Why do rolling chairs have 5 wheels?

[Seranthor]

God damn my leg and elbow hurt.

But anyway, it's pretty obvious that I just fell over and my first thought was "6 wheels would have prevented this".

Does anybody know if there is a specific reason to have a specific number of wheels on the chairs? Would it be too hard to roll or something?

I'm gonna go with stability, less than 5 would increase the possibility of you tipping or fallign over as you moved the chair., and FWIW, sorry you fell. At least you didn't walk into the wall like someone I know did earlier.

[obdigore]

Funny, my first thought would have been "Ow, fuck! My leg/elbow!"

And that is why you are in the Air Force and have not contracted Albino Brain Chiggers. See the difference? :P

[Anakso]

Big and black leather, like the kind you can turn around and say "I've been expecting you." in.

That's the same as mine then
But mines never fallen over, hmm.
Do you roll around a lot? I barely move my chair really.

---------- Post added 2012-12-03 at 06:27 PM ----------

I'm gonna go with stability, less than 5 would increase the possibility of you tipping or fallign over as you moved the chair., and FWIW, sorry you fell. At least you didn't walk into the wall like someone I know did earlier.

I walked into a door today
In the mall those doors that are supposed to automatically open for you were just closing as I approached, they stopped closing with about a 5-10cm gap and I assumed they'd open again but they did not.

[Howard Moon]

This is the one I have, from Ikea:

http://www.ikea.com/de/de/catalog/products/40103100/

It's stability is awesome, probably the most stable I've had. And I frequently lean waay back and put my legs up on my desk when I watch something on YouTube, etc. Never even had the feeling that it's starting to tip, which I often had with my previous chair.

[Anakso]

Ummm Howard. What the hell man!
149 Euros=186 AUD

But... http://www.ikea.com/aa/en/catalog/products/50137208/

My chair was about $180, I could have bought the above if it was about $180!

[wdmshmo]

And that is why you are in the Air Force and have not contracted Albino Brain Chiggers. See the difference? :P

What's wrong with being in the Air Force?

it's the 5 star base creating equal triangles for greater support and weight distribution..

[Howard Moon]

Ummm Howard. What the hell man!
149 Euros=186 AUD

But... http://www.ikea.com/aa/en/catalog/products/50137208/

My chair was about $180, I could have bought the above if it was about $180!

Whoa. Almost 100% in import tax, or wtf??

[wdmshmo]

Well, an odd number provides more stability in one point in that there can't be any right-angular alignment of all the wheels, but more wheels itself also provides more stability in that the allowed space for two wheels to be aligned with each other is smaller. 7-wheeled chair!

Only you'd start to lose effectiveness of weight that the chair can support after six wheels (possibly even five, since you can't get 6 perfect equilateral triangles from six legs.) Cost effectiveness goes out the window.

[Anakso]

Whoa. Almost 100% in import tax, or wtf??

I guess o.O
I assumed Ikea would have factories to make the products here though, to cut costs but I guess they either don't or stuff is just more expensive here!

[Nereth]

Well, an odd number provides more stability in one point in that there can't be any right-angular alignment of all the wheels

Wut?

Tipping happens when CG goes over the support base. Number of legs only effects the effective shape of the base. What's all this right angular alignment stuff?

Op: 6 legs is not gonna add significantly to stability. A pentagon vs a hexagon of equal centroid-to-corner length look pretty similar from an overall area covered perspective, and that's a pretty good analogy for 5 vs 6 leg stability.

[kittenboots]

because the five wheels represent the five points of a pentagram, allowing the chair to channel the powers of hell to either retain it's balance or cause you to fall over in retribution.

duh!

[Deleted]

It could have 27 wheels, if you squirm in it a lot and bend and so on you can still fall.

It has 5 wheels because that is actually its leg and arm, an army with 5 fingers. It's attached to the head of the chair, which is the chair itself on which you stand, It's bend in a straight angle, but not because humans made it like that, but rather because it's an open mouth, ready to eat you.

You see, those chairs are in fact creatures. They are found, thrown meat and when they open their mouth they're thrown in plastic. Then a material cover is put so you don't see the ugly creature under it since you can still distinguish the features.

[Howard Moon]

Wut?

Tipping happens when CG goes over the support base. Number of legs only effects the effective shape of the base. What's all this right angular alignment stuff?

Op: 6 legs is not gonna add significantly to stability. A pentagon vs a hexagon of equal centroid-to-corner length look pretty similar from an overall area covered perspective, and that's a pretty good analogy for 5 vs 6 leg stability.

The argument for more legs = more stability is as follows:

Let's assume that when you fall off your chair, it's because you leaned in the direction between two of the chair's legs. This is where the effective base size of the chair is at it's minimum. See picture below.


(behold my epic Paint skills)

Now take a chair with 6 legs instead of 5. Assuming that both chairs have legs with the same length (radius r), the effective radius between any two legs is longer on the chair with 6 legs (a < b).

So with the 6-legged chair, when you lean in the direction between two legs, your center of gravity has slighly more distance to travel before it causes the chair to tip.

[v2prwsmb45yhuq3wj23vpjk]

Wut?

Tipping happens when CG goes over the support base. Number of legs only effects the effective shape of the base. What's all this right angular alignment stuff?

Op: 6 legs is not gonna add significantly to stability. A pentagon vs a hexagon of equal centroid-to-corner length look pretty similar from an overall area covered perspective, and that's a pretty good analogy for 5 vs 6 leg stability.

The way the weight will be distributed with an even number of legs, aligning it so all of the legs form a right angles relative to the direction of tipping allows tipping to occur more easily. Basically, it's kinda like tipping a box that has the weight distributed perfectly is easier than tipping a box over that has the weight distributed more towards the side opposite the one you are trying to tip it on.

The argument for more legs = more stability is as follows:

Let's assume that when you fall off your chair, it's because you leaned in the direction between two of the chair's legs. This is where the effective base size of the chair is at it's minimum. See picture below.


(behold my epic Paint skills)

Now take a chair with 6 legs instead of 5. Assuming that both chairs have legs with the same length (radius r), the effective radius between any two legs is longer on the chair with 6 legs (a < b).

So with the 6-legged chair, when you lean in the direction between two legs, your center of gravity has slighly more distance to travel before it causes the chair to tip.

Another way to explain it is that the a scale triangle will always have a greater height the smaller the angle created by the two equal sides and each additional leg reduces the angle size. The height increases at a decreasing rate as the angle decreases with a asymptote equal to the length of the equal sides.

[NatePsy]

I guess o.O
I assumed Ikea would have factories to make the products here though, to cut costs but I guess they either don't or stuff is just more expensive here!

I always assumed it was because most of us Australians have higher income compared to other countries. If I'm wrong, somebody else could explain.

[Albert the fish]

make perfectly circular base for chair.
make hollow area inside said base.
fill hollow with multiple "balls" of some sort to allow for movement.

the most difficult part would be designing the "balls" so that they allow for fluid movement in a desired direction instead of being completely uncontrollable.
i can't imagine that this is too difficult a task.
isn't there an engineer anybody knows we can set to this task?

[Deleted]

I don't buy that it's just a cost issue. If it was, then premium chairs would have more than five wheels. They don't.

Having too many wheels makes the chair more clumsy and leaves less room for feet/other stuff. I think having five wheels is just the sweetspot for maximum stability but minimum clumsiness.