Anti Roll Bar question

Ok I’ve been persuaded to by a larger size anti roll bar ie 7/8". However. when it arrived it is not a solid bar, but a tubular one???

Does this seem correct because I thought a roll bar was meant to twist hence a solid bar would be far stiffer than one fabricated from a tube?

Am I missing something here?

cheers

Mark

  • never heard of…Supplied by whom ??

DH

Dont want to say at the moment, but I will off list

Mark,

A tubular cross section is fundamentally stiffer than a solid round section. (stiffness of beams; Q & I values & all that sort of technical stuff)
On older race cars a tubular roll bar was nearly always used.
Short & sweet but I hope it reduces your worries.

John

  • basically agree, but would still want to know who supplies them. Does it come with drop-links as well ? 7/8" is huuuge !

DH

If the outside diameters are the same, this isn’t true. A solid bar of the same diameter is stiffer as there is more material to twist…
A tubular antiroll bar is a way of making a larger diameter and therefore stiffer ‘bar’ which is much lighter than the equivalent stiffness solid bar.

Thanks for the ‘brain exercise’! :slight_smile:

:arrow_right: Matthew

I’ll have to find my copy of “Rorke’s” one day & blow the cobwebs off it :blush:
John

Mark

Do you know the answer to the question?

I don’t…

John :wink:

Unfortunatley I dont know the answer…but I will find out. Somehow???

cheers
Mark

I have a 7/8" tubular roll bar that I used to use when I was autocrossing. I used it in conjunction with a rear bar, uprated springs, etc. It made the handling super crisp, but the setup was entirely too hard for the street, so I removed it. Yes, the new bar came with its own drop links. I don’t recall who supplied it.

Hi John & Mark,
Can confirm (from conducting experiments at physics degree to prove what we already learnt) that…
“for any given diameter, a hollow tube provides greater strength/stiffness than a solid of the same diameter” (even though there will be a lot more metal in the solid bar!)
It’s a fact, so yes a hollow tube would provide better anti-roll control than a solid bar of the same diameter.
Seaandmoor

Hi John & Mark,
Can confirm (from conducting experiments at physics degree to prove what we already learnt) that…
“for any given diameter, a hollow tube provides greater strength/stiffness than a solid of the same diameter” (even though there will be a lot more metal in the solid bar!)
It’s a fact, so yes a hollow tube would provide better anti-roll control than a solid bar of the same diameter.
Seaandmoor

Which begs the question,does the wall thickness have any effect?-How about a 7/8" tube with a 0.005" wall versus a 7/8" bar???

John :wink:

seaandmoor,

Your conclusion is incorrect. A solid bar of the same diameter as a hollow bar and all else being the same, will have greater stiffness than a hollow bar both in bending and torsion. An anti roll bar is subject to both bending and torsion (2-levers). If your experiment showed differently, than something else changed. This difference will be exactly in the material removed, both geometry wise and material wise, according to the principle of superposition.

john.p.clegg,

Yes, wall thickness matters. Material removed from the neutral axis, which for a symetrical bar in cross section is the center of the bar, will have little but some effect. As material is removed farther from the center of the bar or neutral axis, creating a thinner wall, the effects are more dramatic. As the wall section becomes even thinner, different failure mechanisms other than stress come into play, namely buckling.

Bill

I disagree with some of the theories posted above, they are fundamentally wrong!

Take a 1 inch diameter bar of a certain length, fix one end and apply a torsional load to the other end. Measure the angle of twist. Call this x degrees.

Drill out the bar with a 1/2 inch diameter drill and apply the same torsional load to it and it will twist by y degrees.

x will, without exception ALWAYS be less than y

i.e the solid bar will twist less than the hollow one.

The stress in the material won’t change much and neither will the amount of twist but a solid bar is ALWAYS “stiffer” than a hollow one of the same diameter and material.

Obviously weight for weight a hollow bar can be made a much bigger diameter than a solid one and therefore be much stiffer.

Except Bill who happened to post while I was composing my reply :slight_smile: …and Matthew who got it right too :slight_smile:

All of you are beyond me by this time. :confused:

Thanks M100, I guess I was a little quicker on the trigger.

freather,

Anti-roll bar out side diameter is the most important variable. Larger is stiffer.

Bill

it works better than a solid bar because there are 2 diameter surfaces in a tube —ed

This is interesting - I will read the physics whilst watching the F1 this afternoon.
The standard roll bar is solid steel normally and weighs 3kgs and is .675 diameter. The next size up is .8" diameter and weighs more! and there is also a solid steel from TTR 1" diameter - which I think I now need for racing but it will be around 4.2 kgs in weight.
A tubular version, assuming it is well made and from good quality seamless tubing and the bend quality is good could be a winner!
I use the standard bar for hill climbs with 300 lb front springs and the .8 bar for racing - there is a big difference in handling.
I just fitted perspex side and rear screens into my S4 to save about 10kgs in weight but to now get a 1" tubular bar and save another 2 kgs is interesting. So who makes them please?
Richard