Input needed: IRS Ghia Convertible performance street suspension setup

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Input needed: IRS Ghia Convertible performance street suspension setup

Post by Evil_Fiz » Thu Dec 15, 2016 4:43 pm

I would like to thank everyone who responded to my previous thread “Advice needed - Mendeola vs Traditional type 1 suspension”.

After evaluating the advice and opinions provided, and getting the marbles in my head to stop bouncing around, I have come to the decision to stay stock-ish. Please allow me to explain my goals before I pose my questions. The vehicle is a 1970 karmann Ghia convertible and is being built as a reinterpretation rather than a restoration. I intend to add custom touches and improvements in a style consistent with current production vehicles. The goal is a safer, updated vehicle with modern conveniences, appearance, and feel without butchery or detraction from the original esthetic. The car will be a “street only” vehicle.

Goals in order of importance
1) Stability, reliability, and predictability at highway speeds of 70 -85 MPH. I would prefer to build the suspension and drive train for a 100 MPH car to ensure the stated goal.
2) Predictable and nimble handling in the ~1g lateral acceleration range
3) Improved and predictable brake performance (high speed reduction and panic stops)
4) The ability to use the best street tire compounds and widest wheels reasonably possible (thinking Michelin Pilot SS or Toyo Proxes R888)
5) Tasteful body mods to fit said tires (Some fender reshaping is being considered)
6) Availability of parts from reliable, reputable sources, i.e. ten years from now I can still get replacement parts for the upgrades

DISCLAIMER: I am search challenged so feel free to point me towards existing material :oops:

After reading all the threads I could find on STF and The Samba I have generated the following set of questions.
1) Are ventilated rotors recommended or will drilled and/or slotted rotors suffice?
2) Is it possible to have slip-on or bolt-on rotors and fixed hubs, both front and rear, with the configurations below? Which of the two options below would be the better from a parts availability and performance perspective?
a. Willwood 4 piston or 944 front brakes?
b. Wilwood 2 piston with e-brake or 944 rear brakes?
3) Full 944 rear suspension conversion or upgraded T1 with sway bar and kaffer bar? (Thinking Mendeola “Stiffy”)
4) Is there a performance benefit to swapping for early 944 rear diagonal arms/full rear swap?
5) Is the “adjustability” of the 944 rear setup/swap worth the effort for a street car that will occasionally be driven in a spirited fashion?
6) Stock/upgraded torsion bars or coil-over conversion for the rear?
7) I am already planning on caster shims, F/R anti-sway bars, and gas shocks. Recommendations?
8 ) If any are recommended, What chassis and suspension reinforcements can/should I make?

I have an advanced-basic understanding of suspension and braking systems. I am posing these questions here in an effort to avoid costly mistakes and unnecessary expense. I have found plenty of "race" oriented data but I have walked away with the understanding that it does not always translate well to a street setup. I have searched in an effort to avoid redundant posting but I am not finding the answers I need given my current skill set and comprehension. I will probably have follow-up questions so please bear with me.


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Re: Input needed: IRS Ghia Convertible performance street suspension setup

Post by Marc » Thu Dec 15, 2016 7:30 pm

About the fenders: John Kelly was a master sheetmetal man in Seattle who retired some years back (he & his wife now run a bed & breakfast on the WA coast). At one time he offered premade fender flares for 'Ghias that (like all of his work) were a true work of art - those aren't available anymore but he did produce a video that shows how to build your own, you might want to see if he still has some DVDs available. The Seattle mailing address given here is long-defunct but you should be able to reach him by email.

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Re: Input needed: IRS Ghia Convertible performance street suspension setup

Post by ChadH » Thu Dec 15, 2016 7:53 pm

Just a few thoughts without simply parroting stuff I've learned here.

- The Toyo and PSS are very different tires. The PSS is on the upper range of a good summer tire that you can drive daily, get a decent life span, and is very good when compared with more typical all-seasons. You're not likely to get 1G from them except when they are brand new, suspension is set up really well and weather is "just right" The Toyo is definitely more of a track-day tire. It will stick better, but is going to be more sensitive to heat cycling and aging, it might be downright terrifying in the cold or rain, and suffer from overheating and get "greasy" if you drive when it's very hot out. I think, unless you're racing, a good summer compound like the PSS is a better place to start. There's lots of stuff in between too.

- For a street driven car, be careful with going too far with heavy rear spring and swaybar rates. Too much and you'll create a car that oversteers and can be unruly when driven hard. No one likes a car that badly understeers, but on the street, you want a car that's neutral and if anything errors a little toward the side of under steer. Loosing the rear end on a fun on-ramp, and banging the back of the car into the guard rail is not good.

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Re: Input needed: IRS Ghia Convertible performance street suspension setup

Post by Jadewombat » Thu Dec 15, 2016 9:52 pm

Whatever you do, do it in increments until you feel comfortable with higher and higher speeds. I had the good fortune to be able to drive my friend's C2 911 convertible in Germany in some of the no speed zones of the autobahn. We got up to 280k and stayed there (168mph). A hill a mile away down the road you suddenly drive past a couple of seconds later. The car was amazing and felt planted to the road and extremely predictable.

I was pretty shaken up the next day realizing there are very few (in my mind) cars I would trust to go those speeds again that are actually built for it. OK, this is an extreme example, but you get the idea.

Ghias have the aerodynamics to run at higher speeds (vs bugs, which need some tweaks).

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Re: Input needed: IRS Ghia Convertible performance street suspension setup

Post by ChadH » Fri Dec 16, 2016 11:11 am

A really good reference book that I'd suggest. It's fairly general and theoretical, but not "too" technical. ... 0912656468

I had this years ago. I think it's out of publication, but it looks like it's still available used. I actually need to get another copy for myself.

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