Has an independant rearend been done?
09-21-01, 04:25 PM
#1
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Has an independant rearend been done?
I am a 2nd gen fanatic but I just traded a junk 2nd gen for a strong running 84 GS. I am going to do an engine transplant and was wondering if anyone has ever put an independant rear in a 1st gen?
09-21-01, 04:35 PM
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I've seen several cars with 2nd/3rd gen IRS translplants.
expect to pay $$$$, but anything is possible
and your pretty much on your own, as there is no How-To on this one.
expect to pay $$$$, but anything is possible
and your pretty much on your own, as there is no How-To on this one.
09-21-01, 06:10 PM
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http://www.moslerauto.com/cvweb/html/products.htm
Ive been told that this company makes a bolt in kit for our cars. I've heard it's very expensive though. About $5k IIRC.
Mike
Ive been told that this company makes a bolt in kit for our cars. I've heard it's very expensive though. About $5k IIRC.
Mike
09-22-01, 04:58 PM
#5
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Working on it...
https://www.rx7club.com/vforums/show...threadid=21756
I think I'm really going to try! What I'd like to do is fab up a set of subframe links (more like a truss) that bolt the subframe in to where the stock suspension's pickup points are. The subframe from the donor car bolted up to rubber bushings - I can bolt my truss to the IRS's subframe solidly (no bushings), and the truss bolts to the RX-7 body's upper and lower link pickup points on the front, and the Watts link pickup points on the rear. Good thing they used a Watts, this way I have a good rearward mounting point on both sides of the car!
Here's the beauty part - instead of solid-bolting the truss to the body, the truss will use polyurethane stock-type suspension link bushings! They're already the right size, all I'd have to do is weld the proper diameter and length tubing to the ends of the truss and pop the bushings in. This way not only is there less NVH, but it also should be easier on the body and will make up for "manufactuing inconsistencies" (i.e. i can fab up some cool stuff but i'm not PERFECT)
IOW it's sort of like how the Mustang Cobra's IRS is modular - bolts right in to the normal Mustang suspension pickup points.
The downsides, really, are inconsequential - I'd have to work up something for the rear brakes (something I'd have to do anyway with a solid axle), the power handling limit of the rearend is about 200hp (the car would probably fold up by then anyway
), it would have, in effect, TWO rear subframes so it would weigh more than the solid axle (but it's SPRUNG weight, not unsprung like a solid axle is. Besides, I LIKE a rear weight bias. And the car can be returned to stock/the rear suspension can be used in its original application again), the car would NOT be legal in anything but the most extreme autocross classes (it already isn't thanks to other things I've done to the car).
Man I LOVE the rush of inspiration and ideas I get when brainstorming a new concept like this.
I think I'm really going to try! What I'd like to do is fab up a set of subframe links (more like a truss) that bolt the subframe in to where the stock suspension's pickup points are. The subframe from the donor car bolted up to rubber bushings - I can bolt my truss to the IRS's subframe solidly (no bushings), and the truss bolts to the RX-7 body's upper and lower link pickup points on the front, and the Watts link pickup points on the rear. Good thing they used a Watts, this way I have a good rearward mounting point on both sides of the car!
Here's the beauty part - instead of solid-bolting the truss to the body, the truss will use polyurethane stock-type suspension link bushings! They're already the right size, all I'd have to do is weld the proper diameter and length tubing to the ends of the truss and pop the bushings in. This way not only is there less NVH, but it also should be easier on the body and will make up for "manufactuing inconsistencies" (i.e. i can fab up some cool stuff but i'm not PERFECT)
IOW it's sort of like how the Mustang Cobra's IRS is modular - bolts right in to the normal Mustang suspension pickup points.
The downsides, really, are inconsequential - I'd have to work up something for the rear brakes (something I'd have to do anyway with a solid axle), the power handling limit of the rearend is about 200hp (the car would probably fold up by then anyway
), it would have, in effect, TWO rear subframes so it would weigh more than the solid axle (but it's SPRUNG weight, not unsprung like a solid axle is. Besides, I LIKE a rear weight bias. And the car can be returned to stock/the rear suspension can be used in its original application again), the car would NOT be legal in anything but the most extreme autocross classes (it already isn't thanks to other things I've done to the car).Man I LOVE the rush of inspiration and ideas I get when brainstorming a new concept like this.
09-22-01, 05:28 PM
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Sounds like a hella fun project, are there any upgrade parts for the Diff to help support more than 200HP?
I'm sure there are quite a few of these rears around junkyards.
keep us posted on this!
by the way, I know you love drag racing, but I hard IRS is not as good at drags as a solid rear?
I'm sure there are quite a few of these rears around junkyards.
keep us posted on this!
by the way, I know you love drag racing, but I hard IRS is not as good at drags as a solid rear?
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09-22-01, 06:04 PM
#10
Old [Sch|F]ool
The rearend is a Hitachi unit, one size down from the unit used in Datsun 240Zs and late model Subarus (yep, same unit!).
There was a limited-slip version available in the rare RX Turbo... and if I found one of THOSE in the junkyard I'd grab the whole car and not just the rear diff!
It's a good point you bring up about drag racing... and here's the problem. It is difficult to get good anti-squat in an independent rear. You can get all sorts of anti-squat with a solid axle because the torque reacts against the suspension, while with an IRS the torque is decoupled from the suspension, so all you get to work with is using the forward thrust. You do that by putting a down-angle on the trailing links. Problem with that is you get problems with brake hop - braking torques the suspension upward, tire hops off of ground, no grip = no torque, suspension releases, tire hits ground, brake torque yanks tire off ground again... yuck. I'd actually been planning on making the truss adjustable, or at least have two or three different sets of bolt holes, to adjust the height of the subframe relative to the chassis to experiment with the angle of the trailing arms.
The point may all be moot, though, because the stock RX-7 rear suspension has pro-squat rear suspension geometry! This is WORSE than an IRS. Ever notice how the back of the car droops when you accelerate on the highway even though you're not pulling hard? That's the torque reaction COMPRESSING the suspension, which is bass-ackwards for acceleration, but Mazda had other things on their collective mind than drag racing, like getting optimal roll steer and avoiding brake hop like the plague. We have enough trail-braking oversteer as it is!
Another point to consider is the 2nd-gen... they also use a trailing arm rear suspension, and they have the added disadvantage of lots of negative camber + a special link that greatly increases negative camber when the suspension is compressed. The FC guys I've talked to get much better 60' times than me and they do redline clutch dumps, so they obviously have the traction! (altho my car's still faster... Saw a GTUs run a 16.64... but it had a ton of crap in the back so it was hurtin' anyway)
Most of the troubles with IRSs and drag racing is wheel hop, some of which is bad geometry, the rest is gooshy soft suspension bushings. My IRS has only TWO bushings per side, and they're pretty firm despite 180k of use (30k of which was opposite-lock full-throttle PJ style use ) Traditionally crappy IRSs have complex multi-link setups with bushings everywhere.
There was a limited-slip version available in the rare RX Turbo... and if I found one of THOSE in the junkyard I'd grab the whole car and not just the rear diff!
It's a good point you bring up about drag racing... and here's the problem. It is difficult to get good anti-squat in an independent rear. You can get all sorts of anti-squat with a solid axle because the torque reacts against the suspension, while with an IRS the torque is decoupled from the suspension, so all you get to work with is using the forward thrust. You do that by putting a down-angle on the trailing links. Problem with that is you get problems with brake hop - braking torques the suspension upward, tire hops off of ground, no grip = no torque, suspension releases, tire hits ground, brake torque yanks tire off ground again... yuck. I'd actually been planning on making the truss adjustable, or at least have two or three different sets of bolt holes, to adjust the height of the subframe relative to the chassis to experiment with the angle of the trailing arms.
The point may all be moot, though, because the stock RX-7 rear suspension has pro-squat rear suspension geometry! This is WORSE than an IRS. Ever notice how the back of the car droops when you accelerate on the highway even though you're not pulling hard? That's the torque reaction COMPRESSING the suspension, which is bass-ackwards for acceleration, but Mazda had other things on their collective mind than drag racing, like getting optimal roll steer and avoiding brake hop like the plague. We have enough trail-braking oversteer as it is!
Another point to consider is the 2nd-gen... they also use a trailing arm rear suspension, and they have the added disadvantage of lots of negative camber + a special link that greatly increases negative camber when the suspension is compressed. The FC guys I've talked to get much better 60' times than me and they do redline clutch dumps, so they obviously have the traction! (altho my car's still faster...
Saw a GTUs run a 16.64... but it had a ton of crap in the back so it was hurtin' anyway)Most of the troubles with IRSs and drag racing is wheel hop, some of which is bad geometry, the rest is gooshy soft suspension bushings. My IRS has only TWO bushings per side, and they're pretty firm despite 180k of use (30k of which was opposite-lock full-throttle PJ style use
) Traditionally crappy IRSs have complex multi-link setups with bushings everywhere.
09-23-01, 02:16 PM
#12
Old [Sch|F]ool
You must live where the roads are perfect. I don't know how many times there is a perfect corner, EXCEPT for the fact that it's bumpy. Can't take bumpy corners - the live axle gets a bit too live and jumps all over the place. Scary!
And then there's the torquing problem... acceleration lifts the right tire and pushes the left tire down, which makes for inconsistent cornering left vs. right. I wonder if part of Mazda's placement of the Watts link's center pivot was to help try to offset this? I mean, any not put the link on the other side of the car, away from the exhaust heat?
And then there's the torquing problem... acceleration lifts the right tire and pushes the left tire down, which makes for inconsistent cornering left vs. right. I wonder if part of Mazda's placement of the Watts link's center pivot was to help try to offset this? I mean, any not put the link on the other side of the car, away from the exhaust heat?
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