One More Lift Engineering Question

[Jack Olsen]

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

You know, the thing that just occurred to me is that I can position the car either way, depending on what I'm doing. The cool thing about the way I'm installing this thing is that it's just a 48"x48" platform that rises straight up. It will work whether I pull the car in nose-first or tail-first.

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.

He's been right pretty much my whole life.

Can you imagine being the son of an engineer with a 50-year career and only having a philosophy degree (and a lot of professional experience making up, uh... well, 'exciting stories') to draw on when you want to argue with him?

(You know how four-year-olds think their father is superhuman? I still feel that way about my 80-year-old father. He's got a clarity to his thinking that I lack.)

 

[buening]

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?

With your 48" and 26" assumptions, in order to have the car balanced over the pin in the up position the COG would end up 13" from the right edge of the platform. I have a feeling you will have difficulty achieving that with such a short wheelbase that the Porsche has. It is also pretty darn close to the edge if you ask me. Shoving the car to the left, while it stresses the X members more, allows more room for error and neither the car nor the lift would tip over. Most people that I know with scissor lifts just center the lift table under the car and then raise it up. In the up position it is a bit off center by the tracks/platform take up the additional stresses from it being offcenter (to the left in your diagram).

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?

You are correct that it would have less weight distribution to the legs this way, but I would be more nervous sliding that car to the right. You'll have less bearing area on the table and anything added to the right (in diagram) end of the car would have a greater effect in tipping it over

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

Thats about right, but again we are overanalyzing it. These lifts are designed for eccentric loading (car centered on the table) and is the reason why the pin is at the end of the table and not in the center

 

[Jack Olsen]

'Eccentric loading' means a load that is not applied in the optimal and designed-for way, right?

I think I can pronounce the overanalysis phase of this project finished. If I'm taking weight off the front, especially, I can back the car into the garage. Otherwise, my calculations put the COG point 18" into the 48" platform loaded with the engine over the X. Even in my initial test where it wasn't placed that far over, it was very secure feeling when I tried to push it.

I'm going to put an anchor in the concrete at each end, just under the lift. So I can chain it if I'm doing anything that will change the weight distribution or involve a cheater bar used in anger.

I've ordered an annoying red strobe light and a plunger switch so that there will be a compelling reminder to place the dead-block bars in place once the lift is raised.

Now, one last question for you guys who can do math. If these are the weight balance points along with the distance from the rear and front axle centers based on different weight distribution in a 2500-pound car...

%/%--------R/F----Dist to Front/Dist to Rear
50/50 = 1250/1250 - 44.75/44.75
51/49 = 1275/1225 - 45.65/43.85
52/48 = 1300/1200 - 46.54/42.96
53/47 = 1325/1175 - 47.44/42.06
54/46 = 1350/1150 - 48.33/41.17
55/45 = 1375/1125 - 49.23/40.27
56/44 = 1400/1100 - 50.12/39.38
57/43 = 1425/1075 - 51.02/38.48
58/42 = 1450/1050 - 51.91/37.59
59/41 = 1475/1025 - 53.00/36.50
60/40 = 1500/1000 - 53.70/35.80
61/39 = 1525/ 975 - 54.60/34.90
62/38 = 1550/ 950 - 55.49/34.01
63/37 = 1575/ 925 - 56.38/33.12

...what would be the effect on the balancing point (in inches) of reducing (not moving) 100# of weight from the front or rear of the car, assuming the weight came right off of the axle center? It seems like I should be able to figure this out -- but the philosophy training is NOT coming in handy.

 

[Az Scooter]

As long as the center of gravity of the car is between the two fulcrum points, it won't tip without extenuating circumstances. The weight on either end will balance it out, and there is enough weight in the car to keep it on the table.
I would not worry about it, at all. The car weighs less then the limits on the lift. The center point is expanded to the edges of the fulcrums, or pivots of the lift, and the rest is balanced.

 

[buening]

Yeah eccentric loading is loading that is not perfectly centered under the support.

I actually calculate the Dist front/Dist rear as 52.8/36.7 for the 51/49 distribution, but close enough. Removing 100lbs from the back results in a 51.3/38.2. I am summing moments around the centroid load (force x distance). The equation is below:

(Front axle weight)(x) = (rear axle weight)(wheelbase - x)

The wheelbase in this case is 89.5 and the "x" dimension is the distance from the front axle to the COG.

Example: 1025x = 1475(89.5-x). Solving for x = 52.81 Subtract that amount from the wheelbase 89.5 and you get 36.69.

Removing 100lb from the front is: 925x = 1475(89.5-x) Solving for x = 55 and 34.5 to the rear.


This all assumes the weight is applied directly over the axle and not distributed through the frame.

 

[larry_g]

http://excelmathmike.blogspot.com/2010/01/center-of-gravity.html

Buening says it quite well. The above link came up is a search for CG and I thought it interesting as that Lotus picture showed up in another post here.

lg
no neat sig line

 

[Jack Olsen]

Thanks!

Here's a graphic that shows the size of a human (more or less) relative to the lift and the car, and also the approximate change in center of gravity based on pulling 500# out of the back or 200# off of the front.

For the record, I can't even think how it would be possible to pull 500# from the front of a 911.

The distance from the garage floor to the top of the lift is 36" when it's sunk 7" into the ground and is fully extended up.

 

[rwhite692]

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.

^^^^Agree 100%.

The lift table may be able to lift the weight of your car, but, apart from all of your "balance point" concerns (meaning, fore/aft) have you even considered that the lift table may want to collapse from side to side, as the total CG (or should I say, center of mass) of your car will be quite high as compared to where the lift mechanism is?

What design considerations did the lift table manufacturer have in mind when they designed that table? I doubt they included a center of mass at that height and I'll bet they also did not consider any lateral forces.

And, what safety features does a lift table provide, in the event of a failure of the hydraulic jack?

I can see no reason why a proper, low-rise automotive jack "won't work" in your garage. You can use it outside if you absolutely need to get more height, AND it will be useful for servicing your other vehicles as well.

Nice drawings, though.

 

[Jack Olsen]

That's a fair opinion. The fact that this is not exactly what the table was designed for is the reason I'm worrying over it so much in this thread.

But let me correct one thing and answer some of your questions.

It's not a Harbor Freight product at all. It's made by Vestil, a U.S. company in Indiana that's been making material handling equipment like this for 43 years.

Vestil designed the table to work in manufacturing and warehouse settings, where it would be loaded and unloaded by forklifts -- and occasionally slammed into by forklifts. It's overbuilt. It is much less wobbly than the automotive mid-rise scissor lifts I've seen.

In the event of a hydraulic failure, the table's equipped with a brass velocity fuse to maintain platform height regardless of fluid pressure. On top of that, there's a heavy, captive steel bar that physically prevents the scissors mechanism from flattening once the table has been raised.

 

[creativecars]

Jack,
I usually find myself building or modifying things to fit my personal needs and space. Keep up the good work. I think it would be interesting to put a set of scales under the lift, maybe put some 3/4" plywood between the scales and the lift to see how the lift actually distributes the weight as it goes up and down. You can augment the safety factor with jack stands so no one gets hurt. Keep us posted.

 

[jcouch1]

what about making the table to where extensions could slide out and support the outer ends of the car(bottlejack). I would also go with enclosed rollers.

 

[big.jim]

do what all mid rise lift manufacturers recomend " use stands when altering weight distribution , ie removing heavy components"

 

[ron gray]

Your dad is right, bias the vehicle COG as close to the pivot in it's worst condition (max height). If your concerned about the table tipping when in use in this orientation (you shouldn't), you could easily mount some tension wires toward the front. They could sit outside of the lift mechanism, mounted to the bottom, and clip on to the top (something like the cables attached to a pickup trucks tailgate). The cables would react out any movement about the fulcrum. I bet we could design something pretty quick that could accommodate various heights as well! Honestly though, your fine without. Resting on that tabletop has a completely different dynamic than resting on a beam.

 

[Jack Olsen]

Thanks, guys. Here's what practical experience has taught me with it. Of course, the disclaimer is that I'm not an engineer. But there's been a LOT of use so far to support my three observations.

1) There is no way on earth it could ever pivot on the non-captive mounting points. It's one of those 'theoretically possible' things that -- with this particular load (car) in this particular application, is just impossible. (I'm with Ron Gray on this.)

2) There is no way the thing is even shifting on the lift in normal use. The GRIP of the 48"x48" contact patch (with the pad to insure a kind of mechanical conformity) is enormous, relative to the normal puck-at-the-jacking-point connections between an automotive lift and its load. Even with the engine and suspension removed, there's just no budging the thing, even when you're cranking hard on it.

3) It always makes sense to proceed with caution when using a tool for something other than what it was strictly designed for. But this thing was designed and built by a very reputable American company knowing that unpredictable loads would be put on it by (often) brainless warehouse employees, who were also bumping into it with forklifts. It's more stable than any consumer-targeted lift I've ever seen.

I'm loving having it in the garage.