Project 996 Road Rallye: Part 6

We installed the Girodisc 2-piece rotors as the car needed new brakes.  I wanted to save some weight if possible - which we have.  But I want to do down to 17" rims.  We bought a wheel measurement tool to better estimate what offset/width and diameter could possibly work.

GiroDisc weights: 

• Front: 17.95 lb each 
• Rear: 15.25 lb each 

Our stock rotors were worn so I searched for better weights.

OEM-equivalent weights (ours): 

• Stock front: ≈ 23.1 lb each (18.7lb)
• Stock rear: ≈ 14.6 lb each (12.55lb)

Weight deltas (per rotor → per axle → total) 

• Front: 23.1 − 17.95 = 5.15 lb saved per rotor → 10.30 lb per axle
• Percent change: 5.15 ÷ 23.1 ≈ 22.3% lighter per rotor 
•  Rear: 15.25 − 14.6 = 0.65 lb heavier per rotor → 1.30 lb per axle (heavier) 
•  Percent change: 0.65 ÷ 14.6 ≈ 4.5% heavier per rotor 
•  Net vehicle change (all four): −9.0 lb unsprung/rotational mass (mostly from the fronts). 

 What that means on the car The fronts deliver the big win: ~10.3 lb unsprung + rotating mass off the nose, plus larger diameter and better cooling. The rears end up roughly stock-like in weight (slightly up), which is normal for upsized rear rings on two-piece kits.  

Girodisc upsized their rotors vs the stock sizes.  Why?  The rear stock rotor houses a drum brake for the handbrake.  To clear this, Giro have to make their rotor larger.  They then enlarge the front as well to maintain the brake bias.  The benefit is larger brakes for more over heat capacity and performance.  So overall, a good tradeoff - lighter unsprung weight and more performance.

The wheel tool that we used suggests that a 17"x8" rim could work for us at the front.  This would be great as we would like to run tires with more sidewall.  Once the Elephant Racing dampers arrive, we can install and do a proper measurement.

Project 996 Road Rallye: Part 4

Wheels, Tires and Brakes are proving to be the biggest challenge in this build.  Since our goal is to create a car that will work 90% on broken pavement and 10% groomed dirt roads, we are sticking with tire sizes that favor the road.  But we are looking to add compliance through larger side walls as well overall larger diameter to gain some ground clearance.  

Most of the Safari builds run the same size wheel and tire on all four corners and the fifth which they keep as the spare.  On dirt, this can work, as grip is usually minimal.  But our car is going to run on the road so tire size will defer to weight bias.  We wrote about tire selection on a previous Blog: HERE  Since these are rear biased cars, larger tires are needed at the rear.  We will follow what is typically used for the road cars with more sidewall.  Plus our tire should be quieter than one that favors dirt.

We started with tires and then are attempting to find rims that will fit.  I used a resource on the web that will compare wheel/tire sizes and visually shows the potential impact.  The rear option is shown below.  I linked the image to the website.

We have contacted several of our favorite wheel suppliers including: Braid, HRE, BBS and TireRack to see what options we can find.  Ultimately determing clearance with the new Elephant Racing suspension will be key.  We have a tool that should help us make that determination on rim width and offset.  We will post up pix of this tool in action when we get to this stage.

One of my friends, who is Ride & Handling engineer at Toyota, and races 911s, has suggested the Conti Extreme Contact Sport 2 tires.  These seem like a great solution that come in the sizes that I am after: 225/50R17 front and 285/40R17.  

The Girodisc brakes that we have ordered are slightly larger in diameter than stock.  This is due to the fact that  the 996 has a rear (in hat) drum brake for the handbrake.  Giro needed to make the rotor larger to clear the drum and up sized the front to match the rear  - to keep brake bias the same as stock.

We ordered their caliper rebuild kit as well since these have over 100k miles on them.  We will vapor blast the calipers and replace the seals and rubber boots.  New Stainless Steel brake lines are ordered as well as 25 yr old rubber lines don't instill confidence!

We are hosting a Tech Seminar on Nov 8th to show off the build.  We plan to have the tire/wheels selected and have the brakes installed by then.

IMS and engine upgrades related to reliability are also on deck once the car returns from dry-ice cleaning of the under-carriage and suspension area.