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Old 09-08-2020, 06:50 PM   #1
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2021 Interstate 24GT
West Chester , Ohio
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Interstate Swinging Carrier for Large Scooter

Have had several questions about my swing-out Vespa rack for our Interstate. We have now reached the midway of our maiden Interstate trip Ohio-California-Ohio trip (~3.5K miles so far). So, thought it was safe to detail my rig. To be clear....I’ve just built on other’s idea and added my invention to help with the weight of my Vespa GTS 300 (348lbs dry). Here’s the set-up starting at the hitch....
- RakAttach (1up-usa.com) - Large Passenger Side 90 degree swinging hitch has anti-rattle mount to Interstate’s receiver. Only rated for 275lbs....so no problem with bikes or 150 scooters, but needed some help for my larger scooter (see solution I fabricated below).
- VersaHaul Steel Motorcycle Carrier (versahaul.com) - This is one of the most widely used carrier on the market. When I installed this, I flipped the so that the front wheel stop is on the passenger side so when it swings to the curb side, the ramp is away from the vehicle. Note I had to add new stick-on reflectors from Home Depot because of the flip.
- Custom Fabricated Bracket/Wheel - The RakAttach and VersaHaul have no issues with the weight when the system is in the secured position. However, even though it seemed to still be OK when it was swung out.....I thought it could use some help. So, I had my welder fabricate a “L Shaped” steel bracket with a heavy duty swivel utility wheel. So, when unloading the scooter, I put that in the VersaHaul receiver wheel-down. When it swings out, it rolls on the pavement (or slides in the gravel), providing additional support. Note, that it was designed for normal conditions and heights to the ground will vary. So, just use the Mercedes sprinter air suspension to adjust the height.
- Scooter Strapping - I used Rhino USA 1.6” heavy duty ratchet straps and along with Canyon Dancer handle bar tie down grips found on AMAZON to secure the scooter. Additionally, I added two more straps diagonally through the Interstate’s steel skid rings under the bumper to further stabilize sway. If you didn’t notice, I’m into overkill Click image for larger version

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Old 09-08-2020, 07:06 PM   #2
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Quote:
Originally Posted by Rhamilton11 View Post
. . .
. . . If you didn’t notice, I’m into overkill.
. . .


Curious to know the grand total weight of everything you have added, plus the Vespa, and how far behind the rear axle the center of gravity of the new mass is?

Have you done any calculations of the moment arm you have added way back there? Do your MB and/or AS owner's manuals have anything to say about weight/distance limits in this extreme [ "overkill" ? ] location?

Also wondering if you have weighed the total rig, and compared your actual rear axle load with the specs? Are those dual rear wheels, or single? It is hard to tell from the photo.

Thanks,
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Old 09-08-2020, 09:14 PM   #3
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Photos of the Swinging Carrier Mechanics

Maybe should have stated the obvious....many times not unloading scooter....just swinging it out of the way to open the Interstate rear doors.....keeps me in good graces . Here are some pics of the mechanics.....
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Old 09-09-2020, 12:05 AM   #4
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Below is the research I have done. I have not done (nor am I capable of doing) the calculations you mentioned. I can say that we travel with only two people and a scooter. The vehicle is rated for 7. We made it nearly 4K miles with no issues.....but you’re on your own on what works for you. Here are the weights and limitations I am aware of....

Large RakAttach - 55
VersaHaul with ramp - 82
Straps - 10
Vespa - 348
Fuel - 15
_____________
Total Weight - 510 lbs. (below the max tongue weight)

Airstream Interstate (Mercedes Sprinter 3500)
- Max Towing Weight - 5000 lbs.
- Max Tongue Weight - 750 lbs.
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Old 09-09-2020, 03:35 AM   #5
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Thanks for the details. You may be OK in terms of static tongue weight [with a trailer connected to a tow vehicle, and supported by its own wheels/tires].

But you appear not to have considered the geometry of such a large weight -- in motion -- being cantilevered that far behind the vehicle, like a:
-- weight on the end of a clock pendulum
-- stone in a slingshot
-- yo-yo on the end of a string
-- hammer driving a nail home
-- ax head at the end of the handle [while splitting wood]
-- etc.

Weights like this, at a distance from the root end of the action, develop enormous rotational forces.

The exact physics of all this is above my pay grade, but this "Moment" article, and related articles, may help IMO:
https://en.wikipedia.org/wiki/Moment_(physics)

A good example perhaps -- if we wanted to split a piece of firewood, and we stood that piece up on one end, held the head of an ax [only the head] above the end of the wood in two hands, would it not be laughable to think that we could split the firewood by dropping only the ax head down manually without swinging the ax?



Now . . . stand the piece of wood up vertically again, grab the far end of the ax handle, and swing the ax in the traditional manner for splitting wood?

Same ax head . . . much different result!



In my personal opinion, you are playing with fire here.

"Give me a lever long enough, and a place to rest it, and I will move the world?"

Best wishes,
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Old 09-09-2020, 03:54 AM   #6
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PS -- This "Out of Control Sway" thread is a good read IMO . . . for the learning process we lay folks go through, in order to understand the real world in action:

https://www.airforums.com/forums/f23...ay-152451.html

One of the OP's concluding posts, after many pages of the learning process:

[click on orange arrow to to directly to Post #201 in that thread]
Quote:
Originally Posted by cazual6 View Post
. . .
As I look back on this, I am so glad for your advice and others. I was and still a knuckle head, just a little wiser now.
In an emergency evasive action, to avoid a collision perhaps, your entire Vespa contraption could easily become the cart that swings the Interstate horse into an out-of-control-sway, resulting in "overkill" dire consequences IMO.

Good luck,
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Old 09-09-2020, 06:48 AM   #7
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Do continue to provide updates on how this is working for you. That is the same exactly setup I am thinking of getting.

I like the idea of that custom wheel support when swinging open the carrier with the bike...
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Old 09-09-2020, 07:20 AM   #8
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The impact on chassis dynamics is far too complicated for anyone on this forum to calculate. It could be measured with lots of accelerometers or modeled by Damler but I’m more concerned with the hinge mechanism.

I was designing a swing carrier for a cooler and struggling with the design. I’m a mechanical engineer and I couldn’t get comfortable with my back of the envelope stress calculations and my design kept getting heavier. I abandoned the swing mechanism for a much lighter fixed carrier.

Off-roaders often use similar devices to mount spare tires on the backs of their jeeps and they sheer their hinge pins with frightening regularity. The RakAttach hinge has a double shear design (good) and when latched, it looks like the lower arm takes most of the loads off the hinge (good) but the spindle looks thin to my eye for the load you’re carrying when pivoted and is likely made of mild steel (maybe not so good). Your wheel will help with the load but it would be nice if it had a stiff spring to help keep it in contact with rough pavement. I assume the Sprinter’s airbags can provide that function but you’re in a bit of a guessing game as to how much to unload the bags.

I was going to use this hinge in my design:

It has a 1.75” spindle and is a double sheer design. I couldn’t return it so it’s still sitting in a closet. If anyone is interested in it, let me know. It retails for just over $150.
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Old 09-09-2020, 08:17 AM   #9
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RHamilton11, what my husband LB_3 is being very careful in expressing is that we cannot conclude that your type of configuration is safe, but we personally doubt that it is safe. If I were a bettin' woman, I would bet on it failing after metal fatigue has had a chance to set in. The main question is the timeline.

At the very least, in general terms, we agree with Peter (Otra) that there are serious, serious concerns with it, because of the difference between static and dynamic loading. We know what the manufacturer's stated static load limits are for the Sprinter / Interstate. AFAIK, nobody has calculated the dynamic loads that are generated in *actual use* scenarios (as LB_3 noted), but common sense tells us that they are higher than static - probably MUCH higher. And that has major ramifications for what we can carry successfully and safely on our tails.

Omitting a months-long story for brevity, LB_3 and I were originally looking at either buying or creating a very heavy hitch carrier. We backed off and self-corrected after realizing the potential problems with our initial concepts. We settled on a svelte design that carries what we need, but maxes out at around 130 pounds (static), depending on how we are using it (and it's only that heavy for a minority of our travel time). That, combined with a 500-lb tongue weight, gives us enough margin to account for dynamic forces, as far as we can roughly determine.

My husband in his post above said (emphasis mine), "Off-roaders often use similar devices to mount spare tires on the backs of their jeeps and they she[a]r their hinge pins with frightening regularity."

There are multiple additional examples of structural failures happening because dynamic loading was not properly accounted for in the carrying design - and on this very forum, to boot. Here are a couple:

- This poster had his batteries detach at 65 mph and tumble out on the freeway due to an Airstream-designed system that apparently did not properly account for dynamic forces.

- This poster was baffled when his commercial hitch-mounted carrier began breaking away, to the point where he had to lash it into place. He said, "Not sure why this happened, there is no way I had more than 250 lbs and it's supposed to hold 300..." I bet you that the implications of the weight were calculated inappropriately - specifically, the product wasn't designed correctly to account for dynamic loading, and hence it failed.

My point is, these failures occur regularly. Dynamic loading in this context is not as well-understood as it needs to be, so the smart decision is to err on the side of caution.

If your configuration fails at highway speed, not only could that endanger your own life/lives, a break-away scooter could kill or injure someone traveling behind you. Heck, break-away batteries and gear storage boxes could do the same thing. Luckily in the examples I reference above, the breakages were intercepted before total separations occurred and the items were set free to smash into the next drivers down the pike.

We all have different risk tolerance, but personally I would not get behind the wheel of your Interstate while it was in that configuration that you show in the pic.
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Old 09-13-2020, 03:39 AM   #10
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Originally Posted by Rhamilton11 View Post
Have had several questions about my swing-out Vespa rack for our Interstate.
. . .
I hope that you will keep us updated. Earlier posts, expressing doubts and concerns, were done with the best of intentions for your safety IMO.

Happy trails,
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Old 09-24-2020, 04:33 PM   #11
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Hitch carrier for 290lb motorcycle?

I recently purchased a Yamaha XT250, whose wet-weight is about 290 pounds. It was my fond hope to be able to carry the bike on a hitch carrier on a 2017 Interstate GT (3500 EXT). I was looking at something like the Black Widow https://www.discountramps.com/motorc...ier/p/AMC-400/ for a total static weight on the tongue under 350 pounds.

The carrier is not hinged or articulated. Still, reading this and other threads gives me some concern. I would appreciate any feedback that would either relieve my concerns, or warn me away from this configuration.
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Old 09-25-2020, 05:26 AM   #12
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At this point, we have concerns, and we have anecdotal reports of analogous equipment failures that apparently resulted from dynamic overloading.

It might be helpful if someone did a deep dive into parallel usages to see what those users say. Off-roaders, dirt bikers, and anyone else who routinely carries larger-mass objects on their tails.

A proper analysis would take time. I did a quick revisit of tongue weight calculations but the limitation is that they all assume wheels underneath the thing that is on the hitch. So once again, we are back to the problem of having a bunch of analyses of static loading, and nothing useful I can find quickly about dynamic.
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Old 09-25-2020, 06:55 AM   #13
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Originally Posted by InterBlog View Post
At this point, we have concerns, and we have anecdotal reports of analogous equipment failures that apparently resulted from dynamic overloading.

It might be helpful if someone did a deep dive into parallel usages to see what those users say. Off-roaders, dirt bikers, and anyone else who routinely carries larger-mass objects on their tails.

A proper analysis would take time. I did a quick revisit of tongue weight calculations but the limitation is that they all assume wheels underneath the thing that is on the hitch. So once again, we are back to the problem of having a bunch of analyses of static loading, and nothing useful I can find quickly about dynamic.
Thanks.
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Old 09-25-2020, 07:10 AM   #14
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I decided not to take additional risk with the swing away... will stick with just the Vespa carrier... It will push my preparation and planning skills to avoid having to open the back doors... if I can grab it from above, it will have to wait until we get to our destination or I will just have to unload the Vespa to get to it...

Not worth the risk of fatigue on the hitch mounting points and potential damage to the AI.
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