Steel vs alloys wheels

[Harvey]

Hi, long time lurker first time poster. Thought I'd jump in the deep end with my very first post being a new topic!

So, I've just read a news article about Fords new eco-diesel Focus. Quite impressive claimed economy and CO2 figures for such a "normal" car, I'm sure you'll agree. Here's a link to a story about it:
http://www.autocar.co.uk/News/NewsArtic ... ?AR=256396.

Looking at the pictures, it's obvious they've fitted those nasty looking plastic wheel trims to aid the aero (just like VW has done with their bluemotion line). Here's where my knowledge starts to blur, and my question starts to form though. I initially thought "Oh, they've fitted steel wheels to reduce weight" but that didn't make sense in my head.

So, that's my query really: on a car like this (ie, a normal road car that will barely see 100mph let alone a circuit) are steels or alloys more beneficial with regards to unsprung mass and brake cooling? Is brake cooling even an issue on a family car? And, on that note, why don't manufacturers fit something akin to Lotuses solid wheel from the 60s:
http://www.lotusespritturbo.com/Lotus_Type_24.jpg
With a minor redesign could that not be as "aero friendly" as a plastic trim on a normal wheel, with the added benefit of looking slightly nicer than a glorified coaster?


Cheers, Harvey

PS - before someone starts quoting complicated formulae and engineering speak at me, I must say that I am not an engineer even in the slightest. I have quite a mechanical mind and love all things that involve motors, but I am in fact a medic. So please don't bite!

[Just_a_fan]

I doubt a typical road car alloy is much lighter than the steel wheel equivalent. Sure, in some cases e.g. extreme sports cars, they are but they will use more expensive alloys/processes to do so.

Alloys on family road cars are more about style and marketing than anything else. They add cost but little of any real engineering benefit.

As for brake cooling - in a car aimed at the economy-focussed driver, I'm not sure brake overheating is an issue.

[Richard]

Volvo claim that this wheel on the C30 is designed to reduce drag. The press release claims total 10-15% reduction due to the rims, but there are a host of other improvements such as smooth underfloor and radiator that are also part of that 10-15%.

http://www.volvoclub.org.uk/press/relea ... b120.shtml

Reduced air resistance:

- Chassis height reduced by approximately 10mm to help reduce drag

- A front spoiler on the S40 and V50 which is the same spoiler currently on T5 models

- Covered radiator grille. Behind the characteristic Volvo grille there is a wind-deflecting panel that provides better aerodynamics inside the engine compartment

- Wind deflectors in front of the front wheels to steer the airflow

- Aerodynamically optimised wheels with a unique ‘Libra’ rim. The diamond cut finish adds to the unique design and the large unobstructed area that goes all the way out to the tyre makes the rim look considerable larger than it actually is. The total drag reduction of 10-15% is due to the design of the Libra rim/.

- Underbody panels on the Volvo C30 for more efficient airflow under the car

- A unique rear spoiler has been developed for the Volvo C30 which adds both to the aerodynamics and to the visual appearance. The Volvo S40 features the same ‘ducktail’ spoiler as found on the current T5 and D5 models

- New rear bumper on the Volvo C30

http://www.volvoclub.org.uk/press/relea ... b120.shtml

[Richied76]

This is quite an interesting subject for me. Makes me think back to a product Vauxhall did where they cast a steel wheel as per an alloy and then covered it in a plastic cover to make it appear like an alloy. Looked bloody horrid but they had the extra strenght, extra weight to!

Anyway, i think "alloys" are there for one purpose only on regular cars...to look nice. If they are anything other there always on a special "bluemotion" or "eco" model. Thats why F1 being made to go green is such a joke. Manufactures are only interested in doing half a job because they dont want to go all out and make there whole ranges eco friendly.

To your point though harvey I think that cooling isnt such a problem. steel OR alloy they only major influence is the way it affects aero, but i also think the major issue is the wheel arch themself rarther than the wheel

interesting 1st post my man

[alelanza]

Well an alloy 'should' be lighter, thus giving you a more compliant and safer overall suspension