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FW: Making your car go faster, Part 3.2: Tuning the rig

Apparently this never got posted. Trying again...

>-----Original Message-----
>From:	John Browne 
>Sent:	Friday, September 13, 1996 4:00 PM
>To:	'BMW List'; 'bmw-performance@domain.elided'
>Subject:	Making your car go faster, Part 3.2: Tuning the rig
>
>Remember (I'm dating myself, I know it) Carly Simon's song,
>"Anticipation"? That's what keeps F1 drivers (and more normal ones as
>well) on the track. Generally in a truly fast car, you have to crank in
>inputs before the car does something evil. By the time the car is doing
>something bad, it's too late to correct. 
>
>Ok, are those guys psychic, or what? How do they know the car is about
>to do something evil? Here's where we get back to suspension setup.
>Generally the answer is that the driver needs a car which provides a lot
>of feedback, so he/she can feel what's going on. Knowing what to watch
>for as a signal that something is about to happen let's them anticipate
>their way out of trouble. 
>
>Here's a frinstance: as tire lateral loading increases, positive caster
>generates a ever-stronger self-righting force. That is, you crank the
>wheel to make a turn and the wheel wants to return to straight ahead.
>This is one of the advantages to having a lot of postive caster in your
>front end (there's also a drawback, natch). If you have power steering
>(here's where the 02 guys have an advantage) you can't feel that
>righting force clearly, because the boost overwhelms it. Why do we care
>about the righting force? 
>
>Because just before a tire starts to slide the righting force drops
>considerably, and a good driver will notice the wheel get "numb" and
>take corrective action. With power steering, it's really hard to feel
>that numbing in the wheel, if at all. However, the suspension has to
>have most of the micro-compliance (I just made up that term, don't go
>quoting it) removed for this to happen. Otherwise everything just feels
>kind of dead. Micro-compliance is the rubber in the bushings all through
>your suspension that keep a lot of road harshness and noise from
>intruding into your trip to Grandma's. Race cars use rock-hard bushings
>in place of rubber to get two improvements:
>
>1) instant response to even tiny steering inputs, since you're not
>wasting time/energy compressing rubber bushings
>2) total feedback from the road.
>
>So stiff springs and swaybars are the order of the day. 
>
>But not always.
>
>What about when it rains? A wet track means far less traction available
>to the tires, so less lateral loading will be possible. In this case you
>want to be even smoother than before, and so lots of race teams will go
>with a "soft" setup, replacing springs, shocks, and swaybars with softer
>rates all around. 
>
>Sometimes what seems right just isn't. When BMW first started racing
>IMSA back in the 70s with those beautiful CSLs, they found in testing
>that high roll stiffness wasn't the fast way around the track. They went
>with softer and softer sway bars, with the cars rolling more and more,
>and kept shaving seconds off their lap times. Granted, these cars
>weren't exactly NYC taxis when they were done, but they _were_ a lot
>softer. Lap times never lie. 
>
>What about camber? Why is it important? 
>
>Take this test. Take a new pencil, hold the eraser to a smooth surface
>perfectly perpendicular, and attempt to slide the eraser. Now tilt the
>pencil slightly away from the direction of travel and repeat. Two things
>should become apparent: The harder you push down the harder it is to
>slide the eraser (duh!). A tilted eraser is harder to slide than a
>perpendicular. This is called the camber effect, and race cars typically
>want lots of negative camber (tires tilted in at the top) because of it.
>On older, bias ply racer tires, when the tire was loaded laterally with
>lots of negative camber it would actually lay flat on the road. Negative
>camber was added until tire temperatures, taken right after some hard
>cornering, showed even tire temperatures across the face of the tire.
>Modern radial race tires (like BFG R1s) distort differently due to the
>different construction method and thus won't show even tire temps across
>the face. All this tire temperature stuff is vitally important to race
>guys: you see crew workers show temperature slips to drivers during
>practice etc all the time. They learn a lot about their setup from temps
>and so can you, if you have a pyrometer. 
>
>Sadly, too much negative camber ("Marge, I smell a tradeoff coming...")
>eats up the inside tread of your tires from just diving around on the
>street. Lowering your car will induce negative camber, particularly at
>the rear, less so at the front. So before you buy those super-lowering
>springs you might want to think about what they will do to your camber.
>
>What the really big guys do is to set up for a given track on a given
>day. Typically you'll optimize for a given section of track, since a
>setup that makes you fast through one turn may suck through another.
>Some turns are more important than others (see Alan Johnson's book,
>mentioned in part 1) so it makes sense to optimize for them over less
>important turns. This is why teams keep careful records about every time
>the car is on the track, track conditions, setup, lap times, tire temps,
>etc. Data acquisition for an Indy or F1 team is a serious business. As
>rank amateurs we have to do the best we can with a notebook and pencil.
>You also need to develop a feel for what the car is doing, and enough
>understanding of the components and their interrelations that you can
>figure out what changes to try. 
>
>Rule 1: never change more than one thing at a time. Change your tire
>pressures, or change your swaybars, but don't change them both and
>expect to learn anything useful. 
>
>As you get better and better, you'll want a car that allows you more
>control over its attitude (no, I'm not talking about "Yo momma") so you
>can rotate the car easily and point it where you want it. Until you get
>to Formula Atlantic and beyond, this is the hot ticket for getting a car
>through the tight stuff. It takes skill and lots of practice to master.
>
>And the right suspension setup.
>
>John Browne
>BMW CCA
>BMW ACA Puget Sound Region
>M3 LTW (PeeKay)
>Suburban 2500 (Godzilla)
>326 iX (Spunky the brave little car)
>
>copyright (c) John Browne; all rights reserved
>
>