Jack of Hearts

Okay you gurus...bit of an esoteric question but I'm sure some of you guys
(V. Bob, Chris W., Steve W., Bull, Viken, many others) know the answer:

On rake: What effect does adding more, or less, rake than the factory specified 1 degree downslope (measured as 17mm difference between the front and rear suspension points) have on car handling?

I'm assuming it has subtle, maybe not so subtle effects on front/rear weight balance, braking and weight transfer on accelleration. But how critical is it? Do any of you racers change this to gain desirable characteristics?

Also, after cornerweighting, my C2 balanced out at 60 percent rear/40 percent front balance. (Cross weights were equal). That seemed pretty good to me for a rear engine car. But I have no data points on this. Got data?

Come on guys, trot out that arcane knowledge.

Thanks!

Chris

Bull

"Arcane knowledge"? I simply have some years of miscellaneous trivia!

Seriously, in my experience it all depends (don't you just love THAT!). What 911, what use, what suspension, tires, driver, etc., and how much change in the rake? The general answer is that it has to do with balance and the maintaining of balance/management of weight transfer. In general, I would maintain the proper 1% down front rake when changing any of the things mentioned. Otherwise, you will find a change in the handling of the car that has little or nothing to do with the modification you made.

I suppose that if you ran out of all alternatives available for changing the handling of a 911, and still wished to do so, you could then work with the rake/weight distribution/balance of the car. I have never been in that situation.

cabrio993

I am not a guru...but what I understand is that changing the 1% down rake won't affect your weight distribution. Most 993's will balance at 60/40 or close to it.

Also rake becomes important is in aerodynamics in high speed driving. Proper rake will minimize the lifting effect of the air going underneath the car by providing more down force up front.

Bull

How do you change the rake any significant amount without changing the weight distribution??????

cabrio993

Exactly! 1% is not a "significant" amount to affect weight distribution.

Greg Fishman

I think he meant to say 1 degree, not 1 percent rake.
Rake can have a significant effect on handling, BTDT.

cabrio993

My bad...

Steve Weiner-Rennsport Systems

Hi Chris:

Overly simplified, rake does two things (for ALL cars):

1) Allows changes in front lift and overall drag. 1 deg is considered the best compromise between overall drag reduction and front lift cancellation.

2) Permits one to move the rear roll center (in IRS cars) and thus F to R roll couple. Since this this affects lateral weight transfers, corner entry & exit handling behavior is altered. Its a tuning tool (within limits).

Jack of Hearts

Aha! Now we're getting somewhere! Thanks Steve...glad you chimed in. I hadn't considered drag or lift at all. So too much rake adds drag?

How critical is this? For instance, if after corner-weighting one side shows the specified 17 mm difference (front minus rear) and the other side only shows 14 mm (i.e.:MORE rake) is this close enough?

My suspension guy opined that getting the crossweights exactly 50/50 is more important than the ride height measurements.

What think?

Bull

It is unlikely that you will get everything exactly equal. 3mm, being a hair more than 1/8 of an inch, is pretty damn close. It is not enough to effect drag or lift.

Steve Weiner-Rennsport Systems

Chris:

Too much rake adds drag and alters the Front-to-Rear roll couple so that the rear end transfers weight more than the front end in a corner,....not a good thing.

With a 3mm difference (side-to-side) in rake, you are in fine shape,..... Getting the crossweights to 50/50 is ideal and I'll accept a small discrepancy in ride height to achieve that.

Don Magee

Steve, you are a tremendously valuable source of knowledge on this forum. Thanks.

Jack of Hearts

Thanks very much Steve....I will now drive the car with no further fretting....

or, more likely, I will find something else on it to fret about.

chris