More IRS questions

[Guest]

I need some clarification on the wording concerning the IRS.

 

Section 6.1.d reads: ..."Factory IRS" is defined as: (a) the unmodified OEM installed rear IRS cradle that attaches to the chassis and serves as a mount for the center differential and...but the location of the cradle cannot be changed in relation to the OEM mounting point

 

My question is this: The rear suspension subframe of the 99-04 Cobra attaches to the car via a cantilevered bracket. The outer half of this bracket is completely unsupported and flexes significantly, leading to less predictable handling as the entire rear suspension system shifts under the car

 

Are we allowed to redesign this mount to solidly mount the IRS cradle to the framerail without moving the location of the IRS cradle (read: the front mount will be in the OEM location laterally and fore/aft)

 

Below is an example of what I would like to do:

 

 

Once again, my reasoning is for changing the mount is to solidly mount the IRS cradle. Without doing this the IRS cradle is not supported strongly enough and can move under large lateral loading. This movement will cause the rear suspension to actually steer the car with no warning and could become dangerous in some situations. Using this type of mounting will also take some of the lateral load off of the front mounting points which mount to the torque boxes, which are not designed to take lateral loads.

 

Is this type of modification allowed for helping keep the cars safe and controlable at the limit (10/10ths driving).

 

Thank you for taking your time with regard to this matter, we all know that you (AI Directors) are very busy and I look forward to hearing your response.

 

BPT

[JWL]

Brian-

 

We'll chew on it this week and get back to you ASAP.

 

-JWL

[JWL]

Brian-

 

Just to confirm what you're after, you're just adding a piece to the bracket to stabilize it and not shifting the mounting point of the cradle, correct?

 

-JWL

[Guest]

Brian-

 

Just to confirm what you're after, you're just adding a piece to the bracket to stabilize it and not shifting the mounting point of the cradle, correct?

 

-JWL

 

Sort of. I would like to completely redesign the rear upper mount but it would not move the cradle at all (in relation to the OEM location). If the front of the cradle is mounted in the torque boxes (the OEM location) the cradle cannot be mounted anywhere but the OEM location. What I would like to do is to get rid of the rear OEM mount (which can be seen in the pic below, it's the round barrel looking thing with a single bolt holding it to the frame rail, it is located in the pic just below the text reading "Knuckle and Ball Joint")

 

 

I want to cut that off and add a bottom and side plate like the pic in the first post. This design will allow the cradle to be solidly and securly bolted to the frame rail. The location is exactly the same as the OEM location. The proposed design would is much better suited to heavy duty/racing applications than the OEM mount.

 

Is that any clearer?

[Tom Beverly]

I think he wants to eliminate the section highlighted in red:

[Guest]

I think he wants to eliminate the section highlighted in red:

 

Tom thanks for that shot. I cannot find my camera to take a pic of the one I have in my garage but you are right. I want to eliminate the rear upper mounts and replace them with a nice solid to the frame rail mount.

 

Again, the cage would stay in the OEM location as defined by the front mounts (in green in Tom's pic).

 

Thanks!

[Jim P.]

 

 

Sweet!

 

I wonder how hard would it be to put one of those in an early model Mustang? Has anyone done that in a '68?

[Tom Beverly]

Yes.

http://www.dvsrestorations.com/irs.shtml

[johnbasf]

Doesn't 6.1D keep you from putting this in another car? Is there an OEM mounting point in an older car?

 

Sorry for the hijack Mr. Tone.

[Jim P.]

True, but I think that rule applies to AI -

 

- AI/X is exempt, no?

 

I would think that a '68 Mustang w/IRS would definitely be in AI/X...

[johnbasf]

True, but I think that rule applies to AI -

 

- AI/X is exempt, no?

 

I would think that a '68 Mustang w/IRS would definitely be in AI/X...

 

AIX it is Jim. 6.2 says have at it.

[D Algozine]

Brian or anyone who knows,

 

Does the "U" shape of the bracket flex or does the upright flex? I have read about this problem, but I'm not exactly sure what is the weak point. I was considering adding bracing where the bracket attaches to the frame rail, but I'm not convinced it would help. Brian, I think your design is much better.

[swhiteh3]

The OEM design was made that way to impart minimal changes to the frame rail assembly. Althought the bracket comes up, over, and down on the outside of the subframe mounting point, the bolt is essentially still in single shear since the outside of the bracket is not really supported at all. Therefore, when there are major vertical loads fed into the subframe, this bracket tends to flex, allowing the entire subframe to move vertically. It's a flexability in the bracket due it's design limitations - not the framerail.

 

I'd say this modification is certainly within the spirit of the rules. However, the rules as they are currently written says the subframe must remain stock. I'd recommend one of two other options to get the advantage of this modification without violating the rules as they are currently written....

 

The first option would be to use Steeda's Subframe Bracket Kit (http://www.steeda.com/store/-catalog/suspension/IRSsubframebrackeet-150.jpg). This kit supports the outboard side of the OEM mounting bracket. Since it welds to the IRS mounting bracket and not the OEM cradle, I'd interpret that this part would be legal. Maybe JWL can confirm.

 

My second option (which would have little additional advantage, but would satisfy those who are more handy) would be fabricate an entirely new IRS bracket. Again, since your are modifying / replacing the bracket which is not a part of the OEM cradle, but only serves to mount the cradle, then I don't think this is illegal.

 

For those who are not familiar with this set-up, the other pair of mounting points mount directly to the lower torque boxes, which define the location of the IRS.

 

It's too bad that we had to outlaw the KB IRS cradle assembly, and any other IRS subframes that may be in development. Spirit of the rules vs. Protecting us from the slippery slope.... Maybe in the future we'll find a compromise that gives this trade-off some more consideration.

 

Scott

[Guest]

 

The first option would be to use Steeda's Subframe Bracket Kit (http://www.steeda.com/store/-catalog/suspension/IRSsubframebrackeet-150.jpg). This kit supports the outboard side of the OEM mounting bracket. Since it welds to the IRS mounting bracket and not the OEM cradle, I'd interpret that this part would be legal. Maybe JWL can confirm.

 

My second option (which would have little additional advantage, but would satisfy those who are more handy) would be fabricate an entirely new IRS bracket. Again, since your are modifying / replacing the bracket which is not a part of the OEM cradle, but only serves to mount the cradle, then I don't think this is illegal.

 

For those who are not familiar with this set-up, the other pair of mounting points mount directly to the lower torque boxes, which define the location of the IRS.

 

Scott

 

Having looked at the steeda mount on a car I find it to be a rather ineffective bandaid for the issue at hand.

 

The steeda peice does not bolt to anything that is structural and is of inferior design to a properly designed mount. The advantage of fabricating something like i have shown is that it will not flex and it firmly hold the carrier to the framerail. The steeda peice does little (in my opinion) to curb movement of the bracket due to the way it's designed and who it is attached to the inner rear fender housing (sheetmetal). With as fast as the AI cars are getting we should really think about allowing properly designed race car parts to be legal. The steeda part is for the weekend warrior and has little to offer a car that generates the side loads and speeds a front running AI car will.

 

a detailed pic of the steeda/hyland peice can be seen below.

 

 

Can anyone honestly say that is a structural peice or the proper design for an actual race car?

[turbolx]

With a little more area to that piece, it would be a good point to add a "kicker" support from the roll cage to it.

 

Another question that comes to mind is the effects (benefit?) of a raised or lowered rear pickup point for the IRS cradle. Although the torque boxes dictate fore/aft and lateral location on the cradle, it could still be rotated slightly about these two points causing changes in pinion angle and anti-squat. Removing the bushing and support leaves this pickup placement no longer exactly defined.

[Guest]

Changing the angle of the carrier will do nothing beneficial. you won't change the RC or the intersections of the upper/lower arms (SVSA length) but it may change the SVSA height. Without modeling the entire suspension on ADAMS or RBN you have no idea what is happening.

 

The correct way to do it (as was done in the first pic I posted) is to create a jig and build the mount to that jig as to not change the the orientation of the IRS cradle.

 

It's also easy to measure the angle of the cradle in the car as it sits on the ground.

[johnbasf]

If I had the choice it would be the first mounting option. The second pic seems like a band aid. The plate that was added is mounted to the tire well, not real structural. I think the bolts are too far away from the weld. There seems like a lot of room for flexing. JMO

 

http://www.seanhylandmotorsport.com/online/images/IRSsubframebracket.jpg

[mwilson7]

With a little more area to that piece, it would be a good point to add a "kicker" support from the roll cage to it.

 

Another question that comes to mind is the effects (benefit?) of a raised or lowered rear pickup point for the IRS cradle. Although the torque boxes dictate fore/aft and lateral location on the cradle, it could still be rotated slightly about these two points causing changes in pinion angle and anti-squat. Removing the bushing and support leaves this pickup placement no longer exactly defined.

 

Greg,

 

To be fair with the current rules allowing bushings to be unlimited what would stop someone from creating offset IRS bushings similar to what many do to the front steering rack bushings. I really don't see what advantage that this would have because you aren't changing the suspension geometry.

[swhiteh3]

If I had the choice it would be the first mounting option. The second pic seems like a band aid. The plate that was added is mounted to the tire well, not real structural. I think the bolts are too far away from the weld. There seems like a lot of room for flexing. JMO

 

http://www.seanhylandmotorsport.com/online/images/IRSsubframebracket.jpg

 

John-

 

That bracket is in tension and compression. I'd guess that it cuts down any motion at that joint by an order of magnitude. I'd have to disagree with you and say that there really is not that much flexing at all.

[swhiteh3]

Greg,

 

To be fair with the current rules allowing bushings to be unlimited what would stop someone from creating offset IRS bushings similar to what many do to the front steering rack bushings. I really don't see what advantage that this would have because you aren't changing the suspension geometry.

 

If you change the angle of the cradle, you HAVE changed the geometry. You have not changed the location of the pick-up points relative to one another, but you have changed the pick-up points location relative to the vehicle and the ground (reference plane). This absolutely will change the suspension geometry, and therefore characteristics of the vehicle. I'm not saying that there is any room for improving the car with this change - I'd actually tend to think that you'd probably only make the car worse by doing this. But it absolutely will change the kinematics.

[Guest]

Greg,

 

To be fair with the current rules allowing bushings to be unlimited what would stop someone from creating offset IRS bushings similar to what many do to the front steering rack bushings. I really don't see what advantage that this would have because you aren't changing the suspension geometry.

 

If you change the angle of the cradle, you HAVE changed the geometry. You have not changed the location of the pick-up points relative to one another, but you have changed the pick-up points location relative to the vehicle and the ground (reference plane). This absolutely will change the suspension geometry, and therefore characteristics of the vehicle. I'm not saying that there is any room for improving the car with this change - I'd actually tend to think that you'd probably only make the car worse by doing this. But it absolutely will change the kinematics.

 

Scott, you are technically right about the geometry, but it's not the RC (RCH), IC (SV), FVSA, SVSA. About the only thing you are changing is the SVSA Height. If you do a little Trig you will figure out that you are in fact changing the Anti's, very very little. I can't tell you what the effect is unless I had all the numbers but an educated guess is that the change would be almost imperceivable unless you change things by something over 10mm.

 

Since bushings are free this type of change is legal.

 

NOW BACK TO THE ORIGINAL QUESTION!!!!

 

I want to build a properly designed/engineered cradle mount that attaches directly to the frame rail. This sends all the forces into a primary structural member rather than a cantelevered bracket. Do a simple Force Body Diagram and you will see why this is a proper design and a well engineer solution to the issue at hand. I do not want to move the cradle from the OEM position AT ALL. I simply want to attach the unit in a way that is SAFE, STRONG and WELL ENGINEERED. The OEM mount is by defination an afterthought that was designed with a myriad of constrants. Fabbing the mount that I have shown is very very simple work for anyone that puts a cage in a car or installs any number of the suspension systems readily available and when done properly the cradle will sit in the EXACT OEM location.

 

BPT.

 

p.s. JWL i sent you a PM so be on the look out for that.

 

Thaks.

Edited December 19, 2004 by Guest

[swhiteh3]

Brian-

 

You're right - the change to geometry is small, and not neccessarily beneficial. I was just pointing out that there IS a small change in geometry.

 

Since the rules state that the crossmember must be in stock position, it would be my interpretation that you can change the bushing material, but that an off-center bushing would be illegal based on the fact that the cradle must be in stock position.

 

I hope that in the future we can find a trade-off where things like these and "mildly modified" IRSs will be allowed. But for right now, it's unfortunate that the cradle must be kept stock.

[JWL]

Guys-

 

While Brian's solution does look interesting, we are not inclined to allow it at this time. We are OK with adding material to the mount to strenghten it, but chopping off the old one and welding on a new one is not OK as it opens the door to all sorts of measurements for enforcement and other slippery slopes that we'd just as soon not have to work with. I understand that there is a desire to improve the IRS, but until we come up with a way to measure changes and enforce those measurements, we want to stick with the stock cradle mounting points for both the chassis mounts and the suspension mounts.

 

Thanks.

 

-JWL

[johnbasf]

What is NASA's process to determine whether it is a stock IRS? Where is NASA getting the "tech" about the modified IRS? Just curious JWL.

[mwilson7]

I'm VERY interested in hearing the answer to this one as the changes that we made can be measured in mm's.

 

With the current rules all IRS cars are now going to have to be inspected for stock parts. Who is going to be responsible to prove whether the parts are stock or not?

 

Way back when we discussed rules everyone was concerned about not creating more work for the Regional Directors or NASA folks but it would appear that the IRS rule has created quite a bit of workload because now a method for determining exactly what is stock and you must also keep up with any factory changes that might affect those parts that are being required to be stock.

 

It appears we have now gone down another slippery slope which requires tech inspections and tear downs.