RE: unsprung weight

[Luis Marques <marques@ti.com>]

> From: 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%.

Yep.  Just got done upgrading the wheels on my other car.  Stock, worn
205/50-15 tires on 15x7" allow wheels weighed in at 34 lbs.  New 17x8"
wheels and 235/45-17 tires weighed in at 49 lbs on my digital bathroom
scale.

One would intuitively think that a lower-profile tire with the same
diameter would weigh less than a higher profile tire, but the truth is
that 99% of the time you do +1 or +2 upgrades, you also go to wider
tires, so there is more material, plus there's fact that ZR rated tires
are more strongly built with stiffer side walls, etc.  In my case, the
new 235/45ZR17 tire weighted about 25 lbs (about the same as the
wheels).  It's easy to find heavier and lighter wheels.  a 17" rim could
weigh between 18 and 30 lbs depending on width, how much you are willing
to spend and how strong and light you want it.

The additional inertia is generally not a problem in a car with enough
power to smoke them off the line, but I wouldn't put them on a 100 hp
car.  You do give up a little straight-line acceleration for better
cornering.

> 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).

I think these guesstimates are on the low side.  Just calipers, pads and
carriers could easily weigh 10 lbs on each corner.  Rotors?  10 more
lbs.  Hub, hub carriers, bearings, cv joints, 1/2 drive shaft, struts,
1/2 strut inserts, 1/2 springs, 1/2 a-arms, 1/2 sway-bar arms, all add
up to a lot more than 13 lbs.  All that together has to be more than 50
lbs. 

> 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%.

I think total unsprung weight is closer to 90-100 lbs per corner. 
Depending on which wheels/tires you choose, that could mean a 5-20%
increase.  Even on the same size, some wheels may weight 10 lbs less
than others.  HTH

Luis Marques
'87 4kcsq