4x4 Mag - Trade Secrets

Trade Secrets - Tyres

by Nigel Morris

Richard has asked me to write some bits and pieces for 4x4mag . Quite a simple request it may seem, but with running a business (L.A.Supertrux) and writing the news letter for the Off Road and Custom Hilux Club time can some times be in a bit short supply. To get started I have decided to reprint a series of articles which I wrote for the Club news letter, based on technical requirements of building a big truck right. If any of you have any comments or specific questions you want help with, then drop me an e-mail via richard@4x4mag.co.uk and I will do what I can to help, enjoy!

It say's trade secrets above, so you can well expect to get the hot inside line on tyre information. Wheel tech will be covered next month. Tyres are one of those odd subjects, on which I receive a great deal of calls. Now we all know that once the big tyre bug bites you're hooked. But I don't mean calls about how much and when? So much as WHAT and WHY ? It seems that for quite a number of you, it is clear that your tyre size has to change, almost always going bigger, but the technicalities of that change can some times seem to cause you all sorts of trouble. So I will try to unravel a bit of the mystery around bigger tyres, and why they do what they do!!

SIZE- the main point here is that most if not all of the tyres that we buy are "flotation sized" tyres. Now that is not a sales pitch, it's how the industry refers to tyres where the size is given in the following way. Heights, then width, then rim diameter, e.g.; 33x12-50x15. As opposed to a "P metric size" e.g.; 235/75/R15, which refers to the width of the tyre in millimeters, then the height of the side wall as a percentage of the width, then the rim diameter. All clear on that? Next contentious point is that what it say's on the tyre side wall is NOT always what you get! To explain. The tyre companies sell their tyres into a predetermined market, which has established sizes, 33x12-50x15 for example. Now not every manufacturer\rquote s 33x12-50x15 will actually measure 33" tall by 12.5" wide, but they will all fit on a 15" diameter wheel \{we will talk on width next month\}. This can lead to confusion amongst those just getting started, as their mate runs 33's so how come they hit "my" arches? The only way to tell exactly what will fit is to try it on for size. Now just to throw the baby out with the bath water so to speak, experience has enabled me to build up quite a data base on what tyres are the size they say they are, and those that are shall we say pretenders! So if you are considering a change the hot tip is to talk to other owners about what they did, and make some calls to people like me who are in the business of knowing what fits and what hits!!

BALANCE - So what happens when you get those lovely new tyres onto your truck? Well I have to say that last year I wrote a whole technical page on the merits of having tyres properly balanced. I can not over estimate at all, the difference it makes to how bigger tyres feel on the road when balanced as opposed to non-or badly balanced ones. It is money well spent, don't skimp on this at any price!!

PRESSURE - The next major point is pressure, most people run way too much air in their big tyres. Even 33's can happily be run on the road with as little as 18psi in them. The key point to understand here is that it is the volume of air which carries the weight, and not the pressure!! I know it's hard to grasp, but think of it like this. One cubic foot of air can carry X amount of weight. In a given area (a tyre) the area can not increase, but when you compress air you can get two cubic feet of air in one cubic foot of space. With big tyres we have more room, so we don't have to compress the air so much. The only way to get the pressure right is to do some math\rquote s, I know most of you hate math\rquote s and why can't I just print a chart with the pressures on it? Well the reason is this. Each truck weighs a bit more or less than it's friends. Few if any trucks have exactly the same wheel, tyre and lift combination {we're individuals by nature} and so there are no easy reference points if you want it right. On the side wall of your tyre it will say some thing similar to:" Load capacity 3000lbs at 30psi cold". This means that each tyre on the truck can carry a weight of 3000lb if it is inflated to 30psi measured when the tyre is cold. Now you have to take your truck to the local weighbridge, and get it weighed. When you know the total weight, and the information from the side of your tyres, you're ready to do the math' s. Pay attention, as I will be asking questions later!!

Take the total weight of your truck, write it down and put an "A" next to it. Then multiply the maximum weight from the tyre sidewall by four {four tyres under the truck}, write down the answer and but a "B" next to it. Divide "A" into "B" and write the number down with a "C" next to it. Next divide "C" into the maximum side wall pressure "D", and you have the correct pressure for your tyres on your truck. The formula is shown again below to help. Truck weight in lb's =A Side wall max weight x 4 =B B into A = C Side wall max pressure =D C into D = ideal psi. Easy isn't it!! With the correct inflation pressure and having been balanced right, your new tyres will give years of faithful service.

More information to test your maths next month.