The VAG dealer who sold me my car thought I was mad when I insisted, as part of the deal, that the fitted (perfect) alloy wheels were replaced with steel wheels as fitted to the base model.
What he didn't know was that I used to test these and other automotive components for a living. It would be hard to specify a less suitable material than aluminium alloy for a component which will be forcibly driven at kerbs and potholes at speed and which will be fitted to the hub by a means which will incur highly stressed regions in 4 or 5 places. If you bend this material once it's weakened, twice and it will probably fail.
I did get the last laugh with the car dealer though. The two front tyres were approaching the wear limit and I knew that no accredited tyre fitter would remount these tyres onto the new steel wheels. Sure enough, when I took delivery, the car had new wheels and two new tyres.
I can't understand the British obsession with alloy wheels - many up market German cars run on steel wheels in their home territory.
659.
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>>I can't understand the British obsession with alloy wheels
Yes, it's beyond me too!
Number_Cruncher
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>I can't understand the British obsession with alloy wheels
There's a section in my Volvo's instruction book on how to remove the plastic wheel covers, which suggests other markets are less alloy-obsessed than we Brits are. I don't think Volvo sells anything with steel wheels in the UK any more.
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Alloy wheels look good - thats why we like them.
Steel Wheels are for people who see cars purely as a tool from getting from A to B.
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>>people who see cars purely as a tool from getting from A to B.
And think that anyone who sees them as anything else has been well and truly taken in by the marketing people!
Number_Cruncher
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It would be hard to specify a less suitable material than aluminium alloy for a component which will be forcibly driven at kerbs and potholes at speed and which will be fitted to the hub by a means which will incur highly stressed regions in 4 or 5 places. If you bend this material once it's weakened twice and it will probably fail.
It's a good thing that they don't use this material to make, oh, er - aeroplanes - out of, then, or cars - Audi, Jaguar....
I can't understand the British obsession with alloy wheels - many up market German cars run on steel wheels in their home territory.
I assume that they are used to reduce the unsprung weight, however I don't know whether they are lighter than steel wheels by much - anyone else know - 30%? There are also some benefits in terms of slightly better acceleration/braking, and fuel economy.
In our climate, they have various disadvantages, too, e.g. being attacked by the salt that's all over the roads in the winter.
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I don't know whether they are lighter than steel wheels by much - anyone else know - 30%?
Sometimes they are heavier. Difference is seldom great.
There are also some benefits in terms of slightly better acceleration/braking and fuel economy.
Are there? Please explain.
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>> There are also some benefits in terms of slightly better acceleration/braking and fuel economy [using light alloy wheels] Are there? Please explain.
"Flywheel effect". Look at this page: www.pumaracing.co.uk/FLYWHEEL.htm - see section entitled "Other Rotating Components". Basically, a wheel is a store of energy. You need to get it going (less flywheel effect=better acceleration), and stop it (less flywheel effect=less for the brake to stop).
Of course, at constant speed, there'd be no difference from any flywheel effect whether tou were using traction engine wheels or racing bicycle wheels!
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>> Are there? Please explain. "Flywheel effect".
Based on the assumption that alloy wheels really are lighter in weight than steel ones, then.
And actually Fothers, what one might call extremely marginal, too small to measure, and therefore not worth mentioning except wearing one's special illuminated anorak... I agree though that most of us have one that we bring out from time to time.
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Yes and no Lud. During the performance prediction stage of vehicle design, it is fairly standard practice to take into account the rotary inertia, and, usung the **gear ratios and rolling radius, convert it into an extra ficticious mass, which is added to the mass you would measure by putting the vehicle on the weighbridge. This "effective" mass is then used in performance calculations, massively simplifying them by avoiding the need for always casting the equations in both linear and angular form, and solving simultaneously.
However, as the mass and the distribution of mass is remarkably similar between a steel wheel and a "standard" alloy wheel, there's also a vanshingly small difference in effective mass, and hence vanishingly small changes in vehicle performance.
** As you work back through the drivetrain, you multiply the inertias you find by the gear ratio *squared*. So, in first gear, the contribution to effective mass from the engine is really large, and the difference between weighbridge mass and effective mass can be significant. Or, in other words, lightened flywheels only make a significant difference at low speeds.
Number_Cruncher
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Or in other words lightened flywheels only make a significant difference at low speeds. Number_Cruncher
Lightened flywheel doesn't make much contribution to acceleration (although since cars have to go at low speed to reach high speed, any difference helps), but does make quite a difference during downchanges as the engine blips much more quickly.
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Yes, I think you are right - during a 0-60 dash, you don't spend long in first gear anyway, so an imporovement here from a lightened flywheel won't make a great difference to the time.
Flywheels, or at least the thought of them often frighten me. The concept of a large heavy spinning disk with sharp pointy teeth at its edge contained only by a minimal alloyhousing.
Before going anywhere near one with a milling cutter, I would like to have a good set of calcs, or perhaps an FE model which shows that the stresses are still OK! I wonder how many lightened flywheels being sold have been so assessed?
Number_Cruncher
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an average alloy wheel weighs in at 5kg for a run of the mill car
larger family type saloons are usually nearer 10 kg
they therefore on average weigh as much as a steel rim
as one of alun sugars apprentices says "alloys mean nothing to me"in the context of cars,especially when they are scuffed
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>>It's a good thing that they don't use this material to make, oh, er - aeroplanes - out of, then, or cars - Audi, Jaguar....
It's got nothing to do with it.
The loading environment for an aeroplane is nothing like that of a wheel. The required form and function are completely un-related. The aeroplane is subject to a strict maintenance regime, which can include detailled crack detection. Car wheels are subject to a visual inspection by someone untrained in structural engineering or crack detection once a year. A comparison between an alloy wheel and an aircraft structure is completely invalid.
Again, an aluminium car body can't be compared to a wheel. The car body has diffuse loadpaths, and, doubtlessly there are careful procedures for repairing impact damage (which probably make aluminium bodied cars very expensive to repair correctly).
Aluminium responds in an awful way to impact damage, particulalrly local impactsbecause it doesn't respond predictably in the plastic region. Mild steel on the other hand could not be more benign.
In most cases, there are no (or tiny) reductions in unsprung mass - only alloy wheels which are built to be light and have the matching price tag are significantly lighter than steel wheels - the "standard" alloys are virtually the same mass as the equivalent steel wheels. There are no benefits therefore from reduced rotary inertia, and so acceleration and braking are similarly unaffected, as is fuel economy. There may be a small advatage in allowing better cooling of the front brakes, but if you are pushing your car this hard, then other modifications may also be required for safety!
Alloys do offer better control over tyre location, as they are a machined item as opposed to a fabricated steel rim. But, unless you have an in-curable wheel vibration problem, this advantage is rather academic.
Ally wheels are a triumph of style over content, of marketing over engineering, and I don't regard them as an item to be particularly sought after.
Number_Cruncher
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What he didn't know was that I used to test these and other automotive components for a living. It would be hard to specify a less suitable material than aluminium alloy for a component which will be forcibly driven at kerbs and potholes at speed and which will be fitted to the hub by a means which will incur highly stressed regions in 4 or 5 places. If you bend this material once it's weakened twice and it will probably fail. 659.
So I assume you know that their are literally dozens of different alloy specifications then & as someone points out aircraft and various other important items are made from them.
Also the Germans who are one of the most anal on the planet, test alloys fitted to the German cars & they are happy as are insurance co's etc.
Another point is that alloys by breaking very often save more expensive suspension components & the structure they connect to, thereby REDUCING insurance claims & the chances of writing off the car.
Also most Club race cars run on alloys as well every weekend with no problems.
They look vastly better than steels & wheel trims, generally are more precisely round, most important on the bigger sizes used today & also give more break clearance on some.
Paranoid, I thinks so. How much have you killed the resale of your car by having a mdel normally delivered on alloys, having steels instead!
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And I did mean Brake clearance before the pedantic spelling police get going!
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I refer you to my honourable colleague above - couldn't have put it better.
659.
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Hasn't answered a lot of my points actually!
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O.K. simpleton's question time:- if there is little difference between the weight of steel and the various aluminium alloys used in wheel manufacture, then why don't they (car makers) make their wheels from steel to the same designs and use something like a powder coating to provide the finish?
S6 1SW
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why don't they (car makers) make their wheels from steel to the same designs and use something like a powder coating to provide the finish?
... especially since many alloy wheels are "painted" anyway....
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The marketing men have been using "alloys" as a selling tool for years - it's become part of car folklore for the uninformed.
The Trades Descriptions Act would kill this stone dead if anything as sensible as powder coated steel were to be used for road wheels. It would also be difficult (and undesirable) to try to replicate the design of a casting in a pressing.
659.
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I have had steel wheels repaired in the past. Most repair shops had a machine with rollers that nicely turned the rim back to true. Then they did a nice respray so that it looked like new.
The last time was for a lady family friend, new driver, who had hit a kerb, buckling the rim. I took it into a repair yard and had it trued, sprayed, and a new tyre fitted. The next time I saw the car the lady had taken it for servicing and the garage had substituted an old wheel with an almost worn out tyre. It reinforced my predjudice regarding car mechanics.
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As noted above, anyone with an elementary knowledge of metallurgy will tell you that aluminium alloy is just about the worst metal to use for vehicle wheels. Its may be good for certain aerospace applications, but its horses for courses I'm afraid.
As regards the German alloys, I can tell you I've seen plenty of bent and buckled alloys on BMWs and Mercedes. They often buckle or 'flat' on the innerside of the wheel. The spokes are usually right out at the outer side of the wheel, leaving the inner side unsupported.
The joke is of course that the 'big' alloys that are currently in-vogue are usually heavier than the smaller diameter equivalents. Thus a particular Enkei alloy pattern I have in mind weighs 17.5llbs as a 16" wheel, but over 20lbs as a 17" wheel. Add in the fact that lower profile tyres are dispropotionately heavy (because they have to have reinforced sidewalls) and your 17" wheel/tyre combo is significantly heavier than a 16" equivalent of the same running diameter - I would put this at around 40lbs as against 30lbs - i.e. 25-30%, depending on tyre profile etc. The increasing tendancy for numpties to put 18" or even 19" wheels on their cars (cuz they luck kool!) would make this weight increase even more drastic. This is significant not just because of unsprung weight, but because of rotational intertia. Very crudely put (without me doing all the calcs) a 1kg increase in wheel/tyre weight is equvalient to a 5-8kg increase in chassis weight. Multiply that by four and you have a significant (i.e. measurable) impact on acceleration of the vehicle. This is quite apart from any increase in unsprung weight.
A pressed-steel wheel is probably the best option in terms of strength, durability and weight, and these can be made to look quite good (anyone remember 'Rostyle' steel wheels?). However, as with many trends in the car business its all fashion led. You can't beat the physics though...
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>>literally dozens of different alloy specifications
If not hundreds, or even thousnds.
>>as someone points out aircraft.
Yes, alloy wheels are used on aircraft. An ex-colleague of mine spent quite some time helping a company overcome huge problems with fatigue failure of their built up alloy wheels. These wheels were subject to fatigue failure initiated in the area of the through bolts, and as such, these wheels still remain very much life limited items. Every time a wheel rotates under load, it is subject to a fatigue cycle. Steel is a benign material in that at low stresses, the fatigue life is infinite. This is not true for alloy wheels - they *will* fail at some point.
The expensive wheels used for exotica and for motorsport do offer superior performance when compared with standard wheels. However, most punters buying a Fiesta or Corsa might not be happy paying those prices.
>>Also the Germans who are...
I wouldn't be so fast to generalise about German engineers - some are almost human!!! ;-)
Yes, it is true that failing alloys aren't filling the graveyards en masse. To make them anywhere near strong enough, "standard", affordable alloys end up being just as heavy as steel wheels. We have seen the results of catastrophic failure of alloys on this site recently - see the thread posted by BobbyG for an example. A steel wheel will buckle, and in doing so, it will absorb energy. The brittle fracture of an alloy does not absorb energy, leaving it to be absorbed elsewhere - it protects nothing.
>>Also most Club race cars run on alloys as well every weekend with no problems.
Why? I'm not sure what the benefit over steel would be?
Number_Cruncher
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