jgevers

Hi,

Just wondering what people's experience is with rake on the Imprezas.
After playing for while now, I found that (measured from wheel centre to wheel arch) on a Classic, if the rear is 5mm lower, the balance to be about right. On the later ones about 10mm lower at the rear seems to work best. The 05 model seems to need even more.

Any thoughts?

Cheers

Job

911

Totally agree with the classic 'pose'
I once experimented with dropping the nose down (at a race meeting) and the car was really different.
This chamge bought the front 'level' with the rear (I dropped the front by 6mm).
Retoring the height got the balance back.
Should add i did not reset the toe when dropped.

As you know Job, the (early) 911's need a 1 deg rake nose-down measured on the door sill. That is critical too.

All this 'poise' changes on braking and accelleration, so a compromise as ever!
Graham

Danny Boy

iTrader: (2)

I always thought the rake on the impreza as standard was with the rear higher than the front?

Dan.

superstring

Hi Guys

You might want to revisit this thread:

http://bbs.scoobynet.co.uk/showthread.php?t=287036

Unfortunately there are a couple of other threads referenced within that one, which contain a wealth of information on the subject, but don't appear to be active anymore

Edit: Ha! Just figured out to link to those 2 other threads

http://bbs.scoobynet.co.uk/showthread.php?t=198864

http://bbs.scoobynet.co.uk/showthread.php?t=285997

911

Just glimpsed through all 3 threads, and it strikes me that opinions are split (no surprise).
First, we need to know where to measure, because looking at the car on a level road will/may not tell the truth.
On the 911 shell, I'm told by a true expert (Bob Watson Engineering) that you place a level on the door sill. Set the car to 1 degree nose down, job done.

On the Impreza, where is 'level' in relationship to the suspension pick-up points?

Until we know this then you cant say if a car in nose up or down.

When i dropped mine by 6mm at the front just before my second practice run in a hill climb i was shocked just how unpredictable it became compared to the first run, same day, same track etc.
This is well discussed in those threads.
the oversteer tendency became much more pronounced and just as John Felstead states.
The car (for me) was faster with the nose back up by the 6mm.

Graham.

superstring

Hi Graham

According to Whiteline, a "Classic" Impreza has 12mm of forward (positive) rake when there is a 5mm difference, front to rear, in the distance measured from wheel centre to arch (front higher than the rear). For example, 345mm front, 340mm rear. This, presumably, means 12mm as referenced at the suspension pickup points. As I understand, this is their preferred setup on a road-going Classic.

I used to think that raising the front an extra 12mm (17mm total measured) would ensure the car was level. But now I realize that doesn't account for the fact that, as you raise the front, the rear will lower at the same time. Doh! Sooo...level on a Classic is perhaps ~11mm in measured difference front to rear?

John

911

I really rate Whiteline, so their thinking gets my vote.
I'll measure my car tomorrow and compare the 'rake'. Could be interesting!
Nice to 'talk' to a Canadian again, you live in a great country!
Graham.

Danny Boy

iTrader: (2)

I always thought the forward rake was normal because most of the imprezas i have seen have it but mine does not, even curtis at Powerstaion was a bit stumped as to why it should sit this way on standard suspension.

Dan.

superstring

Thanks Graham! Nice to chat with you again too. (I don't frequent the North American boards these days 'cause they're all New Age and I'm strictly "Old School" )

jgevers

What I was trying to find out if anyone had some different ideas from what I found. However you measure rake, wheel centres to arch, suspension pick up points, chassis angle or roll centre height, it still is a comparison between the front and rear of the car. I use the wheel centre to arch measurement because of ease of measuring.

Cheers,

Job

DuncanG

Job, the only person I recall trying to tune this was John Felstead. He came up with the ride-height settings of 335F/340R, ie 5mm rake nose-down measured on wheelarch.

What are you doing by tuning the rake anyway? Is there an optimum angle for the roll-axis that your trying to achieve? Is there any 'best' height for the RCs front and rear? I've seen it written in various texts that the rear RC should be higher than the front, but why?

How does adjusting balance by altering rake compare or interact with adjusting balance by ARBs or spring-rates?


Superstring,
When you raise the front it should have negligible effect on the height of the rear at the axle. There will only be a very small shift in the centre of gravity (fore & aft).

jgevers

Hi Duncan,

By adjusting rake there are a few things to think about like aerodynamics but the main reason for playing about with rake is the difference between the front and rear roll centres. The reason for having the rear roll centre slightly higher than the front is mainly for confidence inspiring feel and handling. The vehicle manufacturers of front wheel drive cars are playing about with rear roll centres at the moment to make the car feel like it has more positive steering. It is a smoke screen though, for most of them it results in lack of traction coming out of corners etc. If you drive the BMW Mini it feels really good but once you start going fast, the car lacks traction (can't get a fast lap time).

If you think of rake and rollcentres to change the tendency of roll and the bars as the control of the roll.

Cheers

DuncanG

So having the front RC lower will give it better grip (less jacking) but make it more rolly?

Having the rear RC higher will stiffen it and tend to reduce rear grip (more oversteer/less understeer) is that right?

Does that not imply that for best performance you want the RCs low for minimum jacking but recover roll-control through stiffer ARBs and/or springs? But then if the RC is too low you get a bad camber-curve with the Macpherson struts.

BTW according to my measurements and using the Susprog3D program (yes I know I'm a geek ) my RCs are about 90mm rear / 60mm front with ride-height of 355/355 (->10mm rake on sill) on my leggy bus. Is that ok?

911

I have just measured my Sti V3:
Front 310mm (arch to wheel centre) and 290 rear.
Thus the difference is 20mm nose high.

I have my new demon AST's fitted in mid Jan 06 so will discuss with Powerstation the ride height 'rake'

Any thoughts?

Graham.

superstring

Hi Duncan

First of all, I have experimented a bit with raising the rear of my car (only by about 1/4") and have noticed it does have a small but measurable effect on the height of the front. When talking about raising or lowering by 5mm or more, I think one should be aware of this and take it into account.

Edit: I should say, as well, that I think that the change in roll centre heights is perhaps more important than any small change in CG(s).

On the subject of roll centre heights, here's what Prodrive's Damian Harty had to say on the subject awhile back:


"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 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 (at the rear) 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."



And here's what (the old ) ScoobySport's Pete Croney said in response:

"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."

Hope this is some interest

John

DuncanG

Thanks John thats very interesting.


Graham, jeez-oh thats low!
" I have my new demon AST's fitted in mid Jan 06 so will discuss with Powerstation the ride height 'rake'"
Drat, I was hoping you were going to do Doune with those underdamped (wrt fast damping) 'fast-road' ASTs

911

Oh no!
The new AST's are remote jobbies with contol independant to compression and bounce.
I look forward to tuning them.
The original AST's with 60/50 spring rates were on the pack's limit, hence 11/12 and 9/12 damping positions in the end, but just so much better than the AVO's.

As to Doune, the mere thought of the Armco's is almost too much, but I really cannot resist the challenge, and to see what the fuss is all about!

Graham.

jgevers

It seems clear that once again, there is no absolute 'best' rake. The balance in the car that I like with 5mm difference measured from wheel centre to arch might be 'horrible' to someone else.

Of course, if a car gets set up for a specific track, for a specific driver, there will be an optimum rake that produces the fastest lap time.

For road and occasional track use I would suggest that rake is chosen to give the driver the most confidence as well as not producing a car that wants to spit the driver into the scenery when the car gets close to it's grip limit on fast corners.

Cheers,

Job

911

I belong in Job's 3rd paragraph.
Confidence is everything when it is you blasting close to the trees.
My Sti just feels so sorted imho, but I am sure that in better hands it would be quicker still....

I have looked very closely at many Impreza's on here and mags, and most have the wheel rim co-incident with the front wing swage line (17'' wheels).
At this ride height you can drive easy over speed bumps etc.


On 16's


Graham.

superstring

Graham, by any chance, do you have a direct side-on pic of your car on level ground?

911

John:
No, but this is the best i can find, new camera then so pic not very good.




Graham.

superstring

Thanks, Graham. Is it just the angle of the shot, or is your car sitting higher in the first pic (AVOs?)?

911

Both are AVO's, but the 'racer' one is with big 205 x 50 x 16 and the lower one is with 205 x 45 x 16 Kumho semi slicks
When Powerstation put the AST's on the ride heights were the same as the AVO's.

Graham.