Questions on adding to the rear of Scamp 16 - Fiberglass RV


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Old 07-02-2016, 03:51 PM   #1
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Name: Teal
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Red face Questions on adding to the rear of Scamp 16

Greetings again,

I am considering adding a small cargo carrier to the rear of my Gem with a bike rack attached. I've read some posts that allude to negative reasons for adding to the rear.

What is the weight or tongue ratio issue associated... I am not an engineer, but know that this is the case, just trying to understand, and learn about what others have done to see if what I am planning would be OK.

Wouldn't some weight in the rear offset the tongue weight? how much would be OK?

Thanks so much for your experience and recommendations.
Teal
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Old 07-02-2016, 04:27 PM   #2
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The rule of thumb is - tongue weight should be 10% or more of the total trailer weight. The weight on the rear could make tongue weight too light.

My scamp 16 tows fine with a bicycle on the rear, but I have relatively tongue heavy side bath model and I keep tongue height slightly below level (nose down).
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Old 07-02-2016, 04:37 PM   #3
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The tongue weight should be at least 10% but no more than 15% of the trailer weight. Adding weight to the rear will reduce tongue weight but it also can produce a pendulum effect with the trailer.
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Old 07-02-2016, 07:13 PM   #4
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Why do you say no more than 15%?
Any science behind that? (assuming you stay within the limits of the tow vehicle of course)
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Old 07-02-2016, 11:08 PM   #5
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Cant believe you'd ask that question. But here's all the answer you need to know....

How Tongue Weight Works | HowStuffWorks

Quote:
Originally Posted by widgetwizard View Post
Why do you say no more than 15%?
Any science behind that? (assuming you stay within the limits of the tow vehicle of course)
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Old 07-02-2016, 11:11 PM   #6
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I don't know about 15% figure but too much weight at front or rear can make a trailer prone to sway. I would imagine the hitch to vehicle would reduce that effect for weight in front of the trailer axle, up until the point it couldn't as in during a panic stop or maneuver. There was a video of a model trailer with weight at different locations around at one time. Front or rear overweight reduced stability. Or too high a center of gravity also.


The rear bumper of some FGRV's is really not sturdy enough without some additional structure to handle much extra weight. My scamp is just attached to a couple of frame ends with welds, I would think reinforcement of the frame might be advisable. The camper is light in weight because they did not over do the steel structural parts. Strong enough, not heavy duty if you get what I mean.
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Old 07-03-2016, 06:37 AM   #7
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Quote:
Originally Posted by RogerDat View Post
I don't know about 15% figure but too much weight at front or rear can make a trailer prone to sway.....
A forward center of gravity is inherently stable.
It naturally inhibits sway.
I was actually looking for some science that more than 15% tongue weight is bad.
I cant believe that it is bad - how much tongue weight do semi-trailers have - 50%? They seem to get along just fine.
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Old 07-03-2016, 07:00 AM   #8
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Originally Posted by widgetwizard View Post
A forward center of gravity is inherently stable.
It naturally inhibits sway.
I was actually looking for some science that more than 15% tongue weight is bad.
I cant believe that it is bad - how much tongue weight do semi-trailers have - 50%? They seem to get along just fine.
Tractor trailer the hitch is ahead of and above the rear axle of tow vehicle. A hitch behind the rear axle is entirely different beast. The weight of a 5th wheel or tractor trailer is essentially on the tractor evenly, the cab without the trailer handles badly due to loss of weight on rear axle. As well as loss of weight on front axle.

If nothing else too much weight on the back of vehicle takes weight off of the front wheels of tow vehicle. That would be the ones that steer and normally provide 2/3 of the braking for the vehicle. That braking distribution was why front disk brakes with rear drum was common in the past. Front is where the weight goes during braking. Applies to trailer too.

In effect a trailer that is nose down with too much weight on the rear bumper can shove the tow vehicle around. Lot of leverage applied. Balanced with 10% on tongue the trailer rides level and tends to spread the load across the axles of trailer and tow vehicle more evenly. It is what a weight distribution hitch does, transfer trailer weight to frame of tow vehicle in front of rear axle so it rides between front and rear for better control.

I can't locate that video of a model being run through different scenarios, and I can't give an exact percentage but I can say that too much weight on front of utility trailer handles badly.
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Old 07-03-2016, 07:31 AM   #9
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Good explanation, Roger.

I will add one more reason not to overdo tongue weight: the front of a trailer frame where the cabin transitions to the tongue is a natural weak spot, especially on some lightweight molded fiberglass designs. A fifth wheel travel trailer, and even more so a semi-trailer, is designed to carry that weight. Many eggs are not.

However, I don't think the 15% upper limit is as critical to towing dynamics as the lower limit. If you check out the Trailer Weights in the Real World database, you will see a number of people towing bumper pulls in the 15-20% range, mostly Casita 17s and large Bigfoots. They all have pretty sturdy frames and are typically towed by trucks using a weight distributing hitch. Here's an excerpt from the spreadsheet with the fifth wheels filtered out and sorted by tongue weight in descending order.

Click image for larger version

Name:	Tongue Weight.jpg
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ID:	96985
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Old 07-03-2016, 08:40 AM   #10
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Quote:
Originally Posted by Teal View Post
...I am considering adding a small cargo carrier to the rear of my Gem with a bike rack attached...
I might consider one or the other, but definitely not both. I would keep the total weight of everything- rack and bike(s) or cargo box and contents- under 100 pounds. You will have to adjust the loading of the trailer to maintain adequate tongue weight.

And Roger is right- the bumper alone is not strong enough. There are a couple of safe ways to make the attachment. Scamp sells a rear hitch receiver kit, which includes an additional frame cross member, so the receiver tube attaches to the bumper and the cross member. I went that route, but by the time I paid for the kit, shipping, and the welder, it got pretty expensive. An easier solution is to buy a bolt-on receiver from Perfect Casita. Wish I'd known about that at the time!

We do carry two lightweight aluminum bikes on the back of our Scamp (about 30 pounds each plus the weight of the rack). A third goes inside the trailer, anchored upside down by the handle bars to our First-Up awning on the front sofa. The one inside is actually the easiest one to load, bounces around the least, and is secure and protected from weather. Just a thought…
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Old 07-03-2016, 08:53 AM   #11
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Many bike racks specify that they are not rated for use on the rear of a trailer. I know of one that broke off, another rear receiver that broke off, neither of them mine. I did have a rear mounted rack bend. Now carry my bikes in the truck.
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Old 07-03-2016, 12:43 PM   #12
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It is a matter of mechanical advantage - if the trailer axle is at the center of the trailer frame [it will be somewhat behind center] the axle is the fulcrum of the lever - 100 lbs on the hind end , it will take 100 lbs off the tongue.
On my trailer - axle to ball is 9ft - axle to back bumper is 5ft - so 100 lb on the back bumper would take 55 lbs off the hitch .
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Old 07-03-2016, 12:56 PM   #13
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Demo from Australia, caravan and trailer weight distribution test

This might be helpful...

https://www.facebook.com/ORD4X4/vide...humbnail_video
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Old 07-03-2016, 04:39 PM   #14
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My 2 cents

My reply addresses what you said specifically.. a small cargo carrier with a bike rack attached. I have a 17 ft Casita that I used a cargo carrier on for a year with NO problems. My tongue weight and back-end weight were balanced correctly and I never had any difficulties until I changed to a cargo carrier made to attach a bike rack that came with it. Despite my best efforts, the combination just put too much weight too far out behind my little trailer. I experienced that out-of-control pendulum action 3 times in 160 miles and acquired a head full of new gray hair.

Upon arrival at my destination, I also found that a steel pin which fit up into the post of the bike rack had sheered off in the first 28 miles of the trip, allowing the bike rack to pivot left to right on the securing bolt, causing the trailer to pendulum. I tried speaking to the distributor whose name is on the item, but they would not hear anything about the safety defect in their product. Would not even discuss it. However, the company I purchased it from will no longer sell it. It shows it as no longer carried.

Here is the name of the combo I would highly recommend you do not buy:
3-Bicycle 60" Folding Cargo Carrier Basket Rack Combo for 2" Hitches
sold by Rage Power Sports on Amazon.com.
RPS refuses to take responsibility for their product or any damages caused saying they do not manufacture them, only distribute them with their name on it. I have also found the same item sold under the name Blue Ox. Buyer beware!

I have seen another type cargo/bike carrier setup that places the bike rack between the hitch and the cargo carrier, but due to the spare tire placement on the Casita, that one would not work for me.
My O-p-i-n-i-o-n and experience is that the combination of the two racks behind a small trailer just extends the weight too far out behind the trailer and affects the overall balance and stability. I won't do it again.
Just my 2 cents.
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Old 07-03-2016, 06:03 PM   #15
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Quote:
Originally Posted by widgetwizard View Post
A forward center of gravity is inherently stable.
It naturally inhibits sway.
I was actually looking for some science that more than 15% tongue weight is bad.
I cant believe that it is bad - how much tongue weight do semi-trailers have - 50%? They seem to get along just fine.
A Semi trailer and tractor have a totally different geometry. The fifth wheel pin is located above and slightly forward of the tractors tandem axle center. Those tandems are built to carry a heavy load. And it does not take weight off the front steering wheels. Same for fifth wheel trailers.

A tow behind trailer connects to the TV some distance behind the rear axle. the hitch weight causes weight transfer off the font wheels to the rear wheels. Too much and you lose traction in front. Especialy bad for a front wheel drive TV.
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Old 07-03-2016, 06:11 PM   #16
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Quote:
Originally Posted by Teal View Post
Greetings again,

I am considering adding a small cargo carrier to the rear of my Gem with a bike rack attached. I've read some posts that allude to negative reasons for adding to the rear.

What is the weight or tongue ratio issue associated... I am not an engineer, but know that this is the case, just trying to understand, and learn about what others have done to see if what I am planning would be OK.

Wouldn't some weight in the rear offset the tongue weight? how much would be OK?

Thanks so much for your experience and recommendations.
Teal
A cargo box and/or bicycle may hide your tail/stop/turn lamps from view to those behind you. Add high mounted LED lamps to supplement, or mount them on the cargo box.
A hitch receiver should have a rectangular tube or channel iron extending forward under the trailer to one of the cross members ahead of the rear bumper.
Compensate for the added rear weight by shifting inside stuff forward., keep you water tank empty - if it is behind the trailer axle - for instance.
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Old 07-03-2016, 07:55 PM   #17
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Those extended bike racks provide a lot of leverage. The bikes weight is essentially pulling down on a pry bar. That prying strains the attachment to the trailer and the joints of the rack/storage fixture. The further back it extends the more leverage it has.


Going straight up from bumper with rack will probably work best. Some folks have put a storage container on the tongue with good results. Norm added some plastic bins under the trailer for storage, sort of like those ones that go under a bed.
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Old 07-03-2016, 09:04 PM   #18
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[QUOTE=RogerDat;596262]Those extended bike racks provide a lot of leverage. The bikes weight is essentially pulling down on a pry bar. That prying strains the attachment to the trailer and the joints of the rack/storage fixture. The further back it extends the more leverage it has.

Many people don't think of the leverage principle. Trailer bounce adds to the effect. I prefer the type rack where the bike wheels set on the rack and a hook clamps over the bike upper frame tube. I think this type rack may reduce some of the leverage. Swagman makes a rack of that type that is rated for trailer rear use.
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Old 07-03-2016, 09:52 PM   #19
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I agree with the info and advise given. I had my own experience with balance and pendulum effect with my 2014 Scamp side dinette plan. Grey hair ... absolutely! With or without rear bike rack. Things like rear bike rack or filling my Dual fresh water tanks multiplied my problems. I've had a bike rack break and damage my bikes.
There are a variety of things I have done that brought the pendulum effect under control.
1. refrain from carrying water in fresh water tanks unnecessarily (instead carry portable water jugs in front of trailer when necessary)
2. Mount bikes on back of tow vehicle
3. Add friction type anti sway hitch attachment (I swear by this)
4. I added shock absorber kit from Orbital Machine Works in Texas. (this has more to do with reducing bounce on multiple bumps than the actual pendulum effect)
The result is safe trouble free towing where I am not having to anticipate trailer behaviour on downhill breaking, unexpected humps in the road at highway speed, turbulence from passing Semis. The friction type anti sway attachment should be a part of any formula you use. The result is total stability and ease of control.
Jim
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Old 09-01-2016, 02:31 PM   #20
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That's a great post! I wish everyone who tows would see that video. It wonderfully illustrates the practical effects and results of a complex dynamic system (comprised of a tow vehicle and trailer operating in changing environmental conditions) that is very difficult to actually understand.

I say this with confidence because I don’t fully understand it myself.

~~

I seem to recall that Bob Miller mentioned having difficulties when he swapped out the stabilizers on his little Hunter Compact II for slightly heavier ones. What seemed like it would be a trivial change turned out to have a definite impact and made the rig more sway-prone.

All of the parts, tow vehicle, trailer, and the loading, play a part in stability or instability of a towing combination. Rules of thumb are generally useful, but not always. As an example, I don't foresee a problem if I were to pull a 1,000 trailer with a 25% or even 35% tongue weight (250 to 350 lbs) on my tow vehicle, provided the trailer and load geometry were suitable and the frame sufficiently strong.

I was recently towing for a ten day trip with up to 420 lbs on the hitch. I travelled over the Cascade Mountains, several other mountain passes, and also drove through some "spirited" winds along the Columbia River on two different days. I never once felt any instability, even when I tested the stability at higher speeds and with a few judiciously applied waggles of the steering wheel.


(I'm not saying that I will continue with 420 lbs on the hitch; it was more a result of not taking the time to closely monitor my hitch weight due to the difficulty of setting up the board-and-bathroom-scale approach. I will now invest in a tongue scale so that I can monitor this more closely.)

And yes, the dynamics of a bike rack and cargo carriers bouncing around on a rear-bumper mount are generally terrible; the movement multiplies the forces greatly. And, locating a load so far from the axle and also behind the trailer axle are both negative factors.

For those like me who still haven’t had enough of learning about this stuff, here's some more interesting information:

http://caravanandmotorhomebooks.com/caravan-dynamics/

"Caravan makers typically recommend tow ball weight as a percentage of the caravan's weight. But caravan dynamics are such that it cannot be."

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