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Old 09-09-2013, 04:24 AM   #1
wurlyvalve
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Default Increasing Ackermann

Hi all,

Has anyone managed to increase the Ackermann (intentionally)? Whilst my car is far from having issues, I always enjoy looking for that last hundreth. Very tight hairpins at a hillclimb that I recently performed at could have just done with slightly more turning. Having campaigned a 7 for a couple of years with everything on the car adjustable, I found that more ack was what fixed this (for my driving style). So looking at options with the Scooby.

Toe out with roll on bump steer toe in is an option. But would prefer ack route.

Put a spacer in between the lower wishbone and the blade / rear bush mount AND fit the top mount in forward position - effectively push the front wheels forward instead of rack back.

Many thanks,
Glen.
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Last edited by wurlyvalve; 09-09-2013 at 04:48 AM.
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Old 09-09-2013, 06:29 AM   #2
Uncle Scotty
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ive been here a long time and have seen all sorts of things talked about....this has come up here maybe 3 or 4 times that i can remember....as far as i know, there is no easy way to do what you wish with your subaru

i do know that some of the drift kiddies make modifications to increase maximum turning angle of the front wheels which may be something to look into and i know that adding a bit of toe out in the rear will make it a bit more playful on throttle

subaru suspensions just dont have much adjustability in the sense you want without going to some extremes to get there
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Old 09-09-2013, 03:19 PM   #3
2wickedWRX
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^^^ What Scotty said. I'm converting my STI to drift with, coming from the 240 scene steering mods are all over the place, but not so much with subies. However, you can cut your knuckles and have them welded back together to increase steering angle. The knuckles I bought were cut and then welded back together and I'll have better steering angle than factory. There is a catch however with cut knuckles, since there cast you need some one experienced with cutting knuckles, an weld the cast so that it will be strong enough to handle the abuse, if the weld is done right the knuckle should handle the abuse. Search for "drift stuff cut spindles" and you should find a few good reads.
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Old 09-10-2013, 03:03 AM   #4
wurlyvalve
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Hi,

Right. Pulled the rack down and had a look at the mounts. It doesn't strike me as an obvious easy fix to move the rack back

I've cut and shut plenty of steering arms in my time. I did this when messing around with the ack on my 7. There is a knack, yes, I got the whole steering arm glowing orange hot when I made the weld and this tended to do the trick. In the UK I can't do this (sort of) as the car will fail its annual government inspection (MOT) for having a welded steering component. Doesn't stop me from having two uprights - one for inspection. But this is not without its pains.

I'm still leaning towards pushing my Com C top mounts to fully forward and then adding the "free caster" mod 10mm spacer plate. The caster changes +ve and -ve will just about cancel out but will leave the wheel 5/8" further forward. A touch more ack....

Cheers,
Glen.
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Old 09-10-2013, 10:57 AM   #5
rexworx
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Not a bad idea...
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Old 09-10-2013, 02:02 PM   #6
Bikelok
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Quote:
Originally Posted by wurlyvalve View Post
Hi,

I'm still leaning towards pushing my Com C top mounts to fully forward and then adding the "free caster" mod 10mm spacer plate. The caster changes +ve and -ve will just about cancel out but will leave the wheel 5/8" further forward. A touch more ack....

Cheers,
Glen.
Very clever.

I have not looked at a legacy bushing in a while, but if its the kind where the tube extends out past the rubber. You can cut and clean up the leading edge of the bushing approx. 2-3mm. Or look for a shorter bushing that fits the control arm (perhaps a GD bushing?). Then install hardened washers at the back end to take up the space.
This will help "push" the control arm forward with the spacer plate at the rear.
I'm not sure if this will work for you, but it may be something to look at.

I'll have to look at my Leggy when I get home to see what kind of bushing it has.
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Old 09-10-2013, 02:15 PM   #7
rexworx
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Im quite sure the gd bushing sleeve is longer. Ive installed a few sets on 04-07 control arms on the older gc and gf chassis. I always shave the front side of the sleeve to move the mounting point forward a lil. this with the free caster mod works great. Even just flipping the pins and not adding spacers...
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Old 09-10-2013, 04:39 PM   #8
wurlyvalve
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Hi,

My lower wishbone is (I think, I bought it 5yrs ago) a GC8 STI bottom arm. I ditched the Leggy arms. Pin already flipped (needed a bit of filing). Think I comprehend the shaving, will need to get my thinking cap on (it's beer o'clock here in Blighty).

As measured at the weekend, I now have 6.5degs caster and 3.25degs camber. I'm happy to lose a little caster if I push the wheel forward to gain ack. Maybe shaving, spacer (6mm maybe) and the com C flip will keep 6degs and move the wheel forwards. I can beat the wing about - she ain't a show car - to clear the tyre.

Also thinking about drilling out the taper in the steering arm, fitting hardened shanked bolt and rose joint to the rack. This way I can get toe in on bump. ie lift the ball up higher.

Just my ramblings,
Glen.
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Old 09-10-2013, 08:29 PM   #9
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i dont think id want to do any of this......no.

i think this is likely to be going in the wrong direction as the positive static caster for these cars is little enough without decreasing it further.....and the proof is that these cars respond so favorably to increasing the positive static caster
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Old 09-11-2013, 03:00 AM   #10
wurlyvalve
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Scotty, then you have missed the whole detail of the point. I wish not to sacrifice caster but to increase ack. That is fine, if you wish not to do this, then simply don't. End of.

Glen.
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Old 09-11-2013, 03:42 AM   #11
Bikelok
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Quote:
Originally Posted by wurlyvalve View Post
I wish not to sacrifice caster but to increase ack.

Glen.
That's what I thought you were getting at. Simply put, you just want to push every thing forward a bit.

Hope it works out for you Glen. Good luck.
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Old 09-11-2013, 04:34 AM   #12
Uncle Scotty
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Quote:
Originally Posted by wurlyvalve View Post
Scotty, then you have missed the whole detail of the point. I wish not to sacrifice caster but to increase ack. That is fine, if you wish not to do this, then simply don't. End of.

Glen.
you cant have both....really....not like that

the GD sti wheelbase is 1/2" longer than the wrx due to increased pos caster built into the front suspension

if you re-orient the front strut tops to move the strut shaft foreward you will decrease pos caster....regardless of moving the rest of it forward as well

seems like you are going about trying to get more steering angle the wrong way and will end up with a poor handling car 95% of the time to make it 'better' for 5%


if you do this....id would very much like to see all the alignment numbers before and after
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Old 09-11-2013, 04:41 AM   #13
wurlyvalve
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Scotty, yes you can. Done it on my 7. The only difference was I moved the rack back on the 7. With the Scoob I need to move the wheels forward. If I move the bottom PUP and top PUP equally the caster angle doesn't change. The ack will increase.

As to whether that is a 5% improvement. That was what I was told continuously with my 7. Yet, my lap times were quicker at every track I visited! Must be my lack of driving style. But it suits me nonetheless.

I can re-post the figures before and after. No problem with that. But I'm not doing this to justify myself to you! And I won't post them up until I have matched the figures, I will keep moving the lower point until they are the same. So it will look a bit boring seeing 6.5 before, 6.5 after. But hey, will do.

Glen.
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Old 09-11-2013, 07:54 AM   #14
Uncle Scotty
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Quote:
Originally Posted by wurlyvalve View Post
Scotty, yes you can. Done it on my 7. The only difference was I moved the rack back on the 7. With the Scoob I need to move the wheels forward. If I move the bottom PUP and top PUP equally the caster angle doesn't change. The ack will increase.

As to whether that is a 5% improvement. That was what I was told continuously with my 7. Yet, my lap times were quicker at every track I visited! Must be my lack of driving style. But it suits me nonetheless.

I can re-post the figures before and after. No problem with that. But I'm not doing this to justify myself to you! And I won't post them up until I have matched the figures, I will keep moving the lower point until they are the same. So it will look a bit boring seeing 6.5 before, 6.5 after. But hey, will do.

Glen.
mmmmmm....well.....myself and another enthusiast who is no longer with us...
managed to get almost +10 caster on an 05 sti(and we could have got more)...but it was undrivable and dangerous with that caster, and the front wheels were pushed WAY foreward to the point of almost touching the foreward wheel arch opening(why we didnt add more at that time)....and the turning circle became HUGE
we came to the concensus that something between +6 and +7 was best, and I prefer between +6 and +6.3

im curious as to the actual affect on the whole suspension dynamics this will have....

its a wonder nobody has tried this before...or maybe they did and just didnt think it imoportant
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Old 09-11-2013, 03:49 PM   #15
Suba_Roo
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I'm sure that you could get MSI to make you a set of knuckles that have steering arms made to your specifications and maybe even adjustable. Only issue is they will cost a fortune.

While you could do it, I'm with Scotty, I think you are hearing hooves and thinking zebras. The 7 is rear wheel drive with the engine behind the front wheels. With these subys with our engine in front of the wheels, unless you've done all sorts of fun stuff to the tranny, you have to think of the suspension design like it's a front wheel drive car. Your project is interesting, and I'm dying to hear if it works in terms of Lap times, but you'll probably use your resources better if you focus on adding more negative static camber, more dynamic camber, and more caster.
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Old 09-12-2013, 03:16 AM   #16
wurlyvalve
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I did originate from Mini's, which are pretty fwd and heavily ack biased. We used to extensively shorten the front tie bars with these, front tyres constantly rubbing the leading edge arch. I don't think the gearbox will care if the front wheels are moved 5/8" forward, they are quite fancy these new fangled gearboxes and have CV joints to allow for this. Have you seen how far the SWRT cars pull their wheels? I have, it's a way (half engine back, half wheels forward).

So you think 3.25degs static -ve camber is lacking? Wow. I was thinking it was excessive. And the tyre temps are pretty uniform now so not screaming at me that I need more contact patch control. In fact, during turn-in, I'm probably too -ve camber, due to the extra camber the caster is giving me with steering lock. It's only turn in front end that I want in tight hairpins, once the car is in the bend (or not a hairpin bend) I am completely rear end limited to cornering grip.

I can post times before and after mods. The trouble is, sneaky little things get added to the car during the off season. So I fear I would be comparing apples with pears. And I don't want to mislead. Bloody weather here prohibits long seasons so I can't easily test back to back at the same track in the same year

Cheers,
Glen.
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