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.