24V AWD Swap Knowledge Base

[nando]

what do you suppose is reasonable?

suppose I use the E46 xi rack (which does offer more front diff clearance). I'd have to move it forward about 1/2" to keep it out of the oil pan. that seems like it wouldn't have a large effect on bump steer?

[LJ851]

what do you suppose is reasonable?

suppose I use the E46 xi rack (which does offer more front diff clearance). I'd have to move it forward about 1/2" to keep it out of the oil pan. that seems like it wouldn't have a large effect on bump steer?

Some argue that some small amount of bumpsteer aids in steering "feel" but that is debatable. I would measure the bumpsteer of the stock setup and attempt to reduce or stay close to that amount. Before any rack location is picked, bumpsteer definitely needs to be verified.

It's hard to guess how much worse BS will be with rack movement and obviously some directions will be much worse than others.

It can go from "this doesn't feel too bad" to "i can't stay in my lane" at a certain point and you obviously don't want to be there.

[nando]

agreed.

btw I took some measurements of the E30 xi rack vs the E46 xi rack a while back, mm's per rev, total travel, etc. I wrote it down but it's buried in my garage somewhere. the E46 xi rack is significantly more responsive than the E30, which is no surprise.

[Nisse Järnet]

Well this turned ot to be interesting :) I can measure what you want!

Meanwile i took this apart a little more:

[flyboyx]

Some argue that some small amount of bumpsteer aids in steering "feel" but that is debatable. I would measure the bumpsteer of the stock setup and attempt to reduce or stay close to that amount. Before any rack location is picked, bumpsteer definitely needs to be verified.

It's hard to guess how much worse BS will be with rack movement and obviously some directions will be much worse than others.

It can go from "this doesn't feel too bad" to "i can't stay in my lane" at a certain point and you obviously don't want to be there.

on that note, it would probably be a good idea to use a rack that has the mounting tabs on the front such as an xi. we probably don't want it any further away than necessary.

[Nisse Järnet]

Btw, what do you think about just lowering the subframe like 5mm? Would that change any geometry too much?

[nando]

I don't think that gains you a whole lot. the sump will still contact the rack.

the problem is it won't help the bump steer issue regardless because your tie rods will be pointing up even more than they were before. and now the control arms are pointing more up too, and the subframe is hanging down lower, etc.

I will say using the E46 xi rack you'd only have to move it forward a small amount (1/2" or less) compared to the E30 rack. the major issue here is the E30 ix subframe needs modified quite a bit for it to work, and I haven't really figured out the tie rod situation yet. if we're doing a custom subframe then the only real problem is the tie rods (steering linkage shouldn't be a big deal IMO).

[Nisse Järnet]

I don't think that gains you a whole lot. the sump will still contact the rack.

the problem is it won't help the bump steer issue regardless because your tie rods will be pointing up even more than they were before. and now the control arms are pointing more up too, and the subframe is hanging down lower, etc.

I will say using the E46 xi rack you'd only have to move it forward a small amount (1/2" or less) compared to the E30 rack. the major issue here is the E30 ix subframe needs modified quite a bit for it to work, and I haven't really figured out the tie rod situation yet. if we're doing a custom subframe then the only real problem is the tie rods (steering linkage shouldn't be a big deal IMO).

Not on my car right now :) 5mm is enough but i might just mod the pan or something else. Spent a few hours lining things up today!

But i think i let the subframe be as you say.

Dont the tie rods from the ix fit the xi rack?

[nando]

No they are too long, because the E46 rack is wider.

[Nisse Järnet]

OK, do you know if the ix tie rods is the same length as E30 rwd ones?

[nando]

No.. but my E46 rack came with tie rods, maybe there's some combo that will work

[flyboyx]

what about rwd ends? i am pretty sure they are shorter than ix ends. maybe e36 inners?

[nando]

maybe, I don't know. I don't have any to mock it up with.

[The Dark Side of Will]

BLUF: Moving the rack forward will not affect bump steer.

Ackerman angle is determined by the position of the outer tie rod end in relation to the pivot (kingpin) of the front wheels triangulated back to the centerline of the rear axle, not the steering rack location. A given strut assembly with a steering arm mounted in the same place on the chassis is going to have damn similar ackerman regardless of where the steering rack is.

Location of the steering rack affects bump steer and there generally is not multiple places to locate the rack and keep bumpsteer in check (within reason)

I can assure you that you want to avoid toe out bumpsteer under compression at all costs !

I also agree that ackerman or positive or negative ackerman is not super critical (within reason)

Not trying to call anyone out here, just trying to help you guys succeed.





tl;dr :
Don't worry about affecting ackerman, focus on keeping bumpsteer in check.

Yes, in a vehicle with Ackerman steering geometry, the axes of the front and rear wheels intersect at the same point for any turn radius.

This is accomplished by having the angle between the steering arms and tie rods such that a given motion of the steering linkage steers the inner wheel further than it steers the outer. This is accomplished by having the steering arms and tie rods at an acute (if I'm looking at the geometry correctly) angle when the steering is straight ahead.
That is the "geometric" steering arm, defined by the perpendicular line from the kingpin axis to the pivot center of the outer tie rod. Obviously the physical steering arm doesn't have to coincide with this line.

Bump steer happens when the outer ball joint and outer tie rod end track different arcs as the suspension goes through its travel.
As long as the rack maintains its current distance above the plane of the control arms--the plane defined by the pivot centers of the inner ball joints and lollipop bushings--bump steer will not be affected by moving the rack fore or aft.

IOW, if you project the control arms and tie rods onto a view that is perpendicular to the plane described above, then they will track the same arc as the suspension goes through its vertical travel.

[The Dark Side of Will]

Some argue that some small amount of bumpsteer aids in steering "feel" but that is debatable.

I disagree totally.

I would measure the bumpsteer of the stock setup and attempt to reduce or stay close to that amount. Before any rack location is picked, bumpsteer definitely needs to be verified.

I'll take the analytical approach I discussed above. I may decide I need to make the crossmembers with provisions to shim the rack vertically. (IE, make the mounts lower than I think they should be and make up the difference with shims so that variations in individual bodies can be tuned out)

There are also bumpsteer correction studs which can be used at the steering arm end. In fact, those will be absolutely necessary if/when I raise the inner ball joints.