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My God, Whats with the metric crap??

jcnoble

jcnoble

NAXJA Forum User
Location
Huntsville, AL
I am trying to upgrade my tires/wheels to 33" and 15x 8" with 3.75 " backspacing. All the tire dealers have rims listed with backspacing in metric, millimeters i think. The conversion is 2.54 cm per inch. Soo 3.75 inch in cm would be 9.54 cm or 95.4 mm?? The place says -20mm is actually 3.75 inches backspacing. Wow am I confused here, I have an engineering degree and I am stumped. Anyone????
 
Dumb question - have you recalculated backspacing from the centre of the wheel? I think backspacing is measured from the inner plane of the wheel, typically (but I can't swear to it at the moment!) and some manufactureres may do it differently, with an "offset" rather than true "backspacing." Zero offset would be with the mounting surface on the centreline of the wheel, halfway between the flanged (at 3.5" for a 7" wide wheel.)

Engineering degree? What discipline? I'm getting ready for MET/Automotive Emphasis, once I finish the three AS's I'm working on (Tool & Die Maker, Engine Machining, and Engine Management & Performance.) How do you like your field, and what's your emphasis?

5-90
 
Im electrical, I guess thats why I am having a hard time. I can calculate what current my fuel pump is drawing but wheel backspacing, hmmm. Still though I dont think this should be difficult to figure out. Also I take exception with your "dumb question" comment. There are no dumb questions just dumb answers. Anyway, thanks for your reply.
 
Yes, thats what the tire dealer is telling me also, I just dont understand how backspacing is measured I guess, and why metric instead of english units? We all seem to talk english units for backspacing on this forum?
 
Because we all think in inches, and the rest of the world thinks in metric. The first thing I do when given metric plans is reclaculate everything into inches.

By the way - the "dumb question" comment referred to what followed, not what preceded. I was talking about myself - just so you know (it's a phrase I use a lot, usually to call out something that I think is obvious, but someone else may not until it's posted.) No reason to take offence - I rarely seem to offend anyone here anyhow. If I'd been talking about your question - I'd have said so, and I tend to be more politic about things like that. I just have a tendency to insult myself at will - although I don't usually allow anyone else to get away with it.

5-90
 
No offence taken. Hey, I'm the first to admit I don't know everything (or anything?) . I just cant stand those who think they do, of which you are not. Anyway, sorry to part your hair. So what was the answer on backspacing , I still dont get it? LOL
 
No problem here - just saw a need (probably misplaced) to smooth things over with you. Like I said, it's a cue - something I think is obvious that you might not.

I think that "backspacing" is an un-official standard - which is often supplanted by "offset." Catch is, I'm not sure which way is positive and which way is negative - but I'm inclined to think a negative offset would be going outwards, and positive would be going inwards (but don't hold me to that.) Either way, 20m/m is a little over 3/4" - figuring from a 3.5" centreline, that would come out to either 2.75" or 4.25" BS - depending on which way they're going. At least, that's my read on the situation. I'd ask for an explanation in "rich dummy" terms - because I tend to simplify things myself, and I remember information in its simplest form. Again, that's just me.

(Oh - I don't have enough hair left to part anyhow. Don't worry - no offence here, either.)

5-90
 
Wheel offset is the distance the mounting surface is from the center of the wheel. On an 8" wide wheel, a 20mm offset is about .78 inches from the center, so 4-.78=3.22 backspace. If it had a positive offset, you would add the .78.

Kids, don't try this at home, I'm a trained proffesional.

P.S. For a 3.75 backspace, convert .25 inches to mm. It's about 6 MM.

http://www.tirerack.com/wheels/tech/techpage.jsp?techid=101 for an explaination.

The only problem is the people who use offset measure rim width oddly, so a normal 8" wheel is only 7.5 for their calculations. It gets confusing. I hate selling
wheels.
 
Yurf. Why do they use "offset" rather than "backspace," then? Just curious - there's some logic to having standards, you know.

So, then, a "negative" offset is subtracted from the centreline, and a "positive" offset is added? I just want to make sure - like I said, I tend to simplify things for my own reference...

5-90
 
Best way to look at this is this way. If you had a 8" rim and the WMS or Wheel Mounting Surface was dead center it would equal 0 offset ( 4" each side of the center line ) or 4.5" of back spacing. The reason the back spacing is higher is that it is measured from the WMS to outside of the rim but the offset, as is wheel width, is measure to the inside mounting surface. So any converting you do you need to subtract or add the thickness of the wheel rim which is 1/2" depending on which way you are converting.
So converting 19 mm to BS on a 8" rim would be 19 mm = 3/4". Half the rim width is 4" minus the 3/4" equals 3 1/4". Then add the 1/2" of rim thickness and you end up with 3 3/4" or 3.75" of BS. :)
 
Sounds good but arent you in Australia? That means everything is backwards from the US. Right hand drive , toilets flushing in reverse etc....So is backspacing the same there?
 
I think you can safely trust GoJeep's explanation. It works the same in the UK where we have RHD vehicles with inch size rims and metric tires and offsets. I prefer the backspacing method but it seems all wheel manufacturers outside the USA use offset which is probably why I have always bought wheels in the USA.
 
Yes, it just gets confusing when there seems to be no standard between manufacturers. Its like when the mars probe crashed because they didnt convert properly from metric to english or was it vice versa? Or here in the US when you buy a 2X4 board it is 1 1/2x3 3/4 or when I measured my new 33" tires they were actually only 32". Has the world gone crazy?
 
jcnoble said:
Or here in the US when you buy a 2X4 board it is 1 1/2x3 3/4......Has the world gone crazy?
Click to expand...

That one is easy. That is a nominal measurment of the lumber before the milling prosses. If you were to go to a lumber mill and ask for rough saw lumber it would actually be 2in. by 4in. But because we like pretty wood that wont give us splinters, they mill it again to make it smooth.
 
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