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RE: Calculating tire pressure's effect on spring rate?
"limted engineering know-how" ??
You've already lost me, but it sounds really neato...
> -----Original Message-----
> From: Scott Fisher [SMTP:firstname.lastname@example.org]
> 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
> 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
> 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
> 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
> modern street radial is to raise the effective spring rate.
> But... what *is* the effect? It's more complex than I can figure out
> sitting here and thinking about the bits and pieces: take the
> square-cube drag law and apply it to the ratio of vertical wheel
> (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
> 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
> 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