Community Forums

Always-RVing · 10-01-2009, 04:01 PM

I see Newmar uses an aluminum frame and Fleetwood uses an all steel frame. My fist thought would be an aluminum frame would be lighter and not rust; however, the steel frame may add rigidity and steel strength that the aluminum would not.

Any thoughts? What do other manufactures use?

Easyrider · 10-01-2009, 05:40 PM

Aluminum is lighter and stronger than steel, but also more expensive.

D in Davie · 10-01-2009, 07:18 PM

You should also look at how the frame is constructed. Newmar spaces their frame members 16 inches apart for added strength. I'm not familiar with how the weight of both types of motor homes is distributed, top heavy or less top heavy. I don't thing the frame would be significant no matter what material was used. JMO

J Walker · 10-02-2009, 07:44 AM

My 2000 Damon Ultrasport is reported to have an aluminum frame for the coach and the MH is surely lighter than most, 22,000 lbs GVWR for a 36' DP. The frame has worked well with no problems. I would not want to have a roll over though and that applies to all MHs except the upscale ones with bodies built like cars with semi-monocoque construction. Some advertise a steel cage around the driver and passenger

speed racer · 10-02-2009, 08:22 AM

Quote:

Originally Posted by Easyrider

Aluminum is lighter and stronger than steel, but also more expensive.

I would like to know what kind of aluminum is stronger than steel? I do know that when they us aluminum in most cases they use more of it than when they us steel. It is my opinion that how they build it is more inportant than what they build it out of.

RustyJC · 10-02-2009, 08:53 AM

Quote:

Originally Posted by speed racer

I would like to know what kind of aluminum is stronger than steel? I do know that when they us aluminum in most cases they use more of it than when they us steel. It is my opinion that how they build it is more inportant than what they build it out of.

It all depends.

The specific weight of aluminum is ~169 lbs/cubic foot, while the specific weight of steel is ~487 lbs/cubic foot. While the ultimate tensile strength (UTS) of, let's say, 6061-T6 aluminum is ~45,000 psi, the ultimate tensile strength of A36 mild steel can vary from 58,000 psi to 79,800 psi. So, in terms of uts, one could indeed make the statement that a typical steel is stronger than a typical aluminum.

Having said that, if one designs two structures for equivalent strength, as you state, more aluminum by volume will be required to compensate for its lower UTS. To oversimplify, I'll need (79,800 / 45,000) = 1.77 times the volume of aluminum in the example above to equal the tensile strength of the same structure constructed of 79,800 psi UTS A36 steel.

Since the specific weight of aluminum is 169 lbs/cubic foot, if I multiply the 169 by 1.77, then I'll need 299 lbs of aluminum to provide the same strength in a structure as 487 lbs of steel would provide, so for a given "strength", it could be said that an aluminum structure weighs 299 / 487 = 61% as much as an equivalent "strength" steel structure.

So, for a given volume, one might say that steel is stronger than aluminum. However, for a given weight, one could just as well say that aluminum is stronger than steel.

The above is greatly oversimplified for illustration only and doesn't get into other considerations such as brittleness, bending strength, fatigue life, etc.

Rusty

speed racer · 10-02-2009, 11:48 AM

I agree with you Rusty but the only problem is MHs are not built out of qubic ft. of aluminum or steel they are built from material at the most 3/8" thick although it be tubular or angle. I am not an expert by no means but how many peices of 2"x2" aluminum would it take to equal the strength of a like peice of mild steel and what would be the result at an inpact point. Don't get me wrong I would prefer aluminum in the body of a MH for the weight savings. I have an 05 Dolphin that is heavy for its size which limits its cargo capacity but it has a steel plate across the front firewall that gives me a fealing of a little security although it is probably not all that secure in a severe frontal impact.

RustyJC · 10-02-2009, 12:00 PM

Quote:

Originally Posted by speed racer

I agree with you Rusty but the only problem is MHs are not built out of qubic ft. of aluminum or steel they are built from material at the most 3/8" thick although it be tubular or angle. I am not an expert by no means but how many peices of 2"x2" aluminum would it take to equal the strength of a like peice of mild steel.

It doesn't matter if it's cubic inches, cubic feet or cubic yards. That's what the specific weight covers - weight per unit of volume. For any given volume, aluminum will be 169 / 487 = 34.7% of the weight of an equivalent volume of steel.

To apply what I said earlier in my example to your posit, for a given 2" x 2" rectangular tube, the cross sectional area (achieved via increased wall thickness) of an aluminum tube would have to be 1.77 times that of a steel tube to achieve equal tensile strength, but the thicker aluminum tube would still weigh only 61% as much as the steel tube of equal tensile strength.

Rusty

Always-RVing · 10-02-2009, 01:41 PM

Thanks for the responses.. I wonder what Prevost, Country Coach and Newal uses for the cage, and I'm sure there's a lot to consider when deciding what metal, thickness, etc to use.

Cruzer · 10-02-2009, 03:26 PM

I'm pretty sure that Newmar follows the same procedure that Tiffin does. Tiffin uses structural steel tubing underneath the flooring for strength. It also uses aluminum tubing in the walls and roof trusses (also on 16" centers) where that kind of mass isn't required. It helps keep the center of gravity down lower and you don't have to worry about condensation or rusting inside the sidewall.

Easyrider · 10-07-2009, 05:00 PM

Rusty obviously knows a lot more about steel and aluminum than I do, his "simplified" explaination is still more than I knew about but to make it even simpler I meant for the same amount of weight bearing ability, aluminum is lighter, therefore stronger if the same amount of aluminum by weight were used to build something as steel, the aluminum example would be stronger. At least that's the way I've always understood the difference between the two in practical terms.

rmmpe · 10-12-2009, 05:29 AM

Then there's that little bother, "Modulus of elasticity" and fatigue.

Truss work, gussets and angular spans do a great deal to provide strength and resist the flexing that contributes to fatigue and ultimate failure.

Short story: Aluminum can be successfully employed to provide strength as great as steel but doing so involves more than just indiscriminately adding it to a structure. The FEA (Finite Element Analysis) programs available today are an invaluable tool when designing an optimum structure. They sure are easier and faster than doing it with a slide-rule, which did not generate graphic illustrations identifying the hot spots. Neat stuff, that.

RustyJC · 10-12-2009, 06:42 AM

Ummm.....perhaps you missed the following from my earlier post:

Quote:

The above is greatly oversimplified for illustration only and doesn't get into other considerations such as brittleness, bending strength, fatigue life, etc.

Yes, I know there are many more considerations when substituting aluminum for steel in a structure. I left them out on purpose, OK?

Rusty

rmmpe · 10-12-2009, 07:18 AM

No, Rusty, I didn't miss your statement. My addition was merely to add a little more info. Not to disagree, or in any way, detract from yours.

Certainly not to allude that yours was in error.