FRAME - Page 3 - Fiberglass RV


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Old 06-22-2006, 09:32 AM   #29
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My inquisitive mind would like to better understand how this is the case.

It's one of those concepts that needs a little explanation to get the neural pathways to connect with it. Even with a bit of a mechanical background and university level physics it took a bit of mental gymnastics to understand how anti sway bars on a car or weight distributing hitches worked.

Sounds to me like you might just be the guy that can explain it. Would you mind helping me understand the concept Don?

Roy
The leading arm axle wheels bonce when the trailer rises & they loose contact w/the road. The trailing axle does not loose contact since when the brakes are applied the trailer squats placing more of a load on the tires, thus keeping them in contact w/the road.

Don
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Old 06-22-2006, 05:46 PM   #30
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The leading arm axle wheels bonce when the trailer rises & they loose contact w/the road. The trailing axle does not loose contact since when the brakes are applied the trailer squats placing more of a load on the tires, thus keeping them in contact w/the road.
Don,

I'd say the exact reverse of what you've said:

While the leading arm axle is trying to make the trailer rise, it will have more contact with the road - the wheel is pressing harder into the road to make the trailer rise.

And while the trailing arm axle is trying to make the trailer squat, it is pressing less onto the road, and so will be more liable to lock up.

Once the trailer has finished rising or squatting, the load on the wheels is the same, whatever suspension it has - all of them have to obey Netwton's 2nd law!

Andrew
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Old 06-22-2006, 09:08 PM   #31
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Don,

I'd say the exact reverse of what you've said:

While the leading arm axle is trying to make the trailer rise, it will have more contact with the road - the wheel is pressing harder into the road to make the trailer rise.

And while the trailing arm axle is trying to make the trailer squat, it is pressing less onto the road, and so will be more liable to lock up.

Once the trailer has finished rising or squatting, the load on the wheels is the same, whatever suspension it has - all of them have to obey Netwton's 2nd law!

Andrew
I've been thinking about this, but don't have an answer yet, maybe if we put our collective minds together we can come up with the answer. All I get is more questions. I suggest we start off using the same example and discussing it from there.

Imagine we are standing curbside watching a trailer go by from the left hand side of the screen to the right. The trailer wheel will be turning clockwise. If the trailer brakes are applied the resultant force on the arm will be clockwise.

For the leading arm, that means the brake force will tend to lift the trailer up at the axle.
For the trailing arm, the resulting force will tend to pull the trailer down at the axle.

In both cases, there will be an increase in force on the torsion suspension, not a shift in the direction of the force.

Is everybody OK with that so far? From here on, I think we need to set some standards for discussion. If not, then things start getting a little too complicated. I wouldn't mind disscussing the effects of some of these things afterwards though.

Lets assume the trailer is connected by a regular hitch, not a weight distributing hitch.

Assume the leading arm has a -ve start angle and the trailing arm a positive start angle as my understanding is these are the typical design. Next we have to determine whether the trailer braking is greater than, equal to or less than that of the tow vehicle. Who wants to pick one and carry on from there?
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Old 06-25-2006, 08:02 AM   #32
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I've been thinking about this, but don't have an answer yet, maybe if we put our collective minds together we can come up with the answer. All I get is more questions. I suggest we start off using the same example and discussing it from there.

Imagine we are standing curbside watching a trailer go by from the left hand side of the screen to the right. The trailer wheel will be turning clockwise. If the trailer brakes are applied the resultant force on the arm will be clockwise.

For the leading arm, that means the brake force will tend to lift the trailer up at the axle.
For the trailing arm, the resulting force will tend to pull the trailer down at the axle.

In both cases, there will be an increase in force on the torsion suspension, not a shift in the direction of the force.

Is everybody OK with that so far? From here on, I think we need to set some standards for discussion. If not, then things start getting a little too complicated. I wouldn't mind disscussing the effects of some of these things afterwards though.

Lets assume the trailer is connected by a regular hitch, not a weight distributing hitch.

Assume the leading arm has a -ve start angle and the trailing arm a positive start angle as my understanding is these are the typical design. Next we have to determine whether the trailer braking is greater than, equal to or less than that of the tow vehicle. Who wants to pick one and carry on from there?
Your point of view seems resonable, but ALL trailer mfgs. have gone to trailing arms. Just call one & ask the engr, dept why they have done this. The principle also applies to cars - when you apply the brakes the nose goes down & helps w/tire contact.

If you get more info please report back to us........Don Meyer, mech engr.
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Old 06-26-2006, 01:55 PM   #33
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The Trillium Outback which I have looked at on a local dealer's lot still has leading arms. It is also the only fiberglass travel trailer in current production in Canada (to my knowledge) which is only 13' overall (the rest are 17' plus). This appears to me to be a choice driven by packaging considerations, as the axle tube can be under the stepped-up rear section of frame and floor only if the arms are leading.
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Old 06-26-2006, 04:20 PM   #34
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The principle also applies to cars - when you apply the brakes the nose goes down & helps w/tire contact.
This seems to be mixing up two related phenomenons - weight transfer when braking makes the nose of a car go down and the same weight transfer increases the load on the front tyres. The nose going down doesn't provide any benefit at all.

Trailing arm suspension does not help with tyre contact - it reduces tyre contact when the brakes are first applied.

Andrew
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Old 06-26-2006, 04:50 PM   #35
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Your point of view seems resonable, but ALL trailer mfgs. have gone to trailing arms. Just call one & ask the engr, dept why they have done this. The principle also applies to cars - when you apply the brakes the nose goes down & helps w/tire contact.

If you get more info please report back to us........Don Meyer, mech engr.
The reason the nose goes down is that center of gravity is above the center of wheels and between the front and back wheels. When the brakes are applied the center of the front wheels become the moment piviot point. The CG attempts to continue going forward but can't so it rises forcing the front of the car down.

In the case of a two wheel trailer the CG is in front of the wheels and behind the coupler. The couple becomes the piviot point. The CG again doesn't want to stop so it attempts to piviot over the coupler putting downward force on the hitch.

If the trailer is properly balanced the CG is in the same realtionship with wheels and the coupler in both cases. It appears to me in both cases the tires will have a tendency to unload, thus no gain in either case.

This is where somebody with a Finite Element Analysis program could if there is a differences between leading and trailing arms.
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Old 06-26-2006, 09:41 PM   #36
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In the case of a two wheel trailer the CG is in front of the wheels and behind the coupler. The couple becomes the piviot point. The CG again doesn't want to stop so it attempts to piviot over the coupler putting downward force on the hitch.

This is where somebody with a Finite Element Analysis program could if there is a differences between leading and trailing arms.
Byron,
Does your '06 S13 have a leading or trailing axle? If leading that makes at least 2 manufactures with leading arms.

The forward rotation of the TV will cause the hitch to lift upwards. I think we might need the Finite Element Analysis program to explain it since Don seems a little tight lipped ( or fingered in this case) with his professional knowledge. {light fingered just does not sound right thought, since it usually means something else}

Roy
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Old 06-27-2006, 12:16 AM   #37
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Byron,
Does your '06 S13 have a leading or trailing axle? If leading that makes at least 2 manufactures with leading arms.

The forward rotation of the TV will cause the hitch to lift upwards. I think we might need the Finite Element Analysis program to explain it since Don seems a little tight lipped ( or fingered in this case) with his professional knowledge. {light fingered just does not sound right thought, since it usually means something else}

Roy
I just took a quick look and it appears to be a trailing arm.

The only reason I can think of at the moment for trailing vs leading is that the tortional forces created during braking would have a tendency to decrease the life of the axel. Trailing arm isn't going to put a lot of tortional stress on the axel, where as leading arm with force the arm to it's maximum downward rotation. I suspect that those tortional forces would overload the axel.

Interesting topic, eh?
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Old 06-27-2006, 07:15 AM   #38
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Trailing arm isn't going to put a lot of tortional stress on the axel, where as leading arm with force the arm to it's maximum downward rotation.
I can't see much logic in this. The torque applied to the leading arm is likely to be lower since most leading arms are used with low ride heights (eg, 'up' axle start angles), so the lever (ground to axle centre) of the braking force is less.

The leading arm isn't going to be forced to its maximum downward rotation, any more than the trailing arm isn't forced to its maximum upward rotation.

Andrew
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Old 06-27-2006, 08:23 AM   #39
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When I ordered my new axle I told Jim Tuthill (Engr Dept @Dexter axel, that my Scamp has leading arms. He recommenged to go w/trailing arms. I have since then ordered the axel & will place it under the dropped box w/the arms facing rearward. The axle will have a o degree angle vs. the current 22.5 Degree down angle.

On the passenger side I had to lower the mounting point 4 1/2". I did this by adding a 5' pc. of 1 1/2" square tubing from the axel area forward past the bend. To this I welded a pc. of 2' x 3" x 1 1/2" tubing to the area above where the axel will go. The overall effect will be lowering the axel 4 1/2" which will raise the trailer 2 1/2"( I got a 2" drop by going to o degree angle vs. 22.5 degree down). I would have not gone to all this work if I had not been advised by 2 axle mfgs. to use a trailing arm axle.

I am now waiting for the axle to arrive.(pictures to folow)

I also talked to Redneck trailer in Tampa Fl. & they refused to make a leading arm axle.

I am just telling my story......My Engineering expertise is not in axle design, however Jim Tuthill, The Engr at Dexterthat I talked to , should be able to give those that want more info ......Dexter axle (574)295-1900
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Old 06-27-2006, 10:07 AM   #40
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I can't see much logic in this. The torque applied to the leading arm is likely to be lower since most leading arms are used with low ride heights (eg, 'up' axle start angles), so the lever (ground to axle centre) of the braking force is less.

The leading arm isn't going to be forced to its maximum downward rotation, any more than the trailing arm isn't forced to its maximum upward rotation.

Andrew

Can you imagine a long stick with a short stick attached with a piviot. The short stick is then pointed toward the other end of the long stick. The short stick has a wheel attached at the end of it. (leading arm). You pull on the long stick and then stop the wheel from turning. What's it going to do?

It's going to reverse become a trailing arm if there are no forces to prevent it.
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Old 08-20-2006, 04:22 PM   #41
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There is rarely a need for a new frame. I am a certified welder & a frame can be repaired a lot cheaper than making a new one. I think you could have more problems w/a new non factory fabricated frame than repairing your current one.

Don Meyer, PhD-Mech Engr

Don:
I was wondering about your frame advice. Along the lines of repair may be better than starting over. I'm going to have to make just such a decision.

[b]Background:
Yesterday the frame on my 76 Tripple E Surfside broke completely on passenger side on the last 200km leg of a 5,000 km trip. Frightening when I think about what could have happened. Passenger side rectangular frame member severred completely a couple of feet aft of the point it disappears under the trailer and at the point that the first "step" in the "ladder construction" is welede to it.

I had it carefully inspected last summer before setting out on a 9000 km trip to Colorado from Ottawa. It was inspected by a recommended welder with long experience in trailers. He had reason to believe he'd get the repair work if he found problems. I specifically asked him to look for stress cracks. I can only assume that it was not easily visible. Since about that time I"ve been hearing mild groaning sounds when stepping on the floor that come from the area of the break. I assumed this was some movement between fiberglass, mounting, and frame.

So, why do you feel that there may be more trouble if a new frame is fabricated than repairing the old. Does incresed gauge of steel not promise greater strength and resistance to cracks, rust etc. When you talk about problems, do you mean difficulty in getting the replacement exactly right, lining up bolting points etc.? Or are there other issues.

I am quite shaken by this experience. I am generally quite careful about the condition of the trailer and the tow car, and felt I had done the due diligence on the frame. So I will have difficulty trusting this frame unless I"m really convinced it ca n be repairred in such a way as to ensure it doesn't happen again.

Your insight on the repair vs. replace issue would be much appreciated as would and that of any other experienced engineers welders as well.
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Old 08-20-2006, 07:41 PM   #42
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Alan, my opinion is that designing a new axle from scratch and trying to get it right the first time out of the box is less likely to succeed than repairing a more-or-less tested design -- Overbuilding creates its own problems, like not enuf flex perhaps subjecting the rest of the trailer to excessive stresses.

Like you, I had a spot on my Scamp that was groaning when I walked around -- It turned out to be a crack across the botton of one frame side just in front of the axle weld (presumably done by a careless welder during an axle replacement; the crack followed an unneeded weld bead) -- I took it to a welder friend and he fabricated a stiffener for that part of the frame.

I suspect the braking arguement of leading vs trailing arms is ultimately a non-issue -- Unlike automotive brakes, regardless of which method produces more braking, when setting up the brake controller, the brakes are set to the point the driver wants (with no care to how much excess braking may still be available).

BTW, my gut feeling is to say that because leading brakes tend to lift the trailer, I would expect them to apply more force to the ground, but I really don't know.

Scamp has switched from leading to trailing axles on the 13' trailer, but I don't know what year that started -- They have also gradually increased the standard 'rubbering' of the torsion axles from 1,200 lbs to 2,200 lbs over several decades.

I would certainly expect the trailing axles to give a better ride than the leading ones -- One need only push a loaded wheelbarrow over an obstacle and then pull it over the same obstacle to note the difference in effort and effect on the load.
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