# does payload weight distribution affect rolling resistance?

Recently came across the following 2-minute YouTube video that claims A Balanced Rig Saves Fuel

While nicely done, it is a marketing video. Does anyone know of any unbiased studies that support the claim?

• The more even the weight distribution on the wheels the better the fuel economy. This is because tires are non-linear. – John Alexiou Mar 8 '18 at 1:12
• @ja72 I guess by "tires are non-linear" you simply mean that if you shift freight around in a previously balanced load to arrive at one that is unbalanced, the reduction of resistance in the one tire is less than the increase in the other...in other words if it were a linear process then the two changes would cancel each other out. – mathematrucker Mar 11 '18 at 19:34
• Say a tire with vertical loading 800 lbs has resistance 8 lbs, with 1000 lbs resistance is 10 lbs and with 1200 lbs resistance is 13 lbs. That is the nonlinearity I mentioned. If 4 wheels operate at 1000 lbs, the net resistance is 2*10+2*10=40lbs. But if the two front tires is at 800 lbs and the two back at 1200 lbs, the net resistance is 2*8+2*13 = 42 lbs. – John Alexiou Mar 11 '18 at 21:20

Consider to simplify the model to two wheels in line as in a bicycle. For nice round figures, the tires are inflated to 100 pounds per square inch.

The load is two hundred pounds and is placed over the rear tire in such a way as to fully load the rear axle and completely unload the front.

The footprint of the rear tire is now two square inches.

In order to accomplish that footprint, the tire has to deflect along the sidewalls in such a manner as to substantially distort the rubber. This distortion absorbs energy that would otherwise be used to propel the vehicle.

A similar situation is in place when a tire is underinflated. I have a cart similar to that shown in the video. When it is heavily loaded and the tires are at 45 psi, the cart rolls easily. One can allow that the tire footprint matches more appropriately the engineering of the tire and that sidewall flex is at a minimum.

When the tire is underinflated (often!) at say, 20 psi, the footprint is greater, the flex is greater and it sure is much more difficult to pull when loaded.

For a vehicle such as a semitrailer rig, heavy loads over one axle would not necessarily cause visible sidewall flex, but it could be mechanically significant.

The above assessment is a simplification, but appears in the video to be substantiated by the rolling cart test process. When you see someone pulling an airliner with his teeth, you can bet the load is evenly distributed and the tires are either properly inflated or are over-inflated for the purpose of the demonstration.

• Thanks for the good explanation. Considering extremes makes it easier to understand what's going on in the less extreme scenarios. The same effect will persist in both---just in a less obvious way in the less extreme one. – mathematrucker Mar 8 '18 at 3:41