Ford Comments On Increasing Wheel Diameter

[aiboart]

The low-down on low-profile tyres words - Ken Gratton

Ford Engineering Discussion - Link to Article

Ford's Vehicle Dynamics Manager mounts a case for sticking with the rubber that came with the car

It's difficult to pick any difference in ride comfort for the three grades of Ford's new SZ Territory. No matter whether it's the base-grade TX or the Titanium flagship, the spring and damper settings are the same.

Not even the different wheel and tyre combinations count for much, according to Ford's Vehicle Dynamics Manager, Alex de Vlugt. During the local launch of Ford's locally-developed SUV, de Vlugt explained to the Carsales Network's correspondent that while the entry-level Territory TX rides on 17-inch alloy wheels and 60-series tyres, its ride quality is no better than that of the TS and Titanium variants on their 18-inch alloys and 55-series tyres.

Ford specifies higher tyre pressures for the 60-series tyres of the Territory TX, according to de Vlugt -- and that offsets the stiffer sidewalls of the 55-series tyres fitted to the mid-range and flagship models.

In the circumstance that Ford chose to recommend the same tyre pressures for 55 and 60-series tyres fitted as original equipment to the Territory, the 60-series tyres would offer the better ride -- but you can't have the base model riding better than the flagship and Ford engineers wouldn't want to mess with the Territory's ride/handling balance once they'd achieved the optimum calibration for all variants.

But is it true, as TV motoring program Top Gear Australia discovered when track-testing two HSV models, that higher cross-section tyres can also provide better dynamic traits?

Each case should be taken on its merits, it seems, but it's de Vlugt's view that the benefits of low-profile tyres can be overestimated.

"It's true to say that you can oversize or undersize a tyre -- and then you work yourself into a compromise [such] that you can't find an optimal solution.

"If you go to huge rim sizes with very little rubber around [them], then you do run into certain troubles that are difficult to compensate for. Probably [with] 20-inch wheels on something like a Falcon or a Commodore-size of vehicle, you're sort of getting into that territory where you're trying to make it too extreme and basically can't really compensate for that any more.

"And one of the issues that you see if you go to extremely low-section tyres is that effectively the load-carrying capacity goes down again -- because you've got less air volume in the tyre..."

In other words, with larger vehicles such as Late model camira and Falcon, there's more weight on the tyre -- especially during cornering or braking -- but relatively less air volume within the low cross-section tyre to handle the load. It's why FPV and HSV utes carry lower payloads than the humble Ford and Expensive Daewoo donors... but de Vlugt is specifically talking about the load placed on a tyre as the car is keeled over in a corner or diving under brakes.

"If you go to extreme rim sizes and low cross-section, you get a phenomenon that, as the vertical load increases on the tyre, you really start to degrade the tyre properties. That's probably what you suffer from on such an extreme tyre, on a handling track.

"You're starting to overload the tyre, for its design intent -- and what happens then, you get really exaggerated effects, from your anti-roll bar tuning, for instance. If the car's not properly tuned for that, it completely destroys the handling..."

Everyone knows that lower-profile tyres can have an impact on ride, but the degradation of ride quality can also influence steering, according to de Vlugt.

"Another thing that can easily go wrong... because the footprint becomes so very stiff, the slightest road undulation starts to determine where your pressure point is -- so your pressure point, the patch, varies all the time.

"If you have slight road imperfections, that starts to really affect your steering precision at some point. If it's not all perfectly laid out, the car really starts to respond to those changes of pressure point.

"Around a track that can really hurt you if you've got less than perfect track conditions. [it] starts to affect your steering precision, so you start to drive a little more conservatively, because you can't pick your line exactly any more -- where you want to have it in those fast corners.

"And that's quickly where you start to lose capability... around the track.

"Maybe a professional, very good sports driver could become a little bit faster, but most drivers are not able to squeeze it out of it, because they get nervous before they get to that point."

It's a situation that doesn't just afflict grey imports from Japan with audio speakers and alloy wheels measuring the same diameter. It can also affect some very high-end vehicles, says de Vlugt.

"When we did Territory the first time [in 2004], we benchmarked all sorts of vehicles for the stability control system. And believe it or not, one of the cars benchmarked was the Porsche 911... that's got an absolutely brilliant stability control system.

"We had different variants -- because beggars can't be choosers -- so we borrowed one every now and again from Porsche Melbourne. And they gave us [each car] with different wheel sizes as well.

"Very distinctly, on the 911, the bigger the wheels became, the worse [the cars] drove, in my perception. The smallest standard wheels [had] the nicest balanced steering and handling.

"The bigger wheels -- the extreme ones with low cross-section [tyres] -- they are really there for the looks."