changing wheel size... what effects does this have?

As posted already.

Note that this changes the gearing as well by the same amount, which alters the way the car behaves in different gears. The most likely problem is that the tyres will, at some point, begin to touch the bodywork, particularly on full lock at the front.

Depends on the size you go to and from.

The thinner the tyre, the worse your ride and handling become on anything other than a glass smooth surface (ie, not a UK road !)

I went from 14" (185/65/14) to 16" (205/45/16) on my last car with no ill effects, and I've gone from 16" (205/55/16) to 17" (215/45/17) on my current car with no ill effects.

It will have approximately zero effect. Your rims are getting one or two inches bigger, but the tyres are getting one or two inches thinner to compensate - net result is that you have exactly the same diameter as before. The bigger wheels will be wider though, so there are limits to how big you can go, before things start rubbing.

Eh ?

Not an option. If you're fitting bigger wheels, then you need to fit lower profile tyres.

What car do you drive, and what size tyres are currently on it ?

snippage

Well, it /is/ an option, but not without buggering up the gearing/speedo reading/handling etc, and adding the chance of much rubbage.

its not my car, so i dont know the exact size, its an L reg astra, with 13 inch wheels my friend wants to just buy a set of alloys off a bigger car and just stick them on, I am tryin my best to persuade him not to!

I've got this funny mental image of 18" wheels with 225/80SR18 tyres!

Peter

It'll work fine, as long as he takes the wheels from another Vauxhall (otherwise the offset will be wrong) and then fits the appropriate sized tyres :)

Which means it's not an option :)

LOL !

Does your friend know if they will fit on OK? You need to know what the correct offset for the wheels should be on the car, then make sure that the offset is the same on the alloys, otherwise you might need spacers. Just because an alloy wheel can physically bolt onto a car, doesn't mean it'll be correct for the car. If your friend doesn't know what the offsets are or doesn't have a clue what that means, I suggest he finds out first. Also, if they're bigger, you'll need to know if they'll physically fit without rubbing on the wheelarches.

Chris.

You can change the rolling radius of the wheels on your car. It's not something you would usually want to do because it won't do you many favours and the insurance company will load your premium, but it certainly can be done.

I'm in the process of changing the rear diff ratio from stock (3.90) to a lower ratio (3.08), which will have a much more profound effect on the gearing / speedo than any likely wheel & tyre swap.

Thing is, though, nobody I know would see any real point in changing the diameter of the wheels. The drawback is they'll rub against the bodywork if larger and wear out faster if smaller, spoil the gearing, and create insurance issues. The plus side is, well, I can't think of any except for the possibility of putting larger diameter wheels on if rallying the car and increasing the size of the springs for extra ground clearance. Track or road, no, I#d only change the wheel size if they were wrong to begin with, e.g. if I had armour plating weighing it down I might go for a larger wheel just to even out the load better, merely widening the tyre would change the contact patch ratio and it would slither about in the wet.

Basically, it can be done but it's a poor idea and probably comes about because someone has some other wheels they want to fit and doesn't realise the issues involved.

Contrasting this with the much more common approach, changing the wheels for larger ones and putting lower profile tyres on is a perfectly valid way to stiffen up the suspension for spirited road / track use.

You'll find that's a higher diff, contrary to how certain posters around here define these things! Lower is more revs for a given speed, higher is less. But close enough :)

Comfort is one plus of larger wheels, or at least, larger wheels mean a more comfortable ride if you're not fitting rubber bands to hefty chunks of shiny crap.

Also it's getting harder to get tyres in certain sizes.

Overall though, I agree. Larger wheels for the sake of larger wheels seems to be just for the looks, as how often do you see cars with aftermarket large wheels running the larger brakes etc that they allow to be fitted?

I can comfirm 45 profile tyres are less comfy than 70. But my wheels are made of some kind of Alloy, not crap, as i don't think that would hold its shape very well...

Although, they are VERY shiny due to the crystal finish ;)

The opposite is true - it's the larger *tyres* that increase the comfort !

If you fit larger rims to a given car, then you'll usually fit thinner tyres - so comfort is reduced.

Yes, that's exactly what they're for.

Er, almost never. I fitted my larger wheels cos they look nice - not for any other reason.

Gee, do you think ? :)

People generally don't buy bigger wheels to fit bigger brakes and fitting bigger wheels does not mean your car will now be much faster than before, so brakes are really nothing to do with it.

I just bought bigger wheels to clear my bigger brakes :) Altho i've gone an inch more than needed ;)

-- Chet

Brakes can be everything to do with it, as someone in a car mod group should know!

If you have a car on 13" rims as standard, with maybe 90hp, then if you get silly and double the power, you might want to think about bigger brakes. Which could well mean having discs far too big to fit under the 13" rims. Therefore you get bigger rims. Common case is Minis - Unless you want to pay shitloads for Cooper 10" setups, to fit front discs in place of drums on the older ones, you use 1275GT or post 84 uprights and discs. But then you need bigger wheels as the 10" ones simply don't have the clearance.

A Dolomite mod is to fit Sierra front discs and callipers to the hubs. It's just possible if a little metal is taken away from the calliper body. So obviously anything bigger will result in needing bigger wheels.

Bigger wheels doesn't have to mean you're going faster, but it can mean you can improve your braking from the speed you're doing anyway. Plus, if bigger wheels mean more grip, that means more braking is available, which means more heat to dissipate, meaning bigger brakes once again aren't such a bad idea. Only applicable if you're actually on the limit though, which of course would be most likely illegal on the public highway ;)

There are times I get tired of having to explain each and every line to the slow children you know! ;p

Larger rolling radius improves comfort. Which means balloon tyres on the standard rim, or more normal sizes on a larger one.

It all depends why you're fitting larger wheels, and what you want from them. You or I might sacrifice comfort and run lower profile rubber to increase grip, but some people might simply want to improve the ride.

Doesn't have to be though.

I must be odd, in that I prefer to change things so they work better, rather than look more nickable then :)