CAI and Thoughts About Wheels

Cold Air IntakeMy old Alfa GTV was very free-reving. You press on the gas and let up and the response was immediate. With the MINI gas-by-wire system, it took much longer to wind down when you let off the gas. Under steady pressure, it responded fine, but with quick changes it seemed to lag. I had read online that this was a function of restricted air intake as much as anything else. To test this, I bought a cold air intake system from with a Green Filter. The kit was very easy to install and only took about 15 minutes with simple hand tools. I was worried the open system would result in increased intake noise, but I can’t say I’ve noticed much of a difference. I do, however, notice a much improved throttle response.

Since I’ve had my Mini, I’ve always appreciated the go-cart like handling, but the 17 inch run-flats provide a harsh ride, especially on the roads around DC. Over the past year I’ve learned much about how the various components of the suspension interact and effect the handling characteristics of a car. I was surprised to learn that the large wheels so popular on cars today are more about “bling” than performance. (“Bling” is a technical term. See: Oxford English Dictionary) Maybe I’m a bit old school, but the large wheels and ultra-low profile tires just don’t look right to me.

One basic approach to suspension engineering is to start from the chassis and go out. What sort of ride height do you want? Stiffness and roll? Sway-bars? How big must the brake disks be? Once you’ve sorted that out, what sized wheels should you have? Remember back to high-school physics where you spun a wheel and used the wheel to spin you on a stool? The same thing applies to automotive engineering. With cars, it’s called un-sprung weight. It’s the total of all those parts that contact the ground and move with your wheels (sway bars, control arms, suspension, wheels, tires, rotors, hubs, etc.) Less weight is better. In general alloy weighs more than rubber, and run-flats weigh more than non-runflats. Heaver wheels/tires take more effort to start rolling, stop, and turn. Wheel size therefore is a compromise between being large enough to clear your brakes, but not too large to present a high side-wall tire which will flex too much on cornering. All this while retaining the same overall diameter of your stock wheel so your speedometer is still accurate.

I enjoy the grip of my Goodyear RS-A Run-flats, but not the harshness of the ride. The R-85 (S-Lite) 17×7 inch wheels and 205/45VR17 tires weigh 50 lbs each. My test wheel is a 15×5.5 inch R82 (Silverstone) wheel with Conti CH95 175/65HR15 tires that weigh 30 lbs each. That’s a savings of 80 lbs in unsprung weight. The trade-off is the loss of 120mm of rubber contact with the road. A 40% weight reduction with a 15% reduction in the contact patch (and possibly traction.) My impression so far is that the harshness is gone from the ride. The car seems to stop more quickly and the steering feels lighter. I’ve also noticed a higher tendency to squeal the tires on corners, but not a feeling that I’ve actually lost traction. That may be due to the dynamic stability control system being more active, but still in control.

What’s next? I’m very satisfied with both changes. I think I want to buy a wider tire, however. I’ve read very good reviews of Falken Ze 512’s (top rating by consumer reports as well.) The 195/60HR15’s are only about $39 each. That size is only a 5% reduction in the contact patch, but the same weight as the Contis. If that works out ok, I’ll sell my S-Lites on ebay.