2015 IRS retrofit?

[CraigS]

FFR can you please look into creating, and offering for rental, a jig to allow us to retrofit the new IRS suspension to our older cars. I just saw that John George's car was done in house so it is obviously possible. I want to do mine next winter.

[CHOTIS BILL]

I would love to do that to my Type 65 that already has the older IRS.

Bill Lomenick

[David Hodgkins]

Craig, I forwarded this to FFR. I mentioned it might be a money-maker for them to set up a retrofit jig. I doubt they go for it but I do think personally it's a good idea , theoretically.

BUT-

For JG's car, you HAVE to think of it as an R&D exercise, taking a known entity and comparing it against an alpha. They used their top guys to do the change and that simply can't happen for all retrofits. They would have to design and build a jig for it and then how many retrofits would it take to recoup that investment? I'm thinking that it's not financially viable.

Personally, I am REALLY interested in the 2015 IRS too. My plan is to sell my MKIII and build a new MKIV. I'll be giving up pin drive to do it, and I LOVE that look, but I guess I'm willing to give up originality for performance.

[Jim Schenck]

We looked at that as we were designing the IRS but it really just wasn't practical. The old IRS is more self contained and can bolt in as a unit where the new one has a lot more tooling to get it located. I wouldn't say it is impossible but just not very likely due to the complexity involved. Cutting out an older IRS would be even another step harder.

[mikeinatlanta]

What is it about the older IRS that one would consider a conversion? I get converting from live axle, but what does the 2015 IRS have that can't be equaled with the older IRS?

EDIT: David, same question to you. What about the new IRS would motivate you to give up pin drive?

[David Hodgkins]

What is it about the older IRS that one would consider a conversion? I get converting from live axle, but what does the 2015 IRS have that can't be equaled with the older IRS?

EDIT: David, same question to you. What about the new IRS would motivate you to give up pin drive?

That's a good and fair question that frankly I don't have a practical answer for, given that 99.5% of my current driving is on the street. I was thinking of getting more into autocross and maybe more track use, and I would say that I would be more comfortable with a "Gen 2" IRS over what I have now. In harsh racing conditions I've seen and heard about the 1st Gen IRS not being as durable as the 3 link. FFR even states that the 3 link is better than the Gen1 IRS for track use. If they decide that the Gen 2 IRS is better than the 3 link, I think that's significant.

Giving up pin drive to get there is the trade-off for me. And this is huge for me because I LOVE the pin drive look! But I understand the geometry enough to know that you need less offset at the wheel and longer axles to maintain better traction, and going that route would be purely for performance reasons. And with my simplistic mind, if I'm going to do another build and step up the performance, why not get the latest and greatest?

[CHOTIS BILL]

I haven’t seen it written but just going by the pictures the rear roll center has been changed which I believe has led to better handling and that is why the new IRS is now a bit faster than the 3 link where in the past the old IRS was a bit slower. Or at least this is what has been written.

Bill Lomenick

[Mike N]

The other very obvious difference is that the upper link is triangulated so you are not reacting brake torque through 'twisting' the lower wishbone.

new-ffr-irs-folio.jpg

On the old IRS the upper link provided a lateral constraint (camber)but very little fore and aft so the top of the knuckle could move back and forth. This did not provide the best wheel control especially under braking. The new triangulated upper link provides a very positive fore and aft constraint and vastly improved wheel control.

[mikeinatlanta]

No doubt the new design is a better one, and I can see the added rigidity of it and certainly agree that FFR is making a good change. What I'm wondering is if it would be easier to simply make some adjustments to the old one vs retrofitting a new one in an earlier chassis. The torsion issue is corrected with spherical bearings and roll center is an easy fix with a welder and drill. Where I'm having the biggest issue is weight. The 2015 design is meant to support track use on a 3,800 pound car. I'm thinking it's way overbuilt for a 2,200 lb car.

To be clear: I'm not knocking anyone's decisions or desires. I'm talking purely from a technical standpoint.

[edwardb]

I too will be interested in FFR's response to the possibility of retrofitting the 2015 IRS setup to other chassis versions. Having the new chassis in the garage right now and comparing to the old, I can only say there are major differences. I'm just not sure whether the option to retrofit is going to be for the masses, and agree with David this was likely a very limited or one-off R&D exercise by FF to get the new IRS quickly on the track.

Couple of observations, much of which may already be well known but will point out anyway. While the new setup is based on the 2015 Mustang IRS, it only uses three components: The center section, the knuckles which are slightly modified and the hubs which are part of the knuckle assembly. The balance of the IRS components are custom components from FF. Because I was early in the game last summer, and the separate components weren't available yet, I was only able to find a complete 2015 IRS pallet. So I saw the entire 2015 Mustang IRS suspension up close. I can say the rest of the Mustang components are very different than what FF designed. Including some elements that are not in the FF design. Assuming the improved performance is real, which I have no reason to doubt, it's hard to say how much is from the actual new Mustang parts versus FF starting with a clean sheet of paper design for their IRS. That combined with the availability of the new components compared to the 20+ year old donor parts I suspect were the major drivers for the change. I can't wait to get it on the road, but it's going to be at least another year. I won't be the best judge though since my other two builds were solid axle setups.

[2bking]

I have the old IRS and would like to upgrade just for the Ford designed pivot assemblies and the better design. The spherical rod ends on the old IRS don't have longevity due to the lubrication issues and are the weak link in the design. I installed the dust caps but I doubt they will provide very much protection. When the time comes for replacing the rod ends, the alignment is a pain.

[mikeinatlanta]

If it's durability of the rod ends, why not use a more durable one?
Like this
s-l1000.jpg

Or these
sky-rod_ends.jpg


Has anyone measured and compared the new and old geometries?

[CraigS]

The other very obvious difference is that the upper link is triangulated so you are not reacting brake torque through 'twisting' the lower wishbone.

new-ffr-irs-folio.jpg

On the old IRS the upper link provided a lateral constraint (camber)but very little fore and aft so the top of the knuckle could move back and forth. This did not provide the best wheel control especially under braking. The new triangulated upper link provides a very positive fore and aft constraint and vastly improved wheel control.

I agree w/ this. There are some on the coupe forum who have modified the upper arm which helps. But that has to be done very carefully so it pivots on an axis parallel to the lower arm pivot axis. And then you still have the very common problem of the axle nuts coming loose. Basically the old IRS was designed to give a better ride in a 4000# car w/ 5 inch wide tires while the new IRS is designed to be run hard in a performance car.

[mikeinatlanta]

I do like the bolt in hub. Does the FFR upper spindle bearing use a heim joint? It looks like one in the drawing. If that's the case, it wont be handling much torsional load from the spindle mounted as pictured. Any lower flex will put an inappropriate side load on the bearing unless the upper arm can float.

Does anyone have any geometry numbers for the old IRS and new IRS? I'm still looking for what would make it "faster".

[Mike N]

I do like the bolt in hub. Does the FFR upper spindle bearing use a heim joint? It looks like one in the drawing. If that's the case, it wont be handling much torsional load from the spindle mounted as pictured. Any lower flex will put an inappropriate side load on the bearing unless the upper arm can float.

The upper bearing looks very similar to the bearings used in the old style IRS lower arms. Loads on the upper arm should be less than the lower due to the distance of the pivot from the axle centerline. I agree that lateral (side) loads on heim ends should be avoided but it worked in the old IRS and I have a similar set up on my front lower A arms that has worked well for many years.

[mikeinatlanta]

The upper bearing looks very similar to the bearings used in the old style IRS lower arms. Loads on the upper arm should be less than the lower due to the distance of the pivot from the axle centerline. I agree that lateral (side) loads on heim ends should be avoided but it worked in the old IRS and I have a similar set up on my front lower A arms that has worked well for many years.

In the old IRS a torsional load from braking would try to twist the lower a arm, which would load the bearing correctly, although not ideally. With the new design, using a triangulated upper arm looks like it will try to pull the ball from the heim with torsional loads.

[Mike N]

Agreed. But remember that as well as the moment loads that are reacted 'correctly' there are thrust loads on the inner heims just from acceleration and braking those are significant too and not ideal for a heim end. Think of it as shear force and bending moment. The shear force from acceleration / deceleration is the same along the whole axle but the moment loads change with distance.

[mikeinatlanta]

Agree. However, ASSUMING an equally shared bearing load and 1G decel and 30% rear braking force, you are talking less than 200lb per bearing. Not a lot, and way less than the torsional loads generated by heavy braking. 1G acceleration would generate about 385 per bearing, still way less than the torsional load braking could generate.

I realize the numbers are a WAG, but good enough to think about the issue.

[Mike N]

Intermittent loading to those sort of forces shouldn't be a concern. I don't know what bearings are supplied with the IRS but for a decent 3/4" bearing the axial load capability should be well over 20,000 lbs. Aurora Bearing recommends that the radial load be kept to less than 10-20% of the axial load (depending on bearing type) which is higher than I expected. Using 10% of the 20,000 lb number and 385lbs gives a factor of safety of more than 5.

[mikeinatlanta]

I'm thinking the axial vs radial is backwards what you stated. Here is a special 3/4" bearing rated for thrust loading. It's not nearly as strong radially (9800) but much stronger in thrust load ability (3500).http://www.mcmaster.com/#8405k391/=110tscs Their standard steel bearing is rated over 28,000 radially but not rated at all for thrust load.
8405kc3l.png


Anyone care to calculate the potential thrust load on the upper spindle bearing of the new IRS under heavy braking? Would require knowing the maximum torque input traction would allow and the distance of the bearing from centerline.

[Mike N]

Yes I had it bass ackwards. A high misalignment bearing mounted on top of the hub with the ball bore axis vertical would be a much better configuration. Just rotating the bearing housing 90 degrees would take care of the bad loading condition.

[2bking]

Anyone care to calculate the potential thrust load on the upper spindle bearing of the new IRS under heavy braking? Would require knowing the maximum torque input traction would allow and the distance of the bearing from centerline.

I'll take a stab at it. Assume 2600 lb car, 1G stopping, 40% of the stopping force comes from rear tires, 24" diameter tires (just to make the math easy) upper and lower spindle bearings are separated by 10", and rear stopping force is divided equally between rear brakes. I'll also assume the upper/lower spindle bearings are symmetrical about the axle, but they are not so the final numbers will be a little off.
Stopping Force on rear = 2600 lbs*.4 = 1040 lbs
Torque from brakes = 1040 lbs*12" = 12,480 in-lbs
Reaction force at spindle bearings = 12,480 in-lbs / 10" =1,248 lbs (+624 lbs upper, -624 lbs lower)
The breaking force is added to these reaction forces, 1040 lbs divided between the upper/lower = +520 lbs upper, +520 lbs lower
Total force upper = +620 lbs +520 lbs = 1140 lbs, 570 lbs left side, 570 lbs right side
Total force lower = -620 lbs +520 lbs = -100 lbs , -50 lbs left side, -50 lbs right side

These numbers are only side loads on the bearings and do not include suspension loads. Unless I got the signs wrong, the upper bearing takes the most force under breaking and sees about 600 lbs.

[mikeinatlanta]

It will be interesting to see how the upper bearing lasts over time with those loads and some wear. Of course, we still don't know the specific bearing being used.

[CraigS]

Are you guys worrying that the ball will try to pop out of the housing of the rod end or that the rod end will snap it's shaft at the jam nut?

[mikeinatlanta]

Ball popping out of housing. Depending on the heim used, they aren't designed for that type of load.

[CraigS]

I agree. I guess the good thing is that the braking load would have the rod end still somewhat contained. I suppose a spacer and large washer on the outside would keep it from coming completely apart even if it did pop. Although I am not sure they would be available in the correct thread, a tie rod end could also be used there. Those are very common in road car suspensions these days.

[Mike N]

Craig. I have heim ends on my front lower control arms and they have worked great and don't show any signs of wear after 7 years and lots of hard use. I'm not sure how they would hold up to repeated pot hole abuse but for track, auto X and regular road miles they don't appear to be any problem. Anyone who decides to use rod ends on critical suspension components should understand that all rod ends are not created equal. A 'cheap' rod end is not going to be a good choice. After much research I found economical quality rods ends http://www.qscomponents.com/collecti...ale-rh-rod-end

[2bking]

After much research I found economical quality rods ends http://www.qscomponents.com/collecti...ale-rh-rod-end

Like most parts the devil is in the details. I'm not saying these aren't "good" joints but there isn't any detail on the ratings in the axial and radial directions or wear life with a subjected load. The load rating shown seems to be a cross section rating of the steel, not the wear surfaces of the injected plastic where the forces should be kept below 1000 psi for extended life. But dirt/debris is the killer of helm joints and the cheaper ones don't have seals to help keep them clean.

[mikeinatlanta]

Like most parts the devil is in the details. I'm not saying these aren't "good" joints but there isn't any detail on the ratings in the axial and radial directions or wear life with a subjected load. The load rating shown seems to be a cross section rating of the steel, not the wear surfaces of the injected plastic where the forces should be kept below 1000 psi for extended life. But dirt/debris is the killer of helm joints and the cheaper ones don't have seals to help keep them clean.

I agree. If I can't see the ratings I don't buy. That's one reason I like the heims at McMaster.

[Mike N]

Mike. I hear you on the ratings but I started with more expensive McMaster parts with 'sealsit' rubber seals and they wore and had clearance within a few hundred miles which made them noisy. The ones I have now have a Kevlar reinforced teflon bearing face and still do not have any appreciable wear or clearance.

[mikeinatlanta]

Keep in mind that McMaster also sells the Teflon lined heims but they are rated. You are correct that they are more wear tolerant for dirt than unlined. They also sell heims with grease fittings which work really well with seals. Basically the seal cavity fills with grease and keeps the dirt out. For me, it's all about light weight above anything else so I get what is lightest.

[David]

David,

I agree, I think the new IRS is pretty trick, and have considered exploring the option of retrofitting. With that said, remember that a FFR with the old IRS and pin drive was able to take a 3rd place at the SCCA Solo II Nationals in AP. . Also, top dog at the Run and Gun event. I promise you that a well setup Gen 1 IRS is more capable than 90% of the drivers out there. There is much more time in maximizing ones setup and driving than changing the IRS out.

I have a set of later Cobra IRS spindles and was thinking of adapting them. They seem to be closer to the 2015 IRS setup and maybe easier to adapt. Just a SWAG though.

David

[CraigS]

Very frustrating that FFR won't release an upgrade kit. I just looked at EdwardBs pics again on the other forum. There is a frame that welds onto tops of the ends of the 4 inch tubes. There is one that hangs behind the diff from the longitudinal tubes. And a few others. The 4 inch tubes look to be standard. The longitudinal tubes are standard as evidenced by having the brackets for either horizontal shocks or 3 link frame welded in place even though they are not used w/ the IRS. I think this would be relatively easily doable by anyone w/ some measuring and welding skills. I just can't see going to all the work to do the old IRS upgrade when this is available, or could be available.