Is bigger always better?

So why am I boring you with all this introspection? Because I am about to discuss a subject that may at first seem to be yet another ‘middle-aged-bloke car enthusiast yearns for something from the past’. I apologise in advance if it comes over as such, but I assure you that the piece is inspired by real technical frustration, not by mere nostalgia. The subject in question is the seemingly unstoppable increase in wheel size…

Right, let’s get the obvious out of the way quickly. You know the score already – big rims and low-profile tyres kill ride comfort, but they look as cool as hell. Everybody knows this, hence you can be forgiven for thinking: why trouble us with it all over again? Most customers vote with their wallets, buy the big-rim option and either don’t know about the degradation in ride, or do know, and simply don’t care.

But let’s go a little deeper than this well-known trade-off and look at some other vehicle attributes affected by your choice of wheel size. We will see that, with one important exception, larger wheels have a detrimental impact on all other performance aspects of a road car.

First off, mass. Besides killing the ride by limiting effective suspension stroke, large wheels and tyres add mass. Unsprung mass. Not good – for ride or for handling. As this mass is constantly turning and burning, it makes a huge difference to dynamic behaviour. Try zigzagging like a rugby winger at full sprint while wearing hiking boots, and you’ll see what I mean.

Big wheels also increase the bonnet height of a car. Most front axle designs need space above the top of the tyre to house suspension and/or structural members. Large wheels and tyres drive bonnet heights up – making for very bluff, truck-like front ends. Among other evils, this complicates pedestrian protection…the unfortunate pedestrian’s head tends to hit nasty hard stuff like engine heads or strut tower tops instead of (relatively) soft things like the sheet metal of the hood or the windscreen glass. And yes, you are better off hitting glass than an engine casting, trust me.

But surely big wheels have some benefits, right? Don’t they have lower rolling resistance? The (pedal) bike fans among us have some inkling of this. Road bikes have used large, skinny, 700mm (28-inch) diameter tyres for many years. Mountain bike tyre diameters have grown in recent years from 26 to 27.5 to 29 inches – driven by the claim that these bigger wheels ‘roll better’ – i.e. have lower rolling resistance.

That kind of feels intuitively right, doesn’t it? And when you consult The Big Engineer’s Book of Hard Sums, the theoretical formulae for RR (rolling resistance) would seem to confirm our intuition – depending which vintage of The Book you have, you’ll find definitions stating that RR is inversely proportional to diameter, or even to the square of the diameter. So, in theory, that 20-inch optional wheel should roll between 10 and almost 20 per cent more efficiently than the entry-level 18-incher.

Except it doesn’t. Disappointingly, the maths, in this case, don’t adequately represent the complicated reality where rubber meets the road. In her recent article on tyres, Joana Fidalgo pointed out that on paper, a tyre’s lateral grip is independent of its width. But she also explained in a beautifully simple way that in reality the maths doesn’t work, and wider tyres do indeed give more grip.

It’s a similar case for rolling resistance – while a larger-diameter tyre should in theory roll more easily, tests of pneumatic tyres on relatively smooth surfaces like asphalt consistently show that it’s simply not the case in the real world – the variation of RR with diameter is negligible, all other factors being equal. Only in edge cases can we measure a reduction in RR with increased diameter – one such case is that of a non-deformable wheel on a non-deformable surface (think train wheels).

Another is the case of the mountain bike or off-road vehicle, where a relatively soft tyre is rolling on very rough ground, with obstacles like rocks and roots. Here, the increased diameter provides more obstacle-clearing ‘leverage’, and genuinely does reduce effective rolling resistance.

So, sorry folks, but big wheels sadly do not make for efficient vehicles – which is why I cringe even more than usual every time I see EVs riding on unnecessarily tall boots.

Speaking about grip: as Joana explained, wide tyres do indeed give more grip. But do we really want more grip? Many of us feel that we are past ‘peak grip’ – modern tyre performance is so good that the limits of lateral mechanical grip are reached at speeds that are frankly no longer responsible on our crowded public roads. So I’m going to suggest the following little equation: wide tyres = more grip = less fun = bad.

While we are listing offences related to excessive tyre width, let’s mention turning circles. Most modern cars have widely spaced front side members for various reasons, including maximising powertrain package space, crash load path management, and ‘frunk’ space improvement. Here’s another of Twohig’s chassis equations for you: wide front side members + wide tyres = limited road-wheel steered angle = crap turning circle = bad, again. Like good ride comfort, small turning circles are something we’ve almost forgotten about. We only remember how bad the turning circle is on most modern cars when we get into something like a Renault Twingo or VW ID.3 and are reminded of how manoeuvrable cars should be.

Okay, time for the argument for the defence of big wheels. There is only one technical advantage for road cars – brake size. Bigger wheels accommodate bigger brake discs, and allow them to be cooled more efficiently. That’s it. End of the case for the defence.

Hang on, hang on, you may say. Maybe there is only one technical advantage, but surely we’ve got to admit they look good? I will not make the error here of commenting on design (we’ll leave that to the experts like Julian Thomson) but I will just mention the original Lamborghini Countach and 15-inch wheels. Having pulled the pin out of that particular hand grenade and rolled it under the table, let’s move on, quickly…

I’ll stop here with the litany of evils associated with oversized wheels. I have not even mentioned the exorbitant replacement cost of large, modern tyres. Nor have I reminded the dear reader of the teeth-grating scrreeerrrrssshhhhkkk of a pristine new alloy rim kissing a kerb, unprotected by its fashionably low-profile tyre. Nor do I have space to address Colin Goodwin’s recently expressed wish for a steel wheel comeback by pointing out that pressed steel works best on smaller diameter rims…

What’s my point here? Well, the point is that I do not think that we have to resign ourselves to this trend, nor accept that we simply have to add reasonably sized wheels and tyres to the list entitled ‘good things past, for which we mourn’. There are precedents to show that excesses that do not deliver lasting benefit to the customer can and do die out. The fins and chrome of the Sixties died out. When was the last time you saw a Seventies-style vinyl roof? Or one of those cool digital dashboards of the Eighties?

The automotive industry can repent of its sins. Logic can win out, and negative trends can be slowed, stopped, even reversed, especially when they are not in the end customer’s best interests.

So let me finish on my usual optimistic (naïve?) note. I’m not just waxing nostalgic about past times and dinky little 10-inch Minilites here. I genuinely see hope of a new generation of engineers and designers, driven by the search for overall vehicle efficiency, rediscovering the customer benefits of smaller wheels. I see future cars having smaller-diameter wheels, narrower, higher-profile tyres and even (yes, Colin!) steel rims. And designers will find ways, I am convinced, to invent new visual languages to make these vehicles look cool as all get-out. Roll on the future, I say.