So just how bad is the FC's IRS?
01-20-05, 05:12 AM
#1
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So just how bad is the FC's IRS?
Maybe this could be in the 2nd gen forum, but they'll be biased anyway. Of course, so will we. But at least we have a right to: we drive the best cars in the world 
My question is really simple, the answer probable isn't. In recent discussions about suspension swaps, esp 2nd gen front subframe in order to install R&P steering, the rear suspension of the FC is always seen as a cheap, bad and useless design. Why? Everybody always states the Miata/MX-5 IRS is better, and though I don't doubt that, I'd much rather compare the FC IRS with what's under our cars now. In other words did Mazda create a better rear suspension for the FC then our live axle, or did they screw up, and is the SA/FB's live axle simply a better device then the FC's IRS?
I have a complete FB in parts, and could build a nice car from it. I also have a almost complete FC in parts (body is scraped) and another FC waiting to be parted out. In other words: all the parts for a IRS project are there while when using a Miata rear suspension, I'd have to buy the parts, making it expensive to just try something. If the thing goes wrong with the parts I have, I hardly loose something, so I can just aswell try. If however, the FC IRS is worse the the FB's live axle, there's little point in constructing a car worse then it's basic donor.
So tell me: why is it that the FC's IRS is such a unwanted thing?
I'd prefer to hear this from people who know both cars, as I never drove an FC, I cannot judge it.
My question is really simple, the answer probable isn't. In recent discussions about suspension swaps, esp 2nd gen front subframe in order to install R&P steering, the rear suspension of the FC is always seen as a cheap, bad and useless design. Why? Everybody always states the Miata/MX-5 IRS is better, and though I don't doubt that, I'd much rather compare the FC IRS with what's under our cars now. In other words did Mazda create a better rear suspension for the FC then our live axle, or did they screw up, and is the SA/FB's live axle simply a better device then the FC's IRS?
I have a complete FB in parts, and could build a nice car from it. I also have a almost complete FC in parts (body is scraped) and another FC waiting to be parted out. In other words: all the parts for a IRS project are there while when using a Miata rear suspension, I'd have to buy the parts, making it expensive to just try something. If the thing goes wrong with the parts I have, I hardly loose something, so I can just aswell try. If however, the FC IRS is worse the the FB's live axle, there's little point in constructing a car worse then it's basic donor.
So tell me: why is it that the FC's IRS is such a unwanted thing?
I'd prefer to hear this from people who know both cars, as I never drove an FC, I cannot judge it.
01-20-05, 07:12 AM
#3
You do know that the rear axle from an FC is not exactly a straight fit? I had the same plans one time, a friend of mine was parting out a FC, and we found out you would have to redesign almost the whole rear of the car.. The subframe up front though, is a smart move. I know one guy that used it when he was converting his 1.st gen to 20B power.
Actually, if i dont remember wrong, nobody has ever went trough with a FC IRS, but there is a guy on this forum trying it out right now. Look for a thread called "Mega roadster buildup"
In my opinion, the 1.st gen axle is easy to fix, reliable up to about 250 HP (unmodded) and interchangeability with other Mazda models are well known, for instance the B2600 diff.
The 2.gen axle is more sophisticated, designed more for comfort than the 1.st gen, more reliable with big HP, but a more complex design usually equals more problems.
In my opinion, the best axle for a street driven 1.st gen is the GLS-SE axle, just a shame they are relativly rare over here in Norway.. And it is a lot easier getting rims for the 4*114,3 pattern than the 4*110!
If you do go trough with the swap, good luck!
Actually, if i dont remember wrong, nobody has ever went trough with a FC IRS, but there is a guy on this forum trying it out right now. Look for a thread called "Mega roadster buildup"
In my opinion, the 1.st gen axle is easy to fix, reliable up to about 250 HP (unmodded) and interchangeability with other Mazda models are well known, for instance the B2600 diff.
The 2.gen axle is more sophisticated, designed more for comfort than the 1.st gen, more reliable with big HP, but a more complex design usually equals more problems.
In my opinion, the best axle for a street driven 1.st gen is the GLS-SE axle, just a shame they are relativly rare over here in Norway.. And it is a lot easier getting rims for the 4*114,3 pattern than the 4*110!
If you do go trough with the swap, good luck!
01-20-05, 07:50 AM
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Well, I don't really thought it'd be a bolt on swap 
It's just a question of how good/bad the system is. I'm pretty sure miata IRS isn't bolt on either... I'm not planning such a build anytime soon either (have plenty of other things to do first), it was more like a "what if" question.
As for GSL-SE axle, you do know ALL 84-85 cars (JMZFB chassis code) in Europe came with this axle? As long as the car is a genuine EU-spec FB (so a S3, and not imported from the US) it'll have this beefier axle, and suspension/braking upgrades. You won't have the same diff ratio's I think, but it'll be as strong as a GSL-SE axle. Our FB's (so S3 cars, S2 in Europe still has a SA22 chassis code, but updated bodywork) came with 12A only, but had most other udates as in the GSL-SE (dash, suspension, brakes,...) except for powersteering, which was never available in EU-spec cars. We did have 4*114,3 patterns on all 84-85 cars, though.
It's just a question of how good/bad the system is. I'm pretty sure miata IRS isn't bolt on either... I'm not planning such a build anytime soon either (have plenty of other things to do first), it was more like a "what if" question. As for GSL-SE axle, you do know ALL 84-85 cars (JMZFB chassis code) in Europe came with this axle? As long as the car is a genuine EU-spec FB (so a S3, and not imported from the US) it'll have this beefier axle, and suspension/braking upgrades. You won't have the same diff ratio's I think, but it'll be as strong as a GSL-SE axle. Our FB's (so S3 cars, S2 in Europe still has a SA22 chassis code, but updated bodywork) came with 12A only, but had most other udates as in the GSL-SE (dash, suspension, brakes,...) except for powersteering, which was never available in EU-spec cars. We did have 4*114,3 patterns on all 84-85 cars, though.
01-20-05, 09:21 AM
#5
I know about the rear axles on the European models. Only problem is that my car is imported from Canada.
It may look like a Series 2 or 3, but it actually is a 80 that has had quite extensive bodywork done to it. The rear lights are "FB" and so on.. I also changed the mirrors and removed the side moldings.
But its still an 80, and has the 4*110 bolt pattern and drum brakes. Not much is left of the original SA now, its kinda a hybrid with a 13B 4 port from a Cosmo/RX5, interior parts from an FC+++
There is nothing wrong with the IRS from an FC, it is a far more modern and comfortable design than the stiff rear on our cars. It should also be able to handle more power and be lighter. You can also get more aftermarket parts for it, because its a more common and newer car. Plus it has coilover springs.
If you are seriosly thinking about a heavy duty axle, may i suggest a Volvo 240, or maybe even a 9" Chevy? They can handle lots of power and are commonly available in Europe, at least in Norway.
It may look like a Series 2 or 3, but it actually is a 80 that has had quite extensive bodywork done to it. The rear lights are "FB" and so on.. I also changed the mirrors and removed the side moldings.But its still an 80, and has the 4*110 bolt pattern and drum brakes. Not much is left of the original SA now, its kinda a hybrid with a 13B 4 port from a Cosmo/RX5, interior parts from an FC+++
There is nothing wrong with the IRS from an FC, it is a far more modern and comfortable design than the stiff rear on our cars. It should also be able to handle more power and be lighter. You can also get more aftermarket parts for it, because its a more common and newer car. Plus it has coilover springs.
If you are seriosly thinking about a heavy duty axle, may i suggest a Volvo 240, or maybe even a 9" Chevy? They can handle lots of power and are commonly available in Europe, at least in Norway.
01-20-05, 09:50 AM
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Fords have 9" ers. Chevy's are 10 or 12 bolts.
The issue with the FC rear is that it is in essence a semi trailing arm design. These toe out in rebound, which is not good for stability. The passive rear steer designed into the FC further complicates things, and may further destabilize the car due to the change in vehicle weight.
You can make a solid axle car handle great, and it would represent the minimum investment. You can also make a semi trailing arm suspension handle great, but you would have to limit suspension travel to keep it in its optimum range.
I beliefve the Miata has a double wishbone/SLA suspension, which offers better control of both toe and camber changes.
In the suspension world, when each is designed properly: Solid Axle <= Swing Arm < Trailing/Leading Arm < Semi Trailing Arm < McPherson/Chapman Struts < SLA <= Multilink
The issue with the FC rear is that it is in essence a semi trailing arm design. These toe out in rebound, which is not good for stability. The passive rear steer designed into the FC further complicates things, and may further destabilize the car due to the change in vehicle weight.
You can make a solid axle car handle great, and it would represent the minimum investment. You can also make a semi trailing arm suspension handle great, but you would have to limit suspension travel to keep it in its optimum range.
I beliefve the Miata has a double wishbone/SLA suspension, which offers better control of both toe and camber changes.
In the suspension world, when each is designed properly: Solid Axle <= Swing Arm < Trailing/Leading Arm < Semi Trailing Arm < McPherson/Chapman Struts < SLA <= Multilink
01-20-05, 09:53 AM
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Originally Posted by mortenf
In my opinion, the 1.st gen axle is easy to fix, reliable up to about 250 HP (unmodded) and interchangeability with other Mazda models are well known, for instance the B2600 diff.
That means the FB's diff will go into the B2600...
Interesting. LSD for my truck. I like it.
Or an even sicker thought... A 6 lug wheel bolt pattern on the RX-7...
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01-20-05, 10:18 AM
#8
At least i know the B2600 4X4 model DIFFERENTIAL fits. Not the whole axle.
Did you know that the gearbox is virtually the same as the old Luce/929 that was made 82-85 (sold in europe as 929) The only real difference is the casing, otherwise the gearing will fit inside a RX7 1.st gen box.
Did you know that the gearbox is virtually the same as the old Luce/929 that was made 82-85 (sold in europe as 929) The only real difference is the casing, otherwise the gearing will fit inside a RX7 1.st gen box.
06-20-05, 01:26 PM
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I'm going to dig this back up on the grounds that:
1.) I just realized that in 1986, the FB was the test mule for the FC IRS. (Though I can't find where I read that, I'm leafing through my Yamaguchi and Matras books as I type.)
2.) I'm looking at different rear axle options for the 1st gen because i'd like matching bolt patterns on front and rear. I can easily get 5 lug on the front...
3.) I have available to me, an FC rear subframe, differential, axles, and hubs. (To Compliment the front subframe I have avalable to me as well... Basically, dismantling an S4 GXL by way of SawzAll.
1.) I just realized that in 1986, the FB was the test mule for the FC IRS. (Though I can't find where I read that, I'm leafing through my Yamaguchi and Matras books as I type.)
2.) I'm looking at different rear axle options for the 1st gen because i'd like matching bolt patterns on front and rear. I can easily get 5 lug on the front...
3.) I have available to me, an FC rear subframe, differential, axles, and hubs. (To Compliment the front subframe I have avalable to me as well... Basically, dismantling an S4 GXL by way of SawzAll.
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