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May 31, 2004 9:38:56 PM

Bilal: Expert; Loc: Manchester, UK; Posts: 1533, Gallery; Registered on: Jun 20, 2003

Chassis dynamics?

Here's one for you infonuts, eg Carlos

In plain English can anyone explain how some cars perform better on the track than others?

I mean I know ultimately you need a lightweight car with 50/50 weight distribution, RWD, and possibly rear engined BUT. After watching the SLR on the top gear test track what got me was the corner which promotes understeer in any front engined car but oversteer in RWD rear engined cars, to the SLR it just turned on a dime and no oversteer just pure grip. How can this happen? (grip)? THE CGT IMO is set up for controllable oversteer ala M3 CSL, although the SLR is very neutrally balanced and has so much grip for a 1768kg front engined, RWD car, how can this be basically?

Shouldn't all cars be setup for this sort of handling for ultimate track times? I know the GT3/CGT and GT2 are much faster around tracks than the SLR but what I basically want to know is a). how did they achieve such breathtaking levels of grip in the SLR format and b). how come cars that oversteer and slide better (GT3's) achieve better lap times? Just explain the chassis features to a newbie...

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May 31, 2004 10:19:43 PM

Carlos from Spain: Rennteam Moderator; Loc: Spain. , Spain; Posts: 21152, Gallery; Registered on: Feb 9, 2003; Reply to: Bilal

Re: Chassis dynamics?

I don't know if I'm one to tackle such a question since I'm just an "affcionado", my expertise are the health sciences

.

IMO the reason the SLR didn't slide in that corner is just because he just got it right according to the available grip he had, nonthing more. In some particular corners you may actually want it to oversteer to make the turn better and if you don't you will be slower around it. Thats why controlable overteer is desirable or why a GT3 with PSM would be slower in a track under a pro driver.

Neutrally balanced just means it set up neither to excessive understeer nor excessive oversteer, however depending on its use or track you may want more oversteer for example, thats why the CGT can be adjusted for more neutral setup, and from factory its setup for more oversteer as we saw from the Top Gear video on which they should of corrected that before the lap for that track. Some cars have excessive underster which, while safe since its easy to correct it, doesn't ussually help take the corner faster. i.e. FWD front-engined cars. Neutral cars may understeer or oversteer depending on the corner and what the driver wants in that corner so he can use it to his advantage. So good sportcars permit so in a controled fashion also so the driver can make use of it if needs to. If the oversteer is uncontrolable when provoked then thats not good however.

Also the engine placement affects the balance since a front engined car will tend to understeer from the added weight in the front wheels, and a rear engined car tends to oversteer more (through can be still tuned for understeer tendency though suspension setups and tire sizes, ex: the 996C4 with stock suspension understeers like a truck but a 996C4 with the M030sport suspension and 265 rear tires tend to oversteer more) Rear engined cars also have the advantage that have a rear weight bias and therefore place the weight over the tractioning wheels like during acceleration coming out of a corner and has a better weight distribution/balance under braking since not all the weight is at the front.

Also 50/50 weight distribution is not the optimum for performance, real sportcars and race cars have rear weight bias (40/60 aprox) because of what I just mentioned with the rear engined car and having more weight behind. Even mid-engined cars are set up to have a rear weight bias not a 50/50, and front engined cars tend to achieve 50/50 only because the engine is at the front (except cars such as the F612 of Maseratti Quattroporte which were made to have a rear weight bias even if the engine is at the front by bring it rearwards a bit). 50/50 weight distribution sounds nice in a comertial but its only good when the car is parked but while driving the weight transfers have to be taken into account and dinamics favor a rear bias. However for non sportcars its better because the more weight in the front the more understeer and the safer for ordinary drivers to drive.

The CGT has in general more "grip" available than the SLR even though the driver may have chosen to slide more and will pull greater G's at the skid pad. Just because the cars is sliding doesn't mean it has less grip, you have to look at the curb speed and how fast he managed that curve overall. The grip depends on many things like the tire compound, tire sizes, suspension efficacy, unsprung weight, curb weight, car's balance, aerodinamic downforce, lift, AWD vs RWD vs FWD, etc. The SLR has approx the "grip", in a skid pad sense, of a GT3 I believe.

Boy, I don't know if I cleared anything up or just got you more confused

maybe some of the pro's around here can explain themselves better

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Jun 1, 2004 2:10:16 AM

Jean: Veteran; Loc: Seoul , South Korea; Posts: 1033, Gallery; Registered on: Apr 14, 2002; Reply to: Bilal

Re: Chassis dynamics?

First, SLR is not a Front engined Rear wheel drive car. It's set up as a Front Midship as the entire weight of the engine lies behind the front axle, which contributes to that fugly long front look.

Depending on the location of the heaviest part (engine) that contributes to location of the rolling center of a vehicle, a car shows a certain handling (and thus rolling) characterisitcs. However, regardless of the engine location or the drive wheels, a car can be set up to under, neutral or oversteer, using different factors such as suspension geometry (commonly known as alignment), spring rates, shock absorber bound/rebound rate, difference of mu(traction) between front and rear tires (or different tire sizes), location of driving wheels, aerodynamics, front and rear track width difference, wheelbase, and etc.

Ideally, the most desirable and fastest handling characteristics would be a neutral setup where in any given corner, your steering wheel is pointing straight, and thus appying foward motion power to the driving wheels. To achieve this, you are drifting your all 4 tires meaning that you're utilizing tires' maximum grip or slightly over them. As I hear modern tires allow about 15% of controllable slip angles over its abilities (correct me if I'm wrong).

However, in a corner, even the weight center reacts to various inputs, such as, decellaration (by braking), lateral acceration (by turning wheels), and acceration (by applying throttle.)

A ideal handling car would be the one that a driver can freely adjust the handling characteristics and react to weight center changes during cornering.

However car companies understand that not everyone is a racing driver, and for the ease of driving, sets up their cars to mild understeer.

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Jun 1, 2004 9:37:31 AM

Bilal: Expert; Loc: Manchester, UK; Posts: 1533, Gallery; Registered on: Jun 20, 2003; Reply to: Carlos from Spain

Re: Chassis dynamics?

I see...

Thanks for you guys' input

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