Raising a Casita SD16

[Steve Rosenberg]

I have a 2003 16' Scamp SD. I wanted to raise it a few inches for some of those no campsite camping spots. Anybody have any suggestions?

Thanks;
Steve

[Roger H]

Steve,

There'll be as many ideas as we have members! What you haven't said is how much you want to raise it... and that'll make a difference in the method you use.

Your profile says "Scamp" but your post says "Casita"?

I don't know what the current axle deflection is on your stock Casita. I raised my Burro 5" by replacing the worn 2000 lb Dexter #9 axle with a 3500 lb #10 with the "high lift" brackets (gain an inch) and a 22.5* down angle. They also offer a 45* down angle. I stayed with 13" tires and wheels.

If yours already HAS the high-lift brackets and the 22.5* down angle axle from the factory (which is possible in a 2003 model) then your next step would probably be looking at spacers between the frame and the mounting brackets.

Another easy clearance mod is larger (taller) tires. That alone may net you an inch and a half clearance if you switch from 13" to 14" tires and perhaps a tiny bit more if you get tall-sidewall 14" tires. Make sure that you still have the minimum 3" "bump" clearance under the fender wells, and that the replacement tires are no wider than the stock ones.

Remember, as with any vehicle, that the higher you make the trailer, the higher the center of gravity and potentially the less well-mannered the trailer will tow.

Good luck!

Roger

[Pete Dumbleton]

Keep in mind that axle mounting spacers will raise the trailer but NOT the axle; whereas taller tires will raise both.

If you can get the bottom of the trailer to match the bottom of the truck, you will find the towing air resistance to be less as air will flow under both with relative ease.

[Brian B-P]

...Another easy clearance mod is larger (taller) tires. That alone may net you an inch and a half clearance if you switch from 13" to 14" tires and perhaps a tiny bit more if you get tall-sidewall 14" tires. Make sure that you still have the minimum 3" "bump" clearance under the fender wells, and that the replacement tires are no wider than the stock ones...

If the replacement tires are the same width as stock, and the same aspect ratio (the "75" in ST185/75R13, for instance), then the one-inch larger wheel diameter means only one half inch extra axle height. Was "an inch and a half" just a typo, Roger, or have I missed something?

By "tall-sidewall" tires and "no wider than the stock", I assume that Roger means a higher aspect ratio. Current trailer tires seem to vary from 75-series (sidewall height 75% of the tire section width) to 80-series (80%), so as much as 5% of the width (less than half an inch) might be available this way.

While excessively wide tires would be bad, most trailers seem to have a lot of room for more width. As long as there is clearance to both the fenderwell and the suspension arm (or spring, in the leaf-spring case), I think moderate extra width should be okay. Really wide would cause too much drag, but trailer tires are usually absolutely skinny by automotive standards.

[Roger H]

Brian, it wasn't a typo. You could realize as much as that depending on the stock size and what you put on for the new size. Sidewall height also grows as a percentage of wheel diameter and overall tire height. I don't have the specs in front of me, and I don't plan to find them, but any tire store can figure out the tallest possible tire combination that will fit in place of the stock 13" tire.

Roger

[Brian B-P]

Brian, it wasn't a typo. You could realize as much as that depending on the stock size and what you put on for the new size. Sidewall height also grows as a percentage of wheel diameter and overall tire height.
...

This is where I think the math is going wrong. The sidewall height is a percentage (given as the aspect ratio, such as "75") of the section width, not of any diameter.

From the Goodyear Marathon size table:
ST185/80R13 - outside diameter 24.7" - static loaded radius 10.8"
ST205/75R14 - outside diameter 26.1" - static loaded radius 11.6" (their smallest 14" tire)

For the 13" tire, my calculated diameter is
185 mm x 0.8 x 2 for two sidewalls (296 mm or 11.6")
+ 13" for the wheel
= 24.6" overall

For the 14" tire, my calculated diameter is
205 mm x 0.75 x 2 for two sidewalls ( 308 mm or 12.1")
+ 14" for the wheel
= 26.1" overall

Static loaded radius means the distance from the tire/axle centre to the ground, while carrying weight, while not moving. This is really the value to compare for ground clearance, but the overall diameter matters for fenderwell clearance.

So it looks to me like the formula works fine, but the compression of the tire under load means that the axle is significantly closer to the ground than just half the diameter. If I compare two tires which are really the same width and aspect ratio, then only diameter difference is due to the wheel size: this is how I get that a 14" tire would raise the trailer only 1/2" compared to a 13" tire which is otherwise similar.

... any tire store can figure out the tallest possible tire combination that will fit in place of the stock 13" tire.

This is a good point - the formulas give nominal values, and every manufacturer publishes tables of actual dimensions. I like to use the formula to find a suitable candidate size (not just in trailer tires) and confirm dimensions with the tables.


Goodyear doesn't make any 14" Marathon as narrow as even the widest 13" Marathon, and all 13" sizes are 80-series while all 14" sizes are 75-series, so the closest I can see is to compare those two in my example, above. The 14" tire is a bit lower-profile (5 percentage points), but also a bit wider (about 10% wider) - the 14" tire would sit the trailer less than an inch higher, mostly due to the wheel (not the sidewall).

Of course, if there is room to go wider, a bit more can be gained: a huge ST225/75R15 would be 1.8" taller than the ST185/80R13. It would also have enough capacity to carry the entire Scamp on one tire (at 65 PSI) - there's an idea: a unicycle trailer!

More realistically, starting with a narrow 75-series 13" tire and going to a wider 80-series 14" tire (increasing width and aspect ratio and wheel size) would make more difference - I guess that's what Roger is getting at.

By the way, I checked Scamp's web site, and don't see an actual tire size - I'm curious what they use.

[Brian B-P]

I realize that not everyone likes doing math, but may want to check out tire possibilities, so here a link to a page with a Tire Diameter Calculator: American Racing - Tech Center. The calculator is the bottom link of the list down the right side of the page. Sorry, the page can be a little hard to read, perhaps depending on your browser settings.

[Lee Hillsgrove]

If that one doesn't work, here is another tire size calculator that I have used a lot - I think it was the original one?

http://www.miata.net/garage/tirecalc.html

[Roger H]

Keeping our perspective focused in the real world here, if you can gain an inch or so by changing wheels and tires, then that's probably the easiest route to try to gain that inch or so of axle clearance. past that, it's just plain difficult to calculate the exact amount of rise you'll get, unless you just block up the axle mounts between the frame and axle. There, every inch of block is an inch of frame rise. That still doesn't raise the axle clearance itself, of course; to do that you still need to change the axle angle of deflection on a torsion axle. Other than that, frame height from the ground is dependent on load, ambient air temp, tire air pressure, tire sidewall flex, original axle deflection, loaded axle deflection, age of the rubber suspension rods in the axle, etc. etc. etc.

Roger