Maximum roof load questions

[slalomskier]

Hey guys, I am going to be roof topping a boat down to miami, which weighs around 300lbs. I am trying to figure out if this is a horrible idea or not. I have done it for short 200 mile trips and less, but this time I am going down to miami from Massachusetts. Picture attached of my rig, and I use three or 4 of the thule cross bars. Any thing I can do to strengthen the factory rack? I am towing two more of these boats behind, and would like to bring a third with me, so this is my only option right now. My usual way of doing this is on my van, but it is already in miami.

Josh

[stupidloose]

I dont see a problem with the weight, especially if your spreading it out over 3 to 4 thule load bars. The problem I see is that it just "looks" bad. You can have everything secured tightly and loaded safely, but I still think with the way it looks you will probably be getting stopped in every state because of the size and appearance of the load. How much overhang will you have out the front and back?

[slalomskier]

I have never been pulled over before with it. the masts are 22ft long, and stick out about 4 feet from the front of the car, which isnt a problem. the front of the boat is about a foot forward of the windshield wipers, and the transom hangs off the back of the boat by about 4 feet as well. I do need to make sure it won't interfere with the trailer I am using.

[bbbuzzy]

Your load definitely exceeds the maximums stated by Volvo and Thule, although I never understood why all the Thule racks I have say there is a 165lb limit. Seems arbitrary. Keep your speed down to reduce the wind drag and "up lift" it creates on your cargo. Good luck.

http://new.volvocars.com/ownersdocs/2005/2005_XC70/05xc70_09.htm#pg161

http://www.rackattack.com/thule.asp

[Ars Gladius]

Although I never understood why all the Thule racks I have say there is a 165lb limit. Seems arbitrary.

Probably because there are limits to material strength and performance in crash scenarios and general vehicle stabiltiy.



I really would not want to put so much weight (300 lbs) so high on the car for anything other than a really short trip. You are significantly changing the weight distribution and consequently the handling of the car. It may not seem too bad during normal driving, but if you have to swerve to avoid at highway speeds, you would quickly find out that your car is not doing what you expected it to do, possibly in a very bad way.

Not to mention a massive hit on fuel economy, especially at highway speeds, you would be better off trailering it.

[Astro14]

It's not just the weight, it's also the moment arm of that long boat...Bumps, manuevers, etc, or heaven forbid, a crash, with that much weight on such a long moment arm and you're asking for some extreme forces that the rack may not be able to handle.

Short trip. low speed, with 300 lbs in plywood sheets (at less than half the length)...I would do that...

But I wouldn't do what you're proposing...I'm with Ars...trailer it...

[slalomskier]

So I cannot trailer it as I am already going to be towing two behind. I am getting paid $1500 to bring it down so am not worried about the gas. I have found in the past that I can drive aggressively with it on top, and as long as it is strapped down tight, there is no difference in car handling other than the noise. The weight of the boat is all concentrated at the center and bottom of the hull, and I can probably get it down to 250lbs. The masts won't be there either. I have seen people put a few hundred lbs in a roof rack without realizing it, so I don't think it will be a disaster. We will see what happens. Knock on wood.

I really just envision this as driving with a big guy on the roof, it isn't an insane amount of weight. I think that I would be more concerned if it was over 400lbs.

Josh

[Ars Gladius]

Here is a very succinct example of why it would be a bad idea to put so much weight on the roof: http://www.youtube.com/watch?v=VHmzZ4-BB7Q (Thx to Lemsteraak)

Great in a straight line, very very bad if you have to avoid something on the road.

And that is without a trailer that will also have a fairly high center of gravity based on the cargo.

[sikbrik]

Can't the 3rd boat go on top of the other 2 that you're already trailering? Any pics of that rig? This looks dangerous. Of course, no worse than hauling it on top of that full size van. I'd pull way over and wait a while if I saw you coming up in my rearview. O_O

Nevermind, just saw the trailer. So it's not 2 next to each other, it's 2 stacked. Weird. Bolt 2 of those trailers together and take 4 boats down with you! Problem solved.

[sjonnie]

Just put a bow line on and you'll be good to go. With that load maybe you want to do two bow lines to each side. Just find a suitable bolt and fasten a loop of webbing under it such that it sticks out the side of the hood. Do one on each side and put a bow line to each one. And take 'er easy :D

http://i12.photobucket.com/albums/a230/WeeHooker/Truck/sub050.jpg

[slalomskier]

Well I made it just fine, though my gas mileage was horrible. Pic attached. Someone hit my door with a grocery cart, but it popped right out and is all good.

Josh


3481

[bbbuzzy]

Glad to hear you made it ok. Pretty amazing load for the vehicle to carry/tow. I guess I won't worry about stuff I carry, as I don't think I'll approach your feat. That's a nasty dent for a grocery cart!

[Astro14]

No kidding! I can't even see the car under all that stuff...:D

[duncan]

Cool pic - what kind of boat is that? Looks like a 420 or 470 from the bow, but hard to say. I've driven those boats all around - they're pretty heavy beasts.

[matt1122]

Here is a very succinct example of why it would be a bad idea to put so much weight on the roof: http://www.youtube.com/watch?v=VHmzZ4-BB7Q (Thx to Lemsteraak)


There is also a very succinct example of why you should steer into the skid. [happy]

[Ars Gladius]

There is also a very succinct example of why you should steer into the skid. [happy]

He was, but once the oscillations started, the car was too unstable to recover.

[matt1122]

If he was steering into the skid, why did the vehicle begin to oscillate?

[Ars Gladius]

Perhaps, "steer into the skid" means something different to both of us?

In my understanding, he was turning into the skid. The rear was starting to come out so he turned in the same direction (rear is going left, you turn left).

You can see the wheel angle change in the video, he was trying to correct but the weight load on the roof made the vehicle very unstable (suspension was unable to compensate and keep the vehicle level) and correcting one skid caused another skid in the opposite direction once the high centered mass of the vehicle shifted to the other side and so on.

Note how much the suspension is compressing and the loss of contact of one or more wheels while the vehicle is still relatively straight (about mid way through).

Under normal circumstances (aka correctly loaded car) the initial maneuver would not have caused that much destabilization, and a competent driver would have recovered.

If he wasn't turning into the skid, he would have rolled it by 00:16 / 00:18.

[Astro14]

If he was steering into the skid, why did the vehicle begin to oscillate?

How many times have you been in a skid...deliberate or otherwise...

The vehicle can oscillate as you overcontrol the skid - steering too much into, or not enough into ... in this particular case, though, the vehicle was too easily overcontrolled because of the high CG...it's more like a loaded van than a car...far less stable...

[matt1122]

If you watch at about 0:08 and 0:10, it appears he must have steered back in the direction the roadway leads after the camera changes angles. It appears the vehicle moves directly towards the camera at 0:10 and the wheels are all facing that way. My understanding was that he should have kept the wheels facing this way at that time, but that he purposefully turned the wrong way as part of the demonstration. It's also likely another vehicle would be there if this happened in real life, preventing the driver from doing what they "should do" to regain control without hitting anyone.

Now that you said "he is turning into the skid," I'm just trying to figure out if that's true and, if not, whether the vehicle would still oscillate in this way if he had. If it does, then I'm curious why this happens. An opportunity to learn. :) Sounds like Astro has indicated that he probably overcompensated. That makes sense to me. I haven't actually heard the audio on the video and had been assuming it was a purposeful demonstration.


As far as my skidding history, which is quite sad...


Once in my XC, when told a friend had died in an accident as I made a left turn. I took the turn way too hard (full throttle) as a result of the shock.
Many times in the T-5R (deliberately), around pole-free parking lots and certain turns with clear visibility and no traffic near home.
Once in the T-5R I had bald tires and took a highway entry ramp too hard.
Once in the T-5R at highway speeds (85mph) to avoid an accident.

I don't think the way the T-5R handled in a skid is remotely comparable to this situation or even the average car, especially given its stiff suspension, high performance summer tires, and low center of gravity, but hey, you asked. I was young and stupid for a long, long time.

[Jorge-789995]

From that video, I learned that the next time I do a slalom course with my XC, I should remove my roof box first.

On a serious note - I think it was obvious that the driver was trying to induce a skid to prove a point that with extra weight up top, you would roll over instead of skid. I do not think the weight caused the skid, I think the weight caused the roll. If the intent was to demo that weight would cause a skid, then I don't think it was an honest portrayal. It looked to me like the oscillation was intentional.

Regarding over compensation. I don't think this is a huge problem unless you are towing a trailer. A normal driver is able to effectively compensate for a vehicle skid. If you add a trailer and the potential oscillation, that is where most drivers cannot compensate effectively.

Keith

[skeith5]

I never understood why all the Thule racks I have say there is a 165lb limit. Seems arbitrary.

Being a European company they would go with metric. 165lbs is 75kg, which seems a little less arbitrary.

Scott

[Ars Gladius]

Matt:

Yes, the video was a demonstration of how the handling dynamic changes with a load concentrated high on the car.

The initial swerve was intentional (eg. avoiding an object on the road), there was no initial skid. The only skidding was later on in the video as the loss of control was becoming more pronounced. I'll hazard a guess that you though it was a skid at the start and he was trying to correct from that?

Jorge:

The demo was not to intentionally slide the vehicle, but show what the impact of a high-up load has on stability when a somewhat 'normal' avoidance maneuver was undertaken.

The initial swerve or two was intentional, however the rest was not and was purely the result of physics. I know it seems hard to believe that a relatively small change compared to the weight of the vehicle can have such a large impact.

[Jorge-789995]

Ars,

Do you speak the language? If so please provide a translation. I'm curious what they are saying.

It reminds me of that Subaru vs. XC video from a few years ago where the XC got stuck and they say something like "too bad Volvo."


Based on the video alone with no translation, I'm not buying that the roll was not intentional. Look at the steering input at 16 and 18 seconds. Those were done either by a completely incompetent driver or with the intention of causing a skid and loss of control. Notice they didn't show the whole sequence including the roll from the outside. Almost any driver could have avoided that loss of control and roll over.


Keith

[matt1122]

The initial swerve was intentional (eg. avoiding an object on the road), there was no initial skid. The only skidding was later on in the video as the loss of control was becoming more pronounced. I'll hazard a guess that you though it was a skid at the start and he was trying to correct from that?

Yeah, that was how I remembered it when I made my first comment about how you should steer into a skid. The same applies to stabilizing the vehicle when the wheels have traction though. By that, I simply meant that if you fight physics, you will lose every time. :)

[Lemsteraak]

I think you guys are all agreed

More weight up high - bad

More driver skill - good

More skid practice - better

I think it is all a matter of degrees. While I have had some really high loads on my roof, I certainly don't like it. Yes, you can do it, doesn't mean you should.

I've worked with a lot of the rack manufacturers like Thule. Here in the states they set a max load of 165 pounds, for roof load and weight of the rack. If you buy the same exact Thule rack from Volvo, max load is 220 lbs. This is because of the higher European standard. The 165 pounds is because of liability insurance, but I think it is wise to keep the max low. It changes the vehicle dynamics and not all drivers are good.

A lot has to do with the vehicle. My experience is that Volvo is top drawer, that is why my wife drives one. I talked with an engineer with Land Rover about the old Disco I and roof loads for expeditions. We were looking at one at SEMA that had something like 450 pounds on the roof, all the gear, jacks, tanks, roof tent, and even a heavy safari basket with mounted axes and shovels. I asked him what LR's rating was, he said 220 pounds, but when I asked him how much so you recommend up there, he replied 135 pounds. After that, you really feel the weight up there. This is on a 5,000 pound truck.

I bought the Volvo XC for my wife after I saw the video you guys are discussing. My take is different. The guy driving was surprised it would roll, I think just about any vehicle would roll in those conditions. We can discuss vehicle dynamics but what I was impress with is the vehicle's structure. That is a strong cage.