New to RV

[Overtaxed]

You are speculating that a DRW is safer than a SRW. When there is a study to prove that I will believe it. There are far too many other single point failures in both designs to support a conclusion that adding a dual rear tire makes any significant difference. I hear quite often that a DRW is better in a tire failure. That may be the case, but only if the payload is derated to the same as the SRW. The moment you go over the SRW limit in payload, the tire failure scenario still leaves you with an overloaded tire that can immediately fail. That is why when I say when there is a study, with real engineering data, then I will believe there is a significant safety difference. As for all the stability arguments, both vehicles have to pass the same stability tests at their weight limits. The only difference is that the DRW trucks have a LOWER requirement for all of the performance rating tests, but their stability at their respective load limits is exactly the same.

The tires on my 450 (rears) are rated for 3975/ea (19.5 in, G rated). My rear axle, towing the 351, is about 7K lbs, so a single tire on each side is still within ratings. No derate necessary (well, not to tow any normal RV), my truck can tow with 2 tires out and not overload anything (but probably not make whoever is behind me really happy as the tires shred into pieces!). And yes, for those towing crazy heavy, or those with less capable tires, a single failure COULD cause another failure immediately, but that's a COULD, not a will. In a SRW, if you lose a rear tire, you WILL have no more rear tires on that side of the vehicle, 100% of the time. What percent does the cascading failure happen on a DRW? IDK, but I'll bet my life it's <100%. I'd be surprised if it was over 1%, but I think we can agree, that's a far less likely situation than losing a single rear tire, especially if your rear tires are rated, individually, to carry the payload you have.

As for all the stability arguments, both vehicles have to pass the same stability tests at their weight limits. The only difference is that the DRW trucks have a LOWER requirement for all of the performance rating tests, but their stability at their respective load limits is exactly the same.

While this may be true (IDK, to be honest), the "respective load limits" are wildly different. Is a 450 towing 34,600 lbs (2020 5er limit) the same stability as a 250 towing 17,300 lbs (2020 5er limit)? Maybe? Honestly, I'd be very surprised if it was, but, it's also neither here nor there. If you want a 351M (which I use because it's what I own) and the max gross weight is 16,800, is it going to be more stable behind a 450 or 250? No need to speculate, I've had both and towed a 351M with it, and even detailed my entire experience with the 250 and then the 450. The answer is "yes, it's more stable". Would my 450 exhibit similar stability to the 250 towing the 351M towing a tandem dual dump trailer that weighed 15 tons? Don't know, but it's also inconsequential to me, I'm not towing 15 tons, I'm towing 8, the 351 doesn't get heavier when attached to the 450, it just gets more stable/better controlled.

You are speculating that a DRW is safer than a SRW. When there is a study to prove that I will believe it. There are far too many other single point failures in both designs to support a conclusion that adding a dual rear tire makes any significant difference. I hear quite often that a DRW is better in a tire failure.

I honestly don't know if such a study exists. But, without an ability to produce such a study, let me ask you, does a DRW truck increase or decrease the points of failure in your view that would lead to an accident? Without statistics to back it up, all I can say, it seems obvious that much of the "really dangerous" stuff that can happen towing has more redundancy in a DRW truck. You've removed some risk, how much, I don't think any of us will ever know, but I think it's inarguable that there are less catastrophic single points of failure in a DRW vs a SRW.

I can't see any amount of rationalization that says it is ok to knowingly drive with an illegal condition on your vehicle. When we get behind the wheel of a vehicle there is some risk involved but we are expected to follow the law.

And now I'm going to take the other side of the argument. There's real question if it's "illegal" to exceed GVWR. Or, put another way, what exactly is illegal, exceeding GVWR, axle rating, tire rating, combination of both/all of them? In my state, you can register a vehicle for many different GVWRs, you just pay more for the tag depending on what you want to be "legal" to haul. My 450, for example, is registered at 11,000lbs, rated (by sticker) to a GVWR of 14,000 lbs, and adding the RAWR and FAWR together nets an even higher number. Adding up the tire capacities gets a stupid high number. Most of the laws I looked up (and I looked at a lot) when trying to figure out "should I do this" with a 250 pointed at RAWR and tire ratings as the numbers that were the "legal limits", not GVWR.

I guess, in conclusion, this is a "you do you" issue. Do what feels safe, if you're close, weigh it, look at your ratings, honestly assess your skill level, your level of fitness (how quickly can you respond) and your exposure (do you tow 100 miles a year or 500 miles a day) and make a decision. I don't fault people towing big rigs with SRW heavy duty pickups, that would make me a hypocrite, I did it, I lived, nobody got hurt, and I didn't feel unsafe doing it. No scary moments at all, although I did find that engine braking could make the rear end squirm a bit, it never felt like it was going to come around on me. Max tow on my SRW was about 20K lbs, I was way under that, under on axles, under on tires, and over on GVWR. Was I legal? Probably not, but I honestly don't know for sure. On the 450, I'm under on everything, so I suppose now I could be considered "legal". Except for one thing, I pull with a Goosebox and because of that, need safety chains. The 450 has these strange bars in the bed to connect safety chains to that the chains from my GB will not attach to, so I had to get U shackles to be able to connect. I couldn't find any that were rated for the weight of my trailer's possibly dynamic load, I'm not sure they exist, so, honestly, I'm probably breaking a law there because the part I need doesn't exist. But only in some states, some don't require chains for the Goosebox. <sigh>.

One thing I will tell you, this whole experience for me, and this is coming from someone who's been towing for 25 years, gave me a tremendous amount of respect and understanding for what commercial truckers go through. It's a blizzard of regulations, one contradicting the next, varying by state (and even by highway in some cases). I read many trucker boards where guys are complaining that the "law" around trucking has basically turned into a "tax" because if they look, they're going to find something wrong and you're going to get a ticket. There are so many laws/regulations, you're almost always doing something "wrong" or "illegal". And that's a crying shame, it really is, because it makes determining if you're "safe" into an investigative reporting segment rather than a "look here and here, if OK, you're good to go". It's a real shame, and threads like this are just a symptom of the problem, there's so much room for discussion and interpretation it ceases to have a clear answer anymore.

[BobKilmer]

For the risk reduction of the DRW, the one area that the risk does increase is there are 50% more tires to fail. That is of course mitigated by the increased chances of a failure are where you have the dual tires. However, given all the other single point failures (like the front tires, the entire front end assembly, both axles,...) I really am not sure there is a significant difference in the overall reliability. I specifically use the word significant because as an engineer I am used to looking at the impact on the entire system. I agree that one failure mode is improved. I am just not sure it makes an overall difference in the likelihood of a catastrophic failure.

DRW trucks do have some obvious advantages. Payload and towing are the most obvious. I would probably own one except for the ride difference between my F-350 SRW and the DRW. We test drove both, and the spouse said she wouldn't accept the ride in the DRW. I had to agree with her on the ride difference, and since I had done the analysis on the towing limits, that made the decision for us. If we drove another truck or newer F-350 we might come to a different conclusion just for the payload differences.

I agree completely on the respect for professional truckers! I have a whole new appreciation for what they go through, not only for the regulations but putting up with everything on the road. I do my best to give them room and respect on the road, regardless of what I am driving or towing.

[DaveMatthewsBand]

The 2 common situations are a sway that gets out of control (that a DRW would have prevented) or a rear tire goes out (that a DRW would have prevented), either leading to a serious accident. While we can debate endlessly on the differences between a 250 and 350 SRW (is it safer to tow, from the perspective of "miles driven per accident" or "seriousness of the accident"), there's not much debate at all between SRW and DRW, the answer is "yes, it's safer"...

Exactly!

It’s obvious and common sense that a dually is safer to tow with. Anyone that uses the “two additional points of failure” excuse doesn’t understand odds. If I had 10 tires sure there’s more points of failure but the odds of me have multiple blowouts is astronomical.
I have a better chance of running over a unicorn.

A dually is a lot safer to tow with period.


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[BobKilmer]

I am sorry, I do understand odds. And statistics and reliability analysis and safety analysis. Having three technical degrees and 38 years of engineering experience leading the design, development, and fielding of multi-billion dollar systems has provided me that background. It also has trained me to look at the entire system and see when one small fix does not make a significant impact in the big picture. That is exactly why I said that adding a second rear tire doesn't address all the other single point failures and doesn't make a significant difference in the overall safety.

If you want to discuss odds, I would look at how many times you actually have a blow out (defective tires like Westlake Es aside) versus a gradual flat tire. After 45 years of driving I have never had a blowout on any vehicle. Many flats, never a blowout. Modern properly maintained tires are specifically designed to NOT do that. A tire typically goes flat with more than enough warning to safely stop, especially when used with TPMS. I would also look at the odds of having a failure of a truck tire versus a trailer tire. Losing one tire on a dual axle trailer immediately puts most trailers in a drastic overweight condition. In the big picture, I don't think adding another set of rear tires to the truck does anything to really significantly improve the safety.

For the dual tire failure, that is what we would refer to in engineering as a cascading failure. The failure of a single tire would overload the second tire, increasing the odds of it having a catastrophic failure. This is very different than having two flats or running over a unicorn. It happens all the time when the system isn't designed to be fail safe.

[Overtaxed]

I am sorry, I do understand odds. And statistics and reliability analysis and safety analysis. Having three technical degrees and 38 years of engineering experience leading the design, development, and fielding of multi-billion dollar systems has provided me that background. It also has trained me to look at the entire system and see when one small fix does not make a significant impact in the big picture. That is exactly why I said that adding a second rear tire doesn't address all the other single point failures and doesn't make a significant difference in the overall safety.

If you want to discuss odds, I would look at how many times you actually have a blow out (defective tires like Westlake Es aside) versus a gradual flat tire. After 45 years of driving I have never had a blowout on any vehicle. Many flats, never a blowout. Modern properly maintained tires are specifically designed to NOT do that. A tire typically goes flat with more than enough warning to safely stop, especially when used with TPMS. I would also look at the odds of having a failure of a truck tire versus a trailer tire. Losing one tire on a dual axle trailer immediately puts most trailers in a drastic overweight condition. In the big picture, I don't think adding another set of rear tires to the truck does anything to really significantly improve the safety.

What other "single points of failure" do you see commonly leading to accidents that's not addressed by DRW? Blowout of a front tire, agreed, no question about it. Blowout of a trailer tire (probably the most likely), no, a dually doesn't "fix" it, but I think anyone here would rather be in a dually with a trailer that's lost a tire than a SRW, that commonly causes a huge sway as the weight transfers, something where more contact patch with the ground on a TV is better. And, of course, you have the other things, not "failures" but more common situations, needing to stop quickly, more tires on the road, more stopping force. Wind kicking the trailer around, more tires, more lateral stability.

I've also never had a blowout, but they do happen, there are plenty of stories (and sadly, sometimes pictures) of the outcomes that can happen from that. Does it happen "often"? No, I don't think it does. But that one I kind of see like wearing my seatbelt, no, don't need it 99.9% of the time, but, when you do need it, wow, it's important to have it.

I agree, DRW is only addressing a few of many failure modes for a tow rig. But it's addressing some of the ones that have the most catastrophic outcomes, which, of course, is important. Not going to keep me from backing into my barn while parking the RV, probably a much more common failure mode, but also much less damaging to people/property.

I don't think anyone here is telling you or implying that you should get a dually. I know I'm not. But at the same time, if someone here came on (and I know this has happened on other boards) to this thread and said "You shouldn't get a dually, you should get a semi, it's safer and better at towing". Guess what? He's right, it is safer and better at towing than any dually pickup truck. And there are failure modes that a semi will handle that my 450 will not, no question about it in my mind, if I had a semi, there are certainly some accidents that I could avoid or better survive than in a dually. Now, I don't have a semi, and the reason is cost and utility, a semi is expensive and it's very single purpose, no dropping into the local town diner when your driving a big rig (in most cases). But I'm under no illusions, a semi is better and safer at towing a big trailer than my dually. We all have to examine those odds for ourselves, some land on a 250 class truck towing a 16K trailer (where I started) and others don't feel comfortable until they are in a semi. But each step "up" in size/wheels/brakes/etc does, without question, increase your safety on the road.

[BobKilmer]

I think you commented on the two biggest single point failures, the front tires and the trailer tires. Additional single point failures exist along the suspension, axles, and drivetrain. It is hard for me to say it is ok to have single front tires and single trailer tires and then rationalize that a dually really helps increase the overall safety. I haven't seen any blowouts, especially on one of the two tires on the back of a dually. You are more likely to have a failure of one of the other tires just due to the fact there are more of them.

For me, I think the dually discussion is very different than seatbelts. Regulations require a seat belt and there is data to show they save lives (although some people still manage to argue that point).

I agree completely with your last paragraph. It is a trade and everyone needs to do what they are comfortable doing. As I mentioned previously, I might have owned a dually if the one we drove had a better ride.

If I can digress for a second, in engineering there are typically two types different design philosophies. One is called Fail Safe and the other is called Safe Life. Fail Safe is just like it sounds, when something fails it is still safe. A Safe Life philosophy is all of the parts should be designed to be safe (not fail) for the life of the product, assuming regular repair and maintenance is done. This is how every vehicle in production today is designed. A good example is the front wheels. In a Fail Safe car if one of the the front wheels fell off the vehicle could continue to run and steer without it. In a Safe Life car, the front wheels would be designed to not fall off as long as they were properly maintained. Fail Safe is almost always safer than Safe Life, but only if the entire system is Fail Safe. It is also almost always much heavier and much more expensive. That is why cars don't have redundant front wheels and trailers have single tires. My point of discussing this is adding the second tires to the rear axle may make them Fail Safe (assuming no catastrophic failure) but the rest of the car is still only Safe Life. The result is the rest of the system (front tires, trailer, hitch, axles,..) are all still the driving factor in safety. Hopefully that didn't make anyone's head hurt. Geek mode off now.

[DaveMatthewsBand]

...adding the second tires to the rear axle may make them Fail Safe...

There it is. You just agreed that adding second tires makes the whole setup safer.

No one said perfect, no one said it resolves all other points of failure.

I don’t have 3 degrees and 38 years of designing multi-billion dollar widgets but that seems pretty clear to me.


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[BobKilmer]

Actually no that is not what I said. Making a single part fail safe doesn’t make the system safer. That is the whole point of my comment. You should read the words I wrote. I said it makes no significant difference. If it did the engineers who built the truck would be equally worried about the front tires failing.

My widgets are what make it safe for you and everyone else to enjoy their freedom You’re welcome.

[DaveMatthewsBand]

My widgets are what make it safe for you and everyone else to enjoy their freedom You’re welcome.

Cool story bro!


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[BobKilmer]

Thank you!