trailing arm question
 

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

Re: trailing arm question
 

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.

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.

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.

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.

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.

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.

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.

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.

also consider anti-dive effects

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

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

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

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.

Randy; Anti-dive at the back???

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

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.

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.

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?

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.

After a little research here on Pelican I've found that CV joints for the '65-'71 models years are NLA...I have to purchase the entire axle assembly.

CV joints are available for the SC control arms though. Does simply changing the '69 axle assemblies outer joint to an SC unit solve the problem though? From the diagram:

http://forums.pelicanparts.com/uploa...1167231940.jpg

It looks like the part that the bolts thread into would also need to be replaced (just to the left of the CV joint in red), which of course I can't find in the Pelican catalog. ;)

Pump the bump.

Anybody been there/done that with regard to the above problem?

Drago,
One thing I regret not covering in that article was information on adapting the various CV joint. I realized later on this would have covered all the issues in converting to alloy control arms. However, there are several iterations of adaptation depending on the MY control arm, CV joint type and gearbox, probably worth another article.

I suppose I took the easy way out with the simultaneous installation of a 915 gearbox. This resulted in a harmonious match with an SC axle assembly.

Yes, individual 901 CV joints are NLA from the usual sources, but it appears Foreign Intrigue carries them:
http://www.foreignintrigue.com/search.asp?radiobutton=2&search=CV+joint

However, it looks like one axle assembly (w/2 CV joints) is cheaper than a single CV joint. If you need 2 CV joints, this is probably the way to go.

These links contain some solutions:
http://forums.pelicanparts.com/showthread.php?s=&threadid=317217&highlight=CV+joi nt+adapt
http://forums.pelicanparts.com/showthread.php?s=&threadid=260451&highlight=traili ng+arms

Sherwood

What I did on mine was get couple of item 12 from a '76, and then had the diameter of the area just outside (to the right) of the splines in the attached picture reduced by 1 mm.

http://www.pelicanparts.com/911/911_...978-83/3-5.JPG

I think that I've heard that the later 915 gear boxes have a different splined pattern on these flanges which will then cause mating problems with the diff. In which case you may do better to replace the entire axle (including the outer stub axles that go through the trailing arm) with a set from a car which uses the earlier style 915 box, like a '76. Be sure to do a trial fitting because I found that my some of the stub axles will interfere on the trailing arm.

Agreed. The CV joints on any given axle assembly are the same on the inboard and outboard ends. So with one donor SC axle assembly, with both ends in tact, it seems to me it should be quite easy to swap out the outboard CV's on your existing axles and wind up with a bit of a hybrid. 901 pattern on the inboard end, newer SC pattern on the outboard. These things are very easy to take apart; it's just one hell of a greasy mess is all.

The real question is whether or not the old and new axles have the same splines. There is a c-clip that holds the inner CV joint over the splined axle end. Pop that c-clip off, and it would be a snap to swap them. Or, if the splines are in fact different, are the inner CV joint pieces otherwise the same? In other words, would an early inner CV joint work with the later outer, with the same ball size and such? Hopefully the splines are the same and that is the easy answer; it would make me nervous to mix and match internal CV parts.

Jeff;
I tried taking that approach when I upgraded my trailing arms and found that the design (spline count and diameter?) of the axles are different, so you can't put a 901 stub-axle onto a '76 axle. I suspect that the same will be true for SC axles.

- John

I was afraid of that, John. So much for the easy answer.

Drago, if you are doing this to get bigger brakes, the "M" calipers that are on there now should be fine for just about any purpose. If you are looking for mounting points for drop links to put a sway bar on it, the Weltmeister that it sounds like you have lined up doesn't use the mounts on the banana arms. It goes to the eccentric bolt on the spring plate / banana arm interface that is used to set camber. So maybe 5-10 pounds a side just isn't worth the level of hassle to change all of this out. Especially when you consider the shock tower clearance problem as well.

That reminds me of a question I've had for a while...

Do all years of the Al arms have the same calipers on them?

- that's the 911 arms, not the 930 arms.

- I know the Carrera pads are a different thickness, but isn't only for the fronts?

Randy,
The rear calipers on al. arms are either narrow A-type (20mm rotor) or wide A-type (24mm rotor) installed on post '84 911s. Both use the same brake pad as the front brakes. The exception are 911s/early 930 turbos with S al. front calipers that use a thicker pad, but the same sillouette.

Sherwood

I think Sherwood is in his cups. But all our fingers slip from time to time.

He knows as well as anyone that all the aluminum 911 banana arms have the same 3" bolt spacing for bolting on an M caliper (not an A, which is exclusively the province of the front strut). I'm pretty sure (but the convenient parts reference doesn't give a part # for the whole banana arm) that all the aluminum banana arms are the same (other than the previously mentioned change in sway bar mounting). And that the wider 3.2 rear rotors were accommodated by widening the M calipers.

My race car (a '68 tub) came to me with the LWB steel bananas. The shock towers had been modified to accept coilovers. The steel bananas had been modified by welding on a lug to the rear of the stock shock mount, and that was where the shocks/coilovers were mounted. I think that was done to clear the Spicer U joints that were used on it (I replaced with CVs pretty quickly - U joints are not good with the angularity involved). But when I switched to the aluminum bananas I don't recall having any problems with the shock mounts. I know (from reading) that there was a change in the top shock mount location at some point, but doubtless the work done to accept the coilovers gave me enough clearance). So I have been in blissful ignorance of any work needed on the banana for certain updates. Can one make up for the "wrong" upper shock mount by moving the lower one out some with some machining?

I really like the fact that the stock mount is unthreaded for a ways. Makes it easier to get the bolt started, as it slides in and holds things before you need to get a wrench on it.

Drago: the thing to the left of the red colored CV in the diagram is just part of the CV boot assembly. Some are two piece - steel flange and separate boot that goes over it. Some are one piece -
the rubber boot is permanently attached into the steel part, which is rolled closed around its bottom. Getting a boot is the least of your design concerns. Once you know what CV you will use, then you can just get the right boot.

Myself, I like the two piece kind of boot assembly. When the boot rips (as it eventually will), you just get another boot and keep reusing the flange. I use a one size fits all boot from JCWhitney. Inexpensive, works great. Just cut off a couple of the biggest accordion bellows parts and it is ready to go.

Walt Fricke

Hi Walt,
Thanks for the mild-mannered correction. Would it suffice for me to say, "I meant M, not A"?

Sherwood

OK, so my '77 Al arms (likely) have the narrow M-type (20mm rotor), assuming nobody futzed with 'em.

I'm missing something here: the arms don't have rotors.

The hubs do, but the rotors all interchange - fat, thin, all fit on the hub. Of course, the caliper has to fit the rotor. But the '77s had the 20mm thick rotor per Aichele's table, and whatever M caliper you have will bolt right up to it.

Having the proper piston diameter could be a different matter - over the years there were at least two rear caliper piston diameters. And then there were the front M calipers, which could be bolted onto the rear by mistake (pistons much larger).

You never know about references, though. Aichele would have one believe that the '81s were the first to get the thicker 24mm rotors. But the Porsche Parts Reference book says (in effect) that the change happened where we all thought it did - in '84.

Walt

Thanks for all the great information guys.

Alas, Jeff is right (again). Not worth the hassle for ~10 pounds. Easier to loose that weight and more off of the driver anyway.

Someday the car will see a 915 and maybe then I'll revisit the aluminum control arm upgrade.

In the mean time she's getting 22mm/29mm front and rear torsions, 22mm and 19mm adjustable front and rear sways with Koni Reds all the way around.

I plan on driving the hell out of it! ;)

And......

you are going with SC front struts with the 3.5" spacing for your SC brakes, right?

Keep in mind, taking 1 mm off of a pair of 915 transaxle output flanges is most likely less then $20 at your local machinest. Buying a pair of used 915 flanges is pretty cheap too. Just because it requires cutting a little metal should not stop you.

That is not what is stopping me. I don't mind cutting chips at all. It's just that I'm pretty much ready to tear into it minus some 'A' calipers and one small Pelican order for miscellaneous stuff like ball joints and tie rods.

The extra work involved in swapping the control arms, at least at this point, doesn't seem worth it to me.

Anyone have a set of 'A' calipers they'd like to sell? :)