Towing capacity and stopping distance calculation guide

[Jim.M]

If your tow vehicle has properly calibrated brakes, in that they come as close to locking as possible without locking, then the stopping distance is the same, save for minor variations in tire friction. I don't know how to answer your question at the end. My car has identically sized brakes on front and back.

hmm, I understand that a coupled trailer and tow vehicle will have the same stopping distance.

And if your trailer can stop itself (uncoupled), in the same or less distance than the tow vehicle (uncoupled), then the brake controller can be adjusted such that the trailer only brakes enough to match the tow vehicle's braking performance.

however, if the trailer has a longer stopping distance (uncoupled) than the tow vehicle, no amount of fiddling with the brake controller is going to keep it from pushing the tow vehicle. An obvious end-case is an unbraked trailer.

most cars and trucks have different brakes on the front and rear axles. common combos: solid front disc, rear drum; vented front disc, solid rear disc;

[rgambord]

hmm, I understand that a coupled trailer and tow vehicle will have the same stopping distance.

And if your trailer can stop itself (uncoupled), in the same or less distance than the tow vehicle (uncoupled), then the brake controller can be adjusted such that the trailer only brakes enough to match the tow vehicle's braking performance.

however, if the trailer has a longer stopping distance (uncoupled) than the tow vehicle, no amount of fiddling with the brake controller is going to keep it from pushing the tow vehicle. An obvious end-case is an unbraked trailer.

most cars and trucks have different brakes on the front and rear axles. common combos: solid front disc, rear drum; vented front disc, solid rear disc;

Yes, that's true. The trailer has a different stopping distance if your electronic brake controller is improperly calibrated, or if the tire coefficient of friction is different from your car. Both of these are likely to contribute even under ideal conditions. It may be possible that the trailer, in fact, has higher friction tires, but who knows? My car has vented fronts and solid backs (same pads and disk diam), but you just said you don't think fade matters, so the vents shouldn't make a difference

[sdiver68]

How do you account for tire contact patch shape and weight transfer effects? Both have been shown to make major contributions to vehicle performance.

[sdiver68]

As to the assertion that trailer brakes don't shorten stopping distances:

http://special-reports.pickuptrucks.com/2011/08/2011-heavy-duty-hurt-locker-brake-test.html



http://special-reports.pickuptrucks.com/2014/08/ultimate-one-ton-hd-challenge-milford-braking.html

[rgambord]

Contact patch shape and size shouldn't matter. However, it is known that rubber is more complicated than other materials when calculating friction. For example, I've read that increasing tire pressure will slightly decrease friction, because rubber has a lower coefficient of friction when more force/sq inch is applied to it. That effect is hopefully irrelevant, as 0.7 is a lower coefficient than the tires on most people's cars anyways, and there isn't a wide variation in trailer tire pressures.

Trailer brakes do shorten stopping distances. What I said is that stopping distance with a trailer with trailer brakes should theoretically be the same as stopping distance without a trailer. In these tests, the trailer with brakes adds to the stopping distance, contrary to theoretical calculations. I have 3 reasons why this is possible:

1. Different tires on the trailer
2. Underadjusted/underpowered trailer brakes
3. Poor brake-force distribution systems on the trucks. I tried to determine whether the trucks have BFD, but I don't know the model years and even then it's not readily available on google.

In the comments:
"Its hard to compare a "5" setting on one truck to a different truck's "5" setting. Also, every trailer is a little different and one may require a different amount of input to get the same brake response, even if still new. Using the same trailer for each test, while certainly more labor intensive and time consuming, would have been preferred to the multi-trailer method used in this test. The "empty" and "trailer brake unplugged" comparisons are better data to evaluate vehicle braking due to these differences."



Here's some helpful discussion on the science of why, theoretically, adding trailer brakes makes your car stop as if the trailer wasn't there in the first place: http://www.reddit.com/r/askscience/comm ... ng_brakes/

[sdiver68]

Trailer brakes do shorten stopping distances. What I said is that stopping distance with a trailer with trailer brakes should theoretically be the same as stopping distance without a trailer.

Whew, my bad I misread it. Still, as you pointed out tests show otherwise.

I'll go back to my 2 basic points. The model you are using omits tire contact patch shape (yes it absolutely does matter!) and weight distribution/transfer. More later

[rgambord]

Trailer brakes do shorten stopping distances. What I said is that stopping distance with a trailer with trailer brakes should theoretically be the same as stopping distance without a trailer.

Whew, my bad I misread it. Still, as you pointed out tests show otherwise.

I'll go back to my 2 basic points. The model you are using omits tire contact patch shape (yes it absolutely does matter!) and weight distribution/transfer. More later

I'm afraid I don't understand what you mean regarding the patch shape. You're saying that two tires at the same pressure, but different widths can affect friction? Or are you referring to tread patterns?

[rgambord]

I found some interesting reading material..

http://the-contact-patch.com/book/road/ ... tact-patch

Very neat. Don't forget to check out the next few chapters. It's not just one page of info.

The basic answer is: tires are complicated, but we use μ = 0.7 because it is a rough approximation for most tires under normal conditions.

[sdiver68]

Tire Traction:
25+ years of racing and having a version of this same discussion on race bulletin boards has taught me that you can't simplify vehicle performance equations as you have. Here's a great thread, including a link to SAE mathematical models on tire traction:

http://www.f1technical.net/forum/viewto ... =6&t=13234

Modern racing simulation games have very elaborate Physics models that dictate performance of the cars. These mathematical models take thousands of lines of code.

Weight Transfer:

All kinds of things can happen with a vehicle-trailer combo. Depending upon the relationships of axles to weight, dimensions,capabilities of the vehicle-trailer and a bunch of other variables, a hard braking situation can move weight around to any of the axles. Some tires may not have any traction at all while others might get overloaded.

Other

With more and more electronic nannies your car might not be safe for unrated loads. For instance my Audi changes it's Traction, Stability, and ABS settings if it senses either a roof rack installed or a trailer being pulled.

Consider the legal aspects of trailering. Yes, your honor I know my 4 speed, 74 lb-ft peak, Mitsubishi Mirage is not rated to tow but I ran these calculations that said I'm good to 2600 lbs, I'm not sure why I couldn't stop...

IMHO

If you want to know what you can tow simply check your tow ratings. If you are skeptical and want to push your luck, check the tow ratings for the same vehicle in Europe.

[rgambord]

Tire Traction:
25+ years of racing and having a version of this same discussion on race bulletin boards has taught me that you can't simplify vehicle performance equations as you have. Here's a great thread, including a link to SAE mathematical models on tire traction:

http://www.f1technical.net/forum/viewto ... =6&t=13234

Modern racing simulation games have very elaborate Physics models that dictate performance of the cars. These mathematical models take thousands of lines of code.

Weight Transfer:

All kinds of things can happen with a vehicle-trailer combo. Depending upon the relationships of axles to weight, dimensions,capabilities of the vehicle-trailer and a bunch of other variables, a hard braking situation can move weight around to any of the axles. Some tires may not have any traction at all while others might get overloaded.

Other

With more and more electronic nannies your car might not be safe for unrated loads. For instance my Audi changes it's Traction, Stability, and ABS settings if it senses either a roof rack installed or a trailer being pulled.

Consider the legal aspects of trailering. Yes, your honor I know my 4 speed, 74 lb-ft peak, Mitsubishi Mirage is not rated to tow but I ran these calculations that said I'm good to 2600 lbs, I'm not sure why I couldn't stop...

IMHO

If you want to know what you can tow simply check your tow ratings. If you are skeptical and want to push your luck, check the tow ratings for the same vehicle in Europe.

I agree with what you've said, but I do want to point out that tow ratings are for warranty purposes, are not standardized, are completely voluntary, and are not legally binding in any way. After extensive searching, I have found no law or mention of law which requires you to tow loads less than what manufacturers rate the vehicle as being capable of. The law in my state is also very clear:

http://coolice.legis.iowa.gov/cool-ice/ ... ut=321.431

1. The service brakes upon any motor vehicle or combination of
motor vehicles, when upon dry asphalt or concrete pavement surface
free from loose material where the grade does not exceed one percent,
when traveling twenty miles an hour shall be adequate:
a. To stop such vehicle or vehicles having a gross weight of
less than five thousand pounds within a distance of thirty feet.
b. To stop such vehicle or vehicles having a gross weight in
excess of five thousand pounds within a distance of forty-five feet.

There are load regulations for commercial vehicles through the FMCSA and other agencies, but not for passenger vehicles. Every government source I have found says that load ratings are recommendations, and that they change when you add more equipment (like trailer brakes, sway control, weight distribution, etc...).

My calculations are a guide. You must still comply with the actual laws. I will not be towing 3000 pounds on my trailer. In fact, my trailer will likely weigh less than 1000 lbs fully loaded. However, I still am installing a 3500 lb axle and brakes (even though the tires are rated to 1000lbs each, and the springs are rated for less) because I feel it is safer. The first thing I will do is test my stopping distance and make sure I comply with the law.

[Jim.M]

What I said is that stopping distance with a trailer with trailer brakes should theoretically be the same as stopping distance without a trailer.

Actually, if you re-read your original post, you said that any vehicle towing any trailer with trailer brakes can stop in 172 feet. I assume if that Toyota Tundra towing the space shuttle pointed downhill and got up to 60mph, they stop in 172 feet?

you should back up these kinds of dogmatic assertions with the long list of assumptions you've conveniently not included in your original post.

Let me help you:

'i define a braked trailer as a trailer with brakes, whose braking capabilities when attached to the tow vehicle, perfectly match the tow vehicle's braking characteristics, thus ensuring I can drop the trailer out of the braked trailer calculations'

Am I close?

[rgambord]

What I said is that stopping distance with a trailer with trailer brakes should theoretically be the same as stopping distance without a trailer.

Actually, if you re-read your original post, you said that any vehicle towing any trailer with trailer brakes can stop in 172 feet. I assume if that Toyota Tundra towing the space shuttle pointed downhill and got up to 60mph, they stop in 172 feet?

you should back up these kinds of dogmatic assertions with the long list of assumptions you've conveniently not included in your original post.

Let me help you:

'i define a braked trailer as a trailer with brakes, whose braking capabilities when attached to the tow vehicle, perfectly match the tow vehicle's braking characteristics, thus ensuring I can drop the trailer out of the braked trailer calculations'

Am I close?

Well, first of all, going downhill isn't what was calculated. Flat and level road is what is referred to in every law I've seen. Theoretically, you could stop a tundra towing the space shuttle in 172 feet, but as has been discussed in the previous couple of comments, tires don't behave linearly. The relationship between normal force and friction is non-linear because of the elastic properties of rubber. The differences between a 1000 lb and 3000 lb trailer is going to be fairly small, and accounted for with using a lower than reality coefficient of friction. The difference between that and a 155,000 lb shuttle is going to be out of the bounds of such a simple calculation.

[Jim.M]

Well, first of all, going downhill isn't what was calculated. Flat and level road is what is referred to in every law I've seen. Theoretically, you could stop a tundra towing the space shuttle in 172 feet, but as has been discussed in the previous couple of comments, tires don't behave linearly. The relationship between normal force and friction is non-linear because of the elastic properties of rubber. The differences between a 1000 lb and 3000 lb trailer is going to be fairly small, and accounted for with using a lower than reality coefficient of friction. The difference between that and a 155,000 lb shuttle is going to be out of the bounds of such a simple calculation.

my main beef with your original post, is that you've made all sorts of shortcuts and assumptions, none of which are explained, and end it with the pronouncement about towing a trailer with brakes:

'In fact, all vehicles can stop in 172 feet at 60 mph. The limiting factors are tire integrity and brake fade.'

So, can you list the assumptions you've made that allow you to proclaim that 'The stopping distance, with trailer brakes, is identical to the stopping distance of the vehicle, without a trailer attached' and of course the '172 feet at 60mph'???

Because it seemed your original post was done to help people calculate how far outside the manufacturers recommended tow limits they could tow. Most of the people on this forum won't call you on your simplistic formulas, but I will continue to do so.

Cheers!