Steel vs. Aluminum Trailing Arms

[Britain Smith]

Just in case anyone was wonder, I thought I would show the differences between the steel trailing arms from '69 and the aluminum ones from '87. The geometry between the two appears to be very similar with exception to the angle on the shock mount.

[69911e]

What is the weight difference?

[Britain Smith]

I will weight them out later tonight...I have to figure out how to get the nut off the spindle to take of the axels.

The biggest difference is the 3.5" bolt spacing for the calipers. I have carrera rotors and capilers now.

-Britain

[304065]

Calzia tells us that the steel arm weighs 16# with the inner stub axle and no outer stub axle. So it weighs more than 16# complete.

He says the Aluminum arm with bearing and no stub axles weighs 12#.

The 87 has a six bolt inner axle whereas the 74 and earlier have a four bolt.

When I do this mod myself (allowed as the geometry doesn't change) I'll measure the actual weight savings. I heard it was 5# a side.

[Randy Webb]

The actual wt. savings is 6.0 lbs. - both arms, incl. swaping the hubs too, and using a NBS traceable scale.

You can figure 1/2 of that is sprung savings.

[Wil Ferch]

Published reports indicate the alu version is also stiffer....

--Wil Ferch

[Randy Webb]

Will, I've heard that before but never seen it in writing. Do you know where that is?

[Leland Pate]

Let's try to get Eric to drill holes in his banana arms to reduce weight!

[Bill Verburg]

Unless the late one is off a 930 or t-look the brake bolt pattern is 3" o/c. Only the short 930 arms use 3.5" and they are in the trailing rather than leading position as they are on 911s

[Wil Ferch]

I believe it may have been in BA's book...maybe even Frere's book, where the add'l stiffness is mentioned.
Bill V... good catch...the 3.5 spacing / 930 issue. Which forces me to ask..aren't the 911 / 930 banana arms different as to geometry...or is it all in the body pivot points?
---Wil Ferch

[Bill Verburg]

Both, the 930 arms are shorter and the pivots are placed differently.

[GeorgeK]

Ludwigsen also mentions the alloy trailing arms are stiffer.

[304065]

Britain, what is your plan for the swaybar mounting hardware?

[speeder]

The only good reason for changing them is the stiffness issue; the weight savings is negligible on a street car.

[TOO RUF]

What year did they change from steel to aluminum? My 74 has aluminum. I'm not sure if they are stock though.

[69911e]

When does the increased stiffness become significant? I assume it only affects camber change...
With waht setup do they deflect sinificantly on street tires, on 225/15 hoosier radials, or not until huge slicks are installed?
Anyone have any data?

[poorsha]

TOO RUF:
74 was the first year for these.

[Randy Webb]

I think it was about '74 - I got mine off a '77, and have not checked my '75 to see what it has.

"the weight savings is negligible on a street car"
- OK, I have to ask the basis for this assertion. Have you done any calculations? any subjective test comparisons?

The wt. loss is in the rear so F/R dist. is affected. It is low on the car so body lean effects are nil. But 1/2 can be regarded as unsprung, so wheel/road effects are affected. I see two potentially useful things here. But my only claim is "potentially useful."

[poorsha]

Seems to me when I replaced mine, I noticed a fair ammount of weight difference. But it was a lng time ago.
The aluminum ones clean up real pretty.

[911pcars]

I measured a 5.5 lb weight difference between alloy and steel. It's the same effect as shaving a few pounds from each Fuchs wheel; only a portion of the lighter control arm would be considered unsprung weight.



The alloy arm above is from a '74-'77 that uses a ball attachment for the sway bar drop link.

Sherwood
http://members.rennlist.org/911pcars