Bigger wheels, Higher Car?

[Fenix]

Hi Peeps

At the mo i'm running some 14" wheels with 175/55/14 tyres on. But i'm planning on sorting out wheels and bodykit sometime this year. I've been given some rather deep M3 sideskirts and when lined up against the car at the mo there is probably a maximum of an inch clearance.

if i put 17's on the car 205/40/17 will this raise the car height off the floor due to a difference in rolling radius? Anybody know what the rolling radius of my current and future wheels is?

[Carlp]

Well i am no expert but think about it this way mate.

14" wheel with a tyre profile of 55 so quite a large sidewall.

17" wheels which is an extra 3" with a tyre profile of say 40.

So instantly if you measure the wheel from the centre to the rim it

is approximately 1.5" more. However this increase is radius is compensated by the lower profile tyre. So the extra radius would probably be roughly 0.5".

Like i say i am no expert but this is how i understand it. No doubt someone will prove me wrong and explain it a bit better and give you a more definitive answer.

Carl

[DaveB]

my 17" wheels were exactly 1" bigger diameter than my 14" standard alloys with 195/45 tyre.

[tiny]

hi

14" rolling dia = 548.1 (for tyres specified)
17" rolling dia = 595.8 (for tyres specified)

ie 108.7% so at 70mph speedo reads 76.09mph

[Carlp]

Well guys thats great i stand corrected and have learnt something new today

[Fenix]

right take it thats mm then.

so 1/2 of diameter gives radius

which means

14" 274.05
17" 297.9

giving an extra 23.85 mm in height so almost an inch higher and almost an inch more filling the arch. woohoo the car will look feckin low and the wkirts should fit.

[Kerr]

i think you will find that wheel maybe too big. You will also find the bigger wheel will ruin your accelaration especially with small engine. I think you want to try for a lower profile tyre. The way to calculate the overall diameter is as follows. When a tyre for instance is stated as a 205/40 *17, the tyre will be 205mm wide and the wall of tyre 40% of the width (40% of 205mm in this case). The wheel is 17 and you convert inches into mm by multiplying by 25.4, therefore, 17*25.4=431.8mm. The tyre size is calculated by (2*tyre width*profile)+wheel diameter. (2*205*0.4) +431.8=595.8mm. Tiny all your figures are right apart from the last. Because the wheel has larger circumference each turn the car will travel further, so infact the car will travel faster than with the smaller wheels.

[Vauxnova]

http://www.fernblatt.com/longhurst/tyre_bible.html

[Fenix]

a: i looked at 18's, 19's and 20's but my budget won't stretch to them so doubt i'll have problems with low profile 17's

b: it's a 1.4 nova saloon if i was bothered about acceleration i wouldn't be driving it

My major concern is having more room below the sills to fit these sideskirts on.

[tiny]

kerr i just used a computer program i got off the web! ah well