Date: Sun, 6 Oct 2002 12:12:01 +0100
Reply-To: Clive Smith <clive.harman-smith@NTLWORLD.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Clive Smith <clive.harman-smith@NTLWORLD.COM>
Subject: Re: Bigger Wheels, Unsprung mass and Calculations
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a) Its not weight, its MASS that we should be talking about, but 'saying
unsprung weight' is OK as long as we know that weight is the gravitational
consequence of mass and that weight has no bearing (sic), especially when
discussing the rotational effects.
b) W.r.t accelerating a rotating object it is Moment of Inertia that matters
(2nd Moment of Mass; so diameter squared is the proportional factor)
c) So we have additional mass to accelerate linearly PLUS additional Moment
of Inertia to get spinning.
d) Don't forget that 15" tyres are heavier generally as well as the wheels,
making the situation even worse.
e) I haven't done some typical calcs to see if the equivalent is 10 x or
not, and it would not be a fixed multiplier, but as a rule of thumb it
sounds possible. Its the difference in equivalent vehicle mass so the
vehicles acceleration would be 200/3000 = 6%, as you say about 19 seconds
instead of 18 seconds with the same gear ratios (which may not be true).
f) Can you notice 6% - NO. Does it matter, probably NOT.
What does matter?
The unsprung mass has to be controlled by springs and dampers.Driving slowly
over bumps and bad terrain won't hardly be affected, but when driving at any
speed other than a crawl WILL. The wheel's displacement will be greater, the
forces put into the chassis via the shock absorbers will be greater and the
shock absorbers will heat up more, be less effective and more easily lose
control of the wheel - the outcome is worse bump and rebound control and
hence less good roadholding, bump tolerance and ride harshness (a well know
side effect of going to larger wheels, whether wider or taller and why I
generally have avoided them like the plague, ditto lowered suspension with
less travel). Its even been known for the shock mountings to eventually
fatigue crack under these circumstances (but not to my knowledge VW's).
My Syncro seems fine with 15" Audi alloys, but then, the Syncro's unsprung
weight at the front is quite high anyway (driveshafts, outboard CVJ's,
strong well designed suspension and bearing componentry, reasonably decent
brake disc and caliper sizes etc) so the OEM shockers will already be quite
well rated and the % change not so much...
... not so for the 2WD Transporter though!
Clive
'88 Syncro Transporter
P.S. Whoever it was responding negatively back there in the threads, don't
knock a little well based theory; I've seen guesswork and blase attitudes
kill people, and am sure given the chance, they and their wives and children
would rather them still alive! Thats why I like VW's, most of the details as
well as the guts seem to have had an ENGINEER cast an eye and sliderule over
them, rather than a smartAlec guessing game designer. Not perfect, but not
crass wanton underdesign as seems to crop up these days, especially when
modifications are involved e.g. bigger wheels just for the looks (and
supposed roadholding benefits).
----- Original Message -----
From: "James" <tornadored@HOTPOP.COM>
To: <vanagon@GERRY.VANAGON.COM>
Sent: Sunday, October 06, 2002 9:54 AM
Subject: Bigger Wheels and Unsprung Weight
> I've been considering doing the 15" wheel/tire upgrade and noticed that
some
> of the replacement SA alloys weigh quite a bit more than the stock 14"
alloy
> (18lbs) (according to GoWesty's specs).
>
> The popular Sunburst SA wheel is listed at 19lbs - only 1lb more than
stock.
> But the Carat2 wheel, for example, is 23lbs...quick math...that's 5lbs and
> 28% heavier than the stock. And the Rhein style wheel is 24lbs!
>
> Which gets me thinking about the effect that weight has on performance. I
> would think the added unsprung weight (again, 28%) must have a very
> significant effect on handling and suspension response. And I've "heard"
> that 1lb unsprung = 10lbs sprung. So, 5lbs more per wheel = 20lbs more
> total unprung = 200lbs sprung. More rough calculation...that would yield
> about a 1 second slower 0-60 because of rotational weight.
>
> I hope I'm missing something here. What are people's real
> world/seat-of-pants results after going big? It almost seems like the
> benefit of the larger/wider tire (in our favorite brand) would be
outweighed
> by, well, weight.
>
> Curious.
>
> James
> 90 Carat
