935 Suspension with Moton Coilovers for the street

[WinRice]

It’s time to let my little project out of the bag. My goal is to build a street legal ‘go-cart’ with some of the best suspension equipment on the market, to be used in weekend runs through the mountains and other fun drives. Another criteria was to maintain the stock 930 look, so the only exterior mods will be a DP style front spoiler to allow for the center cooler. As much as I would like to track this car, my current job and family commitments just don’t allow it. Getting to this point in the project was a big enough battle.

The parts:
1. Moton Motorsport double adjustable coilovers
2. ERP 935 front suspension
3. ERP 935 rear spring plates
4. SRP front and rear sway bars
5. Elephant Bump steer kit
6. Wevo front monoballs and plates
7. Elephant rear sealed monoballs
8. Wevo rear sway bar mounts
9. Wevo transmission mounts (already have the engine mounts)
10. Full Brembo GTP Brake setup
11. All new wheel bearings and seals
12. Custom fabricated tub reinforcing
13. Custom Stainless x-braces and crossbraces
14. Elephant 935 center duct with Setrab STD-272 cooler and hoses
15. Zuffenhaus 9x17 and 11x17 fuchs with a 255/315 Nitto NT-05 tire setup (if the wheels ever get here?)

I got the start of the suspension setup below and added a few more items:

Brand New Complete ERP/Moton 935 Coilover Suspension for 911/930 $8500

I fully realize that a suspension such as this isn’t going to be maintenance free on the street, but most of the wear parts are pretty easy to replace. All the heim ends are typical American thread and are fairly cheap in comparison to other Porsche parts. If I have to replace them every year, so be it.

What started the whole thing:

It started off as good day, but.......

Once I got the pan replaced, the question now became how to reinforce the tub for street use, while not adding unnecessary weight. It’s well known that the 930 tub doesn’t have the stiffness of a 964 or 993 tub, and wasn’t designed for coilovers, but the question was how could I make it better? I’m a structural engineer by profession and used my experience to look at all the loads (at least my interpretation of the loads) being applied the stock tub and what paths they take. In addition, I spoke to several people behind the scenes and got all the opinions I could. Also, plenty of late night bench racing, looking at ‘what if’? And yes, a fully cross-braced cage, tied into the suspension points would be the best, but this a street car.

So here’s the issues I came up with:

1. The 930 tub wasn’t designed for coilovers. Need to get the load path from the strut tops back to the original locations, and reinforce those areas for stiffness.
2. Eliminating the rubber in the suspension will increase the dynamic load into the suspension mount points. Need to reinforce the mount points against fatigue cracks over time.
3. The front 935 suspension, by design, puts much more load on the front mount points than the stock A-arms. Especially under braking. Need to add welds, reinforcement and bracing to the front parts of the tub.
4. Once you add bracing, you’re now moving load to another location that wasn’t designed for it, so that will have to be reinforced.

I’m sure there are a million ways to approach this, right or wrong, the following pics are what I came up with. I’ll try to update as I progress, but that’s pretty slow at this point.



First the popular 'Alarcon' plates. These plates really are ingenious, they not only reinforce the flimsy area around the sway bar mount, they also tie the double layer strut tower into the wheelhouse and all the way to the steering tunnel. If you look close you can see where I drilled holes and puddle welded through both layers of steel at all the panel joints, further tying all body panels together at the strut towers. I cut the center hole 1/2" larger in diameter than the SRP mount and carefully TIG welded around it. These plates look pretty simple, but took plenty of forming with hammers and a sand bag to fit the wheelhouse contours. One tip.....fit and weld the perimeters of the plate first, install the mounts and bend the base metal to square up the mounts and get the smoothest movement of the sway bar before welding around the mount. They don't bend too easily afterwards.

[WinRice]

The 935 front suspension puts much more load on the front '3-bolt' mounts, so I added an upper plate to the front bolt. The pic shows it tacked before the full welds. I also added more welds to the reinforcing plate at the rear two bolts, and at the panel joints around the front mount. Additional welds were added to the bottom of the front mount area. It also helps to square up the weld nuts to the pan before welding in all the reinforcement, they were a little crooked when the replacement pan came out of the box.




The center duct was stitch welded on the inside and outside of the body panels to insure a good transfer of side-to-side forces from the front crossmember to the bulkhead at front of the tub, where the x-brace mounts will be located. You can also see the additional stitch welds I added to the remaining part of the original pan and left longitudinal.




I reduced the width of the center duct to fit the cooler a little better. Steel plates were added to the two 'humps' above each upper corner of the duct, tying them into the stock bumper shock brackets. These mounts will receive a bolt-in bracket from the inside to support the front ends of the x-brace, sandwiching the bulkhead metal between the outer and inner plates, and spreading the load out to the front pan and bumper braces. They also gave a place to mount the front cooler.

[WinRice]

Right side x-brace and cross brace mounts. Most of the bolt-in braces seem to use pretty flimsy end connections, and I felt I needed to get the brace load more centered on the top of the strut, rather than offset like some of the other solutions. What I've found in the past is heim ends don't work very well with double tab connections unless the tabs are very thick or a bushing is used so the hole doesn't enlarge over time. I chose to use bushings that I made from 5/8" bar on a mini lathe. Rather than using a nut on the back side, I machined the back bushing hole smaller and tapped it.




Same side with the heim ends installed. The reinforcement around the lower crossbrace mount was also puddle welded through the wheelhouse to the 'Alarcon' plate to further tie things together and spread the load out.




Left side x-brace mount. The small angle plate at the bottom is to strengthen the connection between the smuggler box panel and the wheelhouse. What looks like a sloppy weld at the wheel house is where I cut small slots and welded the plate to the 'alarcon' plate and the wheel house for a better connection. You can also see additional stitch welds added to the strut tower cover to the wheel house, and at the smugglers box panel to the wheel house.

[Flieger]

Looks very impressive to me. I like the Alarcon plates. Much like RSR reinforcements. The force from the tires causes a tension on the strut brace. The force from the coil-overs would compress them under braking, so you are smart to add an X brace rather than just a lateral link.

[Flieger]

Wow, I just checked the thread with the suspension photos.



Impressive. I wonder how the weights compare to the stock pieces?

[WinRice]

Originally Posted by Flieger

I wonder how the weights compare to the stock pieces?

Definitely quite a bit lighter than stock, but by the the time you add in the weight of the braces and reinforcements, probably a wash.

[spence88mph]

Very impressive, looking forward to seeing more posts!

[Flieger]

Originally Posted by WinRice

Definitely quite a bit lighter than stock, but by the the time you add in the weight of the braces and reinforcements, probably a wash.

But that is "good" weight that adds stiffness and strength. It is also sprung, not rotating or anything.

[ghost1001]

Good stuff....looking forward to seeing this come together....

[SCHNELE]

Nice work.

[totle]

Wow great set of suspension, and great work.
Looking forward to see this finished and hear about the experience when you have tested it out

[WinRice]

Thanks for all the kind words!

I'm in prime, seam seal and Wurth SKS mode right now, so I won't bore you with those details. Once I get that done, I'll get back to fabricating the front x-brace brackets and all the bracing.

[JBL930]

Wow, lots of work going in there, great stuff!

[SCHNELE]

Details are definitely good.

[garibaldi]

You would be wise to shore up the rear mounts as well. I can speak from experience. I tore off sway bar mounts in the rear, cracked the forward spring plate mounts, and actually blasted the entire rear upper shock crossmember off my old 930 when I had it. i had a full 935 suspension I was running on the street. Coil overs, bilsteins, heim jointed everything etc. I literally ripped off that boxed albatross that the rear shocks mount to and sent it piercing through the rear firewall and the whole rear of the car sank. So just beef everything up, you will be moving the stress around to different spots in the chassis

[WinRice]

Originally Posted by garibaldi

You would be wise to shore up the rear mounts as well. I can speak from experience. I tore off sway bar mounts in the rear, cracked the forward spring plate mounts, and actually blasted the entire rear upper shock crossmember off my old 930 when I had it. i had a full 935 suspension I was running on the street. Coil overs, bilsteins, heim jointed everything etc. I literally ripped off that boxed albatross that the rear shocks mount to and sent it piercing through the rear firewall and the whole rear of the car sank. So just beef everything up, you will be moving the stress around to different spots in the chassis

Yep, that's the plan. Once I finish the front, the rear will get the full treatment.

[MikeD930]

Originally Posted by garibaldi

You would be wise to shore up the rear mounts as well. I can speak from experience. I tore off sway bar mounts in the rear, cracked the forward spring plate mounts, and actually blasted the entire rear upper shock crossmember off my old 930 when I had it. i had a full 935 suspension I was running on the street. Coil overs, bilsteins, heim jointed everything etc. I literally ripped off that boxed albatross that the rear shocks mount to and sent it piercing through the rear firewall and the whole rear of the car sank. So just beef everything up, you will be moving the stress around to different spots in the chassis

Christian,
What year was your 930 that had failures? How many miles and how much was it tracked? Was it driven hard on the street? Did you keep your eyes on the rear shock towers for potential failures? Do have any photos of the failure on the front and rear mounts?

Originally Posted by WinRice

Yep, that's the plan. Once I finish the front, the rear will get the full treatment.

WinRice,
I'm impressed with your work. I have full 935 setup with coilovers on my '87 930 with none of the structural reinforcements. It's been that way since 2002. I bought this car in '09. I have been keeping an eye on the rear shock towers and would like to beef up this area up in the future. As for the fronts, that's new to me as for potential cracking so I'll keep an eye on the fronts, too.

On your front mods why didn't you add boxed front strut mounts for racing castor/camber mounts? Regardless, the mods you did looks great. Are going to lose luggage space with your mods?

[garibaldi]

I had a 77 911 turbo carrera, i had about 70 or 80k miles on it when i put on the new suspension. The first night I had it on I tore off the rear sway bar mount clean off the body pulling in a driveway. Now in all fairness, I had never replaced the rear body mounts for the sway bars, and that failure is nothing new for 911s, plus I was running 31mm adjustable f&R bars from SmartRacing. I tracked the car a handful of times and it was an everyday driver for me, I had 425/ 550 lbs springs front and rear. It was a pretty aggresive setup for mainly street use.
None the less, the rear should be shored up. youre putting all of the force upwards onto the shock towers, taking it from the rear torsion bar tube. have you looked close at them? all they are is a stamped steel cup thats sot welded to the top and the cross box is spot welded to the rear engine rails. Problem is everythig is undercoated and shutzed back there so its tough to see any sign of failure. you see all the old racecars 934s, 935s rsrs etc had gussets on that mount. If it were me, Id run a weld around that seam on the cap, gusset it, and add plates to the ends of the cross mount. Mines literally tore off when i hit an expansion joint. The ass of the car just dropped.... and it was a mess to clean up and get straightened out so i could weld it back. I alos had seen one or two cars that I had workied on where the top caps on the rear shock towers were starting to peel off.

[WinRice]

Originally Posted by MikeD94

As for the fronts, that's new to me as for potential cracking so I'll keep an eye on the fronts, too.

I spent several late nights thinking about this, and it dawned on me that on a stock a-frame car the full front-to-back braking force is transferred into the rear mounts through the crossmember. The front mount is just for side-to-side stabilty. With the 935 suspension, the front strut rods and mounts now see the full braking force in tension, and if that force is unbalanced, you can rack the front crossmember side-to-side. The main reasons I'm reinforcing the front mounts and adding the x-brace. Over the long term, it may not ever crack or fail in the stock condition, it just doesn't seem stiff enough for a well functioning suspension.

Originally Posted by MikeD94

On your front mods why didn't you add boxed front strut mounts for racing castor/camber mounts? Regardless, the mods you did looks great. Are going to lose luggage space with your mods?

Boy, did I agonize over that decision! I went back and forth several times between the boxed strut mounts and stock mounts. And I still may regret that decision! In the end, I felt I had enough adjustment with the 935 suspension that it would be fine with more street friendly camber settings, and with a little soft putty I can still waterproof the strut tops. And if this setup doesn't work well, I can go back to torsion bars and a-frames, but with a little better reinforced chassis.

I can still squeeze a couple of soft duffle bags in there for a weekend.

[Steve@Rennsport]

Nicely done, Win!!!!!