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Old 08-28-2014, 11:43 AM   #1
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RV with Tag Braking Distance vs 18 Wheeler?

I was speaking with someone who I always thought was pretty knowledgeable about RV's, when he served up a "fact" I found hard to believe.

He said that Class A RVs ~ even with tags and disk brakes all around, will not stop shorter than an 18 wheeler, even if it's fully loaded.

Could that really be true?
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Old 08-28-2014, 11:52 AM   #2
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From the same speed, I think it would be close.
46,000LB coach with 3 sets of brakes vs 80,000LB rig with 5 sets of brakes.
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Old 08-28-2014, 11:54 AM   #3
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What did he give you to back up his "fact"? I think the trailers on those are hooked to air brakes too, and if that is true a big rig could have 5 axles of braking power... But one fully loaded stopping sooner is interesting. I know we have experienced truckers that RV so I am sure they will chime in soon
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Old 08-28-2014, 12:14 PM   #4
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An interesting fact: Many times I have been told the bigger the ride the greater the "Skid Distance" but... Turns out when computing vehicle speed from skid distance measurements the formula for a "SMART CAR" and a fully loaded 160,000 pound steel hauler..... SAME.

Weight = Mass times Gravity and since Gravity is a constant, (In any given spot) mass and weight thus cancel out.

When you do all the math.. PROVIDED ALL WHEELS ARE BRAING, the formula comes down to (Speed Squared)/Constant*(Coefficient of friction)

And CoF is the same for all vehicles on the same section of road at the same time.

So, once For any vehicle, be it a mini-car or a maxi semi or anything in between.. IF ALL WHEELS ARE BRAKING, Stopping distance is the same.

Other facts: Semi, the trailer brakes come on a fraction of time AFTER the tractor (Fraction of a second) This might add a foot or two

RV's towing cars without an aux braking system.. Can take up to 30% longer to stop (Source US-Gear) I find that excessive but they WILL take farther.. The math is way more complicated.

But for all wheel braking, Only the speed and the road surface matter

And road is same for all vehicles at any given point in time and space.
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Old 08-28-2014, 01:02 PM   #5
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The formula may be the same, but wouldn't the size of the contact patch made by the tires. the tire's coefficient of friction and front to rear weight bias all impact the length of the skid?
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Old 08-28-2014, 01:14 PM   #6
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Yeah, all those things can make a difference. No two tires have the same coefficient of friction, even on the same road surface. And no two vehicle have equally effective brakes either. But rather than make the answer impossible to estimate, let's assume those are more-or-less equal. Same size & type of tire, same good brakes, same skilled driver.

That leaves the big difference as the mass (weight, our purposes). The semi in this question probably has a lot more weight, but also a more wheels braking. If it is twice the weight, e.g. 60,000 lbs vs 30,000, but only 67% more wheels braking (5 axles vs 3), then it is going to take longer to stop. Every time. But if the two were equal in weight, then the one with the most wheels braking is going to stop faster, which means the semi stops first. By a wide margin. You probably won't find many 30,000 lb semi's though, unless empty.
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Old 08-28-2014, 03:40 PM   #7
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There is no simple formula to calcuate accurately stopping distances. example from consumer reports(2011) stopping distances from 60 mph to zero....
Jeep wrangler. 150 feet
Jeep liberty 150 feet
Nissan cube. 149 feet
Acura mix. 126 feet
Porsche booster. 112 feet

Bottom line, I would prefer an rv be behind me rather than semi in an emergency stopping situation.
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Old 08-28-2014, 04:23 PM   #8
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Think of it this way guys. 80,000 pound rig with 18 brakes stopping it vs a 40,000 pound rig with 8 brakes stopping it. The anti-skid system will help IF, and only IF the RV has a good balance for loads on the individual axles. In fact (that word again), RV's do not, which means even less effective braking. Yeah, I believe it.
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Old 08-28-2014, 05:03 PM   #9
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18 wheels would have 10 brakes .
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Old 08-28-2014, 10:48 PM   #10
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Quote:
Originally Posted by MarkofSJC View Post
I was speaking with someone who I always thought was pretty knowledgeable about RV's, when he served up a "fact" I found hard to believe.

He said that Class A RVs ~ even with tags and disk brakes all around, will not stop shorter than an 18 wheeler, even if it's fully loaded.

Could that really be true?
From my point of view I'd rather panic stop my MH from 60mph than either of my 18 wheelers fully loaded (80k). The MH is disc on 2 axles and drum on the tag with ABS and a manually controlled Jake. The three trucks stop differently as well in a panic mode... the KW 18 speed, the International 10 speed, and an Autoshift Freightliner which stops the best of the three. In a panic stop few drivers can manipulate the shifting and Jake effectively. With the autoshift you simply stand on the brakes, the Jake, and transmission downshifting happen automatically. In my coach I have a seperate pedal for the Jake so in a panic stop it's a no brainer. Hard on the binders and the Jake pedal and let the ABS, Jake, and downshifting do their job.

I'm sure there will be conflicting experiences and opinions....
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Old 08-28-2014, 11:06 PM   #11
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Quote:
Originally Posted by wa8yxm View Post
An interesting fact: Many times I have been told the bigger the ride the greater the "Skid Distance" but... Turns out when computing vehicle speed from skid distance measurements the formula for a "SMART CAR" and a fully loaded 160,000 pound steel hauler..... SAME.

Weight = Mass times Gravity and since Gravity is a constant, (In any given spot) mass and weight thus cancel out.

When you do all the math.. PROVIDED ALL WHEELS ARE BRAING, the formula comes down to (Speed Squared)/Constant*(Coefficient of friction)

And CoF is the same for all vehicles on the same section of road at the same time.

So, once For any vehicle, be it a mini-car or a maxi semi or anything in between.. IF ALL WHEELS ARE BRAKING, Stopping distance is the same.

Other facts: Semi, the trailer brakes come on a fraction of time AFTER the tractor (Fraction of a second) This might add a foot or two

RV's towing cars without an aux braking system.. Can take up to 30% longer to stop (Source US-Gear) I find that excessive but they WILL take farther.. The math is way more complicated.

But for all wheel braking, Only the speed and the road surface matter

And road is same for all vehicles at any given point in time and space.
I respectfully disagree. There are too many variables for a one size fits all braking formula based on mass. Disc vs drum is not equal in the real world even before taking into account rotor material, pad material, rotor size, single, double, quad (or more) pucks, number of calipers, and many more factors need to be considered. As noted above not all tires share the same CoF... not even close.

Jump in any late model performance vehicle add weight until it's equal to a Tractor (let's say 30k) and tell me which one you'd rather panic stop from 60mph! Carbon Fiber rotors/pads dual calibers on each wheel with at least 4 pucks per wheel, massive tire footprint with much more adhesion due to rubber compound... the list is endless.

Any thread where braking is discussed is sure to be populated with vastly different viewpoints and theory vs. real world!
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Old 08-29-2014, 12:40 AM   #12
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Quote:
Originally Posted by burgman50 View Post
There is no simple formula to calcuate accurately stopping distances. example from consumer reports(2011) stopping distances from 60 mph to zero....
Jeep wrangler. 150 feet
Jeep liberty 150 feet
Nissan cube. 149 feet
Acura mix. 126 feet
Porsche booster. 112 feet

Bottom line, I would prefer an rv be behind me rather than semi in an emergency stopping situation.
I believe the porshe booster should have been boxster and depends on whether an S or not. That is one of the major differences between them is the braking systems
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Old 08-29-2014, 01:10 AM   #13
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Quote:
Originally Posted by wa8yxm View Post
An interesting fact: Many times I have been told the bigger the ride the greater the "Skid Distance" but... Turns out when computing vehicle speed from skid distance measurements the formula for a "SMART CAR" and a fully loaded 160,000 pound steel hauler..... SAME.

Weight = Mass times Gravity and since Gravity is a constant, (In any given spot) mass and weight thus cancel out.

When you do all the math.. PROVIDED ALL WHEELS ARE BRAING, the formula comes down to (Speed Squared)/Constant*(Coefficient of friction)

And CoF is the same for all vehicles on the same section of road at the same time.

So, once For any vehicle, be it a mini-car or a maxi semi or anything in between.. IF ALL WHEELS ARE BRAKING, Stopping distance is the same.

Other facts: Semi, the trailer brakes come on a fraction of time AFTER the tractor (Fraction of a second) This might add a foot or two

RV's towing cars without an aux braking system.. Can take up to 30% longer to stop (Source US-Gear) I find that excessive but they WILL take farther.. The math is way more complicated.

But for all wheel braking, Only the speed and the road surface matter

And road is same for all vehicles at any given point in time and space.
It has been a long time since I took physics in college. I do believe that part of your explanation is not correct. First of all your weight comparison. Yes weight = Mass X gravity even though gravity is a constant ( at least for this argument) That would not cancel out weight. It just means the weight is going to be a constant for the same mass.

Force is the actual issue to be dealt with. Remember some guy named Newton A body in motion will stay in motion unless acted on by a force ?

The Coefficient of friction is not the same for all tires. There was a reason why my Porsche tires cost a fortune and only lasted for 10,000 miles. They were a silicon based tire and the coefficient of friction was very high which the way I drove it is why I am still here to write this post. The amount of friction a tire has against the road surface is going to be different depending on the size of the tire. There is a reason why they do not have 235x85x16 inch tires on drag racers. Those nice wide tires get a lot better traction

The actual braking force comes from the interaction of the braking components. The amount of friction generated by the brake shoes against the drums ( or brake pads against the rotors) This is an actual opposing force to forward motion. The more breaking components you have the more force you generate to stop.

I am sure you did not mean that all vehicles stop in the same distance. I know I could bring my Porsche down from 60 a lot faster than I can my RV.

In short it comes down to is does the superior breaking power of a semi compared to an RV over come the difference in mass of a loaded semi versus an RV I have not done the math and do not intend to do the math. For those among us that like to do physics problems I anxiously await your results. For my two cents worth I actually think a semi can stop faster. They are going to have 10 brakes compared to 4 on my RV and maybe 6 on a tag axle system ( I do not know if tag axles have brakes) I weigh about 40,000 loaded while a 80,000 pound semi has 2.5 times the braking surface I have.
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Old 08-29-2014, 01:26 AM   #14
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18 wheels would have 10 brakes .
Sorry, I mis-stated my point. The limit is the tires on the ground. A brake set will be able to lock up any dual or single tire set so it's the number of tires against the road and the coefficient of friction that is the issue.
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