Welcome to the second part of my explanation about 4wd rims and tyres. In my previous post explaining 4wd Rims, I mentioned that I run 285/75/R16s on a 16 x 7.5 steel rim with a +10 offset. I don’t by the way, but it’s a perfectly valid example for us to use…
If you’ve read the previous post then you’ll understand the last part… 16 x 7.5 steel rim with a +10 offset. In this post, I’ll explain the rest of it and also some other common terms that you’ll hear regarding tyres.
How To Read Sidewall Tyre Sizes
Tyres are measured in a couple of different ways; imperial (inches) and metric (millimetres). The example above is the metric system… Kind of… Except that some of it is in inches… Crazy I know!!!
Tyres are measured in width of the tread, sidewall size and rim size. In our example:
- 285 is the width of the tyre in millimetres.
- 75 is the height of the sidewall as a percentage of the width of the tyre. Otherwise known as the Aspect Ratio.
- R16 is the size of the rim it’ll fit in in inches.
So there you go. Simple. Thanks for coming. Wait… what? Yyou want more? Ok then…
Calculating tyre diameter
So lets say you want to know the rolling diameter of our tyre. There are two ways you can do this. Lets have a look at the first method.
In our example, our tyre is 285mm wide. The sidewall is simply 75% of 285 (285 x 0.75) which is 213.75mm high. But that still doesn’t give us the full diameter, it only gives us the height of the tyre. The distance from the rim to the road, so to speak.
To get the full tyre diameter, we need to also add the rim size. In our case, it’s 16 inches. There are any number of places on the internet to convert inches to millimetres. But I’ll do it for you this time. Just this once, ok.
16 inches = 406.4mm
So our total tyre diameter is 213.75mm + 406.4mm + 213.75mm = 833.9mm
When talking about tyre diameter, most people talk in inches for some weird reason. So then we convert 833.9mm back into inches… Which comes to 32.83 inches.
But hang on… You’ve seen tyres measurements that don’t seem to look like this at all! You’re right! Sometimes tyres are measured only in inches!
So, for example, your tyre might be a 33×11.5R16. In this case, 33 inches is the total diameter of the tyre in inches. 11.5 is the width of the tyre, and R16 is the rim size. If you did the calculations, you’d find that it’s very close to our original example of 285/75R16
So What About Tread And Tyre Designs?
You have probably all seen the big mud boggers out there, you know the 4WD’s with tyres with tread lugs a deep as a finger. While mud terrain (MT) Tyres serve very well for the purpose they are designed for it might be worth going through the different type of 4WD tyres.
- Highway Terrain (H/T) – Generally Designed for 90% Road/10% Off Road
- All Terrain (A/T)- Generally Designed for 60% Road/40% Off Road
- Mud Terrain (M/T)- Generally Designed for 15% Road/85% Off Road
Let’s break these down
Highway Terrain (H/T):
Surprisingly the 2nd most popular design after the classic All Terrain, H/T tyres feature a close tread design (great for dispelling water) and a compacted sidewall which is all there to minimise road noise and give the best traction on bitumen.
They really are quiet compared to there beefier cousins and are made to withstand higher temperatures and higher speeds that come with highway driving. The big downside to H/T is there next useless off the blacktop so really consider H/Ts if you intending to keep your 4WD in the urban jungle for the majority of its life. The big exception is in the sand where an H/T due to its less aggressive pattern does quite well.
All Terrain (A/T):
By far the most popular choice for most enthusiasts. The Tyre itself is generally stronger and have more aggressive (Medium Void) tread patterns with larger lugs then the H/T counterpart which help to “clean” of debris and mud that can build up between the lugs.
Most A/T tyre will also have rounded shoulders which is better in the sand because it tends to dig in less. Because of fo the lugs design and being a generally stronger tyre, they are less resistant to puncture and take being deflated lower than you standard H/T. They do have some cons, namely the increase n road noise but for most of us this the A/T is the go-to tyre if you intend to do a bit of off-road dabbling.
Mud Terrain (M/T):
These are the special forces of the tyre world, they go where others fear to tread. These tyres have very aggressive tread blocks and very large void which essentially means they will self-clean debris, mud, gravel etc very well and will effectively dig in when turning which will help to keep propelling you forward. I’m not joking when I say there are some places you cannot get to without Mud Terrain Tyres due to the above characteristics.
There are some downsides to this extra capability, Firstly your fuel bill will go up by at least 3% as its harder to get the bulky M/Ts to get and keep rolling. They are also very noisy on bitumen to the point where you will struggle to hear people in the cab, They also tend to wear quite fast on the blacktop too which needs to be considered.
You might be wondering what I run personally. Well, I actually own 2 sets of rims and tyres, One set is the standard setup with a set of desert dueler A/Ts (as a do a bit of gravel driving). My other set is an on steelies with a set of Wrangler MT/Rs. I feel having the two sets gives me the best of both worlds. When it came to setting up my second set, I found the beaurepaires website to be the best value overall especially when it comes to offroading tyres and rims, if you’re in the market I would suggest checking them out.
A Quick Word On Brands
I can appreciate people what to pinch penny where they can, but please remember that tyres are the only thing between you and the road. Don’t skimp on tyres, buy the best you can afford and look for quality European or American brands where there is regulation and standards. There is a lot of cheap Chinese crap hitting the market and do you really want to put your life at risk by putting inferior tyres on your vehicle. I sure as hell wouldn’t
Other Tyre Talk
Some other terms you’ll hear when people are talking about tyres are:
Carcass – According to etyres.co.uk, the tyre carcass is basically the black bit, including the tread, the sidewall, the steel belts if there are any and everything else.
Tread – This is the bit that keeps you stuck to the road. There are all different kinds of tread for all different types of terrain.
Sidewall – This is the side of the tyre. The bit you kick when you’re telling people about your tyres. it keeps the tread away from the rim.
Bead – This is the part of the tyre that comes into contact with the rim. You’ll hear people talking about popping the bead off the rim. This basically means that their tyre has come off the rim and all the air has escaped. It can be difficult to get it back on again if you don’t know what you’re doing.
Footprint – This is the area of the tyre that comes into contact with the ground. One of the best 4wd tricks to get you further is to reduce your tyre pressure. This increases the foot print of your tyre and gives you more traction.
Profile – The tyre profile is the aspect ratio. The height of the sidewall.
That’s basically it really. There are many more terms that you might come across but these are probably the most common.