fd sub-frame idea
 

I've been chewing on an idea for the last month (off & on again) about using a stock sub-frame, however, I'm not dead set on using one - just thinking a bit.

My basic idea is to push the power steering rack forward, not up or down, only forward towards the radiator. What I see wrong with this idea is having to fab the brackets and/or weld additional/extended support to the factory sub-frame. The tie-rods will need to be cut & re-threaded of course. From what I know about the cause/effects of bump-steer, this wouldn't affect the suspension geometry. Now to take this a bit further.. what would stop us from pushing the rack forward & letting engine drop far enough so that the crank pulley is about 10mm above the steering rack? anything?

I've been thinking about what it would take to keep all the accessories on the engine (PS,AC) and the ALT in the stock locations. So, why not push the rack forward to clear the oil pan then drop the engine enough to clear the hood.

Please share your thoughts on this idea (pos or neg).

Why not just using the Defined Autoworks kit? They pretty much have done/accomplished everything you described with their kit -- check them out under the vendor forums.

Please leave the vendors out of the thread as I dont want to turn this into a kit .vs kit thread. :)

Its a personal taste for me, Logan's kit doesn't do what I want and I think I could be more cost effective building this myself, however, if I couldn't weld.. his next upcoming version would be ideal but i'm not that position so I'll do it myself.

moving the rack forward will cause you to lengthen the tie rod "length", which will cause bumpsteer, because geometry will change. Been there, done that, won't work.

http://www.fd3s.net/suspension_front.jpg

I'm not 100% sure that I"m following you completely.
If the rack is pushed forward 1", the arms would be too long. Maybe I missed something or dont fully understand but from the dry-fitting I've done, the rods are too long.

Okay, I know sometimes how I explain things are hard to understand. So I made some crappy pictures to explain it better. This would be on a left hand turn in both pics.
View is from the top.

First up, the moved forward steering rack. In a left turn, it will always make the outside tire have more angle than the inside. Seeing how the inside tire has a tighter radius to follow, this is completely backwards, and the car will understeer very very very very bad. ANd handle worse than anything ever made.

http://i254.photobucket.com/albums/h...nggeometry.jpg

Here is the stock steering rack location in a left turn. The inside tire will have more angle, following the tighter radius of the inside corner perfectly.

http://i254.photobucket.com/albums/h...ndgeometry.jpg

If that helps at all. So the steering rack cant be moved up or down, forward or backward from stock or handling goes to crap.

If your willing to fab a completely new intake manifold and masage the firewall, the 20b will fit directly behind the rack. That's what I'm doing.

I like the pics Logan but are you sure about that? Crispeed is doing exactly that with the 4 rotor Rx8. They are moving the rack straight forward. Also the same thing is done with the S2000 when a 2JZ is put in. Seems to work from what I hear.

The only way I can see around this, is to extend the spindle arms the same distance forward as the rack. This would be nearly impossible, as its very close the the rim already. Maybe with larger wheels a inch or two would be possible with modified spindles, and then you have to deal with a shorter turning radius due to longer arms. In a road race car, a tight radius is not needed as bad, but still seems like alot of work to become even marginal. PM crispeed and see exactly how they are counteracting it.

I'm not opposed to it at all - i'm interested in both ideas to tell the truth.
I also agree with you about moving the rack straight forward. I've read success stories using this idea.

@Logan
I'm not 100% sure how the car would understeer any worse given the rack was centered correctly and the arms were adjusted correctly. I could see that happening if the rack were moved off-center or one arm was longer / bad tie-rod-end, etc.

Mind helping me understand how the car understeers given the rack is centered and tie rods are correct length? I dont doubt you at all - just trying to understand how it works before cutting the firewall or moving racks around.

The difference is Ackerman that's getting changed. In both pictures the rack would be perfectly centered.

I'm using a book that I've been reading for references titled:
On page 207-209, the author points at a few formulas that aid in calculating the Ackerman angle. He goes on to note that at only low speeds where the is zero lateral forces, will it only corner precisely, etc.

The author doesn't touch on any relationship of the rack and spindles (yet!).

So from what I currently gather, it wont matter anyhow as long as I keep the arms in relation to one another correct-length,etc. I think I can back that up by saying "The rack will move the tie-rods only so much regardless of where it is positioned. the maximum length/extension would position the kingpin and rack/arms on a 0* level. the stock suspension clearly isn't designed that way." but i haven't found any evidence to support or deny my claims ;-|