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Mitsubishi 4WD Club Trev's Tip 28. |
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Trev’s Tips
One of the most common upgrades for 4wders is tyre upgrades. This may be as simple as just replacing the existing tyres with something of exactly the same size but with a more aggressive off road tread or it may involve using a completely different sized tyre in the change over.
There are a number of considerations that need to be carefully thought out before taking the big step to change the tyre size to something which is larger than the original. To many of you more experienced off roaders with some form of mechanical understanding, what I’m about to say will be obvious but there a few things to make some of your calculations a bit easier.
Firstly from this point on when I talk about bigger tyres what I am really talking about is the height of the tyre rather than its width. The tyre width really has little effect on your cars performance and handling but its height can have a huge effect.
Taller tyres will 1) Cause your speedo to read differently than you are used to.
An increased rolling diameter of your tyres will make your car travel further each time the tyre turns (with each engine revolution) meaning that if your speedo was accurately reading your speed at 100km/hr before the tyre change then when it is reading 100km/hr after the tyre increase you will actually be traveling faster than this. Whilst you can do your own calculations to predict the change in your speedo reading they will rely on an accurate speedo to start with, which is a very big assumption. Best way to check your speed is to check your speedo reading against a GPS speed reading. Most people have one of these nowadays but if you haven’t, borrow one, it could save $100’s in speeding fines.
2) Rob your car of some power because of the effective increase in the gearing.
Don’t underestimate this. Unfortunately this is a very difficult thing to calculate but hear this. I changed my tyres from a set of 31 inch tall BFG’s to a set of 33 inch BFG’s. Not what I would have thought was a huge increase but my fuel consumption increased by around 5% (car unloaded) due to the engine having to work harder to turn the tyres. Also on one fairly steep hill that I have previously driven up without too many problems the car stalled with the bigger tyres on. I had to go down to low range to get up.
3) Make your brakes less effective.
How’s this you say. For the same reason that taller tyres rob your car of some power with increasing the gearing they will also force you to put more pressure on your brake pedal to get it to stop like it used to. Taller tyres increase the leverage your tyres have between the road and your brakes so you will need them to work harder to counteract this. At best this change will only cause increased wear in your disk pads and rotors, at worst it will make your car unsafe by increasing the stopping distance and drawing attention to itself in the eyes of the police or insurance companies.
The 3 facts above I know will happen for sure but on top of this here is another issue I have heard of that I tend to believe is correct but can not confirm. Due to the increased size of the tyre and resistance to turning it adds pressure to wheel bearings, CV joints and most driveline components (gearbox etc). Also remember that all your tyres need to be the same size so you will need to upgrade your spare as well.
It all sounds very grim but at the end of the day most of us will probably say it is worth the risks to get a bit more off road performance from our cars. The main thing is to be aware of the issues and be well informed when making choices about what size tyres should go on your car.
Now the confusing part is tying to work out what all the numbers on the side walls of the tyres mean so that you can work out how big your new tyres are and how they compare to your old ones.
There are two basic ways tyre sizes are stated. That is metric and non metric.
A common tyre size for a Paj is 265/70R16 (metric).
265 is the width of the tyre in mm.
70 is the size of the tyre profile (distance from the edge of the rim to the top of the tyre). It means that 70% of the tyres width is in the height. ie 70% of 265 is 185.5mm so side wall height of the tyre is 185.5mm.
16 is the radius of the rim in inches which equates to about 400mm.
So the overall height of the tyre would be the height of the rim (400mm) + the height of the two side walls (top and bottom together to make 2x185.5= 371) which would give a diameter of 771mm
A 31x10.5r15 tyre (non metric) is easy to get the height of.
The 10.5 is the width of the tyre in inches and has no bearing on its height. The only figure we need is the 31 which tells us that the tyre is 31 inches tall or about 775mm. The 15 tells us the size of the rim in inches that this tyre will fit onto.
Whilst these calculations are not hard to perform if you want the easy way out go to
http://www.exploroz.com/Vehicle/Tyres/SizeCalc.aspx
This page has an excellent calculator which gives you tyre circumferences, rolling radius and diameter.
It allows you to enter tyre sizes of both metric and non metric dimensions and allows you to compare sizes so you can check to see the differences between the original tyres of your car and your intended purchase.
Take the time to get to know your car by checking out sites like this and seek help from the wealth of knowledge within the club or professionals outside before spending your hard earned cash.
Till next time
Happy 4wding
Trevor Look out!!!! I drive just like you.
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