One More Lift Engineering Question

[Jack Olsen]

In this thread, I laid out the idea of using a hydraulic lift table to raise my race car enough to do brake and tire work on it in my garage.

One issue with my hydraulic lift table is that the two sliding wheels at the end of the side arms are not held captive in their track. In theory, the table top could swing up if a load overhangs too far over the end with the fixed attachment points for the side arms.

Here's an illustration of what I mean:

So two solutions were proposed. One was to make the rolling ends of the side arms captive, so that the top surface was no longer able to tilt.

The other solution was to load the heavier end of the car away from the 'hinged' side of the table.

(Edit: replaced photo with corrected balance point.)

The vertical line in the picture shows the 50/50 weight point for this particular car.

All was well. But I also ran the broad strokes of the idea past my father -- an old-time engineer who doesn't want to see his grandchild-maker crushed under a heavy vehicle.

The first thing he pointed out to me was that the farther I raise the load, the less stable the lift becomes -- in his words, "the stability of the system is compromised with each inch you raise the vehicle."

Fair enough. Nothing earth-shaking about that. And he said he thought the overall idea was viable, which was good.

After watching the video, though, he said it was smart that I "put the fixed pivots on the scissors lift toward the rear of the car where the engine concentrates more of the weight."

That's good, except that I didn't plan on doing it that way anymore. He elaborated: "The pin at the center of the scissors is moving back toward the fixed pins of the scissors as the platform rises. The center of gravity of your car should be over that pin at the center of the scissors at all times." And that: "When the car is at its highest point the CG should be over the fixed pins at the center of the scissors because that's when the system is most unstable."

So, I think it's safe to say he would not be a fan of putting the engine over on the side of the lift with the rollers.

Here's the heavy side over on the stronger side of the lift.

(Edit: replaced photo with corrected balance point.)

Now, I look at this picture and the first thing I think is that it wouldn't help matters very much to make the sliding rollers captive, since the likely tip-back scenario would happen whether or not the table top also flips up. All it takes is for the car itself to turn on a fulcrum point created by the edge of the table itself. The distance between the fulcrum created by the pins and a fulcrum created by the edge of the table is only about an inch and a half.

Given the weight of the car and the center of gravity, it's still unlikely (in my opinion) that the car would tip, even if the front wheels and gas tank were removed.

However... I'm no engineer.

If you guys on this board agree with my father that there's a stronger side and a weaker side because of the movement of the scissors, please say so. If you think he's wrong, then I'll try to make the best decision made on that information and still lift it with the center of gravity farther from the center pin.

If it makes more sense to lift the car with the center of gravity closer to the center pin, then one thought I had was to create supports for the car to sit on that put the weight on the other side of the fulcrum, like this:

(Edit: replaced photo with corrected balance point.)

Another thought would be to have a chain captive in the floor underneath the jack that can be attached to the front section of the car whenever I'm taking off weight from the front of the car. It seems like it would be pretty straightforward to limit movement that way.

But, again... I'm not an engineer. So I did up these drawings and I hope they'll provoke some discussion from you guys who understand this sort of thing.

Thanks.

 

[mytimeyet]

looks like the easy solution would be to make a 'captive' track for the rollers:

_______
O
--------

 

[Jack Olsen]

Thanks. But here's my concern with a change in weight distribution combined with captive rollers:

(Edit: Replaced picture with one that has corrected balance point.)

 

[phy6]

Quickest solution: get a second lift and raise both at the same time, front and back. I wouldn't trust a single scissor lift of that size.

With a single scissor the best you can do is position the center of mass directly above the upper roller when lifted to the work height.

However you support it, I'd want to be able to hang off either end without it tipping or seeming unstable, for wrenching's sake.

 

[larry_g]

When all this discussion started my concern was that it would pivot around the joint at floor level. It looks like you need to captivate the rollers at both floor level and under the table. Where you show the fulcrum in the first picture I would have a lot less concern of the table flipping up that I would have of the problem you show in the last picture (second post) of the car flipping off the table.

In this picture I would consider the whole weight of the car being concentrated on the at the point of where the CG contacts the table. See how far that is from the roller? This is a lever arm that is trying to bend the table at the point of roller contact. Is there enough stiffness in the table to prevent the bending? In use idealy you would locate the CG between the pivot point and the roller.

That said I am not a degreed engineer and cannot do the math for you but experience says this is what you have to deal with.

On edit; you could fabricate some steel sawhorses to slip under the car while in the up position then lower the lift to set on the sawhorses.

lg
no neat sig line

 

[dansmurf]

I am not an engineer but I have put many vehicles on a lift over the years. My biggest concern would be if you were removing parts off the front. Ex, front suspension work. The more weight off the front, the more the CG moves rearward. If that CG gets to the edge of the table, I would be worried it would easily tip.

Story behind my concern. Years ago I put a large truck on a small lift. I could not get it positioned where I felt comfortable so I just barely raised it. I was just rotating tires. I pulled the rear tires first and as I pulled the second rear tire I could feel the rear of the truck rise. I had to put the rear tires back on and remove the front first. Then swap the rears and install the fronts last.

Just my thoughts and experiences. Probably worth what you paid for them.

 

[mikeatrpi]

How many bags of sand would you need to put in the bonnet to move the 50/50 line to the middle?

 

[e-tek]

Good Lord Jack - just get a dedicated scissor lift and be done with it. You've done a good job showing that there's many ways to skin a cat....but there's usually only one right way.

 

[Jack Olsen]

This is fine tuning we're talking about. I've already put the car up and it's more solid than any midrise scissors lift I've seen. I'll do more testing with the front wheels removed -- it doesn't have to be at full height to test how well it's sitting on the platform. I think I'll have a final product that works better for me than a regular midrise scissor lift. But that doesn't mean I shouldn't test it for a lot of different possible situations.

 

[Ironcrow]

The weight distribution on a 911 can't be more than 60/40, not what looks like 80/20 you've drawn.

I'd make the moving rollers captive just because it's good form. As far as which way to load the car, it makes no difference. Do it however your father likes it best.

 

[bobadame]

I would make sure that the hinge pins at the center of the X are grade 8 hardware. If one of those fails it will do so without much warning. I'd also use jack stands once it's up just in case. It just looks spooky to me.

 

[ksdaoski]

reminds me of the "my lotus fell off the lift"

http://auto.commongate.com/post/Lotus_Elise_falls_off_car_lift_at_tire_store/

http://www.motorcitymarauders.com/forums/showthread.php?t=375

 

[Jack Olsen]

I would make sure that the hinge pins at the center of the X are grade 8 hardware. If one of those fails it will do so without much warning. I'd also use jack stands once it's up just in case. It just looks spooky to me.

They're more than grade 8. They're 1-1/2" diameter hardened steel.

These types of tables take hits from forklifts. I'm not particularly concerned about the lift failing. But I want to load the car on it in a way that will be as stable as possible with a 60/40 load.

 

[LWW]

I like your idea of chaining the front of the car down when it's on the lift. If you're going to weld on some channel to capture the wheels you might as well weld a trailer D ring on it to easily attach a chain and hook.

I would also do what others here have suggested and put a "high-rise" ****** jack or a "tall" jack stand under the back of the car when it's up on the lift just to give me some peace of mind while I'm wrenching on those stubborn suspension parts.

Posted in another thread, here's what I'm thinking of:

 

[stroked93]

I would attach a piece of flat steel 1/8th inch at the rear of the table, then when the table was lifted you could swing it down and install a pin on the bottom to lock top and bottom together at the rear. you could probably drill several holes on the base so you could keep the table from tipping at different heights.

 

[stroked93]

once you deal with the table tipping it would be easy to secure the car to the lift.

 

[Jack Olsen]

Well, I have an apology to make. I made a mistake with my graphics, and -- as Ironcrow noticed -- my 60/40 balance line was way, way off. This made the situation look a LOT more precarious (and Lotus-like) than it actually is. I'm going to replace the old graphics in my initial post, and I'll also put them here. The ones you'll see when other guys have 'quoted' me will still show the too-far-back balance point of the car.

Here's the heavy end mounted on the less-supported side of the table.

Here's the heavy end mounted on the more-supported side of the table.

Here's the heavy end mounted on the more-supported side, but with pads. (They hardly look necessary, now.)

Here's the nightmare scenario, which looks less likely with the new balance point.

 

[gsport]

if those are your suggested lifting points for your car, i think you'll be just fine lifting it from there, and not worry about it..

 

[LWW]

I agree with gsport. I would still put that jack stand under the back of the car just for a little added peace of mind though...

 

[Daedalus]

The pads aren't doing any good from a C/G standpoint. It is where it is.

Is the table bolted to the floor? Is the bottom roller pin in a locked in track? I see potential problems beyond what you've mentioned if the C/G were off, but bottom line is I think the C/G has a wide, comfortable berth.

 

[bobadame]

They're more than grade 8. They're 1-1/2" diameter hardened steel.

These types of tables take hits from forklifts. I'm not particularly concerned about the lift failing. But I want to load the car on it in a way that will be as stable as possible with a 60/40 load.

I didn't read the thread that you referenced. I don't know why I was thinking Harbor Freight so in my best Emily Litella voice,"never mind".

If the issue is the possibility of the table tipping, why not make it a non issue by using a couple of jack stands under the heavy end of the car?

 

[kwb]

BTDT with a lift table - I lucked out in that the only time I could get the top to pivot was in a certain circumstance close to where I needed to have top limit. I made a "C" shaped bracket that captured the rollers on the free end to be my upper limit stops and the top half of the "C" kept things from tipping.

In your situation where you have a bit more time to resolve this (I had an ongoing safety issue that had to be immediately fixed) I would make a plate that has a long oval that would capture the roller ends (top and bottom) over the full length of travel.

Capture the top roller and then the top plate, and scissor will pivot about the lower fixed location.

Depending on details of your lift table you might be able to trap the axle between rollers instead of the rollers themselves.

 

[srmofo]

If tipping is your main concern at this point, I am still in favor of pots set in the concrete and straps to keep things from tipping.

I would also add lift points to the table. Just to make sure that it is getting lifted from 2 points furthest from each other. Just like your pic with the red points.

 

[Jack Olsen]

Thanks, guys. I think I saw my initial (flawed) graphic, and it got me more worried than I needed to be.

I'm going to do a few more simple tests to see what the stability is like with the car facing in either direction. And then also test how hard it is to shift/tip the car with it on the table but only raised off the floor by an inch or two.

Looking at this center-of-gravity illustration made me queasy:

Looking at this one makes me wonder what I was worrying about:

 

[AlchemyMetalworks]

Jack...it is my opinion (it is ONLY an opinion, but one based on experience and intuition) that you are going to be fine with using that lift with your car, when the position of your car's CG is near or over the axle/pivot.

My reasoning? I've see far worse balanced loads on single-axle trailers being towed by 1/2 ton pickups down the interstate and arrive safely.

If you think about it, a single-axle trailer is pretty much a similar design. Yes, I'm over-simplifying (a bit). With a trailer, the only thing "capturing" an unbalanced load from tipping, is a stamped trailer tongue on a ball, with a dinky-*** fork and lever holding it. If the trailer is loaded rear-heavy (much like the engine in your car) the natural tendency is to unbalance the designed-in forward weight transfer. The results on a single-axle trailer are far worse, because the loads are dynamic...the mass of the towed object, plus the trailer itself, undulations in the road, swaying side to side, vibrations, etc. In a worst-case scenario, the rearward imbalance wants to lift the rear tires of a truck off the ground.

I would go without the lifting pads if I were using this. Eliminating stacked mating surfaces can go a long ways to alleviating any paranoia regarding a weight transfer in this situation. A tow-hook at the uncaptured end of the table, on the bottom side, and an appropriately sized chain mounted to the floor directly underneath it (at raised height) with just enough slack to affix it, will be more than adequate to prevent any tip.

 

[larry_g]

Jack
For testing your CG lay a 2x4 or 4x4 across the lift table and rise up to the car. The heavy end will be apparent. Adjust the board front or back till you lift evenly or at least close enough that you can push on the bumper and tip the other way. Now you have a real good idea where YOUR CG is. Can you now position the car over the lift table so when you are at full lift your CG is over the the lift arms preferably with the CG centered between the arms?

lg
no neat sig line

 

[Jack Olsen]

Thanks, AlchemyMetalworks. And thanks, larry_g. I was planning on doing exactly that 4x4 trick with my car once the lift was in place. But it makes sense to do it before I've cut any concrete.

 

[McRae]

Jack,

as it seems that the table is quite big, it covers the liftpads, there's no problem. On the new cup cars, which are even more tailheavy, I can hang myself off the ground in the rear when it's up on a two post lift and nothing happens. The liftpads on the newer cars have holes in the bottom, which I had plans on using to do some kind of safetylock to "chain" the car to the lift. The transport tables at Weissach has that feature, so thats where I got the idea, but I have since then realised that it's not really necessary, it's stable enough anyway.

 

[ghnl]

One issue with my hydraulic lift table is that the two sliding wheels at the end of the side arms are not held captive in their track. In theory, the table top could swing up if a load overhangs too far over the end with the fixed attachment points for the side arms.

IMO the bigger issue is that you are trying to make the lift table perform a job for which it was not designed.

Could it be done with reasonable safety - probably.

Wouldn't it be safer/better to get the tool that was designed for the job?

 

[Jack Olsen]

I agree that it's always prudent to use the tool designed for the job. But in this case, there isn't a lift that would work in my particular garage. And the hydraulic lift table design happens (I hope -- that's the point of this thread) to work with my particular car.

I have two benches along the wall of my garage that fold down. Even if I mounted a traditional midrise lift below the surface, I wouldn't be able to stand in front of those benches.

 

[MoonRise]

Jack,

You got a nice price on a very nice lift table.

But that lift table is NOT really the correct tool to lift a car that overhangs the table itself.

Is the lift strong enough to lift the weight of your Porsche? Seems like it.

But just lifting the weight load is not the whole issue.

You have a nice garage (make that a VERY nice garage), a nice Porsche, and a nice lift table. But that doesn't mean that all those things go together.

Nice Porsche in the nice garage, yup.

Nice lift table in the nice garage, yup.

Nice Porsche on top of the nice lift table in the nice garage, IMHO that is a NOPE.

You know when you are doing something or thinking of doing something and that little voice that you can sometimes hear whispers "Something doesn't seem right ..." and occasionally shouts "Danger!"? A -whole- lot of folks seem to be hearing that voice in this situation.

And yeah, those 'original' CG pics just SCREAM "NO!!!!"

The edited/corrected pics look much better from a CG on top of the lift table standpoint, but the load is -still- overhanging the lift table. By a lot.

Then you get into the issue of the top of the lift table being 'hinged'.

Then you get into the issue of the scissors mechanism of the lift table shifting its relative location underneath the table top as the lift rises. And what questions that raises about the load CG and the table support points and the scissors arms and pins and what not.

Put the rated weight on top of the platform with the load CG roughly in the 'middle' of the table top and not overhanging the table top and that lift will pretty much be trouble-free strength AND stability wise.

An overhanging load with a (possibly) shifting CG is -not- what those lift tables are made or designed for.

Then you have to put safety stands under the car, front and back once you raise the car. And tie-down straps/chains front and back once the car is raised. With floor pots (anchor points) that you have to still install (not brain surgery, but still more 'stuff' to do before you can do 'stuff' on your car). And blocking/pins/braces/props/what-have-you on the lift mechanism as well before you 'can' work underneath the platform or the overhanging load (Owner's Manual, Page 16, Danger Note #8, also the same Danger note as a decal/sticker on the lift itself).

I'm all for repurposing and DIY and so forth, but there are just too many 'iffy' and 'made-do' and such issues/questions that I keep feeling unsafe and just 'iffy' about this whole plan/idea.

IMNSHO.

Could it work? Maybe. And that is the problem, the answer is 'Maybe' with a bunch of safety concerns/issues and not 'Sure, no problem'.

Suggestion? Just call Vestil directly and ask THEM about the applicability of lifting a Porsche 911 (that overhangs the table by a LOT and has some major weight components at the far points of the overhanging object) on top of that size and model of lift table and working on said Porsche while it is on top of the lift table. Contact info right in the owner's manual, on the web, and on safety sticker #6 on the unit itself (Page 16 in the manual for the location of sticker #6).

 

[darkk]

??? You guys worry too much.

 

[Jack Olsen]

Well, I'll point out that automotive midrise lifts rarely ever secure their lower rollers and many of them do not secure the uppers either. On top of that, the automotive lifts have cantilever type arms that let you increase the leveralge on the 'wrong' side of the fulcrum.

The owners manual for the Vestil lift never prohibits overhang other than to caution against having too much on the hinged side of the table. I'll point out again that my exact lift table can be purchased with a 92" table top on it. Same mechanism, just a lot more 'overhang.'

That said, I do appreciate the concern and the cautions. If someone asked me this same question, I'd tell them to take things very slowly and that -- for the record -- I was against it.

 

[Photo]

Jack,

Enough talking, let's get that puppy installed into your garage-mahal!

Lane

 

[Jack Olsen]

See, that's just the problem. Work's going to keep me from doing the actual installation for two or three more weeks. So all I can do in the meantime is worry over it.

However, I just took a quick break and decided to see where my car's actual balance point was. If you're a 911 owner, keep in mind that my car is probably different than yours. It's been lightened considerably, has an all-fiberglass front end, has non stock suspension mounting points, has the wrong year's transaxle, engine, brakes, seats, has a cage, etc., etc. It's also had the entire drivetrain moved forward about 1-1/2".

Still -- this lift is for my car and my car only.

With a little less than 1/2 tank of gas indicated, but with an aluminum jack and some other things in the trunk, I rolled it far enough back in the garage so that I could get a 2x4x1/8" length of rectangle tubing underneath it. To keep the beam from falling over, I clamped on some support.

Then I jacked the car up and lowered it down -- four times in total. I started slightly behind the jack point because I've got a heavy engine in a light car. The final point where I could easily rock it back and forth surprised me.

Here it is, perched and balanced:

<iframe title="YouTube video player" width="480" height="390" src="http://www.youtube.com/embed/JPAyXYlAFVQ" frameborder="0" allowfullscreen></iframe>

And here's where the weight balance point shook out:

So this is how it will sit on the lift:

It measures out to a point 36-1/2" from the rear wheel hub center and 53" from the center of the front wheel. That's a weight distribution of 59/41 F/R, which is slightly better than a stock 911.

 

[Photo]

Jack,

I believe you will be fine.

Are you planning on putting a metal (angle iron) lip around your opening when you install your lift? If so, please heed some advice. I used one around my opening and made one mistake. I should have had it galvanized as it's started to rust with the moisture from melted snow up here in the great white north. (And yes, it was painted in an anti-rust paint last summer!).

You may not have the running water issues that I have, but you're a heck of a lot closer to the Pacific Ocean than I am.

Good Luck, and have fun!

Lane

 

[buening]

I wouldn't worry about the table itself tilting since there isn't any table overhang to the right of the table hinge. The car would tilt first before the table does.

I agree with your father about the center of gravity being located over the pin, but there is one problem. With the C.O.G. located over the pin in the fully extended position, when you lower the car toward the ground the C.O.G. of the car will no longer be over the pin, rather it'll be towards the left (in your diagram). I would err to having the C.O.G. located over the pin in the bottom position so that it ends up being to the left of the pin in the fully extended position, minimizing the chance for the system to overturn. You have the bottom tracks to help prevent overturning when the load gets shifted to the left too much. You can put a mark on your car's frame for the C.O.G. point, to help you in locating the car on the lift in relation to the pin in the lowered position.

What is the distance between the arms at the fully extended position (between table roller and hinge)?

 

[Jack Olsen]

Since the drawing is from the specification drawings for the unit, and the top is 48" across, I'd hazard that the 'spread' from roller to hinge goes from about 44" (lowered) to 26" (raised).

To my eye, it looks like my car's COG would move from about 3" to the left of the center pin when raised to maybe 6" to the right of the pin when lowered. But lower down is when the structure is stronger, isn't it?

Here's the corrected image with the car facing the other way.

If the center hinge ends up in the center of the lowered platform, then this configuration would appear (to me) to have less optimal weight distribution both lowered and raised. Am I looking at it wrong?

To put them close to each other, here's the version that my father favors:

 

[robertlynk]

Jack I think this is the optimal position because of the cantilever action especially if you remove front end components. if you remove the engine it move closer to center line

 

[larry_g]

The reason that dad is correct is that both lift legs (the X) carry weight. In your prefered version the CG is to the left of the X therfore two legs on the left of the X are in compression and the legs on the right are in tension.

lg
no neat sig line