Need help with determining thickness of steel bars

[sinkjk]

TL;DR Summary

Need help with determining thickness of steel bars.

I got a lift which goes into the trailer hitch in the back of my car for my wife's scooter. Only problem it was made for a wheelchair. I found the extension arms so I only need to come up with the 4 cross bars. When I lift the unit, the tires of the scooter and the weight will sit on two crossbars in the front and the two crossbars in the back.

So the question: How thick would I need to have my four 1 1/2 inch steel flat bars which will be 28" long for them to carry the weight of the scooter which is 160 lbs?

 

[Baluncore]

Welcome to PF.

Only problem it was made for a wheelchair.

The attachment was designed for a wheelchair. There will be a maximum weight specified for the wheelchair, what is that maximum weight?

Do you know the make and model of the attachment?

We need a diagram that shows more details, such as what you mean by crossbars.

 

[sinkjk]

The extension arms are made for the weight so they are not a problem. The lift is also made for the scooter weight so it is not a problem.

If you look at the picture you can see the crossbars that come on the unit, that is if you buy it made for the scooter.

The only question is: does it need to be 1/4", 3/8" or what for the bars to handle the weight of the scooter without bending?

 

[Baluncore]

If you look at the picture you can see the crossbars that come on the unit, that is if you buy it made for the scooter.

I see parts of dark things in the picture, some could be called crossbars, but my powers of remote mind reading, are unreliable.

Do crossbars go across the vehicle, or across the scooter?
What are they attached to, with what spacings?

Without a diagram, you are failing to communicate.

 

[Lnewqban]

This may help:
https://www.bruno.com/scooter-lifts/out-sider

Welcome, @sinkjk !

What do you call a wheelchair and what a scooter?
We need dimensions besides weights.
Are you going to weld those steel flat bars to the lift?

 

[sinkjk]

I'm sure you are smarter than me, but you are trying to make it a lot harder than it is. Please look at the picture again. Look at the bottom. You can see the bottom bar going from left to right. On each side of the reflector, you can see a screw. That is attaching the arm past the center platform. These I have and have installed. Now look at the far right. You can see the bar that goes from front arm to the back arm. There are two of these on both sides, the tires ride in-between them and they carry the weight while traveling.. The bars I need will be screwed onto the right and left arms going from front to back by a 3/8" screw. I hope this makes it clearer.

 

[Baluncore]

I hope this makes it clearer.

No, I am as confused as before.
Front, back, left or right, are relative to what ?
Before I guess at an answer, I need a diagram with dimensions.

 

[berkeman]

I'm sure you are smarter than me, but you are trying to make it a lot harder than it is.

No, we are trying to be sure that any advice we give you won't result in your scooter falling off the back of your vehicle on the highway and causing injury/fatality accidents.

Short leash -- do a *much* better job documenting your question or this thread will be closed.

 

[Vanadium 50]

The only question is: does it need to be 1/4", 3/8" or what for the bars to handle the weight of the scooter without bending?

The bars will bend. There is no way around that. The only question is how much, and how much you can tolerate. That depends critically on how things are attached, and what the worst case load is - which may well be when you are putting the scooter on or taking it off and have an unbalanced load.

The bar bend is not your problem. Your problem is the attachment points. They are likely to yield long before the steel does.

 

[sinkjk]

I appreciate you trying to answer. I will be able to watch the attachment points to see that they are not loosening, so I guess I am not as worried about that. I am attaching them using 5/16" stainless steel screws with lock nuts, just like the company does. I can tolerate the metal bending, I just don't want the metal to bend to the pointing of breaking. Am I just better off using the 3/16" bar to keep bend to a minimum and then no chance of the metal breaking? I don't have to worry about any breakage when loading or unloading as the bars will be flat on the ground at that point. As you might guess, the company wants to charge me an arm and a leg if I get them from them.

Again, thanks for trying to help.

[Mild insult removed by the Mentors]

 

[Vanadium 50]

Quiz: if you have a 3/8" bar attached by a 5/16" screw, which will yield first? (And do you really have only 1/32" of steel around the screw hole?)

 

[sinkjk]

No it would be a 1 1/2" wide bar by 3/8" thick

 

[Nik_2213]

Having wearily wrangled both my beloved wife's electric wheel-chair and 'Herbie', her nimble electric buggy, may I suggest you carefully consider the load's weight distribution. ? The batteries are really, really heavy. If you can, put them nearest the hitch.

Hmm, is this frame a side-loader, where buggy is across back of vehicle, or end-on ? Latter gives a much, much bigger 'arm' to forces on hitch and frame. Either way, I suggest you look at the robust build needed to tote a Skidoo or jet-ski lest your 'wheel-chair' carrier flex, fatigue then fly apart at Murphy-worst time...

FWIW, I bought a custom trailer to carry my wife's buggy. Its entire flat-bed pivoted 90º L/R on the commercial A-frame chassis to allow the tail-ramp to be optionally lowered kerb-side. Yes, it was seriously over-built, but towed so well I was glad of the indicators' repeater alert...

Added: One nice feature was that, unladen, the trailer, minus ramp, was designed to be stood on its tail for storage...

 

[sophiecentaur]

I hope this makes it clearer.

I would magine that you intend to make some sort of sketch of your planned job. Just post that. The photo is not helpful.

FWIW, I bought a custom trailer to carry my wife's buggy.

How does that affect your parking? Having used a trailer. I used to find that roads and car parks are not very trailer friendly. But
, of course,, if your application suits a trailer then no problem.

 

[Nik_2213]

I was familiar with our family's clam-shell-bodied 'camping' trailer, having towed that thousands of miles, and the buggy-trailer was even better behaved. Sometimes we had to use designated RV / trailer 'long' parking bays, some-times mini-bus/MPV bays, sometimes tuck into HGV / trucker bays. The latter seriously amused adjacent truckers given their six-axle 40+ tonne semi rigs. Sometimes, I'd find vacant car-bays on double-row end and roll right in. Where necessary, I'd get two bay-tickets from parking toll machine.

One way-station offset 'car' rows to prevent people just driving through. As we were stopping overnight at the site's motel, I had to use three bays. Square in first row, straddling two of next. Upside, this left two half-bays beside trailer, delighting a pair of weary bikers. Another site refused to let us park end-on 'connected'. So, I put car in one bay facing soft-verge, uncoupled trailer and rolled it into adjacent bay, Briskly screwed ground anchor into soft-verge, locked trailer coupling to that...

Much consternation ensued as a succession of drivers thought the bay was empty, got half-way in, found themselves facing the trailer tail-ramp's multiple warning triangles...

 

[sophiecentaur]

I was familiar with our family's clam-shell-bodied 'camping' trailer, having towed that thousands of miles, and the buggy-trailer was even better behaved. Sometimes we had to use designated RV / trailer 'long' parking bays, some-
. . . . .
Much consternation ensued as a succession of drivers thought the bay was empty, got half-way in, found themselves facing the trailer tail-ramp's multiple warning triangles...

Which indicates that a short load shouldn't really be on a trailer. (Also, short trailers are pigs to manoeuver in reverse.)

 

[Nik_2213]

With respect, much depends on the proportions and relative wheel-bases of tow-vehicle and trailer, plus the c/g position, hitch-loading and 'swing centre' of trailer.

As I mentioned, this trailer was well behaved. In fact, it did not wriggle, snake or dance, ran wondrously 'true'.

 

[Baluncore]

With respect, this thread is not about trailers, it is about short vehicle extensions to carry specific loads.

 

[Nik_2213]

My apologies for wandering so far tangentially.

A thought: I've seen vehicles, usually SUVs, that have both a 'wired' tow-hitch on the centre-line, and a bare second in line with driver. Latter for eg 'boat-ramp' work ?

What-ever, having a second mounting support, now closer to heavy battery pack, would both stabilise the 'tray' and provide significant reduction of 'moment arm' forces.

 

[Baluncore]

What-ever, having a second mounting support, now closer to heavy battery pack, would both stabilise the 'tray' and provide significant reduction of 'moment arm' forces.

From the chassis of the vehicle, the structure will spread in stages, out to the load. It is unnecessary to make any one stage heavier and stronger, if the stages on either side are mismatched. To minimise cost and weight, we start analysis at the load, then migrate towards the vehicle.

The first step is where the tires of the load rest on the flat bars. If those bars are tight and have no sag, then the end connections of the flat bars will fail, the welds will crack. That comes about by the triangle of forces on each flat bar.

Once those bars are bent to reduce their tension, there comes the question of the front and rear members being pulled together by the load on the flat bars. Something must oppose that tension, and still attach to a vehicle connection. It seems the flat bars are an unnecessary addition, since the compressive component could support the load without the flat bars.

I'm waiting for a diagram before doing the numbers on the design.

 

[Joe591]

The problem with this is even if you size the steel bars correctly, you end up with everything else being undersized. Like a guy that always skips leg day. Great upper body strength, but very skinny legs.

 

[Ranger Mike]

1/4" thick steel bar ( unknown elasticity so I used 65,000 ) over 28 inch length with midpoint load of 25 lbs. defect beam 0.25" but this is in a non supported free state. Neither end of the bar is welded.
I took and old 1/4" bar suspended it between two spaced blocks of equal height and placed the load at 14" midpoint, the bar deflected 1/4". Again neither end was anchored. I did same procedure with 2" wide 1/4" bar and got only 0.125" deflection, again not anchored.

 

[Baluncore]

... if we up the mid point load to 160 lbs. it deflects 0.080"

I was more concerned with the axial tension in the 1/4" thick bar, (the triangle of forces), and how that will distort the structure that supports it. If you minimise the deflection, you maximise the tension, and the damage to the frame when there is a vertical bump. The wheels need to apply the load directly onto the tubular frame, or be supported, hanging in deep catenaries.

 

[Ranger Mike]

If the wheelchair carrier has a rated weight capacity that will carry 160 lbs. then the 4 bars 1/4 thick will be more than enough if properly welded to the frame. If the carrier is for the true wheelchair and has only a 75 lbs., capacity you better rethink things.

just for general info
we use steel 1/8" thick straps, 1" wide to mount a 20 gallon fuel cell in a race car. 20 gallons x 8 lbs. per gallon is 160 lbs. and we are racing!

C

 

[Baluncore]

If the frame is already built, then rather than adding steel plates, it would be worth considering a fabric mat, laced to the frame, like a slack trampoline. The cost, and the weight of such a solution, will be less than the steel bars.

If the mat is stretchy, the load will be distributed over, and suspended within the frame, not resting directly as point loads on the frame. Frame distortion due to tension in the surface will be minimised by allowing some sag in the mat. The impact due to bumps in the road will be reduced, both on the frame and on the payload. Besides scooters and wheelchairs, the frame can also be used for moving other low-mass items, that can be loaded from the ground surface.

 

[Ranger Mike]

if this was a solid axle trailer your flexy solution may have merit, but we are talking about a vehicle that has suspension and already deals with road bumps and chuck holes. Adding a stretchy mat that could fail at any time is a liability and not an advantage. Especially with acid rain quickly rotting any material fabric.

 

[Baluncore]

Adding a stretchy mat that could fail at any time is a liability and not an advantage.

That is an irrelevant rhetorical truism.
The mat would clearly not be selected to fail at any time. Have you never bounced on a trampoline, sailed a catamaran, nor trusted a safety net?

Adding mass, to make something more rigid, especially if it is welded, not bolted, is poor design. Being "heavily constructed" is amateur. Mass is a design negative, not positive. The frame provided, that must support the bars, is more lightly constructed than the bars. That is clearly a design mismatch, that may be solved by the use of a mat.

I am trying to make the point that there are alternative low mass solutions, without welds, that can be optimum solutions. You used the example of a flexible strap.

we use steel 1/8" thick straps, 1" wide to mount a 20 gallon fuel cell in a race car.

Those straps would terminate in a clamp, with a threaded fastener. You would avoid the HAZ, by not welding the ends of the straps to the body.

 

[Ranger Mike]

The fuel cell mount is fabricated to fit the fuel cell steel case and must be bent on a sheet metal brake. This is not flexible strapping. It is then welded to the chassis as required by the sanction body rules. Would you want a bolt breaking during a race! Fuel cell is never mounted to the body as roll over crashes bend the auto body components to destruction.
Granted you can spend hours developing a low mass hi tech solution. I would choose to go the most economical way, simple is better. Form follows function. We are not designing for the space shuttle, it is a 160 lbs. cart carrier! Weld in the bars and run it. It may be , in your opinion, a poor design but it is strong, simple and effective.

And yes, I spent a lot of time in a parachute, as a jump master and have seen the effects of parachute malfunctions due to canopy, shroud lines inversion nets. Nylon and fabric have a shelf life a lot less than plain old steel bars.

You have a penchant for gratuitous remarks like " That is an irrelevant rhetorical truism.
The mat would clearly not be selected to fail at any time. Have you never bounced on a trampoline, sailed a catamaran, nor trusted a safety net?"
Why do you get so personal? I never attacked you or made disparaging comments. If I did it was not my intent.

 

[Baluncore]

Why do you get so personal? I never attacked you or made disparaging comments. If I did it was not my intent.

I am looking for alternative solutions. Rather than evaluating my suggested alternative, you immediately set out to demolish it rhetorically. I see that as a destructive attack.

 

[Ranger Mike]

you may be looking for an alternative solution. The Op is not. 1/4" bar vs 3/8 bar. go with 1/4-inch bar and weld it

 

[Baluncore]

I see no urgency to make a snap decision, since the OP has been gone for six weeks. We can leave all the possible solutions open. Others will find and read this thread, they deserve a range of rational solutions.

1/4" bar vs 3/8 bar. go with 1/4-inch bar and weld it

The limiting factor is tension, not deflection. It would be sufficient to go with 1/8" strap, wrap it over the arms and bolt it.