trailing arm question

[bbaer6676]

Im getting ready to start my track car project. I know I need to replace the factory arms from the 1970t to get the larger caliper spacing. Do I go with the alum arms from 74+ or is it really worth it to use the 930 arms with the camber boxes? And are all the alum arms the same through all the years?

brian

[jluetjen]

Originally posted by bbaer6676
Im getting ready to start my track car project. I know I need to replace the factory arms from the 1970t to get the larger caliper spacing. Do I go with the alum arms from 74+ or is it really worth it to use the 930 arms with the camber boxes? And are all the alum arms the same through all the years?

Depending on your application and planned tires, the 74+ arms may be fine. Some questions that will help to give a better answer....

1) Are you planning on ultimately racing the car or not? If so, what sanctioning body and what class?

2) What is going to be your engine spec, or how much HP are you planning on putting through the chassis?

3) Based on #1 and #2, what size and types of wheels and tires are you planning on using?

Lots of people are tracking cars using the original style trailing arms, or even SWB cars and having a great time and going fast. The standard configuration suspension has more then enough handling to provide a lot of thrills if set-up correctly and if driven fast enough. If your track driving/racing experience is such that the non-turbo geometry is holding you back from getting the last few 10th's of a second, then I'd say go for it. Otherwise, in my book you're just moving money from your wallet to the suspension that would most likely be better used doing a few extra track days.

[trak ratt]

Except for an all out race car I‘d stick with just the standard aluminum control arm upgrade. Later alloy arms used bigger bearings and stub axels/flanges but require other component mods too. IIRC on pre ’72 911s you need to make a notch in the arm and change shocks but the mods are relatively easy. Nice explanation of the mods in the April ‘96 Excellence magazine. Saves something like 5.5 lbs a side.

[bbaer6676]

i am going to be running 9 X 15 front and 11 x 15 rear. The car will be autoX and DE nothing sanctioned. I would like the engine to be a 2.8 but will probably just be a 3.0 right now the tires are yoka's. I think you might be right about the money not being well spent on the 930 suspension for what I am looking to do.

[Chuck Moreland]

The narrow body AL arms are not all the same.

74-77 used a ball-stud attachement point for the sway bar. 78+ used a bolt on.

74-75.5 use 4 bolt stub axles, 75.5+ use 6 bolt. You can swap out the stub axles between early and late to make what you want.

[BK911]

I tried to put '74 arms on my '71. They bolted right on, but the upper shocks hit the shock tower. Some material would have to be removed from the control arm to move the bottom of the shock a little outboard.

I heard you can either mount the shock upside down or remove the cover and everything will fit fine, but I haven't tried it. I decided to stay stock.

[BK911]

Also, I am not sure what you mean by larger caliper spacing. I thought all the rears were the same. The fronts are probably 3" on the T struts, but S's, SC's, and Carrera's use 3.5". Some T's and E's have the S struts with the 3.5" caliper spacing.

[CLE]

There are several reasons I would try to get you away from the Turbo arms.

First, they are getting hard to find used and the cost is really going up like crazy so, if you ever need a spare it will be even higher.

Second, you will have to change your inside pick up point to accomodate the 930 arms.

Third, when you change the pick up points this increases your active camber gain which is exactly what you don't want to use Radial Tires. Those arms were build so they could run them on the 934's and 935's which both had Bias Ply Slicks which the camber gain is designed to work with.

Forth, they are heavier.

Fifth, If you plan to run stock offsets on the 11 inch wheels and use 930 flares, they won't fit because they also have a 22 mm more positive offset so, it's like the come with a built in spacer

One good thing about them is they have a lot stronger bearings for side loads but, the others seem to work fine

What brake spacing are you looking for? the 74 on up arms have a 3 inch as do the 76 and 77 930 arms but the 78 on turbo arms actually have a 3.5" spacing (same as a front S strut) It's easy to see the difference, the 78 and later mount on the rear side of the axle and the earlier ones are just like the 74 arms and mount on the side in front of the axle.

[Evans, Marv]

First, your wheels sound outragesously wide. Second, like said before, you will need to have 1 inch machined off of the steel and alimunum part (boss ?) of the swing arm where the bolt goes in to attach the bottom of the shock. You will also have to cut some of the bolt off to compensate for the machining. You will also have to use jacks to get the swing arm & bottom of the shock to line up perfectly to get the bolt threaded in. At least I did. Maybe somebody else will chime in with some tips on that, since I'm finding I have to do that part again. Have fun.

[randywebb]

also consider anti-dive effects

[911pcars]

Originally posted by Evans, Marv
First, your wheels sound outragesously wide. Second, like said before, you will need to have 1 inch machined off of the steel and alimunum part (boss ?) of the swing arm where the bolt goes in to attach the bottom of the shock. You will also have to cut some of the bolt off to compensate for the machining. You will also have to use jacks to get the swing arm & bottom of the shock to line up perfectly to get the bolt threaded in. At least I did. Maybe somebody else will chime in with some tips on that, since I'm finding I have to do that part again. Have fun.

Marv,
Mostly right on. The boss of the alloy arm to remove is aluminum, and about 1" is removed down to where the steel-threaded insert begins. I don't remember shortening the lower shock bolt at the time. Maybe I just sourced the bolt length needed instead of starting with the OEM bolt.

Article is here.

Sherwood

[bbaer6676]

11 x 15 is the wheel size from the rsr, there are many people on this board running 11's in the rear.

[bbaer6676]

For some reason I thought that the earlier steel arms had different caliper spacing than the alum ones. Appears that I was wrong.

[Evans, Marv]

Sherwood,
It has been awhile since I had mine done. I think my trailing arms are '74s. At any rate, they are pre-'77s. I think the steel insert came right out to flush with the surface of the aluminum and was not threaded down about an inch. I remember the machinist commenting to me his cutter was chattering or something, maybe because of the two metals when he machined it. Anyway, it shortened the depth by an inch, and I can't remember if I changed to a different bolt with a shorter or no shank.

[jluetjen]

Originally posted by randywebb
also consider anti-dive effects

Randy; Anti-dive at the back???

Or are you thinking about "anti-squat" or reducing the lift at the back under braking ("anti-lift?")?

[jluetjen]

Originally posted by CLE

Third, when you change the pick up points this increases your active camber gain which is exactly what you don't want to use Radial Tires. Those arms were build so they could run them on the 934's and 935's which both had Bias Ply Slicks which the camber gain is designed to work with.

CLE, I agree with most of what you said, but this part doesn't sound right. "In the day", the 930's from which these trailing arms are being taken from used wide radial tires. Sure a lot of the 934's and 935's used Goodyear bias plies, at least in the U.S. Dunlop 350/700-19's were used on the Factory 935's according to a R&T article of the time, and I believe that they were radials. But the revised geometry goes back to the Carrera RSR 2.8

In general, radials can accept more negative camber (and even are "happier" with it) then bias ply tires because of the relatively softer sidewalls. But the reason that Porsche went with the revised geometry is because they were generating (among other things) too much positive camber at the back when under extreme cornering, which was causing the rear of the car to loose grip. So the revised geometry did two things:

1) It raised the rear roll center a bit which made the car stiffer in roll at the back.
2) It increased the (negative) camber gain in roll which helped to keep the outside tire from rolling over onto the outside edge.
3) Increased the anti-squat at the back.

But these all come into play if you are really hustling the car hard with big sticky slicks. If you're not in that situation, then I doubt that the suspension change is going to make a big difference. There are usually "bigger fish to fry" in the driver's compartment then in the suspension.

But I agree with the recommendation that to go to Turbo arms in bbaer6676's case is most likely money mis-spent right now.

[Drago]

OK, I read Sherwood's article and I have one question:

I'll be installing later style SC control arms on my '69T w/ the CV joint 10mm bolt attachements to the inner and outer drive flanges. Since the control arms are SC units they have the 8mm attach bolt provisions.

What, if any, solution do I have? Does anybody make CV joints with 10mm bolts on the inner flange and 8mm on the outer?

The author of the article states that he did this conversion on his '69 but he doesn't state what his solution is to this problem (short of transaxle replacement or using '74 (only) control arms).

Thanks in advance for any and all responses.

[gestalt1]

Drago, you could buy axle assemblies for an SC and replacement inner CV joints for your 69. replace the inner CV on the new axles with the 69 type CVs.

[Drago]

Originally posted by gestalt1
Drago, you could buy axle assemblies for an SC and replacement inner CV joints for your 69. replace the inner CV on the new axles with the 69 type CVs.

Thanks gestalt, that makes sense.

The pesimmist in me must ask if it really is this easy?

[jluetjen]

Originally posted by gestalt1
Drago, you could buy axle assemblies for an SC and replacement inner CV joints for your 69. replace the inner CV on the new axles with the 69 type CVs.

Yeah, that's essentially what I needed to do when I put the '76 trailing arms on my '69. The only catch is that the 915 axle flanges had a 1 mm larger diameter where they ride on the bearings in the side-plate/transaxle case. So I had to have my local machinest turn them down by 1 mm. They've worked great since then, but I've never taken them apart in the few years since I've made the change to inspect them. I hope to do that at some point later this coming year and will post what I find at that time.