SCCA ST* Classing question (Miata content)

Hi all

As you probably know, poly bushings are great for a few reasons (cheap, stiffer than rubber), but they tend to bind up and not work well. A potential workaround is to install grease zerks, but then you have continual maintenance to keep them working.

There's a guy on MT making a setup to retrofit off-the-shelf poly bushing kits with smaller sleeves and bronze bushings. These should theoretically solve the binding issue.

So here's my question - would this violate 14.8.b, which states

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 standard 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.

I'm inclined to think yes, due to that last bit about "a similar component involving internal moving parts", but I'm not sure. It seems that under the strictest interpretation of this rule, poly bushings themselves might violate it - correct me if I'm wrong, but aren't the stock bushings bonded to the metal sleeve?

Should I just skip all of this and go for delrin?

Here's a link to the sleeves, in case you're interested

http://www.miataturbo.net/suspension-brakes-drivetrain-49/miata-bushing-megathread-heirarchy-diy-delrin-dimensions-info-discussion-87573/

What class are you trying to verify this for? If its for STS or STR, then poly bushings are fine.

Sorry if this was unclear.

This is for STS. The retrofit thing he does uses a normal poly bushing, but uses an undersized sleeve, so allow for an oil impregnated bronze sleeve bearing. So instead of 2 pieces - the poly, then the sleeve, there's 3 - the poly, then the bearing, then the sleeve. You're not increasing the amount of metal relative to bushing material, since the sleeve is smaller to accomodate.

cmcgregor wrote: Sorry if this was unclear. This is for STS. The retrofit thing he does uses a normal poly bushing, but uses an undersized sleeve, so allow for an oil impregnated bronze sleeve bearing. So instead of 2 pieces - the poly, then the sleeve, there's 3 - the poly, then the bearing, then the sleeve. You're not increasing the amount of metal relative to bushing material, since the sleeve is smaller to accomodate.

If, somehow, the metal content is the same as a stock bushing, you're good as long as it's still cylindrical and pointed at the same angle as OEM. You're not adding multi-axis motion with your sleeved sleeve nor are you deliberately using a sloppier bushing material (uh...bubble gum?).

I'd be more worried about how much metal can you get away with. Measure the OD and ID of the metal cylinder in the stock bushing. That's how much metal you have. If those values aren't the same after you split that piece into two rings and fill it with bearings, you'd need to prove that you aren't adding more metal, which would not be fun (CAD? Volume displacement measurements?).

If it comes up, given the dimensions of everything I can get you an answer on volume of metal in old vs new in a matter of minutes. That's easy as far as the math goes.

The volume of metal is no greater than that of the poly bushings as sold by energy suspension, or whoever. The holes in the bushing aren't enlarged to retrofit them (with one exception). That's not to say that the poly bushings are technically legal, I don't know, but plenty of people use them. I was more worried about this being considered a change in bushing type, since you're adding a bearing surface. I can see it argued both ways

RedGT wrote: If it comes up, given the dimensions of everything I can get you an answer on volume of metal in old vs new in a matter of minutes. That's easy as far as the math goes.

I'm hoping it's kosher because I want some for myself!

I've always wondered if that "amount of metal" mentioned in the rules was mass or volume. :)

Both.

Brett_Murphy wrote: Both.

I would say that meeting the rule by volume meets the spirit of the rule. Not an expert, just saying. If someone ran these i would not object. I also currently run rubber bushings because I am lazy and none of the benefits of a bushing swap seemed worth it. Heck, i have run near-identical prepped and aligned cars, one with poly bushings one with rubber, and couldnt tell the difference nor did my relative speed to other people change in any measurable way.

Interesting.

I probably won't buy them this year, but good to know they're probably fine. I agree poly isn't necessarily better than stock rubber, but they are cheaper if your bushings need to be replaced anyway. And, since both Miatas I've prepped have had over 200k on them, they've needed to be replaced haha