I'm engine swapping an NB Miata frame right now (check my Pentastar Spider thread for specifics) and have a consideration I'm wrestling with. The oil pump hangs down under the front of the block, RIGHT where the NB steering rack is. Excluding "move the engine forward", "move the engine back", and "move the engine up", a suggestion was made to swap the spindles from side-to-side and convert to a rear steer rack with a new custom oil pan. This threw a few red flags for me, but I'm hoping to hear from the more experienced to make sure I'm on the right track.
The Miata chassis is lowered, resting on solid, fixed suspension. Ride height is appx. 5-5.5" at the bottom of the pinch welds (fenders are long gone, can't get those measurements).
It appears that swapping spindles side to side won't change anything EXCEPT brakes (no problem) and moving the steering arm to the back, dropping it an equivalent distance to where it currently sits upward.
The theory as discussed is that setting up a rear-steer rack with the exact same width as the current rack would be necessary, as well as moving that rack backwards and down the exact same amount as the steering arm/tie rod pivot point to simulate the same conditions as stock. Something about that doesn't feel right though, as I'm pretty sure the steering arm will now swing upwards during turns an equivalent amount to the front-facing arm swinging down, although they will both rotate about the same axis.
I did find a slick tool, VSusp.com, to help me visualize this, but I'm not with the car and can't take any measurements right now.
Can anyone chime in on this? Is it feasible, even if I have to re-spec a new rack width/position?
Interesting. My first thought is that as long as the steering rack location is a mirror of the front rack location (relative to the lower ball joint), bump steer should be in the same range but reversed. It's an oddball idea but I think it will work.
I can't think of why you'd want to move the rack vertically other than perhaps some minor amount for bump steer fine-tuning.
In reply to Keith Tanner :
The steering arm appears to angle upwards from the hub to the outer tie rod pivot. If reversed, it would angle downwards. Moving the rack down in height equal to the difference in height of the tie rod pivots would theoretically keep the tie rods parallel to their current position, just a smidge lower. This is my theory.
Do the Miata uprights have any caster built into them? If they do the steering would become "entertaining".
DeadSkunk (Warren) said:
Do the Miata uprights have any caster built into them? If they do the steering would become "entertaining".
There does appear to be a smidge, although I wonder how much I could adjust out if it's too much. Edit: found a post saying the axle center is 8.5mm/.33" forward of the balljoint centerline. Not sure how much that changes things but worth considering.
In reply to DRProjectCentral :
That actually looks like more than I was expecting. I'd think (I don't know) that amount of offset would introduce some serious instability. There's a quarter of an inch offset on the picture and I can't know how it scales. The axle centerline is normally above the steering axis by that offset amount. Swapping sides would put it below. You might be able to dial in enough caster to still have the contact patch trail the steering axis, but the steering effort and feel would have to be affected. I'm not sure I would try it, but maybe someone smarter than me will chime in.
Edit: I think this would throw off the self-centering effect big time. The weight of the car hangs below the steering axis because the axle is above and forward of it. So the car's weight is helping the self-centering. If you swap sides the axle will be below and behind the steering axis, where it won't be hanging anymore. It will be a balancing act.
You would need a pair of rear-steer uprights from another car. Then they would have the offset in the correct direction. But what's the chances of anything existing that will bolt up to Miata A-arms? Zero, I think. Is a Mustang II front steer or rear steer?
DRProjectCentral said:
Edit: found a post saying the axle center is 8.5mm/.33" forward of the balljoint centerline.
so yeah, flipping that side to side is going to contribute to steering instability, as the wheels will want to flop about like the wobbly casters on a shopping cart.
DRProjectCentral said:
In reply to Keith Tanner :
The steering arm appears to angle upwards from the hub to the outer tie rod pivot. If reversed, it would angle downwards. Moving the rack down in height equal to the difference in height of the tie rod pivots would theoretically keep the tie rods parallel to their current position, just a smidge lower. This is my theory.
I don't follow your theory. The vertical location of the tie rods won't change relative to the ball joint. It'll still angle up (or down, depending on where you're starting from). You're not talking about rotating the spindles, just swapping them side to side. The correct vertical location of the rack won't change, because the vertical location of the steering arms doesn't change.
As for the caster, that's just a matter of tweaking the control arm design you end up using or maybe moving the upper pickups. At that point, you ARE rotating the spindle. You'd just have to watch out that you didn't run out of range in the ball joint. Might add some interesting geometry as you turn the wheel, would that be jacking?
....and going to rear steer would mean finding a proper steering rack, with geometry that won't introduce bump steer. You need to make the motor fit around the steering, not make the steering fit around the motor..BTDT
Luckily, the width of the Miata rack (pivot to pivot, the important dimension) is fairly normal so it shouldn't be too hard to find something else. A rack from a RHD Miata mounted upside down might be the easy button :)
Keith Tanner said:
DRProjectCentral said:
In reply to Keith Tanner :
The steering arm appears to angle upwards from the hub to the outer tie rod pivot. If reversed, it would angle downwards. Moving the rack down in height equal to the difference in height of the tie rod pivots would theoretically keep the tie rods parallel to their current position, just a smidge lower. This is my theory.
I don't follow your theory. The vertical location of the tie rods won't change relative to the ball joint. It'll still angle up (or down, depending on where you're starting from). You're not talking about rotating the spindles, just swapping them side to side. The correct vertical location of the rack won't change, because the vertical location of the steering arms doesn't change.
As for the caster, that's just a matter of tweaking the control arm design you end up using or maybe moving the upper pickups. At that point, you ARE rotating the spindle. You'd just have to watch out that you didn't run out of range in the ball joint. Might add some interesting geometry as you turn the wheel, would that be jacking?
The spindles have to turn 180 degrees around the kingpin angle to put the steering arms in the right spot. When they do, it will lower the tie rod end relative to it's earlier position. If I just swap them straight across, the hubs will be inside the car.
To Skunk's point, I'll start looking into rear-steer uprights to see if I can make something work. Fabbing up some control arms shouldn't be too hard, as well.
The irony of all this is the Fiat this is all going under is already a rear-steer, and those uprights have removable steering arms. It's common to see people accidentally reassemble them flip-flopped from side to side and complain of poor steering characteristics. It makes me wonder if the Fiat uprights are close enough to the Miata uprights, then I can machine a custom steering arm with the tie rod pickup wherever I want. Hm....
I did understand that the hub would be on the outside, yes :) But you're right about the kingpin angle.
Apart from messing with spindle trail usually you also get anti-ackermann as steering arms on front steer spindles generally are angled outwards towards the wheel (while rear steer spindles have them angled inwards)
If spindles just swap sides and the steering arms stay the same, you get anti-ackermann and that is no fun on a street car.
The only reasonably successful swaps I have seen involve bolt-on steering arms (or in rare cases, heated and bent) and a spindle with no trail to speak of.
therealpinto said:
Apart from messing with spindle trail usually you also get anti-ackermann as steering arms on front steer spindles generally are angled outwards towards the wheel (while rear steer spindles have them angled inwards)
If spindles just swap sides and the steering arms stay the same, you get anti-ackermann and that is no fun on a street car.
The only reasonably successful swaps I have seen involve bolt-on steering arms (or in rare cases, heated and bent) and a spindle with no trail to speak of.
This^. We would run C3 Corvette spindles on metric chassised stock cars and it involved bending the steering arm quite a bit. On yhe plus side it saved a little weight, got a better front hub, and you could get less bumpsteer the minus was that it was a project to do so we rarely planned ahead to make spares.
Also check if there's much kingpin inclination built in. Some spindles have quite a bit which could make swapping them interesting.
