another axle question

[KA]

Hi All,
My new axle is unloaded here on the porch and I notice that the cross bar (not sure if I'm using the correct terminology here) has a slight arc to it. It seems to be curved so that it is a little higher in the center. This is a Dexter #9 torsion axle. The shop where I bought it is closed for the weekend so I can't ask them until after the holiday. Is this arc normal or do I have a bad axle?
Any ideas?
Thanks.
Kris

[Steve_Cox]

Kris,

It is called positive camber. positive camber leans both tires on the axle outward from the center of the vehicle, and as the load increases it puts the wheels and tires at a more neutral camber position for even tire wear. Hope that helps. Are you having fun???

[angib]

The curve in the axle makes the wheels/tyres lean outwards slightly at their top - this is intentional and correct.

I think the reason that all axles have positive camber is not to do with loading, but because it makes the trailer track better.

For exactly the same reason the vast majority of cars have slight positive camber on their front wheels, to improve the steering.

Andrew

[KA]

Hi Steve,
Thanks for answering my question. I like the sound of "positive camber". It is much more reassuring than "negative torque", "high tension stress load" , "excessive heat warp" or any number of other things that I can dream up in my state of mechanical ignorance. My fun level on this project increases as I learn on each baby step of the journey. Thanks again for taking the time to answer my question.
Kris

[KA]

Hi Andrew,
Once again, I thank you for answering another of my endless questions. I could not do this build without your help. You have a lot of knowledge and info and it is very kind of you to share.
Kris

[asianflava]

Yep, it's so the trailer tracks better. When it is installed, you won't even notice it.

[KA]

Thanks, Asianflava!
Kris

[Steve_Cox]

The curve in the axle makes the wheels/tyres lean outwards slightly at their top - this is intentional and correct.

I think the reason that all axles have positive camber is not to do with loading, but because it makes the trailer track better.

For exactly the same reason the vast majority of cars have slight positive camber on their front wheels, to improve the steering.

Andrew

Thanks Andrew for the correction.

But..... I got this info from the Dexter Axle web site FAQ....

" Axles are typically built with a pre-determined bend in the tube that compensates for the expected deflection under load."

But they could be wrong as to why they do it, we just know for sure that they do it...

[asianflava]

I've heard that reason also, but I have a problem with the physics behind it.
How will it deflect when the axle is supported on the ends and not in the middle. There is nothing in the middle to push down and flatten the axle out.

[apratt]

I've heard that reason also, but I have a problem with the physics behind it.
How will it deflect when the axle is supported on the ends and not in the middle. There is nothing in the middle to push down and flatten the axle out.

I know you wouldn't think the axel will bend in the middle, but it does!! How I know..... I had a large utillty trailler and I overloaded and didn't reallize it untill I got behind it and then I notice the top of the wheels where tilted inwards. Shocked I got on my knees and looked under the trailer and seen the axel was bowed downwards in the middle. Needless to say I unloaded half the stuf I had in the trailer!!! Whew!! I thouht I ruined my axel, but it went back to ok.

[Nitetimes]

I've heard that reason also, but I have a problem with the physics behind it.
How will it deflect when the axle is supported on the ends and not in the middle. There is nothing in the middle to push down and flatten the axle out.

It may seem that way but I have replaced a lot of axles because they were straightened out. It is unlikely to happen to one with a TD on it unless it's pretty heavy. Most of the ones you see flattened are on trailers that have been seriously overloaded.
There may not be anything in the center pushing down but you still have leverage working on it.
Don't know much about physics but I have seen what weight will do to things.

[asianflava]

Yup I guess so, who knows when you overload it. Thinking about it again, it can bend when there is enough weight on it, with enough weight anything can bend.

We are talking about torsion axles aren't we?

[angib]

"Axles are typically built with a pre-determined bend in the tube that compensates for the expected deflection under load."

I stand duly corrected, and promise I will try to remember that in future!

On the issue of how to bend an axle, here is the loading diagram - the ground is pushing upwards on the axle well outside where the trailer frame is pushing downwards on it, so that puts a twisting action on each end:



You can easily get exaclty the same sort of bend, but the other way up, by putting a thumb and forefinger on each end of a 12" ruler - get a 2" offset between each thumb and forefinger and you can snap the ruler.

No, I'm not buying you a new ruler because you've just snapped yours....

Andrew

[Nitetimes]

We are talking about torsion axles aren't we?

Yep, they can be straightened out too but it is less likely than a sprung axle when you consider that the spindle is not actually attached to the axle tube itself. What I usually see happen to them is that the spindle will snap off the arm.
It's a miserable job trying to replace it too.

On the issue of how to bend an axle, here is the loading diagram - the ground is pushing upwards on the axle well outside where the trailer frame is pushing downwards on it, so that puts a twisting action on each end....

Andrew that's exactly what I was talking about.