Racing ARB, rear mount front bars - Alfa Romeo Bulletin Board & Forums
 
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post #1 of 5 (permalink) Old 02-20-2014, 05:23 PM Thread Starter
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Racing ARB, rear mount front bars

I am planning on making a rear mount front antiroll bar for my Vintage Berlina and for my GTV as well. The Berlina's spring rates seem about right but I think the car stills rolls way to much.

I have looked up the threads I can find here on the BB for info and have not found a lot of specific information.

But I think I can go with what I've got so far.

Briefly, I am looking at the Speedway Engineering 1.25" OD hollow bars. 36" long, .120" or .188" wall thickness.

What I am looking for is what anyone is currently using and why.

Wall thickness?

Arm style and length?

Distance from the lower control arm pivot to the new ARB attachment point? (to figure out motion ratio)

Any way to make this style of bar adjustable?

Here are my notes from my current 27mm bar:

Berlina/105/115: 105/115 Alfa front ARB on my Berlina is 1.085 and has an effective length of 30.75 and the arms are 11.0 long. Motion ratio is .54
Rate at mounting LCA is 443 lbs/in. Rate at the wheel is 239 lbs. /in.

Greg

1972 Berlina Vintage Racer, 1987 Milano Platinum (LeMons Car), 1982 GTV6 EP Race Car, 1974 GTV Vintage Race Car, 2012 VW GTI stage II, 1966 Corvette Coupe 327-350, 2007 Triumph T100 Bonneville. Dearly departed: 1971 Fiat 124 Spider, 1983 Saab 900 Turbo, 2004 Saab 9-3 Aero, 1995 BMW 540i Sport, 2014 Audi A4 (swamped)
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post #2 of 5 (permalink) Old 02-21-2014, 05:27 AM
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Rear Mount Sway Bar

Greg,

There were posts on this a long time ago. See if this link works.

http://www.alfabb.com/bb/forums/spid...-sway-bar.html

Dave

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post #3 of 5 (permalink) Old 02-21-2014, 05:12 PM
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Quote:
Originally Posted by GregSef View Post

Briefly, I am looking at the Speedway Engineering 1.25" OD hollow bars. 36" long, .120" or .188" wall thickness.

What I am looking for is what anyone is currently using and why.

Wall thickness?

Arm style and length?

Distance from the lower control arm pivot to the new ARB attachment point? (to figure out motion ratio)

Any way to make this style of bar adjustable?

Here are my notes from my current 27mm bar:

Berlina/105/115: 105/115 Alfa front ARB on my Berlina is 1.085” and has an effective length of 30.75” and the arms are 11.0” long. Motion ratio is .54
Rate at mounting LCA is 443 lbs/in. Rate at the wheel is 239 lbs. /in.
Direct numerical comparisons can be made with some straightforward maths.
Bar diameter raised to the 4th power gives an arbitrary number that can be compared to another bar diameter also raised to the 4th power.
Tho these numbers will only be directly comparable if the ARB is of the same design and the spring section of the bar is the same length.
If the spring section of the ARB is made longer, it gets softer, so a percentage difference of length should give the correct numerical comparison.
The same should apply to different arm lengths. Shorter arms will have less leverage over the spring section of the ARB, so will effectively make the ARB stiffer. So a percentage comparison should again be applicable.
And given that you'd probably attach the ARB to the standard position on the LCA's, the motion ratio will stay the same so doesn't need to be allowed for in the calculations.

Converting everything to metric numbers:
27^4 = 5314401 units

1.25" x 0.120" wall thickness
= 31.75^4 - 25.654^4
=1016190 - 433132 = 583058 units

The hollow bar section is about 15% longer so, if my maths is correct, should be about 15% softer. So the new ARB would have a calculated value of 495599 units

A measurement of the new arm lengths would be needed to complete the calculations. Again the percentage difference would need to be applied to the 495599 units to be able to compare it to the original ARB.
As you can see, the longer ARB spring section is already softer than the 27mm bar in the original position, so the arms would need to be considerably shorter to get the ARB stiffness back up to the 27mm bars value.

Slowly Progressing Vortech Supercharged 1990 Alfa Romeo 75 Potenziata. Out of Action Twin-Charged 1988 AW11 MR2. Current Daily Driver, The Glorified Taxi 2006 BF FPV F6 Typhoon.

Last edited by Duk; 02-21-2014 at 05:22 PM.
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post #4 of 5 (permalink) Old 02-21-2014, 06:02 PM Thread Starter
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Thanks for the reply but I am looking for the exact dimensions of what people are running for these bars.

Length of bar from arm to arm?
Length of arm measured at a right angle to the ARB?

These bars are attached to the ball joint bolts so the motion ratio is better versus the .54 for the standard attachment point on the spring pan.

I am good with the math and have the exact wheel rates calculated for the roll stiffness of the standard front bars.

Greg

1972 Berlina Vintage Racer, 1987 Milano Platinum (LeMons Car), 1982 GTV6 EP Race Car, 1974 GTV Vintage Race Car, 2012 VW GTI stage II, 1966 Corvette Coupe 327-350, 2007 Triumph T100 Bonneville. Dearly departed: 1971 Fiat 124 Spider, 1983 Saab 900 Turbo, 2004 Saab 9-3 Aero, 1995 BMW 540i Sport, 2014 Audi A4 (swamped)
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post #5 of 5 (permalink) Old 02-23-2014, 11:08 PM
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