it and the internal windings turned brown from the heat. With a little more stress, it would have burned out. I removed some of the added weight to limit it's load to four heavyweights. The motor still runs hot, but not hot enough to damage itself.
If you want to add weight to a loco, you need to determine how much weight to add. Use too much and you put the loco at risk. Use too little and you don't get the desired improvement in performance.
The easiest way to test for weight is to get a bunch of 1, 2 and 4 oz lead fishing weights and put them in a zip-lock sandwich bag. Drape the bag(s) over the loco body in a position such that the added weight is evenly carried by the driving wheels and test it. It is easy to add and subtract weight until you determine the correct amount. If the loco slows down noticeably but continues to pull, this is an indication that you've used too much weight. A loco should be able to slip it's drivers WITHOUT the drivers appreciably slowing down when the movement of the loco is impeded by grabbing the coupler and holding it. Wheel slippage without the wheels slowing is a safety release valve, it prevents the loco from lugging. When a loco lugs down and slows under load, it starts to draw a lot more current and the motors will run very hot. You might not notice the impact of the heat until the motor melts itself from its mounts or burns up.
If you have a current meter, place it in series with a track lead and watch the current as you add weight. If you notice more than a 20 or 25% increase in loaded current, you might want to stop there.
If the loco has regular plain bushings for its load bearing axles, you might want to limit the added weight to just a pound or so. More and you'll start to wear the bearings. Make sure that you pay special attention to the bearing lubrication.
Once the optimal amount of weight is determined, you'll need to open up the loco and to install it. Try to get the weight balanced above the driving wheels if you can. On a diesel, an even better location is the fuel tank. This will bridge the weight between the trucks and keep it low. The weights can be glued down with your favorite structural adhesive.
If the loco is small or the places that weight can be added are oddly shaped, then use lead shot. Get it from a sporting goods store (one that sells guns and shotgun shell reloading supplies). It usually comes in 25 lb bags, enough to last awhile. #8 or #9 shot works well. I attach it with a 50/50 mix of TiteBond II wood glue and water although regular carpenter's glue or even white glue will work. Pour the shot where it needs to go and spray with the mix and then let it sit for a day. It will be firmly bonded.
Another good source of weight is a load of batteries used in a battery powered radio control conversion. You don't get much say on how much weight you will add though. If you want the weight to do any good at all, you have to add it inside the loco, above the drivers. If you add them to the tender or a following car, you make more weight for the loco to haul and reduce it's performance.
This page has been accessed times since 17 Nov 02.
© 2002-2010 George Schreyer
Created Nov 17, 2002
Last Updated November 13, 2010