Suspension set up questions

[sbmackie]

Current set-up in 1982 SC used 70/30 autocross and track/street:

18.6 front/26 rear torsion; stock sways, but have 22 front/21 rear Carrera sways to be installed soon. sport Bilsteins rear, unknown but allegedly sport Bilsteins front.

Considering: 21.6 MM hollow torsions front. Plus's and minus's?

The previous owner installed the 26 MM torsion in rear to "eliminate some understeer". So, going back to the 21.6 up front will return the car to the stock 5MM "spread' (18.6/24)

Could also keep same 7MM spread if you will by installing some 27 effective rate hollows in rear.

I like the balance of the car at this point. Nobody in my local club has the same set up I am considering.

I've read the article on this site.

All inputs gladly accepted.

Scott

[Chuck Moreland]

You said you like the balance now. What problem are you trying to correct?

[barney911rs]

My only words of advice is to either install the stronger after market rear sway bar mounts, or make sure you install larger torsion bars to match the larger sway bar. I've seen more then one person install large sway bars and not increase the torsion bars. Remember, they work together. Any suspension load not handled by the "stock" torsion bar is transferred to the sway bar and next thing you know, the mount has been ripped from the chassis.

I've been running 30mm torsion bars and a 22mm rear sway bar on my 73 that is pure track, not saying these are sizes you should use, and I still have the stock sway bar mounts and they are holding up just fine.

[aigel]

The 21.6 hollow bars are 21 mm effective? I recommend you go with 27 in the rear then. That's what I had on my SC and the balance was great.

George

[sbmackie]

I am seeking somewhat better track/autocross stability and less body roll.

Good points on the rear sway mounting brackets. I will check with my wrench. Although, I already have 26MM torsions in the rear, so I think I am ok there.

Does anybody know the weight difference between the hollows and the solids?

Will the heavier duty sway in the front damage the sway bar mounting brackets if I don't go with a heavier torsion bar?

Yes, I think the 21/27 in the SC will be a good mix.

Thanks for everybody's input!

Scott

[aigel]

I do not know the actual weight difference, but it is several pounds for sure. You can do a back of the envelope calculation if you estimate the volume of the drilled portion and multiply it with the density of steel. If I do that, I end up with anywere between 2 and 5 pounds per bar for reasonable (on the top of my head) dimensions. Naturally, if the bar is bigger, the weight savings will be bigger. I had all hollow bars on my SC.

George

[sbmackie]

I just got off the phone with Brian at Sway away. He says figure a roughly 11 pound weight loss going hollow.

Scott

[aigel]

Great. That's probably all 4 corners. Some people claim that the drilled bars also have slightly different spring characteristics:

"There are significant differences
between gun drilled, and solid
torsion bars other than their
weight. The hole in the center of
gun drilled torsion bars creates two
surfaces instead of one, allowing
the bar to react differently than a
solid bar to deflection. Gun drilled
torsion bars have a faster spring
rate than a solid torsion bar, which
means they rebound, or spring
back, quicker."

source: http://www.racemaxims.com/pdfs2/torsionbarfacts.pdf

George

[petevb]

Wow. The quote on hollow T-bars is some impressive marketing hype. There is no truth to it whatsoever, but it's impressive...

"Faster spring rate", huh? Buyer beware. Hollow T bars can be hardened inside and out, increasing the spring's lifespan if done right. I can't see any engineering reason for that spring back rate claim, however.

[Ira Ramin]

It's on the internet, so it must be true.

[Chuck Moreland]

To maintain the same balance, you don't want to maintain the same millimeter "spread". The relationship between diameter and spring rate is not linear, it is to the 4th power. It's further complicated because changing 1mm on the smaller front bar is a bigger percent change than 1mm on the larger rear bars.

If you really want to maintain existing balance while upping the spring rate, you need to look at the ratio of the equivalant spring rate - not the spread of the diameters.

Right now you have this

18.8 front : rate =110 in/lbs
26 rear : rate = 160 in/lbs

Current ratio = 110/160 = .69

Let's try stepping up to a 21mm front bar.

21mm front : rate = 174 in/lbs.

Rear rate = 173 / .69 = 251 in / lbs

Turns out a 29mm rear bar is about 254 in/lbs and would be pretty close. So if you really want to step it up to 21mm in front and maintain the same balance, you should go 29mm rear.

Know that 29mm is more rear bar than is usually used with a 21mm front. Just as 26mm is more than is usually used with 18.8mm fronts.

21/27 and 21/28 are more common combinations.

[aigel]

There may be a dynamic difference between a hollow bar and a solid bar but it should be very small. That company web page speaks of a "quicker" spring rate. I thought that spring rate was just force/distance with no time aspect to it?

Cheers, George

[sbmackie]

Chuck,

You said:

18.8 front : rate =110 in/lbs
26 rear : rate = 160 in/lbs

Current ratio = 110/160 = .69

Let's try stepping up to a 21mm front bar.

21mm front : rate = 174 in/lbs.

Rear rate = 173 / .69 = 251 in / lbs

Found a good reference to 'splain it more on Rennlist.
I am looking for some 28MM effective rate hollows.

You much smarter man then me.

Scott

[rfuerst911sc]

I'm running 22 front 30 rear solid bars and love the way the car handles. Also have Carrera 22/21 sways with Bilstein shocks.