hey guys i am trying to get my hands on a set of wheels from AMC here and am halfway there of getting them.
anyhoo, the rims i have on now are 205/65 R15
and the rims i will be getting have rubber on it of 235/45R17

what i would like to know is if i buy these rims(+ tyres), would it be the same rolling resistance as the 15's i have on now? becuase i dont want to buy these and find out there smaller or something.

any help or info would be very greatly appreciated :D

obviously bigger rims = wider tyres = Greater rollin resistance.

rolling resistance or rolling diameter? there is more tyre on the road with the 235's so there should be more resistance, if u mean rolling diameter grab a calcy and figure it out.

rolling circumfrnce (whatever) is the same, total tyre size around is the same.

sorry sorry sorry thats what i mean
what i meant to say was this(paint picture down the bottom)
i was wondering if i got the 17's would this size increase at all (i thought it was rolling resistance but obvioulsy i am wrong :redface: :redface: )

Wider tyres will produce more drag. Also your new tyres diameter will increase about 4.5% so your speedo would read lower than actual but in the whole scheme of things, is acceptable as the speedo requires an accuracy of plus minus 10%.

Also what's the difference in the weights of these wheels? As their mass moment of inertia would change, which is the resisting torque the engine requires to overcome to accelerate or decelerate (for brakes) the weight of the rotating parts. This is the physics of the mechanical system before any consideration of contact with the road.

This mass moment of inertia is very sensitive, as whatever your result is multiplied by 4, and reducing or increasing the radius of the mass from rotating centre gives an exponential result that can severely affect accelerating/breaking performance and fuel consumption of a car, if its way off.

Now, with consideration of contact with the road, it becomes a delicate balance: small, thin and light tyres are easier to turn but these tyres lose grip very quickly. Thick, bigger and wider tyres give you more grip but their moment of inertia become very much heavier to turn by the engine.

Manufacturers usually pick an optimum point and design around that for safety, drivability & performance.

Now, with consideration of contact with the road, it becomes a delicate balance: small, thin and light tyres are easier to turn but these tyres lose grip very quickly. Thick, bigger and wider tyres give you more grip but their moment of inertia become very much heavier to turn by the engine.

Actually the amount of contact area is not as important
If you have two tyres of the same compound yet one set is twice as wide as the other set all other things equal, you will not get any increased grip in simple situations (not turning etc)

I know tyres are very complex, but looking at it simply, the compound determines the 'grip'.

Larger tyres allows you to use a softer compound though, this is where the extra grip comes from. It is hard to find small high performance tyres.

Peace

Tim (Flame suit on)

sorry sorry sorry thats what i mean
what i meant to say was this(paint picture down the bottom)
i was wondering if i got the 17's would this size increase at all (i thought it was rolling resistance but obvioulsy i am wrong :redface: :redface: )

no wont change, ur going to bigger wheels smaller sidewall tyres so it all works out the same. there are tyre size calculators on the net for u to have a lok at but it wont change

Try this site. You just put in the size and it does the hard stuff for ya :thumbsup:

http://www.miata.net/garage/tirecalcold.html

Or here (if you want the "new" one.)

http://www.miata.net/garage/tirecalc.html

Or here:

http://www.powerdog.com/tiresize.cgi

Or here:

http://www.1010tires.com/TireSizeCalculator.asp?action=submit

than you richard M and ol' Fart - they were great websites which told me what i needed to know. my speedo will be out by approx 0.6mph - 1.5mph




and to Joukowski - you Mr know to much about tyres!

Thanks for the websites though it was just what i needed to know!!!