Largest, easiest chunk of weight would be aluminum calipers. Eric did a run but I don't think there was ever a huge demand. I think it was the price that held people back.

I think part of the reason a stock 914 is such a joy to drive is the light, narrow tires. They may not provide ultimate traction but feel light and smooth and let the car "dance" over the road.

"I've been testing factory and aftermarket Porsches on road and track since 1997. Long ago lost count of how many cars, track days, and road tests. Along the way, the cars that hit the sweet spot and were "soul connected" were the ones that stood out. They were often less is more, such as a 986 2.5 that was just plain more fun to drive than a 993 Turbo. Or the 987.2 Boxster Spyder, which was far more than the sum of its parts—and probably quicker over the road in the hands of more drivers than a contemporary GT3. And, dare I say it, more fun despite a merely wonderful engine instead of that insane Mezger.

There are a lot of other examples, but the 914 stands tall in this regard—and that goes for virtually any good 914.

I get this. I've ridden and raced bicycles all my life. I ran a bike shop for 6 years so I rode hundreds of different bikes than just mine.
Most of the bikes I owned were top of the line, great bicycles and did there job well. Two of them were outstanding and I have not been able to duplicate the feel.

So maybe we should be aiming to hit that sweet spot with a 914. I've never driven a 911RS but it seems to get described in that way. So maybe we need to make a 914RS.
A magical riding car that can be duplicated.

The factory trailing arms were designed with narrow tires and limited grip as the parameters. With 50 years of tire evolution, you can put enough sticky tires on a 914 that will make it exceed 1G lateral loads. The trailing arms were never designed for that kind of load, and are going to start failing from a combination of metal fatigue and age.

An aluminum trailing arm, similar to the 911 one, would be a great investment. If I were doing it, I would incorporate a mounting pad for a monoblock (Boxster) caliper, all the required provisions for the park brake (stop block, bellcrank, altered cable mount, etc..) and new fairly hard durometer rubber bushings.


But it is beyond a hobbyists ability to build a upgraded trailing arm like this.

Someone with some deep pockets would have to build and sell it.

Well said! Doesn't exactly look profitable given a upper bound of ~100,000 vehicles and mabye 22,000 rabid fans (or whatever latest membership is here).

Now consider the number of those that would be willing to spend $$$ to make the "upgrade".

For me a good chunk of the 914 appeal is that it is what it is. Faster, more reliable, great handling cars are all around us in this day and age. Agree a 914 stands tall given its age but again, it is what it is.

This one would take a village to get done, but I think it's within the realm of possibility between our contacts and even some of the talent here. And I may have a "cost no object" buyer who might underwrite the most expensive pair (the first ones), as he was already thinking along same lines when I mentioned them. He's getting over Covid, and we were talking cars to keep him distracted. He's got a lot of other stuff, but loves his 914. And if a number of us signed on for a group buy…

Four billet aluminum ends won't be cheap, but they should be doable. Best way would be to incorporate choices such as which of three bushings/bearings up front and ears for 914, 930, or 986 calipers in the rear—leaving it up to the customer to move to 911 e-brake, RSR, or spot calipers. Or no e-brake.

Carbon or lightweight steel blades would probably be the "hard" part—after proper/safe design and FEA, that is.

Maybe one could get an mechanical engineering grad student to undertake a project using some of the new generative design software and DMSL and print stainless or titanium trailing arms with the highest strength to weight ratios:

https://www.pinterest.jp/pin/541698661419803565/

Maybe not the cheapest, but you might get something pretty wonderous for $1400 per arm (give or take)

Autodesk has a new software out to design these type of organically perfected devices:

http://www.withinlab.com/case-studies/new_index18.php

How's this ?
A single large monoball attached at current outer front trailing arm mount.
A rigid narrow cast or box type trailing arm that extends past the hub
and includes mounts for upper and lower un-equal length rear links.
The links extend inward toward the transmission and attach to a fabricated
hoop forward of the transmission mounts.
The loads are now through three mounts intead of two and are spread over
larger areas.
Full disclosure, I'm not an engineer nor could I plot the geometry.
Can't draw so well either.
Nonsense ?

Typical Subaru rear suspension.

Open to all ideas, as this is all about spitballin' at this stage.

With that said, and pursuant to @mb911's post above, I actually think the suspension concepts of the 914 are pretty good. Sure, multi-link, double A-arm, etc are better when it comes to camber gain, etc—but 914s are plenty fast and able to process bumps even by modern standards. The light weight and long wheelbase definitely help. And, to my mind, the best solution is a part that bolts on—so it's a reversible mod. That broadens the customer base. Cutting the tub is a nonstarter for a lot of folks.

I reached out to an overqualified acquaintance who might be able to help with some early feedback on ballpark weight loss vs rough costs for the first pair of trailing arms (development, setup, creation) and then more in small qty. We'll see if he laughs me out of the room.

Meantime, also reached out to one of the 2-3 techs I would hire to rebuild/narrow/reinforce/five-lug/add 911 e-brakes/repaint/re-bush/re-bearing/etc for what he might charge. Asked him to do so on the basis of one stop, so no procurement on my side. First response: "$2500?" Then, as the scope of the work set in, "Could be more like $5k, with no brakes other than the parking brakes." Remember, this is checkbook math, not DIY—and some won't need to do some of those things, and can subtract accordingly, but checkbook math is the fair comp and I suspect $3500-5500 is a decent guesstimate for rebuilt/modded 50yo steel trailing arms.

Ben/MB911 might be able to give a good estimate since I think it’s in his wheelhouse
@MB911

I would need you to put things down on paper or a picture.. I am more a picture book kind of guy.. Then I could give a real estimate.

Cut down Boxster rear hub assm and just weld aluminum sheetmetal and tubing to copy the 914 arm mounting points/shock. Easy peasy and cheap.

Or maybe bolt the Box Assm to the AL trailing arm which would allow for toe and camber at the hub via spacers.

I'd also use a mass produced sleeved rubber bushing from another car/truck that presses in, example the Nissan Hardbody ft upper susp arm.

Whoever makes them can send me a set as payment

Having gone down the path of custom trailing arms (still mounted on my car), the more I studied the stock design the more sense it made. I also took a stab at aluminum, but only got as far as machining the bearing carriers. See post #32 at: http://www.914world.com/bbs2/index.php?sho...tive&st=20#

Andys

As a point of reference this thread on the weight difference between steel and aluminum 911 trailing arms is instructive:

http://forums.pelicanparts.com/porsche-911...arm-weight.html

Im sure its been documented somewhere on here before but as a point of reference using my bathroom scale: a OE 914 BARE (no bearings no shaft nothing) trailing arm is 15 lbs.

Anyone have a 914 arm they can post laying the same way this 911 arm is oriented?

Lets see what the two look like.

Click to view attachment

^ Now we're cooking with some gas.

Why replicate/redo/machine the trickiest part—the wheel/bearing carrier—when an arm can be made to accept a 911 or 986 wheel carrier? 986 is appealing as they're plentiful used or new, and engineered to deal with loads from 255/17 summer tires up to MY2000~, and 265/18 summer tires from MY2000~ on, when mounted on a 2900-3200lb mid-engined car. Wheel bearings availability is probably just fine and will be for a long time, and they're ready for 986 rear calipers and incorporate an internal drum e-brake. For the truly nutty 914 builds, there's provision for a lateral link (to the transmission?) and even a wheel-speed sensor for traction control.

Aluminum 911 trailing arms Rick just posted are out there new and used as well, but are probably in greater demand for obvious reasons. Might be sliced and diced to work, however, and also mounts 911 calipers/e-brake.

That would leave design of the blade/arm and front end. I suspect the outside cost on a pair of new arms has to be $5,000-7,500 before even a small group would consider them, and there will have to be advantages besides weight in order to make them compelling. Could it be done?

986 wheel carriers, Porsche-engineered, made in Italy by Brembo. Looks like a high-quality part. Available for $120-200 used, thanks to two per 996 junked and four per 986 junked.

I almost forgot about Porsche's cost-saving move to design these to work at the front or back of the car (LF/RR or RF/LR), which allowed them to use just two castings for all four corners of the 986. Wheel bearing, 986 caliper mounts, and integrated e-brake are all very nice—and I suspect this piece will be hard to beat for weight vs strength. Not sure what to do about that shock mount—could it be put to use via machining or custom shocks, or does it need to be removed?

A couple more images.

The 986S and 986.2 went to a larger wheel bearing, I believe in conjunction with the 18-inch wheel approval (or near it), but the early setup was engineered and approved for a 255/40ZR17 tire at the back of a 3,000~ pound Boxster, and a 225/40R18 summer tire at the front of 996s. I remember a Porsche engineer saying this design was also an answer to problems with the front wheel bearings used on the front of 964 and 993 race cars—which had reached their limits with 18-inch slicks.

My thought was to see if a 911 arm could be sourced and used as is with an adapter to the suspension ear and mount on the 914 chassis without cutting the car or the arm. My concern is the triangle the 911 arm forms when adding the flat bar may interfere with something. The 914 part does not form a triangle. Likely for a reason.

That 986 hub looks inviting...

Suspect the far wider triangle of the two-piece 911 trailing arms is afforded by the narrow transmission ahead of the axle line in a 911—the only way to do that in a 914 would be to reverse the trailing arms in a 914…an idea Porsche thankfully abandoned with its very first mid-engined sports cars in the early 1950s.

914 arm is triangulated, though to a much lesser extent…but that still needs to be considered and replicated.

Agree about 911 arm + modded 986 wheel carrier + some form of triangulation (that maybe doubles as a toe-adjust). Not sure it will be lighter if it's going to be cost effective, but it might be stronger, newer, and integrate a 986 e-brake, 986 caliper mounts, and wheel bearings in the bargain. I've never liked the idea of slicing the 914 trailing arm to add the 911 e-brake and caliper mounts. I know a lot of people have done it, and successfully, but I have also seen those conversions go wrong. 986 front calipers, meanwhile, are easy to adapt, and probably the best technical solution for a 914 (modern, light, stiff, cheap, plentiful, available for more and less power thanks to 986S, and set up for a mid-engined car rather than a rear-engined one). Only count against them is they don't look as period or as cool as 930 brakes…but the latter are big $.

I can't draw to save my life, but the Rennline outer 911 arms are only $250 a pair, while it seems like a nice pair of 986 carriers with e-brakes run $200-300 a pair. So the challenges will be figuring out mating the 986 carrier to an (existing? 911 outer?) trailing arm, the forward pivot/mount, hub placement, body clearance, and triangulation. Oh, and the stub axle, too. See? "Easy." Had an offline conversation yesterday, and a good engineer to speak with this about was named.

I wonder if the 986's "pinch" strut mount can be removed so a trailing arm can be made to attach the carrier and also locate the 914's lower coil-over mount.

There are a lot of interesting starting points, from mild to wild, and perhaps one of these vendors could modify something they've got to suit and also provide integration for triangulation to the 914's inner mounting ear.

Elephant offers a surprising number of choices—but I guess it makes sense as there's a lot going on at the back of a 911…

Was expecting these 959s of 911 spring plates to be $4-5k. They're priced at $2500. So the range from Rennline to 959 appears to be ~250 to ~2500. The wilder Elephant arm also makes a possible point (or two) for triangulation evident. Might have to be flipped side to side and/or modified to fit on a 914 to avoid the curved arm from interfering with the body.

Would I pay $2500-4000 for an arm like this, in a 914, vs $3500-5000 for rebuilt arms with all the tricks? Yes.

And yet another option from Elephant…

Not sure I see the need for ride-height adjust with all of the kits available to do the same with 914 rear dampers, but then again, I still haven't bought one of those sets. Might be nice to fine-tune the ride height and even corner balance the car with my less than exotic Bilstein HD/Weltmeister setup—particularly if it's part of the bargain in a trailing arm upgrade. No need to replace springs I like with threaded sleeves and new springs of the same rate.

A while back I looked into redesigning the 914 rear suspension for my racecar. The options are a bit limited due to the relatively wide flat 4 or 6 engine being in the way. A 911 trailing arm would be a great option, if you could get it to fit. But there isn't room for the inner mount.

Dual wishbone type suspension is an option, but of course would mean a lot of modifications to to the chassis. I've seen it done on a few tube frame 914 racecars. I've also seen stouter trailing arms like the ones on Sheridan's 914.

The challenge is twofold for the 914 - the trailing arms flex, and the chassis in the rear suspension area needs reinforcement too.

In my case I added a lot of tubing to the car when I built it to strengthen the chassis. The trailing arms are just stock one with the welded reinforcement kit. Someday when I re-do the rear suspension, I'm planning a custom trailing arm with the adjustments at the hub, similar to what Sheridan did.

My GT conversion.
Is this lighter and stronger ?
If the trailing arm were re-shaped (narrow + taller ) could it accomodate the narrow
body wheel / tire requirement ?

Gawd that's Purtty!

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

Whoa! Never saw these. Very cool, and I'd be curious re: your questions.

I have no doubt that Porsche engineered (and overbuilt) the 914's steel trailing arms down to a price. I doubt it did so with a 50- to 100-year service life in mind, but there's no doubt they're long-life parts. I cannot remember hearing of a failure, but they were reinforced for racing by Porsche and others.

I do wonder what goes on inside of the boxed sections, as it's "unlikely" they were treated for condensation/rust any better than the 914's longs and center tunnel were. What did yours look like inside when they were sliced up, Rory?



Some very nice work indeed on Rory's car. That car keeps impressing me, visually and technically—how has it not been nominated for COTM yet? (Makes mental note.)

I've been considering the reinforcements below next time my engine comes out. The inner mount reinforcements are from Tangerine Racing, while the outer is from Patrick Racing. Like the idea of reinforcing the outer mount with the long, but wonder what an engineer with relevant experience might say—and don't love having to remove the rocker panel to adjust if that's what would be necessary. The factory did some interesting things with rocker panels on some of its rally cars, including leather straps to allow the back of the rocker to be peeled open for field service. Looks cool on a Monte Carlo works GT, but zero interest in that for a road car.

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.

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

@groot Any thoughts?

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.

^ Really nice upgrades, Rory.

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… 

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.

Me? Lots of thoughts... about what?

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

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.

^ 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.

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.

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

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.

next time it will be chromoly and TIG

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.

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.

While repairing damage to my 914's passenger rear quarter caused by a texting driver crashing into it ,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.

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
Click to view attachment

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.



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!




No apology needed! 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.

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 ?

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.



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



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.



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.