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Old 07-17-2015, 12:53 PM   #21
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Will you sign the copies at Nashville this year? Kind of like a book signing...
Of course, just like Goshen last year, for a price.
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Old 07-18-2015, 03:28 PM   #22
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Originally Posted by 05CrewDually View Post
Using the ramp your not just supporting the weight of one axle. You're lifting the weight of both tandems on one axle. It doesn't take a degree to see this.

Losing a tire had nothing to do with this. Again, a flat tire is still supporting it's weight, just now it's on the wheel. Plus the other axle is supporting it's share.

Actually, lifting a tire off the ground by the trailing or leading axle is doubling the load on that one stub axle.

As for proper jacking, place the jack under the spring plate or capture a u - bolt in the jack head and jack it just high enough to lift the tire to be removed.
I have used this way on my boat trailer for years. But I can see how the ramp way is much easier and safer. With the bottle jack you still need to carry boards to put underneath and the bottle jacks I've had have leaked hydraulic fluid all over my truck bed so it liooks like I will bring a ramp along on my trip west in September.
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Old 07-18-2015, 04:44 PM   #23
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Originally Posted by 05CrewDually View Post
You're lifting the weight of both tandems on one axle. It doesn't take a degree to see this.
Vector analysis of the forces involved would show otherwise.
Sometimes what seems "logical" on the surface is not so mathematically.

While I don't have a "handy graphic" to show how a five point load share (4 wheels and the hitch weight) is redistributed when one wheel is removed, I do have one for a WD scenario that should make the point of weight transfer to the other side more clear.

Say we have a 10000 pound 5th wheel with a weight distribution of 80/20 where 20 % of the weight is supported by the Landing Gear (or hitch when connected to change a tire roadside). The Camper has 2 5,000 pound axles each supporting 1/2 of 8,000 pounds and each tire supporting 2,000 pounds each.

Remove one of the tires and the 8,000 pounds is now supported by 3 tires or 2,666 pounds (and a small fraction transferred to the landing gear/pin).

One axle will be supporting 5,332 pounds (a 6% temporary overload) and the other 2,666 pounds. The 6% overload is well within the elastic overload (Shock Load) parameters for torsion axles and within the deflection overload (Shock Load) for spring suspension axles.
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Old 07-18-2015, 05:02 PM   #24
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Originally Posted by Herk7769 View Post
Vector analysis of the forces involved would show otherwise.
Sometimes what seems "logical" on the surface is not so mathematically.

While I don't have a "handy graphic" to show how a five point load share (4 wheels and the hitch weight) is redistributed when one wheel is removed, I do have one for a WD scenario that should make the point of weight transfer to the other side more clear.

Say we have a 10000 pound 5th wheel with a weight distribution of 80/20 where 20 % of the weight is supported by the Landing Gear (or hitch when connected to change a tire roadside). The Camper has 2 5,000 pound axles each supporting 1/2 of 8,000 pounds and each tire supporting 2,000 pounds each.

Remove one of the tires and the 8,000 pounds is now supported by 3 tires or 2,666 pounds (and a small fraction transferred to the landing gear/pin).

One axle will be supporting 5,332 pounds (a 6% temporary overload) and the other 2,666 pounds. The 6% overload is well within the elastic overload (Shock Load) parameters for torsion axles and within the deflection overload (Shock Load) for spring suspension axles.
Herk, you're totally missing it. Your talking about 5 points in a tandem axle scenario, leaving 3 tires on the ground.
The game changer is when you use one of these "ramps" you're totally supporting the 2 points of load from one side all on 1 point stub axle. So the one stub is supporting both weights which is overloading not only the axle but the tire, and spring with double the weight it normally carries.

Your illustration proves my point. The weight has to go supported some where. When you use the ramp it's all going to the one stub that is on the ramp since the other stub is off the ground.
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Old 07-18-2015, 05:22 PM   #25
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If one needed to remove tires/wheels to put in metal valve stems......what if you put a ramp on both sides so two tires(1 on each side) could be removed at once....anybody have thoughts on that?
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Old 07-18-2015, 05:33 PM   #26
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Originally Posted by Herk7769 View Post
Vector analysis of the forces involved would show otherwise.
Sometimes what seems "logical" on the surface is not so mathematically.

While I don't have a "handy graphic" to show how a five point load share (4 wheels and the hitch weight) is redistributed when one wheel is removed, I do have one for a WD scenario that should make the point of weight transfer to the other side more clear.

Say we have a 10000 pound 5th wheel with a weight distribution of 80/20 where 20 % of the weight is supported by the Landing Gear (or hitch when connected to change a tire roadside). The Camper has 2 5,000 pound axles each supporting 1/2 of 8,000 pounds and each tire supporting 2,000 pounds each.

Remove one of the tires and the 8,000 pounds is now supported by 3 tires or 2,666 pounds (and a small fraction transferred to the landing gear/pin).

One axle will be supporting 5,332 pounds (a 6% temporary overload) and the other 2,666 pounds. The 6% overload is well within the elastic overload (Shock Load) parameters for torsion axles and within the deflection overload (Shock Load) for spring suspension axles.
Very well explained as to how it really is. Good job Herk.
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Old 07-18-2015, 09:53 PM   #27
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Originally Posted by jtownstumpjumper View Post
If one needed to remove tires/wheels to put in metal valve stems......what if you put a ramp on both sides so two tires(1 on each side) could be removed at once....anybody have thoughts on that?
Assuming you ramped the same tires (both front or both back) on each side you would then have a 3 point support with a larger portion of the load shifted off the tires and onto the hitch/landing gear.

In this case each tire would support "about" 1/2 of 2/3 of the weight assuming the CG is equidistant from each support. The wheels will support more the closer the wheels are to the front support and have less "share" the further back.

The math gets complicated as the CG shifts around using varying distances between supports. Also if the wheel locations are such that the CG shifts aft of the supporting axle (possible but not likely) the camper will tip on its "butt" if not restrained.

To simplify the tandem configuration in my previous example, I put the CG of the wheel supported load in the center of the box formed by the 4 wheels.

In the "wheel off" scenario, the CG gets shifted slightly towards the two good tires and just forward (assuming you are changing a rear wheel) of the axle box center due to the support structure changing from a "box" to a "triangle".

Note: I am not disputing the supporting tire/axle will be in shock load; just that it will not be as severe as a 100% overload (the sum of the two tires before lifting) as previously postulated.

The camper itself will "lean" into the flat due to the good wheel on pavement compressing and "squashing" out slightly to support the additional weight.

If the good tire inherited 100% of the flat tire's weight (in the scenario above), the 2000 pound load would become 4000 pounds and would surely blow the tire (typically rated at 2250 pounds on the axle above). In most cases the "good" tire is just fine after you change the blown one (unless it suffered the same impact damage that blew the other) and you will get 100% of the remaining life expectancy.

Personally, I would still just change one at a time using the wheel lift system.
If I wanted both tires on a side off I would use a bottle jack to lift the frame, add axle supports and then release pressure on the bottle jack to support the camper while I worked on the wheels.
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Old 07-18-2015, 10:20 PM   #28
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There are two assumptions conflicting here. One group is assuming that two ramps are in use eliminating one axle from weight distribution when changing the tires and the other assumes there is one ramp in use during the tire change leaving three points in contact to carry the load (ignoring the hitch) Since the load is temporary to complete the tire change (~30 minutes) and load is shifting but still being carried by three of the four tires, (A situation similar to when a jack is in use) I believe the risk to the running gear is minimal.
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Old 07-18-2015, 10:21 PM   #29
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Originally Posted by comfun1 View Post
Will a car ramp work?
Depends on how heavy your unit is. I have/had a set of car ramps that I use to change my oil and filter on our car..... Had the bright idea to use one ramp to get my camper wheels up in the air.....
Started to pull my unit on the ramp and flattened it the height of an inch

For my unit... Noooo, it will not work
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Old 07-18-2015, 10:29 PM   #30
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There are two assumptions conflicting here. One group is assuming that two ramps are in use eliminating one axle from weight distribution when changing the tires and the other assumes there is one ramp in use during the tire change leaving three points in contact to carry the load (ignoring the hitch) Since the load is temporary to complete the tire change (~30 minutes) and load is shifting but still being carried by three of the four tires, (A situation similar to when a jack is in use) I believe the risk to the running gear is minimal.
Not conflicting; just confusing. The two ramp thoughts were in answer to StumpJumper's question, the other was in response to CrewDually.

The short answer is that as long as the ramp can support the load, the axles will be fine.
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