Carbon Fiber Wheels Article

Its not so much the unsprung weight - as the rotating mass creating polar inertia.

 

I saw that, and I have been preaching this for years as people put bigger and heavier wheels and tires on their cars and say how they ride and handle so much better. Kept telling me I had no idea what I was talking about.

 

I personally have been going -1 or -2 on wheels for the track for years now. I don't play in the NBA so don't need dubs

 

Big wheels like sunroofs is a tyrany foisted on the public by dealers and manufacturers.
Even out of the sportscar realm where accelration or turning not paramount, they ruin ride.

i dont get it at all, but I guess big wheels are our eras version of Vinyl roofs.

 

Great comment! Personally, anything over 17" scares me due to pot hole damage.

 

https://www.youtube.com/watch?v=IcqcSuIKcKk

 

recently bent an oem bmw 17" rim after hitting pot holes....your concern is very real.

 

There are real engineering reasons for bigger wheels. They have primarily to do with what is called the scrub radius. The scrub radius is the distance from the center of the tire to the kingpin line intersection with the ground. The idea being that it is better to get the the scrub radius down to a low number because that is what causes steering kickback when you hit bumps. The bigger the scrub radius the more kickback you get during impacts, and the steering forces don't increase as much during cornering. This was all discovered by Chevrolet when they were reworking the front suspension for the Corvette and the designers found that they had to go to 17 inch wheels from the 16's that were on the previous years car.

In order to get the scrub radius down you need to move the ball joints out into the center of the wheel. If the wheels are small they are going to hit the control arms at high steering angles, so that's the first reason to make the wheels bigger.

But if you move the ball joints out into the center of the wheel the brakes are going to be inside the wheel. Which means that in order to have big enough brakes the wheel diameter is going to go up.

If you look at old racing cars of the late 60's to get big enough brakes you had to have the brakes hanging out inside of the wheels and consequently these cars had a big scrub radius.

The bottom line is that bigger wheels and lower profile tires are what is required to get steering and braking up to the standards we expect today.

That said, depending on the weight of the car you shouldn't need wheels any bigger than 17 inch or 18 at the max in the front. Bigger than that in the back are just for style and to try to make the car "look right".

And yes, pothole damage is much more frequent with lower profile tires so there is a downside, and of course inertia is greater with bigger wheels,so there is a ride and acceleration compromise, but the upside is less kickback, and of course much sharper steering from the shorter sidewall.

Most enthusiasts will give up a bit of ride harshness for better steering response and bigger brakes, so that is the compromise that gets down to the engineering team, and the result is bigger wheels to make all that work. Perfectly logical when you understand the engineering involved.

 

That sounds like a balance problem, or alignment or runout problem, not wheel size.

 

As you say you dont need more than 17 or 18, these are sizes considered teeny these days.

Yess too small like too big wheels create subotimal compromises, and its an interesting point about kingpin line intersection. But wheel size today has gone way beyond engineereing compromise and is a function of fashion and "styling". I seem to remember an interview with BMW engineers bemoaning how marketing was insisting on bigger wheels to the detriment of ride, and handling.

One wonders whwther consumers really want this as a stock non option or whether marketing sees a big wheels as big margins and therefore insists on it.

To the extent that CF gives you the better of everything they have to be a great improvement. Possibly they require some owner care in terms of curbs and other hazzards to wheels, but for a sports car they sound like a big improvement. I know they are lighter than mustang 350 wheels, but are these 350R wheels much lighter than really good alloy wheels?

 

In about 10 years we are probably going to see the rebuilding/restoring hot rod shows on TV reverting back to more sensible wheel sizes under the guise that monster wheels suddenly look so dated.

 

LOL

Maybe one day in the future we will be able to buy high performance street cars with optimal wheel and tire sizes.



Hopefully the silver lining in the big wheel fashion saga will be that CF wheels for our track cars will be affordable.

 

Oh yeah, the early 1990s. Street rods with pastel paint jobs and tons of billet.

 

We are talking about the real world with potholed and patched streets. Sports cars are one thing but what are the "real engineering reasons" for sedans and SUVs. The whole thing has gotten quite ridiculous.

According to a thread I read awhile back on eng-tips, the suspension engineers at the OEMs hate these things as they trash ride compliance and NVH. How is it that in the 70's Porsche 930s and MB 6.9s were able to thunder down the Autobahns at 150 MPH on 70 series tires? The large rim and low profile tire has become absurd on non sports cars. Designers are marketers are to blame. All image and no substance.

 

I agree, unfortunatly we may also have a customer base who are all about image and no substance in their cars. How to explain the decline of BMW as product while its sales rose.

 

I'm no fan of oversize rims and I feel that there is a big push for too big a rim on many cars.

And while those Porsche's and MB's could blast down the autobahn at 170 mph on their 15 inch rims that's about all they could do. Today many sports car owners take their cars to the track and even if most of them don't owners want cars that they feel are "track capable". Magazines take virtually every sporting car to a track and wring them out and report on what they find. The program managers aren't going to be embarrassed by a report in "Car and Drivel" that their high performance car ran out if brakes in a few laps. Those Porsche 930's and MB's could handle the autobahn just fine, but if you took them to a track day they'd smoke the brakes in about 10 minutes of full out track duty. Back when those cars were produced, there was no such thing as track days and those cars, without a serious brake upgrade would never cut it today.

Face it, today's modern cars are much more track capable than the 930's ever were and that's a design requirement for every serious sports car (Porsche's included) made now. Bigger wheels, on the order of 18 inches are pretty much a requirement if you're going to fit big enough brakes for track work, and that requirement didn't exist in the early 80's.

Before the 1984 Corvette, there was no such thing as tires bigger than 15 inches for anything but trucks. GM vehicle dynamics engineers had a specific set of requirements fore steering response for that car and according to a good friend who was there at the time, they really asked a lot of the tire companies and really only Goodyear thought they were serious and came up with the "Gatorback" which was a revelation in handing and steering response at the time. The move to 17's was required when they went to "zero scrub radius" front suspension in 1988 and tires were one step further. And yes the NVH guys hate them, but the brake engineers need the diameter and they generally win that argument. Also steering sharpness that you get from lower profile tires has become a design requirement. I went from tall 70 tires to +1 low profile tires on my wife's sport coupe and I was amazed how much quicker the steering became and that's something you simply can't get from tall tires.