dgeesaman

iTrader: (7)

Max has some good points. And his analysis of the rotational inertia is spot-on - weight near the center does not affect inertia as much as the outer edges. So the hub and wheel is the least effective place to remove weight. If you're interested in improved torque delivery (acceleration), then shopping for a lighter tire will pay off much greater dividends. Regardless, we'll see a couple of FDs in a ricer magazine or car shows with carbon fiber wheels - I'll bet that's exactly where they expect to make their sales.

I don't agree that a carbon fiber wheel could be made at 8lb. If I was designing one, I certainly wouldn't sign any contracts agreeing to meet that goal. Maybe 12lb. (Carbon-Epoxy composite is about 30% lighter than aluminum). To get to 8lb, you'd have to make massive reductions in material which means either tedious directional lay-ups to use the fiber for maximum strength (which IMO would be well out of the $10k price point), or make it weak. And Carbon man aptly pointed out that in any case, wheel damage to carbon fiber isn't a dent or out-of-round - it shatters. So you would likely lose the whole car if the wheel failed at speed.

There is another advantage - overall the weight of the wheel is part of the unsprung mass of the suspension, and the wheel and tire have the same effect on unsprung mass. But your bigger brake package (if needed, due to the reduction in heat sinking) will eat that up quickly.

And just one last caveat: they mention weight savings, but they dont' mention what they are comparing to.

One last note: carbon _fiber_ is an outstanding conductor. (It's the same basic material as diamonds - only the diamond is a 3d crystalline structure and the fiber is a 2d crystalline structure - that's why it is so strong). But the epoxy that the fibers are contained inside aren't very conductive and reduce the system performance greatly. There are epoxies that conduct heat very well (e.g. graphite heatsinks), but I without extra research I have to assume they don't adequate strength for wheels.

Bottom line, with $3k for a good set of forged aluminum wheels, and $800 for a top-notch set of tires, you'll have another $6k left for massive power upgrades to the car. You could even buy a helium filling kit and save a little more unsprung mass. You'll leave the guy with carbon wheels in the dust.

Dave

carbon man

Graphite may be a good heat sink but Carbon Fibre is not. We use Carbon Fibre as a heat barrier for a lot of products we produce because of the lack of heat transfer we get from Carbon Fibre.
The resin to bond the carbon fibre together is another sience, we can produce hi-tensile products down to very maluable products depending on the resin used.
A hi-tesile resin/carbon products will shatter when broken but it does have a high strenght to weight ratio.
A low tensile resin can take high impact repeatedly without failure but does have a lower strenght to weight ratio.
I believe the biggest gain from a lighter wheel and tyre combination is the reduction in un-sprung weight, this directly relates to avalible traction (drive or steer)

Carbon Fibre was first made by Kodak by accident they found the residue from burnt photographic film had extremly high strenght, Carbon fibre is made from burnt accetate.

Ian.

jic

iTrader: (1)

stripling

i had no friggin' (been watching the new battlestar galactica too much!) idea the tire weighed so much :o

Q1: Why don't people just use the stock wheel for track? Are the sides too tall? If so can't you just get a 16" with 20 or 30 a/r sidewalls?

Q2: I am pretty sure someone said the weight of the wheels don't effect the stopping power of the car as does the traction of the wheels. The rub is this - the MORE the wheels/tire combo weighs the more inertial weight you have to stop. 3000 pound car plus 6000 pound of rotational inertial weight is 9000 pounds to stop an FD from 70 to 0. Take off 8lbs / wheel and that 9000 pounds sinks to under 8000 pounds. THUS the TRACTION on a given contant TIRE remains to the advantage of the lighter wheel. IOW, traction X stops 8000 pounds faster than 9000 pounds as 9000 pounds uses up the traction per ... foot, yard, whatever... faster than 8000 pounds. NOW if you upgrade your brakes and tires WITH the heavier rims then sure they'll perform the same, but stick carbons on that upgrade package and bada-boom... better stopping distances. I THINK this thinking is correct, no?

Q3: One and for all, do rims really transmit heat away from the brakes? I checked with a racer buddy of mine who told me "less than 1% on motorcycles, less than 3% on cars under brake-fade racing conditions"... is this true? if so, it's negligable!

Q4: Where the hell can you actually BUY carbon wheels? I find motorcycle carbon wheels mass produced now but cars? No joy thus far.

Q5: If you POLISH your stock FD wheels good and shiney, how much weight do they lose? Probably not even a pound

Q6: HELP! Isn't there something about "rolling resistance" being diminished with a larger (say 19" rim) than with a ... say, 13" rim? The question here is... why do people like bigger rims? a) bigger road-patch b) less rolling resistance and c) less wear on the bearings is what i was lead to believe... am i wrong AGAIN???

HERE IS MY SOLUTION FOR THE ULTIMATE DRAG CAR for the CARBON WHEEL problem: Use those go-cart CF wheels that guy posted about a page back I bet those suckers weigh NOTHING and the tiny diameter wheels will give you nothing but torque! Yes, you'll have to shift outta 1st at 7m/h but hey! I wanted torque, right?

stripling

OH CRAP

I forgot...

Q7: JUST HOW LARGE of a wheel CAN you put on an FD? I've seen 19" out there, but do they have to cut out the wheel wells to prevent rubbing or change the springs? I've got a stock-suspension setup on my FD, how big can I go? 375/45/19? OH! And at what point do the wheels stick out further than the fender in back?

stripling

OH CRAP x2

Q8: I saw an FD in Provo, UT a couple years back... it had STOCK 17" rims! My stupid rims are only 16" ... did they make 17" stock FD rims? his looked JUST like mine but 17" ... ya know... so, i stole 'em!

not! i wish...

you guys know what these rims are/were, how much they weigh and do they come in 18"

Q9: Can you get rx-8 rims in a different offset to fit the 7? I love those rx-8 rims i'm stupid like that

7racer

A1-9.....Search

fd_neal

iTrader: (4)

Stripling ill answer a few of the questions here...

1) ALOT of people use the stock wheels for the track. The ones who dont are either looking for the lightest wheel they can in a stock size OR want to run a tire wider than a 245. Also sidewall height has nothing to do with it, too short of a sidewall will hurt you just as bad as too tall a sidewall... really tho anything from a 30-50 series will be totally capable depending on width.

2) Can you lock your brakes up at speed, or more importantly threshold brake? I know i can (and so should any FD with brakes in good condition) therefore rotating mass isnt affecting your stopping ability. No matter how much a wheels rotating inertia is its still only a couple pounds per wheel to the overall mass of the car so if you can modulate brake power to the threshold then the brakes are overcoming the inerta, witch i should add is nowhere near 6000lbs.

3) Yes, a metal object is bolted directly to another metal object. there will be heat transfer.

4) no idea, they arent worth the money.

5) you wont loose any weight. maybe a few grams per wheel while you smooth them out. but nothing noticable

6) Why would a bigger wheel have less rolling resistance. Rolling resistance is determined by the tire not the wheel. if you have a stiff smooth rolling tire of the same width and OD then the rim size isnt gonna change the rolling resistance. On the other hand if you have a crappy tire that deforms alot under the weight of the car even when properly inflated then yea its gonna have more rollign resistance.

people go to larger rims for looks and or available wheel/tire sizes. Its pretty easy to find 285/30/18 tires and 18x10 wheels. try finding something that wide in a 16" wheel and tire combo... not nearly as easy as the 18's.

as long as you have the correct offset the bearing load will be identical to stock, however the heavier the wheel/tire combo the more wear on the bearings regardless of load, and larger wheel/tires are always heavier given that your shopping in the same market. ie a chrome 16x7 wheel will be heavier than a light weight 18x10. but a lightweight 16 will always be lighter than a lightweight 18.

7) Im sure you could fit 20" given you find tires, ive never looked nor do i care to. the widest you can fit without significant work is a 285

dgeesaman

iTrader: (7)

1) People don't use the 16's on the track because they are heat-soaking the stock brakes and need a bigger brake. To fit a bigger brake, you need a bigger wheel. The reduced accleration and changes in handling are much less important than the ability to stop at the limits of your tires. The bigger brakes don't reach such high temperatures and allow consistent performance throughout the track session. Note that in shorter events (autocross, drag) it's not an issue since the brakes don't get that hot - in these events the stock wheel/tire size is very popular because it's designed very well for this.

The tuning (ricer) culture has adopted this as a style, even though most of them don't get bigger brakes to take advantage of the change.

2) (This subject gets very complicated, fast. I'm not clear on your physics background, and I fear we will make no progress in this subject without that) The weight of a wheel affects performance in several ways:
a) Light wheel/tire travels over bumps more readily, improving handling
b) Light wheel/tire spins up and spins down more readily, improving accleration. Physics calls this property Moment of Inertia - it's not a rolling resistance, it's resistance to changing speed. It does not provide any practical improvement in braking, because braking is tire limited, not torque limited, which is FDNeal's point about threshold braking. The brakes should not be at their limits, but the engine will be.
c) Light wheel/tire reduces overall car weight, improving accleration and braking - just like anywhere else on the car, a 2800lb car is faster than the same car at 2820lb. (This is the classic F=m*a, where F comes from the engine, and m is the car's weight, and we want A to be bigger)

Stopping distance is not a significant factor in going to a bigger wheel. Factor B becomes irrelevant, and factor c) is very small (5lb/wheel is still only .7% of the car weight).

I think you're trying to compare two systems:
1) stock wheel, stock brakes
2) larger, lighter CF wheel, larger brakes, lower profile tire

In theory, you could use a CF wheel to allow more brake without hurting the weight and moment of inertia. However, it would take some very lightweight wheels, and very well-cooled brakes to do it. The cost, difficulty, and complexity of pushing this system are in the range of Formula 1. To do it at any lower level is currently an exercise in excessive risk or money.

3) There can be no once-and-for-all conclusion. Do wheel pull heat away from the brakes - absolutely yes. Is it signficant - I can't say for sure without doing a heat transfer study, and that answer would be specific to each application. Motorcycles generate much less braking heat than cars, so comparisons there are mostly irrelevant. IMO, if I was racing and a carbon-fiber wheel was presented to me as an upgrade option, I would at least do some tests to ensure the CF wheel did not increase my brake temperatures.

4) I have no idea. I've never looked, and in my limited knowledge I've only seen them on motorcycles. I don't think any race car wheel uses it. It's possible that for car applications, which see very high wheel loads and temperatures, carbon fiber remains inferior to a lightweight and strong metal like forged aluminum.

5) A few grams. It's inconsequential.

6) a) No, diameter does not change contact patch. Width does - we have left width out of this discussion up until now. Width has the same general tradeoffs as increasing diameter, except you're gaining tire grip.
b) Rolling resistance is more based on tire stickiness and drivetrain than anything else. I think you're confusing this with the increase in Moment of Inertia, which reduces acceleration. And bigger wheels have increased moment of inertia so that's not the reason.
c) The bearings will see less wear from shock loads because the car isn't going acceleratings as fast. Generally it's not an issue since any form of going faster (cornering, braking, acceleration) will put extra stress on the bearings and suspension.

People do bigger diameter wheels either because they think it looks good, or because their racing heat-soaks the stock brakes. The same with wider wheels - they think it looks good, or they've found it improves their lap or quarter-mile times.

Dave

KevinK2

regarding delivered power:

Stock 16" wheels are about 16 lbs. If you use 8 lb carbon wheels, you save 8 lbs dead weight at each corner, and another 5 lbs per corner of effective dead weight due to inertia. Total up savings, and it's 52 lbs. For a 3000 lb car with driver, that's like having 1.7% more engine hp or torque.

Rx8 wheels are same 50mm offset, but only 7.5" width.

------------

These are the epoxy - carbon fiber -aluminum cycle wheels I designed. Lance used them in most all his TDF time trials. For geeks, some of the carbon fiber had a 50E6 psi modulus.

http://www.hedcycling.com/wheels/hed3.php

dgeesaman

iTrader: (7)

Kevin - wow, neat product. I take it that these are built for stiffness, and not strength limited?

Dave

KevinK2

Yup, stiffness and weight. The wheel spokes are NACA tail airfoil shapes, 3/4" width. I wanted to at least match lateral stiffness of a standard (mavic-40) spoked wheel. Bicycling Mag featured them a decade ago, and had a pic of me with an FEA display of the wheel on my old dell 486 ... dark ages of FEA. I knew out PR guys would not mention me, just noting a cray computor was used .... but I had my name in big letters on the screen, lol.

sereneseven

ok i have to say a few things...First of all i've worked in profesional sports car racing for a while as a dampener tech and have seen a lot of crazy **** tried in the hopes of gaining tenths of a second...wheels are huge for more reasons than weight, the rigidity of a wheel can be its biggest improvement (a certian daytona prototype team tested some new wheels for bbs last year at watkins glen, between wheels alone which wieghed almost exactley the same they picked up 4/10ths sec.

I have not seen anyone one bring up the diference between motorcycle rims and car rims so i will...on a motorcyle the majority of forces applied to the rim are in line with the stongest part of it, the spoke. on a car rim the majority of forces are applied in a lateral way.... because of this a car wheel inherintly flexs more than a motorcycle wheel. carbon does not like to flex I have seen carbon tubs come apart in as little as three races.

maxcooper

I stopped using the stock wheels initially because I wanted wider tires. Now I have brakes that require larger-than-stock wheels, too.

Your mental model of this issue is flawed. If you can lock the wheels, the rotational inertia of the wheels is not a problem. You are correct in that the mass of the wheel adds to the overall mass of the car, but only in the 5-lbs-per-wheel range. It is no different than the same weight added to another part of the car. And for the money, you could shed a lot more mass elsewhere (titanium exhaust, etc.).

They do. I would be very surprised if it is only 3%. But if you are shelling out mad cash for carbon wheels, I am sure you could solve this problem with big brakes (risk of undoing the carbon wheel weight/inertia advantage here, though), ducting, spray, fluid circulation, etc.

-Max

carbon man

the weight reduction IS VERY important on "UN-SPRUNG" weight, for example when I worked on super-bikes some teams (including the one I worked for) would go to the expence of titanium hollow axles for a weight reduction of a few ozs. This weight reduction has no effect on inertia or torque, this has an effect on un-sprung weight only.
To control the suspension the lighter the un-spung weight is the easier it is to control the suspension and the more avalible grip from the tyre.

Another way to look at this is the more load/weight pressed down on a tyre the more grip becomes avalible up to the point the tyre is overloaded and the less load/weight is applied on the tyre the less grip is avalible, when we measure the amount of grip that is gained by adding a set amount of weight and we then meassured the amount of grip with the same amount of weight reduced we would see less grip.
So if we measure the grip level when the wheel is bouncing slightly the amount of grip avalible is a lot greater than if the wheel is bouncing lots. The lighter the wheel, tyre, brakes and susspension is the easier it is to control and the more grip we can achieve.
Most motor sport catagories have in there rules, a rule for the minimum wheel weight and this is usually set to a standard race alloy wheel.
I worked on a car team which got there 3 piece wheels made light, the spun rim was 1/2 the thickness alloy than the normal race wheel and after every race the wheels had to be re-rolled as they did slightly buckle with use, we did win the chamionship with that car again that year (4th year in a row). None of his competitors notice the light weight wheels, if they were carbon fibre people may have noticed.
We used carbon fibre brake rotors on the superbike before they were banned and it was not for better braking over steel rotors it was for lighter un-sprung weight and a small amount of gain from a reduction in inertia and gyro.
An easy test would be to put your tyres on the same size rim as you have on your car but heavier and see if you can notice any difference.

Ian.

stripling

max... i've enjoyed reading your posts. you're a serious minded fd buff and i respect that.

oh, and i loved your web-site... and i want your race wheels... i just don't know why you don't use those suckers as every-day wheels!

oh, and i want your suspension setup.

any suggestions on modern-day setups for wheels (like yours made most modern) and suspension?

stripling

ONE LAST THING about wheel weight and traction / intertia...

FACT: IF a 30 lb. 16" wheels spins @ 70m/h it's inertial value is 2400 pounds of force
FACT: IF you attempt to stop that vehicle the stopping values RELETIVE to the traction is (total wheel inertia) + (vehicle weight) or around 12000 lbs @ 70m/h
FACT: IF you lock-up your brakes the inertial value drops to 0 BUT you traction goes to HELL
FACT: IF you have super-duper brakes it will not matter if you try and stop that vehicle quickly 'cause your BRAKES are not your bottleneck it's your TIRE TRACTION
FACT: TIRE TRACTION is BETTER the less weight it has to fight, sprung or un-sprung

and most of all:

FACT: an RX-7 going 120 m/h WITH or WITHOUT anti-lock brakes stops 2.2 car lengths shorter distance WITH CARBON WHEELS than with stock wheels.

and, of course, fact: rx-7 with locked-up brakes stops the same distance with carbon as with steel or aluminum or flinstone stone wheels... which is over twice the distance of a controled, non-skid stop

so, all this arm-chair physics talk has been fun BUT... the fact is, carbon fiber wheels:

1) stop you faster
2) start you slightly faster but ROCKET you faster once you hit 80 on your way up!
3) manuver you better esp. @ fast speeds
4) save on fuel average 2-5%

disadvantages?

1) they won't take heat from your brakes at all (this is negligable since wheels don't really take much of the brake's heat)
2) they shatter on curbs or impact
3) they cost a hell of a lot
4) they aren't shinney like chrome or polished so they don't get slutty oriental panties off as fast - but, like, who cares... if it's not cauc-asian ... hmm, i need to think of something clever for that --- and!
5) they'll give other driver's errections when they see 'em if they know what they are... not good unless you drive a bright pink FD with tassles on the rear-view

stripling

yo mamma

FDNewbie

iTrader: (10)

They're the stock 99spec 17" rims. PM me if you're interested in 'em.

~Ramy

FDNewbie

iTrader: (10)

Well, thought I'd bring this back to life, because there's a company now offering a selection of carbon fiber wheels They range from $1600 - $2000 per wheel, and are somewhat lighter than the average wheel in their width/diameter. http://360forged.com/. They've teamed up with Dupont Registry, so I guess they must be good...

~Ramy

DaveW

Remember, that since CF wheels are dark gray, they will absorb heat from the brakes by radiation, possibly weakening them. Several F1 teams have had CF rear suspension failures due to excessive heat.

ptrhahn

iTrader: (9)

Not MUCH lighter... my 18x10" street wheels are 19.8 lbs. ... their 19x10" weighs 21.5 lbs. All things being equal, you usually see a 1 lbs increase in weight for inch of diameter, so it's really in the same ball park... and you can get lighter weheels than mine.

FDNewbie

iTrader: (10)

Peter, you quoted MY post in your reply...READ IT I didn't say they were "MUCH" lighter. I said "...and are somewhat lighter than the average wheel in their width/diameter."

I'll also add that (if I'm not mistaken), their wheels are pretty light for 3-piece wheels. My SSR GT3 18 X 10" wheels were considered "lightweight" 3-piece wheels, weighing in at a hefty 26 lbs a pop!

Heisenberg

iTrader: (3)

You can get the Mazdaspeed Mags for cheaper then that. Its just to bad they don't offer them in various sizes and offsets.

None the less those are pretty nice wheels. I just would never spend that kind of money on them.

XxMerlinxX

iTrader: (2)

Dymag makes true carbon fiber rims, not overlaid steel. I don't know pricing, but I know their 18's weigh around 14lbs. They're kind of ugly though, I'm not a big fan. System01 used to make carbon fiber wrapped wheels, at around $200 a wheel IIRC, but I don't think they're around anymore. "Carbon fiber" car rims are pretty scarce. There's too much R&R that goes into making a car wheel out of C/F, so that puts off a lot of companies. The price of carbon fiber also fluctuates greatly, especially as of late with Airbus, Boeing, and the US military buying up a bunch of it, so I would imagine that advertising such rims at a set price for any amount of time would be rather hard.