RE: unsprung weight

[Eaton Dave <dave.eaton@minedu.govt.nz>]

generally speaking, upsized tyres add weight (and rotational inertia, but
that's another story). some published weights from the dunlop sp9000's
illustrate this (again, manufacturers rarely publish this information).

so, for example we have a ur-quattro: -
15x5 et35 tsw's @8.4kgs and 215/50x15 sp9ks @9.4kgs and we have 17.8kgs or
39 lbs

now, we go to 17x8 wheels: -
17x8 et35 tsw's @10.8kgs and 245/40x17 sp9ks @10.9kgs and we have 21.7kgs or
47.7 lbs

so, an overall increase in the weight of the wheel of over 20%.

how much does this affect overall unsprung weight?  well, certainly not as
much as 20%, but way more than 3-4% as has been stated.  the major
components of unsprung weight are the wheel/tyre, the brakes (rotors and
calipers), and the suspension pieces (not all the suspension of course as
some of this is sprung).

as a guestimate, you'd be looking at around 3kgs for brakes (7 lbs), and
maybe 6kgs (13lbs)  for the suspension (this would be too much i suspect).

now re-run the figures and you have an original unsprung weight of
17.8+3+6(=26.8kgs) or 59lbs per corner.  upsizing the wheels and tyres
results in 21.7+9=30.7kgs or 68lbs per corner.  total unsprung weight
therefore increases from 107kgs to 123kgs or 235lbs to 270lbs.  this is an
overall increase of 15%.

hth,
dave
'95 rs2
'90 ur-q
		-----Original Message-----
		Date: Wed, 17 Feb 1999 15:02:12 -0800
		From: "Bernard Littau" < >
		Subject: Re: unsprung weight

		Dave writes:
		>the 15x8 et35 weights 8.4kg,
		>the 17x8 et35 weights 10.8kg
		>
		>thus an increase of 2.4kgs (5.3 pounds) per wheel.  to put
it another way,
		>by going +2, you have increased your unsprung weight from
the wheel alone
		by
		>30%.
		>
		>of course, the tyre weight would be on top of that.

		The tire weight would not necessarily be strictly additive,
as you imply.  I
		would assume that a 50 series tires weighs less than a 70
series tire, all
		other things being equal, no.  It may be a wash in terms of
total mass.