How the Ford GT Team Selected the OE Rubber
The often asked “why Goodyear?” question prompted me to write some of the facts and background regarding our decision to choose Goodyear for the tires on the GT. Also, I have included some high level tire performance fundamentals, which further explains tire differences and tradeoffs.
Criteria
Criteria for tire selection is large and I won’t try to capture them all here, but the criteria includes:
• Durability / reliability including at top speed
• Handling – wet and dry
• Stability
• Agility
• Response
• Predictability
• Steering feel
• Tire Weight
• Ride
• Noise Vibration and Harshness (NVH)
• Wear
• Aesthetics
• Cost
Predictive Design Ability
The very short nature of the FGT program required us to get most everything “right” the first time as we had no time for iteration. Therefore, predicting the best direction was important on the FGT, which added this predictive ability criteria for the program in general, including the tires.
The Options
- Bridgestone/Firestone – As noted on the Forum, Bridgestone/Firestone was not an option at Ford since it was soon after the Firestone tire issues at Ford.
- Michelin – Michelin was a top candidate, however, they kept pushing run flat tires and the BFG brand. Run flat tires hurt performance, ride, weight (very important rotating and unsprung weight) and NVH. We told them we would not utilize run flats, but Michelin persisted. I also told them we did not want BFG because its image and brand did not fit the FGT like Michelin.
- Continental – Continental was somewhat in the running, but did not stand out versus the others on any specific dimension.
- Goodyear – Goodyear was a long-time partner with Ford and had done well in the recent past for Ford and competitors including Ferrari. Further, Goodyear had a couple of very strong guys in the predictive analysis department that we felt would be important.
The Selection
Michelin persisted with pushing run flats, BFG came back with very high cost. Continental came back with a little lower cost than Michelin, but with few perceived advantages at the time. We felt Goodyear had a strong edge on predictive ability, had done well recently as noted and they came back with extremely aggressive pricing. We could not make a stronger argument for Michelin and Continental so the FGT program choose Goodyear for all of the reasons noted. Not just cost.
Tire Development Reality
Goodyear’s predictive capability was not at the level expected and needed, for multiple reasons. In the end, we (McGowan, Walsh, White, Cullen and myself) spent significant effort with Goodyear to achieve all of our targets. We were successful.
VMax (Top Speed) Testing
A note on top speed ability, since this was mentioned on the Forum, incorrectly, as reasoning for the choice of Goodyear versus others. All of the major tire manufacturers could hit our Vmax tire testing at the +8 psi (40 psi) we specified above 150 mph. None of them could make it at 32 psi. Ford’s tire Vmax testing is very, very difficult.
Tire Sizing
We set the overall widths based on performance and refinement including agility. Tire width is directly related to performance with negative tradeoffs including agility and rut wander/camber sensitivity with increasing width. The overall vehicle benchmark F360 Modena was on 215’s and 275’s. We beat the F360 Modena, with ease, by 2 sec per 50 second lap, on our tiny handling track in which horsepower was not a factor because there are no straights. And the FGT is 250 pounds heavier. All of us in development thought the F car was faster subjectively because it felt so on edge all of the time and took significant effort to go fast while keeping it from spinning out. FGT’s superiority came a little from the chassis/suspension system, but primarily from the much wider tires that we had worked so hard to make work well with the car – to be progressive and drivable at the limit and take complete advantage of the tires’ and car’s ability. True synergy – make the whole greater than the sum of the parts.
Widths Continued – Performance Dry, Wet, Cold and Package
Bucket of Grip – The Bridgestones that most are running on the FGT are nearly 13% wider at the front (265 versus 235) and nearly 10% wider at the rear (345 versus 315). All else being equal that is effectively 10% to 13% more grip, which is a lot. We call that a bucket of grip in racing. That is 1.1 G versus 1.0 G cornering capability, if everything else was equal. These statements regarding tire width are backed up by the FGT versus F360 example and continually in the real world.
However, all else is not equal – the Goodyears are a better wet weather tire and have more open tread…less rubber on the road than the Bridgestone, which adds to the traction and grip deficiency of the stock Goodyears versus the Bridgestones, beyond the width deficiency. We (me, McGowan, Walsh) prioritized wet handling a bit on this high horsepower car without traction, yaw or stability control.
This type of grip deficiency based on width and tread pattern cannot be made up for by compound. And if a company attempted to try compensate with compound there are significant tradeoffs including wear, response (lateral stiffness) and predictability at the limit.
Cold weather – rubber and the tires have a “glass transition” point where the tire rubber effectively changes to hard plastic. Each tire is different. But summer/high performance tires’ glass transition point is typically between 40 – 50 deg F. In addition, the supercharged FGT engine is very sensitive to air temperature and cold temperatures make big horsepower (around +50 horsepower or more at 40 deg F versus 75 deg F). Combine the bigger than normal horsepower with “plastic” tires without traction, yaw or stability control and you have a handful to manage.
Final Notes
As noted, we were worried about rain, but in reality it seems the rain ends up being less of an issue with the FGT because people seem to be much more aware that traction and grip is at a premium in the wet. However, drivers don’t seem to be nearly as aware of the large reduction in traction and grip mixed with big horsepower on a dry, but cold day.
The lack of driver aids and the super charger’s sensitivity to temperature (much higher than a naturally aspirated car and many turbo cars) are the main reasons for the FGT being more difficult in the cold than some competitors at the time and since. Yes, the Bridgestone’s should be noticeably better in the cold than the Goodyears for the same reasons I point out that they are better in the dry at “normal” temperatures (width and tread pattern).
Age – all of this is compounded by the fact that most seem to have their first experience with the Goodyears when they are multiple years old. Tires continue to cure from the day they are built. Sun/Ultraviolet light and heat cycles accelerate this process. Two to three year old plus tires lose a lot of grip from new regardless of wear. Jeff and Ralphie use their Goodyears and change them out for new within a couple of years since they need the wet (and snow) capability so they end up liking the Goodyears best for their cross country travel.
I agree with “public service announcement” reminders about being careful in the cold….with any high horsepower car, especially the forced induction GT for reasons mentioned. The Goodyears, when new or the same age as others, are an equal tire for their width and wet weather performance. The Bridegestones are a superior dry and cold weather tire due to its much greater width and greater rubber on the road tread pattern (not as good for rain).
Hopefully this helps the FGT owners better understand the tire development and selection process as well as some tire performance fundamentals.
Scott Ahlman, President of Ahlman Engineering, Inc, with 20 years of automotive industry experience, leads Ahlman Engineering automotive product development and motorsports engineering services.
Ahlman’s success and reputation in racing earned him a lead role for chassis design, vehicle dynamics and system engineering/architecture of the 2005 – 2006 Ford GT 200 MPH Supercar.
