Bigger tyres can make your 4×4 better off-road, but you can open yourself up to a series of complications. Here’s why you need to think carefully before slapping bigger takkies on your vehicle.
Once you’ve got a pukka 4×4, it won’t be long before the urge to fit larger tyres will begin to consume you. You’ll take one look at your mate’s Wrangler with its fat 35-inch takkies, and your own vehicle will start to look like a Smart car. You’ll develop a debilitating inferiority complex. You’ll question your own worth. Every obstacle will become a taunting reminder of just how much more capable your mate’s big-wheeled Wrangler is. Eventually, you’ll come to simple conclusion: a pukka 4×4 without bigger tyres isn’t much of a pukka 4×4 at all.
Don’t let anyone lie to you. There is only one reason why people fit bigger tyres to a vehicle: it looks cool. They’ll tell you all sorts of stories about how it improves ground clearance and traction and flotation, but these are mostly just things they tell themselves to justify their actions. It’s really about looking cool.
That said, we’re fully aware that you’ll probably need to justify the massive expense of fitting bigger mud-terrain tyres to your spouse, accountant and bank manager, so we’ll go into a bit more detail about the actual benefits of bigger tyres.
Firstly, it improves ground clearance. We’re not talking about gaining 200mm, but you get a small improvement. If you fit tyres that are 25mm (1 inch) bigger in diameter, and you probably don’t want to go more than 25mm bigger, your vehicle will be raised by 12.5mm. Importantly, it goes up by half the diameter, since only the radius has an impact. All that matters is the distance from the centre of the wheel to the ground.
Bigger tyres will also improve traction. A wider tyre will provide a bigger contact patch, and this can really help when you deflate them in sand. A bigger tyre (but with the same rim size) will also provide a bigger sidewall, which means that you can deflate it more than a low-profile tyre.
Let’s get technical
So, it is indeed true that bigger tyres can improve your vehicle’s off-road capability, but don’t expect a dramatic change. Also, it can be argued that the negatives greatly outweigh the positives.
A lot happens between the moment you press down on the accelerator and the moment your vehicle’s wheels start turning. Torque created by the engine must first be transferred through the gearbox, transfer case and differentials before reaching the wheels. To ensure that this happens as efficiently as possible, vehicle engineers have created ratios for all three that have been perfectly set up for what your vehicle looks like in standard specification. Fit bigger tyres, and everything changes. As the radius (from the centre of the wheel to the ground) increases, the efficacy of the moving force (generated by the drivetrain) decreases in relation to it. In other words, the larger the tyres, the more drivetrain performance is hampered. If your vehicle didn’t have a lot of oomph to begin with, you’ll really notice the difference. You’ll also notice that it doesn’t feel the same off-road. Where first gear low-range might have been short and strong, it’ll now be weaker.
The only solution is to try and get back to that balance of ratios that the engineers found in the first place. This can be incredibly hard (and expensive) to do, and you might never achieve that perfect balance again. Also, turning bigger wheels requires more torque than turning standard ones, so some performance loss is inevitable. A good rule of thumb is to expect a loss of around 3.5% in torque for every inch (25mm) you go up. That’s why you don’t want to go up more than 25mm, or 50mm at the absolute most. If you go up a bit, the vehicle won’t instantly fall apart. The change in performance will probably be imperceptible. Just know that there will be an impact. Every action has a reaction.
A reduction in performance and strain on the drivetrain aren’t the only issues to worry about, either. There are a host of potential concerns, especially if you’ve gone up drastically in tyre size. Here are some problems that will crop up:
- Your speedometer and odometer will be out. Both are calibrated to your tyre size, so upping the diameter will throw these out. It’s not a big problem, though, since they can be recalibrated quite easily.
- Bad brake performance. As tyre diameter increases, breaking performance decreases. Your brakes are designed to handle standard tyres, not massive ones. The solution: upgrade your brakes.
- Increased fuel consumption. With your engine working hard to get those big tyres going, fuel consumption will take a hit.
- Higher centre of gravity. When it comes to dealing with seriously scary obstacles, the most capable 4x4s are those that combine great ground clearance with a low centre of gravity. The higher your vehicle’s centre of gravity, the better the chances that it’ll topple over. Fitting bigger tyres ups your centre of gravity.
- Tyres touching the body. If the tyres are too big, they will touch the inner guards of your wheel wells when you turn sharply. Go up slightly, and this won’t be much of an issue. Go up too much, and it becomes impossible to turn sharply. When you’re on an off-road trail where the suspension pushes the tyre up into the wheel well, your vehicle can be badly damaged. The only solution is to modify your car. It could mean pushing out the firewall, enlarging the wheel wells and fitting modified wheel arches.
Down the rabbit hole
What should be clear from the above is that buying bigger tyres can send you down a slippery slope that leaves you angry, bruised and penniless. Chances are, opting for bigger tyres will necessitate a bunch of other upgrades and tweaks. If you’re willing to do it, you can indeed eventually own one incredibly capable 4×4, but it will be an expensive process. There are companies that offer these upgrades (Arctic Trucks is a well-known example), and they can help you create a big-wheeled monster. People are often surprised at what these companies charge for an upgrade, but the info above hopefully makes things clearer. It’s not just about fitting bigger tyres and modifying the arches. There are many, many important things to consider. To do it well takes a lot of time and careful configuration.
If you’re not interested in going down the long road towards a proper big-wheeled 4×4, then you need to ask yourself if it’s worth going up just a size or two. Perhaps it is. As stated, ground clearance will improve a bit. But more important than size, however, is the tread and pressure of a tyre. If you’re going to be doing a lot of off-road driving, invest in all-terrain or mud-terrain tyres. An aggressive tread can make a surprising difference. Also, play around with tyre pressures. Figure out the best pressure for every off-road situation. Know exactly by how much you should drop pressures when tackling rock, mud and deep sand in your particular 4×4.
Text: GG van Rooyen