Wheels are only as strong as the components used to build the complete wheel. A motorcycle wheel needs an axle, wheel bearings, center spacer, seals, outer spacers and a hub (either cast or CNC machined 6061-T6 billet aluminum) to hold these parts in perfect alignment. If any of these parts are out of specification, or out of alignment, the central portion of the wheel will, at best, wear prematurely, and at worst, may suffer a catastrophic failure that will destroy the hub. Customers have brought me wheels with wheel bearing failures that have destroyed the bearing seating surfaces. I see 4-5 rear wheels per year that have the sprocket mounting lugs broken or shattered because they ran the wheel with loose sprocket bolts and nuts. Needless to say, replacing a hub is very expensive. A few notes on hubs: Cast hubs are lighter than billet hubs. Cast hubs are “strong enough” for the majority of motorcycle riders. KTM started machining the centers of their cast hubs in 2008 and many customers mistakenly assumed their hubs were billet hubs. Billet hubs are stronger but can be significantly heavier than cast hubs due to the forging process that creates the denser alloy billet blocks (vs. casting) which are then CNC-machined into the final hub. It should be noted that most aftermarket hub manufacturers use the same 6061-T6 alloy, so the only differences in hub strength are due to the hub design itself. Since the mass of the hub is concentrated near the axle, increased rotating mass is not as critical as the increase in unsprung weight from the heavier hubs.
Moving out from the hub, the next essential component is the spoke kit. Spokes are typically made from carbon steel with chrome plating, 302 or 304 series stainless steel, and recently Titanium. Carbon steel spokes are stronger and less expensive than stainless, but weigh 2-3 times more than Titanium. Titanium is incredibly strong, resists stretching and has a very stiff modulus of elasticity that causes it to spring back into its original forged shape. Plated carbon steel spokes are the least expensive, stainless steel a modest amount more, and Titanium spokes nearly double the cost. Quality spoke kits will run $65-$200 depending on metal. Add $10-$20 per set of powder coating. Just be aware that powder coating doesn’t stick as well to stainless steel as to carbon steel, and Titanium really doesn’t require coatings since another property of Titanium is excellent corrosion resistance. BTW, Titanium spoke kits are 8-10 ounces lighter that OEM spokes, three times stronger and run about $225-$250 per wheel. So if you are absolutely serious about minimizing unsprung and rotating weight, Titanium spokes are an incredible, but expensive, option.
Next inline are the nipples that thread onto the end of the spokes. Nipples are made from aluminum alloy, plated carbon steel, brass or stainless steel and come in square or spline drive but most fit snuggly in the rim holes. Rims are typically drilled for 6.4mm, 7.8mm or 8.4mm diameter nipples. Minis use 6.4mm nipples and big bikes typically use 7.8mm or 8.4mm (up front) and 8.4mm rear nipples. The nipple diameter is larger than the spoke wrench needed which can range from 5mm to 6.8mm. Nipples are typically $0.75-$1.00 each, but add $0.50 to $1.00 for color anodizing per nipple. Note that only alloy nipples can be anodized. In order to make a wheel strong, spokes must be long enough to engage most, if not all, of the nipple threads. The junction of the spoke, nipple and rim is absolutely critical to wheel strength and makes a huge difference between spokes that constantly work loose and ones that retain their tension for years.
And, finally, we get to the rim. Rims are made from plated steel or from 7000 Series “aircraft-grade” aluminum alloys. Contrary to popular belief, rims are not cast as one continuous metal part, but are extruded to form a spiraling “corkscrew” that is sliced in 360 degree sections to build a rim. The ends of the rim are compressed together and TIG welded to form the final rim shape. The welded seam is then ground and polished and then the rim is subject to the science of heat treatment. Heat treating 7000 Series alloys greatly improves the rim’s ability to resist bending, and when done properly, make the alloy behave a bit like spring steel. When done improperly, the rim will either be too soft or too brittle. D.I.D. and Excel have mastered the heat treating process and make (in my opinion) the best rims in the world. Some rim manufacturers tout their rims’ “hardness” as a key selling point. While hardness is important, rims that are not heat treated properly can be too hard and become brittle and subject to cracking – usually at the weld seam.
Excel makes several grades of rims. Starting with their OEM line and the Notako series, moving up to their Takasago line and topped off with the Takasago A60. DID also has three product lines, starting with the DID (Standard or OEM) Dirtstar, the Dirtstar LTX and capped off with the DID STX. Scout out the starting lines of any pro level Motocross event and you will find that 90% of the teams will run Excel A60s or DID STX rims mainly due to the extra strength of these two rims. For Supercross, folks are much more conscious of rotating mass, so you will see DID Dirtstar LTX, Excel A60s, and a few Standard Dirtstar and Excel Takasago rims.