wheel offset (was Re: Anyone want some new wheels?)

Post by **zorruno** » Fri Feb 02, 2001 3:39 pm

I have 16" wheels with 45mm offset. I use 5mm spacers for the rears (=
40mm) - I have played with the spacers off then back on again. A slight
increase in cornering judder sometimes on uneven surfaces (is this what they
refer to as bump steer? - or is this front wheel tracking only?) but balance
on smooth road surfaces is improved IMO. I like the 40mm better.

The possibility of wheel bearing failure with smaller offsets and/or larger
wheels has been discussed on the US lists (over and over), but I think this
is rubbish. There are a number of people around the world running low
offsets on an MX5 Miata. I know of a couple running 32mm and 15" wheels,
and one with 35mm and 16". One guy runs 27mm on 13" rims for the track.

A couple of wheel/performance gurus in the US have stated that offset should
be decreased as wheel widths increase anyway to keep approx the same width
of rim on either side of the hub-plane.

I'd buy the 38mm, and I doubt 35mm would be a problem at all if you found a
nice/cheap/light enough wheel, and the wheel/tyre combo fit in the guard OK.

H.

Gary Morrison · Post by **Gary Morrison** » Sat Feb 03, 2001 12:29 pm

A couple of wheel/performance gurus in the US have stated that offset should
be decreased as wheel widths increase anyway to keep approx the same width
of rim on either side of the hub-plane.

As offset is measured from the centre of the wheel, I think that as wheel
widths increase the geometry would still require the same offset.

Colin? We talked about this a while ago?

Gary

Colin · Post by **Colin** » Mon Feb 05, 2001 8:54 am

Hi all,
Yes Gary I recall our conversation.
The centre is always the centre. This being the case I believe that the
width of the wheel does not influence the offset.
ie: a std wheel is 6in wide with a 45mm offset measured from a point 3
inches from either side. 14 inch diameter
my wheels are 6.5 inches wide with a 45mm offset measured from a point
3.25 inches from each side. 16 inch diameter
This means in both cases that the center of the wheel is 45 mm offset in
both cases.
The main reason for reducing the offset is because the wider you go the re
is a chance of fouling the coils. Reducing offset moves the centre of the
wheel towards the outside of the car.
Mx5 suspension is very easily upset by changing the offset. If you wish to
pottle around town on a sunny day then it doesn't really matter. If you wish
to pottle one weekend and really use the car on windy roads the next,
retaining the offset is fairly important. Lower offsets also place greater
strain on bushes etc.. increasing wear.
I don't think the diameter makes any difference either. The centre is always
the centre.
A friend has a publication with a good section on wheel/ tyre combos. I will
see if I can get it scanned.
Cheers
colin

Post by **zorruno** » Mon Feb 05, 2001 2:45 pm

The centre is always the centre. This being the case I believe that the
width of the wheel does not influence the offset.

Yup - I have rethunk that logic, and yours is quite correct.

however I'm told the 1998/9 MX-5 stock offset was 40mm for 15" wheels -
(The track increased on the 98/99 and overall width by 4mm) The stock 14"
were 45mm offset however. Anyone know what the stock offset is for 2001 16"
wheels? (http://www.mazda.co.nz/models/1012.html shows that the track
increased again on the 2001 - and I suspect this is because of an offset
change)

I don't think the diameter makes any difference either. The centre is
always the centre.

definately agreed.

Mx5 suspension is very easily upset by changing the offset. If you wish to
pottle around town on a sunny day then it doesn't really matter. If you
wish to pottle one weekend and really use the car on windy roads the
next, retaining the offset is fairly important. Lower offsets also place
greater strain on bushes etc.. increasing wear.

I think there must be more to it than wear increasing. Not too many MX5
drivers drive with the revs below 5,000 RPM just to reduce wear on the
engine

And although it is agreed that wear on bushes etc may increase, I
don't think at +/- 10mm you'll have to replace anything noticeably earlier.

Obviously YMMV, but I think it is more to do with the vector loading,
causing changes to unsprung weight i.e. the further out the centre of the
wheel is from the hub the more unsprung weight factors apply (hold a brick
out on a stick - forces are greater when you increase the stick length)

Increasing the track has performance benefits - so does decreasing unsprung
weight. I believe the balance of these two items (plus many other factors
obviously) is what is important.

OK then... I did a bit of searching, and came up with a couple of opinions.
Some technical, and some less so...

quote from the performance handbook (sorry - not the best source of info...)
"...most aftermarket wheels have a 35-37 mm offset! A 10 mm difference will
still bolt on & look correct and you can get away w/ it most of the
time...but if you are a critical enthusiast it could result in unacceptable
quirks."

other opinions of owners running lower offsets
http://realbig.com/miata/2000-07/1413.html 30mm offset in Germany...
http://realbig.com/miata/1998-04/3632.html ...and sold by Mazda
http://realbig.com/miata/2000-05/657.html 38mm offset
http://realbig.com/miata/2000-01/2749.html 27mm for race use
http://realbig.com/miata/1999-11/1330.html 38mm offset
http://realbig.com/miata/1998-08/2280.html 27mm
http://realbig.com/miata/1998-06/4098.html 35mm
http://realbig.com/miata/1999-11/1327.html
Seems to agree with my uneven surface experiences
http://realbig.com/miata/1999-11/1331.html and this
http://realbig.com/miata/1996-04/1245.html and this with 35mm
http://realbig.com/miata/1995-09/255.html
and this - also reasons for -ve offset in a vehicle
http://realbig.com/miata/1995-09/277.html reasons for +ve offset
http://realbig.com/miata/1997-09/2198.html different offset front to rear
http://realbig.com/miata/1997-02/285.html
handling should improve with smaller offset...(2nd hand info)
http://realbig.com/miata/1996-04/1297.html
a negative opinion - likes the 45mm offset
http://www.miatapower.net/cgi-bin/wilma ... msg00687.h
tml?line=38#hilite
arguement for decreasing the offset with larger wheels
(url may split to 2 lines)

A friend has a publication with a good section on wheel/ tyre combos. I
will see if I can get it scanned.

Tirerack has good info about how offset is measured.
http://www.tirerack.com/wheels/tech/offset.htm
They also have lots of other wheel/tyre technical info.

Gary Morrison · Post by **Gary Morrison** » Mon Feb 05, 2001 3:04 pm

zorruno you're right about the 40mm offset on the later cars, I have seen a
list showing this on the net, but I can't think where.