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      05-01-2009, 09:58 PM   #68
Cleveland
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Drives: M3 coupe
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Post The answer is a little complicated...

Quote:
Originally Posted by GregW / Oregon View Post
Do you have any clue as to how a 1" wider rim could only be 0.2 lb heavier? Very strange. Are the centers identical?
Greg,

I can explain this phenomenon for you from a manufacturing perspective...

The front (19x8.5) and rear (19x9.5) wheels are using essentially the same CENTER, combined with additional CNC milling on the rear wheel spokes, (separate from the "back milling" on the outside of the spokes), that yields an overall weight that is almost identical to the narrower front wheel. For structural integrity reasons, the outer rim (barrel) cannot be used to shave off weight on a street car applications. It has to come off the more durable forged center section, which has a few key areas that can be milled without sacrificing the structural integrity of the wheel.

A solid 3-D CAD design, innovative manufacturing and engineering refinements, extensive FEA stress analysis simulations, and impact and radial load testing of physical prototypes are required to make this happen.

This is assuming a company actually HAS a professionally-trained CAD (Soilidworks) wheel designer, and a Structural Engineer who has experience in the wheel industry. Don't fool yourself into thinking everyone has these tools at their disposal.

Remember this...

The weight of an aftermarket wheel can be manipulated in a number of different ways.

Some manufacturers use sound Engineering principles and innovative techniques to reduce the weight of their products. (without sacrificing quality to any great extent) Other manufacturers take 'calculated risks' to reduce their wheel weights and cap their overall costs. (USING THIN SPOKES AND OUTER RIM BARRELS FOR EXAMPLE)

A low-offset no-lip aftermarket wheel will have a modest impact on the final weight, because the ends of the spokes go all the way to the outer edge of the wheel barrel. This adds additional MASS...

Note how the LM-R wheel spokes are almost perpendicular to the inner barrel. (virtually straight up and down) You could put a straight edge on that wheel center and see almost no slope angle. The LM-R also has a step lip in the 20" sizes. The spokes are also very short. Essentially the same centers are used in the 19", & 20" diameter wheels. The outer barrels are step-lipped on the larger diameter 20" rims...because they have to be. (to make up the difference in size) The 19" wheel would of course have no step lip.

But the actual LM-R wheel center is really only 18" in diameter, so a bit of creative engineering is at play here. The outer edge of the center section has a 1/2" gap all the way around the wheel.

Technically...this is not a reverse lip or a step lip wheel. It's a "tweener".

1/2" + 1/2" on either side of center bore = 1" total

That is how you get a 19" wheel (using an 18" center) WITHOUT using a step lip...



This is not necessarily a bad thing, just making a point here. Not every wheel diameter has a dedicated forged center section.

Every wheel manufacturer uses a "trick" or two in order to mass-produce modular (2 or 3 piece) wheels. They are somewhat forced to do this, so they can offer all the different designs in their catalog to the largest number of customers. Forged centers are usually good for a 2" diameter spread. (max)

Anything more than that...and the smaller center makes the whole wheel look TINY.

It's prohibitively expensive for wheel manufactures to build a dedicated wheel center for every single size diameter they offer. (17", 18", 19", 20", etc.) As you can imagine, this would cost a small fortune. And only the very wealthy among us, would be able to buy these products. This is a standard industry practice to reduce forged wheel manufacturing costs.

Again, just clarifying a point. None of this has any bearing on the overall quality of the wheel, if sound Engineering principles and high-grade materials were used in it's construction.

The lipped wheel or 'step' lipped wheel usually has a center section that is basically FLAT (little or no concavity), and the overall weight of the wheel is reduced due to the shorter, flatter recessed spokes.

Flat recessed wheel spokes = shorter distance = less material = less weight. (generally speaking)

These are small but important details, that are often overlooked by the average wheel buyer...and even some retailers.

There is nothing earth shattering about all this...it's just rarely noticed by the general public.

The entire wheel weight debate is a bit overdone on internet message boards to be perfectly honest. The weight of any particular wheel is certainly not the most important detail, but I do understand why consumers harp on it to a certain degree.

The truth is...it's very hard to compare two wheels from different manufacturers that do not share the same size, width, offset, raw material grade, material construction techniques, and overall design. Many times you end up with flawed data comparisons that aren't based in reality.

It's unfortunate that so many aftermarket wheel enthusiasts focus on wheel weights, like it going to kill them to buy a wheel that weighs .75 pounds more than a competing product. That's going overboard in my opinion...unless you are building a dedicated track car where every ounce of weight counts.

Yes, weight is an important part of wheel purchasing decision, but some individuals place wayyyyy too much importance on this one aspect of wheel manufacturing. The lightest wheel on the market, is not, and cannot, be the strongest wheel on the market. You can't have it both ways...

A little more MASS isn't necessarily a bad thing sometimes. (in moderation of course!) It also buys you a little more security in the long run. (my opinion)

I would just like to see a more 'common sense' approach to this subject in the future. It's important to keep things in there proper perspective guys. That's all I'm trying to say...

Cheers.
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