Vehicle tire load vs stopping distance

[theboom]

Forgive me if I have not posted this in the right place but I have a quick question. Say if you have a trailer on a truck, let's say a semi trailer, that has 2 axles on it. You measure the stopping distance from a certain speed. You then add another axle to the trailer that has brakes on it with no additional weight on the trailer other than the few 100lbs that the axle itself weighs and test it again from the same speed in the same conditions. If assuming the brakes are fully capable of locking up the tires regardless of the weight on them, will the 2 or 3 axle trailer stop quicker? Basically I am asking if the extra axle helping stop will stop quicker or will the extra axle taking weight away from the original 2 axles make them not able to help stop as much resulting to the same stopping distance or maybe even less due to the extra weight of the 3rd axle?

 

[.Scott]

The stopping distance is determined by the speed and the coefficient of friction between the tires and the road. So, normally, there should be no difference.
If the brakes are actually being locked (as you described), then the extra axle could have the benefit of melting less rubber - and thus keep the coefficient of friction from being reduced. So, depending on exact speeds, weights, tire contact area, and tire material, there could be a reduction in stopping distance with the additional axle.

Also, with more tires on the ground, wear on each tire as a function of braking and driving is reduced.

 

[rcgldr]

For tires, the coefficient of friction decreases somewhat as the load increases, known as tire load sensitivity. Wiki article:

https://en.wikipedia.org/wiki/Tire_load_sensitivity

 

[theboom]

The stopping distance is determined by the speed and the coefficient of friction between the tires and the road. So, normally, there should be no difference.
If the brakes are actually being locked (as you described), then the extra axle could have the benefit of melting less rubber - and thus keep the coefficient of friction from being reduced. So, depending on exact speeds, weights, tire contact area, and tire material, there could be a reduction in stopping distance with the additional axle.

Also, with more tires on the ground, wear on each tire as a function of braking and driving is reduced.

I was simply stating that all brakes were capable of locking up to imply that they are able to hit max grip in all tires, not that during the test they would lock up. All tires in the test would have ABS. So does that change your answer?

 

[theboom]

For tires, the coefficient of friction decreases somewhat as the load increases, known as tire load sensitivity. Wiki article:

https://en.wikipedia.org/wiki/Tire_load_sensitivity

I have been looking at explanations of that and it still kinda confuses me. They say that with increase of load would decrease the friction but then why do drag racers try to get all the weight to transfer to the rear or do they put the engine in most supercars in the back to get more weight over the rear tire to help accelerate faster. Also wider tires on cars have been shown to decrease stopping distance in addition to other performance benefits and an extra axle would basically be the same as wider tires because = more rubber on the ground overall.

 

[.Scott]

I was simply stating that all brakes were capable of locking up to imply that they are able to hit max grip in all tires, not that during the test they would lock up. All tires in the test would have ABS. So does that change your answer?

That makes melting much less likely. So the coefficient will stay the same.

I would have to guess about the effects of the ABS itself. My guess is that the extra axle will not change the stopping distance.

 

[.Scott]

@theboom : I notice that the wiki article you cited discusses slip during cornering turns. That would create odd deformations in an inflated tire - and would expose different portions of the tire surface to the road.
I am not sure if there would be corresponding problems when braking with an ABS.

 

[berkeman]

I have been looking at explanations of that and it still kinda confuses me. They say that with increase of load would decrease the friction but then why do drag racers try to get all the weight to transfer to the rear or do they put the engine in most supercars in the back to get more weight over the rear tire to help accelerate faster. Also wider tires on cars have been shown to decrease stopping distance in addition to other performance benefits and an extra axle would basically be the same as wider tires because = more rubber on the ground overall.

Can you give some context to your questions? are you designing a trailer, and trying to decide how to configure it?

 

[theboom]

Can you give some context to your questions? are you designing a trailer, and trying to decide how to configure it?

http://www.cowtownsleepers.com/images/safetyhitch1.jpg

Someone online was talking about this and they said it would increase braking performance by 50%. I started thinking that that may not be true. I wanted to ask about the actual physics around it to see what would really be the benefit if any.

As you can see the dolly thing takes the weight off the back of the truck and puts it on the new extra axle. This is where I got my question if adding a second axle on a trailer without changing the weight would change the stopping distance. Yes you have a second axle to brake but in my mind you taking the weight off the back of the truck makes the rear brakes on the truck not be able to do very much resulting in no increase in stopping distance? Am I correct in assuming that? With no weight transfer to the rear truck axle, the rear axle will get lighter under braking like in a car. Another thing to think about is that the dolly adds 1300lbs to the total weight of the rig. The brakes on modern trucks and the dolly are top notch so no difference there.

And if it’s about rubber on the ground how about this dolly on a single rear wheel truck vs a dully without the dolly as they have the same number of tires on the ground. Would this dolly on a dually further increase braking performance or would weight transfer and the extra 1300lns cancel the extra axle out? What about if they moved the mounting point forward a bit in the dolly so that 50% was on the truck and 50% on the dolly? Same basic thing because your still taking half the weight off the truck?