Thursday, February 23, 2006

T#18: Big wheeling

You might surmise from this it's proving to be a long week. And in case I don't get time to mention it in it's own post (a very real possibility at the moment):

I'M OFF TO THE GENEVA MOTORSHOW ON MONDAY.

I have press accreditation and everything - very excited. Ahem. Anyways, thank you.



Big wheeling

Ok, ok – so I know this has, in fact, been Done To Death. But. I figured, what the hell, stick an oar in – my site, my opinion, and all that. Plus, I have a final few nuggets of information gleaned from “Keith” the Jaguar engineer.

So, wheels. Alloy wheel – big ones. We all know how good they look – which is why they end up on the cars in the first place (via the design sketch. And the marketing department). We also all probably know a number of detrimental things about them, right…

Contrary to popular blaster belief, bigger wheels generally do not improve vehicle handling – more on this in a moment, but basically, they add weight and remove ride comfort. In case that requires further explanation: more metal means more mass for the suspension to control, and; lower profile rubber reduces the bump absorbency added to the ride by the pneumatic cushion that doubles as the tyre. Changing the rolling diameter (i.e. by upping the wheel size) also impacts on the accuracy of the instruments, and is probably catastrophic for the environment, too. I’m kidding about that last part. But, Hey! They look good.

And this is the thing – they do look good. So where’s the sensible solution, or compromise? Magazine’s (Evo’s a good example, but they’re not alone) are increasingly flagging up the difference the optional increased alloy-size makes to the ride comfort of the vehicle they’re testing. Off the top of my head, this has been a notable issue for the last generation BMW M3, and the new Mini, though Audi hasn’t escaped criticism either (and we all know how fantastically exciting those look running the maximum possible rimmage). What exactly is the excuse for this?

There is a matter of expense, but why isn’t this a problem of the past? The weight issue is resolvable – better materials and improved construction can make larger diameter wheels at least as light as their smaller circumferenced brethren. Lighter weight means improved efficiency – the cost differential will pay for itself. Hold on, you’re thinking, wasn’t that VW’s reasoning behind the 3Litre car – and exactly how many people bought that? Basic error in that instance: the 3L was a stripped out Lupo, whereas the alloy is all about style. People will pay for this – especially if you frame it as a performance enhancement.

As for the ride issue, surely suspension technology is sufficiently sophisticated by now that taking the bounce out of the tyre isn’t an impossibility? In fact, wouldn’t minimising this variable actually improve ride control? Take the inconsistency – which will change with type and brand – out of the tyre by making it as small a part of the ride equation as possible; if the tyre variable hardly exists you can tune the suspension to act with greater accuracy and consistency. Or maybe I’m just talking nonsense. I suppose at least the local alloy “reconditioning” service will thank me the first time you curb the dubs on your Kia Picanto…

Alright, that’s enough from me; time for some thoughts from “Keith” on this subject.

First up, he was very keen to stress exactly how much tyre choice has an impact on a car’s handling. Those four little tiny contact patches – the rubber you use for those probably make more difference to the way your car behaves than anything else. Car manufacturers specify a particular tyre for a reason, because it best suits the ride/handling balance of their vehicle, in relation to the market it’s aimed at.

However, it goes further than that, because often the car maker will specify a tyre to its own OEM requirements. Meaning that the tyre you buy off the shelf at the local fitters most likely won’t be exactly the same as the one the car rolled out of the factory on, even if it is wearing all the same badges. Apparently, this doesn’t often come down to differences in compound but car companies can and will ask for sidewalls and banding to be altered to the benefit of a particular vehicle. Is this cheating? It certainly seems a bit pointless if the tyre that makes your car work best is not going to be easily available later on.

He also hinted at how engineers are put under pressure by their marketing department in order to get cars to accommodate bigger rims and wider tyres. This comes back to the public perception that bigger wheels not only look better, but go better too – the car rides harder, so it must be sportier, right? And as for the tyre width, while it is obviously worse to under-tyre a vehicle than to over tyre it, there are circumstances where wider tyres can actually reduce grip instead of improving it. Over-tyring a car can wreck the handling, inducing understeer, tramlining and general lack of pointiness. March’s Octane magazine has an article on the early 90s Corvette, and it notes how the width of the front wheels were reduced over its lifetime – starting at 275 section, reducing all the way to 245.

So, in short: consumers, stick to the manufacturer’s recommended boots; manufacturers, get around to engineering a proper solution to such stylistic excess – go on, make us all happy.


Links:

T#16: Driver aids? [internal]

Driver Aids? Post Script [internal]

Evo magazine

Octane magazine


Picture, by pure coincidence is via a post made by Jalopnik today. That car is currently for sale on eBay. Click here for more photos...and even a short video. I daren't say anything else.

1 Comments:

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