Hi-Lift Jacks

[MagicMtnDan]

There was a question about how the jack is able to support the vehicle stably with such a small base. The reason is that the jack is supposed to only provide a force straight up. All lateral forces are supposed to be handled by the other contact points... in that respect it is no different from any other mechanical jack... if you can roll off a scissor jack or a bottle jack it will do exactly the same thing.

I didn't intend to have my comment about physics be a question about how the jack can support the vehicle stably. I was trying to make a comment about the jack it's lift range (tall) and the small base. The point of my comment was supposed to be a concern about how the long (tall) lift bar has to be perpendicular to the tiny base and how challenging that is as the jack is used to lift a vehicle so high up. That's been my concerned since these conversations started and were demonstrated today (unfortunately).

 

[Falcon]

It's my belief Dan that the small base is by intent. As Frog says, the high-lift is designed to work in one direction... straight up. Think about it, if Jason had a big wide jack base that wouldn't move... the result would have been that when the truck rolled forward, it would have pulled of the jack point and fell. As designed, it just did a slow fold to the front. I would much rather have that than it pulling off the jack. Just my thoughts, but if I would have engineered it, that's what I would have done.

Falcon

 

[BigJ]

Agreed regarding the base to height concern; think suspension bridges. Relatively tiny footprint, enourmous height and superior stength when propertly supported by exterior contact points.

I think I'll try this again today. I'll try playing with the chocks first. I'll also use a hydraulic jack and lift one of the front wheels first with her powered down in 4h, then try to manually spin the wheel. If it doesn't move, I presume that's as good as we can get, in terms of preventing rolling?

If all that checks out, I'll give the hi lift another go.

frog I'll also triple check the angle of the standard before engaging the weight. But I have to wonder... We want it perpendicular to the load, not to the ground, right? In this case they happen to be the same thing (truck on flat ground) but 'out there' if the truck were hung up on a rock, on a hill...

 

[MarkT]

One thing to consider... geometry... when frog says that "the jack is supposed to only provide a force straight up" he is correct.

But think about what happens when the jack is used to lift the vehicle high, especially from the side. The lifting point the jack is placed under moves not only up, but away from the jack the higher you lift. The geometry change destabilizes the jack as you lift high. The way around this is to lift a few inches. Put blocks under the vehicle. Lower and reset the jack position. Lift. repeat.

Now think about the forces when lifting one side of the truck from the rear. The angle of J's truck is putting a slight side force on a very tall post with a small base. Had the truck not moved forward... it very well could have tipped sideways once the tires came off the ground. Because again, frog is correct. "all lateral forces are supposed to be handled by other contact points". Once the rear of the truck is off the ground, there would be nothing to resist the lateral force created by the angle of the truck trying to push the top of the jack sideways.

Other jacks are certainly not immune to these same forces. But most other jacks do not have the capability to lift so high in one lift without blocking and resetting. And other jacks usually have more stability due to their base to height ratio.

 

[MarkT]

Forgot to mention one thing...

A bottle or scissor jack has the all the lifting forces in line with the base. But if you look at a hi-lift, the lifting point is offset from the post that provides the lifting force. This offset provides a "tipping force" that other designs do not have.

 

[MagicMtnDan]

One thing to consider... geometry... when frog says that "the jack is supposed to only provide a force straight up" he is correct.

But think about what happens when the jack is used to lift the vehicle high, especially from the side. The lifting point the jack is placed under moves not only up, but away from the jack the higher you lift. The geometry change destabilizes the jack as you lift high. The way around this is to lift a few inches. Put blocks under the vehicle. Lower and reset the jack position. Lift. repeat.

Now think about the forces when lifting one side of the truck from the rear. The angle of J's truck is putting a slight side force on a very tall post with a small base. Had the truck not moved forward... it very well could have tipped sideways once the tires came off the ground. Because again, frog is correct. "all lateral forces are supposed to be handled by other contact points". Once the rear of the truck is off the ground, there would be nothing to resist the lateral force created by the angle of the truck trying to push the top of the jack sideways.

Other jacks are certainly not immune to these same forces. But most other jacks do not have the capability to lift so high in one lift without blocking and resetting. And other jacks usually have more stability due to their base to height ratio.

Ah clarity. We eventually all got there together in post #44

 

[Falcon]

Man. I feel smarter just hangin' with you'all <G>

Falcon "Lift me up, before you Go Go"

 

[BigJ]

I set out to try again but didn't. I tested my improvised chocks and I ended up not at all confident in their ability to hold. We definately will need some serious chocks for these tires.

I also jacked up the front driver tire (using a hydraulic jack) and verified it spun freely in 4x2, ebrake on. I then lowered back down, engaged 4x4hi, drove around a bit, engaged the ebrake and relifted. Our trucks do indeed stay locked in 4x4 when shut down. However, I was able to spin the tire approx 6" before hearing the tcase bind and stop the wheel from spinning. I rolled it back and forth several times to be sure; there's a good 6" of play in there before lockup.

Given both of those discoveries, I thought it best to not push my luck lifting with the hi-lift again.

I think Mark's right about this. No matter what, we really should be lifting a little bit, blocking, reseting and lifting a little bit more. I think this is true regardless of the lift of choice. The temptation to do otherwise is obvious with the hi-lift, but I think if self control is exersized, the hi-lift can be as safe as any other jack.

I've certainly learned a lot thanks to all this. And I won't say I feel comfortable using any jack with this truck yet. But I will say I have ordered a lift mate, and I'll be trying it out when it arrives. On this end of things, I have a feeling this accessory might be just the ticket that justifies carrying a hi-lift. If all the same problems exist for us with any jack, but a hi-lift can raise a tire...

To be continued.

 

[MagicMtnDan]

Another great benefit of this forum is when people (like you Jason) are willing to share information (even when the experiences aren't completely positive), other members can learn from their experiences. It's always better to learn from others' mistakes as it should save us from making them (not saying that Jason made any mistakes).

Thanks Jason, for sharing unselfishly with us. We all learned because of it.

 

[BigJ]

^^That's exactly the goal Dan. Information sharing, in all its (positive and negative) forms.

I do think I made a mistake yesterday. Two actually: 1) not chocking the front tire(s) and 2) not realizing how much weight was coming off the ebraked rear driverside tire. It was just plain luck I walked away from that damage free.

But I learned a lot too. Mark's advise really makes sense to me, and I don't think it would have rung so true without me getting out there and ******** up the way I did. Also I think frog's thoughts on the lift mate are dead on. I can really see how that accessory transforms the hi-lift into something that starts to make sense for us.

Its all good... Dad was right.