Space invaders

But – strap in, Andrew, here we go – there is one aspect of a car that seems purpose-designed by the automotive gods to put even the most equable designers and engineers at each other’s throats. It is the wheel-fender gap – that gap between the top of the tyre and the wheel arch opening. Unless you drive a high-riding 4×4 of some kind, I can almost guarantee you that this gap on your car was the cause of heated arguments between the designers and the engineering geeks.

I myself may even have been guilty of harsh words and finger-jabbing once or twice on this subject. I will draw a veil over the project(s) in question, to protect both the innocent and the truly guilty. But I will have to plead guilty to having raised my voice and said some very nasty words about the clearance – or lack thereof – between the tyre and the lip of the wing, or ‘fender’ if you prefer. But why is this simple gap such a battleground?

Well, it’s unfortunately an area where the two professions are almost fated to clash heads. I hereby ask forgiveness in advance from my remaining designer friends for the massive generalisations to follow, but bear with me – they are generally true on most passenger cars. Most designers will wish for a near-zero gap between tyre and arch. They would like the arch opening to simply follow the outer envelope of the tyre, in side view. Sadly, this tyre envelope is probably already quite a large one, modern automotive design fashion tending to favour bigger and bigger wheels and tyres.

So why do the engineers not simply indulge their creative colleagues in the studio and trace the outline of the tyre with the surrounding sheet metal and plastic? Maybe just leave 5mm all around for luck, and adjourn to the company canteen for tea and medals all round?

Well, because a larger wheel-fender gap is required for several good technical reasons. Let’s start with the pretty obvious ones. Tyres move – up and down in the case of all four wheels, turning around the steering axis in the case of those at the front and even moving a bit fore-and-aft under longitudinal impacts like pot-holes or curb strikes. And they can’t touch anything aggressive as they do so. A tyre can survive a temporary ‘kiss contact’ with a plastic wheel-arch liner, but it will not survive dynamic contact with a sharp ABS cable or brake line bracket.

So the ‘tyre envelope’ has to be designed for extreme wheel displacements – not just the car sitting nice and straight and level as in the showroom (or design studio) floor, but on the limits – close to full lock, suspension in full ‘jounce’ or at the limit of its upward travel. This is close to a loss of control situation for most drivers – think of trying a ‘save’ a pretty severe departure from controlled flight: it’s the last moment at which you need that front tyre to touch something sharp and eviscerate its sidewall.

Most designers will have heard this story before, so they will reluctantly roll their eyes and accept there must be some margin for this. But now things get tense. The engineer looks at his comfortable shoes, coughs a bit and says ‘snow-chains’. Cue more dramatic eye-rolling on the part of the poor designer. Like it or not, some provision needs to be made for chains – either the full-size generic ones, or at best, the super-low-profile types that have now become available for more ‘sporting’ cars.

But now things get tricky, because we start getting into areas where the performance of the car are in direct conflict with the aesthetics. Thankfully, this is not always the case – aerodynamic cars are (often) good-looking ones, for example. But wheel travel is a loaded gun. The tyre envelope, when we take into account its travel in all dimensions, will resemble what French engineers call ‘la patate de roue’ – yes, the ‘wheel potato’ – and it really does look like a somewhat flattened spud. To reduce the wheel-fender gap, we need a smaller potato. Designers will often start by challenging the steering angle – if we reduce the steering angle, the front tyre envelope reduces, right? Better wheel-fender gap! Right – but now we have a terrible turning circle: not good for the end customer.

Next gambit – let’s reduce the vertical wheel travel: shorter springs, stiffer bump stops; all good, no? No. Any chassis engineer worth his or her salt will tell you that, like power and cubic inches, there is no substitute for millimetres of travel when it comes to suspension tuning. Reduced travel has no benefit at all – it forces the chassis teams to increase spring rates, turn up the damping, and resort to very ‘steep’ bump stops.

Modern cars already present the chassis folks an almost impossible challenge – more mass to control, bigger wheels and lower bonnet-lines – both of the latter limiting the available suspension travel. If we now also aim for cigarette-paper wheel-fender gaps, we can wave goodbye to any semblance of ride quality. Mr Mel Nichols of this parish recently reminded me of those by-gone French cars that handled so well, but also seemed to have the secret of riding beautifully. Peugeot 405, anyone?

Have a look at the wheel size and the wheel-fender gap: it’s not an accident that you could put four fingers in there. It’s also not an accident that a BMW M4 on 20-inch rims has very small wheel-fender gaps – maybe one pinky finger only? The BMW’s ride has been set to ‘stun’, however.

The stage is set. Tempers flare. Matters are escalated to Chief Designers and Chief Vehicle Engineers. Hard words may be said behind closed doors. But it’s now up to these folks to trash it out, find the right compromise and come to a sensible agreement on this simple but oh-so-contentious gap.