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Registered
Join Date: Jan 2004
Location: CT
Posts: 11,565
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Stiffness Hollow vs Solid T Bars
I've been doing some research on hollow T bars on this bulletin board and in the PCA archives and I've not been able to find anything on the relative stiffnes of a hollow vs solid tbar of the same diameter. There have been several references to the weight advantage of the hollow bars and some vague mention that a hollow bar will be less stiff than a solid bar, but nothing specific.
I know that the stiffnes of a solid bar is porportional to the 4th power of the diameter. My engineering intuition also tells me that the stiffnes of a hollow bar should be proportional in some way to the difference between the inner and outter diameters. I would guess that the stiffness is proportional to the 4th power of the outter diam minus the 4th power if the inner diam. This may seem arcane to some of you however, I have a set of hollow 28 mm bars that I am planning to install in my RSR Clone project car and I am worried that they may be too stiff for my street driving. Does anyone have the answer?? |
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04-28-2004, 02:05 AM
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For the same outer diameter, the hollow bar will be slightly less stiff than the solid bar. Use the equation J=pi/32(D^4-d^4) to calculate the polar area moment of intertia for a hollow bar (vs J=pi*D^4/32 for a solid bar) and then the theta = Tl/JG equation to determine relative "spring rates".
The short answer is that - depending on your definition of stiff - I don't think 28mm bars (hollow or not) will be "too stiff" for street driving. Mike
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Mike 1976 Euro 911 3.2 w/10.3 compression & SSIs 22/29 torsions, 22/22 adjustable sways, Carrera brakes |
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04-28-2004, 03:24 AM
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Original Owner
Join Date: Dec 2002
Location: San Antonio, Texas
Posts: 1,907
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The hollow bars cheat by .5mm or so, so that their "effective" spring weight is the same as the equivalent solid bar.
The difference is weight. That's why you buy hollows.
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tsuter 78 911SC Turbo Targa Thaaaats Right!! |
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04-28-2004, 04:31 AM
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Join Date: Nov 2001
Location: a few miles east of USA
Posts: 3,393
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tom
i am using 31 hollow and they are fine on the street.
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Rich  '86 coupe "there you are" |
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04-28-2004, 05:03 AM
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Somewhere in the Midwest
Join Date: Oct 2001
Location: In the barn!
Posts: 12,499
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I've got 32 mm hollow tubes in the rear with Sport shocks...not to stiff for me liking...the key is to match the shock to the spring rate.
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04-28-2004, 05:58 AM
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Back in the saddle again
Join Date: Oct 2001
Location: Central TX west of Houston
Posts: 56,316
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Yep, some vendor has a chart that says what the similar solid bars are. I think tsuter is right, the hollow bars are about .5mm larger to get the same stiffness.
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Steve '08 Boxster RS60 Spyder #0099/1960 - never named a car before, but this is Charlotte. '88 targa  SOLD 2004 - gone but not forgotten
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04-28-2004, 06:17 AM
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Registered
Join Date: Apr 2002
Location: Santa Clara, CA
Posts: 5,668
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Vendors of hollow bars publish the size as "equivalent" diameter.
A 28mm hollow has the same spring rate as a 28mm solid, but if you get the calipers out you'll find it is not 28mm. They do this so you don't have to do the math.
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Chuck Moreland - elephantracing.com - vonnen.com Last edited by Chuck Moreland; 04-28-2004 at 12:07 PM.. |
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04-28-2004, 12:02 PM
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1976 Porsche 911
