RE: Calculating tire pressure's effect on spring rate?

[Ed Kellock <EKellock@placemakers.co.nz>]

"limted engineering know-how"  ??

You've already lost me, but it sounds really neato...

:-)

Ed

> -----Original Message-----
> From:	Scott Fisher [SMTP:sefisher@cisco.com]
> Sent:	Friday, October 02, 1998 10:30 AM
> To:	Audi List
> Subject:	Calculating tire pressure's effect on spring rate?
> 
> While we're talking about obscure handling effects, I've wondered
> about
> something off and on for a long time, but especially lately.
> 
> On my first blast through Highway 9 in my '83 Coupe back in June, I
> noticed an unhappy wobble in the right rear.  Felt like a shock not
> quite right.  I decided to check out the suspension and see what I
> could
> come up with.
> 
> Well, wisdom costing nothing in the application and everything in the
> acquisition, I checked my tire pressures after I got home.  Sure
> enough,
> the tires were pretty low, 26 psi in all except the right rear, which
> was 21 or so.
> 
> A quick stint with the air compressor brought them all back up to 38
> PSI, where I was MUCH happier with the car's handling, steering,
> transient response, stability in corners -- you name it.  Yeah, they
> thumped a bit more, but it was MORE than well worth it.
> 
> And it got me thinking.  In the right rear, I nearly doubled the air
> pressure in the tire, and one purpose for raising the air pressure in
> a
> modern street radial is to raise the effective spring rate.  
> 
> But... what *is* the effect?  It's more complex than I can figure out
> by
> sitting here and thinking about the bits and pieces: take the
> square-cube drag law and apply it to the ratio of vertical wheel
> travel
> (that's the travel of the rim relative to the ground, not the wheel
> relative to the chassis) over the cumulative surface area of the
> inside
> of the tire... it may not *quite* be Newton's three-body problem, but
> it's beyond my limited engineering know-how.
> 
> So.  Anybody got a formula that says "raise the tire pressure X% and
> it's like raising the spring rates Y%"?  (I'm guessing it's, oh, on
> the
> order of X = 10Y, but that's pure seat of the pants...)
> 
> (And in case there's any question about what I'm REALLY up to, I'm
> trying to convince myself that yeah, I really WOULD enjoy the car more
> with stiffer springs, even if they WERE a little more rumbly... :-)
> 
> --Scott Fisher
>     Sunnyvale, CA