driving with one wheel/tire/axle off the ground

[jesusgatos]

I did what you are talking about, it BARELY got the tire up, and I mean BARELY!
The weight of my super singles were heavy, and the spring pack has slop for verticle when you unload it, plus it is hanging by a couple spring leafs, which are not as stiff in that load direction.

BUT, it would be enough to do if you took the wheel/tire off. I wouldn't do this if you had to keep the tires on and didn't want them hitting the ground. For a flat tire, sure.

Btw, chaining the axle up enough so it hits and upper stop eliminates the chain moving around too much, but what, and where that stop is needs to be addressed.

Ah, first-hand experience. Good point about the tires on/off. Might not get around to building the axle chain-up brackets I designed and posted about earlier in this thread, only because think I'm planning on bobbing that truck now. Maybe for the 5-tons though?

 

[Hammer]

Ah, first-hand experience. Good point about the tires on/off. Might not get around to building the axle chain-up brackets I designed and posted about earlier in this thread, only because think I'm planning on bobbing that truck now. Maybe for the 5-tons though?

This is on a class 8 semi truck to chain up an axle when needed.
Made it nice when I was towing them from the back. Easy to hold the axle up when I lifted the truck off the ground and not rip the air bags.

 

[mudguppy]

The second part of your post I quoted is correct, and conflicts with the first part of your post. Agree with the second part, but not the first. Most leafsprings are not going to exert the same force in either direction. ...

So you're saying that a piece of spring steel has different spring rates depending on which way it bends? No.

... Sure, at some point it's going to lift the middle axle, but how high do you think we'd have to jack-up the rear of the truck? Betch'a mountain dew it's more than just a few inches...

yeah, that was my last point - the range of travel that the bag would need between full compression as to not interfere with the rear axle offroad and full extension to push the rear far enough down to lift the intermediate would be huge. Sorry, I thought I made that perspective clear.

 

[VPed]

As someone else already said, singled tires can be run flat (on the rear) for some time with little issue (first hand experience). We also now know that the bottle jack experiment barely lifts the tire. How about not actually desiring to lift the tire, instead only wanting to remove enough weight off the flat to eliminate damage from excessive sidewall flex. The axle is still connected to the drive train so the flat should be rotating about the same as the rest. 2 cents.

 

[jesusgatos]

So you're saying that a piece of spring steel has different spring rates depending on which way it bends? No.

Yes, that's exactly what I'm saying. A bent piece of spring-steel is going to have different spring steel is going to have different spring rates, depending on which way it bends. Not just because of the way it's bent, but the way it's captured and loaded. That's true of individual springs, and even more true of an assembled spring-pack because of the way they're constructed. They're designed to be loaded in compression, and subjecting them to loads under extension (say if you were to install them on the vehicle upside-down) would change the spring-rate and damage them. Dunno if the weight of the vehicle on the lifted axle and suspended wheels/tires (if you could get them off the ground) would be enough to damage these particular springs in this particular application, just saying that it's something to consider. And I'm really not trying to be argumentative, but I've spent a considerable amount of time learning all about how leafsprings are made and tuned. Actually wrote an article for off-road.com about it a long time ago.

yeah, that was my last point - the range of travel that the bag would need between full compression as to not interfere with the rear axle offroad and full extension to push the rear far enough down to lift the intermediate would be huge. Sorry, I thought I made that perspective clear.

Gotcha, must have missed that. Could solve that problem by mounting the airbags on modified links like the lower links on desert race trucks? Think the bigger issue is how high you'd have to lift the back-end of the truck to get the middle axle off the ground, and what that would do to the leafsprings, being ever-extended into such an extreme arc.

As someone else already said, singled tires can be run flat (on the rear) for some time with little issue (first hand experience). We also now know that the bottle jack experiment barely lifts the tire. How about not actually desiring to lift the tire, instead only wanting to remove enough weight off the flat to eliminate damage from excessive sidewall flex. The axle is still connected to the drive train so the flat should be rotating about the same as the rest. 2 cents.

Taking the wheels/tires off would be one way I could see that might work, but I like what you're suggesting even more. As long as the reason that axle needs to be out of service doesn't keep the wheels/tires from rolling.

 

[mudguppy]

Yes, that's exactly what I'm saying. A bent piece of spring-steel is going to have different spring steel is going to have different spring rates, depending on which way it bends. Not just because of the way it's bent, but the way it's captured and loaded. That's true of individual springs, and even more true of an assembled spring-pack because of the way they're constructed. They're designed to be loaded in compression, and subjecting them to loads under extension (say if you were to install them on the vehicle upside-down) would change the spring-rate and damage them. Dunno if the weight of the vehicle on the lifted axle and suspended wheels/tires (if you could get them off the ground) would be enough to damage these particular springs in this particular application, just saying that it's something to consider. ...

I know that theoretically this can be correct - however, I'm considering, for practical purposes, the bottom couple/few leafs in the deuce pack are bent so little that there is not a significant change in rates from one side to another. we're not talking about a cheby 1/2 ton truck spring with a significant arch...



this point, I think is moot and silly, as we've both outlined there are so many other issues to be resolved. super-singles or not, I'm confident that the last one or two captured leaves would be able to support the <2000 lbs of intermediate axle and wheels/tires.



I still like your chain-up brackets - I think that's the way to go.

 

[VPed]

Chain-up brackets would be the cheapest way to go, and would be highly effective. I have however, debated (with myself) the possibility of using air bags to slightly unload one or the other of the rear axles. I have thought of doing this to reduce the bind while turning and to be able to tailor the turn radius. For example, in a parking lot, loading the front rear should reduce turning radius. Effectively a short wheel-base vehicle. On the highway, loading the rearmost would add to steering stability, like a longer wheelbase vehicle.

 

[jesusgatos]

That seems like a pretty good idea. Maybe even more useful on something heavier, like a 5-ton?

 

[Hainebd]

I know that theoretically this can be correct - however, I'm considering, for practical purposes, the bottom couple/few leafs in the deuce pack are bent so little that there is not a significant change in rates from one side to another. we're not talking about a cheby 1/2 ton truck spring with a significant arch...

View attachment 468198

this point, I think is moot and silly, as we've both outlined there are so many other issues to be resolved. super-singles or not, I'm confident that the last one or two captured leaves would be able to support the <2000 lbs of intermediate axle and wheels/tires.



I still like your chain-up brackets - I think that's the way to go.

it is not only the axle but 1/2 the load on that side of the truck and bumps combined with speed just amplifies the load on the chain. So normally the axle weight would be on the tires and the sprung weight of 8,000 pounds total would be shared among 4 points almost even. Now tie up one axle. Chain is holding 2,000 rear and 2,000 of unload truck so min is 4,000 chain now hit a bump with the tire on the ground and that force pushing the tire up is equally pushing down on the chained up axle. Easily 8,000 load on chain. I would scrap the idea except for emergency.

 

[VPed]

I wouldn't be so quick to scrap the chain idea. The springs and bogie are working in the direction originally designed when an axle is chained. But you effectively cut in half the spring rate. The static weight of the truck at that point is easy to handle with a chain. Hitting a bump will increase the load on the chain but only as transmitted by the spring pack. There would be a lot of capacity to absorb shock-load afforded by the springs. Shock loads are what breaks most chains. It is easy to size chain for static loads but since chains have little give, shock loads can quickly become excessive without something to dampen them.

 

[jesusgatos]

Can't see how using appropriately-sized chains in an emergency-type situation would be unsafe. Like VPed pointed out, they're loading the suspension in the right direction and the springs should absorb all but the most violent shock-loads. Maybe a combination of chains and airbags would be the ultimate solution, but it seems unnecessarily over-complicated to me.

 

[mudguppy]

chains. You're original design looked plenty beefy and quite effective.