From: darling@simlab.arc.nasa.gov on behalf of Dave Darling [darling@simlab.arc.nasa.gov] Sent: Thursday, April 08, 1999 3:20 PM To: tooltime1@mindspring.com Cc: 914@rennlist.org Subject: Re: [914] tires At 12:14 PM -0800 4/8/99, tooltime1@mindspring.com wrote: >> The 195/50s lower the car considerably (~1 inch over 205/50s) >>give a harsher ride, lower gearing, and better feedback from the road. > >What is the diameter or height on a 195/50 as compared to a 205/50? My >trans has be re-geared so the 60 series tire would be a better option for a >steet tire. Maybe a 205/55 as a good compromise. I am using 22.5 inch >Sports car specials on the track. These tires are for street only. I'm sorry, I had a brain-fart. I meant that the 195/50s were about 0.25" shorter than the 205s, but that's just from the numbers. (195*0.50 = 97.5mm sidewall; 205*0.50 = 102.5mm sidewall.) I had a factor of two in there (tire is on each side of the wheel) and then applied it twice. Oops. The 205/60 is nominally 123mm sidewall, while the stock tires (at 165/78) are nominally 129mm. You can do the math for the gearing, etc. >Ride height not that big of a deal since I am going to cut the struts and >use shorter shocks in the back. This will get the car down and solve the >handling problems. Ummm... I don't think there's any need to do that! The shocks themselves don't really affect the ride height, except when they bottom, and you definitely don't want them doing that! Use the aftermarket springs that everybody sells. You can pick your spring rates, plus they usually lower the ride height. And the adjustable spring perches can help as well. For the fronts, you just adjust the torsion bars, or re-index them if you need a bigger change. AFAIK, I have stock torsion bars and 140#/in rear springs. The driveshafts in the rear are very close to horizontal, which is suposedly where they should be. My front A-arms are just about horizontal as well, which is also supposedly where you want them. No cutting of anything was required. Of course, your mileage may vary. >Which sidewall profile gives the widest contact patch in relation to the >tread width? In general the lower the better. I don't know about specifics, as I never measured my contact patch. I will say that the 50s did stick better than the 60s, as well as providing a lot more feedback. But I only did the A-B comparison with the A509s, and that was comparing worn-out 60-series to new 50-series. I *think* they stick better even when you take the wear into account, but no real back-to-back testing to show that. I would think sidewall deflection is one of the major problems with most street tires anyway. A number of years ago, some friends did some test- ing on a 928 with street tires to determine optimum tire pressures. They were quite surprised. When they let air out, the cornering power got worse! My friend said that they were at 40 PSI and traction was still improving when they ran out of daylight. That says to me that the sidewall compliance was more of an issue than the contact patch. That's why I overinflate my street tires for autoXing. (~36 PSI.) I'm not quite brave enough to run them at 40. And yes, I do wear the centers out more. Competition tires and race tires are a different matter. They are generally designed with stiff enough sidewalls that the contact patch is the dominant factor. For my next set of street tires, I'll be getting ones that are too skinny and too tall. I want a more reasonable ride and gas mileage for the long trips to and from the track. Different priorities for different folks. --DD Dave Darling 74 914 2.0 (VROOM!) darling@simlab.arc.nasa.gov <--- OR ---> ddarling@wgss.com "914: The Porsche Picnic Basket. A lid that opens on each end, and a handle in the middle.." -- CHD