Handling after changing Rims on my Bora

[wai]

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

Not necessarily. It might, but only if you were being put off cornering fast because the tyre sidewall was flexing too much and causing the car to have a poor feel (wallowing). Ultimately, it does not matter whether you have F1 type aspec ratios or rubber band tyres, it is the coefficient of friction between the tyre and road surface that determines the grip level. Wide tyres allow the tyre manufacturer to use a softer compound that is more grippy, but wears faster giving the same wear life as a narrower harder compound. Unfortunately, for road use, this is not always done as in the main, these tyres never ever get anywhere near the limit of adhesion and wear life is a much better selling point.

[gringo_54_oval]

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

[kaanage]

Umm, no. The tyre profile does not (and indeed cannot) influence the coefficient of grip of a tyre (which is largely dictacted by compound, road surface, tread pattern and the size of the contact patch).

What lower profile tyres do actually achieve in terms of handling is to let you feel more accurately what's going on at the road surface - lower profile is generally related to sidewall stiffness, and what this in turn means is that the feeling the driver gets is that grip becomes slide in a short period of time. Higher profiles/softer sidewalls tend to obscure the feeling of this moment (and thus feel to have a smoother transition from grip to slip) because the sidewalls flex and allow the car to shift laterally even though the tyre surface has not begun to slip significantly - so you feel the chassis of the car start to move outwards in the corner before the tyre actually lets go, meaning the transitional period "feels" longer.

Not necessarily. It might, but only if you were being put off cornering fast because the tyre sidewall was flexing too much and causing the car to have a poor feel (wallowing). Ultimately, it does not matter whether you have F1 type aspec ratios or rubber band tyres, it is the coefficient of friction between the tyre and road surface that determines the grip level.

EXACTLY - so many people here confuse steering response with grip. Lower profile tyres => increased sidewall stiffness => faster steering response - this may result in increased cornering speed as a function of increased driver confidence but that's all.

It's the same with people fitting stiffer antiroll bars/springs/dampers - the faster steering response fools them into thinking the car is more capable so they are willing to push it harder (outright cornering grip actually decreases due to decrease tyre compliance and increase load transfer in this case unless wider/softer tyres are also fitted)

[goodfella]

I almost went with the same style wheels on my Bora, but settled on 17's instead, must say with an inch or two out of the ride, they will look perfect.

Cant comment on the ride changes, other than work with pressures and get the tyres run in.

[dylan8]

OP in simple english!
...

haha this!
so many numbers and facts in this thread.

Wheels look great, unless you are going to track the car or do some serious driving i wouldn't be stressing to much.
You almost always pay the price for looking good.

[wai]

One other thing that you also need to consider.

When you go to low profile tyres, the new tyres will have a significantly different damping coefficient, and so your suspension needs looking at. Things like the spring rate and damping rate will need to be changed to suit the new tyres, as well as any sway bars because of the new spring rates, otherwise you can end up with a much worse handling vehicle.

You can have major issues if all you do is change one thing and leave everything else the same.

[kaanage]

Tyres are self damped - which is just as well as there is no way for the cars dampers to control them.

The additional unsprung weight of wider wheels/tyres is what leads to the damping ratio being upset - especially if you also do the "upgrade" of stiffer lowering springs at the same time while leaving the dampers as stock.

Roll rock brought on by stiffer antiroll bars would be disconcerting but it basically doesn't happen (the car gets all slidey and pitches in violently into potholes and across bumps before this happens)

[gldgti]

Tyres are self damped - which is just as well as there is no way for the cars dampers to control them.

Not so - tyres have plenty of effect on the suspension, and the suspension has plenty of effect on the tyres - you're talking about directly connected mass/damper systems. Tyres do have sping and damper characteristics, but you can change these with inflation pressure and temperature very easily, and this has a big effect on the suspension.... what I understand from your statement is that you are indicating tyres are critically damped, which they definately are not.

The effect of increasing wheel size and decreasing tyre profile on an otherwise unmodified suspension will be an increase in harshness over bumps and an inability of the vehicle suspension to keep the tyre in the ground over bumpy surfaces. This is because the standard suspension is "softer" and has 2 problems - the new tyres react faster to the bumps (they accelerate the suspension faster because the tyres hare much harder) and with more energy (the wheels/tyres are heavier) so the total suspension travel for a given bump is greater but also slower than before.

This is why to keep the frequency response of the original system, both the damping and spring rates of the suspension need to increase.

OP - bora looks sweet as

[kaanage]

Sorry, I wasn't clear enough - I meant that the damping coefficient of the tyre itself cannot be controlled in any way by the dampers.

The increase in tyre stiffness will increase harshness but has no effect on whether the suspension's damping coefficient/factor/frequency (all the same thing just measured in different ways) can control the tyre/wheel. All the additional stiffness does is inject sharper impulses into the spring/damper mechanism which does not modify the response.

But you are correct about the increase in unsprung weight requiring more spring rate and damping to control as effectively. But increase in damping alone could retain control but almost nobody (on this forum) would uprate dampers with stock springs.

[wai]

Sorry, I wasn't clear enough - I meant that the damping coefficient of the tyre itself cannot be controlled in any way by the dampers.

You had me worried with what you said for a while. I did my thesis on this subject in my final year of engineering

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. The same goes for when you change springs for aftermarket ones. The whole lot has to work together otherwise you can end up with something that does not handle too well in what might be considered as "normal" conditions.