Wheels can make a lot of difference tot he handling if they are selected correctly and matched to your suspension and tyres and driving style.
1] Wheel diameter
A lot of people think going bigger in size is a good thing; you can put bigger tyres with lower profiles giving you more contact patch and lesser flex in the side walls etc. These things are just a few very minor "pros" as compared to the cons.
Firstly the gains made by stiffer low profile tyres will not be felt unless the suspension is also upgraded and "setup" accordingly( will cover the setup part in diff post)
Stock suspensions are designed for comfort, reliability, ease of maintenance and cost effective for selling the car at a particular price...
(A) Loss of ride comfort
Most of the times the big wheel-low profile tyre combo only introduces a bumpy ride, puts extra load on the shocks since we have now lost the cushioning provided by the slightly higher profile stock tyres.
(B) Final drive ratio (drive ratios details will be taken up in diff post as it will need a bigger explanation)
Another most overlooked con is - loss of accearation, bigger wheels alter the effective final drive ratio. The engines are designed to make torque at a particular rpm, the gearboxes are designed and ratios selected for each gear based on this effective powerband and the size of the wheel. If you use bigger diameter wheel the gear ratios for each gear become taller.
E.g. if your car was designed to have T1 Nm of torque @ n1 RPM and travelled at X1 MPH in 3rd gear
Now since the drive ratio is changed the car will now be doing X1 MPH at a lower RPM. This means now your engine is turning at an RPM which is lower than what was originally designed by the factory, which again means you are not in the power band and will be accelarating slower. Max accelaration happens when the engine is making the max torque within a paticular range of RPM and is able to transmit it via gearbox for a specific size of wheels.
Overall the car will be slower in doing 0-60mph,0-100 kmh etc
(C)Contact patch
This will increase with bigger dia wheels if the tyres are also bigger, which means if you select lower profile the effective circimfrence of the tyre will remain closer to stock... but if you try to go for bigger tyre with biger wheels you will get a bigger contact patch which means more rubber touching the road.
This could even reduce braking distance by 1-2 feet, which is good.
(D)Unsprung wieght
Anything not heldup by the suspension and shocksis unsprung wieight - like axles, differentials and wheels.
The unsprung weight can increase slightly with bigger wheels and tyres., this increases decreases the responsiveness of the suspension over bad roads.. suspension is not able to react as fast as possible to this extra weight of wheel and tyre.
Apart from comfort factor the car will not feel responsive to quick steering changes
(E)Rotational inertia
Bigger wheels and tyres are heavier and add to the rotational mass, it makes the engine work harder to spin that extra weight of the wheels.; this is in case of ricers putting oversized bling rims on crappy underpowered FR cars.
(F) Cornering
People may argue that bigger wheels and tyres may corner better due to rigidity of the sidewall (lower profile) and more contact patch they can get more grip.
More grip means they can go harder into corners and carry more speed while turning...You will get more "G" during the cornering.
Ok - how about this argument - since you are now suddenly having so much extra grip can ur suspension handle this ? during cornering the outer tyre is now having so much traction it will cause the entire weight to transfer to this side which will cause the inner wheel to not have any weight or even be lifted off the surface of the road..slightly reducing the contatc patch or even lifting off completely.
This will now reduce the overall grip as the inner is now not being effectively used wjhile going atound corners.. can the suspension handle this exyta grip is my question..
2]Offset/ET
Let's start this simple and later add the more technical calcuilations..I'l be going slowly here as it get's complicated pretty soon.
There are 4 things involved in order to understand this:
(a) Hub face :This is the mounting surface on which the actual wheel is bolted on with lug nuts; hub us what u see when u remove the wheels.
(b)Centre line of the wheel : The centre of the wheel when seen from top of the car .. the centre line will be parallel to the hub face.
(c)Plane on which the the lug nuts are located on the wheel: The nuts are not always on the centreline line of the wheel. this plane is determined by the shape/design of the wheels. This will also play a role in moving the distance between the hub surface and the wheel centreline
(d)Distance between the (a)hub face and (b)centre line : This will become your offset.
Imagine looking at the car from the top , we are considering the front right wheel
The hub face is on the left side, the wheel centre line is on the right side.
Consider the wheel as a sleeve which mounts on the hub.
How deep the sleeve/wheel covers the hub is determined by where the mounting surface of hub meets the inner face of the wheels mounting surface.
After mounting if the wheel has more +ve offset(more gap between the centreline and hub face) the wheel wil sit deeper into the wheel arch i.e. closer to the body.
Visualise it like this - more the offset. more the gap between mounting surface of hub and wheel mounting surface, therfore in oder to make them touch we will have to push the wheel further towards the hub to make them meet, this means towards the body of the car
Worst case : It will touch the inner part of the suspensing/steering mechanism and begin to rub.
After mounting if the wheel has -ve offset(less gap between the centreline and hub face) the wheel wil sit outwards in the wheel arch i.e. away from the body.
Worst case : It will poke outside of the wheel arch and put extra load on the suspensing/steering mechanism
This means on stock rims we can use widest tyre we can want till the point will have sufficient gap between inner surface of tyre and the suspensing/steering mechanism.
If we still want to go wider we will need to replace the stock wheels with a diff wheel which increases the gap between the body and tyre..ths is where ur ET comes in.
3] Suspension and steering geometry
Camber, toe in toe out, steering axis inclination, scrub radius.. <to be continued>
4] Gear ratio (Final drive accounting for effective circumfrence)
<to be continued>
