Lets Talk: SCCA and Poly Bushings

[Whiskey11]

I wanted to have a discussion on Poly bushings in control arms for SCCA autocross in ESP and STX/STU (Eventually). For those outside the SCCA, the problem is that the SCCA does not allow the lower control arms to be replaced with anything. The bushing rules for ST are:

B. Suspension bushings may be replaced with bushings of any materials (except metal) as long as they fit in the original location. Offset bushings may be used. In a replacement bushing, the amount of metal relative to the amount of non-metallic material may not be increased. This does not authorize a change in type of bushing (e.g., ball and socket replacing a cylindrical bushing) or use of a bushing with an angled hole whose direction differs from that of the original bushing. If the Stock bushing accommodated multi-axis motion via compliance of the component material(s), the replacement bushing may not be changed to accommodate such motion via a change in bushing type, for example to a spherical bearing or similar component involving internal moving parts. Pins or keys may be used to prevent the rotation of alternate bushings but may serve no other purpose than that of retaining the bushing in the desired position.

A few problems to consider there:
First: We can't run anything resembling a Delrin spherical bearing since we have to still accommodate articulation via compliance of the bushing.

Second: We can't increase the metal to non-metal ratio.

I think the second is going to be hard to protest, and a wheenie protest at that but I suppose it is protestable. As it stands right now, I can only find one company that produces a poly bushing for the stock lower control arms and that is Prothane. The kit # is 6-313. From the design of the kit it would appear to be a mistake to run it in unmodified form for autocross duty so I have two ideas for how to "fix" that problem but I'd like some input from people here with more specific knowledge on suspension bushing design.

Here are my thoughts on the modified bushing (yellow lines indicate holes drilled in the bushing):




Idea 1 is the easiest to implement and should allow for some articulation in the vertical plane (axle articulation) but there will be some bind associated with it. Idea 2 would have a different effect on bushing bind, and I think it might reduce it over the first design.

The only other thought I had would be to make a 3 piece bushing, the center being the highest durometer, the outers being a lower durometer with cutouts similar to the factory bushings meaning holes through the outer two parts of the bushing in the upper and lower portions (where they are on the ideas above) to allow it to articulate more free while retaining the fore/aft stiffness necessary to combat bushing wheel hop and longitudinal location of the axle.

The reason I ask is because the Torque Arm I plan on purchasing (well any torque arm) is going to be removing a longitudinal locating part of the suspension (the UCA) and that presents itself with the added problem of keeping the axle centered in the fender wells with the stock control arms.

Anyone have any better thoughts that are SCCA legal?

 

[dontlifttoshift]

I don't know if I would spend the time drilling the bushings. The arms are hourglass shaped and I think that you will find they twist pretty easily.

I also think you will find that you will want those bushings to be as stiff as they can be. The LCAs are the only thing pushing the car forward. Take the minimum penalty in "roll stiffness" for maximum gain in drive off the corner. You can make up for the added roll rate a hundred different ways. You are going through all this work and expense to get more drive off the corner, right?

 

[Whiskey11]

I don't know if I would spend the time drilling the bushings. The arms are hourglass shaped and I think that you will find they twist pretty easily.

I also think you will find that you will want those bushings to be as stiff as they can be. The LCAs are the only thing pushing the car forward. Take the minimum penalty in "roll stiffness" for maximum gain in drive off the corner. You can make up for the added roll rate a hundred different ways. You are going through all this work and expense to get more drive off the corner, right?

It would probably take about 10 minutes to drill all four bushings for better articulation and costs nothing but time to do (I have the tools or access to them)... a drill press for the vertical stuff and a guided hand for the other. My car doesn't have a lot of body roll, but it's enough that I think a poly setup without care to allow things to articulate could be a potential problem. I'd prefer to keep the twisting from happening in the LCA itself as that sounds like needing to replace the LCA every few years. Yes, they are cheap on the forums, but replacing bushings all the time could get tedious!

So finding a solution that is stiff in the longitudinal direction while allowing articulation is the ultimate goal. I priced out sheets of poly at McMaster-Carr and it isn't horribly expensive. They have 5 different durometers of Polyurethane from 60 to 95. A 95 center with softer outers could be done as well.

 

[dontlifttoshift]

I imagine it would take cycles in the millions to wear out the control arm itself due to twisting. I really do believe it was built that way on purpose.

As stated in the rules, you will have to pin the bushing to the control arm somehow to keep the bushing from rotating and your newfound softness ending up somewhere you didn't intend it to.

The Gt500 arms aren't legal, right? I thought I read that you said that but i don't recall why.

 

[Whiskey11]

I imagine it would take cycles in the millions to wear out the control arm itself due to twisting. I really do believe it was built that way on purpose.

As stated in the rules, you will have to pin the bushing to the control arm somehow to keep the bushing from rotating and your newfound softness ending up somewhere you didn't intend it to.

The Gt500 arms aren't legal, right? I thought I read that you said that but i don't recall why.

The rules state that the bushing may be pinned but does not say it must be pinned. I do agree though that some method of keeping the bushing from rotating the wrong way might be desireable and I will have to think of something to prevent it as I dont think friction between the bushing face and the axle/chassis mounts will be enough.

One other thing I need to think of is the offset in the arms and if running a larger offset would reduce roll steer more. My car is lowered enough to have noticeable roll understeer and it would be nice to take some of that out if possible.

The GT500 arms are not legal in ST unless you have a 2011+ car and convert all the necessary equipment over to make it a Brembo Mustang since that is the only ST legal car that comes with them. Update/backdate in ESP makes it legal to do without a wholesale package change because of the Brembo Mustangs.

This is a good discussion, hopefully others will chime in and we can come up with a diy solution that meets the wonky rules of the SCCA.

 

[dontlifttoshift]

The rules state that the bushing may be pinned but does not say it must be pinned. I do agree though that some method of keeping the bushing from rotating the wrong way might be desireable and I will have to think of something to prevent it as I dont think friction between the bushing face and the axle/chassis mounts will be enough.

My understanding of urethane bushings, at least from the hot rod side of things, is that the center steel sleeve should be locked by friction to the chassis and axle mounts. The bushing should be free to rotate around the center sleeve. More often than not, that is not the case, and the center sleeve isn't wide enough for that to happen without preloading the bushing itself a lot.

One other thing I need to think of is the offset in the arms and if running a larger offset would reduce roll steer more. My car is lowered enough to have noticeable roll understeer and it would be nice to take some of that out if possible.

Off setting the bushings will have no effect on roll steer. The chassis side and axle side mounting positions are fixed so offsetting the bushings will only change the location of the control arm but the geometry will stay the same.

The GT500 arms are not legal in ST unless you have a 2011+ car and convert all the necessary equipment over to make it a Brembo Mustang since that is the only ST legal car that comes with them. Update/backdate in ESP makes it legal to do without a wholesale package change because of the Brembo Mustangs.

I didn't realize that, so my '12 brembo car is legal if I installed the GT500 LCAs? Seems odd.

This is a good discussion, hopefully others will chime in and we can come up with a diy solution that meets the wonky rules of the SCCA.

Yeah, I love this stuff. Other than the classifieds, this is the only section of the forum I pay attention to.

 

[Whiskey11]

My understanding of urethane bushings, at least from the hot rod side of things, is that the center steel sleeve should be locked by friction to the chassis and axle mounts. The bushing should be free to rotate around the center sleeve. More often than not, that is not the case, and the center sleeve isn't wide enough for that to happen without preloading the bushing itself a lot.



Off setting the bushings will have no effect on roll steer. The chassis side and axle side mounting positions are fixed so offsetting the bushings will only change the location of the control arm but the geometry will stay the same.



I didn't realize that, so my '12 brembo car is legal if I installed the GT500 LCAs? Seems odd.



Yeah, I love this stuff. Other than the classifieds, this is the only section of the forum I pay attention to.

You and I are on the same page about how a poly bushing should be setup and the roll of the center sleeve. On the rollsteer front, I thought that splaying the control arms results in a change on how much roll steer the axle sees due to LCA inclination in sideview. Surely there is some ability to offset them further in the stock mounts to achieve some effect.

The GT500 arms are legal for Brembo cars because you already have those control arms on your car, that's why.

 

[lost won]

I didn't realize that, so my '12 brembo car is legal if I installed the GT500 LCAs? Seems odd.

I think what Whiskey 11 might have meant is that your '12 Brembo already has the same LCAs as the GT500.

OOPS! Whiskey is fast. Beat me to this reply.

 

[dontlifttoshift]

Whiskey, are you talking about moving the control arms side to side looking at it from top/plan view?

Gt500 control arms. If we are talking about these then I do not have those on my car.

 

[Whiskey11]

Whiskey, are you talking about moving the control arms side to side looking at it from top/plan view?

Gt500 control arms. If we are talking about these then I do not have those on my car.

Yes, sorry, top/plan view and adding more offset than factory. I doubt I could offset the.bushings enough in side view to take out roll understeer that way, there is over an inch of.height difference there.

As for the rear LCAs, are you sure you dont have them because its part of the Brembo package for 2011+ cars or at least it is supposed to be.

 

[Vorshlag-Fair]

You make it sound like only Prothane sells a poly bushing for the S197... ??

We've had this Energy Suspension lower front Control Arm Bushing set for sale for the S197 since August 2011.

​

Install gallery is here: http://vorshlag.smugmug.com/Instructions/Eibach-poly-LCA-bushing/

As for the increased articulation, well... looking at your post you never refer to the REAR control arms, so I thought you meant the FRONT bushings. Which matter a LOT more for suspension geometry control, which matters more than the rear. But for the rear...

They have an UCA bushing set and a Panhard bar bushing set. But no lower control arm bushings. A-ha! That's why you said only Prothane. I'm a little slow, but you did leave out the rear LCA detail, which wqe had to figure out the hard way.

OK, sorry... proceed.

 

[Whiskey11]

You make it sound like only Prothane sells a poly bushing for the S197... ??

We've had this Energy Suspension lower front Control Arm Bushing set for sale for the S197 since August 2011.

​

Install gallery is here: http://vorshlag.smugmug.com/Instructions/Eibach-poly-LCA-bushing/

As for the increased articulation, well... looking at your post you never refer to the REAR control arms, so I thought you meant the FRONT bushings. Which matter a LOT more for suspension geometry control, which matters more than the rear. But for the rear...

They have an UCA bushing set and a Panhard bar bushing set. But no lower control arm bushings. A-ha! That's why you said only Prothane. I'm a little slow, but you did leave out the rear LCA detail, which wqe had to figure out the hard way.

OK, sorry... proceed.

Yeah sorry if that wasnt clear. This post was intended to discuss all poly bushings compliant with SCCA rules with the starter being on the rear LOWERS of which Prothane is the only kit I know of for the stock arms. I looked on Energy's page and saw none either.

 

[Justin_H]

Besides the bushings and the color of the finish, what distinguishes the GT500 rear arms from the GT arms? Seems like a reasonable case could be made that they are just a bushing change and paint, which isn't restricted.

 

[Whiskey11]

Besides the bushings and the color of the finish, what distinguishes the GT500 rear arms from the GT arms? Seems like a reasonable case could be made that they are just a bushing change and paint, which isn't restricted.

The arms themselves are different as IIRC the arms are slightly thicker on the GT500 arms and thinner on the GT's. I think it's more than just paint and bushings.

 

[Norm Peterson]

You need to know exactly what the Prothane bushing shape is and what the dimensions of the inner sleeve are - I suspect they differ somewhat from your sketch. And what any outer sleeve measures.

I doubt you really need to drill the holes clear through. Once the remaining "solid" section drops down under half the length of the outer sleeve (specifically the length over which torsional LCA moment loading is applied), you're down under 20% of the moment stiffness . . . which is approaching the OE situation. I do see a need to drill more holes in the upper and lower quadrants, so that you've got some void area out at least to the vertical projections of the inner sleeve.

The work I see on the faces is protest-proof, as both ends of the OE inner sleeves extend well past the rubber. Worst case for the drilled holes - fill them with nerf foam or something similarly soft.

Perhaps fitting a grease zerk could fulfill the pinning function. Or if that's seen as protestable (by providing another function), run a same-thread bolt or plug except for when you're re-lubing the bushings. Yeah, that'd be a pretty damn weenie protest, but then again I'd be afraid of how any "clarification" might end up reading if you tried to get the lube function accepted (like anybody would ever lube anything while out on course).

I've done all of those bushing mods or variations on them before, on some bushing or other. In some applications it is possible to remove enough poly-induced roll stiffness via these bushing modifications to clearly notice in terms of lateral head toss driving over one wheel bumps and depressions. No numbers, but it'd have to be significant in terms of in-lb/deg rear roll stiffness. Whether the S197 responds that well depends on what the bushings actually look like and how far you take the modifications.


Hint: do some OE metal and non-metal volume calcs - I think there might be a few more things you could legally do.


Norm

 

[Whiskey11]

You need to know exactly what the Prothane bushing shape is and what the dimensions of the inner sleeve are - I suspect they differ somewhat from your sketch. And what any outer sleeve measures.

I doubt you really need to drill the holes clear through. Once the remaining "solid" section drops down under half the length of the outer sleeve (specifically the length over which torsional LCA moment loading is applied), you're down under 20% of the moment stiffness . . . which is approaching the OE situation. I do see a need to drill more holes in the upper and lower quadrants, so that you've got some void area out at least to the vertical projections of the inner sleeve.

The work I see on the faces is protest-proof, as both ends of the OE inner sleeves extend well past the rubber. Worst case for the drilled holes - fill them with nerf foam or something similarly soft.

Perhaps fitting a grease zerk could fulfill the pinning function. Or if that's seen as protestable (by providing another function), run a same-thread bolt or plug except for when you're re-lubing the bushings. Yeah, that'd be a pretty damn weenie protest, but then again I'd be afraid of how any "clarification" might end up reading if you tried to get the lube function accepted (like anybody would ever lube anything while out on course).

I've done all of those bushing mods or variations on them before, on some bushing or other. In some applications it is possible to remove enough poly-induced roll stiffness via these bushing modifications to clearly notice in terms of lateral head toss driving over one wheel bumps and depressions. No numbers, but it'd have to be significant in terms of in-lb/deg rear roll stiffness. Whether the S197 responds that well depends on what the bushings actually look like and how far you take the modifications.


Hint: do some OE metal and non-metal volume calcs - I think there might be a few more things you could legally do.


Norm

Thanks Norm, I'll look into it when they arrive. Do you think I would need to do this to both ends of the LCA or would just the axle side be sufficient?

 

[Norm Peterson]

Both ends.

A true link (what you're trying to approach) has zero moment stiffness at both ends. Torsionally, if you cannot eliminate virtually all of that moment at one end as with a spherical, you need to minimize it at both ends. For the lateral bending mode of the LCA, even one spherical is not enough to eliminate this stiffness component.


Norm

 

[Whiskey11]

Alright, I got the poly bushings in today and they are nearly hard plastic so I'm assuming they are a high durometer, probably 95. That almost universally guarantees needing modification of some method. To give you an idea, here is what they actually look like courtesy of Summit Racing:

The instructions that I received with the kit were for the earlier Mustangs (the picture of the axle shows quadrabind mess) but I'm pretty sure these will work correctly for us. I'll have to pull the control arms to make doubly sure but I'm confident they will work.

That said, for the modifications, I'm thinking three relatively small holes, maybe 1/16th in diameter in the bottom and top portion of the bushing to allow for articulation in axle rotation but not in the horizontal direction. I've purchased a second set of control arms from a member here which I will be using as a back up pair in case I screw these guys up.

 

[Norm Peterson]

Dunno if 1/16" holes will be enough, but it's better to start too small and work up.

I'd specifically plan on revisiting the hole size. IIRC, I started at 1/8" x several holes top and bottom . . . and still went bigger.

Find a drainage gutter or possibly a driveway entrance that does not have a discrete 'step' that you can slowly drive across at an angle (the sharp bump will confuse what you want to notice). Note how much your head and upper body get tossed laterally. Start with everything OE so you have a sense of how much stiffness the poly bushings added.


Norm

 

[a50cobra]

Whiskey, any updates on this?
I was about to start a new thread before I ran across this one (thanks Search). I have been following your autox thread but don't see the mention to this upgrade.