Tyre Size Choice

[stevieturbo]

I have never seen a spec for sidewall stiffness. All brands will be different.

And I'm not saying widest will offer most grip, because that is not always true and also depends on wheel width. And tyres too wide for the wheels, can give less progressive grip and oddly less feel.

Whilst maybe not so relevant to a more lazy 4wd and your suggested off road use....it's still interesting about wheel width relative to tyre.

https://www.youtube.com/watch?v=1MDq23scaWw

Really, there is no single best tyre for you, because there are so many variables. There is just no best. Because someones perception of best, will be different from anothers

In one sentence you're asking about tyres for off road...then another you're looking at performance road tyres. As you can see, it's all over the place...and for most part, you're massively overthinking, or worrying about it.

Just buy good quality tyres and get over it.

Reviews tests on YT for off road/SUV type tyres, or as said before, just plain bog standard normal good quality tyres, which is most likely what would be better suited

Or get multiple sets of wheels and tyres you can change for the conditions and use

Edited by stevieturbo on Sunday 17th November 16:58

[MustangGT]

LotsOfLaughs said:

Right, so it sounds like a slightly wider wheel with a standard tyre, or a significantly wider wheel with a slightly wider tyre, will be an upgrade in every way?
(Irrelevant sidenote, this is a big no-no for offroad applications because it leaves the sidewall very exposed to punctures/damage/etc, especially at lower pressures)
So based on what youve said, going from a 215/45r18 on an 18x7j wheel to a 235/45r18 on an 18x8j wheel will do everything, because the tyre width increase (20mm) is less than the wheel width increase (25.4mm, 1") so it gives both more grip and a more progressive breakaway characteristics?
Is there any way I can find the specs for sidewall stiffness? Say if I wanted to know which, between a Pilot Sport 5, a Potenza Sport, or a Rainsport 5, had the stiffest sidewall?

Changing from a 215 to a 235 would indicate a change to a 7.5" width wheel, most advice seems to be a max 225 tyre on a 7" wheel. By going to 235 you change the contact patch pattern and can change the mechanical grip available if you lower the tyre pressure a bit.

Regarding sidewall, the simplest way to consider it is the load rating of the tyre should reflect the sidewall stiffness to some extent. The higher the load rating the greater the weight that can be supported by the sidewall.

However, given one of your main drivers is increased ground clearance I would suggest your first port of call is to swap out the current suspension for the standard 'normal' spec for other models in the range. Changing the tyre size alone will give you a few mm extra, but not really significant. The increase in size is equivalent to a 2.7% increase in rolling circumference, thus still being okay for your speedo, I personally do not consider anything above 3% to be reasonable.

[LotsOfLaughs]

stevieturbo said:

I have never seen a spec for sidewall stiffness. All brands will be different.

And I'm not saying widest will offer most grip, because that is not always true and also depends on wheel width. And tyres too wide for the wheels, can give less progressive grip and oddly less feel.

Whilst maybe not so relevant to a more lazy 4wd and your suggested off road use....it's still interesting about wheel width relative to tyre.

https://www.youtube.com/watch?v=1MDq23scaWw

Really, there is no single best tyre for you, because there are so many variables. There is just no best. Because someones perception of best, will be different from anothers

In one sentence you're asking about tyres for off road...then another you're looking at performance road tyres. As you can see, it's all over the place...and for most part, you're massively overthinking, or worrying about it.

Just buy good quality tyres and get over it.

Reviews tests on YT for off road/SUV type tyres, or as said before, just plain bog standard normal good quality tyres, which is most likely what would be better suited

Or get multiple sets of wheels and tyres you can change for the conditions and use

Edited by stevieturbo on Sunday 17th November 16:58

As I said, the offroading point is irrelevant in this case, although possibly useful if considering winter tyres?

So to summarise what I picked up from that video (well worth watching, thanks!) the most important thing is that the tyre size is well matched to the wheel width. Then, wider = better in the dry, for any combination where the wheel and tyre sizes are well matched.

MustangGT said:

Changing from a 215 to a 235 would indicate a change to a 7.5" width wheel, most advice seems to be a max 225 tyre on a 7" wheel. By going to 235 you change the contact patch pattern and can change the mechanical grip available if you lower the tyre pressure a bit.

Regarding sidewall, the simplest way to consider it is the load rating of the tyre should reflect the sidewall stiffness to some extent. The higher the load rating the greater the weight that can be supported by the sidewall.

However, given one of your main drivers is increased ground clearance I would suggest your first port of call is to swap out the current suspension for the standard 'normal' spec for other models in the range. Changing the tyre size alone will give you a few mm extra, but not really significant. The increase in size is equivalent to a 2.7% increase in rolling circumference, thus still being okay for your speedo, I personally do not consider anything above 3% to be reasonable.

Id go for a 7j or an 8j for an 18" wheel, 7.5j isnt common in that size, but is very common on a 17". 235s on an 8j wheel is within the guidlines, with 245s being the maximum, likewise for 215s on a 7j, which is standard. I don't see what Im loosing by going for the 235/45R18 on an 8J, compared to a 215/45R18 on a 7J, other than possibly slightly worse wet performance.

I may get a second set of tyres, for winter, and those will be 215s on a 17x7 wheel, in the tallest aspect ratio thats reasonable. (215/60R17 at a guess)

As for chaning the suspension, I probably will get coilovers eventually. I dont want to fit standard springs, because then the rear will be softer than the front, and itll understeer. If I change the fronts too, itll roll like a boat, and Id then need much thicker ARBs, by which point, coilovers are cheaper, and have adjustable dampening and rideheight. Really what I need is some kind of air suspension in the rear, but not the showcar kind. Airbag helper springs wont work because the shock sits inside the spring.

[Smint]

Earlier you mentioned underbraking being a regular complaint from non STi Subaru owners, i think thats because its relatively easy to upgrade to STi parts, which suits the wants of many Subaru owners who didn't pay the STi premium but wish their model to look like one.

Our previous H6 Outback and the current Forester 2.5XT were not fitted with STi sized brakes and whilst i'm hardly a track day enthusiast by keeping those brakes well serviced they proved more than adequate whenever harsh braking was called for, Subarus arn't stupidly overservoed like so many other cars, what might feel like inadequate brakes at first simply require more pedal pressure than many run of the mill cars.

For your rough road needs two suggestions, either fit standard Legacy springs and shocks for raised ride height or swap the car for a standard Legacy or even Outback/Forester which will have the ground clearance you need.

[LotsOfLaughs]

So - A bit of an update;
I've had a closer look at the tyres, and Ive noticed that they've all worn on the outer edge. The outermost groove is below the 1.6mm legal limit, but the rest of the tyre is fine.
Before anyone suggests an alignment, I had one done a few months before getting the tyres. Ive also noticed that the fronts have worn more than the rears, on the outside at least. Also, Ive been slightly over-inflating them, to 35psi, so they arent under-inflated either.
I googled why this might be the case, and Ive read two theories that could be true. Firstly, the tyres have a softer compound on the outside to give more grip when cornering, and thus wear out on that side faster. The second is that the sidewall of the tyre is too soft for how I drive, and so its "rolling" when cornering and wearing out much faster, and I need a more "sporty" tyre.
So, in light of this information, I'll probably steer clear for Rainsports 5s, and spend the extra on Pilot Sport 5s. Is there anything else I should consider? Should I possibly get 225/40R18s instead of the 235/45R18s I had planned on getting since they are cheaper by about £70 a tyre.
Thanks

[SlimJim16v]

It may be that you need some/more negative camber on the front.

[LotsOfLaughs]

SlimJim16v said:

It may be that you need some/more negative camber on the front.

Is there a way I can work out my own alignment specs? I presume I wont have any problem asking a garage to align it to specs I ask for rather than what their computer says?
Also by "work out my own specs" I mean calculate mathematically, rather than doing lots of RnD and testing...