Can someone please explain what offset means please, in relation to rims :-/

MikeDundee wrote on 24. Sep 2007 at 21:13:

Can someone please explain what offset means please, in relation to rims :-/

How far in (or out) the wheel is...
PCD is the diameter of the circle that passes through the bolt holes...

I believe its how far in or out the rim sits on the hub, thats why front wheel drive rims will fit an Omega but sit a long way into the wheel arch because it is a different offset to the rear wheel drive set up.

OK folks thanks for that, so given that I have just acquired offset 38's will they just go straight on without any of those spacers thingey's or  that?

All you'll ever need to know ....   perhaps ...

http://www.carbibles.com/tyre_bible_pg2.html

(page 1 is the same treatment on tyres !)

:)

Entwood wrote on 24. Sep 2007 at 21:25:

All you'll ever need to know ....   perhaps ... http://www.carbibles.com/tyre_bible_pg2.html (page 1 is the same treatment on tyres !) :)

Great stuff, interesting reading, still none the wiser as to whether the offset 38's are OK as irmscher only show offset 30's for the omega :-/

omega will take offsets between 35 and 45

wrote on 24. Sep 2007 at 21:41:

omega will take offsets between 35 and 45

Thanks for allaying my fears Will-Floyd :-[.......... :y

MikeDundee wrote on 24. Sep 2007 at 21:46:

wrote on 24. Sep 2007 at 21:41: omega will take offsets between 35 and 45 Thanks for allaying my fears Will-Floyd :-[.......... :y

So a tyre at 235/40....is 40 the offset? ::)

No, 40 is the profile of the tyre (height)

no, the 235/40 only relates to the tyre

235 being the width in mm? i believe the 40 is the sidewall measurement, given in a percentage figure, of the tyre width?

MikeDundee wrote on 24. Sep 2007 at 21:49:

MikeDundee wrote on 24. Sep 2007 at 21:46: wrote on 24. Sep 2007 at 21:41: omega will take offsets between 35 and 45 Thanks for allaying my fears Will-Floyd :-[.......... :y So a tyre at 235/40....is 40 the offset? ::)

Tyres don't have offsets .. only wheels do .. :)

Entwood wrote on 24. Sep 2007 at 22:01:

MikeDundee wrote on 24. Sep 2007 at 21:49: MikeDundee wrote on 24. Sep 2007 at 21:46: wrote on 24. Sep 2007 at 21:41: omega will take offsets between 35 and 45 Thanks for allaying my fears Will-Floyd :-[.......... :y So a tyre at 235/40....is 40 the offset? ::) Tyres don't have offsets .. only wheels do .. :)

So what size tyre goes on these then? :-/

http://cgi.ebay.co.uk/Irmscher-Sport-Star-18-Alloy-Wheels-Vectra-Astra-Omega_W0QQitemZ260162122203QQihZ016QQcategoryZ9888QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

225/40/18 should be fine

what you got on your current ones?

ask sir miggy what he has on his

wrote on 24. Sep 2007 at 22:08:

225/40/18 should be fine what you got on your current ones? ask sir miggy what he has on his

Thanks again :y......235/40/18 on current rims.....does Sir Miggy have sport stars then?

no but he does have 18" alloys, if they're 18x8 like yours are, you should be ok with the same size tyres as his

if anything, having 225 instead of 235 will keep your speedo as close to accurate as possible, over 2.5% either way is illegal if i'm not mistaken

bare in mind your speedo should have been calibrated in line with the OEM alloys your car had from new

if you had 16s on 225/55/16 then your 235/40/18s are only 1.33% out which is fine, but 225/40/18 is 2.55% which is too much, you might be fine putting your existing tyres on the new wheels

wrote on 24. Sep 2007 at 22:22:

no but he does have 18" alloys, if they're 18x8 like yours are, you should be ok with the same size tyres as his if anything, having 225 instead of 235 will keep your speedo as close to accurate as possible, over 2.5% either way is illegal if i'm not mistaken bare in mind your speedo should have been calibrated in line with the OEM alloys your car had from new if you had 16s on 225/55/16 then your 235/40/18s are only 1.33% out which is fine, but 225/40/18 is 2.55% which is too much, you might be fine putting your existing tyres on the new wheels

Yeah just been looking at that on the carbible thingy, what a nuisance :-/

Mick - not looked at the carbible link so apologies if this is a repost....

tyre numerical sizes are a hybrid of 3 different units, ie
235 - is the size/width of the treadface in mm's.
45 - is the height of the sidewall as a %age of the treadwidth, ie 45% of 235mm or 105mm roughly.
17 - is the rim diameter in inches.

Therefore if you increase the diameter of the rim from 17 to 18 inches, you need to reduce the sidewall height (take a lower %age profile) to keep the same rolling diameter of the wheel and tyre.  It gets further complicated by increasing the width of the tyre, obviously......   :(

235/45/17 is effectively the same size as 265/35/18...... :o

If you check this link - http://www.miata.net/garage/tirecalc.html - it has a size calculator so you can add/exchange to your hearts content and still get the same rolling diameter and see how far out the speedo etc will be....   :y

but you want to make sure your tyres arent too wide for your arches ;D

hotel21 wrote on 24. Sep 2007 at 23:43:

Mick - not looked at the carbible link so apologies if this is a repost.... tyre numerical sizes are a hybrid of 3 different units, ie 235 - is the size/width of the treadface in mm's. 45 - is the height of the sidewall as a %age of the treadwidth, ie 45% of 235mm or 105mm roughly. 17 - is the rim diameter in inches. Therefore if you increase the diameter of the rim from 17 to 18 inches, you need to reduce the sidewall height (take a lower %age profile) to keep the same rolling diameter of the wheel and tyre.  It gets further complicated by increasing the width of the tyre, obviously......   :( 235/45/17 is effectively the same size as 265/35/18...... :o If you check this link - http://www.miata.net/garage/tirecalc.html - it has a size calculator so you can add/exchange to your hearts content and still get the same rolling diameter and see how far out the speedo etc will be....   :y

Cheers for that :y

wrote on 24. Sep 2007 at 23:54:

but you want to make sure your tyres arent too wide for your arches ;D

no I don't think the arches are wide enough for the tyres ;D ;D

MikeDundee wrote on 24. Sep 2007 at 23:58:

wrote on 24. Sep 2007 at 23:54: but you want to make sure your tyres arent too wide for your arches ;D no I don't think the arches are wide enough for the tyres ;D ;D

unless you get greater offser, thus making your wkeels sit further out, giving room for a wider tyre, at least on the back, but a certain amount of wheel protrusion from the arches is also illegal i believe, and you wouldnt want wider front ones because you wouldn't be able to turn :D

If the wrong offset with a wider profile tyre then the inner edge of the fronts connects with the struts   :'(  .....

always best to stick with standard :y

tis cheaper on insurance :D

hotel21 wrote on 25. Sep 2007 at 00:06:

If the wrong offset with a wider profile tyre then the inner edge of the fronts connects with the struts   :'(  .....

According to Will-Floyd MBE the rims just purchased should be fine :y....and if they're not i'll just blame Will, and then curse myself for a couple of weeks ;D ;D

MikeDundee wrote on 25. Sep 2007 at 00:14:

hotel21 wrote on 25. Sep 2007 at 00:06: If the wrong offset with a wider profile tyre then the inner edge of the fronts connects with the struts   :'(  ..... According to Will-Floyd MBE the rims just purchased should be fine :y....and if they're not i'll just blame Will, and then curse myself for a couple of weeks ;D ;D

then i shall take this opportunity to point out that you bought the wheels before I said anything  :y ;D

wrote on 25. Sep 2007 at 06:13:

MikeDundee wrote on 25. Sep 2007 at 00:14: hotel21 wrote on 25. Sep 2007 at 00:06: If the wrong offset with a wider profile tyre then the inner edge of the fronts connects with the struts   :'(  ..... According to Will-Floyd MBE the rims just purchased should be fine :y....and if they're not i'll just blame Will, and then curse myself for a couple of weeks ;D ;D then i shall take this opportunity to point out that you bought the wheels before I said anything  :y ;D

I'll ask you first the next time :y

MikeDundee wrote on 25. Sep 2007 at 07:54:

wrote on 25. Sep 2007 at 06:13: MikeDundee wrote on 25. Sep 2007 at 00:14: hotel21 wrote on 25. Sep 2007 at 00:06: If the wrong offset with a wider profile tyre then the inner edge of the fronts connects with the struts   :'(  ..... According to Will-Floyd MBE the rims just purchased should be fine :y....and if they're not i'll just blame Will, and then curse myself for a couple of weeks ;D ;D then i shall take this opportunity to point out that you bought the wheels before I said anything  :y ;D I'll ask you first the next time :y

and if i'm not certain i'll just say i dont know :D

wrote on 25. Sep 2007 at 12:47:

MikeDundee wrote on 25. Sep 2007 at 07:54: wrote on 25. Sep 2007 at 06:13: MikeDundee wrote on 25. Sep 2007 at 00:14: hotel21 wrote on 25. Sep 2007 at 00:06: If the wrong offset with a wider profile tyre then the inner edge of the fronts connects with the struts   :'(  ..... According to Will-Floyd MBE the rims just purchased should be fine :y....and if they're not i'll just blame Will, and then curse myself for a couple of weeks ;D ;D then i shall take this opportunity to point out that you bought the wheels before I said anything  :y ;D I'll ask you first the next time :y and if i'm not certain i'll just say i dont know :D

I never knew in the first place :-/...and am still uncertain :o