Shazaam's Tech Library - Carbon Wheels
A useful technical article from guest contributor Larry Kelly of San Diego CA (aka Shazaam!).
Whether the bling is worth the premium price, I’ll leave it up to you to decide.
One thing that impresses me is that BST states that their wheels "are suitable for street use." Marchesini on the other hand say that their magnesium wheels "are for racing applications ONLY and are not intended for street use."
So, in an era where anyone can sue everyone for anything, it's impressive to see a manufacturer like BST that is confident enough in their design and quality control to bring a product to market without trying to skirt the liability issues and cover their butt.
I also think that it's important to remember that the probability and safety consequences of hitting a super-pothole or a curb are the same for both a carbon fiber wheel and an OEM wheel.
So, if you're going to get too squeamish about this technology, you should probably do so BEFORE you get on a motorcycle.
Designing and testing a full carbon fiber wheel is a fairly sophisticated engineering problem. You need to use finite element analysis to accurately predict the stresses in the rim, spoke and hub area for all loading conditions. Worst-case load combinations associated with the weight of the motorcycle and rider, plus dynamic loads associated with combinations of acceleration, deceleration, rotation, turning and road surface conditions have to be directionally combined in a conservative analytical way, using appropriate safety factors, to come up with the initial design.
The main problem is, how to quantify the dynamic loads for the road surface. Do you design for the deepest pothole? At what speed? How do you, and who decides, what a standard pothole looks like? So you see the analytical dilemma. If you hit a big enough pothole, hard enough, you'll damage any wheel.
An equally important issue is the fatigue strength of the wheel, a measure of the wheels ability to survive cyclic loadings without developing cracks that will eventually lead to failure. A wheel is subjected to millions of cycles of the above loads in combinations that cannot easily be anticipated or analytically modeled.
The way around this analytical dilemma is to test the initial wheel design to an established standard intended to ensure a safe design under MOST road surface-induced impact loads. The two carbon fiber wheel manufacturers, Dymag and BST, certify their wheel designs for street use by passing dynamic fatigue testing to BS, TÜV and impact testing to JWL standards. The USDOT only requires only that each wheel manufacturer conform to the Tire & Rim Association standards to be DOT compliant. None of the magnesium wheels have these certifications.
With any new technology there's a number of unknowns. Magnesium wheels, early in their development, developed stress cracks adjacent to imperfections in the cast material and required regular examinations by their owners using dye penetrants (to reveal cracks) to assure that the wheels remained safe for use. Because of their current high cost, carbon fiber wheel experience is anecdotal at best within the motorcycle community, and I wouldn't interpret the manufacturer's (and their retailer's) silence as an indication that these wheels don't fail. But again, there are a several hundred sets of wheels out there so no news may just be good news.
I'm sure that the manufacturers have done their best to produce a good product. But what happens when you hit a super pothole? There's a lot of experience with magnesium wheels to know that you bend the rim to varying degrees, sometimes losing tire air pressure. Carbon fiber doesn't bend at it's strength limits, it cracks - sometimes catastrophically, so they may not show external signs of damage to warn of a later failure. Some owners have reported rim damage during mounting tires. What that means in terms of riding safety, I'll leave it to you to decide.
Balanced against all that, the amount of weight savings with carbon wheels over magnesium wheels is HUGE: about 3.5 lbs. for the front, 4.5 lbs. for the rear (Dymag vs. Marchesini.) The carbon wheels are as dramatic an improvement over magnesium as mags are to stock wheels. Better turn-in, suspension behavior, braking, and acceleration. Keep in mind too. that you can negate these benefits by choosing a heavier tire (or get the same benefit by choosing a lighter tire.)
With all that said, I bought a pair of Dymag sport magnesium that have a weight savings of 2.6 lbs. front, 4.6 lb. rear wheel weight savings over the stock Ducati aluminum rims. The handling improvement is akin to putting power steering on a car, and quite frankly, I have no interest in having even lighter steering. So for me, carbon fiber rims would be a waste of money. An unexpected downside is that my 916 is more susceptible to crosswinds and is more easily diverted by road grooves. Less gyroscopic force, and all that. A carbon wheel would be worse in this regard.
To me, carbon fiber makes much more sense for the track where there are few road hazards, and especially for race bikes where overall weight reduction is paramount. Although some race series initially prohibited their use, mainly because of high costs, BST carbon fiber wheels have been used in Moto GP for well over four years now and are currently approved for use for CCS and WERA racing. Since most new equipment innovation is first race-proven before being introduced for street use, I'd be still be cautious about street use, but carbon fiber wheels are definitely here for the track.
Remember that BST now has two versions of their wheels on the market: street & race versions. BST felt their street wheel was overbuilt enough that they could build a half-pound per wheel lighter version for racing with lighter spokes and outer rim. Knowing of course that race wheels, like in the case of magnesium wheels, often end up on the street.
In any event, the CF wheel manufacturers provide for extensive traceability of the materials and processes used to make these wheels, so make sure that you stay registered with them in case of a safety recall.
In regard to specific first-hand experience, the US sole importer of BST, motowheels states:
Both aluminum and magnesium wheels will get damaged on moderate impact. We, and everyone else, see this damage all the time. We sell a roughly equal number of magnesium and BST carbon wheels. In a sample of over 600 sets, we have only replaced one BST wheel for impact damage. We replaced over 25 aluminum and 15 magnesium wheels in the same 12 month period. All the BST wheels in North America came back through us, so I'm sure we have not missed any. I'm also sure there are other aluminum and mag wheels that did not go through us, so it is safe to assume that there are a lot more damaged aluminum and magnesium wheels out there to add to the total. moto
Carbon Fiber Wheel Failure
The question whether carbon fiber wheels are safe comes-up regularly. The manufacturer’s fabrication quality, testing standards, road hazard survivability, problems in inspecting for internal damage, and general suitability for road or track use all gets discussed. There’s a lot of anecdotal information out there but rarely does an actual failure get discussed. Here’s one case.
Accident Summary
“I was directly behind him. His wheel exploded at 120 MPH between
turn 7 and 8 at Thundehill raceway. The bike had never been dropped, dumped
or crashed before. BTW, the hoop is torn in half for anyone who hasn't seen
it up close in-person.” 900CR
Failure Investigation
I was not there when it happened. I found about this a few days after it happened and received a series of photos about a week and half after I heard about it. I'm not a composite expert nor am I an expert in accident re-creation, but I have spoken to a lot of qualified people that have seen the photos.
Rider error was ruled out as the rider was a very experienced track rider. Wheel was checked by the shop and deemed to be in good condition for the last year of intense track use. Records show the serial numbered wheel to be fault-free.
The engineers that I spoke to feel the tire lost air and caused the bike to lose control which resulted in the bike tumbling end-over-end at 120 mph+ (speed reported by rider).
Here is what they pointed out from some other photos that were submitted:
Axle around wheel nut is cracked from an impact. Energy can not pass through a broken wheel which indicates the bike sustained serious impact while tumbling and was likely intact at the time of impact.
No damage or marks on the underbelly, exhaust or deep sump oil pan to show the bike hit the ground from a wheel coming apart while upright (the deep sump is only 3-4 inches of the ground)
No marks from wheel on the inside of the swingarm or bottom of exhaust either.
Dirt is packed into the exhaust pipes indicating it hit on its ass end where it has no give at all (note condition of forks/triple clamp and front wheel). Witnesses say it packed about 6 to 8 inches of dirt in the pipe. The suspension and swing arm has no flex at this angle. A wheel with no air in it will have to absorb all the energy. It is obvious that it hit very hard on its ass end.
Spokes ends show no impact damage indicating it hit the ground. The paper thin outer cosmetic layer is still intact. It was not ripped off but was severed on the ends near the rim due to trauma of the outer rim from hard direct impact.
No marks on the caliper or rear rotor of hitting the ground as a failed wheel wheel on a upright bike would typically show.
There is no doubt that the wheel take a huge hit as the rim has a deep fracture
from a high energy impact with possibly no air in the tires.
Outer rim shows significant impact damage in one spot causing the distortion
of the rim to rip the spokes off.
No marks on the inner rim from rotor or caliper impact like you would see on a spinning wheel that came apart.
Owner indicated he felt the bike drop down momentarily and then going down before he can react to it. If the "wheel exploded" while upright, it should not have such a direct high energy impact on the outer rim, and would show more impact on the bike's undercarrage and surrounding area.
I did some research on the wheels:
BST is the most tested wheel by independent facilities than any other wheel. It was raced in MotoGP for nearly two years before it went on the market.
Wheel rim test exceeds 10,000 NM for impact.
Each spoke is tested to exceed 8,000 NM before ripping off laterally.
Rim Technology tested the BST wheel as stronger than an OEM wheel.
Wheel is fully TUV, JWL and DOT certified. Facility and production process is also TUV approved. BST is an aerospace company that builds CF helicoptor blades.
Each wheel is cured in autoclave for 5 hours and then tested before it moves on. Wheel is x-rayed at factory to show proper curing and lay up. Each wheel has several page history that can be traced through its serial number. This particular wheel was traced through the serial number and it showed no flaws.
Some people are assuming the spokes are "glued on the rim". This is not the case. The BST wheel is a one piece carbon wheel. The carbon fibers run from the rim to the spoke. The spokes are not laminated to the rim and will not tear off with less than 8,000NM per spoke. It would require a huge impact to break the rim off the spokes. It makes sense that the spoke would tear off with impact as energy does not like to make 90 degree turns.
It is unlikely that a wheel that could take all that testing could fall apart or explode going in a straight line. There is very little stress on a wheel when going straight. It is more likely that it came apart on impact.
We may never know what happened. BST has offered to send a composite engineer to the USA to examine the bike, the tire and the wheel. The engineers want to know as much as anyone. If there was some evidence that the wheel was at fault they would pull it from the market. So far they do not have any physical evidence of that.
I know a lot of people have reservations about carbon wheels and are quick to blame the wheels when they see the broken carbon wheel like that. Most people's experience with carbon is limited to the CF fender that they have on the bike. The fender and the wheel has about as much in common as an aluminum screen door has to an aluminum wheel. It is two entirely different processes and materials.
I think the owner of the bike has hired an independant party to find the cause of the accident. I have not heard any results yet. If any one is interested, I can share the findings with you.
Submitted by Motowheels
(sole US importer of BST carbon fiber wheels)
You can read the full thread here:
http://speedzilla.zeroforum.com/zerothread?id=36291&page=1
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