Rear suspension...

[aarvig]

So, I am baffled. Everyone seems to think that IRS is the comfort option and the better suspension. However, I have talked to the folks at Factory Five and have been told by them that the three link suspension is far superior, that it has been significantly more engineered and that it is a better ride than the IRS. :confused:
I've been told that by two of their employees. This afternoon, I was told that the IRS is equivalent to a 1970's corvette and the 3-link is equivalent to a 2014 mustang. Everytime I have told them my intended use (road cruising, looking for comfort with the edge towards performance) I have been hurried away from any ideas of the IRS and told the three link is the way to go. I was also told this afternoon that they are in discussions to improve the IRS ride quality and reengineer it. When I pressed for more info the employee wouldn't give me any more. Strange. I am not sure what to think about this but it seems that what their website and marketing information says about the IRS is the opposite of what everyone who works there says.

[CHOTIS BILL]

That is strange. I did hear talk at one time about possibly raising the IRS roll center to be more compatible with the front roll center maybe that is what is being discussed and they want to discourage people from buying IRS kit until it is changed.

Just a guess,

Bill Lomenick

[chopthebass]

I was told by an employee (who recently left FF) that IRS is a more comfortable ride and better handling. He went on to say that a 3-link setup will be 95% as good as IRS. What the heck that means I don't know. Sounds like a lot of personal opinions and who you talk to on the day.

[CraigS]

Those comments from FFR are crazy. I have driven an IRS MkIV w/ the FFR supplied konis and springs. I have ridden in an MkIII or early MkIV w/ 3 link and FFR Konis and springs. There is no question in my mind that the IRS is the better ride. Those 350 springs FFR started shipping w/ the Konis for 3 link are too stiff in my opinion. In theory on a smooth road over dull bumps and undulations we could get a spring/shock combination for a 3 link that would give a ride comparable to IRS. But, as soon as you throw sharper bumps in the middle of a turn into the test drive, I see no way IRS won't be better. The comment that chopthebass quotes are a lot more in line w/ my experiences.

[AC Bill]

I think it depends on how comfortable you want the ride, and the type of driving a builder intends. As you listed, you should be fine with the IRS.
If you planned on more serious track and drag strip type driving, I'd go with the 3 link over the IRS.
Personally, I still find the 3 link ride very comfortable. I have 15" tires, so a little bigger sidewall, which helps smooth things out a bit.:)

[Jim Schenck]

The IRS is a better ride for sure, not only because it can absorb bumps with each wheel individually but also because it is sprung much softer. (The wheel rate is softer not the actual spring) The three link has an advantage over any IRS system in forward grip and the difference in overall handling is just down to tuning. Because of the very short driveshaft, the solid axle cars have to be run with less suspension travel and end up on the stiffer side. The IRS does not have this limitation and we can spring it softer to give the better ride, so we effectively tune the two suspensions from the factory to give more of a difference because that gives people more choice in the type of car they build. Tuning the IRS to be stiffer (with bushings, shocks, and spring rates) can make it at least close to the 3 link in terms of smooth track handling and for sure better on rougher tracks or streets. With the three link the option to soften it up for just cruising is more limited because of the driveshaft angles getting to severe with more suspension travel.

[edwardb]

The IRS is a better ride for sure, not only because it can absorb bumps with each wheel individually but also because it is sprung much softer. (The wheel rate is softer not the actual spring) The three link has an advantage over any IRS system in forward grip and the difference in overall handling is just down to tuning. Because of the very short driveshaft, the solid axle cars have to be run with less suspension travel and end up on the stiffer side. The IRS does not have this limitation and we can spring it softer to give the better ride, so we effectively tune the two suspensions from the factory to give more of a difference because that gives people more choice in the type of car they build. Tuning the IRS to be stiffer (with bushings, shocks, and spring rates) can make it at least close to the 3 link in terms of smooth track handling and for sure better on rougher tracks or streets. With the three link the option to soften it up for just cruising is more limited because of the driveshaft angles getting to severe with more suspension travel.

This is great information and clarifies a number of aspects. Always good to hear straight from the experts! Thanks Jim.

[aarvig]

Thanks for clarifying Jim. I won't mention names but the two employees that I am chatting with when I have called up should be educated on this topic. What you said makes 100% sense. I even told the second employee how strange it was that the three link was recommended over the IRS for street crusing. Anyway, thanks for clarifying.

[superjoe]

Does everyone here get the IRS also from FFR or their are some other suppliers sell the IRS?

[aarvig]

I have only been able to find the IRS portion through Factory Five. In fact, I believe parts of it are welded into the frame when the frame is built. You will still need to get the differential, cv joints and spindles from another source. I know for sure that Gordon Levy is able to acquire them and rebuild a nice unit for you. You want the T-bird rear end I think...not a pro on this yet...:D

[pcww]

I , too, appreciate the comments from Jim. BTW, every post purchase interaction I have had with FFR has been first rate. I have nothing to gain from my comments but one of the factors that led me to FFR was factory support and I have not been disappointed. Henry

[Bren]

I have only been able to find the IRS portion through Factory Five. In fact, I believe parts of it are welded into the frame when the frame is built. You will still need to get the differential, cv joints and spindles from another source. I know for sure that Gordon Levy is able to acquire them and rebuild a nice unit for you. You want the T-bird rear end I think...not a pro on this yet...:D

If I remember correctly, it's the Cougar Mark VIII that has the "desirable" aluminum pumpkin. Aside from that, the other thing to look for is whether the diff is limited slip or not.

[Jim Schenck]

Bren,

It's the Lincoln MKVIII that has the aluminum center but usually those are not limited slip. The factory T-bird limited slip can be installed in the Lincoln carrier and some aftermarket units will work but you have to check and make sure they have the grooves cut for the axle snap rings to pop into or the axles won't stay in place. Also on both T-birds and Cougars you have to make sure the diff is for sure an 8.8, some of the cars had the smaller 7.5 rear although most of those also had drum brakes. (8.8 or 7.5 will be cast into the housing) The sure bets are the supercharged V6 cars, they are all limited slip 8.8s with disc brakes.
These parts are starting to get scarce but we will be addressing that before they become impossible to find.

[aarvig]

P
Bren,

It's the Lincoln MKVIII that has the aluminum center but usually those are not limited slip. The factory T-bird limited slip can be installed in the Lincoln carrier and some aftermarket units will work but you have to check and make sure they have the grooves cut for the axle snap rings to pop into or the axles won't stay in place. Also on both T-birds and Cougars you have to make sure the diff is for sure an 8.8, some of the cars had the smaller 7.5 rear although most of those also had drum brakes. (8.8 or 7.5 will be cast into the housing) The sure bets are the supercharged V6 cars, they are all limited slip 8.8s with disc brakes.
These parts are starting to get scarce but we will be addressing that before they become impossible to find.

Jim, Here's where I am curious. When you say that we will be addressing it...do you mean we are going to redesign it completely to use a differential or we are going to start producing our own aftermarket differentials. What is the timeframe we can expect this change to come in? The reason I ask is that I don't want to plunk down the change right now if something new and improved is coming down the pipe.:o By the way, I fully understand you may not be allowed to answer this...just thought I'd ask.

[Benji]

P

Jim, Here's where I am curious. When you say that we will be addressing it...do you mean we are going to redesign it completely to use a differential or we are going to start producing our own aftermarket differentials. What is the timeframe we can expect this change to come in? The reason I ask is that I don't want to plunk down the change right now if something new and improved is coming down the pipe.:o By the way, I fully understand you may not be allowed to answer this...just thought I'd ask.

Rampant speculation but wondering if they are adopting it to use the new IRS diff from the new Ford Mustang maybe.... since it is being mass produced now, it maybe a better and possibly even cheaper option.

[CraigS]

I haven't researched for details but there are also a ton of Explorers w/ IRS. Possibly some of their components could be used.

[Kpt112]

I just ordered my kit this week with the IRS option. I have been having a really tough time finding then Thunderbird rear differential. I found 1 with a 197k miles on it out of state. With the knuckles this part of the build is getting expensive. Does anyone have a lead on where I can get the parts?38765 thanks in advance.

[NavyChief]

I just ordered my kit this week with the IRS option. I have been having a really tough time finding then Thunderbird rear differential. I found 1 with a 197k miles on it out of state. With the knuckles this part of the build is getting expensive. Does anyone have a lead on where I can get the parts?38765 thanks in advance.

I've done a lot of searching around due to incomplete info on this and came up with this:

Forte has a complete IRS option (easiest and best)
http://www.fortesparts.com/axle-rear-end-parts
Breeze has the complete ford racing spindles with bearings and hubs as a kit (source your own diff)
http://www.breezeautomotive.com/details.php?prod_id=378&cat_id=37
Jegs sells the same thing as breeze (same prices so I would use Breeze)

The diff is not that difficult to find and I'm not sure why people keep referring to the Mk8 as having the aluminum lighter diff. All the ones I've come across are aluminum so not sure why. I'm always learning new stuff though maybe a different design?

You could source a 99 Cobra diff as they are 28 spline and you would be good with the FFR supplied halfshafts
You could source a later model Cobra 03/04 with 31 splines and use the Driveshaft Shop to get 31 carrier/28 hub halfshafts as they sell them for the FFR kit. http://www.driveshaftshop.com/kit-car-misc-parts/factory-five
You could source a cheaper Mk8 open diff and rebuild with a limited slip.

or...

You could try to find the Tbird diff components if you pray to the right gods and perform the proper sacrifices and rituals.

[RickGT40]

When I ordered my base kit last summer I got the IRS option. I then found a Mark VIII rear suspension drop out - hub to hub with brakes - on Craigslist for 225. I looked on Ebay for a while and found a Thunderbird differential with the 3.27 stock ring and pinion for 125 delivered. I ordered new hubs and bearings for the uprights and new bearings and seals for the diff for about 250, and then had a local shop put the center section together for 175. total was about 850 plus brake parts. If you look around you can find the parts you need.
The Mark VIII had aluminum housings mostly with 3.08 open diffs. The Thunderbirds diff you need will have a Ford id tag with S511B as the model id. I believe the "B" represents the REV level so there may be an A or C there, but it will still by a T-bird diff. The housing on T-birds are cast iron/steel.

[Kpt112]

Thanks for the advice, never thought of Craig's list. I got a lead on both a 1994 and 1997 T-Bird as soon as I looked. Here is hoping.



When I ordered my base kit last summer I got the IRS option. I then found a Mark VIII rear suspension drop out - hub to hub with brakes - on Craigslist for 225. I looked on Ebay for a while and found a Thunderbird differential with the 3.27 stock ring and pinion for 125 delivered. I ordered new hubs and bearings for the uprights and new bearings and seals for the diff for about 250, and then had a local shop put the center section together for 175. total was about 850 plus brake parts. If you look around you can find the parts you need.
The Mark VIII had aluminum housings mostly with 3.08 open diffs. The Thunderbirds diff you need will have a Ford id tag with S511B as the model id. I believe the "B" represents the REV level so there may be an A or C there, but it will still by a T-bird diff. The housing on T-birds are cast iron/steel.