Handling after changing Rims on my Bora

[kaanage]

The damping coefficient of the tyre sidewall is completely independent of the dampers. It is internal to the tyre, however when you look at the entire suspension system, changing one component can have an effect on the system and so you need to be aware of this when making changes.

While the tyre sidewall stiffness has some effect on the system as a whole, the effect is basically to add to compliance of the system as a whole. The self damping of the tyre is high enough to treat it as a totally damped member that almost only affects the rate at which forces are transmitted to the suspension (the magnitude and action distance are affected to a lesser degree).

In theory, for maximum grip, you should lower your spring rate as you stiffen the tyre (+ reduce the damping to match this rate reduction) but in practise, the change in tyre compliance when changing tyres is small enough to ignore unless the profile is radically altered (in which case the suspension is likely to be changed quite radically anyway for the other reasons already discussed in previous posts). You MIGHT factor it in if you were running an open wheeled racing car with multiple sets of springs and antiroll bars and double or triple adjustable dampers if changing tyre brands.

F1 cars go the other direction by having such stiff suspension (to minimise camber changes due to roll + to maintain ground clearance due to downforce loading at high speeds) that the tyre compliance forms a major component of their suspension movement - most people would be shocked that F1 cars run 80 series profile tyres.

In the context of this thread where the OP has made a relatively small change in tyre profile, it is the other factors that have primarily affected the car - tyre compliance alone would only have made it react to steering a little faster and ride more harshly.

[Manaz]

I don't recall saying that the tyre profile influences the coefficient of grip.

That's exactly what you said. You said lower profile tyres allow better cornering speeds, and that's simply not true, as cornering speeds are not dictated by any aspect of the sidewalls.

... which at the end of the day, allows you to corner faster like I said - does it not?

No it does not. The tyre lets go at exactly the same time, determined by angular force over area and coefficient of grip. Again, the profile of the tyre has no influence. The type of feedback changes depending on sidewall stiffness, but the point at which the tyre lets go does not. And it's that point which determines your cornering speed.

[Manaz]

One thing to note when evaluating tyre and suspension performance together is that lower profile tyres pass more low amplitude/high frequency movements to the springs/dampers. This is why low profile tyres and hard suspension results in a jittery ride - higher profile tyres are able to absorb (to some degree) small changes in the road surface profile, leaving the suspension to deal with only larger road surface variations.

Also when considering suspension, you have to realise that your springs have a dual purpose - the first is to keep the wheel in contact with the ground, the second is to keep the chassis off the ground. It is primarily the spring which absorbs impact from road surface changes - this is why progressive rate springs work (they allow quick deflection for low amplitude/load deflections, but as loads increase they firm up (to prevent body roll in corners or "float" when there are larger undulations). Spring compression is controlled primarily by spring rate. Dampers are primarily there to control the rate at which springs expand after compression (to prevent bounce associated with sudden unloading of the spring or return of force as the spring uncoils after it absorbs compression energy), though they can also be used to change compression rates as well (by resisting compression force encounted by the spring) - this is more unusual (and generally only available as something you can influence on 3-way adjustable suspension - 1-way adjustable is height adjust only, 2-way adjustable is height plus rebound).

And of course, to counter body roll, you can increase swaybar stiffness (often adjusted by changing size) - it's not uncommon for cars that race on very even surfaces to have massive swaybars but relatively soft suspension.

None of which at this point is relevant to the OP unless he's also looking at suspension changes.

[EdwinConan]

None of which at this point is relevant to the OP unless he's also looking at suspension changes.

Not relevant to me for now, haven't got money for suspension. But thanks to all the replies, I'm learning a lot from this thread, I'm not a guy who knows a lot about cars mechanically, and desperately want to know all these.

[Diesel_vert]

Well, I found a perfect example which runs contrary to what I said earlier - all I had to (re)ask myself was, would a car fitted with road tyres in a 225/40 R18 fitment handle better than if it were fitted with track tyres in a 205/55 R16 fitment... even if deep down I already knew that it would be extremely unlikely! Indeed, the OP's current tyre re: Kumho Ecsta KU23 is a demonstration of that, i.e. a low profile tyre that appears to be focused on low NVH.

Another example is F1 cars - the aspect ratio itself certainly isn't a barrier to massive cornering speeds, is it?

I suppose my thinking was that usually with a top tier brand (Bridgestone, Goodyear, Michelin, Pirelli, etc.) the further up you go in their product range, the higher the performance. That it's usually accompanied by a lowering in aspect ratio and an increase in section width doesn't mean one should make assumptions on how it affects the tyre's performance circle, without actually knowing its design or underlying structure.

Not relevant to me for now, haven't got money for suspension. But thanks to all the replies, I'm learning a lot from this thread, I'm not a guy who knows a lot about cars mechanically, and desperately want to know all these.

It's a great hobby - though it can get expensive. Fun though.

[Yevvy]

OP in simple english!
38 - 40 psi and then lowered =
WINNING!

^ what he said

i run 39psi (cold) all round

[kaanage]

Another example is F1 cars - the aspect ratio itself certainly isn't a barrier to massive cornering speeds, is it?

Part of that is the regs which don't allow wheels with a larger diameter than 13". The F1 cars would probably corner a bit faster with slightly larger (say 15") wheels so they could go down from 80 to 60 series at the front but I seriously doubt that they would drop to ultra low profiles even if the regulations allowed it. And they would have to soften the suspension to suit.

At the end of the day, ultra low profile tyres and large diameter wheels on a car not really desgned for them are more a fashion statement than a performance boost (and can make the car slower in real terms as well as harsher/noisier).

[Diesel_vert]

Part of that is the regs which don't allow wheels with a larger diameter than 13". The F1 cars would probably corner a bit faster with slightly larger (say 15") wheels so they could go down from 80 to 60 series at the front but I seriously doubt that they would drop to ultra low profiles even if the regulations allowed it. And they would have to soften the suspension to suit.

I love how they always change the regs to slow the cars down, yet the brilliant engineers always manage to find a solution that allows them to make up for the lost speed.

At the end of the day, ultra low profile tyres and large diameter wheels on a car not really desgned for them are more a fashion statement than a performance boost (and can make the car slower in real terms as well as harsher/noisier).

It would certainly be easier and better value for money by just upgrading the tyres, but a problem faced by those who have 195/65 R15 or 205/55 R16 tyres, is that some performance tyres are only available once you upgrade to 225/45 R17, 225/40 R18 (or even 235/35 R19, depending on how much of a tyre snob one is).

I think the OP should choose his next tyre wisely.