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Old 01-06-2002, 12:01 AM   #1
superstring
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Location: Victoria, British Columbia
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Black Diamond Pearl

Lightbulb Car rake and handling: just for the record

I've read on a number of posts people asking about their cars sitting low in the rear, mine included (This applies to both "old school" and "new age" cars). The usual response is: don't mess with the rake, you'll screw up the handling. Below is the meat of a correspondence between me and Pete Croney (Scoobysport UK) that I thought might be of interest.

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 Shield's (whom I'm sure you know) once told me that Impreza's 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.

Pete
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Old 01-06-2002, 12:05 PM   #2
d00f
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Anyone with a gtec and a large parking lot want to screw with their rake and post the results?

-Michael
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Old 01-07-2002, 08:17 AM   #3
LUME
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Quote:
If you draw an axis through the roll centres, the point where it touches the ground is where you get the most grip.
??? The most grip? What does this statement mean? To me, this is like saying "the point where your virtual swing arms attach gives the most grip". It doesn't make sense. Can you ask him to explain this?

--Roy
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Old 01-07-2002, 07:09 PM   #4
gumby647
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What's rake? I don't have any leaves on my car. I know castor, camber, toe and at one time I might have known what ackerman was. I know what rake and trail are on motorcycles but I don't know how that would relate to cars.
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Old 01-08-2002, 01:49 AM   #5
scotty305
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I think he means that if the front of your car sits lower to the ground than the rear, there will be more weight on the front tires, and you will get better grip.

Subaru, for one reason or another, has opted to make the front of the car sit higher off the ground than the rear. This could be to put more of the weight distribution to the rear, forquicker turn-in, or it could be to increase ground clearance. It could be to lengthen the front springs, to allow for a softer, more comfortable ride. There are a lot of things that automotive engineers have to think about when designing cars to be driven by the general public.

TEIN s-tech springs lower the front of the car more than the rear, so that the front sits about as high off the ground as the rear. I don't know if this improves handling or just looks cool. It looks pretty cool, though..

Something to look into: does the WRC car have the same setup? I mean, is the front higher than the rear, are they equal, or is the front lower than the rear?


-scott-
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Old 01-08-2002, 08:36 AM   #6
Austin
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Quote:
Originally posted by gumby647
What's rake?
The angle of the car from front to rear. Usually measured by laying a level in the door jam, because the door jam is flat.
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Old 01-08-2002, 01:35 PM   #7
ConeMasher
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Forget rake you're hitting it from the wrong angle...
cornerweight the car if you're serious.

-- ConeMasher
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Old 01-08-2002, 02:37 PM   #8
d00f
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You could still cornerweight the car but the real question here is what percentage difference between the front and back gives the best handling. Another way to look at it which has been posted here is by the amount each end is lowered or raised.

-Michael
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Old 01-08-2002, 05:34 PM   #9
Austin
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Rake is just another thing to be considered when corner weighting the car - less important than actual corner weights.
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Old 01-08-2002, 06:28 PM   #10
driggity
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It seems to me that subaru has the rake set like it is for a reason and you'd probably want to lower the front and rear by the same amounts to keep it like this. Unless of course the stock rake is better for comfort, but not as good for performance. Of course this is probably a moot point unless you have coilovers and can choose how much to lower each end.
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Old 01-08-2002, 11:32 PM   #11
superstring
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When I bought my Prodrive suspension kit from Mike Shields about four years ago (at the time it was the ONLY Prodrive kit, it's now called the GT), I was surprised by the fact that it lowered the front of the car only about 1/2" and the rear about 1 1/8". The optical effect is that the rear sits lower than the front (negative rake), whereas with the stock setup the rear sat a bit higher than the front (positive rake).

I didn't really like the "look" but when I asked Mike about it, he said that this was how it was supposed to be and, specifically, Prodrive had designed the kit with very careful attention to the relationship between the front and rear roll center heights to achieve the "best" handling. He said this relationship is very important in a car with strut suspension and that "Impreza's handle better when they're squat in the rear". He also said that some people go for the "look" when lowering their cars and they end up compromising the handling.

As I said in my original post, I've read more than one post in the past from people asking about their cars looking "weird" because they sit low in the back. Obviously, it's really only those people who can adjust the height of their suspension (eg coilovers) that have the option of doing anything about this.

BTW LUME: If the rear roll center is higher than the front roll center, the roll axis will slope down toward the front and meet the ground somewhere at the front of the car (and vice versa). I'm sure this is what Pete is talking about, but other than that my technical understanding about roll centers isn't very complete!
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