Martin S.

all know, stiffer bars reduce body roll. If you go from a 17mm bar to a 20mm bar, what will be the improvement, is it linear? Well no...I just lifted this material from Grassroots Motorsports based on a post I read earlier concerning the new H&R sway bars, supposedly 26mm front and 24mm rear. I thought that the TRGs were big with 25mm front and 22mm rear, well...Torsional (or twisting) motion of the bar is actually governed by the equation: twist = (2 x torque x length)/(p x diam4 x material modulus.) And since the diameter is in the denominator, as diameter gets larger, the amount of twist gets smaller. Which, in a nutshell, means that torsional rigidity is a function of the diameter to the fourth power. This is why a very small increase in diameter makes a large increase in torsional rigidity.

(Warning: The concept is solid however this math above and below is suspect...it is being checked out )

For example, to compare the rigidity of a stock 15mm bar to an aftermarket, 16.5mm one, simply use the equation 16.54/154. Some quick math yields the figure of 1.46. In other words, a 16.5mm bar is 1.46 times as stiff-or 46 percent stiffer-than a 15mm bar of the same design.

Add just one more millimeter to the diameter of the bar-for a total of 17.5mm-and the torsional strength skyrockets to 85 percent stiffer than the stock 15mm bar (17.54/15.04 = 1.85).

PedroNole

Martin-

Thanks for that analysis. With all the different bars, sizes, etc. you've helped me understand how far I need to go right now and, as important, how much I need to spend.

TroyN

Cool stuff! By the way Martin, if you join NASA you get a subscription to Grassroots Motorsports - I saw you were going to renew in your other post.

Martin S.

Now you tell me on the free subscription...I already subscribed for two two (2) years, oh well .

jrgordonsenior

Martin,
I'm sorry but that math isn't correct.
16.54/15.04= 1.09 or 9% stiffer
17.54/15.04= 1.16 or 16% stiffer

Sorry, this newby does not want to appear argumentative but the math just didn't look right to me.
JRG

Martin S.

As stated in the original post, "I just lifted this material from Grassroots Motorsports". It is not my math..I don't even have a calculator with me today. But now I have looked at the math..it does not compute for me either. However this much I know is true. If you replace a 22mm sway bar with a 24mm sway bar, an increase of about 10%, the sway bar will be much stiffer than the 10% increase.

As was writtren above, "And since the diameter is in the denominator, as diameter gets larger, the amount of twist gets smaller. Which, in a nutshell, means that torsional rigidity is a function of the diameter to the fourth power." Me thinks there may be truth between these lines.

I will send the quote off to Cup Car for his feedback. He knows these things...once I receive the feedback, I will re-post, fair enough?

jrgordonsenior

Absolutely!
Again, I'm not trying to be an *******, just noticed the math was incorrect and that was important to the discussion.
I'd like to hear your take on those bars if you buy them versus your TRG's. Are you going to Willow?
JRG

Martin S.

I will be at Streets of Willow...and no offense, your comments were appreciated. I know the concept is solid...the math cannot be quite duplicated. My car (See avatar) is 326 LP, stop by and say hello.

mdd

What Martin said originally is accurate. The effective spring rate (stiffness) of a swaybar is proportional to the bar diameter raised to the 4th power. If all other variables remain the same (bar length, arm length, material selection, etc., a 10% increase in diameter results in a 46% increase in stiffness.

Check the Bosch Automotive Handbook, 4th edition, page 284 for the complete equation and diagram.

Martin S.

Thanks Mark for the clarification. I think it is important for Rennlisters to know the facts behind what we pay so dearly for...example, 993RS sway bars at over $1,200.

It seems that the H&R 26mm front and 24mm rear are indeed monster bars. BTW I have just ordered Bosch Automotive Handbook, 4th edition from Amazon.com

agdamis

Cost of these H&R sway bars guys?

mleds

I had the bars but on my vehicle and am content (the car is torsionally much stiffer and the bars make no noise over bumps or drivways. I will be at Willow and live in LA if you want to check them out). Under $800 was the price.
Michael

graham_mitchell

He just didn't use the correct notation. He meant:

16.5^4 / 15^4 where '^' means 'to the power of'

chris walrod

Great topic, Martin. A spreadsheet with all available ARB's (anti-roll bar) for the 993 would be cool bit of info to have.

ARB's are really an unavoidable bandaid for the independant suspension system. It coulples two independant system that conceptually wish to be totally independant. Ideally, one would want the least amount of sway bar stiffness that one could get away with.

Martin S.

Chris,

I have heard this before...go to monster springs and as litlle bars as absolutely necessary. Of course the car set up with 650+ Lb. front and 1,100 Lb. rear would nearly undriveable on the street, at least for my tender ****.

And thanks to Graham Mitchell for clearing up the math. Graham writes, " He just didn't use the correct notation. He meant: 16.5^4 / 15^4 where '^' means 'to the power of'"... as Chris writes, is this a great place, or what!

Best regards,