economiser
10-29-2014, 04:40 AM
The car is an '83 RX-7, the first generation, chassis-wise. To get the power steering the car's owner requires, I have to swap the front end from a second-gen, an '87 in this case.
At first this looked to me like adjustable anti-dive. Great! But how much?
So the "frame rails" are part of the unitbody, and the crossmember bolts to the bottom of that. This is how many amateurs do the swap, spacing the rear by 1/2"-1" due to the curve of the rails. But I got a tip: "The bottom of the FB frame rails are about 1.5" lower then the FC. When you put in the FC sub frame the front roll center will be "under ground" (on a FC at normal ride height the control arms go up hill toward the center about 5/8"). To low of a roll center equates to lots of weight transfer to the outside tire, dartness/twichy handling and bump steer problems. Now if you section the ends off of the FC subframe and plate it to fit between the frame rails in the proper location for ride height you can correct the roll center. You would also have to section the frame rail to clear the tie rods. A roll center that is to low will give quick transitions turning left to right. Do not mistake quick transitions for good a "good" handling car. "
I feel comfortable doing the extra work, I'm just not sure what angle I should try to end up with, longitudinally, for the lower arms.
I'm hoping to extend the wheelbase about 1/2" to gain some caster, and to help weight distribution and hood clearance. I really don't like to see front wheels looking behind the center of the wheelwells, either.
No thoughts yet about how much tire-to-fender gap vertically, but enough for speed bumps, ditchy driveways, and the most mild of dirt roads.
Please advise, comments and questions welcome. More pics will be made as you tell me what you need to see.
If noone can advise, I'm thinking I'll try to start with the arms front-down about 3-4 degrees for a baseline.
As for the roll center, Anyone have any reasons why I shouldn't just raise the crossmember the 1.5" suggested? More? Less?
At first this looked to me like adjustable anti-dive. Great! But how much?
So the "frame rails" are part of the unitbody, and the crossmember bolts to the bottom of that. This is how many amateurs do the swap, spacing the rear by 1/2"-1" due to the curve of the rails. But I got a tip: "The bottom of the FB frame rails are about 1.5" lower then the FC. When you put in the FC sub frame the front roll center will be "under ground" (on a FC at normal ride height the control arms go up hill toward the center about 5/8"). To low of a roll center equates to lots of weight transfer to the outside tire, dartness/twichy handling and bump steer problems. Now if you section the ends off of the FC subframe and plate it to fit between the frame rails in the proper location for ride height you can correct the roll center. You would also have to section the frame rail to clear the tie rods. A roll center that is to low will give quick transitions turning left to right. Do not mistake quick transitions for good a "good" handling car. "
I feel comfortable doing the extra work, I'm just not sure what angle I should try to end up with, longitudinally, for the lower arms.
I'm hoping to extend the wheelbase about 1/2" to gain some caster, and to help weight distribution and hood clearance. I really don't like to see front wheels looking behind the center of the wheelwells, either.
No thoughts yet about how much tire-to-fender gap vertically, but enough for speed bumps, ditchy driveways, and the most mild of dirt roads.
Please advise, comments and questions welcome. More pics will be made as you tell me what you need to see.
If noone can advise, I'm thinking I'll try to start with the arms front-down about 3-4 degrees for a baseline.
As for the roll center, Anyone have any reasons why I shouldn't just raise the crossmember the 1.5" suggested? More? Less?