[Trak Ratt]

Content Warning:I’ve given in to peer pressure and changed the chart below to properly express the relationship between hollow torsion bars size and their equivalent solid brethren. This is without checking the referenced Performance Products catalog that is at my residence. If after reading my corrected posting the responses from my closest friends and compatriots make little sense you have only yourself to blame for reading any of my posts. Thanks in advance for your criticism and rancor

Originally Posted by david riley

I’d always thought that hollow torsion bars were lighter and slightly stiffer than the equivalent solid bars. Over the last year I’ve had several “heated” conversations to the contrary with several people. Of course, the brain cells I needed to prove this mathematically have either long since died in alcohol induced euphoria or are no longer accessible due to over e-mail and Internet usage

So here is the data on the more common hollow torsion bar sizes from page 106 of my “spring Summer 2005 Performance Products” catalog. I have the 22mm fronts and 28mm rears in the SC and they have performed very well for many years

Relative size Actual size as purchased from ??
Front
21mm - 21.6mm
22mm - 22.5mm
23mm - 23.5mm

Rear
26mm - 27.6mm
27mm - 28.5mm
28mm - 29.4mm
29mm - 30.2mm
30mm - 32.0mm

[Jazzbass]

Hollow bars are lighter, but not stiffer that the same size solid bar. From the Elephant Racing site:

The size given is a solid-bar-equivalent. Actual hollow bar measures about .5mm larger.

I think your chart is backwards. Actual size 21.6mm hollow = 21mm relative size (that is, the same stiffness of a 21mm solid bar).

[BobNovas]

Dave - the idea with hollow bars is that the outer section of the bar does most of the "work" and the inner section does very little. When you twist the bar, the outer section has to move a lot more than the inner section - so the inner section provides very little of the torque. You can take away the inner section and not change much. A little, but not much.

By the way, if you take a torsion bar and wind it around a cylinder, you get a spring. And, guess what, the spring works exactly like the torsion bar, it's just different shaped. Compressing the spring twists the cross section of the rod that you make it out of - just like twisting a torsion bar.

[Vicegrip]

e the surface. Chris is right, the chart is backwards. Most hollow bars are sold on the equivalent size not the actual size which is a bit bigger than a same torque solid. Less confusing to all us ex pot head email users I guess. Much of the torque resistance is produced near the surface of the T bar or spring. This is one reason to not to nick up or let rust attack the surface.

[Trak Ratt]

Originally Posted by BobNovas

By the way, if you take a torsion bar and wind it around a cylinder, you get a spring. And, guess what, the spring works exactly like the torsion bar, it's just different shaped. Compressing the spring twists the cross section of the rod that you make it out of - just like twisting a torsion bar.

And spring “rates” are measured the same way as torsion bar “rates”. It’s just the answer is usually given in “pounds” for springs and millimeters for torsion bars. But what is the formula? And doesn’t the wall width impact the stiffness. Come on some one, show me the money :P

[Vicegrip]

Shudup and fix your chart already!

Random ramblings.

T bars are springs. Springs are T bars. T bars and springs can be preloaded. They are different too. Preloading a spring reduces its total travel. Pre loading a T bar does not.

T bars don't get in the way. That is, they don't have to be in the path of susp travel. Springs in most common systems are in line with travel. Springs will fully compress, go hard tight and become the bump stop. T bars can go full arc and the suspension is always the limit.

Springs can be progressive. They can have a thinner set of coils that compress first and when they hit each other the thicker coils come into play. T bars can't do this, they are linear progression only. I guess you could make a T bar that was inside of another T bar that was setup so the small inner T bar would go to a stop that would be part of the outer bar. This would be a progressive system but not as simple as a well made spring.

So... T bars. Not progressive and thickness for thickness, are about the same the alloy used being the variable factor.

Springs. Progressive, can have different rates depending on coil diameter and how progressive they are not just cross section thickness.

The method used for rating each is the simplest that conveys the information needed. Springs have to be measured in compressive pounds and T bars can be measured in dia if you know what thickness translates to in pounds of compression.

Take a T bar and drill a hole in it and it will have a lower rate. Stay same dia and have a hard time translating apples to oranges to us. Make it a little bit bigger around to compensate for the lower rate. Sell it to folks that want to be able to think apples to apples. Don’t need to give a long formula to convert just let us know what it acts like. Drilled 28.6 = 28 and so on.

Best part. Pitch them as if it they are far better than solid so folks will spend more $ on them.

[Trak Ratt]

Originally Posted by Kurt Mickelwait

Shudup and fix your chart already! )

Ok, it's not MY CHART, I accept that the hollow bars ARE less stiff than solids.
That said, does anybody have or can point me to the formula for converting bar mm to spring lbs? I already know it depends on length and width and probably has some sort of relation to pie (and ice cream ) and uses some silly looking character with a hat or something….
This might actually be useful for converting to coil overs or just coming up with the equivalent spring rates for when talking to Ricer guys. I can always ask Linda and she would drop what ever she was doing to work it out for me, but then you guys will be complaining about not being able to register on time

[BlackTalon]

You can buy this program and play around a bit... http://www.springwizard.com/torque.htm

[BobNovas]

Try http://www.miata.net/sport/Physics/1...moothness.html, around the middle of the page.

[BlackTalon]

And from Pelican: "The formula for calculating spring rate is: k=d^4G where k=spring rate, d=diameter, G=modulus of elasticity (constant for steel)."

For a hollow bar, you probably need to substitute d1^4 - d2^4 in the above equation. The modulus of elasticity is the one thing I am not sure of, as "G" is normally the shear modulus (and "E" is the modulus of elasticity). For steel, E = 30 x 10^6 psi, and G = 12 x 10^6 psi.

Also, the maximum shear stress = (the torque load x shaft radius (or wall thickness))/ the polar moment of inertia

The polar moment of inertia = (pi x d^4)/ 32 for a solid bar, and (pi x (d1^4 - d2^4)/ 32 for a hollow bar.

And the twist angle for a given torque load = (the torque load x the length of the torsion bar)/ (polar moment of inertia x shear modulus).

If all this is too overwhelming, ask Bob Novas to crunch the numbers!