Guys,

I need a little help. I'm trying to piece together bits of information from various threads but am having difficulty getting a clear picture of what needs to be/ can be reinforced on the chassis when the suspension is stiffened.
Basically, I'm in the process of transforming my C4 into a F class racer/NASA GTS racer. The car rarely sees the public roads unless its going for a fill up. Otherwise, its track/ race at this point. One of the upgrades this month is to stiffen the suspension to RSR Bilsteins/ springs at 700-900# range (not sure how stiff yet), monoballs, TRG sways. Overall x3 or so stiffer on the corners. Also, motor , transmission are out now, so probably going to go with WEVO for motor, ? for tranny. While this should transform the car in terms of handling, I understand that it comes at a cost to the pick-up points on the chassis as the road stresses are delivered more intensly to the frame.

From other posts, I know that the motor mount bar needs to be reinforced. But other than that, I'm not clear. But what about other areas, such as the motor mount holes on the rear frame, the shock towers, control arm/ trailing arm points? From the Factory Manuals, it looks like the RS rear sway bar mounts are "reinforced", and obviously the seam welding elsewhere in the rear helps. Logically, the 964 Cup Car enhancements ought to reflect Porsche's experience with chassis failure points. If I understand correctly, the Cup Car and the RS (not RSA) tub are the same.

Anyone with experience doing some of these chassis mods on a previously stock C4 or C2? Anyone with experience on these areas failing , cracking from increased stresses induced by stiffer suspension + monoballs + racing?
Thanks in advance, just trying to get an idea of what may fail in the next year or 2.
Of course what can be done vs what the budget will allow vs what the rules will allow are all separate.
Dave

 

Do a search on the user name
Geoffrey

 

Thanks Bill

 

The 964 chassis is significantly stronger in every respect over the earlier chassis. I am not sure how significant the seam welding on the Euro RS chassis are over a stock chassis since it is limited to the suspension areas only. There are a lot of C2 being raced and I've not heard or noted any significant damage due to stress over time. The exception is a 3.8l 850hp RS America based turbo car with 13" tires, etc. That particular car's rear suspension carrier is beginning to show stress where it meets the rear frame rails. However, it is a car running slicks with lots of HP, and has been backed into a tire wall twice so I don't know source of the stress damage.

On my racecar project, I seam welded every panel in the entire car and made the chassis noticably stiffer when on the rotisserie. I'm running 600/700lbs spring combinations but have not yet driven the car. I expect it to be responsive and stiff. I also have suspension points tied together with the cage and triangulated in the front with a stiff non adjustable GT2 Evo strut brace.

For a stockish C2 running on 'R' tires, I would not worry about the chassis. The first thing would be to install a proper welded in cage, however in PCA stock classes you cannot tie the suspension points together. I'd install a strut brace, the cup rear transmission mount, and solid engine mounts. Reinforce the engine carrier, and watch the solid engine mounts for cracking around the ears from vibration and age. You'll be happy. I'd visually check the car as best as possible realizing that the heavy seam sealer on the chassis will make it difficult to see any cracking or separating of the chassis. The area I'd be most concerned about (and I'm not concerned) would be the rear shock tower cross member.

 

Geoffrey,

Thanks for the information and advice. I just spent the last 2 hours rereading about the development of you race car in the posts . What a beauty!!!!! While I had been following along with the development this past year, it actually makes better reading going straight through. Lots of good info there.
Thanks for the advice. That's exactly what I needed to hear and the direction I will go. From what I am understanding, the engine carrier will be the only thing that needs welding. Otherwise, the inherent durability of the modern frame coupled with the relative slippy grip of the R compounds vs slicks, and the relative low HP should keep a C2 or C4 out of trouble (except in cases of driver error!) Makes sense to me.
I guess that I will have to put a welded cage on the wish list, instead of my bolt in "cage in a box" that I have now - oh well, the price was right. Strut baris in now, so I just need to locate a solid mount for the G64 transmission. I don't think that the G50 mount will fit, will it?

http://www.carreracupusa.org/street/.../Seamwelds.htm

Thanks,
Dave

 

Dave, you can always weld in your "bolt in" roll cage... I have a SafetyDevices cage which is very well made - is that what you have? My car in no way feels "flimsy"... compliance in tires and bushings weighs in way before chassis flex, that's for sure. In 10 years of racing cars I haven't heard of a welded unibody component cracking apart or coming loose. I've seen suspension struts fail on older cars, but not 964's...

Just a note about the SD cage - I was surprised to see how much my cage vibrates separate from the chassis - if I were to put my finger between the cage and the pillar between the windshield and the driver's door, I'd loose my finger! But that's at 145mph through a turn

I reinforced my engine carrier because I like welding and look forward to welding projects... and because there are some reported failures mentioned on this forum with firmer mounts, and I've got the WEVO mounts installed. The G50 solid mounts or stiffer mounts won't work. You can make your existing tranmission mount solid, but there's no-one out there that sells a solid mount. It's pretty easy to fabricate yourself if you have the fab tools, or you can convert your existing mount into a "semi-solid" mount which makes it slightly harder than the factory cup car mount. A G64 is already stiffer than a G50 since it really has two mounts thanks to the front differential being mounted to the chassis also.

 

A note about cages. I looked for a long time into a variety of cage options. Obviously cost is a factor, but I chose a custom cage for several reasons. I wanted the cage as tight to the chassis as you can get. This allows welding to the chassis which eliminates the issues Garrett was mentioning. Also, it allows you to have the most visibility, especially when looking into a corner. Third, it gives you more interior room since it obstructs it less. And fourth, I wanted a TIG welded cage, not MIG welded. If you can swing it, that is my recommendation. Mine was about 1/3 more than a SD weld in cage "kit"

 

Garrett - Unfortunately, I have a autopower cage - bolted to the floor panels and not the sills. The decision was one of necessity as I needed a cage for my first NASA race. I had a DAS bar, but realized 3 days prior that the rules required a cage! Fortunately, the guys at Autometrics found the autopower cage in the "spare parts bin" and I was able to paint and install it in time. They had pulled it out of a car that had upgraded to a custom cage. The cage fits poorly with large gaps at ceiling and A pillar, and I'm not happy with the bolt-to-floor feature (you could actually see the sheet metal bend as the bolts were tightened!!!), but its much safer than the DAS bar. It does make the car "stiffer", even with the loose fit. In hindsight, I can't believe that PCA allows BARS in its stock race cars.
Anyway, I would have prefered the Safety Designs, given the option. However, I may upgrade to full weld in custom in future.

Geoffrey - Your cage looks fantastic - Thats the ideal fit and attachment. I find that mine, with the poor fit, really does block visibility at A pillar, and in rear view. Also, its a head buster on entry/exit. I am not sure if PCA allows welding to A pillar in stock class? Does any know the answer to this?

Thanks,
Dave

 

The rules do not specify that the roll cage cannot be welded to the chassis at the A/B pillars. The only differentiation between GT and Stock class rules is that in Stock, the cage must be within the passanger compartment ie no tying into suspension points.

 

Geoffrey - thanks for the info - custom weld in next year???

Garrett - Details on "manufacturing" the transmission mount for the C4??? I'm interested.

Dave