Thread: Big Tyres: Pros and cons

Originally Posted by Rick122

How good of you to pick parts of my post to use as the full basis of your argument. Maybe we should list every scenario you could possibly encounter giving the pro's and con's of each tyre, and then list every tyre available and rate it against each scenario. Don't think it's going to happen. You like big tyres and are prepared to live with the compromises, good for you.

Not like the 'big tyres are for *******' brigade doesn't do the same thing. In fact, I find that camp to be far more aggressive.

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Originally Posted by Rick122

How good of you to pick parts of my post to use as the full basis of your argument. [...]You like big tyres and are prepared to live with the compromises, good for you

Sorry you feel hard done by. Wasn't selectively quoting you to do you or your case harm. Just focusing on what was relevant. You wrote what you wrote. There is no backing away from that.

Something is either "about the only place" or its not.

And: no, I am not a proponent of "wider is better". Thought I have been careful to make that clear.

I am against people saying "narrower is better nearly all the time" or the mirror of that - "this one or two scenarios are the ONLY places where wide might be useful".

Originally Posted by PAUL200

Hi all,

I see a lot of 4WDs in Australia with large tires and lift kits etc...

Does installing big tires and lifting the suspension have much impact on the fuel economy? I know it must have some but has anyone ever measured it?

Another thing I have observed is that the military don't install all this stuff on their vehicles which makes me wonder whether it actually increases off road capability much? Are all these lifted 4WDs with big tyres just poser cars?

Anyway, to help the original poster...

My td5 disco 2 came with a set of 265/75 16 muddies, which now almost all the time are replaced with 255/60 18 ATs.
There is a noticeable change in fuel economy and response - and would guess could affect mechanical longevity if kept on long term.

You can get a manual td5 disco to the top of Billy Goat's Bluff on stock 29" HTs... but they will be ripped to shreds after. A set of ATs or similar will cop the abuse for much longer.

In my experience, mechanical and environmental sympathy are at least (if not more) important than tyre size.

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Originally Posted by DiscoMick

Isn't the length more important than the width with bigger tyres?

Traction is important.

How much traction is available varies for a lot of factors like: the surface of the road/ground, wet/dry, soft/hard, the tyre's rubber compound, tyre pressures, tyre size, vehicle weight etc.

If we say for any vehicle many of those factors are a constant. i.e. a vehicle won't change weight, tyres, in the blink of an eye: then the variables are simpler, largely just ground/trail condition and tyre pressures (size of contact patch)

Now let's just hold the ground/trail condition and tyre pressure (contact patch size variation) as constants for a bit too.. Loosely... And change one of the earlier fixed parameters: the size of the tyre.

A tall narrow tyre, aired up, will have a certain length and width of contact patch. Let's say it's a 6" tyre. However long the contact patch is (adjusted by tyre pressure) largely the WIDTH of the contact patch will not change. 6"

A tall wide tyre, aired up, will have a certain length and width of contact patch too. Let's say it's a 10" tyre. However long the contact patch is (adjusted by tyre pressure to match the LENGTH of the 6" wide tyre) - the WIDTH will still be 4" wider than the 6" tyre

No question. No room to move. In this scenario the area of the contact patch for the wider tyre is larger (I.e. length X 10" is a larger patch than length X 6" ).

Does this matter? Of course it matters anywhere that there is any traction to be had on a reasonably firm surface. From tarmac to blue ice.

On soft surfaces. Soft sand. Soft mud. Other factors kick in. Things like how deep into the surface the tyre sinks. Every added millimeter depth adds to the length and relative steepness of the perpetual and virtual "ramp" the tyre has to constantly run up while making forward progress. Until underbody grounds and no one is going anywhere.

Does wide or narrow perform better in soft conditions? You decide for yourself. I have seen and used both wide and narrow tyres in soft conditions and don't see a lot of difference.

On any firm surface where by definition there is no "digging in for more traction" No contest. Wide

Narrow tyres are best, eh?

I guess that's why tracks are so narrow...

[ame]https://www.youtube.com/watch?v=P4RmS_xkszU[/ame]

One of the common arguments trotted out by those who stick to old chestnuts like "narrow tyres are always (or nearly always) better than wide" is the formula that says
"Contact patch area = weight / tyre pressure"

This formula leads to thinking (popular teaching in science/engineering classrooms) that says wide or narrow makes no difference - Contact patch will be the same if vehicle weight and tyre pressure are equal.

(Of course in my previous comments I talked about unequal tyre pressures. Specifically: adjusting the pressure in the wide tyre (lowering it) to match the LENGTH of the narrower tyre's contact patch... The wide tyre patch is still wider thus greater contact area)

So let's now tilt the equation back to equal pressures. Same contact patch area according to the formula. Which begs the question: if the contact patch is the same why do the narrow pundits say a narrow tyre better? Doesn't this (same contact patch area) put the lie to the argument that a narrow tyre has a smaller contact patch which means higher ground force down on that patch to give better dig down ability and higher traction to boot because more load on a smaller area means better traction... (Breathless!)

C'mon... You can't have it both ways. Ok the narrow tyre pundits who are clever then avoid a fight that can't be won. Yes, contact patch is the same. Vehicle weight is the same. So ground force is the same, traction is the same... Static traction.

This then leads to comments like - "BUT....!":
- while the contact patch area is the same there is still a difference. The patch is wider than long in a wide tyre. Good for lateral (sideways) forces. The narrow tyre patch is longer than wide. Better for traction in the straight line where it's more needed. (They will argue)
- and... If the wide tyre were to be aired down to increase contact patch area: Available friction is not related to contact area as the weight is spread across a greater area and so no improvement in traction. (They will argue)

And maybe if the formula holds true... And if physics classes are all spot on in the theories... Narrow tyre pundits win. Narrow is better. Maybe.

What if the theory was bollocks? All the above "narrow is better" logic falls apart and some better understanding of why the reality we observe on the race track and in the jungle (i.e. that wider tyres do just fine thank you) seems to run counter to the classroom wisdom.

How would someone empirically determine whether the theories are bollocks? Real testing?

Devour this:
Fact or fiction? Tire contact patch size is determined mostly by weight and tire pressure. | Performance Simulations

Just to be a fungi and poke fun at one eyed comments in general:

Anyone here want to say hi lift jacks are dangerous and no one should use them?

******* with big tyres and lifts would need one for whenever the barge gets a flat at the mall car park...

Originally Posted by PAUL200

Hi all,

I see a lot of 4WDs in Australia with large tires and lift kits etc...

Does installing big tires and lifting the suspension have much impact on the fuel economy? I know it must have some but has anyone ever measured it?

Another thing I have observed is that the military don't install all this stuff on their vehicles which makes me wonder whether it actually increases off road capability much? Are all these lifted 4WDs with big tyres just poser cars?

I tilted at this in some of the many lengthy posts in this thread already... But just to bring it to focus for you...to answer part of your question:
Bigger tyres are one of the few things (mods) that will increase ground clearance to axles/diffs. (Another would be fitting portal type axles)

More ground clearance does make an improvement to off-road capability (not getting hung up on obstacles so much). Of course this is NOT needed, and truly a ****, for those who's off-road excursions are limited to the local mall car park.

It's just a fact of life that beyond certain vehicle specific limits larger tyres can mean lifts and other work may be needed.

I do want a bit more ground clearance for my driveline. Trails I drive have already seen me dragging diffs over rocks and damaged all my side steps (so I removed them - see pic, still with stock 235/85/16 tyres).

My intention is to only go as large (tall) as I can without doing any lifts. So I think research says max is 255/85/16 for a 2013 Defender. That's what will be my next tyre around end of this year.

Sorry I cannot respond to the fuel economy question - never collected that data myself.