Advice on 20" wheels

[gordon24]

Hoping to get some help on my next Porsche buy. I'm driving a 2002 C4S Cab now but ready to get into a 997.2. I really like the look of the C4 with it's wider rear fender flare so likely to go C4. Also want the HP and braking on the S so narrowing to a C4S. As I'm shopping I've come across a really nice C4S, white, full black leather with silver stitching and 20" wheels. The wheels seem to fill the wheel wells nicely and completes what is a sharp and fairly distinctive look. However, I recall a Sharkwerks post that pointed to the 997.2 suspension being better tuned for 19" wheels.

I will track the car but not race it.

Thoughts?

Thanks,

Gordon

[White987S]

Easy, get a second set of dedicated track tires mounted on a set of 19 inch wheels.

Edited October 16, 2013 by White987S

[gordon24]

perfect solution really if the 20's are good on the street, thank you.

[White987S]

perfect solution really if the 20's are good on the street, thank you.

The 20's will be just fine on the street with good rubber and proper road force balancing. All the new 991's are now coming standard with 20" wheels.

[gordon24]

Thanks, good point. I think I'm set.

[Mijostyn]

Hi Gordon,

The only reason to have 20" wheels is so that they will fit over bigger brakes, which you will not have. The car is designed to operate with wheel tire combinations of a specific rolling radius. In order to maintain this when you go to a larger wheel you have to use a tire with a smaller aspect ratio (less side wall) This is very bad in street cars for several reasons. The tire becomes a real pain in the a-- to mount increasing the likelihood of damaged rims, you are more likely to bend a rim hitting pot holes, the tires will be more expensive which at 10K miles a pop is something to consider and most importantly they will not handle as well. That is right. Take a look at any formula 1 car. BIG side walls! Race cars can operate with smaller brakes than street cars because they are about 1/2 the weight. Plus cosmetics are not so much of an issue with race cars. Go to Singer Vehicle Design. What do they put on arguably the greatest 911 ever made? Custom 17 inch rims with big side walls. (when I win the lottery)

The 991 uses a larger rolling radius set up with bigger brakes. The side walls are about the same.

The only thing special about Hunter Road Force balancing is that it might be a little more idiot proof, but not much. A good technician using a simple balancer can get every bit as good a balance as the same tech using a Hunter. My point being is that it is not the balancer that saves the day, it is the Tech. Don't worry about finding a good balancer, Worry about finding a tech that knows what they are doing. These are signs that the tech knows their business.

1) The tech never balances a cold tire that has been sitting on the ground for more than 5 minutes. High performace tires flat spot as they cool down. This can throw the balance off by 1/2 once! He will take the car out, warm up the tires and put it immediately on the lift. There the tires can cool off all they want.

2) The tech never runs his machine in round off mode. If all you see on the machine is 0, 0.25, 0.5, 0.75 etc. Run away. Porsches need their wheels balance to < 0.1 oz. What these machines mean when they say "0" is not "0" but rather < 0.25! With round off mode off the machines report weight in hundredths such as 0.37, or 0.12.

3) The tech has to use fractions of wheel weights to get the balance right. Tape weights come scored in 0.25 oz increments. If all you see used are whole 0.25 oz segments...run away. Although you can wind up on a whole segment in most cases you have to use a fraction of one segment to get the balance right such as 3.25 segments ( about 0.82 oz)

Edited October 18, 2013 by Mijostyn

[gordon24]

wow, different perspective and very specific which gives me a lot of confidence in your answer. I hadn't really considered the sidewall thickness in a full context; car geometry and wheel preservation trump looks in my book. I think I'll continue to shop for something with 19" wheels then.

I live in the Seattle area, are you aware of a "good tech" near me?

Thank you for the thoughtful reply.

Gordon

Edited October 17, 2013 by gordon24

[Mijostyn]

Don't mention it.

Living in New Hampshire I'm afraid not. But I would start with your local Porsche independent. You might ask your local PCA chapter for a recommendation. Then just hang out and watch the tech in action. Warm tires, machine set to report in hundredths, and odd sized weights, then you probably have a winner. Performance shops are going to be much more sensitive to this. Regular cars can be over 1/2 oz out and the driver would never know. Not in a 911. At about 0.2 oz the steering wheel will start shaking usually around 75 mph. Take your hands off the wheel and watch it dance!

[kgoertz]

Thanks Mijostyn. That was an informative post. My wheels have never been perfectly balanced and I think I'll try a local speed shop with your advice. Nice.

I was also wondering about the impact of low side-walls on a 911 and I would be interested in understanding why cornering would be better with a larger side-wall.

The PO had installed (beautiful) 19" rims on my 996 which was really only designed for 18". At one point I was running 315/25R 19 on the rear which is getting pretty thin. I've been looking around to downgrade to 18" rims mostly to improve ride and handling but I'm not sure how it would impact track/spirited driving. I assume the impact would be similar to downgrading from 20" to 19" on a 997.2.

[Mijostyn]

Hi kgoertz,

First, I have to make one distinction. Racing cars have low aspect ratio tires. It is just that the tires are so wide they still have plenty of side wall. It is the absolute size of the side wall that causes alterations in handling, ride and load carrying capacitiy. Ride and load carrying capacity are easy and intuitive. As the side wall gets shorter the side wall becomes stiffer and the ride gets harsher. As the side wall gets shorter there is less room for deflection of the treat before the side wall is completely pinched transferring additional load directly to the rim...bending the rim. Handling is a little more complicated. First the tires act sort of like lateral shock absorbers. Lateral give in the side walls absorbs lateral stress transients which might otherwise cause the tread to loose grip. Imagine hitting a severe bump without any suspension. You wind up airborn! But, with a suspension you control the transfer of these forces to the car and the car stays planted. The next factor is shear stress. As lateral forces build on the tire in a turn the tread deflects towards the inside of the turn essentially pulling the car in sort of like Prosche's new 4 wheel steering. A larger side wall allows higher sheer stress before the tire looses grip. These issues are exactly why we use smaller wheels/larger side walls for snow tires.

Having said this, too much side wall is just as bad. All high peformance tires have relatively small aspect ratios and side walls. Really big side walls give a bouncy ride and soggy handling in high performance cars.

Stick to 18" on your 996. 997.2s run with 19" wheels. Going to 20" wheels is a down grade in performance. If you own a Honda with 60 series tires, 1+ ing the car is no big deal but our Porsches are always designed at the edge of tire technology and it is better and safer to stick to the factory set up.

Edited October 19, 2013 by Mijostyn

[gordon24]

A bit delayed but thank you for the added insights and perspective ... interestingly you and Mijostyn guide me to the 19" wheels ... each with slightly differing arguments which absolutely affirms I will steer clear of 20's, unless (or until) I can start shopping for a 991 designed for that size.

thanks again to all, this is a great place to learn about these cars.

[kgoertz]

Thanks Mijostyn. Nice explanation. I'm still looking for a nice set of 18" rims to downsize mine and your argument helps justify the cost.

[Mijostyn]

Kgoertz,

May I suggest OZ Alleggerita HLTs. These are GREAT wheels for the money. Beautifully made and very tough. They only weight 16 lb. The Italians know how to do two things better than anyone, leather and metal. Just look at the castings in a Ferrari.

[Yash]

Mijostyn and others, thanks for the knowledge.