Midwest Classic Insurance

Visit our community sponsor

Thanks Thanks:  0
Likes Likes:  0
Results 1 to 20 of 20

Thread: Rear suspension...

  1. #1
    Senior Member
    Join Date
    Dec 2013
    Location
    Twin Cities, MN
    Posts
    139
    Post Thanks / Like

    Rear suspension...

    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.
    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.

  2. #2
    Senior Member CHOTIS BILL's Avatar
    Join Date
    May 2011
    Posts
    427
    Post Thanks / Like
    Blog Entries
    1
    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
    Chotis Bill

  3. #3
    Senior Member chopthebass's Avatar
    Join Date
    Oct 2013
    Location
    Calgary, Canada
    Posts
    941
    Post Thanks / Like
    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.

  4. #4
    Senior Member CraigS's Avatar
    Join Date
    Feb 2011
    Location
    Blacksburg, Va
    Posts
    4,707
    Post Thanks / Like
    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.
    FFR MkII, 408W, Tremec TKO 500, 2015 IRS, DA QA1s, Forte front bar, APE hardtop.

  5. #5
    Senior Member AC Bill's Avatar
    Join Date
    Feb 2011
    Location
    Vancouver Island BC Canada
    Posts
    1,877
    Post Thanks / Like
    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.

  6. #6
    Director of R&D, FFR Jim Schenck's Avatar
    Join Date
    Mar 2011
    Location
    Wareham
    Posts
    444
    Post Thanks / Like
    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.
    Jim Schenck
    Factory Five Racing

  7. #7
    Senior Member edwardb's Avatar
    Join Date
    Feb 2011
    Location
    Lake Orion, Michigan
    Posts
    10,545
    Post Thanks / Like
    Quote Originally Posted by Jim Schenck View Post
    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.
    Build 1: Mk3 Roadster #5125. Sold 11/08/2014. Build 2: Mk4 Roadster #7750. Sold 04/10/2017. Build Thread
    Build 3: Mk4 Roadster 20th Anniversary #8674. Sold 09/07/2020. Build Thread and Video. Build 4: Gen 3 Type 65 Coupe #59. Gen 3 Coyote. Legal 03/04/2020. Build Thread and Video
    Build 5: 35 Hot Rod Truck #138. LS3 and 4L65E auto. Rcvd 01/05/2021. Legal 04/20/2023. Build Thread. Sold 11/9/2023.

  8. #8
    Senior Member
    Join Date
    Dec 2013
    Location
    Twin Cities, MN
    Posts
    139
    Post Thanks / Like
    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.

  9. #9
    Junior Member
    Join Date
    Nov 2014
    Posts
    14
    Post Thanks / Like
    Does everyone here get the IRS also from FFR or their are some other suppliers sell the IRS?

  10. #10
    Senior Member
    Join Date
    Dec 2013
    Location
    Twin Cities, MN
    Posts
    139
    Post Thanks / Like
    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...
    Hurricane HM-2016, 9" Ford w/3.50, 427W, TKO 600.

  11. #11
    Senior Member
    Join Date
    Mar 2014
    Location
    Pass Christian, Mississippi
    Posts
    122
    Post Thanks / Like
    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
    MK IV complete kit delivered August 21st, 2014. Ford Racing 427w with X heads. IRS. Upgraded Wilwoods, 6 piston front and 4 rear. Tremec t600 and Ram Dual clutch. Holley Terminator EFI.

  12. #12
    Senior Member Bren's Avatar
    Join Date
    Dec 2013
    Location
    Build: Sugar Land, TX; Live: Madison, WI
    Posts
    271
    Post Thanks / Like
    Quote Originally Posted by aarvig View Post
    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...
    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.
    Build Blog
    #8203, carbed 427W (611 hp), TKO 600, IRS
    Delivered: May 27, 2014
    Mechanically "Complete": October 1, 2014

  13. #13
    Director of R&D, FFR Jim Schenck's Avatar
    Join Date
    Mar 2011
    Location
    Wareham
    Posts
    444
    Post Thanks / Like
    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 Schenck
    Factory Five Racing

  14. #14
    Senior Member
    Join Date
    Dec 2013
    Location
    Twin Cities, MN
    Posts
    139
    Post Thanks / Like

    Red face

    P
    Quote Originally Posted by Jim Schenck View Post
    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. By the way, I fully understand you may not be allowed to answer this...just thought I'd ask.
    Hurricane HM-2016, 9" Ford w/3.50, 427W, TKO 600.

  15. #15
    Senior Member Benji's Avatar
    Join Date
    Feb 2011
    Posts
    358
    Post Thanks / Like
    Quote Originally Posted by aarvig View Post
    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. 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.

  16. #16
    Senior Member CraigS's Avatar
    Join Date
    Feb 2011
    Location
    Blacksburg, Va
    Posts
    4,707
    Post Thanks / Like
    I haven't researched for details but there are also a ton of Explorers w/ IRS. Possibly some of their components could be used.
    FFR MkII, 408W, Tremec TKO 500, 2015 IRS, DA QA1s, Forte front bar, APE hardtop.

  17. #17
    Member Kpt112's Avatar
    Join Date
    Feb 2015
    Location
    Bucks County PA
    Posts
    98
    Post Thanks / Like
    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?image.jpg thanks in advance.

  18. #18
    Junior Member
    Join Date
    Jun 2014
    Posts
    23
    Post Thanks / Like
    Quote Originally Posted by Kpt112 View Post
    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?image.jpg 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/deta...=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-ca...s/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.
    Last edited by NavyChief; 02-15-2015 at 08:06 AM.

  19. #19
    Junior Member
    Join Date
    Jul 2014
    Location
    Elgin, SC
    Posts
    25
    Post Thanks / Like
    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.

  20. #20
    Member Kpt112's Avatar
    Join Date
    Feb 2015
    Location
    Bucks County PA
    Posts
    98
    Post Thanks / Like
    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.


    Quote Originally Posted by RickGT40 View Post
    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.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  

Replica Parts

Visit our community sponsor