77 911S w 207k miles: Suspension Rebuild

[AFB24911S]

Post suspension rebuild updates/product reviews:

-Elephant Racing Poly Bronze: make the car a little jouncier, you can feel and hear them clunking a little over hard bumps, but that isn't as noticeable after a while and the suspension moves very freely. Super predictable, very happy with them as part of the complete suspension package.
-Stock Bilstein HD dampers: good enough for me, for sure, the car is never under damped.
-21/17mm rear/front torsion bars: good, as stiff as a newer car but no stiffer. I may yet raise the ride height a bit and go softer though, to retain the vintage feel. As is, sportier than my newer Alfa Romeo Giulia in the canyons and tighter than anything I have butt dyno memory of without being punishing.
-Rear sway bar: really good to have, holds the ass end of the car together, was expensive to buy a new bar and have the upgraded rear mounts welded on but ties the car together very well.
-Steering rack: didn't rebuild it, don't feel like I needed to.
-Turbo tie rods: couldn't feel a difference, don't think I'd do those again in retrospect for the sake of saving $. Mine were fine.
-Rear trailing arm bushings: those were worth doing, the old ones were SHOT and that can't be good.

Now that the car handles so nice, I'm heating up the motor in the canyons so the next upgrades are a front oil cooler and a radio upgrade. Nothing has fallen off the car even though I'm a novice mechanic. So if you're wondering whether it can be done, it can! Patience, tools, and determination are all it takes. Pay professionals for the tough stuff like heavy engine work and transmission stuff if you must, but brakes, suspension, axles, etc. are all within the realm of the ambitious home mechanic in my opinion.

[AFB24911S]

One year on, and I've taken the car to an autocross! Happy to say that the suspension has continued to suspend the car above the road successfully, no failures or breakages.

More notes on feel/stiffness, post-first autocross: it's super communicative and confidence inspiring until a mid-corner bump. If the surfaces you want to go fast on are uneven, like the Colorado Air and Space Port, for example, consider starting with a softer torsion bar and seeing if you want stiffer.

I have to say I vaguely regret going as stiff as I did, but not enough to do anything about it yet. The car was fast, way faster than I expected, and so much fun. As a total novice, I was only 6 seconds off the pace of a 20-year track day/HPDE veteran with slicks, which feels pretty good.

If I get off my butt and find the money next summer, I'll borrow some sensors from an engineering friend before and after swapping in softer torsion bars. Data would be super cool to have.

[AFB24911S]

I have read a bunch more forum posts about suspension modifications, and I have learned about the importance of shock valving! Thank you to the folks on this forum who explained the importance of it better in other posts, especially this one and especially @Bill Verburg:

https://forums.pelicanparts.com/porsche-911-technical-forum/876392-bilsteins-stock-suspension-hd-digressive.html

It took me a few times to read those graphs and understand what was meant by digressive vs linear or progressive, so I thought back to physics and controls to make it all make sense to me.

Force = mass * acceleration

In our case, the mass is the unsprung mass of the wheel, brake caliper & rotor, wheel bearing, and suspension arm(s) holding the wheel to the car. The two items acting on our unsprung mass are the damper and the spring (torsion bar). They exert force in different ways.

Force of spring = -k * x where k is a constant inherent to the spring and x is the position measured away from resting; the further the spring goes away from its resting, neutral position, the more force it exerts. This is why mild cornering produces mild roll and aggressive cornering produces aggressive roll.

Force of linear damper = c * v where c is a constant inherent to the damper and v is the velocity of the mass, our wheel. As velocity increases, or as shocks become more abrupt, damping force increases. That is why, on my car with linear dampers, the sharper the bump is the more aggressive the damping reaction is. A little body roll in a corner doesn't incur much damping force, but a sharp impact over a pothole causes lots of force. Linear shocks can be overdamped, where they produce more force than necessary to bring the mass to its resting place to the extent that it takes longer for the mass to come to rest. Or, they can be underdamped, where they produce so little force that the mass bobs back and forth for a long time before it comes to rest.

Check out this cool video showing how changing those variables impacts motion: https://www.youtube.com/watch?v=PwlntnWtqJc

Now here's why digressive damping rules for sports cars. With digressive dampers, the "C" constant in the equation becomes velocity dependent: at high velocities, a digressive damper will exert less force and the "C" constant will be lower. At low velocities, it will exert more force and the "C" constant will be higher. The main example of low velocity suspension movement is body roll; we want less roll for better traction, so high damping force to counteract roll is good. The main example of high velocity suspension movement is bumps; we want optimal damping force, not too much and not too little, over bumps to bring the suspension to rest as quickly as possible and, therefore, regain grip as quickly as possible. A mass is "critically damped" when, for a given spring and a given displacement, the time it takes for the mass to come to rest is as short as possible. It doesn't rebound back and forth, but it doesn't resist the motion so strongly that it keeps the mass away from equilibrium.

According to the experts at Elephant Racing and 3R Racing/Performance, my linear dampers are the primary cause of the stiffness I'm experiencing over potholes and expansion joints; I just spoke with Chuck of Elephant Racing and Brian at 3R Racing & Performance (the local Elephant Racing installer). They both think the 21mm front and 27mm rear bars are not stiff at all, and that going to digressive valves is the way. The only question now is whether re-valving the Bilstein HDs I bought three years ago or buying new Von shocks is cheaper. The Vons have spherical joints on the bottom, which is better, but they are shorter than the Bilsteins, which means they're better suited for lowered cars. I want to go higher on ride height just as a way of avoiding scraping; too many steep driveways in the mountains of Colorado.

So, more research to be done, but definitely going for digressive shocks and definitely keeping my torsion bars and poly bronze bushings. For now. More to come, I will be implementing one form or another of these digressive shocks soon! I'm definitely going to get some video on my new GoPro of the wheels over certain bumps before and after this change, I may get wild and crazy and see if I can't find a sensor that I can stick on the suspension arms to log movement and measure the reactions before and after.

Science! Nerd-dom! Performance! Wallet? Better go make more money...

[PeteKz]

That's a great video. Thanks for posting it.

[AFB24911S]

Digressive shocks with custom valving installed, and WHAT A DIFFERENCE! The car rides so much better than before. As the technology promises, the car is soft over harsh bumps like railroad tracks and potholes but does not lean very far in the corners.

Another note: the suspension was lower than I thought when I first put the car back together, it was about a half inch lower than euro. Now, at euro height, it is still sporty but never bottoms out during normal use.

Those shocks are expensive, and getting the car corner balanced is also expensive, but this suspension is like nirvana now. It rides softer than my 2020 Alfa Romeo Giulia over bumps but rolls less in the corners too. I recommend it for Colorado roads or better, but I would think that even softer torsion bars might be better on midwestern or east coast roads. More notes to follow after track driving and road trips. I may go to adjustable sway bars later on.

[Funracer]

I live in FL and save my big jobs for late fall and winter due to high garage temps in the summer.

Love your thread but just realized you have been doing it for 3 1/2 years! I was hoping to get my suspension bits done in a couple of months over this coming winter. It seems I may be too optimistic. Maybe I should do the front one year and the rear the following year?

Also your 77 is lighter than my 83 so not sure I can use all the same parts.

Inspired by your results but discouraged by the timetable.

Thanks for your thread. I will be referring to it often.

[AFB24911S]

Don't be discouraged, Funracer; I did not take 3 years on the suspension work, I bought the wrong shocks and only now have gotten the right ones installed. I took about 3 months to do the suspension work and did a bunch of "while you're in there" brake, axle, and other restoration work at a novice pace back in 2020/21. Then, I had new shocks installed when I was unsatisfied with the conventional Bilstein HDs.

About weight difference: for what it's worth, all the F and G body cars came with 19mm front and 23mm rear torsion bars. Porsche must not have thought the weight difference between our cars was enough to warrant a different choice. Nowadays, spring rate and damping rate are preference choices with these cars. I think my setup is fine for my use case, but I still might try softer torsion bars at some point just to see if I like them more for daily use.

A couple top tips that will help make sure you finish in just a couple months:

1. Wash the underside of the car as best you can before lifting and removing wheels. Makes the work go faster.
2. Be prepared to have to press the torsion bars out of the front control arms; have a shop with a hydraulic press on speed dial.
3. If you're going after the bushings in the rear trailing arms, use the Elephant Racing tool even though it costs what seems like a fortune for simple parts.
4. Get an electric ratchet to make removing and installing bolts & nuts go faster.

You should be able to knock it out in 3 months, but if you're going stiffer than stock budget for the Vons shocks and skip the Bilsteins.

[Bill Verburg]

here are the effects w/ different chassis & corner weights, Bilstein shocks