Modern trailing arms for the 914?, 986 carriers/calipers/e-brake, more adjustability, more tire? Options

[eeyore]

@groot Any thoughts?

[rgalla9146]

As for Petes question about internal condition of the trailing arms.....
mixed answer. Upper surfaces were perfect (dip painted even ? !) some bottom areas had some surface rust.
They are not a closed chamber, they all have holes manufactured in. My only regret is I didn't use seamless chromoly tubing.
Notice the grinding on the brake adjustment tube. That is necessary for GT spaced calipers to be centered over rotor.

[horizontally-opposed]

Rory also has some suspension console reinforcement that you can see in the pic.

Simple triangulation of inner ear and in the case of the rear trunk increased
cross section of transmission crossmember.

^ Really nice upgrades, Rory. (IMG:style_emoticons/default/wub.gif)

[horizontally-opposed]

Some of that 911 gear looks like a Rube Goldberg devise. I having a hard time imagining those parts not flexing (twisting) under a torsional load.

Haha. Can't disagree, though I am not qualified to comment. With that said, the $250-350~ pair of spring plates with ride height adjustment might be a better starting point than $2,500 moon gear. Narrower, too. And if camber can be adjusted at the wheel carrier…or just stick with the shims, which work fine.

One thing I am noticing in looking at images of the 901/911 tub is that its upside of more room inboard for that banana arm appears to be offset by the way the 901/911 tub drops down—the 914 has more room for its one-piece trailing arm. Need an engineer to tell us which is "better," if one is, but if there's one thing I have learned while studying Porsche, it's that the 914 benefitted from another 3-5 years of learning at Porsche—not to mention the skyward engineering ambitions of a young Ferdinand Piëch. Yes, that one—the 917, Quattro, and Veyron guy. Also, the Phaeton… 

[horizontally-opposed]

And then you consider the work that people are doing to 914 trailing arms, more often than not reinventing the wheel on their own as they do so…with the photo below as seen in Armando's wonderful "The last new 914-6 GT thread" over on the Bird Board…

Poking around, several shops offer rebuilt trailing arms from $850~ with no bells and whistles to $1800 with 911 e-brakes & four lugs or $2400 with 911 e-brakes and five lugs.

[groot]

Me? Lots of thoughts... about what? (IMG:style_emoticons/default/evilgrin.gif)

I expect the 911 blade-type semi-trailing arm was more expensive (more complicated, more parts, etc), so the 914 version is cost-reduced version of the semi-trailing arm and took advantage of the 914 package space. They are both less than ideal for proper race cars... but, it can get worse.

In the end, they both have the same limitations (toe/camber/roll center/static camber are all intertwined). Camber gain is minimal and the roll center gets way too low with increased negative camber.

Responding to the original question with some commentary:

• Move the brake line
• Tire clearance can be pretty minimal at the trailing arm, I'd be okay with 5mm
• If you can't get to 5mm, you may consider scalloping the trailing arm, while adding some stiffening actions

[914forme]

I have been thinking about this a lot, and came up with several solutions but they achieve nothing. As just posted there are a bunch of issues with the rear arms on both the 911 and the 914.

I had a set of the best arms ever made from Tangerine Racing. And they had a bit of clearance issues with 17 rears. Chris said dent the gusset he puts in there.

Here is my thoughts going thiner you have to increase strength in some way. Thicker walls and internal gussets but these do nothing for the inherent geometeryissues that come with running the arms.

If you want lighter get a set of arms made out of Titanium. That would also allow you to narrow them.

If you really don't care what they look like under the car, a series of laser cut sections could be made and significantly reduce the thickness of the area while adding to registry.

If you did this out of aluminum then you could weld on a modified 986.2 rear console and all the great bits that go along with it.

Lot of TIG time to build it.

In reality have someone 3D model it, send it off to be 3D printed as a sand cast mold, and then build the parts of your dream. You get one shot with the mold. But it works.

[barefoot]

I have been thinking about this a lot, and came up with several solutions but they achieve nothing. As just posted there are a bunch of issues with the rear arms on both the 911 and the 914.

I had a set of the best arms ever made from Tangerine Racing. And they had a bit of clearance issues with 17 rears. Chris said dent the gusset he puts in there.

Here is my thoughts going thiner you have to increase strength in some way. Thicker walls and internal gussets but these do nothing for the inherent geometeryissues that come with running the arms.

If you want lighter get a set of arms made out of Titanium. That would also allow you to narrow them.

If you really don't care what they look like under the car, a series of laser cut sections could be made and significantly reduce the thickness of the area while adding to registry.

If you did this out of aluminum then you could weld on a modified 986.2 rear console and all the great bits that go along with it.

Lot of TIG time to build it.

In reality have someone 3D model it, send it off to be 3D printed as a sand cast mold, and then build the parts of your dream. You get one shot with the mold. But it works.

Making trailing arms out of titanium or aluminum won't necessarily buy you anything of value.
Stiffness is the major design requirement for a trailing arm and for a given mechanical design (ie the present dimensions od the 914 arm)
While aluminum is only 34% as dense as steel, it's only 34% as stiff as well, so specific stiffness (Tensile modulus divided by density) is 106 for 7000 series alloys.
6-4 Titanium alloy is 56% as dense as steel boy again is only ~62% as stiff, so specific stiffness is 101.
carbon steel is more dense, but much more stiff, so specific stiffness is 106. so for a given geometry you'd have to make the wall thickness much thicker in aluminum to achieve the same stiffness, same for titanium, so no weight savings.
Only changing the arm geometry (like a bigger box section, or the clever tube inserts seen in these posts) can improve it's stiffness.
Higher strength alloys don't improve stiffness, just allows greater deflection before permanently bending.

[horizontally-opposed]

^ Good inputs.

Would be curious for an engineer's take on Rory's very cool looking solution, but figure that's only added labor on top of the rebuild/reinforcement services available for $1800-2400 once a 911 e-brake is grafted into place. Also wonder how much weight that saved, Rory? I sure dig the look…

$2000-4000 for a pair of modified 50yo trailing arms would seem to open up possibilities for new trailing arms—and perhaps some weight savings come instead from 986 or 930 calipers instead of two-piston steel Ate calipers and/or two-piece rotors. But it's going to take an engineer to see a smart way forward.

[eric914]

Ive just skimmed though this thread but it presents an interesting. I don't believe that the 911 spring plates see any lateral load though, it is just transferring the force generated by the torsion bars. The aluminum suspension arm takes all of the lateral loads. In the 914 the spring plate would be eliminated and a coil over shock used in its place.

[rgalla9146]

^ Good inputs.

Would be curious for an engineer's take on Rory's very cool looking solution, but figure that's only added labor on top of the rebuild/reinforcement services available for $1800-2400 once a 911 e-brake is grafted into place. Also wonder how much weight that saved, Rory? I sure dig the look…

$2000-4000 for a pair of modified 50yo trailing arms would seem to open up possibilities for new trailing arms—and perhaps some weight savings come instead from 986 or 930 calipers instead of two-piston steel Ate calipers and/or two-piece rotors. But it's going to take an engineer to see a smart way forward.

Doooh! I didn't weigh before and after. But....
I wanted to add stiffness in a different way.
The commonly available kit was not appealing to me.
Our cars have virtually no multi-layer metal features or reinforcements.
They do have complex shapes and boxes which provide very strong lightweight
assemblies.
My picture shows what was removed and what was not added.
What was added was maybe 1 1/2lb. of heavy wall 2" and 3" tubing.
The stiffening kit and removed discs weigh 2lbs 14oz.....~3lbs then
1 1/2 lbs went back on.
So a net loss of 1.5 lbs per side. Unsprung.
What really matters would be performance in a torsional rigidity test.
That can not be done with a postal scale

[stownsen914]

If you're fabricating and really want light, I believe the best racecar fabricators use chromoly. It's not lighter than low carbon steel, but you can use thinner wall to get the same strength. I suspect most just use DOM steel since it's easier to work with.

[rgalla9146]

If you're fabricating and really want light, I believe the best racecar fabricators use chromoly. It's not lighter than low carbon steel, but you can use thinner wall to get the same strength. I suspect most just use DOM steel since it's easier to work with.

(IMG:style_emoticons/default/agree.gif) next time it will be chromoly and TIG

[914forme]

I have been thinking about this a lot, and came up with several solutions but they achieve nothing. As just posted there are a bunch of issues with the rear arms on both the 911 and the 914.

I had a set of the best arms ever made from Tangerine Racing. And they had a bit of clearance issues with 17 rears. Chris said dent the gusset he puts in there.

Here is my thoughts going thiner you have to increase strength in some way. Thicker walls and internal gussets but these do nothing for the inherent geometeryissues that come with running the arms.

If you want lighter get a set of arms made out of Titanium. That would also allow you to narrow them.

If you really don't care what they look like under the car, a series of laser cut sections could be made and significantly reduce the thickness of the area while adding to registry.

If you did this out of aluminum then you could weld on a modified 986.2 rear console and all the great bits that go along with it.

Lot of TIG time to build it.

In reality have someone 3D model it, send it off to be 3D printed as a sand cast mold, and then build the parts of your dream. You get one shot with the mold. But it works.

Making trailing arms out of titanium or aluminum won't necessarily buy you anything of value.
Stiffness is the major design requirement for a trailing arm and for a given mechanical design (ie the present dimensions od the 914 arm)
While aluminum is only 34% as dense as steel, it's only 34% as stiff as well, so specific stiffness (Tensile modulus divided by density) is 106 for 7000 series alloys.
6-4 Titanium alloy is 56% as dense as steel boy again is only ~62% as stiff, so specific stiffness is 101.
carbon steel is more dense, but much more stiff, so specific stiffness is 106. so for a given geometry you'd have to make the wall thickness much thicker in aluminum to achieve the same stiffness, same for titanium, so no weight savings.
Only changing the arm geometry (like a bigger box section, or the clever tube inserts seen in these posts) can improve it's stiffness.
Higher strength alloys don't improve stiffness, just allows greater deflection before permanently bending.

I while I agree with you on your basics, it is the design that forms the ability to make the part structurally sound beyond the pure metallurgy, You have to know what you're doing with the Alloys to make this work.

I was never implying it would have been built the same as the 914 stock steel carbon arm.
But that you are limited in the design due to the factors placed onto via the chassis.

[914forme]

Pete While I find the topic intriguing I wonder what the real end game is.

Tire technology has outpaced suspension design over the last 50 years. So now unless you racing competitively in auto crossing at the national level I would not worry about 10mm of sedition width.

Slap on a set of DOT R compound tires, add a set of fender liners, and go drive. The fender liners are needed to avoid the small upward dents you get from all the rocks flying up under the fenders. As little as people drive their 914s you get years out of the tires. And well they are way better than the Dunlops my dad had to choose from in 1976 when he got his 914-6.

Remaking the arm, while possible would exceed the 2-3K you would spend to have an arm customized. Via Chris.

Or you can do Rory's design and it works also, just to a lesser degree than Chris' solution, but it does provide a solution that can easily be done in your garage.

I had an idea on how to duplicate Chris efforts, but chose to pay him for his intellectual property. Sometimes it is worth supporting the vendors that make this hobby what it is, and the community.

[914Toy]

While repairing damage to my 914's passenger rear quarter caused by a texting driver crashing into it (IMG:style_emoticons/default/mad.gif) ,repairs required a good used replacement full quarter panel and trailing arm. I recall one "expert's" comment that the trailing arm strength and design included minimizing damage to the tub in the event of such damage. This worked for me. So, perhaps strengthening the trailing arms along with weight reduction should not be done for our street cars, but may be helpful for track cars.

[burton73]

Sorry for my mess on my pouring table but this is a PMB (E) modified strengthened trailing arm done 10 years ago. Weight 25.5 LB with 930 turbo Stub Axels, 911 early parking brake, Elephant Polly Bronze and well what you see.

Bob B

[horizontally-opposed]

Pete While I find the topic intriguing I wonder what the real end game is.

Tire technology has outpaced suspension design over the last 50 years. So now unless you racing competitively in auto crossing at the national level I would not worry about 10mm of sedition width.

Slap on a set of DOT R compound tires, add a set of fender liners, and go drive…

End game is:

1) "Right-sized" footprint: I've had virtually every 15-inch tire size that will fit into a narrow 914 under my car. Current rubber is Avon CR6ZZ, a vintage race/rally tire available in three compounds, so about as grippy as you'll get. 185/70 all around is just okay; with RS 2.7~ power, I'd like to run a similar tire package without resorting to M471 flares or a repaint. Suspect there are others in the same boat with 2.7s, 3.0s, 3.2s, etc

2) Performance and looks; 215/60 seems to me about right for what I am after in terms of performance, and I think it would also look great on the car. 225/50R16 also offers fantastic performance and looks great tucked into the back of a narrow 914.

3) Tire choice; 205/60R15 tire choice is far from great; factory tire sizes offer prospect of more availability & choice over the long haul; 195/65 & 215/60 or 185/70 & 215/60 offer period looks and at least three great options for the street. If there's space for 225/50R15 or 225/50R16 in the rear, unlocking some great R-compound tires, that's a bonus. My test fit suggests modified trailing arms and a bit of fender pull might make it work.

4) Cost to rebuild/modify old arms: Prospect of spending $2000-4000~ to redo another set of 50yo trailing arms with little to no technical upside isn't interesting. 911 e-brakes and aluminum calipers wasn't appealing, so I wanted to get some feedback from the community on whether the time has come for another option.

Fully agree on rewarding vendors in the 914 community, and have sent a fair bit of business their way over the years, but perhaps a new trailing arm might be more profitable for them than modding old ones? Fortunately, smarter people than me (!) are spitballin' this now. Carbon was an early casualty due to setup costs as well as actually popping them (not to mention liability and potential inspection/longevity/etc), and I suspect Ti is out due to $$, too.

While repairing damage to my 914's passenger rear quarter caused by a texting driver crashing into it (IMG:style_emoticons/default/mad.gif) ,repairs required a good used replacement full quarter panel and trailing arm. I recall one "expert's" comment that the trailing arm strength and design included minimizing damage to the tub in the event of such damage. This worked for me. So, perhaps strengthening the trailing arms along with weight reduction should not be done for our street cars, but may be helpful for track cars.

Glad you raised this, as it was on my mind at one point. Worth paying attention to if there's a solution—as I'd far rather lose an arm than a car!

Sorry for my mess on my pouring table but this is a PMB (E) modified strengthened trailing arm done 10 years ago. Weight 25.5 LB with 930 turbo Stub Axels, 911 early parking brake, Elephant Polly Bronze and well what you see.

Bob B

No apology needed! (IMG:style_emoticons/default/biggrin.gif) Another great input & data point. 25.5 with all that we see there is not bad, not bad at all. Maybe the goals shift to cost of upgrade ($2000-4000~ rebuild vs $250-500 custom spring plate + triangulation of some sort + used $100 986 carriers, etc) with any weight saved or additional clearance for a 215 or 225 tire as gravy.

[mepstein]

I'm still confused when you say $2-4K to redo the trailing arms. Rory's mods are cool but I doubt they make much difference in handling and I can't image a narrow body street car needs stiffer trailing arms.

There's a lot of low hanging fruit on a 914 that can be improved before you spend the big bucks on diminishing returns.

There's no way to fit 225 in the back without fender mods.

Michalin TB15's ?

[horizontally-opposed]

I'm still confused when you say $2-4K to redo the trailing arms.

Seeing $1800-2400 listed on websites for "standard" rebuilds with 911 e-brake and/or stiffening, and suspect that's a result of jigs and knowhow. Had a fabricator I like suggest all that plus reinforcements and scalloped for a bit of tire clearance plus the 911 e-brake etc could run $4000-5000.

Do like Rory's setup, but if the knife comes out, I'm going to 911 e-brakes too—and I've seen those go wrong.

I can't image a narrow body street car needs stiffer trailing arms.

Agree. If I redo mine (again), I probably won't reinforce.

There's a lot of low hanging fruit on a 914 that can be improved before you spend the big bucks on diminishing returns.

Agree also—but 31 years in, I'm through a lot of the low-hanging fruit short of a lightweight battery (on the list) and non-steel body parts (probably not for this 914). There are actually a few places I will add a bit of weight to increase usability—mainly radio and heat. But I like the idea of offsetting that…and am slowly (!) planning my next suspension rebuild.

There's no way to fit 225 in the back without fender mods.

It's been done with what I'd still call minor fender rolling/pulls—and not just once or twice. I've seen 215/60R15, 225/50R15, and 225/50R16 on the back of NB 914s over the years. Sometimes hacked, sometimes not—and sometimes not at all. One 914 six conversion claimed to have 225/50R16 under stock rear fenders—not sure I believe they are completely stock, but it's clear they used 16x7 Fuchs with custom offsets.

As for Michelin TBs, 215/55R15 could be good for some cars—and are certainly viable for the front of an M471 car. But they're a bit "short" for a narrow body (to my eye), and I've heard from friends who ran them on street cars that they aren't so easy to live with—being noticeably worse than the Avons in that regard. Tried them on a 914 M471, and they sure offered sweet steering and plenty of grip. Same owner wasn't so happy with them later on.