-=uk99=-

have got a set on but the backs are higher than the front will this afect handeling what should they be set too
thanks
Trev

swedey

same here chap higher at the back,(classic scoob) , dont think it does to be honest seen loads of cars higher at the back, allthough had a brown trouser moment saturday doing 120(private road of course!)and the back seemed to get light and started to almost lose grip?!... not good, why did you ask??

mark@wrx

Don't think that having the back higher is good for handling. Most cars you see are either level or slightly lower at the back. Being 'nose down' would put more weight onto the front suspension and also the steering.

FASTER MIKE!!

thats how mine are mark. if anything its slightly lower at the back.ie gap between the wheel and the arch. seems to handle quite well also look the bolloxs
its so low now that when it went in the garage to have a look at the brakes, the mechanic could`t get it on his hoist

mike

[Edited by FASTER MIKE!! - 4/14/2003 10:30:13 PM]

superstring

-=uk99=-

This is a question that has interested me for some time, and, if you have the time to read the following , I think you may find the answer(s) you're looking for .

The first is an email exchange between me and Pete Croney, whom I'm sure you know! The second is a reply to a question posed by me from Prodrive's Damian Harty from the ScoobyNet thread of the same name. BTW I have it on good authority that a difference of 5mm front to rear in the tyre/fender gap (front higher than the rear, measured from wheel centre to fender lip), translates into 12mm of positive rake; e.g. front 345mm, rear 340mm. So, using this as a guideline, a difference of ~17mm front to rear would mean the car is level.


First Pete Croney


Me:

I've noticed, in pictures, that your cars are set up with either no rake or a slightly POSITIVE rake. As I understand it, "conventional wisdom" has it that Impreza's handle "better" (I assume this means less understeer) when they have a slight NEGATIVE rake. Mike Shields (whom I'm sure you know) once told me that Imprezas handle better when they're "slightly squat in the rear". I'm no expert on roll centre heights and I was hoping you might clarify this issue for me.

Pete:

I know Mike well. I'm not sure how he arrived at the "rear squat" set up
though. Mike ran Leda on his car and loved it. We have always recommended
a small amount of rake as this makes a dramatic increase in the levels of
front end grip.

If you draw an axis through the roll centres, the point where it touches the
ground is where you get the most grip.


Now Damian Harty:

Me:

I own a MY98 Impreza 2.5RS (2 DR coupe/Type R bodyshell) with a Prodrive "Performance" (fast road) suspension kit, with which I'm very pleased.

However there's one thing about the kit that's always puzzled me. The rear of the car sits slightly lower than the front; in other words, the car has a slight amount of negative rake. (I'm not just talking about tyre to fender distance) The dealer who sold me the kit assured me that this was normal and that Prodrive had developed the kit "with particular attention to the front/rear roll centre relationship" and that "Imprezas handle better when they're a little squat in the rear".

Can you comment on these statements? I'm particularly interested in knowing what sort of roll centre relationship Prodrive was trying to achieve and the technical reasons for it. Also, once this relationship has been established, how much room (if any) is there to fiddle with it before you mess up what was intended - i.e. if I were to somehow raise the rear of my car by say 1/2 inch, would that change the handling balance in any significant way?


Damian Harty:

Roll Centre Q: When a car is near the lateral grip limit, the relationship between front and rear roll centre is very important in determining the way breaks away in response to small imperfections in the surface. If the roll centre is too high then the breakway becomes more aggressive. In general we'd prefer the front of the car to break away before the rear since it doesn't lead to a spin. Our circuit cars adhere to this principle, too. Therefore when we're near the lateral grip limit we prefer a slightly lower roll centre at the rear than the front. This can produce slightly odd behaviour during turn-in and so we use the dampers to compensate for this a little and "hold up" the rear of the car during turn-in - I think I talked about the different phases of turn-in in an earlier post on the subject. So basically, your man is right - ride height is a cheap way of adjusting roll centre height in a strut-based car and as long as we pay attention not to compromise travel and on-centre behaviour by re-valving the dampers, then we end up with an improvement in limit handling breakaway. If you raise the car by 1/2 inch then you'll notice the initial turn-in might well feel a little more secure, however, you may find the breakaway behaviour at the limit a little less benign. By all means try it somewhere where you have space not to damage anything.

*******

Hope this helps!

John




[Edited by superstring - 4/15/2003 5:47:36 PM]

Tone Loc

iTrader: (1)

My car sits lower at the front..... i really don't like the look of cars 'sat on their ****'. Looks like you've got a boot full of shopping. Having spoken to Leda they say they prefer to set the front lower too.... as do most competetion cars i've seen (BTCC, F1 etc). That's just my preference tho.

Tony.

Pete Croney

There is much more to this that just the gap, tyre to arch.

You have to consider suspension location, wishbone angles and the car's weight distribution. Then factor in what will be acceptable damper rates for fast road use and you can start to adjust all of the variables to optimise turn in, mid corner grip and the overall "feel" of the handling.

Damian mentions rear squat on the touring cars and using a very stiff rear set up to improve turn in. On the track, this would be very good and would allow breaking deep into a corner without running wide when applying the power. On the road, such a set up would be awful to drive as the back end would be hoping and skipping all over the place.

In fine tuning the rake that we use in Leda installations, I spent a lot of time on the road and on track working out what the best compromise was. Those that have been in my own car, or other Leda equiped Imprezas, will know that are very stable and transition into slide is very neutral (front/rear balance) and very predictable.

stockcar

as a general rule a very slight rake of (no more than) 10mm - lower at the front than rear - is a good rule of thumb on almost any car!! (measured off the chassis/ body!)
this has the effect of sitting the car level when under hard acceleration
i would never recomend sitting a road car back down, due to the weight transfer during acceleration/ braking which would most likely make the car unstable........as will having the nose too extremely raked!!
alyn - as performance